Originally Posted by sloboats
I have a broken board and need info building new daggerboards, I was going to use the good board for a mold
, but not sure on the core
. My first thought was to have the bottom 2' solid glass and the rest 8lb closed cell foam. I do not want the board riding up in the trunk and it has to be sacrificial as not to hurt the trunk. Appreciate any ideas or help.
The general modality of thoughts on boards is this: You want the trunks to be stronger than the boards, as the boards are far easier to fix after a collision
than are the trunks. Plus which, broken board wont allow water
into the boat, whereas broken trunks will.
You may want the first several inches, to a foot or so (max) to be solid, so that the tips wont be damaged if/when the boat is resting on them on the hard
to dry out.
From there, you'll want stiff, strong, foam cores. And for the bottom few feet to maybe be a bit weaker than the rest of the boards, so that the tips are sacrificial for groundings, & severe impacts with a floating object. Albeit, that (strength) differential, has as much, or more to do with the board's layup
schedule, as with the cores.
For in my studies on the matter, generally, the core is the same strength/density throughout the entire length of the board.
Also, the vast majority of the semi-custom, to custom designs out there, have purpose designed systems which cause the board to retract vertically when it strikes a heavy object at speed.
It'd be worth talking to your boat's designer(s) about this. As well as some custom designers, to look into the possibility of retrofitting such a system.
Because the primary reason for the automatic kickup system, is to cut down, or fully eliminate any damage to the trunk when you strike something. In addition to cutting down on the damage which is caused to both the board & the trunk.
And also, most semi-custom, to fully custom multihull
designs of any size, have replaceable crash boxes/blocks (inserts) which fit into the trunk, along with the board itself. Right behind it, & also wrap around the board's aft end, to aft third.
With them being made out of high density foam, designed to absorb such impacts.
Then in bigger boats, they also have integral composite structures, built into the foam crash blocks. Which are designed to crumple when the board strikes something with sufficient energy. Just like a crush zone in an automobile.
First the foam takes some of the energy of the strike, & if it doesn't absorb it all (along with the board kicking up), then this composite & foam crush structure takes the rest of the energy from the impact.
So then, again, they're one more component which mitigates damage to the board itself. And all but eliminate any damage, to the trunk & boat. When working in conjunction with the board's kickup system.
It sounds as if, perhaps, your boat may not have these. But talk to the manufacturer about such. And also, you'll need to find out from them, the following about the board;
- The core's specifications.
- The layup
schedule for the board. Including the orientation of each layer of fabric
- The specifications or each layer & piece of fabric
used in the original construction of the board. As fabric strengths, even for the same type & weight of fabric, can vary HUGELY from one fabric maker to the next.
As in it's physical properties can be 2x or more, greater, from one brand to the next.
Also, the type of sizing on the fabrics, which is what helps the resin bond to them, again, can vary quite a bit.
In terms of their performance (as they form an integral part of the physical strength & stiffness properties of the fabric itself).
And also they govern what types of resins bond well to them, & which ones do not.
- The specifications of the resins used in the board's construction. As their physical properties vary from one to the next also.
- Whether or not you'll need to vacuum bag the skins onto the cores, when laying up the board.
- If the resins used, need to be post-cured, once the initial cure is completed. And if so, what the "recipe" for this is. Meaning, at what temperature, & for how long do things need to be baked.
- Also, you'll need to know to what NACA foil sections the finally finished board needs to be faired & templated to.
- As well as what type(s) of finish(es) can be, or need to be applied to the board.
For example, can you/should you use graphite, mixed with epoxy
, to flow coat the board with when it's finished. As such coatings make the board slipperier with regards to moving it up & down inside of it's trunk.
Although the choice to use such final coatings, also depends upon how much, if any, of the board which you leave in the water
when you're not sailing.
Because if you leave much, if any of them down, then you'll be wanting to use some type of antifouling in lieu of putting a graphite coated finish onto them.
And not to make you loco, or to purposefully adding even more work & complexity to the job. However, now might be a good time to talk to the boat's manufacturers about the possibility of adding crash blocks/cassettes to your board trunks.
Or changing their aspect ratio, & or foil section, etc.
I hope that that helps. Although I know that so much in the way of details regarding daggerboard construction, probably complicates things quite a bit. In addition, of course, to being a LOT of information to read & digest.
Feel free to run any questions by me which you might have. About the above, or about the topic in general. And too, Kurt Hughes http://www.fastcomposites.ca/site/
has a Lot of information about boards & trunks, on his website, as well as in much of the other things which he has written. Ditto on John Shuttleworth http://www.shuttleworthdesign.com/index.html
That, & with some digging, I can come up with some companies who build such items. Ditto on firms which carry the materials that you'll need.
is one, for example.
: A good website to study most of these topics on is www.compositesworld.com
And might I recommend doing a smaller, less pricey project
or two first. Using both the materials & techniques which you'll be using to build your replacement board.
Also, FWIW, Kurt Hughes
, who with all of his boat building experience (which is substantial), & his considerable design expertise, had the rudder
for his personal, 40' trimaran
, built by a company that specializes in such items. Back when I first met him, in the mid '90s.