iom yacht foils

Achieve Your Potential in Competition IOM Sailing

Vickers RC Sailing

Producer of the V11 Competition International One Metre Design by Ian Vickers.

Our workshop opened in 2014 with the V8 IOM. Since then we have produced hundreds of hand crafted IOM yachts in the Vickers design range. Yachts that have helped IOM competitors around the world achieve their potential and exceed expectations in competition.

We are currently offering the V11, the innovative design that finished 3rd at the 2019 World Championships in Port Alegre, Brazil.

When our clients return, they know they are tapping in to smart, competitive technology in IOM design and build. Stiff, Lightweight boats, that are Easily Assembled, Fast and Forgiving to sail. Join the fun!

Improve your results at the club and the regatta, with the fast and forgiving V11 from Vickers RC Sailing

Set Up and Sail

The V11 detailing is simple but effective. In conjunction with the V11 Set Up Guide, assembly is straight forward, and vital tuning information assists you to quickly understand and master your V11's potential

Vickers RC Sailing build processes are well refined and thought out.

Our boats are light, stiff and clever,  keeping to a simple and effective philosophy around detailing

The V11 design is a hull, foils and bulb package to compliment the full range of conditions experienced in RC Sailing. The hull design has full volume forward and features the familiar chines and tumblehome of the modern IOM for reduced weight and windage, as well as good tracking characteristics. The V11 features our peaked style foredeck for efficient water shedding when buried downwind and a geometrically strong shape for the forestay and mast ram areas.

Foils are slippery and strategically positioned with the rig to balance the yacht. The V11s forgiving balance through the wind range, allows the skipper to focus on the race at hand, and not rely on micro steering to maintain a consistent VMG.

The rig sits solid on its deck stepped mast step, complimented at deck level with an encapsulating mast gate arrangement over the mast ram. The rudder tube is rigidly supported inside the moulded servo tray. 

Hull construction is from E-Glass Fibreglass Cloth and High Grade Epoxy Resin.

The foils are Carbon Fibre Layup with High Grade Epoxy Resin.

Paint system is a 2 pot Urethane and the boat is thoroughly post cured before finishing.

The V11 Features-

Plastic main hatch access. 

Ready to accept RMG Winch and Standard sized Rudder Servo

Bow Bumper attached.

Hardware is Bantock including an adjustable mast ram, adjustable mainsheet post and tiller arm.

All necessary fittings attached for standing and running rigging.

The V11 comes out of our workshop in Beach Haven, Auckland New Zealand and is ready to accept rigs and electrics.

We also make sails that can be added to the order, or ordered separately.

So hit the enquiry button to discuss delivery times and options.

We are ready to help.

Essential to success in sailing comes down to the  Rig and how it is set up and tuned. The V11 comes with a Set Up Guide that covers all the expert information needed to set up like the guns. It specifies the hardware, construction and tuning information to get your rig and sails set up and sailing near optimum straight off the bat, so you can enjoy your racing and punch above your weight.

Include Sets of sails from Vickers RC Sailing and enhance the experience.

Vickers Rc Sailing is back taking orders again for the V11

We look forward to hearing from you 

Thanks for submitting!

• Classifieds
• Remember Me Forgot Password?
•   Boats Sailboats Build Log How to make sailboat rudders and keels in 6999 easy steps

Page 1 of 11

1

	. In that thread I show several techniques for casting lead bulbs from simple plaster molds to complex CNC aluminum machining. In this thread I will be doing some similar things, but will not be rehashing some of the intricacies that I have already delved into there, such as the aspects of CAD design and CAM processing. Similarly, I won’t be describing EVERY technique available, just the ones I use for my own projects. Hopefully, this will make things a bit easier on me as far as documentation so I am hoping to be able to get things done in 6999 steps instead of 7000 this go around! I also have another build thread going that details the building of the Alioth, a 3d printed IOM. . I will be showing how I make the foils for that boat in this thread. The plan is that these threads will all start to merge together into a finished project as I progress. The bulb and foils threads showing construction techniques for those parts and the boat thread showing the installation. I am keeping these aspects separated because, as in the casting thread, I work on several casting projects. Likewise, I will be doing more than just the Alioth IOM foils in this build thread. Just as in the other threads, I am starting out with a blank slate. I am no expert in making these things and in fact have never made an RC boat rudder or keel fin before. I think this is very important to point out because I want YOU to be able to say to yourself, “If this idiot can do it, maybe I can too!” That being said, I have some past experience that I am bringing to the table. I have a 25 year background as a professional woodworker and bench jeweler. Nearly three decades in computer art and design work. I have a shop filled with WAY more than the average amount of tools. If there is something in my head that I want made, it is generally gonna get done. One of my key past experiences that will apply in this thread is that I have a history of building with composite construction. I used to be as much into high performance sailplanes as I am currently into sailboats of the same caliber. If you were flying a Taboo GT or XP-5 discus launch glider a decade or so ago, I probably made your fuselage. I was also making DLG carbon tailbooms and made the first fuselages and booms for the Mark Drela XCBD (Cross-country Bubble Dancer). I even built my own CNC 4-axis foam cutter from scratch to make cores for composite sailplane wings and rudders. So, yes I am new to sailboat foils, but will be bringing a lot to the table. I hope to have some fun and do a lot of learning. I also hope for participation and advice from others that have already been down this road. I cannot stress how important it was in my bulb casting thread to stand on the shoulders of others. I will be doing a lot of the same here. I am not an engineer, I just write the manuals.
	.

	.

	Graham Herbert thinks nature may have figured out a good shape for his IOM foils already.
	.

	So, there are some definite starting points mentioned above. I have found that there are many roads to take and each builder has their own little twist to add. I am not sure there is a perfect solution, but half the fun is in trying to find one anyway. Someone is screaming at their screen right now. “OMG how can you put forth info on designing a rudder for a boat without doing the dozen calculations required for proper this that and the other effect of blah blah blah., etc.” Look man, I am not here to present a dissertation. I am here to show you how I built a pretty decent IOM rudder that is probably gonna work out fine. See the previous post for my thoughts how things are going to be OK even if you don’t do the math. This is just a prototype whose sole purpose is to verify that I can make a thing that looks about like an IOM rudder to start with. Think about this thread as a vocational tech. class mixed with arts and crafts, not a science class. It is the way I know how to work. Yeah, I took Honors Physics in High School, but even in an Honors class somebody had to get the lowest grade in class and barely pass. Not ashamed to say that in 1991 that person was me! This keel fin is by Gabriel Le Duc. He is doing some nice work in Southern France and I have been following his progress on FB.
	.

	Rudder drawing in jpg format. pdf
	.

	Here are experiments from weeks ago showing early 3d print tests leading up to this. Is that a vacuum chuck plate and CNC carved foam cores on the left? Sure is, but that will come later. Right now, we are in printer land!

