A Sunday afternoon spent at home bimbling rather than sailing led me to wonder about moisture content in hulls. 

I'm seeing figures of moisture content for GRP hulls as being a maximum of 2 or 3% of the hull weight. Admittedly these referred to yacht hulls more or less constantly immersed. Anyone know if this is a similar figure for Foam Sandwich hulls? It didn't sound a lot.

What might a typical Foam Sanwich hull take on over 10 years standing in a dinghy park and racing at weekends? 

Once a FS Merlin hull has absorbed some water / weight, is there a way of reversing the process - anyone tried it?

I've not come across a recent Merlin that has put on weight that could be put down purely to moisture absorbtion. Back to the Future was reweighed after it's bow drop and refit and it had put on weight but it's pretty obvious where that had come from. I never thought to weigh the fittings before they were attached but the figures married up with what I expected.

Jon Gorringe's boat (3712) had not put on weight after it's mods on reweighing either. Not 10 years old, but it's not often you get a boat that old for a re-weigh.

Do you think you have a problem with 3586? Don't forget it's been painted. From new it would have had 8-12kg of lead.

Hi Chris, no reason to think so - she's almost completely stripped out now, and by next weekend I should have a completely bare hull and a bag of new fittings and string to put on her. It seemed like a perfect opportunity to weigh the hull and fittings separately as a before and after. 

Don't have the certificate yet but she still carries around 8-10Kg by the size/look of it. Some lead has been taken out, I assume to compensate for the bow drop and bilge keel mods.

I've got to take all the paint off next anyway, so if leaving it like that for a few weeks would help shed a couple of kilos of moisture I was willing to try it!

Was the moisture issue confined to the WOD generation of boats?

I don't know very much about the WODS, but it's pretty obvious that there is a lot more structure in them than the FRP CT hulls regardless of builder. They are also contructed in polyester resin which is heavier than the epoxy used in modern hulls.

When an FRP hull is damaged and the laminate punctured the surrounding foam will absorb water, especially if the boat is sailed and it's under the water line as it will be forced in.

Having seen some quite tasty damage to boats over the years one merlin in particular springs to mind. I don't know quite what had happened to it, but the keel was soaking wet from the forefoot cradle back to the centrboard case. this was dried out, repaired and the boat re weighed. It weighed in at what it should have done according to the certificate, much to my suprise.

A litre of water weighs a kilo, and a litre of water is an awful lot to hide as absorption in a boat.

So summing up, I don't beleive that modern FRP hulls that have not had massive and badly repaired damage absorb a significant amount of water. Of more significance is paint - especially regripping the floor -, extra fittings and rope and thinga like padded toe straps and buoyancy bag covers.

Heaven Sent weighed 101kg at the champs with at least 3kg of correctors still in, 19 years old.

All good news -  a testament to the build quality and materials - as Chris says, adding layer upon layer of grip and paint won't help. I guess 2% or 3% over 10years might not be far off, which is far less than I've absorbed over the last 10 years.  Maybe I'm shaking the wrong tree here....

we reckon a repaint (just keying the exising material with 600 paper) sprayed 1lt of 2k + hardenener,gives 2 good coats coverage= 1kilo in weight, thats why we take off most of the existing material,a good 240 longboarding removes most of the old stuff,finish prep with 600 for a finih base = virtually no weight gain,.....as with most things,you get out what you put in....

Wearing a Marine surveyor's hat, the percentages that you see on moisture meters are pretty meaningless, most were designed for measuring damp in buildings.  With a yacht hull, which is in most cases solid laminate, when looking for an osmotic condition, the moisture ingress is only likely to be into one or two layers of the outer laminate.  Carbon and/or kevlar laminates might throw up spurious readings due to conductivity or just because kevlar can act as a wick.  When considering a Merlin I guess that a moisture meter might identify whether there is water ingress between the laminate and the core, in which case it is likely to show locally extremely high readings.  I would be extremely surprised if an FRP boat takes on any weight in its lifetime through natural causes.  A non epoxied wooden boat ... that may be a different story