Extra rudder at rear of keel

JumbleDuck

JumbleDuck

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In the yard at Port Bannatyne is a yacht, Phendra, which from the CYCA handicap lists I gather is a Guy Thompson T31. As well as a perfectly normal looking rudder where you'd expect it, she has another rudder (or rudder like object), almost as big. down the trailing edge of her fin keel. I've had a look at details of a couple of T31s for sail online. I can't see any sign of the keel-rudder on the only underwater profile picture I can find (it's a bit small and dark, though) and there doesn't in either case seem to be any obvious control system for it visible.

I'm intrigued. Is it a trim tab? How is it operated? One of the yard guys working on her moved the tiller about and only the normal rudder moved, so the keel one doesn't seem to be coupled. Did all T31s have these?

What, in short, is going on?
 
The keel trim tab doesn't seem to have been fitted on all T31s and/or possibly some owners removed them if they didn't use them. I believe they were operated by a separate lever from the rudder with the purpose of setting it to offset leeway and weather helm. Having sailed a T24 (which doesn't have a trim tab), I suspect that many considered it a refinement they could easily live without. The basic underwater shape is very similar and on a beat they settle into a well defined groove.

Rob.
 
Many thanks, both. All now explained. Do any racing yachts allow their keels to be rotated to match the angle of attack to the tack they're on?
 
Some dinghies have what are known as gybing centre boards which pivot to change angle of attack, I've sailed on one yacht with a trim tab, tricky thing to adjust as too much tab creates drag. So angle against speed pay off.
 
we had a gert big trim tab on a RORC boat in the late 60s. There were measurement advantages one year, and disadvantages the next, so it was locked off and sealed up.


The only time it was really useful was one COwes Week when we were crashing down onto the start line off the Squadron and I'd put the boat too far in front by about 5 seconds. Opposite lock on rudder and trim tab resulted in a huge braking effect and lots of wash, and very slow forward progress. We got away with it and made a decent start:). Frightened the boats behind us, though.
 
Pladdatoo said:
The T31 wasn't the only one to have such a device, (some of ) the Centurion 32's also had them.
Click to expand...

And the Shipman 28. Mine had it but as the boat was raced it's been glassed over and the size of the rudder increased.

As I understand it these boats were designed to run at fairly large heel angles (24-27 degrees for the Shipman) and therefore had a fair bit of weather helm due to the centre of pressure of the sails being off centre. The trim tab was used to tune this out to help the helmsman. Lift to windward would then come mostly from the trim tab and not from the rudder.
 
I believe the 12metre class when they were used for Americas Cup typically had a trim tab. In at least one case operated by an inner wheel of the helm. In one case I believe when the rudder failed t was used for steering. However I imagine this would be of limited power. A trim tab on aircraft surfaces are always on the trailing edge to reduce control surface loads so these on the 12m were very different. I always imagined that the trim tab on the rear of the keel would be more akin to flaps on a plane where the camber of the foil can be increased to provide more lift. In the case of the trim tab on a keel it would introduce some assymetric camber so that the keel could provide lift to windward. This would counteract leeway indeed could provide a negative leeway. On this theory I wonder why we don't see trim tabs more often. I have wondered about fitting a trim tab to my little boat but a lifting keel makes it all a bit tricky. Easier to think about than do.
Of course a trim tab must introduce drag if it is doing real work but then drag at or above hull speed wont make so much difference but less leeway will get you to windward quicker. Sorry all my own theories olewill
 
William_H said:
I believe the 12metre class when they were used for Americas Cup typically had a trim tab. In at least one case operated by an inner wheel of the helm. In one case I believe when the rudder failed t was used for steering. However I imagine this would be of limited power. A trim tab on aircraft surfaces are always on the trailing edge to reduce control surface loads so these on the 12m were very different. I always imagined that the trim tab on the rear of the keel would be more akin to flaps on a plane where the camber of the foil can be increased to provide more lift. In the case of the trim tab on a keel it would introduce some assymetric camber so that the keel could provide lift to windward. This would counteract leeway indeed could provide a negative leeway. On this theory I wonder why we don't see trim tabs more often. I have wondered about fitting a trim tab to my little boat but a lifting keel makes it all a bit tricky. Easier to think about than do.
Of course a trim tab must introduce drag if it is doing real work but then drag at or above hull speed wont make so much difference but less leeway will get you to windward quicker. Sorry all my own theories olewill
Click to expand...

As you say, more like a flap. The trim tab is quite large in relation to an aircraft trim tab. It can clearly be seen in the diagram in the entry for the Shipman 28 on Sailboatdata at http://sailboatdata.com/viewrecord.asp?class_id=878.

A trim tab incurred a penalty in the calculation of the IOR rating, multiplying the rating by 1.0075. On my boat this would have taken the IOR Mark III rating from 20.75 to 20.91. This would have taken the IOR Time Multiplication Factor from 0.8912 to 0.8928, so not a lot of difference. That is about 6.5 seconds per hour penalty.


I have no idea how trim tabs affected IMS or IRC ratings, if at all.
Rod
 
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