	.

	I just want anyone that hasn’t been there before to go check it all out. The doc you want is in “Other” and called Center of Effort Location - fin or rudder. I will also have the PDF for direct download at the bottom of this post. I won’t re-type the instructions, as they are fairly clear on the process. Here is a detail you should pay attention to. Notice the line chosen as the bottom width of the rudder is not at the bottom of the rudder. The rudder tip has a radius to it and Sailsetc. has picked a location about midway through that curve. If you also notice that if you cut the tip off that line, you could about fit that tip piece in the gap near the leading edge that line. This eye-balled line is a bit of a guess. Likewise the location of of the rudder stock axis is said to be placed “no less than 3mm ahead of the center of effort point”. OK, but is there not a “no than Xmm ahead” limit. So, I chose a spot that was 4.5mm ahead of the CE because that as also the location of the thickest point of the rudder airfoil to give maximum room for the rudder stock. When typing up this post I did not want to get complex, but wanted to do more than just throw out the term Center of Effort and devote a post to finding it with no other info at all on why it matters. Luckily, I found some info presented at at a simple level without using any math titled The Physics of Sailing. Take a look here. Mr. Pierce says that CE and CLR represent the centroid of the average forces that are contributed by the foil. So Sailsetc. has provided a way to find that by estimating the location of the centroid (Center of Effort) of the area of the rudder or fin from a 2D view. I used their instructions to find that location with my CAD software. But I also have the ability to push a button and have the software find that exact centroid point using the 3D shape and did so after doing it “manually” and compared locations. Well dangit, they aren’t in the same place. Rhino3D says the 3D volume centroid is about 10.9mm behind and 7mm above the Sailsetc. 2D area centroid position. Hmmm. I proceed to the next step in the instructions which tell you to draw a line from that point to the leading edge and bisect it. I do so and have a minor jaw drop when I see where the midpoint of this line is. If you recall the CE point calculated by the Sailsetc. 2D method places the rudder axis “no less than 3mm” forward from the CE. My computer has taken the complete 3D data of the rudder and found that axis line at this EXACT 3mm offset location. It is kinda cool that you can bypass some advanced math by essentially just saying “move this point ahead about 3mm in the 2D view to make up for the contributions of the 3D volume and you will probably be alright”. pdf
	.

	Finally, a picture! Here is a mold half, fresh off the printer.
	.

	This looks like an unfinished sanding job, but this is the point you can stop at.
	.

	You can also order from Smooth-on, which is where the Amazon stuff ships from anyway, so check to see if you can save buying direct. Make sure your molds are free of any dust, grit, and crud. I mixed up a 15cc batch of epoxy for each mold and did them one at a time so I could keep the epoxy flowing and runny. I only coated the tops of the molds. No reason to do the sides and back. I used a 1/2” shop brush to apply it all over the top. It helps to focus on getting it a bit “rubbed in” to the plastic. The epoxy may try to pull away from the plastic in a few spots, but working it in just a bit will get it to behave. Make sure there are no puddles, thin spots, or brush hairs (or your hairs) on the surface. Then take several passes down the long axis with your brush, smoothing and evening the coat out while working in full length strokes from tip to the open end of the mold so that excess epoxy is removed as I go. What works nicely at this point is to apply some gentle hot air while holding the mold vertically. This will help surface air bubbles to expand, pop, and self level. It will decrease the viscosity and helps excess epoxy slide down and off the mold. It can also help you smooth out runs and sagging. I have a small heat gun that works great for stuff like this and is fantastic for heat shrink as well. I use it for all kinds of stuff. CAUTION: Do not overdo the heating. You can burn the epoxy or soften the mold. You can cause curing to begin immediately. You can cause tiny bubbles become huge, hardened lumps. What you can get away with will depend on your heat source. Test out your technique on a sample print before you coat the molds. I have quickly flashed a propane torch over epoxy for a second or two just to pop bubbles. I have also spent a minute of time working on flattening a run with a low temp heat/air source. Experiment until you have some experience. Now stand your mold vertically and leaned back a bit so no epoxy accumulates at the open base. It is fine for it to run off the end and drip freely. You can pop the runs off the mold base with a chisel later. Don’t be too disturbed with how it looks. My pictures below will show you several mistakes, such as a trapped hair in the black mold. Also, hardened bubbles because it was hot as Hades in my shop that day and the epoxy was bit too stiff by the time I remembered I needed the hot air gun. I even had epoxy build up the open end because I forgot to tip the molds back once I had my area cleaned up for the day. The first coat of this stuff almost always looks like poo, even on my best days. It will come out fine in the end. Looks ugly? Yep. No worries. It will get taken care of. First coat is on. Not pretty, but it is even.
	.

	This is the black mold from the previous post after being having the first coat sanded, but before the 2nd epoxy coat. I told you it was going to come together! Don’t sweat those scratches. Coat #2 will manage them.

RCGroups Message

Quick Reply
The following errors occurred with your submission

Thread Tools

Similar Threads
Category	Thread	Thread Starter	Forum	Replies	Last Post
Discussion		MtnGoat	Life, The Universe, and Politics	50	Aug 03, 2022 06:31 PM
Build Log		will_newton	Sailboats	89	May 22, 2022 04:43 AM
Question		BerkannY	Ornithopters	5	Apr 08, 2022 04:38 PM
Discussion		spencer911	Sailboats	12	Oct 20, 2015 02:03 PM
		Paul Penney	Scratchbuilt Indoor and Micro Models	209	Apr 20, 2009 03:42 PM

• Electric Flight
• Advertising
• Our Sponsors
• Review Policies
• Terms of Service
• Privacy Policy
• Site History
• Mark Forums Read
• Member Search
• Upcoming Articles
• Do Not Sell My Data
• Manage Consent
• Back to Top

• 1 (current)
• Product Name

RG65 Carbon Keel & Rudder

Universal carbon reinforced keel & rudder set can be used for our Vanquish 65, Noux 65, Siri 65 and all…

RG65 Fin & Mast Case

Fin case for our RG65 Keel with incorporated mast slot allowing mast to be positioned in a 30mm range and…

IOM Carbon Keel No. 1

Universal carbon reinforced keel which fits our Vanquish, our Terminator, Noux II, Siri, Maniac II, Laerke and other IOM yachts.…

IOM Fin & Mast Case

Fin case for our IOM Keel No. 1 with incorporated mast slot allowing the rig to be positioned and raked…

IOM Carbon Rudder Type 1

Carbon reinforced rudder for IOM class boats. This rudder can be used for our Terminator, Maniac II and other all…

IOM Carbon Rudder Type 2

Carbon reinforced rudder for IOM class boats. This rudder can be used for our Vanquish, Noux II and all other…

Marblehead Keel Fin & Rudder Combo - Type 1

Carbon fiber reinforced keel fin & rudder combo - fits the Sagitta and other Marblehead yachts. Keel Fin : - length…

Marblehead Carbon Keel Fin - Type 1

Carbon fiber reinforced keel fin for the Sagitta and other Marblehead yachts. - length 630mm - chord 75mm to 50mm …

Marblehead Carbon Rudder - Type 1

Carbon fiber reinforced rudder for the Sagitta and other Marblehead yachts. - length 205mm - weight about 50g…

T-foil Rudder for Mini40 boats

Our T-foil rudder reduces the tendency to pitchpole, it is recommended for beginners while learning to sail their multihulls and…

Asymmetric V-foils for Mini40 boats

Our asymmetric V-foils generate more lift than symmetric V-foils, they can be used to lift the boat to sail on…

Please read our GDPR statement

Privacy policy and general data protection, terms and conditions of use.

