Foiler Design

Ah Ha!

Charlie, I think I understand: the foil strut combination does not move as it does on the Trifoiler but the foil moves relative to the strut. Right?
If thats the case you may be able to use a wand because the all moving foil strut combo(trifoiler) takes more physical force to move than does the flap.And this set up may not require much force to move at all. Whats your sailing experience with moving the foil at high speed-does it move easily by hand as I would think-or harder?
How much angle of incidence change up and down is allowed by the mounting? How free moving is the foil/mounting?

 

Q: Charlie, I think I understand: the foil strut combination does not move as it does on the Trifoiler but the foil moves relative to the strut. Right?----

---- um, I'm not sure because I don't know how it moves on the Trifoiler. The strut is locked in place on the beam to the outrigger float, the foil simply changes angle of attack as it sits in one spot below the strut.


Q: If thats the case you may be able to use a wand because the all moving foil strut combo(trifoiler) takes more physical force to move than does the flap.And this set up may not require much force to move at all. Whats your sailing experience with moving the foil at high speed-does it move easily by hand as I would think-or harder?----

---- It moves easier. Much like a bicycle is easier to steer once you get up to speed.


Q: How much angle of incidence change up and down is allowed by the mounting?----

---- I've never measured. The joystick has numbers running from 1 to 10, with 5 being neutral, approximately. By eye, it looks like 10 to 15 degrees in either direction, positive and negative.


Q: How free moving is the foil/mounting?----

----I'm not sure what you mean here. . . The foil where it attaches to the strut? The foil is not free moving at all, it only changes angle of attack, no other movement at all. The strut doesn't move either, it's just locked in and simply moves as the boat moves.

The point where the foil swivels is a bit closer to the leading edge than the trailing edge. The rod connects on the aft side of this spot where the foil attaches to strut.

C

 

wand

Charlie I'm fairly sure a wand would work; you can e-mail me with your snail mail address and I'll send you a sketch of an idea. I'll try to describe it here.
The wand will rotate on an axle. Attached to the axle there will be a lever consisting one short lever and one longer lever. This can be made out of one piece of glass or aluminum.The short part of the lever should be about one - two inches(just guessing: this distance is determined by the distance it will take to allow there to be around 60 degrees of movement between MAX NEG LIFT and MAX POS LIFT by the wand-approx. one half the distance between where the push rod intersects the foil and the pivot of the foil; the shorter the more movement and mechanical advantage) from axle pivot to the attachment point of a pushrod direct to the foil. A line between the cl of the axle and cl of the pushrod attachment point should be horizontal with the foil in neutral(+ 2 or 3 degrees I assume). The longer part of the lever should be 45° up from the shorter one(at neutral) and around 18-to 24" long. The end of this lever is attached to shock cord that pulls the lever aft and the bottom of the wand forward as the boat lifts.As the wand rotates aft because of water pressure before foiling it will push the pushrod down raising the front end of your foil creating lift. As the boat starts to lift the shock cord will pull the lever aft and the wand bottom forward lowering the front end of your foil and reducing lift.
You can attach the wand to the axle in such a way that you can adjust wand length; a combination of wand length and shockcord tension will control altitude. You can increase the complexity by devising a way to get the wand out of the water in nonfoiling conditions thereby reducing drag.
You'll have some experimenting to do but this ought to get you real close to right.
E-mail me at: lorsail@webtv.net with your snail mail address for the sketch.

 

Steve:

Does this mean that the foiling technology can be applied to non-planing hulls?

 

I think there's a case to be made for foils being most appropriate for non-planing hulls.

There's nothing as efficient at low speeds as a slender displacement hull. Since foils can be retracted, they allow the hull to "shift gears" just like the rig does through sail changes. Even if the foils are fixed and not retractable, the boat still changes modes between displacement sailing and flying. The drawback is the boat is still carrying the weight of the foil system even when retracted.

A planing hull is essentially a fully ventilated foil operating at the surface. So it pays the fixed foil penalty in wetted area when sailing at non-planing speeds. A foil could potentially provide the same dynamic lift with less wetted area.

So it's possible for foils to improve low speed performance because the hull can be designed for the low speed range and have minimum wetted area. It doesn't need to be designed for speeds above take-off because then it's out of the water. There may also be some weight gains because the hull doesn't have to be designed for the pounding of planing.

Whether or not there's a net gain for foils depends on the details of the design and how it's operated.

 

A number of years ago a novel approach to foil control was described to me. I've waited to see it implemented, but haven't to date. It's simple, has infinate variations and some decided advantages, I think . Probably also some disadvantages, but that's why I'm posting it here:

Presume an airplane configuration; forward foils near CG with flaps; rudder foil slaves to fwd foils' lift (a simple configuration for illustration; of course there are better ones). This is similar to a Rave's layout, but the forward foils' flaps aren't connected to anything--they're free to trail.

