Foil Shapes for Low Wind Speed Venue?

I posted this on “that other forum site” and didn’t get much in the way of responses, so I am trying here:

Fairly soon I will be building a US1M whose design decisions are all slanted to a low wind venue (scientific drifting to 3 mph and puffy).

The hull will have a low prismatic coefficient and minimum wetted surface, but I am uncertain if any changes should be made to the standard 6% max chord depth for the fin and about 8%-9% for the rudder, with the rudder area being about 27% - 30% of the area of the fin. These “norms” seem fine for a boat that sees 5 - 8 mph wind and the resulting boat speeds, but this boat is going to be going much slower.

My thinking is that the boat will make more leeway at the lower speeds and the fin will need to be thicker to prevent separation at the larger angle. At lower speeds reducing form drag is less important than reducing the surface drag what input I got was that I should use the same 6% depth of chord, but use a shorter chord to reduce wetted surface.

Has anyone thought this through or done any research on this?

Any help would be appreciated.


Hi FC -

I had this bookmarked from a long time ago, and perhaps it may have some answers. Not sure how current the “new” thinking is, but there are a few areas that might be of help. I haven’t done any testing or evaluations - so from that point, I guess you are on your own.

Good luck - and hope there is a tidbit of helpful info in the article.


first, the ancient experts told us that the immersed appendage area should stay around 6.5% of the Sail Area and should be be splitted in 2/3 for the FIN and 1/3 for the rudder. All that are empiricall obsevations after many years of designing.
With low winds this area may go up to 7.5%.

Second the Fin/Rudder profiles shall be in accordance with the expected speed to obtain the needed lift. Personnally I would suggest 7.5% as minimum for the Fin and 12% for the rudder.

Important to note that the rudder is entering “stall” conditions very often and producing loss of speed due to drag.
This phenomenon occur most of the time when turning a buoy or change of direction, but rarely admitted by the skippers !

Some used fin with 4.5% profiles !!!

If then the wet area is reduced acting on the appendages, then one should expect more drift. Compromise !


Thank you both for the replies.

I will have the fin and rudder areas as well as the locations from the designer (a respected IOM designer - can’t say who just yet) so I will not alter those, but after searching further, I found something on the SailsEtc website where Bantock was discussing the history of the foil sections he has used;

Q How has fin design changed over the years?
A Before 1992 we used fins to a 9% thickness/chord ratio section by Helmut Quabeck that seemed ideal for low speeds. A big step forward was made in 1992 when we switched to a 7% t/c section by David Hollom. At the same time we adopted the construction method we are still using and which gives an amazingly high stiffness to weight ratio. The gains made by the 1992 fin fuelled big performance gains resulting in several major championship wins. In 2000 we experimented briefly with a 6% t/c version of the 1992 section. It had some advantages but had lower stiffness. …


Q Is SAILSetc now making a deeper/narrower rudder moulding?
A Yes. All our rudders have been relatively short and wide in the past the 358 series (Ed. 8% and 9%). The reason for this is that their drag is lower than deep/narrow rudders. For a given thickness of stock they can also have a lower thickness/chord ratio which also keeps their drag low. All the tests I have carried out with earlier designs of deeper/narrower rudders have shown perceptible speed loss in light airs and this has been a cost that I have felt not worth taking. …

Bantock goes on to explain how he has changed the designs over the years to accommodate the IOM venues that generally have significant wave action. Since my racing venue is an inland lake with virtually no wave action, I believe I can discard the later modifications an stay with the lower profiles.

From this I can conclude that Claudio is again on the correct path. I think I will make the fin profile about 9% and the rudder the same area as per the plans, but about 9% or 10% profile.

Most successful boats try to be good in all conditions and may be slightly biased to either higher or lower wind speeds. This design is going to be totally to the low wind side with few compromises even to mid A rig conditions or higher. Yes, it will be a special purpose boat, but hopefully one the exactly matches the expected conditions.

Thank you both for your thoughts.


9% for the Fin is too much !!
better to stay around 7.5% for low speed sailing.
For the Rudder I would suggest not less then 10%


I agree with the rudder at about 10%, but why do you say to keep the fin at 7.5% when Bantock had success with a 9% fin at low speed? Can you explain your reasoning?