PLEASE READ THE FOLLOWING TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS SITE. By using this site, you signify your agreement to these Terms and Conditions. If you do not agree to these Terms and Conditions, do not use this site. RCSails  may modify these Terms and Conditions at anytime.

This site is copyright protected.

THE MATERIALS IN THIS SITE ARE PROVIDED "AS IS" AND WITHOUT WARRANTIES OF ANY KIND EITHER EXPRESS OR IMPLIED.  RCSails  DOES NOT WARRANT OR MAKE ANY REPRESENTATIONS REGARDING THE USE OR THE RESULTS OF THE USE OF THE CONTENT OR OTHER MATERIALS IN THIS SITE IN TERMS OF THEIR CORRECTNESS, ACCURACY, RELIABILITY, OR OTHERWISE.

TO THE FULLEST EXTENT PERMISSABLE PURSUANT TO APPLICABLE LAW, RCSails  DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING, BUT NOT LIMITED TO IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NON-INFRINGEMENT. RCSails DOES NOT WARRANT THE ACCURACY, COMPLETENESS OR USEFULNESS OF ANY INFORMATION CONTAINED ON THIS SITE. RCSails  DOES NOT WARRANT THAT THE FUNCTIONS CONTAINED IN THE MATERIALS AVAILABLE ON THIS SITE WILL BE UNINTERRUPTED OR ERROR-FREE, THAT DEFECTS WILL BE CORRECTED, OR THAT THE MATERIALS, THIS SITE OR THE SERVER THAT MAKES THEM AVAILABLE ARE FREE OF VIRUSES OR OTHER HARMFUL COMPONENTS. YOU, AND NOT RCSails  ASSUME THE ENTIRE COST OF ALL NECESSARY SERVICING, REPAIR AND CORRECTION.

UNDER NO CIRCUMSTANCES, INCLUDING, BUT NOT LIMITED TO, NEGLIGENCE, SHALL RCSails  BE LIABLE FOR ANY SPECIAL OR CONSEQUENTIAL DAMAGES THAT RESULT FROM THE USE OF, OR THE INABILITY TO USE, SITE OR ANY DOWNLOADED MATERIALS, EVEN IF RCSails  OR ITS REPRESENTATIVE HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. IN NO EVENT SHALL RCSails   TOTAL LIABILITY TO YOU FROM ALL DAMAGES, LOSSES, AND CAUSES OF ACTION (WHETHER IN CONTRACT, OR OTHERWISE) EXCEED THE AMOUNT YOU PAID TO RCSails  IF ANY, FOR PRODUCTS PURCHASED ON THIS SITE.

APPLICABLE LAW MAY NOT ALLOW THE EXCLUSION OF IMPLIED WARRANTIES, OR THE ABOVE LIMITATIONS OF LIABILITY, SO THE ABOVE EXCLUSIONS MAY NOT APPLY TO YOU.

Links to Other Web Sites

Submissions.

You are solely responsible for the content of any comments you make. You agree that no comments submitted by you to this web site will: (i) violate any right of any third party, including copyright, trademark, privacy or other personal or proprietary rights; (ii) be or contain libelous or otherwise unlawful, abusive, or obscene material or constitute the misappropriation of the trade secrets of any third party; and (iii) disparage the products or services of any third party. You agree not to submit any personal information (other than your email address or user name) through email sent to other users or messages posted on this site by you.

Termination

• RG65 Yachts
• 2m - Multis
• Mini40 - Multis
• MultiONE - Multis
• 65M - Multis
• Options / Shipping / Insurance
• Classifieds / Used Sailboats

• Shipping & Returns
• Privacy Notice
• Conditions of Use

The first thing to understand is that the fin, acting under the water, is balancing the sail forces developed in the air. As the wind comes up and the sails develop more drive, the fin has to develop "opposite" lift in order to keep the boat more or less on course. In order to develop lift to oppose and thus balance the sail forces, the boat must sail at an angle in the water, an angle we otherwise know as leeway. Typically, an IOM might sail at a leeway angle of about 3 degrees in No.1 rig over much of the wind range.  (Leeway develops for boats with "simple" symmetrical fins.  If the fin has an effective trim tab, or has articulated multi-elements, it can develop lift at zero leeway.)

The amount of lift developed by a foil is given by the formula

( D * Area * Coeff of lift * Speed )

D is the density of the fluid.  In air, D is about 1.2 if area is measured in square metres and speed is measured in metres per second.  For fresh water, D is 1000.  Let's imagine our IOM is sailing in a 4 m/sec breeze, and the boat is moving along at, say, 1.1 m/sec.  We are in No.1 rig, with a sail area of about 0.6 sq.m, and if the sails are developing a coefficient of lift C of around 1.0, total sail lift (in Newtons;  around 9.8 Newtons to a force of one kilogram) is about

( 1.2 * 0.6 * 1.0 * 4 ) = approx 6 N

of around 0.2, then total appendage lift is about

( 1000 * 0.05 * 0.2 * 1.1 ) = approx 6 N

The two values match reasonably...  Of course, I've chosen representative, but realistic, values to obtain this balance.

There are three parameters which affect the amount of lift force the fin develops, as shown by the formula:  fin area, lift coefficient, and boat speed.  The boat speed is (more or less) given, so you are left to trade off the lift coefficient against fin area.  More area, more lift, lower coefficient of lift needed, and so less leeway.  The picture is trying to illustrate that the fin force comes from the fin "flying" through the water at an angle of attack equal to its leeway.

Off the wind, the fin is a drag appendage, and on the run more fin area is bad because of wetted area drag.  While beating, fin drag comes from two sources:  wetted area, and induced drag.  Induced drag depends on the square of the fin lift coefficient, and more area gives lower loading and lower induced drag for a given amount of lift.  On the wind, more fin area is generally good.  (But see "Update 2 (b)", below, for Graham Bantock's comments.)  So the actual fin area you use is a compromise between the performance you want on the wind, and off the wind.

