Wet Core

[phsailor]

Well I decided to take care of a crack on the floor near the seat of my newly purchased Spindrift DS1. I ground down the top layer only to find a wet cardboard material.
I assume that is balsa wood.
Under that material I did a tap test and all sounds good. I ground some floor test spots several inches from the crack. The balsa material was very wet.
Any advice on what to do about this situation? Can I let balsa dry and just put a few layers of glass on the ground areas and call it a day.

[K.C. Walker]

It looks like no one wants to get involved with this thread. A wet balsa wood core is bad news, I'm afraid. When you have rot it means it's probably been wet for a while. With luck it hasn't wicked too far from the original crack but the longer it has been let go the more likely that the water has wicked pretty far from the crack. To fix it right you need to keep cutting until you find dry solid core.

There is one way that I read about to dry out a balsa wood core if it's still bonded to both skins and therefore still structural. You can pull a vacuum on it for several days. This could be tricky but it might be possible. This is the way surfboards are repaired when they get dinged.

A former national champion for the DaySailer, Dave Keran had a Rebel with a balsa core and had similar problems. I don't remember how much he had to cut out, but it was a lot more than he was expecting.

[seandwyer]

Hey, Guys,

You know, I haven't peeked inside my core because I'm terrified of what might be in there. But I have a question: how much structural support does balsa really give? I know it certainly represents something--much more than the nothing that some boats have in their floors, but isn't most of the structural support in the rectangular box of fiberglass that was laid up around the balsa? Couldn't the rotting balsa be seen as something akin to a lost wax process? I know it would be better to have something solid and I know that if I cut into mine and found soggy pulp I'd be compelled to redo the whole thing--but is is really worth the effort? And, what's the worst that could happen if you leave that soggy mess alone?

Just some thoughts.

[GreenLake]

Sean, if I understand the explanations for these things correctly, then what the "stuff in the middle" contributes in a sandwich is to a) separate and b) connect the two portions. Now, if you have a narrow (hat) profile of a stringer, then your conception of a "lost wax" process is not too far off the mark, because the necessary separation and connection (transfer of shear) is taken over by the sides.

For large flat areas of double hull, you probably can't rely on the sides alone, otherwise the performance reduces to that of separate single layers. (More or less). Also, of course, you may want to remove soggy balsa for weight reasons.

Balsa probably goes through some stages, where "merely" wet balsa may still provide the separation and perhaps at least some of the connection, but more deteriorated parts won't.

[Mike Gillum]

Sean,
Wet core is a hidden cancer like dry rot in a wood boat or missing a cylinder (or two) in your car's engine. You know you have a problem and while you're still able to get from Point A to Point B you're simply postponing the problem allowing it to spread and get worse.
Your Spindrift may actually be cored with a cardboard-like material instead of a Balsa Core as I came across a cardboard core in the cabin top of SLI #14028 last summer when I cut the thru-deck Harken for the 20:1 Boomvang.
In that particular situation I cut the hole slightly bigger as well as over-drilled the fastener holes, thoroughly taped everything off and back filled all of the penetrations with WEST Epoxy to seal so that water wouldn't penetrate the core at a later date.
Most of us prefer to sail/race rather than work on our boats but even "Plastic/Tupperware" boats aren't immune from deferred maintenance.
Mike Gillum DS #2772

[seandwyer]

OK, OK--you guys have convinced me. I already pretty much agreed, but was hopeful for news pointing away from another "core project".

So let's say you don't know if the core is wet or not. You've noticed nothing that would make you believe the glass envelope has been penetrated and hull appearance and performance seem normal. At 45 years of age, should we assume water has most likely found its way in and do a little exploratory surgery? Start drilling test holes here and there in the most obvious places? Being the same age as the boat, I can tell you that doctors seem to want to do similar things to me, but I feel like this is a bad idea. Well, I WANT this to be a bad idea. Even if I found something, it's too late in the year to bother with it with only 2 months to go before I hope to be afloat--but maybe next winter.

