Wet Balsa Sandwich

Razorfish

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Not appetizing is it?

When fitting some deck gear on a friend’s boat yesterday we realised that the swarf from the drill was waterlogged. On closer inspection it appeared that the core of the deck (which appeared to be balsa) was very wet.

I would like to ask the panel:

What are the implications of this?

And

What, if any, is the cure?

Cheers,

Andy
 

snowleopard

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the implication is the fairly rapid onset of rot, after which the sandwich becomes two independent floppy skins of GRP and loss of all rigidity.

the cure is drastic- cut off one skin and re-build the sandwich, preferably using closed-cell foam. unfortunately solutions based on injecting resins to replace the balsa or making up a solid laminate will add a lot of weight and affect the stability calculations.

as it's not an accident but 'wear and tear' or lack of maintenance, there won't be any comeback from the insurers and unless it's a very recent boat and the ingress of moisture can be traced to poor workmanship during build, the manufacturers won't help.

sorry to be the bearer of bad tidings but this has to be very bad news. the only possible silver lining is that it may not yet have spread too far. you'd need to do some test drillings around the area to find out.
 

Swagman

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Suggest you ensure it is balsa and not a foam core before really loosing it - they can look alike when only bits are dug out.

If it is balsa - the above respondants have it right. Sorry.

If it is foam, you may be more fortunate in that the issue could be localised - and CAN then be sorted by allowing the area to dry (lots of holes over area on the inside skin) using either hot summer days or blowers.

Then injection of new resin.
Enough pumped to bridge any voids twix the inner and outer skin and thereby regain the needed rigidity. Do ensure you fill all voids completely - a space left will only be a future water catchment spot.

Then bog / repair holes.

Then track down how the water got in there.

That may involve removal and resealing of deck track, winches - anything bolted close to the locality of the problem.

Whilst you have them off - maybe worth checking in each bolt hole to see if the holes were ever 'sealed' properly. I do not mean the silicone under the item, but the foam between the two skins. If you can see / fell foam on the inside of each hole - then then sorry - they need added work also.

It's easy - but takes time. Use a small L shaped tool to rout out foam between the skins around the hole (1/2 inch will do) and fill that with solid bog. You wish to get the the two skins being bridged around the hole - so if water ever does get back below the deck fitting - it will not enter the foam (but simply appear below deck).

Hope this helps................
 

squidge

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Hi , its a common problem with balsa but may not be so bad as i found out on a limbo 6.6 i used to own.The problem is usually found around fittings or fractures where water can creap in. It dose mean cutting out the rot and drying out as much as you can. If you can tow her try asking you local commercial vehicle repair centre to see if you can use a truck painting oven. Over a weekend speeds up the job no end. Once Dry fill as suggested above but you may wish to beef it up a bit around deck fittings with epoxy and mat/strand then finnish off by refitting the bit of deck you cut out, gell coat fill , sand and deck paint. Done well it will never be seen.
Good luck
 

snowleopard

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if the manufacturers are still in business they could tell you what material was used for the sandwich. 90% of production builders make above-waterline sandwich with balsa. a few use foams and of those, the best use PVC foam which is virtually impervious to water.
 

Swagman

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Yes - agree with Snowleopard above and if it is quality foam core the water will not harm it. Do not think the water weight will do any harm unless you are a v competitive racer.

If it is razor cut foam then note the ingress may not be as close to the point that you drilled.

If razor cut foam and not really saturated and rolled well with resin (guess smaller yacht would not be vacuum bagged) then cappilliary action can suck water right through the length of a yacht. I've seen it get in on one forward side of a deck, and appear when a hole was drilled 4 metres away!

Fun, eh?
 
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