= C / (3.14 AR)

where "AR" is the aspect ratio of the fin.  High aspect ratio fins give less induced drag.  If our AR = 4 fin is developing C = 0.2, then we can estimate C is about 0.003.

The aspect ratio also affects how well a given aerofoil section performs in "real life".  A higher aspect ratio means the section lift coefficient retains more of its value when used on a real foil according to the formula

= C / (1 + 2/AR)

where C is the coeff. of lift for the whole fin, and C is the coeff. of lift for the fin section.  This difference in lift coefficient is due to the difference between 2D (aerofoil section) and 3D (complete wing or fin) effects.  So what?  Well, if we have a fin with AR = 4, and we need C = 0.2 to balance the sail forces for a given fin area, then we are looking for the section C to be about = 0.3.

The total drag of a fin comes from two major components -- induced drag (or drag due to lift) and profile drag (drag due to the shape and size of the foil).  These two major drag components could be thought of as "active" and "passive" drag.  Then, within "passive" or profile drag, there are two further components -- drag due to the cross-section being presented to the incident flow, and wetted surface area drag due to the friction drag of the surface of the foil.

Here is a graph of the lift to drag polars for four symmetric aerofoil sections, showing how drag and lift develop with angle of attack, that is, leeway.  The points come from tests in a wind tunnel at around Re = 60,000 -- low speed in other words.  These four sections are all realistic candidates for your fin section.  Let's see which one you might choose.

Let's just check our Re regime before plunging ahead.  How do we know that we shouldn't be looking up graphs of section performance where Re = 300,000?  The Reynolds number relates to the "characteristic" length of the item we are looking at, measured in the direction of flow.  Our fin probably has an average chord of 80 mm, and this is its characteristic length.  The formula for Re is

where n is the kinematic viscosity.  For air, n = 0.000015, and for water .000001 when L is measured in metres and V in m/sec.  For our fin, then, L = 0.08 and V = 1.1, so Re = approximately 88,000.

We can see that the test sections stall beyond an angle of attack of 7 degrees (except for J5012) -- lift remains fairly constant, but drag shoots up.  In the working region below stall, from an angle of attack -- leeway -- of about 1 degree to about 5 degrees, we see that the lift coefficient rises nicely while the drag stays fairly constant, C between 0.015 and 0.020.

The drag from the NACA 0009 section looks pretty competitive, but this is deceptive -- while the NACA 0009 is a 9% t/c section, the NACA 64A010 is a 10% t/c section, as is the SD8020, and the J5012 is a 12% t/c section.  So the NACA 0009 should be showing a little less drag than the others because it is thinner.  You can see that the J5012 section shows drag around 22% higher than the other three sections, consistent with it being at least 20% thicker.

The second graph shows the drag values (rather crudely) adjusted for section thickness.  It is now clearer that, when comparing like with like, that the SD8020 section has lower drag than the NACA 0009 section for angles of attack up to around 5 degrees.  So you would choose the SD8020 section in preference to the NACA 0009 section, BUT ONLY if you were confident that your leeway would remain below 5 degrees almost all the time.  If your fin needed to develop a lift coefficient greater than around C = 0.3 (and hence C = 0.45), then the SD8020 would be a bad choice.  For example, if you wanted to experiment with a low area fin, instead of "normal" sailing at a leeway of around 2 or 3 degrees and a C = 0.2 you might be sailing with a leeway of around 3 to 5 degrees and a C = 0.3.  When a puff comes in and the fin needs to increase the lift it develops, drag would go up more and, in addition, the fin would stall earlier, if you'd used the SD8020 section instead of the NACA 0009 section.

The NACA64A010 section is worth a little attention.  It is a "laminar flow" section, with maximum thickness well back at around 40% of chord, while the NACA 0009 has max thickness at around 28% of chord, with J5012 at 34% and SD8020 at 27% of chord.  However, the NACA 64A010 has poor drag performance (where we need it in the region of C = 0.2), almost certainly due to the fact that it is designed for a quite different flow regime at much higher Re.  On the other hand, you might want to try it for a low area fin, because drag at C = 0.35 is much improved.  But see "Update 2 (a)", below, for Graham Bantock's comments.

The overall section drag coefficient, C , at C = 0.3 for the SD8020 section when scaled to a 9% t/c, is about 0.0135 (reading off the second graph).  To a first approximation, we can take the overall fin drag coefficient to be equal to the section coefficient, that is, we can estimate C = C = 0.0135.  If the SD8020 was our chosen section for our fin, and we earlier estimated C = 0.003, we can estimate that the "base" drag coefficient of the fin due to wetted surface and profile is about 0.0105.  So induced drag is rather modest here, at about 25% of total drag.

With the exception of the NACA 64A010, these aerofoil sections show moderate "drag buckets" at low angles of attack -- their section drag does not increase much as lift increases.  That is what you want from your foil section, and this is what you would ask from your aerofoil section designer.

We can see that the J5012 section is not yet approaching stall when at 7 degrees angle of attack.  Very roughly, most symmetrical sections reach stall at an angle of attack that is similar to their t/c % (and, they reach a maximum value of their C that is also approximately equal to 10 times their t/c), so we would expect to see the J5012 stall at around 10 or 11 degrees having developed a maximum coefficient of lift of around 1.0 or 1.1.  So as a fin gets thinner it will stall at increasingly lower angles of attack, and will develop lower maximum coefficients of lift. The exact point of stall depend upon the fin section, the Reynolds regime, and the nose radius. My understanding is that a fin with a relatively blunt nose -- ie significant nose radius -- will not stall quite so eagerly as a fin with a sharp nose, but will show somewhat higher drag.

So where are we?  You will begin your search for the ideal fin by first deciding the sort of coefficient of lift you want it to "normally" develop, and the maximum coefficient you think you are likely to need before you accept that the fin will stall.  This decision will help determine the section thickness because we know that thinner sections will struggle to develop high lift coefficients, and will stall sooner at lower angles of leeway.  If you want a thinner section in order to minimise drag, you know that you will probably need a larger area fin, which simply sends drag back up again, and you will have to iterate around your design space for the compromise between fin area and section thickness that suits you.  While doing this, you will search for a section which offers you lowest drag in the region you are interested in.  Some sections offer this at lower coefficients of lift, some at higher.  Finally, you might play around with the nose radius of the section you like to tweak its drag and stall characteristics.

  Will Gorgen has e-mailed me:

Certainly. I guess I was wanting to bias the discussion towards fin area rather than leeway angle. That is, given the force that the fin needs to develop to balance the rig, my experience of fin design is that, having gone via the lift curve slope you then go on to determine the area you want.  I didn't say it, but perhaps it might be worth saying, that as far as I can see, leeway in and of itself isn't of much consequence. It doesn't really matter whether the boat's leeway is 2 degrees or 4 degrees or whatever, and it isn't usually a design issue to target or manage some specific amount of leeway. That is, a little leeway or a lot of leeway doesn't matter in itself -- what matters is the drag price you are paying for the lift you are generating.

