Daggerboard cores

I am building new daggerboards for my Macgregor 36 catamaran to replace my erstwhile and apparently poorly designed boards that broke at the hull exit (of course). Lots of mistakes on the earlier ones: Essentially, for a 450 mm chord, I made a couple of fiberglass I beams about 125 mm apart, glued on A600 corecell, shaped the foil and vacuum bagged the skins. Neither I beam was at the maximum width, and you can guess what happened. Examination of the stubbs shows evidence of the board crushing and then breaking off.

For the new boards I want to skip the endless fairing, and am building a female mold in the manner recently described in a pdf by Kurt Hughes. I can use the same mold to vacuum bag 4 skins, and then join them to make two boards.

So what do I do for the core? I want to vary the skin thickness to have the maximum carbon at the hull exit point, and laying in a rigid core seems impossible. I am open to suggestions. I have thought about just building up a solid section of fiberglass about 100-150 mm wide on each skin, and routing it flat when I trim the leading and trailing edges. That might be heavy, but a) I know it would be virtually incompressible, and b) would automatically fit the uneven inner surface of the skins.

In my research, I read of people talking about pourable epoxy foam for this, but all I can seem to find is urethane foam, which I understand is unsuitable. I came across two other ideas that seem attractive for weight and cost issues. One was to make up a bog of epoxy and microballoons, which really is a sort of epoxy foam. This could fill the cavities around my solid glass sheer web, or perhaps be the entire core. Anybody have any experience with this?

An even lighter, and cheaper, core was suggested in a boat design forum: using styrofoam beads. Normally I would not consider styrofoam, but the guy claims a good pedigree for the idea: "the epoxy /Styrofoam mix is an idea i picked up from Kiwi multihull designer Malcolm Tennant as it was something used to fill rudders and daggerboards on his cat designs. The materials are easy to get,the styrofoam beads are what is used in a bean bag chair, or should be available in a craft or fabric store for next to nothing, with the epoxy you want to use the slowest hardener. Ive used this method for rudders on a Searunner 31 tri, a macgregor 36 cat and monohulls as well as several bulb keel struts.
Steve."

Has anyone else ever heard of this? I can get a large supply of 1-1.5 mm beads pretty cheaply, such as here: http://www.milkbottlefill.com/. Seems there is a whole milk bottle collecting community on the web. Who knew!

Sorry for the length. All thoughts appreciated.

 

I will be using pour foam for my boards, 8 lb to 16 lb foam. To beef up your boards put a diamond shape lay up of glass , carbon or whatever your using inside the board. I will pull my dagger board plans out in the morning and find out the number of lay up,s and location of the diamond. Rick

 

Gooday 'rberrey' Don't know what your background is - however unless you want to make a full time profession of building 'boards' I'd look very carefully at what you choose to use when building the project that you've described above - & the way you have mentioned.

'Pour-faom' is a floatation foam - a gap filler - an insulation medium - model carving substitute - - BUT - it is not a structural product. That's just not what it was formulated to be.

Any of the many brand names of structural foams - start at around 130 kgs per cubic mtr. & top shelf one go up to 180 kgs/cu/mtr - anything less - WILL NOT WORK - long term or be dependable ! ! For a 'high-performance' multihull - you'll also need a very substantial strong-back of a 'box section' of carbon laminate - then wrapped with a strong carbon laminate skin & then infussion molded or vac baged plus heat cured, then maybe you'll get close.

All that is - if your serious - have a top-shelf racing multihull - - - lesser rudders/rudder production are for lesser performance multihulls.

Get into - crew.org.nz site & ask the same question & you'll no doubt get the right answer. There's also a good rudder & comments in 'sailing anarchy' if you want to learn - where it's really happening. I have no faith in what your suggesting. Oooops!! Ciao, james

 

I,ll have to write my designer and tell him pour foam wont work. Rick

 

Hi Rick,
I would be very interested in any information you are willing to share that you feel does not violate your designer's intellectual property, particularly anything about the boards lamination schedule.

I don't quite understand the "diamond shape" Do you mean 2 dimensional or 3 dimensional, and what orientation and size? I think this diamond shape is probably some sort of sheer web for the boards, but I can't quite visualize what it looks like.

Thanks,
Larry

 

Center line of the diamond top to bottem is 1/3 width of the daggerboard, the diamond is 1/2 baggerboard width side to side. Diamond to extend 1 1/2 DB widths above the keel and 2 widths below the keel.Lay up for glass is one inside lay up on board and one outside lay up on diamond. The foam will have to be dished out where the diamond is before the first lay up. Pour foam may not be structural , but 16lb closed cell pour foam is 1 lb more psi than my renecell. My outside lay up and inside are close to the same, so I wll use 17oz 45/45 biax and 18oz 0/90 biax lay up twice, with 3oz or 4oz S-2 glass for a finish cloth. Rick