[K.C. Walker]

Sean,

The good news for you is I don't think you have a balsa wood core to worry about! I was thinking that you had a low enough serial number boat that it would be a non-chored boat. None of the O'Day made boats have a core, aside from the possibility of having stringers with a core of balsa.

[seandwyer]

KC--Thanks for clarifying! I actually was referring to stringers all along assuming they were what you guys refer to as a core. So I haven't got a core after all--but maybe some rotten stringers. Do we worry about those, or not?

So a core is a balsa layer the breadth of the floor between two layers of glass?

phsailor--what have you got?

[GreenLake]

Sean, a stringer is a narrow, strip-like reinforcement that usually goes fore-and-aft. "Core" the way it's used here refers to the middle layer in sandwich construction of panels. The sandwich construction doesn't have to go the width of the hull.

In terms of mechanics, the stringer acts like a beam, the cored sandwich acts like a plate. These have somewhat different characteristics.

For stringers, especially if very narrow, the sides can be constructed in a way that allows the stringer to be hollow and yet function as designed. In which case any core material is icing on the cake. The wider the stringer gets, the more support it would need in the middle. In principle, if you desperately wanted hollow stringers you could place two narrow profiles side by side.

Having wet balsa in a stringer is still going to add weight to your boat (and it could freeze if you store your boat outside in a place where frost occurs). Any failure of the balsa might well be less of a total failure, because the sides can carry at least part of the load. Still, Mike's analysis holds.

The good news is that redoing your stringers is a limited project (after all, they are rather limited in extent).

[Dave Keran]

A wet core is trouble in many ways. Structurally the core ceases to provide the rigidity that you need and wet balsa is heavy. Leaving it as is will only make matters worse because. The Neither of the cl. When I discoved the wet balsa in 11026 I chased the problem with a dremel with a short bit. Working outward from the crack I drilled holes until I got dry balsa without any water. Then I routed out all the area that was damaged. You will know if the balasa is rotten as it gives off a distinctive smell. In the case of 11026 the rotted area ran from the aft edge of the CB trunk on the port side back within a couple of inches of the transom with. Latterally the affected area was up to eight inches away from the centerline. It was a very irregular shape. I've got pictures of the whole process if you want to see what I did. You can e-mail me.

Dave Keran

[seandwyer]

Hey, guys,

Been a while since I've been on here--and been a long time since this thread was current. I guess I've been lucky! Anyway, several years ago we were discussing the replacement of cores and stringers. I recently found what I believe to be a stringer that has rotted out and I'm wondering if this is an emergency or whether I might just sail the summer and worry about it next spring.

This is an '68 O'Day DS-1. The "stringer" I'm talking about is not one of the main ones but the smaller ones running next to the base of either bench seat. I've ground an opening near the aft end of the bench and the fiberglass box where the "stringer" should be is empty and wet. There was a piece of paper and a lot of dirt in the void. Does anyone know if this was most likely wood or just cardboard they used to lay up the fiberglass with? I know the main stringers in the floor are balsa.

Also--in the old core project thread (which I can't find anymore--I think KC Walker did that) I think we discussed a kind of foam that could be used in place of balsa. Does anyone remember the name of that foam?

Thanks!

[GreenLake]

Some stringers get their main strength from the laminate forming a U channel. In that case, missing whatever core was used is not an issue (ever). The situation I mean is one where the stringer isn't too wide. Some people (supposedly) did things like use a half of the cardboard from the center of paper towel roll for laying up reinforcements.

With the "core project" there were large flat sections, so essentially panels. In that case, there isn't a wall there that can transmit the force between upper and lower level as in the case of a narrow stringer. That's where the foam comes in. It keeps the two layers of laminate separated and that gives them their strength.

'68 is quite a bit younger than mine, so I don't have first and experience I don't think mine has any stringers at all. That may be a later invention.

The "Core Project" thread is very much preserved. I haven't looked for your foam, but if you follow the link you should be able to find it.