Yup. I have comments on planform on another page ( ), but will probably leave them unchanged for the moment. I'm getting the feeling that I might want to take a closer look at section design and talk about the pressure distribution along the section and how that leads to more or less drag due to separation and turbulence.

where the keel should be placed, it might be useful to list the design objectives that are trying to be achieved with the proper keel placement.

To start, I think the most important thing about positioning is that it is a balance issue -- did we get the fin in the right place in relation to the rig, or does one or other need to move forward or aft so the helm is more or less neutral or gives a touch of weather helm?

Thereafter, as far as I know whether you want the rig/fin package placed relatively forward or relatively aft in the hull is a quite different issue where we are playing with the amount that the centre of effort of this package is ahead or behind the centre of buoyancy of the hull.  This leads to differences in ability to hold course, ability to turn quickly, ability to track acceptably in waves and/or following seas, how much the boat trims down when pressed on the beat, or nose dives when pressed on the run.

Finally, this combines with the issue of what kind of fin and bulb configuration do we want -- "T", "L", or reverse "L". If we want the bulb to cause some fin twist when heeled then we want "L" or reverse "L" and therefore we want the rig/fin package somewhat forward or somewhat aft of where it would fall for a "T" configuration.

  Graham Bantock has a couple of comments:

(a)  You say that reducing chord to get C up to 0.35 where C is lower looks useful.  However, that would reduce Re, and that is usually bad news for C .

Yes, the side-effect of reducing the fin area, for a given draught, is to reduce the chord, and therefore to reduce the Reynolds regime.  As we go to lower Re, C rises rather than staying constant, and rises quite significantly.  For example, for the SD8020 profile, we saw that C = 0.0135 for C = 0.3 when Re = approx 60,000.  Well, if Re = 300,000 (a very common test point for aerofoil sections), C = approx 0.007, half what it is at the lower Re regime.  So we might guess that if Re now drops to 40,000 because we reduced the chord 33%, then C might be as bad as, say, 0.02 (but we don't know for sure, since Selig does not report data below Re = 60,000), that is, 50% higher.  Ouch!

is proportional to C and AR .  Induced drag is proportional to C and fin area.  Doubling the chord halves the C , doubles the area, and halves AR.  Thus induced drag remains the same.  What you write is an oft repeated error that appears in some high power sources apparently.

Given that, on the wind, we want to balance the sail force without driving the fin into a high-lift regime, I said that more fin area is generally good.  This isn't as clear-cut as I originally thought.  More fin area will certainly serve to lower the coefficient of lift, and also the coefficient of induced drag, but not by as much, because for a given draught the increased area gives us increased chord and lower aspect ratio.  Then, the amount of induced drag will not change that much because we still need to multiply the only somewhat lower induced drag coefficient by the now increased lifting area to get to that actual amount.  We might have reduced one term, but we've gone and increased the other term, and are back, more or less, with the drag where we started.

Foil Generator

We take advantage of the techniques used at the high end of the yacht design industry to generate shapes that fill the requirements given in our design brief. The main objective of these techniques is to automatize as much as possible the iterative process inherent to any engineering problem, especially in selecting the right foil shape for the working regime of the boat. In the case of the appendage development, we work with some of the already well known genetic algorithms to select the foil that fulfills the design requirements. The iterative process imitates natural selection by breeding new shapes from the best foils generated in the previous generation. Our in house software produces random foil shapes each generation that are tested with a CFD panel code, the foil shape of the best of them are then varied for producing different variations or mutations of the original shapes that are tested again. After a few minutes, we can produce thousands of shapes were only the best ones are selected. These newly shapes are then translated to the desired 3D shape of the appendage and tested using some of the best RANS CFD codes available to validate the findings in the 2D optimizations. This way we avoid free improvisation and save time in building prototypes that can be left apart before even being built.

Every design we build is an opportunity to improve the optimization algorithm as we validate our design methodology by logging data on the water. This way we have a rational argumentation for improving our designs in future loops, which is one of the main points defining the sailing lab concept.

Privacy Overview

• Remember me Not recommended on shared computers

Forgot your password?

IOM as a trimaran

By Richard98 May 15 in IOM

Recommended Posts

Been playing around with multihulls for some time now; various configurations of cats and tri's. Here is latest effort and was intended to use an old One Metre hull with a "bolt on" conversion to a lightweight trimaran. Not a new idea, I know. Paul Goddard has an excellent version based on the DF95.

Anyway, a few pics of what can be achieved with simply printed floats and a few printed brackets. Boat can be sailed conventionally as a One Metre: remove the fin and ballast, clip on the hulls, connect the longer rudder and just sail as a trimaran (will need a stub fin as a centreboard) Or add foils to the outboard brackets and try foiling. This is Mini40 legal but a bit shorter than the max allowed and a tad more convenient.

Weighs in at just under 2Kg all up sailing weight. I have not sanded the printed floats and they are printed in 4 parts. This set was assembled with thickened epoxy, hence the visible joints. I have printed another pair and joined with clear epoxy and will retro fit these after fully testing.

I do not intend to have this measured or propose any "minor" rule amendments to make this legal. Just a good way to use an old IOM hull and have some fun.

Link to comment

Share on other sites, eric finley.

Hi Richard, nice development project!  Few questions.

What 1 metre hull is it based on?

Are you printing the pontoon hulls yourself? If not where are you obtaining them?

Foils and rudder source?

What wind speed do you need to achieve foiling? 

If not foiling how large a fin is required? Any ballast? In this mode how does it perform in comparison to its base IOM hull?

I presume a rescue boat is required when sailing in case of capsize. Glad to see your experimental/development is not diminished. Where are you sailing this yacht?

Are you still playing with the American one Metre?

Hi Eric, This is an old One Metre development hull. An extreme design idea that never really worked. Any One Metre would be good though. With the displacement reduced to sub 2Kg a typical IOM will only be on about half of it's WL and "sitting on it's rocker" hence a manoeuvrable platform as opposed to a full WL. Added bonus of plenty of reserve buoyancy. I am printing the floats and also the X beam brackets and foil / rudder incidence adjusting clamps. 

Not a lot of wind is required to foil, as proven with the same foils on a heavier cat configuration. Light wind -  remove foils and slot a short, stub fin to act as a centreboard. Fit smaller rudder and sail conventionally. No ballast required = very nippy ! Don't forget that only one float is in the water at any time with a low wetted area.

As pictured with the orange prototype floats, the foils are in the fixed incidence tubes and it was apparent that the high aspect rig moment cancelled the incidence. Next sail will see them fitted in the adjustable, outboard, brackets visible in the shoreside pic. Incidence can be quickly adjusted. The rudder is already mounted in the same bracket on the aft beam.

