Daggerboard questions

We have a Trident Trimaran: 30 ft, about 900 kg, 32 sqm sail main+jib. Our last summertrip with nice winds had an overall average of 8 knots.

Our centerboard of roving covered wood (probably low quality pine) is very wet, heavy and likely a bit rotten. Its shape in the water is similar to a 45-50 degree pizza-slice with a radius of 100 cm. We have rather poor tacking. The width of the board casing is 40 mm. The casing extends into the cockpit, so hoist a daggerboard and store it in a dry place when the boat is not saling. We can relatively easy rebuild the casing.

Our plan is to build a daggerboard with the following specs:
NACA 0010, chord about 39 cm
Length underwater: 110 cm – and about 45 cm above the bottom
That gives a size about the present centerboard, but with a much better profile.

We are unsure about making the board straight or make its shape epileptical. The latter is complicated but lighter and lifts as much.

We are also in doubt about building materials:
One alternative is red cedar 43x43 mm pieces glued into a large plank and routed into shape and then covered with 3 layers of 220 g UD glass plus some perhaps 250 g roving matt. It seems strong, but heavy + a constant risk of water intrusion.

A second alternative is to use 75 kg/cubic meter foam (divinycell H) with make a high desity central beam (250 kg/cubic meter) covered in carbonfibre (Carbon fibre braided sleeve Ø 60 mm – approx 41g/meter). After shaping 3 layers of 140 g UD carboncloth plus some 200 g roving matt are applied. That should give a very stiff and light board, that will not soak up water. We are unsure whether this foam+carbonfibre will have the needed strength.
Cost in this case is less of an issue. It is more serious to lose a lot of sailing if the board breaks.

 

That is a heavy laminate on the stripped cedar; I'd go for two layers of light box weave glass with four layers of uni directional carbon over the thickest section of the board, top to near bottom, then quick fill and fair, then one more layer of box weave, fill, polish wet and dry finish. If you're worried about strength, use Port Orford cedar instead of Western red for the core. The latter is brittle compared to the former, which is one of the very strongest for weight boat building woods known. Elliptical tips ate not difficult once you start.
The foam and glass laminate also seems excessive to me ... but then I'm a weight conscious nut.
I've done similar wood/glass/carbon layups as listed here for many boards and rudders and never had any problems with water ingress.
If you're going to plow into a hidden reef, it doesn't matter what materials you use ... there will be a mess.

 

Whatever you do, do NOT make it epileptical (epileptisk)! Elliptic (elliptisk) is far better!

(Undskyld Anders, men jeg kunne ikke lade være!)

 

If the daggerboard is a section of a circle it is a really weird shape. Centerboards with a pivot point sometimes have a similar shape. If it is a pivoting centerboard, NACA foils would only work in one position.

 

The old school approach is to use mahogany strips with a couple of epoxy/glass layers for abrasion. Very heavy by Gary's standards but really hard to break as the strength isn't dependent on the laminate. Another way is use ply sides joined with a nose section sprung around a central spar for load and joined together on the back edge . Fill with foam and epoxy glass the exterior with your carbon on the leading edge.

 

thanks

For all your comment, and sorry about my poor english geometry: elliptical is of course the right shape.

To Gary: Is it a problem to mix glass and carbon in the laminate. I mean all to load is then taken by the carbon.

I have found Ian Farriers material list for the f-22 trimaran

"Daggerboard
Cedar Core (economy) - 50 x 65 (2” x 2 1/2”) 7 @ 1.9m (6’ 3”)
OR Foam Core (lightest) - 430 x 1930 x 49mm thick (17 x 80 x 2”)
with HD inserts as per Sheet 36 0.004 cu.m.(0.14cu.ft.)
330-400gm (10-12oz) DB knitted fabric (45-45° fibers) 4.8sq.m. (52sq.ft)
100/4” wide 400/430gm (12/13oz) Unidir. tape (fibers at 0°) 16.7 lin.m. (55’)
Epoxy, or vinylester 5kg (11lbs)"

It allows for two layers of DB knitted fabric plus about four layers of UD tape at the thickest point/along the spar.

We are forced to built somewhat thinner 40mm not 49 and slightly shorter in order not to interfere with the boom. So this laminate looks reasonable.

Anders

 

There's no need to build an elliptical board. A tapered board with the tip 40% - 50% of the root chord would perform very close to an elliptical board, and your chance of producing an accurate and fair section shape would be much higher.

A straight board with no taper will also perform quite well. The important thing is the board be deeper than your existing board. The increase in depth will reduce the drag more than the change in planform shape.

 

Anders, you mention not interfering with the boom. On my little cat, the daggerboard has an articulated handle which folds over when the board is up. As long as a lot of the board is within the case when the board is fully down, this seems to be adequately strong.

 

I have found Ian Farriers material list for the f-22 trimaran

"Daggerboard
Cedar Core (economy) - 50 x 65 (2” x 2 1/2”) 7 @ 1.9m (6’ 3”)
OR Foam Core (lightest) - 430 x 1930 x 49mm thick (17 x 80 x 2”)
with HD inserts as per Sheet 36 0.004 cu.m.(0.14cu.ft.)
330-400gm (10-12oz) DB knitted fabric (45-45° fibers) 4.8sq.m. (52sq.ft)
100/4” wide 400/430gm (12/13oz) Unidir. tape (fibers at 0°) 16.7 lin.m. (55’)


What would be the weight difference between these 2 boards, one with cedar and one with foam?

Bob

 

Why is there all this consternation about weight.
The board displaces its own volume of water, and water is much heavier than wood. Even a board made of heavy wood and heavily glassed will support most of it's own weight when it is fully down. When the board is fully up a large part of the board is still immersed below the waterline. And who sails downwind with the board fully up? A well designed multi will tack downwind with its board half down faster than the same boat sailing dead downwind with its board fully up.

 

Well, not thinking about the total weight of the boat, another issue with a very heavy dagger board is moving the thing.

They are not easy to move around.

Also, the board is never fully submerged when fully down. It's about 1/3 of the board, at most, below the lowest point of the keel, plus the distance from the keel to the waterline that is in the water. The rest is weight you have to carry around in the boat.

When the board is fully up, there is nearly none of it in the water, since catamarans have a very short distance between the waterline and the deepest point of the keel.

 

My wife and kids, keen sailors all, cannot easily pick up the board blank I made out of wood. After glassing and painting they will have a hard time moving it about when we rig the boat. What! You say I shouls be moving t when rigging? Man, I might spill my beer, and since I live in Bavaria "THAT" would be a crime.
Besides, in a few years the girls won't want me hangin around, "Just deliver the boat and help us put it in the water, Dad"

Bob

 

Crumbs Bob. It must be a really big board. What is the size of your boat???

 

Most of the strength comes from the layup of the outter skin, espcially if it has carbon fiber in the layup, so the foam serves to save weight.

the wood has the advantage of being more durable in the case of impacts or point loads since the wood will not crush as easily as the foam. But the wood is not as easy to form, and weighs more too. You might consider forming the LE and TE out of wood, and perhaps the tip, and than used foam for the bulk of the core. More work but will make a reasonably light and yet durable dagger board.

I would also go with a straight taper rather than elliptical plan form. The performance is almost the same, and much easier to make accurate (so likely will have better performance). Also easier to make accurate repairs of dents as well. Also, with a square tip rather than a sharp edge as you would have with an elliptical plan form, the tip will be a lot more durable for both storage and the occasional grounding.

going with a high aspect ratio dagger board should noticeably improve your performance. Make sure where the dagger board meets the hull you have a close fit, that will improve performance as well.

Good luck.