Partially raising a dinghy centerboard

[amolitor]

I overheard a remark to the effect that a particular dinghy seems faster with the centerboard partly raised. This leads me naturally in to wondering what the effects of raising it actually are.

Assuming that it's raised only a bit, so it's just cocked back but presents much the same wetted surface, I get:

1) center of lateral resistance moves back. Reduce weather helm/increase lee helm.
2) in terms of hydrodynamic lift, the fin starts to look like a shallower, longer board with the same width. Longer in the fore-to-aft dimension, that is. I don't know what this does to lift, but the "shallower" part probably means more drag for the lift. Given that the highest-tech fins around are very deep and very short (fore-to-aft) I assume that all of this is Bad.

As you raise the board further and more of it disappears into the hull, drag should drop off, but your tendency to make leeway will go up pretty dramatically as well, and you can't point.

The only advantage I can see (at least from a hypothetical point of view) is that the boat's balance should change subtly, which might increase efficiency somewhere else in certain conditions.

Also, I suppose there might be some speeds at which the longer/shallower fin might lift better?

[Phil Locker]

Assuming this is happening in light to moderate winds, it could be he has an imbalance between the rig & the centerboard (leading to too much helm to steer a straight course, and the resulting drag from the rudder).

On the more adjustable dinghies, setting mast rake to the wind conditions is common. The whole system works together... more wind===> more mast rake, sheeting further out from centerline, and bringing up the centerboard to keep everything in balance (also because you're maxing out your righting moment and can't carry any more power).

If a tuning sheet is available for the boat, start with the mast step location (is it correct fore/aft), then the mast rake, etc

Just my two cents,
Phil

[frankofile]

In addition to what's been said already, raising the centerboard raises the center of lift, reducing the capsize moment and making the boat more stable. Very effective for depowering in big wind.

[mattotoole]

The primary advantage is of course less drag, but at the expense of leeway (upwind and reaching) and tracking (downwind). So it's a tradeoff, and every racing dinghy is different in terms of how much it's worth to fiddle with the board.

Most Laser sailors raise the board as much as possible going downwind, and at least a little while reaching, even on a close reach. Some other, really fast boats can use less board going upwind as the wind picks up, because the board generates more lift per area at higher speeds. This is why some catamarans seem to have such small boards -- like fighter jets have small wings for their weight. While racing Australian 12' skiffs and Cherubs many years ago, we used less board as the wind picked up. But raising the board created boat handling issues (board in the way, etc.), so with the 12 footer we just used a smaller board. With some boats, raising the board is more trouble than it's worth. Some simply won't steer straight. The additional rudder input creates more drag than the board does.

As for the finer hydrodynamic/design issues, Phil has explained them well.

[amolitor]

Err, I should have specified. This is a swing-type board, not an up-and-down daggerboard. "raising" it starts by simply swinging it back, so wetted surface remains mostly constant. I think drag will actually go up a little, but I'm not sure.

[Phil Locker]

... which brings us back to rig balance.

[and yes, drag probably will increase due to effectively decreasing aspect ratio, as well as the fact that a sweptback planform has higher drag than a vertical one, and while we're at it the tip shape has probably become somewhat ugly compared to when the centerboard is presented vertically].

re: the comments about reducing board area upwind in a breeze, for daggerboards its all about decreasing wetted area & reducing moment arm as lift increases dramatically with velocity and you only have so much righting moment available. Center of effort of the foils still moves aft, though not as dramatically as with a swept-back centerboard (wetted area of daggerboard is reduced, while area of rudder remains constant).

[John ilett]

The Moth class does it backwards in comparison to most classes. On a regular moth (without hydrofoils) in conditions over 12 knots you would start to raise the dagger centreboard upwind about a foot at the most to depower and push it full down for stability off wind.

As the hulls are so narrow they still get a reasonable bite in the water upwind and you don't have to let out so much sheet that the sail is backed.

