Automatic keel trim tab?

If you read the gybing board thread-esp. what Tom Speer says-you'll see that if you rotate the whole keel including the bulb there will be NO DIFFERENCE in how the keel/bulb assembly goes thru the water-the difference is how the HULL goes thru the water. In fact, the boat doesn't point as high but its vmg can be greater-the hull rotates bow to leeward and now the hull is going straight thru the water with almost no leeway. The angle between the CL of the rotated foil and the CL of the boat is the approximate angle of attack of the foil. Sails can be slightly eased...

 

I am probably wrong but I thought that gybing boards change the angle of attack so as Doug said the boat aligns itself to the direction traveled and as a results has less drag. I thought the idea for asymmetrical boards and or trim tabs was to increase lift to windward. Maybe a system using this mechanism on a gyibing keel could be devised.

 

Timothy, gybing boards don't really change the angle of attack. The AOA remains the same and the hull rotates. A keel foil with a trim tab is trying to do what the gybing board does because when the back end of the tab moves to leeward it changes the effective AOA of the foil it is attached to.
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It seems to me that with a model the simplest solution would be to rotate the whole foil in a trunk. Many models install the keel fin+ bulb in a trunk molded in or glassed into the hull. On BTG's boat the trunk would be slightly wider and the head of the fin would be shaped to work like a gybing board using both the bulb weight and water pressure to move the whole thing 2-3 degrees from tack to tack. A pin integral to the keel fin and placed at the center of pivot of the assembly could be attached to an epoxy glass piece that straddles the sides of the trunk. Depending on the weight of the keel bulb,a miniature thrust bearing could be used or just a couple of delrin washers.
A sliding "V" shaped piece of epoxy glass attached to a servo could be moved aft to center the foil downwind....

 

I interpreted the explanation backwards. It works like all the rest but his method of control is novel and should work. The question is, has anyone developed actual data to show that such devices improve VMG? The idea to put it on an RC model sounds good.

It will decrease righting moment though.

 

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In some of the stuff I linked above I'm sure there is data. All the IACC boats
use a trim tab as did the 12's before them. The Volvo ocean racers use toed in asymetrical daggerboards that do the same thing(one at a time). You're right about reducing RM but it is minor on most boats that use it.
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Just out of interest and to further illustrate your point: The inventors of CBTF(about to win another Sydney-Hobart on Wild Oats, Alfa or Maximus) have a little known patent that intentionally does exactly the opposite of what you suggest except with great effect. The system is based on the original patent where the twinfoils develop all the lateral resistance for the boat as well as generate turning force. Except on the new patent, the canting strut (+-55 degrees or more) has a trim tab on the back end. The tab is not used to improve windward work at all but is used to generate DOWNFORCE when the keel is canted(back end of tab UP). This downforce is RM and allows, approximately, a 20% reduction in ballast that a canting keel boat would normally carry. Since a canting keel boat already has up to a 50% reduction in ballast compared to a fixed keel boat the predicted performance is quite high.

 

That makes sense.

 

Sorry Doug I am still a little confused . Would not a toed in asymmetrical dagger board maintain its angle of attack , change the angle of attack of the hull produce lift to windward, and with increasing heel reduce righting moment, in affect the same as a gybing board with a trim tab ,or am still failing to grasp the concept?

 

Timothy, read the thread I referenced above. A toed in daggerboard does the exact same thing on one tack as does a gybing board or trim tab on both tacks. They allow the bow to rotate a little to leeward so that the hull is no longer moving a small amount sideways upwind. But the foils are going thru the water in the almost* the same way they would without the trim tab or without being toed in(relative to the hull cl) and without being angled(relative to the hull cl) in the case of the gybing board.
In the case of the trim tab visualize the effective angle of attack of the foil this way: if the back end of the trim tab is deflected,say 10 degrees, to stb draw a line from the leading edge of the fin to the back end of the trim tab; the angle that line forms with the center line of the foil/boat is the effective angle of attack of the deflected trim tab+fin on port tack(tab to stb).
Hope that helps.

* almost because in the case of the fin + trim tab the actual effective foil section changes with deflection of the tab-same princible ,though.
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Tom Speer,from that thread:

"Cambering the keel is just like gybing it. Camber changes the zero lift angle of attack. But once again, the total lift has to stay the same. So if you increase the camber**, the change in zero lift angle of attack is just like gybing the board. And it would have the same effect of rotating the bow off the wind to align it more with the course through the water.

The difference with camber compared to gybing is that camber affects the profile drag, while gybing is just another way of changing the angle of attack. With camber, you can place the minimum profile drag at a nonzero lift coefficient, so you can center the drag bucket about your operating point. So aside from acting as a second rudder for big maneuvers, minimizing the profile drag is what I see the AC crews doing with their keel flaps."

** Deflecting the tab=changing the camber=changing the effective angle of
attack of the keel fin.

 

Thanks Doug . I think I have it now. Sorry to be off topic but I am curious. For shallow draft and downwind work, would it be feasible or worth while to design a lifting dagger board like keel in a partially rotating trunk, something like a rudder in a drum?

 

Tom, Doug,

I do remember the long and exhaustive thread on jibing boards from some time back. I was mainly trying to get to an understanding of what BTG was proposing. The hull of the boat will rotate to be more in line with the direction of travel. If a hull is designed for this, there may be a minor bit of improvement. On the other hand, some boats, particularly hard chine ones get some lateral resistance from the hull. All hulls have lateral resistance and if the hull is in line with the motion, this will be lost. Since the foils can be more efficient in L/D than a hull, the lifting foil is a net plus. Worth the effort. Maybe, especially in match racing where boat speed trumps everything else..

 

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Timothy, you don't need a foil to rotate downwind-but you probably want it centered. I'm not sure the complexity is worth it for shallow draft non-racing boats but you'd have to decide case by case...

 

Yes, but gybing keels are already developed- I wanted to try something new... Will it work? I hope so, anyway- I keep up innovating and try to evaluate my imagination horizons .

Regarding the wings for creating more righting moment- as far as I remember similar solution was developed by Wally design team... But nobody applied it on a scale boat ... (the wing I'm talking about is the orange one on this render) http://www.wally.com/templates/popupReD.asp?Titolo=Wally&idImmagine=3776&idOrdinamento=2&idNews=0

THE SECOND IDEA:

Kind of evaluation of the idea of ,,canting wing for righting moment increase" (CWRMI, how sophisticated ;p ) is to put at the end of such a wing a bulb without lead inside. Such wing will work on leeward creating more righting moment on quite big arm. Also such ,,empty bulb" may also contribute to the overall displacement of the boat, and allow to ,,move" the displacement from the hull into torpedo shaped bulb. The benefits should be in :
* lower shape drag of the hull by making it possibly smaller
however...
*wetted surface increases... ;/
I'm not sure will the benefits overweight the drawbacks...but... curious about your opinions

 

BTG-sounds like you're headed toward a trimaran-good place to go!
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Just to give the proper credit: the wing you showed was developed by DSS-Dynamic Stability Systems: http://dynamicstabilitysystems.com/#/profile
DSS was developed by Hugh Welbourn-here is an interview: http://www.harken.com/Interviews/DynamicStability.php

 

Actually I'm going (eventually) to apply this device on a monohull... Will it work? Any chances? Worth playing?

 

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Frankly, if you use a device that contains buoyancy at some distance from the hull supported by a structural member attached to the hull AND the buoyant part is always immersed, when it is to leeward, then I think that by almost any definition you have created a trimaran. Maybe I'm misunderstanding you?