Dagger boards versus shared lift

[Bruiser]

Ok the Hobie 16 thread was quite interesting. Can you all help me on how far down to run dagger boards for optimum shared lift in different wind conditions. Also upwind versus downwind, as well as with spinnaker down wind..

Thanks for any knowledge

[Bruiser]

So no body knows nothing?

So I am guessing everyone runs their dagger boards down all they way all the time?

Going up wind in light air, is there an advantage to raising the windward dagger board to eliminate drag?

How about the windward rudder? Or are we messing up the shared lift formula?

In heavy air, windward board up?

How about down wind, boards all the way up or just part way?

How about on a reach, as boat approaches 20 knots of boats speed, do the boards still need to be all the way down or does drag increase to a point to negate its lift?

I know I play with changing these positions on the board all the time, but I have no idea of whether there is any difference speed versus losing lift.

Anybody even have a single thought?

[J Drew]

Hey Rich,
I can't believe this isn't a hot topic. I know everyone has their own approach to it, but I think mine does a lot better with the boards slightly up (less drag)

I know that sometimes, in light wind, I can do pretty good with them all the way up, even up wind. A little slide with very little drag, seems to work ?
Good question about the rudders and shared lift?

I'm sure there's a scientific approach to it.
Wish I knew what it was ??

[havliii]

I'm in.

Downwind, I'm boards full up, at any angle, at any speed. I can't see any reason to want the drag or need the lift.
Upwind, if I have active crew, windward board up, leeward board down until the helm goes to near perfect balance, as the board drops into the water you can and should feel the effect on the helm. I rarely use full down, half to three quarter nearly always balances the helm and points as high as she'll go. (if I'm being lazy I leave the windward board down)

The physics of all this I will leave to Bill but due to leeway I am guessing that the board changes the flow direction ahead of the rudder, thus the rudder is 'seeing' a deflected stream of fluid when the board is down. Again, I am guessing here, but this is why you should feel a change in helm pressure as soon as you begin to lower the boards.

FWIW I sail a SC19 and a SC20 same strategy for both boats.

[Bruiser]

Thanks guys, I am thinking similarly.

I noticed at higher speeds and wind, that maybe the boards do not need to be as deep going up wind. Again, I have no numbers just a feel. I have not spent much effort getting the windward board up for up wind. Seems like a good idea but I do not know if it is actually figured in for "shared lift" design etc.

On the down wind sailing with spinnakers up, I was thinking some board down might help with getting windward hull to lift easier. Not sure if we want that, or by how much? I remember the "wild thing" being a big deal and making a large difference in speed on the smaller boats.

thanks,

[havliii]

Bill once told me in a previous thread flying the hull is a matter of weight management nothing else. Trap to leeward and you will fly a hull in any wind. Pop the hull and watch the speedo jump immediately, it works.

[Bill Roberts]

Hi Rich,
Today is the first time I have seen this thread. I see much confusion between shared lift and no shared lift and monohull sailing and catamaran sailing. Shared lift is built into the design/construction of the boat. You will notice that SC and ARC boats have the CB trunks further forward than other brands of beach cat. The SC15 and 17 carry 50% of the sail side force on the rudder. That is why the rudders are so large. In a test once I tested small rudders on a SC17 and the boat was much slower to windward and it was harder to tack. It also did not point as high as a standard rudder size SC17. There was more slide slip in the path sailed through the water with the smaller rudders.
Now, where to put the daggerboards? AS we all know the daggerboards generate a side force opposite the side force from the sails. When the sails are trimmed for sailing to windward, boards full down. Dagger board area required under water is related to sail side force and inversely proportional to dynamic water pressure, 1/boat velocity squared. So, the greater the sail side force the greater the board area is required underwater. The faster the boat goes, the less board area is required underwater. Sail side force is max at max righting moment and both boards are needed down at this speed. As you crack off to even a slight reach the boat speeds up dramatically. Let's say we were sailing to windward at 10 knots. (Remember 10**2 is 100.) Now let's crack off to a close reach at 14.1 knots. (Remember 14.1**2 is 200.) Now we have twice the dynamic water pressure so we need half as much board area. Let's go! Windward board up. Now let's reach off a little deeper and speed up to 20 knots. Now the dynamic water pressure is 400, doubled again, so raise the leeward board to half up and let's go a little faster still.

