Keel Trim Tab design

I am looking for info on Trim Tab design.
Existing bulb keel has approx. a chord of 1.5 times the span and a thickness of about 15%.
I don't want to increase the draft.
I hope with a tab to be able to decrease the keel area and thickness.

What sections are suitable for use with a tab?
How big should the tab be?
How much could the keel area be reduced?

Peter

 

Do I understand correctly that the keel is wider than it is deep? That seems strange for a bulb keel.

If that is the case, why would you want to add a tab? It would allow a modest reduction in keel area, but it's not addressing the major source of drag, which is due to the shallow keel.

Instead of a tab, how about a keel with a centerboard that is entirely housed within the keel. It would give you shallow draft when you need it, and better performance when you have the depth to drop the board.

 

Tom,
Yes,
Length is 1.5 times the depth.
Bulb was a retrofit.
Are you saying that there is no gain in effiency to be had with a tabbed keel?
Is a tandem keel a better option?
I don't want to increase the draft.
Peter.

 

Performance depends on the lift/drag ratio of the hull. The lift on the keel is determined by the sail trim, not the keel design or whether the tab is deflected or not. A tab will alter the angle at which the hull goes through the water, but it's the drag that determines the velocity made good.

So you need to attack the drag, and, there are two main sources of drag on the keel - skin friction and lift-induced drag. The first is mostly due to the wetted area. If you have more area than you need, you might be able to reduce the drag by shortening the keel's chord. For keel sections that have a distinct low-drag region by virtue of laminar flow, a tab will center the low-drag region about the operating lift. But that his hardly the kind of keel you have.

The induced drag is inversely proportional to the square of the depth of the keel. You can get some increase in the effective span of the keel with winglets, etc., but fundamentally the way you decrease this drag is by making the keel deeper. Since induced drag is the lion's share of the drag of the keel when going to weather, a modest increase in depth can make for a significant reduction in the drag of the keel. Much more than you'd be able to get with a tab.

A tandem keel of the same depth will have basically the same induced drag. There may be some drag reduction by virtue of cutting out some of the wetted area to make it a tandem keel, but I'm skeptical that it would significantly improve the performance.

As they saying goes, "If you're hunting elephants, you have to go where the elephants are." You basically have a low-performance keel, driven by a constraint on the depth. The best way to improve the performance is to relax that constraint.

 

Tom,
It looks like I am stuck with what I have.
Can answer one more question for me.
It appears that some foil sections are more suitable for the trim tab application than others. Which ones are those?
Thanks,
Peter

 

It depends on the application and the section design.

Sections that have a distinct drag bucket can benefit from a tab so as to move the drag bucket to match the operating condition. But the drag bucket is generally due to laminar flow, and you're not going to have laminar flow in water over bottom paint.

Sections that have a steep increase in the aft portion of the section are not suitable for a tab because the tab will cause the pressure to increase more rapidly (assuming a positive tab deflection), and result in earlier separation at the trailing edge.

Finally, deflecting the tab, especially for low-aspect ratio keel like yours, will result in yawing moments that have to be trimmed by the rudder. This is likely to be the biggest benefit (or deficit) from a tab for your application. A positive tab deflection will tend to turn the boat away from the wind. I assume the boat has weather helm, and there may be some benefit from using a combination of tab and less rudder to trim the boat. This is not something that can be determined by looking at the section shape, as the same size tab will have approximately the same effect on the lift and moment for different section shapes.

 

Sorry to interupt, but I wonder if there has been a miscommunication in language. Or perhaps I'm way off the mark.

Phum, from memory you own a NIS with a swing keel. Mind my dodgy drawing, but does your keel look something like this where the length or draft is 1.5 times the chord?



Cheers
Mojo

 

Mojo,
Some people collect cars,stamps etc. I collect boats.
The NIS. is only one of my collection. At last count I had 4 TS's (2 wood 2 composite) 2 sportsboats(old) 2 yachts and many models.
The Post was re. one of the yachts.
Have fun in warm Townsville
Peter

 

My mistake, sorry Phum.

Sounds like a fine collection of boats.

I'm actually over in bloody cold Manchester, Uk at the moment. I'm going to struggle here in winter.

Cheers
Mojo