If Bantock says so do not esitate ! All depends on what you and Bantock consider low speed .
The important is ensure that the Fin sailing at about 4.5° angle of attack the profile will not enter to Stall.
I think even that a chord at 7% is fully adapted and compatible. Lower the thickness less drag !

I see thickness debated, but are there any thoughts on airfoil sections? NACA 4 digit, 6 digit laminar flow, or something for low reynolds numbers like some of the symmetric Selig sailplane sections?

Or does it not matter that much?

Graham Bantock of Sailsetc sells fins and rudders that he has developed with the help of some high powered designers, but according to what I have read (See this article regarding 505 dinghy foils unless you have a CNC created mold, the tolerances are so small as to overwhelm any effect of different sections beyond staying close to a NACA 00XX.

Since the commercial fins and rudders are all designed for top of A rig conditions using 6% fin and 8% rudder profiles and I don’t have access to a CNC mill (nor really the inclination to make a mold for a one off boat) the foils will be hand shaped. I will try to make them NACA profiles, but at this scale even a few ten-thousandths error will make them way off, I don’t think it matters much beyond putting the max depth of chord in the right place.

If it’s for a “one off” boat, more than hand shaping - where getting a ten-thousandths error is actually really good shaping - why don’t you follow Eric Rosenbaum’s method?
he wrote a nice tutorial for a US1M keel, similar to his RG65 tutorial, a while ago, it was published in the last AMYA magazine dedicated to the US1M … Jim may have a PDF copy or you can ask Eric directly I guess … Changing the profile thickness and chord should be pretty easy.

I have been using Eric’s method ever since I saw it in Model Yachting in his RG65 article. I had a few missteps, but now have the technique down fairly well. The fin will be made that way.

However, unless you want a rudder that is squared off on the bottom, it is difficult to get the taper on the bottom to meet the right way with the TE with that technique It is much easier to make the rudder from layers of thin ply, like door skin, and then use the difference in the colors of the layers to determine the profile and then cover it with glass and epoxy. We paint our foils white here because it is easier to see when they pick up aquatic weeds. It is a shame to cover all that nice carbon, but function or form!

Ahha! I apparently missed the rudder part. In this case you are right. Although I did make some rudders a while ago with a similar method (Eric’s fin construction), I followe the one in the US1M construction guide… The rudder had (has I’m still using it occasionally) a ODOM shape (it was for my old mistarl) I just taped the two half’s together (made out of 1/32 ply) and the inserted a bent shaft inside (steel rod with a brass sleeve) and then adjusted the shaft position to get a somewhat foil-shaped rudder … glued everything and shaped it by sanding the edges, it worked …

I am still considering exactly what shape to make the bottom edge of the rudder.

Can’t remember where I read it, but the author asserted that unless you can make a very accurate elliptical Spitfire wing like shape (done with a CAD program and CNC milling) the home builder is better off using a squared tip as the flow separation points are critical and a hand drawn shape will cause uneven boundary layer separation and a great deal of drag. As the profile moves from a longer to a shorter chord toward the tip, because of the scale, it gets progressively more difficult to keep the correct max depth in the correct location and the proper shape. A squared off tip has more vortex issues than a properly formed elliptical tip, but less than a poorly formed elliptical shape.

Bantock and Creed have made large gains in boat performance over the last several years, mostly from improved foil profiles as CNC milled molds have become common, so this sounds logical. That leaves me undecided regarding what to do about the tip. Any suggestions?

you can always build a rectangular rudder with appropriate (or desired) camber and then simply glue on a cross piece of waterproofed ply to bottom of rudder. No need to build a wide set of “wings” - just wide enough to prevent flow separation off and downward from the side of the rudder blade/foil

ADDED: I’m not sure either of them (Bantock or Creed) plays in classes where “winglets” on rudders are allowed - their main class at present seems to be the IOM class which prohibits any sort of horizontal surfaces on rudder blades. If in doubt drop an email to Morrelli & Melvin multihull designers and ask them for comment. Yes - multis are a lot faster than a mono - but that would tend to emphasize the need to keep ALL of the flow attached. Dick

I wrote myself about the elliptical form being the one producing the best lift/drag coefficient. It is not my idea but read on various books. The Spitfire was one of the application, this plane was climbing higher the the others planes due to the wing efficiency.
An elliptical form is easy obtained by taking or drawing an ellipsis and then divides by 4 the shape. The 1/4 of the shape is a perfect elliptical rudder.
As Dick says, on my Trap 65 I used the winglet on the rudder. This feature helped a lot to reduce nose down but the wet area increased.

