Author Topic: Keel trim tab performance  (Read 2477 times)

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Offline claudiof

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Keel trim tab performance
« on: January 15, 2007, 10:02:26 AM »
Trim tabs can be seen here and there on protos but not much on Minis. I heard one Mini had such a device (Peter Heppel in 1999), but don't know how much the trim tab helped, if it helped at all.

On paper though it looks very interesting: 5 degrees more can be expected close-hauling and additional performance can also be gained broad-reaching with a negative trim angle. It's also simpler and lighter than a canting keel. Anybody familiar with this option?

Offline mathieu verrier

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Re: Keel trim tab performance
« Reply #1 on: January 15, 2007, 13:08:07 PM »
hello,
to me the trim tab is an improvement on fixed keels only. When a keel is canted, it results on the keel having  an angle of attack and thus a force which in some case can result in increasing (because of the upward component) the heeling moment and maybe (very rarely) some times reduce it, which is almost impossible to compute or see unless a dedicated towing tank testing is done. Thus, a trim tab could solve this problem but you don t know how much angle you need and in which direction you need it to act to both increase the side force and reduce the heeling moment. So if there is one but you are not sure how to set it, it is of no use.
Hope this helps

Offline Charles Bertrand

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Re: Keel trim tab performance
« Reply #2 on: January 15, 2007, 15:46:30 PM »
Hi,

Trim tabs are usually applied on fixed keel as a way to produce more lift with minimum extra drag. Fitting a trim tab can enable to reduce the foil section thickness as well as the chord (and therefore wetted surface area) of the keel. It is true that you can save some leeway angle by using a trim tab but your sailing angle will always be as good as your sails (jib sheeting angle) enables you.
Downwind, they can indeed be used with a negative incidence to have a more downwind course.
However, it's use is less evident for reaching sailing conditions, which seems to be the most frequent point of sail in offshore racing. This is the reason why you tend to find trim tabs on IACC and other "round the cans" yachts, but not so often on offshore racing yachts.

They have yet also been applied more rarely on canting keels. On the Coockson 50 for exemple, the trim tab enables the keel to produce the required lift to compensate the sailing side force although it is canted and therefore does not require the use of a canard or daggerboards. The yacht sails upwind with a moderate cant angle and can use the power of the extra cant angle at reaching, where sideforce is not so much required.
Trim tabs on canting keels could also be used theoretically to increase the righting moment of the yacht by generating some downward lift with some negative incidence, when the keel is highly canted. However, this would inevitably produce some extra induced drag and requires the use of canards or daggerboards.

Cheers,

Charles

Offline NicoG

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Re: Keel trim tab performance
« Reply #3 on: January 15, 2007, 19:07:54 PM »
Most Cooksons think about retrofitting canard/daggerboard.

trimtabs need delicate decisions, not easy when you are tired :)

Offline Charles Bertrand

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Re: Keel trim tab performance
« Reply #4 on: January 15, 2007, 19:55:32 PM »
True, the latest one to be launched has gone for that option...
http://www.sailinganarchy.com/index_page2.htm