T-foil

[national]

Anyone fancy sharing their thoughts on T-Foil rudders?
I take it the idea is to lift the stern? does this not just bury the nose?

[catri26r]

I have sailed my catri 26 for 5 years now, and it is equipped with no less than
5 foils, 3 Tfoils and 2 bruce foils.
My experiece so far is that the Tfoil on the rudder (actually the designer prefer to call it a steerable daggerboard) has a non-dive function like on some motorcycles.
If have noticed that when sailing beside multihulls without foils, they are moving up and down, were my boat is sitting still.
The Tfoils on the rear of each outrigger is doing same function.
I guess it works like on an airplane, if you take off the stabilizing wing on the rear the airplane will not fly very stabile.
The Bruce foils in front only provide lift.

[granite]

If you are talking about dinghy's such as the Int 14, then upwind with the foil lifting you move your weight back to bring the trim level agian. It basically decreased the effective displacement of the boat.

Downwind with the angle adjusted to reduce the lift then they have a stabilizing affect keeping the bow out of the water and making the boat smoother to sail.

[smakinen]

I want to get/make a T-foil system for my I-14. I prefer the design of adjusting the pitch of the foils relative to the rudder rather than adjusting the fore/aft swing of the rudder with fixed foils. This system requires a moveable rod inside the rudder to adjust the pitch of the foils. To do this, I think I need to make a new rudder by creating a mold from the existing rudder. If I create a two part mold, then I can put a column inside the rudder where the rod will go. Hope all that makes sense.

I need to figure out how to make a two part mold acurately so the two pieces can be joined together to make a nice smooth rudder foil shape. Suggestions?

If someone knows of existing rudders, rudder molds, etc. That would be great instead of doing it all myself.

[tspeer]

There are different reasons to add a foil to a rudder. One is pitch control. The other is to modify the stern wave and effectively lengthen the boat.

Paul Bieker has written a good description of the latter design philosophy. His foil is mounted much closer to the surface than a T foil:


A key design parameter is to keep the span of the foil at least as wide as the planing surface of the hull. Otherwise, it would be more efficient to generate the lift with the hull instead of the foil.

[smakinen]

That is a good point regarding the functionality. The systems that I've seen, with pitch adjustment, have horizontal foils mounted at a similar position on the rudder (as accurately as I can judge from pictures). Do you know the difference in position on the rudder? Position for modifying the stern wave versus position for pitch control?

Do remember where Bieker's article is? On the web, magazine? I might have seen it, but not remembered this aspect/missed the point.

[TimClark]

Hey Steve, so you're looking at a T-Foil for this season? Nice!! For the mold, I could come up with the design of the rudder and t-foil, and for a mold I have access to a CNC router that we could use to make a female mold for the rudder using dyvincell(sp?) foam. I have been working on a few designs lately that have incorportated the use of a T-Foil, and I have read up about them.

P.S.-Are we gonna go out this weekend or is the boat still in repairs? Forecast looks nice...

TC

[smakinen]

Sounds great. I didn't realize that assistance was so near. I jumped to posting to a completely public forum before asking within my new local sailing community.

Will email you offline.

[tspeer]

Originally Posted by smakinen

...
Do remember where Bieker's article is? On the web, magazine? I might have seen it, but not remembered this aspect/missed the point.

It's on the I14 site.

[catri26r]

Dear National, you buy the lifting theory great! - now imagine what you can use this lift for -> move the load on the boat(crew)to the rear resulting in less weight in the front of the boat = less dive achived.
In addition to this if the boat pools forward lifting out the rear end, the aft T-foil is lifted out of water too if designed well, and the T-foil has now lost ALL it's up lifting force and the boat falls back down safe and sound.
If the T-foil on the rudder is mounted on the very bottom of the rudder this may not work well.

[sigurd]

A key design parameter is to keep the span of the foil at least as wide as the planing surface of the hull. Otherwise, it would be more efficient to generate the lift with the hull instead of the foil.

Could you explain more? I'm thinking that for the same lift, the planing surface must be larger, and has bigger Cd, so at high speed the shorter span foil could have less drag than the hull, even with more induced drag?

[smakinen]

This is a good question. I'm not sure about the answer, I'd like to read what tspeer has to say/write. I would think...the t-foil creates a system for interactional hydrodynamics, where the combined effect of the hull and tfoil is more efficient. Biplanes are designed this way, so that the lift to drag ratio for each wing is higher due to the presence of the other wing. The total system is not more efficient than the hull alone unless the horizontal foil plan is as wide as the planning hull surface. The t-foil will also improve the drag characteristics by improving the "exit" of water aft of the hull by decreasing the cross-sectional area of the wake.

[tspeer]

Originally Posted by sigurd

Could you explain more? I'm thinking that for the same lift, the planing surface must be larger, and has bigger Cd, so at high speed the shorter span foil could have less drag than the hull, even with more induced drag?

Yes, that's true, one speed and above. But why accept larger induced drag?

If you make the span large, you have less drag in marginal planing conditions, and if you lift the hull enough reduce the wetted area by twice the foil planform area, you've gained on the skin friction, too.

You'll notice Bieker's foil is as wide as the bilge.

[granite]

The foil is normally about 30cm behind the transom so will be running in the area of water that makes up the stern wave.

Perhaps if you make it too wide you will have part of the foil in the stern wave and part outside could there be turbulence between the areas of water that would cause an increase in drag?


A second less important but practical concern is that if the foil is wider than your boat and your car it is difficult to transport so you wont take it sailing.

[sysfx]

Originally Posted by catri26r

...if the boat pools forward lifting out the rear end, the aft T-foil is lifted out of water too if designed well, and the T-foil has now lost ALL it's up lifting force and the boat falls back down safe and sound.
If the T-foil on the rudder is mounted on the very bottom of the rudder this may not work well.

Gunnar,

When the stern is lifted out of the water, a T-foil on the rudder's bottom generates downward lift that helps the boat return to horizontal.

Foils placed closer to the surface enable lift (and drag) to disappear sooner, probably resulting in a comparatively smoother ride. They also increase the effective waterline length.

Comparing bottom and surface foils with the same section, area, span and incidence, the foil closer to the surface will work at a larger angle of attack because the flow near the surface tends to follows the boat's bottom curve, while the flow deeper tends to be horizontal.

Rgrds,
Luiz