Now, attach 2 wands to the flap itself. Presume these are very thin, streamlined, and are mounted vertically--one up from the flap and one down. Presume they are oriented 180 degrees to each other.

The drag from the wands are equal, they offer no torque on the flap axis. Now, shorten the lower wand. The drag from the wands is now unequal, the flap deflects downward, the foil lifts.

As the upper wand exits the water's surface, it's effective length becomes shorter; until at some ride height its shortened length equals the lower wand's torque on the flap and the foil goes to neutral. If the foil rises too high, the shortened upper wand offers less drag then the fixed length lower one, the flap rotates up, the foil's lift goes negative.

End of story. No moving parts other than the flap/wand assemblies. No adjustments, nothing.

Of course there are several obvious problems; the wands ought to be swept back both to avoid ventilation and to offer some damping of the system. There is additional damping as the flap tries to rotate quickly--the wand-tips offer much additional drag through the moving water, damping over-fast response. Damping is necessary to deal with "countouring;" the ability to sense the average surface without reacting to every little wave.

The lower wand is in danger from shallow water--so replace it with some other drag-producing device--the absolute drag on the wands can be quite small, so overall added drag can also be quite small.

Several variations are possible, preloading the flaps with springs (this makes a speed sensitive version, however. Opposed wands offer balanced flap operation which is speed independent--even if one replaces the shorter lower wand with equal length wands but an asymetric foil section to gain the lifting bias). It should be possible to design wands/flaps which offer too little force to actuate the flaps at low speeds (hullborn), yet work properly at higher speeds. The concept might be simple enough to utilize multiple self-controlled foils for better efficiency (some lift at low speeds while other wands don't yet have sufficient force to actuate high speed foils; then the low speed foils can exit the water (a la Decavitator), as the high speed foils come online. Multiple small (really tiny) wands per flap might offer more damping ability; also might make it easier to deal with surface countouring issues.

Last, this approach, if the wands and flaps are properly sized, ought to better deal with the orbital motion of water molecules as the foil enters and exits waves than other surface-sensing systems--a major problem with foilers flying faster than following seas.

Did I miss anything?

Dave Culp

 

Yes, and I think the gain is probably more significant than becomming completely foilborne. I have played with rudder foils on at least two boats with interesting results. A relatively small foil on a catamaran rudder will essentially eliminate pitchpoling as a capsize mode. Similar results have seen the Moth and I 14 classes significantly refine and advance themselves.

 

I've read of an even simpler system that was implemented by Hydronautics. A T foil has a flap that is free to rotate as you describe. Extending vertically from the flap is an arm or wand. In the Hydronauics version, it was a piece of sheet metal folded to form a V cross section that got wider toward the top. The wand was long enough to extend above the surface of the water when flying. The wand was located at the center of the flap, just behind the strut. A cam on the strut could adjust the forward-most movement of the wand, and thus the most negative flap deflection.

The hydrodynamic hinge moment on the flap made it want to deflect upwards. The moment on the wand wanted to deflect the flap downwards. As the wand emerged from the water, its frontal area decreased, lessening its moment and allowing the flap to deflect upwards, reducing the lift. If the boat flew lower, more wand was immersed and the added drag deflected the flap down, increasing the lift.

Both the hinge moment and the hydrodynamic moment from the wand were proportional to speed squared, so this mechanical feedback system automatically adjusted its gain as a function of the speed of the boat. However, I suspect the boat did tend to fly higher with speed, since the lift coefficient would be lower with speed, and this means a more negative flap deflection. The changing width of the wand resulted in a nonlinear gain as a function of flying height.

This is about as simple a system as I've ever seen. The flap/wand unit was all one piece and the only actively moving part. It's the same system you describe, but without the lower wand.

I believe the report detailing this system is on the AMV CD#2 from the International Hydrofoil Society.

 

It's exactly the same system, with one of a number of "other" ways to bias the flap for positive lift. I'm guessing the 2-wand version is less sensitive to velocity effects, but the one-wand-plus-bias is more robust. Does anyone know if somebody has built and tested these methods?

Is there an inherent weakness, which might describe why any other system exists?

Dave

 

Drag? Bradfield wands are draggy enough, and they're not perpendicular to the flow, nor have as much exposure.

And then there's flexibility. Bradfield's system also allows the operator to adjust the set point of the feedback system by changing the bungie preload. The Fastacraft Moth kit puts the wand at the bow where it will add lead to the feedback, allowing the foil to start reacting to a wave before it gets to the foil itself. Also by retracting the wand one can eliminate the sensor drag completely for displacement sailing. An appropriate up-stop on the flap would result in near zero lift for minimum induced drag at the same time.

BTW, a modern aft-loaded section would put plenty of hinge moment on the flap, so the lower wand is probably unneccessary.