I have a rescue tug (cheaply made from liteply) and will also fit a floater at the masthead to prevent full inversion.

Sailing at Colwick Park, Nottingham.  US One Metre still available and have pinched the rig for the trip. Might give it a go again.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Already have an account? Sign in here.

• Existing user? Sign In
• All Activity
• View our Suppliers
• Purchase a Supplier Package
• MYA Website
• Leaderboard
• MYA Districts
• Create New...

Clubs	Members	Boats
37	164	274

​Description

Frank Russell Design

R/c and model yacht design, plans, boats, sails..

The  Ellipsis… IOM represents a different approach to what is now “conventional chine” boats,

The truth is chines do not do what designers say they do. They do not “grip” the water or generate lift, or  stop the boat going sideways in any way.

The main application of a chine on modern IOM’s is to easily allow a boat to have tumble home aft by separating the topside shape from the bottom shape. This aft tumblehome In combination with much fuller bow sections, reduces the “in out wedges” of a hull which stops the hull from trimming bow down when heeled. This hull “balance” , allows the hull when heeled to sail in a straight line rather than curve to windward and allows the foils to stay at an optimum angle of a attack without further skewing the hull. This makes the boat easier to sail and reduces drag. 

Other effects of the chine are the ability of a designer to make the stern sections flatter than they otherwise would be and gives the hull a volume bump just above the waterline increasing initial form stability and at the bow increasing acceleration. The chine may also allow a cleaner separation of the water flow aft at speed, but this is debatable.

The Ellipsis… approach is to remove the chine completely but to retain the volume distribution and excellent hull balance of a chine boat. This produces a much simpler and easier to produce hull in both glass and wood. The Ellipsis… is very well balanced upwind and can be trimmed to give a negative feel right though the useful range of heel angles. When running the Ellipsis’s full bow gives the boat excellent nosediving prevention ability and exceptional acceleration in gusts.

The Ellipsis… being narrower on the waterline and has less immersed hull surface than many chine boats. The deck is slightly wider allowing a wider stay base and better rig control.  The flat deck with a raised “bubble” section allows the jib foot to be very low in the boat.

The Ellipsis… is also unlike many chine boats, and very pretty boat .

Assembled Boats:

Assembled Ellipsis IOMs, ready to rig and fit radio are now available from ARS Composite Freelancer in Thailand. 

These cost $900US plus delivery and can be ordered by contacting:

FRD IOM SAILS

Updated:27/11/22

• Gift Certificates
• My Account  
• Sign in or Create an account

Currency Displayed in

• Dragon Flite 95
• Hales Micro
• Sail Accessories
• Insignia Sailcloth
• Film by the Foot
• Sailmakers Full Roll
• Special Tools
• MX Components
• Dragon Force RG65

Hover over image to zoom

  Product Description

Brighton Boat Works is proud to be a US distributor for the exceptional new IOM the MX16 .  Hull, Production tooling have been designed and produced by Maurizio Morbidelli 's Italian Company MX Components. This boat comes from the factory complete with Hull, Deck Fin-box,Deck accessories, Carbon Fiber Foils; both rudder & Keel, Carbon fiber coated Ballast. Additional included deck accessories are a radio pot, Rudder arm, mast ram and main sheet riser, and silicone nose bumper. Just add servos and rigs (we are happy to provide those as well!!) and you are good to go racing! This Item is Special Order Please allow 10-12  weeks for delivery.

THe MX16  is the next evolution of the extremely successfull Goth Hull design.The MX16  features a narrower beam and revised forward deck volume. The new design is complete with a new keel, rudder and bulb design. The MX16  is available as either a single color boat or a 2 Color boat(Hull and Deck). Any Color may be selected from the RAL Color Chart. That chart can be found at http://www.ralcolor.com/ . Please specify the desired color when ordering. Color is added to the boat in the mold to insure an excellent finish without the additional weight of a gell coat.

The deck, hull, fin box and deck accessories are assembled at the factory. all openings to the hull are factory cut. Additional Accessories for this boat include:

1. Black painted Carbon Fiber Rudder

2. Black painted Carbon Fiber Keel

3. Carbon fiber coated and black painted Ballast

4. Mast Ram

5. Sheet riser

6. Battery/Radio Pot

7. silicone Nose Bumper

8. Rudder steering arm

9. Installed Nickle plated Sheet guides and Back stay mount.

10. Servo tray is predrilled to receive the RMG Drum Sailwinch. THe MX14 can be outfitted with an arm winch if desired.

  Find Similar Products by Tag

  find similar products by category,   product reviews.

Write review

Write Your Own Review

We promise to never spam you, and just use your email address to identify you as a valid customer.

This product hasn't received any reviews yet. Be the first to review this product!

Popular Brands

• Variant Marine Hardware
• Brighton Boat Works
• Jelacic Sails
• View all brands
• View all categories

All prices are in USD .

Click the button below to add the IOM - MX16 to your wish list.

Colin Thorne's Radio Sailing Technology

• Drag Measurements on an International One Metre Yacht
• Thoughts on Chines Aug 2013
• Methods for estimating heeled resistance (Dec 2013)

Estimating the hull drag of an IOM Yacht (August 2014)

• Ballast Bulbs
• Keel Deflections
• Measuring the Drag Difference Between Two Hulls
• Simple Towing Gauge
• Observations at IOM Nationals 2011
• Part 1 Balance of a Yacht to Windward
• Part 2 Balance of a Yacht to Windward
• Part 3 Balance of a Yacht to Windward
• The Concept of Sailing Regimes
• Heel effect on wind and water angles (Nov 2013)
• Part 1 Sail Dynamics: The Concept of Automaatic Rigs
• Part 2 Sail Dynamics: Automatic Rigs
• Mast Section Stiffness
• Luff at back or side of mast Aug 13
• Wind Speed and Gradient Observations at Lake Julienne (Jan 2014)
• Skip to content
• " onclick="window.open(this.href,'win2','status=no,toolbar=no,scrollbars=yes,titlebar=no,menubar=no,resizable=yes,width=640,height=480,directories=no,location=no'); return false;" rel="nofollow"> Print

  1.0 Introduction

Hulldrag.zip is the spreadsheet, click on it to download, ignore any message about updating from other spreadsheets .

This article presents a downloadable spreadsheet for estimating the drag of IOM hull designs. All that is required is to enter some numbers  from a normal design program into a space in the "Input Data" sheet and the spreadsheet does the rest, giving graphical output for up to three hulls at once to allow comparisons. No mathemeatrical knowledge is required.

The spreadsheet was made using Windows 8.1 and Office 13 but will work on modern Apple computers, and has been shown to work on Windows7/Office 2003 PCs.  The method can be applied to other model yachts but the accuracy would be uncertain, particularly in the skin drag calculation which may underestimate the drag of smaller yachts such as RG65s.  In addition, most Marblehead and 10 Rater designs are too slender for the residual drag part of the calculation.

