Vane for submarine prevention

Recently, several posts have mentioned the idea of a vane for submarine prevention. In one case, it is attached to the rudder, and was probably to small to have an effect, and also probably illegal. In another case, it was mentioned briefly, and dismissed as not worthwhile.

It is raining today in NJ, so I decided to work out the numbers. It appears that a wing section of reasonable size, mounted upside down under the stern, could actually supply enough downward force to negate the pitching moment of the sail.

I have assumed a Razor hull, with measured drag coefficient of 0.514 oz/kt^2, and a sail COE about 10 inches above the water line, and a measured pitch stability of 27 oz-in/in without the bulb. The LCF is assumed to be 7 inches aft of the prow. The hull drag was used to determine the force on the sail. Drag effects and pitch stability from the bulb were also included.

I have found that a cambered wing with 1.25 inch chord and 5 inch span will cancel the pitching moment of the sail. It is assumed that the wing is placed at zero angle of attack, and has a lift coefficient of 0.5.


  1. I am not infallible; it would be good if someone else checked my numbers

  2. The wing must be mounted at least 2 inches under the water line to avoid cavitation

  3. The wing will have some drag, which will adversely affect windward performance. The zero angle of attack should minimize induced drag. The wing also adds wetted surface. I currently believe that wetted surface is a very small component of Footy drag

  4. A gust will pitch the nose down before the hull picks up enough speed to cancel the effect. If the wing comes up too far, the game is over. But if it is mounted low enough, its angle of attack (and force) will increase, and it will regain control.

  5. The wing will increase the pressure under the stern, possibly creating a stern wave that will help propel the boat, and helping to cancel any increase in drag.

  6. The wing may be attached to the rudder, if the rudder does not extend beyond the box (but may need a rule interpretation)

  7. It was assumed that the drag is a simple quadratic. This is obviously not true, and severe gusts will still be a problem.

  8. A lift coefficient of 0.5 was assumed at zero angle of attack. Wing sections exist that can achieve this, but all the data is at much higher Reynolds numbers. So this may be an issue. However, I have found a paper by Thomas Mueller at Notre Dame, which has some data on thin cambered airfoils at very low Reynolds Numbers, which gives some encouragement in that area (we may need a small angle of attack, perhaps 3 degrees).

  9. Heeling will obviously have an effect. It will probably reduce weather helm, and may cause lee helm. It may require re-positioning the mast.

Nevertheless, it is a rainy day in NJ, with time to consider strange Footy modifications. It may give the readers of this forum an incentive to pray for sunny weather in NJ.

Walt this is just like the effect achieved by the foil on my bottle boat rudder.
As the bow dips the angle on the foil increases and drives the stern down to help the bow up ~ works well

Come Sun come, come Sun come
Sam’s at the gate with a frosted cake
Come Sun come

That’s the only incantation I know for sunny weather, it’s never really worked all that well so don’t hold your breath. I like Andy’s stand.

Happy Yachting - Kip

Andy, methinks I smell an ill-controlled experiment. If the behaviour you observe and the foil are actually cause and effect, Walt’s calclations are grossly pessimistic. He is suggesting that you need a foil with a chord of about 32 mm and a span of about 225 mm - in other words, something the size of your bottleboat’s rudder. I don’t know what you are seeing, but I very much doubt whether it’s New Super Wonderfoil. :devil3:

Looking at Walt’s figures, they are at least superficially OK to my eyes EXCEPT that they assume that drag is a simple quadratic. Since much of the submarining problem arises precisely because this is not the case (gradient of drag curve goes up, increase in power has to go somewhere, line of least resistance is to defy gravity (submirine: think about it) rather than go faster).

On Marbleheads, which are nothing if not adventurous, rudder foils are used to a limited extent. They are definitely not ‘must haves’. This despite much much lower displacement/length ratios - i.e. flatter drag curves with increasing speed. They are also MUCH faster than a Footy in absolute as well as relative terms. Since the absolute lift per unit area of the foil is pretty much proportional to the square of the absolute velocity, this strongly suggests to me that such devices are not worth a candle on a Footy. Note ABSOLUTE velocity and lift PER UNIT AREA. There are no special dispensations for being the littlest: quite the reverse.