Now let's slow down going to windward. The wind has dropped off and we can only go 3 knots to windward. The sail side force is small but the dynamic water pressure on the board is down very dramatically. At 10 knots it was 100. At 3 knots it is 9 or one eleventh of what it was at 10 knots. The boards are highly loaded at 3 knots and asking for more help, "give me more area, I'm on the verge of stall, help".

Downwind: A catamaran sailing downwind with spinnaker has the spinnaker trimmed in snug against the leeward side of the boat and the main is on or near boat centerline. The mainsail thinks it is going to windward. The boat has much sail area with a sideways projected area. Side force is large. The boat is moving right along, good speed but not flying a hull. Therefore leeward board full down windward board up. Wind picks up some and boat fly's a hull continuously and now leeward board half up. If not enough board is down, the skipper can see and feel the boat side slipping. When this is the case, full board down.

If you are going to sail the boat downwind like a monohull, spinnaker way out and main out sailing square before the wind, no boards needed.

[Bruiser]

Good stuff, thanks Bill

[Bill Roberts]

Well I'm glad Bruiser read the discussion. Did anybody else read it?
Did readers understand it? Am I wasting my time?

[havliii]

Hi Rich,
Now let's slow down going to windward. The wind has dropped off and we can only go 3 knots to windward. The sail side force is small but the dynamic water pressure on the board is down very dramatically. At 10 knots it was 100. At 3 knots it is 9 or one eleventh of what it was at 10 knots. The boards are highly loaded at 3 knots and asking for more help, "give me more area, I'm on the verge of stall, help".

please explain "highly loaded at 3 knots" how are they highly loaded? per your quote "the sail side force is small" the load on the boards has to be small can't be anything else or the system is unbalanced. Obi wan is this a typo?

[J Drew]

Ok, I got it. Might have to study it once or twice
Thanks Bill

[Bill Roberts]

OK, The "highly loaded" relationship that I am talking about is a ratio. It is the ratio of the sideways force on the daggerboard divided by the ability of the daggerboard to generate an equal and opposite side force. The daggerboard's ability generate side force varies with the square of the speed at which the daggerboard is moving through the water. So at 10, the max daggerboard side force is proportional to 100. At 3 the max daggerboard side force is proportional to 9 or 1/11th of what it is at a speed of 10.
The sail side force does not decrease as rapidly as the daggerboards ability to generate side force at low speeds. So, at low speeds it is possible for the daggerboards to be asked to generate a side force greater than they can handle. This is what is meant by "highly loaded".

Think of an airplane coming in for a landing. Just before the wheels touch at very low speeds, the wings are at max lift, verge of stall,"highly loaded". Now, fly that same airplane at 40K ft and 500 mph and the wings are perfect, operating at best lift to drag ratio, no where close to stall. What made the difference? The speed at which the air is flowing over the wings.

[J Drew]

So what about the shape of the dagger boards?
If we are talking about lift and side force, why are they symmetrical, instead of foil shaped (top view not for foiling purposes)
I would think that a foil shape would be more productive

[Bill Roberts]

Drew you are correct. A non symmetrical board shape, like an airplane wing , will produce more lift per square foot of area than a symmetrical shape and it will make less drag. So, why don't all beach cats have non symmetrical board shapes? With non symmetrical boards you would sail on one board at a time, the leeward board. This means every time you tack or jibe you would have to reverse boards. By this I mean the board that was up is put down and the board that was down is raised up. This makes the boat more labor intensive to sail and complicated. So, let's keep it simple and go with symmetrical boards.

[J Drew]

Ok, I can understand that, plenty of info to practice with