Blisters above waterline

[anjolain]

When I was preparing the boat for the winter I noticed some small blisters (diameter 2-4mm) on stern of the boat near the centerline. They are mostly in shape of a line starting from 10 cm above the waterline and continuing up to 55 cm above waterline. Below waterline the hull looks normal so there is no blisters visible.

They look like osmotic blisters to me. But what is confusing that they are so far over the waterline, so I hope that they are something else. The boat is a Hallberg-Rassy which I believe has a good track record of not having a osmosis. I would be interested to hear your experience on blistering above waterline.

 

[Phoenix of Hamble]

They'll either be the result of Wicking or paint failure... Most likely the former.

Wicking is where water has crept up fibres through capilliary action from lay up issues.

Its not that uncommon, but does imply layup problems, although not usually serious.

 

[jwilson]

They'll either be the result of Wicking or paint failure... Most likely the former.

Wicking is where water has crept up fibres through capilliary action from lay up issues.

Its not that uncommon, but does imply layup problems, although not usually serious.

Not sure it will be due specifically to wicking. Apart from the quite common sight of small blisters in the boot-top splash zone, blisters above the waterline are more often due to leaks down from above (fittings/capping rail etc) or to fresh water being trapped against the hull by covers, prop pads or similar.

Are you talking about the transom itself, or the hull area nearby?

 

[Evadne]

When my boat got "osmosis", I had small blisters above and below the waterline. Since being epoxied from the waterline down I see the odd blemish above the waterline, but nothing below. Has your boat been epoxied?

(My problem was diagnosed as a porous gelcoat, due to bad lay up which led to wicking. not osmosis, but more often than not labelled as such. It didn't become an issue until she inadvertently had to spend two winters afloat, she was a little over 25 years old at the time.)

 

[anjolain]

Not sure it will be due specifically to wicking. Apart from the quite common sight of small blisters in the boot-top splash zone, blisters above the waterline are more often due to leaks down from above (fittings/capping rail etc) or to fresh water being trapped against the hull by covers, prop pads or similar.

Are you talking about the transom itself, or the hull area nearby?

Yes, I am talking about the transom. I don´t think that leaks from above would cause the blistering. There might be some fresh water trapped in cockpit locker against the hull near the waterline, but not 50 cm above the waterline.

 

[jwilson]

Yes, I am talking about the transom. I don´t think that leaks from above would cause the blistering. There might be some fresh water trapped in cockpit locker against the hull near the waterline, but not 50 cm above the waterline.

Odd.. There is usually some reason for localised blisters, usually something trapping water for a time inside or outside the area. Water in the bilges or lockers does not usually cause blisters first on the outside, though it's a good way of getting "high moisture readings" on survey.

Localised blisters can be something within the layup itself, which is virtually impossible to determine without destructive cutting open.

I'm not saying it is this, but just as an example imagine a greasy or oily streak on the surface being moulded (perhaps between gelcoat and first layer of glass, or 1st and 2nd layer of glass). This would leave a non-bonded line hidden within a perfectly good-looking final hull surface. Now bolt a fitting somewhere it's hole meets that line, let the drilling stress the laminate slightly, and have a non-perfect waterproof seal around the fitting......

Du Plessis, author of the most definitive work on GRP construction, lays the blame for blisters primarily on bad moulding, and as this work is normally done by error-prone/hangover/angry/about to be fired humans, it is both common and random, hence some twenty year old Seabasher 40s getting "osmosis" and some not.

 

[anjolain]

It turnout to be wicking just as "Phoenix of Hamble" wrote. The blisters above waterline are solid and cannot be opened with a sharp object. But then I removed antifouling below the waterline and found two small blisters right below the "wicking lines". I could burst them and brown liquid came out. So it is osmosis, case closed.

Problem is now that I do not know extent of damage. There is no sign of wicking anywhere else, and there is no visible blisters below waterline. However, the two blisters that I found at transom were not visible either under the antifouling. There are few places were the antifouling and primer? has worn out and white gelcoat is shining. And there is no signs of blisters either. What I am hoping is that this is just very localised problem and the hull is otherwise healthy. I would appreciate some suggestions how to continue with this problem? Sodablasting is not allowed in the harbour so I cannot clear the whole bottom to see how it is.

 

[catalac08]

On a couple of boats I have had there has been blistering which has developed where there have been name decals. No other sign of blistering anywhere else on the boats above or below the waterline. These show no liquid when opened up and were about 3mm on each boat.