As I type this, I think I begin to see the snag. Walt is assuming a simple quadratic drag curve. But the lift curve of the foil at a constant angle of attack is also a simple quadratic. Therefore a foil system that will maintain level trim at ‘cruising speed’ (remember you can’t turn the thing on and off) will necessarily have sufficient lift to cope with any speed-induced bow-down trim. I other words, as the burying force goes up, the lift from the foil rises at the same rate, increase the angle of attack and you get a positive correction.

However, since the assumption about the drag curve is wrong, this is not the case and I think that the thing falls at first base. :graduate::graduate:

Now somebody, prove I’m wrong :zbeer::zbeer:

Angus, your comments are most likely correct. This was an academic exercise to occupy a rainy day. But it will work a little better than you assume, because when it starts to hit the wall of high drag, the start of nose-down pitch will increase the angle of attack. This doesn’t completely solve the problem, because the vane will stall at about 10 degrees, which corresponds to only a 1-inch bow-down attitude. This will in turn create an extra high drag below the hull, increasing the nose-dive. So the vane wants to be deep, but not too deep.

With regard to Andy’s boat, he may have a much lower COE (due to stronger winds and smaller rigs), which would in turn allow a smaller wing, and he may be seeing some small effect. In our inland waters, we are lucky to get a perceptible breeze, so we use outlandishly tall rigs with high COE’s.

Obviously, this is something that is probably not a world-beating break-through, but it was fun to look at, and it may be worth a try.

My attitude in retirement is “Don’t do anything unless it is fun”. Fun includes all things involved with understanding boats.

Walt, please don’t get me wrong. I’m a ‘mere’ 56, but I’m badly disabled and I’m never going to race a sea-going yacht again and neither am I going to design a Fastnet Race winner - which was my greatest ambition when I was 16. Therefore all my boats are similar in spirit to your work: i’s an interesting idea that might work, but if ir doesn’t - thn what the hell!

Keep up the good work. Full perfectly free to pour cold water on my ramblings is well: its thebest help anyone can give!!


Angus, I am very grateful for your inputs. As you have seen, I like to look at various crazy ideas, most of which turn out to be impractical. So it is important to have some intelligent people provide comments. It also saves a lot of work in avoiding experimenting with things that have no chance of working.

Another rainy day, so I have looked at the probable drag effect of the vane. Its Reynolds number is around 20,000. It looks like the L/D ratio of the vane may be as low as 8 (or worse). This will reduce the effective net thrust of the sail by 30%, which is an unacceptable hit to windward performance.

Another deleterious effect is that the downward thrust of the vane increases the effective displacement of the hull by 3 ounces at hull speed (and much more at higher speeds); however, this might still be better than having the stern come out of the water, reducing the water line length while increasing the frontal immersed area.

With regard to Andy’s vane, a closer look at its size shows that it is probably indeed too small to have a noticeable effect. But he says that he does see an effect of going nose-down, then coming back up. This is probably unrelated to the vane. It is most likely the natural dynamic response of the hull-keel combination. When a gust first hits, it pushes the bow down and starts to move the hull forward, but the bulb lags behind (causing more pitch rotation) because its mass is way below the hull. The bulb quickly catches up to the speed of the hull (it has no other choice; they are connected), and the bow comes up a little. It acts like a damped pendulum with a period of about one second.

I’m a little confused as to exactly what the foil is supposed to be correcting. Are we talking about reducing “hobby-horsing” due to wind variations & wave action or are we talking about reducing downwind “nose-dancing” or both??? I just really have my doubts as to whether any submerged horizontal static fin is going to improve overall boat performance. Now, if it were some kind of wing keel with dynamically controlled “elevators/ailerons” powered by two servos with helicopter gyros to control the action. Gyros are commonly used on scale R/C submarines to give much smoother running & diving action. In full sized dinghy sailing the key to performance is to move your big butt around. Moveable ballast on a Footy would be a pretty fun thing. Just imagine the wacky moveable ballast & moveable underwater appendage systems that people would come up with. That would be “development”. Check out the Aquataur Models R/C version of a Musto Skiff. <>> Just think how much fun a Footy version of that would be. Unfortunately, the current rules forbid such radical design. Would an “unlimited” class be out of line (the hull has to fit in the box)??? I for one would really like to see what people could come up with. Maybe, the saying “build it & they will come” fits pretty good.

Happy Yachting - Kip