[frankofile]

My frame of reference for depowering by raising the centerboard is a modern 505 board which has the pivot point approximately 4 inches forward of the leading edge. When these boards are rotated aft, much more of the board is also raised out of the water than with the old style board that has the pivot point within the board itself. Still, with the modern style board, the center of lateral resistance moves aft and the rig is moved aft also partly to compensate (partly also to open up the jib angle).

Phil has made some of these boards and may have a drawing to post in case anyone is interested in what they look like since my description probably isn't very clear.

[SeaDrive]

A boat moving fast needs less foil area (or less lift per unit area) than a boat moving slowly. Boats are designed with a board big enough for light winds, and can sacrifice some lift in strong winds. Raking the board will reduce it's frontal area, and also lengthen the chord, both of which will redue resistance. This complements well with the increased weather helm in strong winds.

[Andy P]

Originally Posted by SeaDrive

AThis complements well with the increased weather helm in strong winds.

In Cherubs it's a bit backwards too re dagger board lifting and weather helm.

The board is lifted a bit upwind in a breeze to reduce drag and heeling, but downwind it's always down so you can get the max power from the asymmetric kite. ( and the crew can't fall over it in the gybes if it's down )

Also opposite is that we get a little lee helm upwind in strong winds - because we can sail the boat flat or heeled to windward, the chines give an effect. Combined with the main flat twisted and eased, means that the jib is working harder than the main, so lee helm.

pic of 5o5 foils with the fwd pivot.
The main reason for this is to get a longer board.
The board which is not measured must fit in the case ( which is measured). The extended pivot means that the board fits higher and extends further fwd in the boat when completely lifted. Therefore the board can be longer outside the boat when fully down.

I think the bigger kite on 5o5s has made it faster to reach the runs with the pole on the forestay ( like an asymmetric) with the board all the way down.

[Phil Locker]

Originally Posted by Andy P

In Cherubs it's a bit backwards too re dagger board lifting and weather helm.

The board is lifted a bit upwind in a breeze to reduce drag and heeling, but downwind it's always down so you can get the max power from the asymmetric kite. ( and the crew can't fall over it in the gybes if it's down ).

The Canadian Fireball guys that went to the Worlds in Italy 2 or 3 years ago came back muttering about being off the pace downwind in a breeze, as the hot new technique was to raise the centerboard nearly completely, trapeze wayyy back, and sail hot angles... effectively planing their way downwind with lots of leeway. Different strokes for different boats... Hard to keep the boat upright with no flopper-stopper though.

Originally Posted by Andy P

The board which is not measured must fit in the case ( which is measured). The extended pivot means that the board fits higher and extends further fwd in the boat when completely lifted. Therefore the board can be longer outside the boat when fully down.

And happily the pin extension means that the center of effort happens to remain about the same with the shorter chord board, keeping everything in balance without other significant changes in boat tuning.

Back to upwind... this post reminded me that some 5o5 guys like to rake their boards FORWARD by about 5 degrees in lighter air. Same for the pokie little InterClub dinghy too.

Phil

[CT 249]

On sailboards, you really, really notice how much of a capsizing effect the foils have. On a longboard with a centreboard is more than 15 knots or so, the lift created by the centreboard is angled slightly up (since you sail these boards with the lee rail down) and the centreboard can easily rise to the surface and throw you and the board over. So we have to cure that by raking the board aft, reducing draft and exposed area etc as said above. Don't even TRY to go downwind with the board downwind in planing conditions, you'll go straight over.

Shortboards find the same thing; FW boards, with their 70cm high-aspect fins, are tough in a blow partly because the fin is always developing sideways lift, which with the slightest bit of heel becomes vertical lift.

Given the small size of sailboard foils and the capsizing power they generate, it seems obvious that there must be a significant amount of capsizing force from dinghy foils - you just don't notice because it feels the same as the rig forces, pretty much.

[Andy P]

Another 5o5 board ( FINLAND ? )

this has an extremely long and narrow board, with very long pivot.

The board also operates within a sleeve, so that the board can lift as a daggerboard.
The board when lifted is completely above the case, but requires the mast to be deck stepped.

I think this arrangement has now been banned.


Normally, 5o5 gybing boards operate with a few degrees forwards rake, and lock when lifted to 90 degrees or more.