The drag of a boat through the water is generally considered to consist of::  

Skin drag , that is the drag that comes from the water rubbing on the hull, this is based on normal naval architecture methods modified to suit the observed drag values described in Hulls/ Drag Measurements on an International One Metre Yacht on this website.  

Residual drag which is mostly wave making drag, and is often called that.  This is based on the Delft Yacht Series method described in Keuning (2008).

Details of the calculation methods are given in the Appendix. The method for residual drag is based on a more recent Delft Series method than that in Delftship or Freeship neither of which can accommodate normal IOM shapes because they are based on wider models, and the minimum IOM water line beam for which they are valid is 200mm as compared to just under 170mm for the more recent method in the spreadsheet.  In addition, these methods cannot deal with the very low Reynold’s numbers that apply to the skin friction of model yachts, whereas this spreadsheet is specifically tailored to suit these low Reynold’s numbers.

It must be realised that, though it is important, the upright drag is not the only factor in hull performance.  Some other factors are heeled drag, pitch tolerance (including nosediving behaviour), and wave impact drag.

The spreadsheet uses hydrostatic values that most design programs such as Delftship, Freeship amd MacSurf, will provide.  If a program is available that will give these parameters for a hull when heeled, or at a different pitch, this spreadsheet may be used to give an estimate of the drag in those situations as suggested by Fossati (2009).  The author uses Hullform 9P (this is now a free download) for this because, although it is harder than some to get a fair hull, the hydrostatics package will calculate the hydrostatics values for the hull at different heel and pitch angles.

The rest is up to intuition, judgement and experience! 

2.0 Using the spreadsheet

There are five sheets in the spreadsheet:

  2.1 Input Data

This is where data is input.  The spreadsheet can handle 3 sets of boat data at a time.  This sheet also has an area for archiving data for many boats.  To use the workbook copy the hydrostatics data onto the input data sheet in columns D E or F, lines 4 to 16. Remember to give your hull a name.  Check the warnings columns K, M and O to see if the data falls within the limits of the Delft Series, if it does not, the accuracy will be compromised, though small deviations are probably OK. Do not alter anything but the column of data.  The green area shows the units required and gives a description of each factor.  Data for boats that are not wanted for comparison can be stored in the archive area. It is recommended at first to retain at least one of the better boats that come with the spreadsheet for comparison.

Two parts of the input data need some clarification: the first is the displacement and the other is the definition of LCB and LCF.

          2.1.1 Displacement

Since an IOM weighs 4 kilograms it might be thought that this would be the displacement to enter into the spreadsheets, but this is not so.  The displacement entered is a volume not a mass and so is in cubic metres for one thing, the other is that the keel rudder and bulb displace water and hence the hull does not have to support the full 4 kilos.  The weight of two different keel/bulb combinations were measured in air and water and the displaced volume calculated.  The results are given in Table 1.

Table 1 Displaced Volumes of keel and bulb

Keel	TS2style	Craig Smith keel and Sails etc. Bulb
Max Chord, mm		83.5
Bottom Chord, mm		83.5
Thickness %		6
Weight in Air, g	2484	2488
Weight in Water, g	2147	2198
Displaced Volume, m^3	.000340	.000290

Most modern boats have keels much the same as in the second column and the displaced volume for an IOM weighing 4kg in air can be calculated as in the next table:

Table 2 Displaced Volume Calculation

Headings	Fresh Water	Salt Water
Total displaced volume,	.004	.0039024
Displaced Volume, keel and bulb	.000290	.000290
Displ. Volume, rudder	.000025	.000025
Required Displaced Volume of Hull	.003685	.003587

In practice it is close enough to use .00369 cubic metres for fresh and .0036 for salt.

There is a common perception that unless a boat is designed to 4kg it will float low with its ends in the water when they have carefully been designed just to clear the surface. In fact, a boat so designed will float about 2.5mm higher than in the design program, and this is barely enough to allow for the surface tension of the water raising a meniscus around the hull.

The spreadsheet relies on referring to the true displaced volume of the design in question, that is either .00369 or .0036 and using the hull parameters for a displacement to 4kg will give thoroughly misleading results. Most common design programs will allow the draft to be changed to give the correct displacement without changing the shape at the end of the design process, and the parameters for this condition should be used in the spreadsheet.

The author prefers to design for the actual displacement and raise the stern clear of the water by 4 or 5 mm which is the practice with most successful boats. Most also raise the bow.

          2.1.2 Inputs LCB and LCW .  

For the purposes of this calculation these are measured from the “forward perpendicular”, which can be taken as the forward end of the waterline.  Some design programs measure these dimensions from the transom (e.g. Freeship) and so it is necessary to correct these before entering the values in the spreadsheet.

          2.1.3 Scaling

Most design programs are intended for full size boats 10 metres or more long, and many run into numerical problems with model yachts: for example Freeship rounds off the displacement to the nearest kilogram.  To get around this it is best to design the shape at, say, 10 metres long and then alter the dimensions before putting them in the spreadsheet.  For lengths divide by 10, for areas divide by 100, for displacements divide by 1000, and for righting moments divide by 10,000.

    2.2 Calculation

This sheet is the engine of the workbook. It is write protected so the formulae cannot be inadvertently changed.

    2.3 Drag Plots

This sheet presents the drag versus speed results in graphical form. These are updated automatically as soon as a new boat is entered.  A high and low speed graph are provided to give a bigger scale for the graph.

T here is also a % faster than graph (See Figue 2). This compares the boats whose inputs are in columns E and F on the Input Sheet with the boat whose input data is in Column D (called “Boat 1” here, the identity of Boat 1 is given just above the chart).  If the boat is slower than boat 1 then the numbers are positive, but if the boat is faster than boat 1 the value is negative. 

Thus if it is desired to get a boat faster than boat 1, negative values are wanted!  In the example, Sky is not very good as boat 1 is faster for almost the whole range, on the other hand Target 0.52 looks to be a bit better than Boat 1. The graph is set to show differences of +- 5%, however the scales can be changed if required, to do so click on the vertical axis and select "Format Axis" then alter the range. 

    2.4 Plots

This sheet has been prepared for the more sophisticated users! The intent of the data on this sheet is to allow a study of the components of residual drag. These are updated automatically as soon as a new boat is entered.  The other part of drag, skin friction is pretty much determined by the wetted surface, but residual drag is more complex.  The sheet shows the contribution of each term to the residual drag.  Where the values are positive they are adding to drag and where negative they are reducing drag.  The relative magnitude of the terms is also worth studying.  It might be thought that improving a boat is just a matter of reducing terms that add to drag and increasing those that decrease it.  In practise it is more complex because when the shape is altered to optimise one term is will also change the others!  Each of the terms is described below:  

Term 1 LCB/LWL : The graph suggests that the further aft the centre of buoyancy is  the better except at high speeds.  This is a big contributor to the total drag through most of the range.

