Trim tab

[slowpoke]

Does anyone know anything about trim tabs on the trailing edge of the keel? I've seen them on a few boats, what are the advantages and disadvantages? Thanks for ed-jamikatin' me!

[admin]

I've only seen them on powerboats as a way of getting the arse out of the water and on to a plane, the motor being powerful enough to get the boat out of displacement mode. Have you seen them on sailboats? Seems like they'd only introduce more drag.

On the other hand, it might be a good way to introduce more LOA for those pesky size-queen handicapping systems that want us at 19 feet

[i550sailor@aol.com]

This is probably the best article I have seen written about the subject.

http://www.sponbergyachtdesign.com/Keel%20and%20Rudder%20Design.pdf

[lohwaikin]

I think Rocky is referring to a "mini aileron" at the lower trailing edge of the keel foil (just before the bulb) which can be set from the headstock of the keel on the cockpit floor. The aim is probably (most likely) to induce some right moment while sailing beam reach.

I've been thinking about that lately....
Just wondering how long should this "tab" be, and the means of moving/fixing it, can it be automated via reliable mechanical means.

I have also been thinking about a heavier keel lately...

Cheers,

[jray]

Loh, I sent you a pm. Maybe some help on the single handing stuff.

[admin]

Sorry, was in a hurry and missed the "of the keel" part.

Carry on.

[ryderp]

A trim tab on the keel basically serves the same purpose as Chad's rotating keel - it lets you increase the angle of attack of the foil while keeping the boat pointing in the same direction. However (no free lunch and all that), increased lift also means increased induced drag. I like simple over complex, but if I were going to have an adjustable keel, I'd probably go with Chad's approach.

Phil

[Chad]

A trim tab can be an easier method to achieving a gybing keel when there is ballast involved (like the i550). My gybing keel takes a little added complication to make it work. It uses the pressure from a deflector on the jib sheet to make it gybe, and a string has to be pulled to center it. I've been out a half dozen times now with good crew, and we're still not using it since other things about the boat require our attention still.

A while back I noodled around with a trim tab design that I ran through Javafoil and it seemed like it "beat" the standard NACA 0012 by a little in all conditions- less drag with the tab centered and no angle of attack (straight run), but more lift and better Cd when the flap is deflected (upwind setting). Here's where I left it...

FlappedFoil.pdf

(6.37 KiB) Downloaded 245 times

[slowpoke]

That looks interesting Chad, but how much of the length of the keel do you think it would need to run to be effective?

[Chad]

I wouldn't do less than the whole trailing edge length.

Again let me warn that this needs to implemented in a way that is automatic. If it takes crew attention, it will be forgotten or will cause other important things to be missed. There are ways to do it so it's automatic (using jib sheet tension to tack the flap, perhaps spin halyard tension to center it as the winning I14 did in Long Beach a few years back), but these all take time to rig in the parking lot, your crew won't understand the spaghetti, and you'll spend the first half-dozen sails just explaining the how-it-works aspect, rather than the how-to-go-fast stuff. I'm a geek for this stuff, but you need to sail with like-minded geeks to make it all go!

[micah202]

I wouldn't do less than the whole trailing edge length.

Again let me warn that this needs to implemented in a way that is automatic. If it takes crew attention, it will be forgotten or will cause other important things to be missed. There are ways to do it so it's automatic (using jib sheet tension to tack the flap, perhaps spin halyard tension to center it as the winning I14 did in Long Beach a few years back), but these all take time to rig in the parking lot, your crew won't understand the spaghetti, and you'll spend the first half-dozen sails just explaining the how-it-works aspect, rather than the how-to-go-fast stuff. I'm a geek for this stuff, but you need to sail with like-minded geeks to make it all go!

....sounds like a change of tune....lesson learnt?