Further, the performance of a sailing model is due to the skipper thumbs by 50%, the 50% left is split in sail tuning at 40% and 10% hull construction.


The US1M allows just about anything for a rudder as long as it is less than 14.25" below the hull (one rudder only, however) so a winged rudder is permitted and would not be difficult to build.

Or, expanding upon your wooden tip idea, I could make a rectangular hardwood tip, form it to fit inside the hollow carbon skinned straight rudder and then shape the tip using Claudio’s suggestion to draw the tip shape. Saturating it with epoxy after sanding it smooth should give control over the shape and make it water resistant.

I guess the question is which will have better performance at the low speeds for which the boat is being designed. A winged rudder will have more surface area and thus more drag and the shaped tip may have flow issues with increased drag. As you say, I would keep the wings small - just enough to add an “end plate effect”.

The only way to tell would be make both and do some two boat testing to determine which is better.

Every winglet equiped rudder I have ever seen, is being used on FAST boats. Moths, A-Cats, AC72, etc.

I would think the extra drag in low winds due to surface area, plus no need to keeping the nose up when going downwind, would make it a non issue.

Great debate, let me add some thoughts.
I sail the national championship league here in Slovenia, we have prevailing low wind conditions, like here:

My boat is a wide, for the class, heavy boat(1850g), with 5500cm2 of sail.
It’s keel is an aluminum plate, 3mm thick, the front edge is an elips, the back is sharp. Primitive and simple.
The rudder has 11% of chord thickness.
This heavy and wide boat is faster in light winds than #93(second in the championship), which weighs about 1500g, and has lower wetted surface, and also approx 5500 ccm

My boat wins against the odds, reason is the “thumbs” (claudio says 50% of performance)
But that is irelevant for this discussion.

2.reason is the small drag from the keel, the other boat has a smaller chord keel, but its thickness is about 8-9%.

Rudders are similar, my bulb is a bit bigger than his, my bulb is longer, and has the same diameter.

In my opinion it is best to go for a very thin keel, as thin as possible(if you have structural limitations like IOM boats), and a rudder of about 11% t/c for all round conditions, and a rudder of about 8% for mid range conditions.


Thanks for the insight.

Last year, I built a very light, hard chined, thin ply US1M with a 3 lb (1350 g) rather than the customary 4lb (1800 g) bulb and found it to actually be more difficult to race. It accelerated better than the other boats because of its lower mass, but it would also decelerate very quickly as well. Because of its lower momentum at the same speed as the other boats, it could not “cheat up” around a mark if you were too low and would not coast through a lull, but rather would stop dead in the water and have to re-accelerate in the next thermal puff. Conclusion: a certain amount of mass is helpful in light and puffy conditions, so I am going for minimum hull weight from careful construction and take what is saved there and put it in the bulb.

The hull plans are not available from the designer yet, but in discussions he said it would have “a low prismatic coefficient and minimum wetted surface”. That probably means a fine bow and stern with the displacement created by a wide beam. I will of course consider what he recommends for foil profiles, but he is more a traditionalist and may say to stay with the standard 6% for the fin and 7 to 8% for the rudder.

I think I am going to start with a max length short chord keel to keep with wetted surface down and keep the necessary stiffness in a sudden puff, but make it rather fat at around 8% to hopefully reduce the flow separation at the expected larger AOA at slow speeds. The rudder will be a bit longer chord and shorter than normal for the class, base upon Graham Bantock’s suggestions.

I can see it now…this boat will probably have both multiple fins and rudders before I’m done tweeking the design. LOL


Not sure about more leeway at lower speeds (smile)… Leeway is what allows the fin lift to balance the sail lift. At lower boat speed, sail lift is smaller, and so is the need for fin lift. I guess I would expect leeway to decrease rather than increase in this case. Have I gone wrong somewhere, do you think?