Term 2 Cp : This is a measure of how the buoyancy is distributed, a large Cp has the buoyancy out towards the ends and away from midships while a small Cp indicates the reverse. A high Cp is generally linked to better medium high speed performance.  

Term 3 is (Displaced Hull Volume)^2/3/AWP : The displacement is effectively fixed, so a large value of AWP (area of the water plane) looks good at high speed, but such values are associated with high wetted surface and adversely affect other parameters.  

Term 4 is BWL/LWL : Clearly narrow boats are favoured except at 2 to 3 knots.  

Term 5 is LCB/LCF : the ratio of the distance of the centre off buoyancy from the forward end of the waterline to that of the centre of area of the water plane. Low values of this ratio are beneficial though the whole speed range.  

Term 6 is BWL/Tc: the ratio of the waterline beam to the draft.  For most of the range this wants the draft increased and the waterline beam reduced.  

Term 7 is Cm : This describes the shape of the midships section, a triangular section has a low Cm and a rectangular section a high one.  There is a difficult choice here because a small value is good up to about 3 knots and then bad at high speed.

Col Thorne August 2014

Appendix Basis of calculations

A1.0 Residual Drag

This is based on the Delft Yacht Series as in Keuning (2008).  It is an empirical method based on the statistical analysis of lots of tests rather than one built up from first principles of fluid mechanics.  Definitions for the input data for the equations of the method are given in Table A1 and in  cells A5 to A16  of the  Input Data  Sheet.  These parameters are provided in the output of most hull shaping designs.

                             

The equation for this method is:

The parameters a0, a1 etc. have been derived statistically and are given in Table A2 as a function of the Froude Number Fn.  This is a general value indicating the speed relative to the boat size.  To calculate the speeds for your boat, use:

v=Fn*√(9.81*L)

Where v is the boat speed in m/sec and L is in metres. For most IOM designs this will give speeds from around 0.9 knots up to about 4.6 knots.

Because this is an empirical method, errors can occur if it is applied to boats too different from those tested. The most serious limitation for IOM designs is the waterline beam.  The lowest waterline beam to waterline length ratio in the tests was 0.17, so some loss of accuracy can occur for really skinny hulls.  The other ranges are shown in the Input Data , Columns G, H and I.            

In addition, the Delft series did not include any mpdels with chines.  If heeled hydrostatics are used then the effect of the chines on the hydrostatics will be covered but not the fluid dynamic effect of the sharp corner.

A2.0 Skin Friction Drag

    A2.1 Calculating Reynold’s Number

A discussion of the skin drag as compared to conventional methods is described on this website in Hulls/Drag Measurements on an International One Metre Yacht , and will not be repeated here. The first step is to calculate the Reynold’s Number for each of the boat speeds that arise from the residual drag calculation. The Reynold’s numbers give an indication of the nature of the flow round the hull at each speed and are calculated as shown in the equation below.

Reynolds Number, Re=VbL/v

                        where:

Vb= Boat Speed in m/sec (= 0.514* Knots)

L= characteristic length (0.7*LWL for most hulls), m

ν= kinematic viscosity, this varies with temperature and salinity

The data in Table A2 is from ITTC Recommended Procedures 2011 rev 2 .

Table A2 Properties of water

Temperature	Fresh, density, kg/m^3	Fresh, Kinematic Viscosity m^2/s	Salt water, density      kg/m^3	Salt water, Kinematic viscosity        m^2/s
10	999.7	1.31*10^-6	1027.0	1.36*10^-6
15	999.1	1.14*10^-6	1026.0	1.19*10^-6
20	998.2	1*10^-6	1024.8	1.051*10^-6
25	997.00	0.893*10^-6	1023.4	0.937*10^-6

 It is interesting to note that the viscosity, which is part of the Reynold’s Number, varies quite a bit with temperature.  Because I live in a temperate part of Australia I normally use 20 degrees but in Europe or North America I understand 15 degrees is more commonly used. Once the Reynold’s numbers have been calculated the skin friction coefficients can be calculated.

     A2.2 Skin Friction Coefficients

The equations for the skin friction coefficient are given below.  The usual values are the ones in brackets but to make the calculation give the right answer it was found necessary to increase the usual coefficients by 40%, hence the 1.4 factor.  The reason for this is not clear, though it is speculated that it may be due to surface tension.

    A3.3 Calculating Skin Friction

Once the coefficients for the particular boat speeds have been calculated the total skin friction drag can be calculated as follows:

A4.0 Total Drag and comparison with measured values

To obtain the total drag, the friction drag and the residual drag are added together for each boat speed. Thus the total drag at each speed is:

The first set of drag measurements was reported in Hulls/Drag Measurements on an International One Metre Yacht on this website.  Since then some other measurements have been made and these are compared with the values calculated as above.  The results are provided in Figure 1. The agreement is good for such a difficult problem.  It should be noted that the natural scatter in the test results is actually more than the difference between reasonable designs, but fortunately in calculations like this, the prediction of the difference between boats is commonly much more accurate than the absolute values so the lack of great precision in the experimental data does not mean that useful comparisons cannot be made between designs using the methods above.

If the necessary parameters can be obtained for the boat at different pitch and heel angles the method can be used to look at this aspect.  Often hulls that look similar when upright and on their waterlines are quite different when heeled.

Keuning, J.A., and Kargert, M.; A Bare Hull Prediction Method Derived from the results of the Delft Systematic Yacht Hull series extended to higher speeds; International Conf on Innovation in High Performance Sailing Yachts, Laurient, France, 2008

Fossati, F. Aero-hydrodynamics and the performance of sailing yachts. Allard Coles Nautical London 2009

Happy designing!

Col Thorne August 2014  

• About the author

We have tried many different shapes and sections for rudders and all boats are now built with the standard D4 rudder design

We are sure its best to have a lightweight rudder as we believe its best to have less weight at the ends of the boat so the rudder is built of foam and glass. We finish all our foils with epoxy resin and graphite. This surface is sanded with 1500 wet dry sandpaper and is very easy to maintain and keep clean

• Search for:
• Entries RSS
• Comments RSS
• WordPress.org

Cross-cutting (Global)

• Data and Resources
• Take Action
• 2030 Agenda

About Migration

Mission of the International Organization for Migration (IOM) in Moscow 4, Stasovoy street 119071 Moscow Russian Federation

+7.495.660 77 82 [email protected]

For questions related to medical examination: +7.495.660 77 84 [email protected] ,  [email protected] For more details visit  medical examination page

Migration updates 

Subscribe to IOM newsletter to receive the latest news and stories about migration.