rotted inner layer of GRP hull

[sukibe]

I have been doing some work on the inside of my GRP boat and noticed some rot between linner and outer layers of the hull. There seems to be a one quarter inch layer of some kind of wood, maybe balsa which has rotted in several small areas, maybe one square foot in size. I am not sure how to repair it but have dug out some of it and tapped around the area with a hammer and found several more hollow sounding spots. My thinking is to dig it all out and replace the rotted wood with glass mat and then glass cloth over the affected areas. Any ideas or suggestions welcomed.

[SailDesign]

sukibe - remove all the rotten balsa, and replace with new. Then glass back over with a similar laminate to what was removed.
Send the rotten balsa backto Baltek, who refuse to believe that this can happen to it. I mean, I like balsa and all, but it does rot if you expose it to water.
Steve

[gonzo]

Are you sure you identified the wood species correctly? What part of the boat is this core on?

[sukibe]

it is inside the bottom of the hull and partway up the sloping side, seems to have been put there to stiffen up the hull. When I peeled off the top layer of glass which was loose I found black muck underneath. I continued pulling off loose glass and found what looked like one quarter inch thick by about one and one half inch squares of end grain wood, I went to the baltek site and found they manufacture an end grain balsa procuct. The hardest part of the project now seems to be that the rot is not in nice uniform squares but in irregiar shapes with areas of solid material in between.

[Willallison]

Who was the boat manufacturer?
If you are unavle to establish exactly what kind of core it is, then perhaps they can tell you.

[gonzo]

The solid material is resin. Scored cores, whether wood or foam, end up with those solid columns which help with compresion strenght and adhesion between laminates. You have to grind the bottom laminate to get good adhesion anyway, so it won't be a problem. The proper repair is to remove all the damaged core, grind the laminate , put a new core and laminate the inside layer. Vinylester has much better adhesion than polyester and will give a better secondary bond.

[sukibe]

What would you use for a core? I don't think I want to use wood again. What kind of foam would you recommend. To answer Will, the manufacturer of the boat is out of business, it is an '82 United Sailing Yacht, the same boat that I am converting from sail to power. Another problem which I didn't mention before is that the center of the hull bottom is stiffened by a piece of 3/4 inch ACX plywood, about a third of that has been affected by the same laminate failure as the balsa core and has rotted, I am digging that out too and will replace it. Thanks for the advice