replacing balsa core

The "correct" way to do it is to cut out relatively large panels of GRP skin on one side till you find the limit of the water damage, then clean out the rotted core, replace it and then replace the GRP skin. If you cut out relatively large panels and do it neatly, you can probably do a neat job of putting the original skin back, retaining the profile and surface finish. Those Fein type multifunction tools are probably best.

You can replace the core with more balsa, but there are high density foam alternatives that are less susceptible to repeat damage. If the damaged area is small and close to a skin fitting, you may be able to excavate the damaged core without removing skin and just back-fill with epoxy. People report success using a cheap Allen Key sharpened on a grindstone and clamped in the chuck of an electric drill to mill out the rotted core between the skins.
 
maby said:
You can replace the core with more balsa, but there are high density foam alternatives that are less susceptible to repeat damage.
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It depends what the structural requirements are; balsa is a lot stronger than all but the most expensive foams, and waterlogged foam isn't much more durable than balsa. It's also a lot easier to get polyester to stick to balsa than it is to foam.
 
You should also note the thickness of the balsa as I have both that and foam to do this type of repair and they are not the same thickness.
 
Repairing from inside the hull is a bit messy but the outer gelcoat and any non slip patterns are preserved and original.

If using foam to replace the balsa, ensure it is 'closed cell' foam, it will not breakdown if water ever gets inside, unlike Balsa.

To find the area effected I drill 6 mm holes from the underside about 50 mm apart on a grid pattern, check the material that the drill pulls out for any sign of moisture, mark the hole with a marker pen if dry. this will give you an outline of dots to follow when cutting out from below deck. Keep the cut out. t can be used again to cover the repair.

The new core material should also be drilled with holes on a grid about 50mm apart, this will ensure no air is trapped between the deck and the new core material, any excess material should squeeze out through the holes. the same applies to the original panel, you simply wipe over the holes to fill them.

I prefer replacing Balsa with foam, it's easy to shape, easy to fit and will not break down in the future. But it MUST be 'Closed Cell'.

Good luck and fair winds. :)

.
 
maby said:
If you go ahead with it, report back - that is how we all learn!
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Yes I will , the boat in question is a Mirage 270 bilge keeler and the survey (according to the seller) quotes
"A survey report conducted in April 2012 is available for inspection. The boat has suffered from ingress of water into the coachroof and the balsa core needs replacing hence the low asking price. The boat is sea-worthy and this problem does not affect or undermine the strength or structure of the boat."
this I wondered about as there must be a reason for having the core;)
Time will tell.
 
Presumably the affected bits of the deck don't have load bearing or structural things attached to them; the practical consequence is restricted to having a floppy deck underfoot.
 
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