Catamaran Dagger Boards

[Inquisitor]

I am thinking about dagger boards for a small beach catamaran (~20’). I want it to rotate up if it hits something. However, I didn’t want the friction penalty or the extra weight of the walls inside the hull of the slot. The two views illustrate my premise. The first is the normal version. The second is based on them being attached on the inside walls of the hulls.

Benefits

1. Get rid of the pocket walls (the red lines). Some reinforcement is required on the inner walls, but hardly as much as is eliminated.
2. There is no slot drag.
3. When up there is no drag at all.
4. When flying a hull the leeward dagger is close to vertical and thus more efficient.

Detriments

1. I’m guessing some kind of interference drag might be present since there is a gap between the hull and dagger. Anyone have any idea qualitatively how bad this might be (e.g. better or worse than the slot drag) ?
2. Assuming structural aspects are taken care of, are there any other issues I should be concerned with?

Latching Mechanism – Showing the side view and the lever. What is a recommended way of latching this down so that it doesn’t swing up by water friction, but does swing up hitting a rock?

Foil Shape - I thought about using a asymmetric foil to improve upwind performance. I have “Theory of Wing Sections” by Abbott and Doenhoff. Is there a preferred foil shape for something in this class?

[Deering]

I've had some similar ideas for a project still in my head...

Possible destriments:
1. The low-pressure side of the foil may draw air down from the surface, causing it to stall. The hull acts as a barrier to air migration in slot-mounted boards.
2. You may have problems with debris getting caught in the slot since it crosses the water line.

Latching - a few possibilities:
1. Bungie cord
2. Rope with suitable jamb cleat
3. A push rod secured on a friction latch that pops loose or slips under adequate load

If you're going to do that, why not just do a single, larger board pivoting down from the center of the bridgedeck?

[tspeer]

I think you should consider putting the boards on the outside of the hull, like conventional leeboards, instead of on the inside. That way the hull itself would brace the board against the moment from its sideforce and the pivot would be loaded in tension. The way you've drawn it, the pivot must react the entire moment in bending - not an easy task.

I suspect you put the board on the inside so it would be angled outboard, instead of inboard as would be the case if you mounted it on the outside with the hull shape you've shown. This could be alleviated in two ways: by giving the hulls a fuller sectional shape, and by "bending" the board.

The fuller shape would reduce wetted area, too, but I suspect you've drawn the shape you have because you'll be building in tortured ply. But the Tornado was designed to use tortured ply, too, so you should be able to use similar sections.

There's no law that says the board has to lie all in one plane. Especially if you're making it from composites, you could have the head of the board lying against the hull and the blade bent outward. At the bend, you'll want a high density core to take the crushing stress there. When fully retracted, the blade will be sticking out from the side of the hull instead of lying flush, so that may be a problem when docking. But the board could also be raised to the vertical position, putting it over the dock instead of bearing on it.

[Inquisitor]

Originally Posted by Deering

Possible destriments:
1. The low-pressure side of the foil may draw air down from the surface, causing it to stall. The hull acts as a barrier to air migration in slot-mounted boards.
2. You may have problems with debris getting caught in the slot since it crosses the water line.

I’m glad I posted… I hadn’t thought about either of these two. Hopefully, I’ll be sailing in relatively clean waters so the latter won’t be as large an issue, but the first might be pretty bad. It probably would create a nice vortex and drain sucking noise right under the tarpaulin… not a nice sound on a nice summer day!

Why not single centered dagger? – 2 reasons - it’s a simple beach cat…so there is no structure to hold it there and second when flying a hull, the center would be near 2.5 feet (more) out of the water. You’d be losing effective wing at the time you need it most.

[Inquisitor]

Originally Posted by tspeer

I think you should consider putting the boards on the outside of the hull...
The fuller shape would reduce wetted area, too, but I suspect you've drawn the shape you have because you'll be building in tortured ply. But the Tornado was designed to use tortured ply, too, so you should be able to use similar sections.

The main reason is… I didn’t want the ugly things on the outside of the hulls for the world to see.

I do want a more rounded shape and was using a Hobie Fox as a guide. I am using tortured ply. I have another thread going, quizzing about how much torturing I can do to a ply. Do you have any URLs for the Tornado… a Google search brought me back some pictures and events, but little technical info. I’d like to see what I can gleam off it.

If I can get as much rounding as I want, I plan on the shaft/bolt being much lower on the hull. Thus the lateral load caused by the leeward dagger would put the shaft in tension while compressing the upper portion against the hull.

Activating it is becoming the biggest issue. A direct connected lever would be under the tarp and adding mechanism to relocate the lever is adding the weight which is the main reason for putting it on the side.

Maybe I need to re-evaluate the whole thing. Doing a simple sliding dagger sounds like it may be lighter after all. I was a little worried about hitting things.

[farjoe]

Originally Posted by Inquisitor

I do want a more rounded shape and was using a Hobie Fox as a guide. I am using tortured ply. I have another thread going, quizzing about how much torturing I can do to a ply. Do you have any URLs for the Tornado… a Google search brought me back some pictures and events, but little technical info. .

checkout thebeachcat.com site for a download of the full tortured wood tornado plans.

if you join the Blade catamaran group at yahoo there is a whole series of excellent pictures about this construction method.

guzi

[Inquisitor]

Amazing... and I paid for plans to a 16'.

[tspeer]

You might look at Reino Urala's ideas for a daggerboard that can kick up.

An even simpler idea is to put the pivot well ahead of the board, which is shaped like an inverted "L". This requires a slot that is longer than a daggerboard, but shorter than a centerboard. As the board pivots about the forward axis, it moves up as well as back, cushioning the shock if you run aground. However, if you take a strike just below the hull (floating debris, for example), there isn't much tendency to rotate and the load is taken by the board and the pivot.

If you draw concentric circles centered at the pivot and passing through the ends of the slot, that defines a ring from which you "carve" out the planform of the board. The more forward the pivot, the deeper the board you can shape to fit through a slot of a given size. Unfortunately, placing the pivot low is also important to maximizing the depth of the board. Putting the pivot at the bottom skin leads to something akin to a long centerboard box in the hull, but a much shorter hole through the skin for lower drag - and the hole is completely filled with board when the board is all the way down, just like a daggerboard.

The traditional solution for daggerboard collision is to place material that can be crushed behind the daggerboard. The energy from a collision goes to crushing the material, which is then replaced. This can be built into the box, or attached to a the bottom of a flexible batten that is inserted down the aft side of the box after the board is lowered.

[Inquisitor]

Originally Posted by tspeer

You might look at Reino Urala's ideas for a daggerboard that can kick up.

The traditional solution for daggerboard collision is to place material that can be crushed behind the daggerboard. The energy from a collision goes to crushing the material, which is then replaced. This can be built into the box, or attached to a the bottom of a flexible batten that is inserted down the aft side of the box after the board is lowered.

All great ideas! I’m definitely back to putting it inside… now just have to digest all these options. I do like the high aspect design ideas best.

[yipster]

fresh from under the history button: multihull keels and daggerboards check the tab "news and articles"

[ron17571]

Unless your really gung hoe for speed id suggest looking at the hull shape of an old prindle 16,i never had any problem pointing into the wind or coming about.I guess i just dont like things that hang in the water and bring you to a sudden stop or snap off,when i started to read your post leeboards were my first thought.KISS thinking.