Using hydrofoil for race kayak stability

I don't see how two fully submerged T-foils will help with stability. You either need active control (e.g. wand controlling elevators as per moths), or surface piercing foils. Given you already have outriggers this latter would seem easy to achieve, resulting in geometry a bit like hydroptere in miniature. This would also be well suited to using individual wings/heli blades in a cantilever arrangement.

 

So the idea is that a small, equal amount of lift on both sides will keep the boat level and I am hoping will have less drag and weight than a conventional ama. The fore aft attitude of the boat should remain pretty constant as it is a 20' long hull and will be used on flat water. The T-foil upright will be half submerged and half above the water line.

 

If the tfoils are fully submerged(1-2 chords below the surface) and set at zero degrees angle of incidence(parallel to the loaded waterline at speed), then, when the boat tries to roll, the angle of attack changes on the foils so that the boat is automatically righted. Set up this way the foils produce no lift unless they need to so total drag is less than if they were set-up for constant lift.
And the boat needs to be moving!

 

Unfortunately that doesn't work as you need some differential force to generate a righting moment. For example with amas as you heel over one is submerged further and so generates more buoyant (upwards) force. This can be achieved with surface piercing foils because as you heel more foil span is submerged and so more lift is generated (and vice versa on the other side).

With fixed T-foils the lift from each is constant as the boat heels (except for rather fast roll rates) so no righting moment is generated; they therefore add no stability. It is true that as the hull is not being lifted clear of the water longitudinal stability is less of an issue, though it is still important that the lift be in the correct place lengthways or trim will be off, adding drag.

Edit: crossed posts with Doug. I disagree that the foils will generate useful righting at reasonable rates of roll. Exactly the same effect could be achieved with a (large) skeg.

 

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No-the foils will be much more efficient than a skeg due to their position outboard and their design. A foil system that develops constant lift on a boat like this when you're not trying to fly the whole boat is simply creating unnecessary drag. Many foilers use a rudder tfoil that is set to zero to start with and automatically develops lift or downforce as required w/o human intervention.
The differential lift/downforce created only when it's needed, by even the slightest roll, will tend to keep the boat level at the lowest drag.
Foils need to be symmetrical sections and the boat needs to be moving!

 

Doug
Whilst agreeing that this will give a degree of roll damping, I'm going to disagree that this gives any useful stability. To give stability, the righting force needs to vary as a function of roll angle, not of roll rate. Now I realise that lift falls off close to the surface (that effect was exploited by the Russian river hydrofoils), but it still needs lift to be constantly generated, which a neutral foil angle will not do.

Simon

 

Simon, it seems to me that if the boat is moving along and something causes it to want to roll that the roll movement will change the angle of attack of the water on each foil causing one to lift and the other to generate downforce? It's only a neutral foil angle when not rolling.
In thinking more about this: if the crew slightly moves to one side with foils arranged to be at zero degrees AOI the foils may slightly dampen the movement but they won't correct the boat to level if the crew maintains his new position. But I'm pretty sure that if the crew maintains his original position with the boat level, the foils will try to correct any roll caused by an outside force with the boat constantly moving at speed.
In other words, I think the foils would enhance dynamic stability.

 

Agreed, that's what I meant by roll-damping, but it won't restore the craft to a level position if heeled (i.e. no static stability enhancement even if moving).
Simon

 

Gentlemen, thanks for this really interesting discussion. My thinking behind this is somewhat akin to the Russian river vessels in that as the boat rolls the upper foil will loose lift whilst the falling foil will maintain it so producing a righting force. My foils are intended to be shallow and as they are on quite long out riggers they will reach the surface with only a few degrees of roll. I agree that primary stability is most likely to be through the damping effect caused by the foil surface area moving up and down through the water. In my application I can't see that the foil will alter its angle of attack to the water in roll and get the lift/downforce effect that Doug refers to? So I am still likely to need both foils generating a little lift unless I resort to some sort of active system.

 

Roll damping with the foils is what would happen when ,with the boat not moving and fully loaded, you manually rock the boat and then release it.
BUT: the dynamic stability enhancement I refer to(that will only occur with the boat moving) changes the angle of attack of the foil relative to the flow and thereby causes the foil on one side to lift and on the other side to create downforce. The point of this is to enhance dynamic stability with the lowest drag possible. Setting the foils up to develop lift all the time when you're not trying to fly the boat is a formula for increased drag.

 

Doug...yes so if I read you correctly there are 2 resultant forces created by the movement of the foil (forward vs down and forward vs up) that gives lift and downforce? As you say this could be the perfect solution as there is no major drag when the boat is moving level. I'm going to build and test quite soon.
I've got some really nice heli blades that are uniform profile and should be perfect for a prototype.

 

Thats the thing: its so simple to test. Not so simple to set it up correctly. You'll need to know exactly how the fore and aft trim(pitch trim) changes when you are paddling. Then you have to devise a method of setting the foils up to be at zero degrees angle of incidence when you're going. I think it will work well assuming the area of the foils is sufficient-but not too much.

 

Doug: we are referring to this dynamic effect as "roll damping" rather than "stability enhancement" because the force always acts to resist and slow a roll. This may be enough for the OP in that it would make the boat less twitchy, so their ability to keep the boat upright by body motion might be enough.

The problem is that the two T-foils also resist a roll motion towards upright, so they do not improve stability in the conventional sense (a righting moment at non-zero heel angle). A pair of surface piercing foils (at zero angle of attack if desired) would provide both the roll damping and steady state effects. That is, they would resist any roll, and provide a righting moment whenever the boat is heeled*.

I simply don't see any advantage to T-foils over this arrangement, structurally or functionally.

*Specifically, surface piercing foils would provide a moment proportional to both roll rate and roll (heel) angle, submerged T-foils only respond to roll rate.

 

The problem with surface piercing foils may be the angle that they are set at. It seems to me that if they're set at 45 degrees it would take roughly twice the area to give the same response as horizontal t-foils? And the outrigger would have to be longer to get the center of lift in the same place as the tfoils.

 

Heli blades, thats what I like to hear! Good on ya!
I have a few used blades myself for just such purpose.

If you instal ailerons, you can control them with an inexpensive, levelling servo, auto-magically!
Yes, you'll need a small battery.

While you're at it, fill in the rest of the wings trailing-edge ( between the ailerons ) with flaps for improved overall speed performance by means of a smaller wing.