Ohhhkay, lets see if I can do an explanation here.
And let me digress. See, some years ago, the Bright Idea for fixing busted boards was to drill into the busted board ends and stuff in a wooden dowel, ‘it will act like the stringer’ the eedjits would say. But the problem was that it would almost certainly fuc# up the alignment of the two pieces. “No problem” they said, and did a very oversized hole and poured in resin/filler powder mix, stuffed in dowels and set the pieces of the thing more or less aligned in some sort of half baked jig.
As is my wont, I ranted and raved a bit about just what a bad idea that was. And being me, I did it with examples.
See, they put something hard in something soft: a cylinder of hardened resin plus filler plus dowel into foam. By way of explaining, I’d use a handy model: Irish Coffee. Coffee, sugar, Irish whisky ( ideally Bushmills) and whipped cream on top. Generally comes in a tall glass mug with a straw. So, you take the straw in both hands and, excerting all your strength, drag it through the whipped cream. .
Funny thing. It didn’t take all your strength, now did it. You can move that straw real easy. Hard things in soft things, not so good. At least not as structure. Don’t do it.
Order me another one, I’ll repeat the demonstration. I don’t think you really understood how that worked, yeah, another Irish Coffee, mister bartender, we’ll make sure…
Who says teaching engineering isn’t fun.
Right, you have something fairly hard ( the fin hardware) and something quite soft ( the original foam) plus something else hard ( the glass skin of the board). You want to put them together so they stay together and stay in place. Not an easy problem.
Now, to foams.
First, the home depot home and garden spray foams. Look, they are meant to be insulation. Low density, so they insulate well, being mostly encapsulated gas.Very soft, structurally. Not awfully strong.And not real controllable. Want to have some fun? Put that stuff in a sealable but not that strong container. Use quite a bit. Blows it apart. Asd an aside, they have industrial spray foam setups for insulating houses. A high expansion type fore new construction, they spray it against the outside sheathing and let it expand inwards and trhen there is a low expansion type for retrofits on existing houses with finished walls. They tried it with the regular high expansion foam, it blew the walls apart,
Next, the two part expanding foams. The densities vary, I’m not sure how they do it but if I was building it I would vary the amount of gas ( CO2?) the reaction produces, the less gas the more dense and conversely. Lovely stuff, homogeneous for the most part, but again, don’t overstuff the cavity, you get bulges at best. In the boat biz, we often foam void spaces to give the Coast Guard required reserve buoyancy requirement. But when somebody doesn’t read directions and puts too much foam mix in…lets say it gets ugly. A lot of rebuilding needed.But it’s pretty tough stuff, has some adhesive properties.
And with all these you have something not very dense or stiff - the foam the board is made of- an d something pretty hard that['s taking a structural load ( the fin gizmos, boxes or plugs or whatever) and you need to make the transitions without any abrupt changes and ideally, not making it into a major project… That’s hard. Ideally, you’d have a foam that’s not very dense at the outside where it contacts the foam and quite dense where it is sticking to the fin whatsits. Fairly stiff, etc, etc. And you could do it on one shot, rather than several layers plus drilling and so on.
Now, JRandy has a very good idea, use gorilla glue to glue in some pieces of dense foam. Sand to shape, drill, done. Maybe glass and tie the fin hardware into the bottom glass. If I had Hans’ access to pour foam, I might do several pours, fill completely with the lightest foam, drill that well oversize, denser foam, drill oversize, densest foam, drill that pretty close and mount your fin mounts, ideally bedded in glass that ties into the board glass and there you are. But these are ideal jobs, the best possible ways to do it.
Okay, what I’m suggesting is something to save some steps. The gorilla glue expands, a little or a lot, depending on how constrained it is. If you can do it ( a fair anount of glue, clamp the fin hardware in place so the foam can’t move it or expand past it) you could get dense foam at the hardware that expands/gets less dense when it gets to the foam in the board. And if you do it right, it;s one and done, not layering the stuff. I’d do it that way if I was doing a repair job for somebody else. But I want to get the job done fast, low cost to me, and I’m a cocky SOB with maybe more faith in my abilities than I really should have. .
What should you do?
Depending on your access to stuff, and we all do that, I’d suggest you go with either glued in dense foam or a single pour of the two part foam, I might go to medium density rather than the lightest. Foam pieces are a lot of work, but maybe easier to come by for you. Pour foam- the minimum quantities of that stuff are expensive and way way more than you need for this one job. If you have a boatbuilder buddy with leftover components from a job, good. I know such people, As the saying goes, if all ya got is a hammer, everything is a nail.
Hell, if it was me, doing my own,. I might just fill with pour foam, glass over it and then glass on fins. But that’s me.I notice the original foam bits in there are blackened, prolly due to leakage around the fin plugs. You want to prevent that,
That wasn’t a whole lot of help, now was it?
doc…
