Sailing Downwind . . . Centerboard Up or Down for Max VMG ?

[SailFastTri]

We generally don't go DDW in light air because deep reaching is faster, so unless the wind is strong we tack downwind when we have room.

When deep reaching I will usually raise the centerboard to deliberately induce more leeway and take a course lower than the heading would indicate. It also helps keep the sails full. (Dragonfly trimaran, no keel at all).

I've never determined whether it's faster (in terms of VMG) to raise the centerboard and induce more side-slip downwind or to keep it lowered for better tracking and more SOG but higher/longer course. Anyone have thoughts about this?

(Not asking about DDW, as DDW would be a clear decision to raise it.)

[shipofools]

Downwind with chute, I am assuming you are sailing 90 anyway? Never really go DDW myself.

I leave it down, since it helps with tracking, gives me a pivot point, and most importantly I always forget to bring it back down when I round up.

Racing, I might bring the board up, but once again, it sucks to forget it up at the mark.

Cheers,

[estarzinger]

Originally Posted by SailFastTri

I've never determined whether it's faster (in terms of VMG) to raise the centerboard and induce more side-slip downwind or to keep it lowered for better tracking and more SOG but higher/longer course. Anyone have thoughts about this?

I would have thought Up = less wetted surface = less drag = faster.

[shipofools]

Originally Posted by estarzinger

I would have thought Up = less wetted surface = less drag = faster.

agree, but if you are running 90, which is the ideal VMG for downwind on a fast tri with chute, you are being set, therefore the board is helpful.

in real light winds, ddw I might bring it up, but most times I find I can make better vmg by not going as deep.

i guess the question I have for the op is what is your sail inventory like?

screecher, chute, reacher, etc

[SaltyMonkey]

Up, or some people leave a little down. Everyone on the transom. Surf

[estarzinger]

Originally Posted by shipofools

agree, but if you are running 90, which is the ideal VMG for downwind on a fast tri with chute, you are being set, therefore the board is helpful.

I agree up to the blue.

The set is positive (eg toward the mark), so you would need a bunch more straight line speed to make up for it (if you put the board down and stop the set), and my guess is that your straight line speed would be down (because the drag is bigger than the lift) not up. But that surely depends on the design and boat speed.

My suggestion is the OP call dragonfly. they race these things and must have a clear empirical answer to this question. It would be easy to answer with two boats side by side.

[Tri]

Why don't you just try it out? If conditions are stable go 15 minutes with the same sail setup, course, etc with the board down and measure your VMG Waypoint and do the same with the board up. If you do this a couple of times you should be able to see if there is a difference. If there is no difference then I would leave the board down because of the issue of getting the board up and down (while sailing).

Cheers,
Achim

[admiralslater]

MY happy angle is 120 apparent and i like a lttle board down maybe a foot or so as I find the boat is just that little bit sharper in response . We fly symmetrical, as you can see in my avatar

[senormechanico]

DDW almost never but if so, I pull the board all the way up.
I usually gybe downwind with apparent at 90 degrees. Half a board and zero helm.

[44'cruisingcat]

I only use board when I'm going upwind.

[Factor]

I have adopted a new approach to me sailing to save time and learn more quickly its called WDPS (*) - I think your answer lies there.



* What does Polux say

[autumnbreeze27]

our board bangs when it's down unless there's enough lateral force on it to hold it in place, so we only have it down when going upwind and the wind is strong enough.

If anybody figures out how to keep their board from banging let me know!

[Tri]

Originally Posted by senormechanico

DDW almost never but if so, I pull the board all the way up.
I usually gybe downwind with apparent at 90 degrees. Half a board and zero helm.

Is 90 apparent you max VMG downwind? Do you know what your target angle is for max VMG downwind? Do you sail it with board up or down?

I am currently trying to find out what the target windangle is for max VMG downwind without polar diagrams.

Cheers,
Achim

[senormechanico]

Originally Posted by Tri

Is 90 apparent you max VMG downwind? Do you know what your target angle is for max VMG downwind? Do you sail it with board up or down?

I am currently trying to find out what the target windangle is for max VMG downwind without polar diagrams.

Cheers,
Achim

For us at least, 90 apparent is the fastest down wind as well as the most comfortable ride (most of the time).

We have a VMG meter among our Raytheon instruments. Wind angle and knotmeter are both pretty well calibrated.

If we do go DDW it's in light air and we do it with no main and just an Asymmetrical chute, board up with the main halyard attached to the end of the boom acting as a backstay.

[Tri]

Thanks, what does 90 apparent translate to roughly in true wind angle on you boat?
Cheers
Achim


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