 

Rave wands are 1" square and nearly 2' are immersed. I'm thinking of wands the size of automobile aerials, streamlined. They could be truly tiny, if the flap were balanced. There's no need to make them perpendicular to the flow either. In fact there's good reason to sweep them back.

Is this a feature or a bug--er, patch? Trifoilers never had these, and I've always questioned their purpose on the Rave. What's the advantage, if you had self-adjusting foils, would you want these?

I've wondered about this. Ketterman feels that the lead is crucial--even it's length/timing. OTOH, Raves and most other Bradfield boats don't use it at all--yet seem to fly just fine. What's the real scoop?

Lots more mechanism, many moving parts, much weight/cost/potential for failure. I note that in real-world boats, the sensor is rarely retracted; only de-coupled, and left to drag through the water. If the tiny wands could be made small enough, and could be designed not to actuate the flap at slow speeds (preload?), you could get almost the same advantage--with no additional parts at all. I bet the weight savings alone would make it superior.

Dave

 

I was mistaken - the report is not by Hydronautics, it's Grumman. And it is on the IHS AMV#2 CD. The reference is:

Muncie, Robert C., "Development and Testing of Fully Submerged Hydrofoils With Drag Vane Control Installed On 15 ft Runabouts," Report 63-13-M-(M20)(R), Grumman Aircraft Engineering Corporation, 25 April 1963.

"Conclusions

"1. The sea state capability of the fully submerged foils exceeded expectations. The ability of the 15 ft runabouts to remain foilborne in 1 1/2 to 2 foot waves shows good potential for the fully submerged vane type, trailing edge flap control.

"2. In general, and for the fully submerged foils tested, the smaller aspect ratio high camber foil provides a better rough water lift to drag ratio and better sea state capability. The higher aspect ratio foils provide better smooth water lift to drag ratio and a smoother ride.

"3. For the configurations tested, the fully submerged foils provide a better rough water and high gross weight lift to drag ratio, and better sea state capability, than the "Sea Wings" surface piercing foils. The "Sea Wings" a better smooth water lift to drag ratio at low gross weights, a smoother take-off and better turning capability.

"4. Increases in gross weight have been least detrimental, to significant smooth water performance parameters, for the fully submerged aspect ratio 3.00 foils. Performance degradation was somewhat greater for the aspect ratio 1.85 fully submerged foils and largest for the "Sea Wings" surface piercing foils."

Some other pertinent quotes:

"It is important to realize that, for the tested configurations, the "Sea Wings" surface piercing foils have a much higher loading than for the fully submerged foils...Accordingly they have less foil wetted area and less foil friction drag in smooth water which accounts for their higher top speed capability at the minimum gross weights. At the higher gross weights, the "Seq Wings" still have less wetted foil area, but due to the increased immersion, they begin to use the relatively inefficient (high dihedral angle) of the surface piercing element...Strut drag increase with gross weight for all configurations was about the same because immersion increase was about the same for all configurations."

Note: the drag vane fully submerged foils ran deeper with gross weight because a higher flap deflection was required to support the increased weight. This required more vane immersion at the same speed.

"Maximum gross weights for the tested configurations have not been limited by flow breakdown (ventilation or cavitation) on the main foils. Instead they have been limited by the inability to trim the craft to higher nose-up pitch angles before reaching the point where insufficient thrust was available to force the craft to higher speeds. This is strongly dependent on the hull shape of the outboards, realizing that changes in main foil incidence angles were not used to favor take-off capabilty."

There are a number of qualitative observations about the behavior of the two foil types that are quite interesting in addition to the quantitative data presented. For example, drag vane ventilation made the fully-submerged foil takeoff a two-stage process.

 

Dave,

I like the system you are describing, for it's simplicity, but there may be a good reason not to sweep the top wand back. As the wand emerges from the surface it will rotate forward. If it is raked aft, more of the wand will emerge from the surface, further reducing the wand area. I suspect this may cause self excitation. In this respect, there is probaly a good case for raking the upper wand forward to improve self damping.

Mal.

 

(litterally) Flying 18 footer...

Just got the latest Australian Sailing Mag (for those who don't know it, it is the magazine to have if you're a sailing nut in Australia), open up to the third page, and what do you know... someone has put wing-tip (aka. Brett Burvill's Windrush Moth-style) foils on an 18 and sent it litterally flying. Is this the way forward? Is it just another rediculous project? Who knows! Either way, perhaps this is one testiment to the fact that the 18s and 16s should still be a development class as they used to be!

 

Hmmm, you've a good point, but it works the other way when the (or both) wands are submerged (presuming an upper and a lower wand): When the flap deflects upwards, one wand is "laying down" and the other is "standing up," creating a positive feedback. Once the upper wand breaks the surface, this feedback reverses... hmmm, is this maybe the reason we're not seeing this setup on boats?

Dave