Wet hull - no visible signs

[AFRANKS]

I know that associated subject of the dreaded pox has been raised many times but here goes ....
(Boat Mirage 28 1980 new Rig 2000, engine 2005) Scenario:
Originally bought boat 10 years ago, survey found the readings of the hull to be high 70 - 100 boat was in the slings etc. Hull had been 'protected' with two coats Gelshield previously. Recent survey no change still a wet hull. The hull has never been cleaned/ blasted back in my lifetime. What does the panel think to these approachs:

1.Slurry blast hull back to original surface - if no blisters (here's hoping!) leave to dry over winter consider covers heaters etc. prime, antifoul relaunch etc

2. If blisters present investigate mobile professional fix cost prohibitive.

3. Fill blisters cary on sailiing.

 

[Pasarell]

Do you have any blisters?
Gelshield used to have a sachet of aluminium paste to add to the second coat to give a different colour and that will probably affect the "moisture" reading.
If you don't have any blisters, and the Gelshield is in good condition I would leave well alone and not interfere with what is probably a perfectly good hull

 

[Jonnysailboat]

Fibreglass hulls aren't waterproof, they absorb a surprising amount of moisture which passes safely through the laminates and evapourates on the other side. If the moisture content of a hull is measured in the slings, presumably just after having been lifted from the water, the meter readings will be high, it can actually take months for a hull to dry out throughly (depending on various factors!). Chances are there's absolutely nothing wrong with your hull.

 

[tyce]

you would have to be mad to remove what is probably a good coating of epoxy unless you had a real good suspicion of underlying problems.
leave well alone forget about it and sail the boat

 

[lw395]

Osmosis is the passage of water through the laminate. This isn't actually a problem. Hydrolysis (in this context) is water breaking down the resin. More of a problem. I would agree that no blisters = no problem. I can recommend Tony Statton Bevan (apology for spelling!)'s book on the subject.

 

[essexboy]

Could you take readings inside the hull below waterline and compare ?

 

[billskip]

Initial readings should only be used as a reference...never take a reading without cleaning the area of antifoul as antifoul is a bit like blotting paper and will absorb any moisture in the air and also retain moisture longer. all also depends on the humidity where you are ...you could very well be concerned about nothing..and involve yourself in a lot of unnecessary work and expence ...

FWIW next time boat out for winter clean off all antifoul as soon as practical and leave clean over winter.. then compare readings by a "set zero" reading taken from a few high positions on the freeboard and readings taken below the WL.

you can get different readings on either side of your hull just because one side is "sunny"and the other in "shady"

Relax take it easy and treat the blisters if you have any and you dont need moisture meters to see them.

 

[Birdseye]

First of all a caveat - what follows is my understanding as an average yottie and deffo not an expert. You need to check facts from a lot of sources and then make your own mind up.

What is commonly referred to as osmosis in boats is in fact the chemical breakdown of the resin under the influence of a soup of water dissolved chemicals. Blisters are a symptom just like spots are a symptom of measles. But blisters arent the problem (other than cosmetics or blisters of a heroic size) and you can have this chemical breakdown going on without a sign of blisters or with blisters so shallow you wont see them unless you know what you are looking for. And indeed you can have blisters without osmosis too.

You check the moisture of a laminate after giving it a good wash in fresh water and a chance to dry off any surface dampness. Then you compare the readings above the water line (the datum reading) with those below. Then leave the boat for a decent time (ideally a couple of months) out of the water and again take readings. If the moisture hasnt changed much and is high, then its a good indicator of a wet laminate and likely osmosis. On the other hand, all laminates get wet in water and if it dries out on land then its a good sign. Moisture readings in the slings dont really show much on their own. High moisture + blisters make osmosis likely but even then a surveyor will look for other things and talk of "possibilities" rather than certainties.

DIY treatment is very possible if the boat wont bear the cost of a pro job.. Its easy to get a hull blasted or peeled by specialist companies, you can do the hull washing etc yourslef and you can apply the epoxy yourself. You can hire meters to check moisture. My guess is that for a 26 footer you would be looking at £1000 TO £1500 FOR A DIY. main problem is likely to be finding somewhere where you can do the work and can keep your boat ashore for the necessary time (months if out in the open). I've just done this blast / paint process myself except that in my case the coat was preventative on a dry hull and therefore fewer coating layers. Total cost was about £850 for a 34 footer..

I'm no expert so if you want to learn what its all about try www.passionforpaint.co.uk

 

[maxi]

Hi, if the boat was in slings, does that mean that she had just been lifted from the water - if so simply ignore the readings and sack the surveyor.

As an ex Mirage 28 owner who still sails a Mirage 35, I have never seen an osmotic boat moulded by Thames Marine - then again I have not seen them all.

Your surveyor should qualify the moisture readings aginst a calibration figure, the numbers that you give on their own are meaningless without knowing the baselines.

If you cannot see blisters I would suggest that you leave well alone, but do try to get some common sense out of your surveyor. High moisture readings are far from being a reliable indication that your boat is osmotic.