Is this osmosis?

[emo]

Is this osmosis ? late 1970's hull with solid GRP lay-up - South of England Boat Builders - Rival . left on sea lough in british isles for number of years and appears to have been heavily fouled before recent cleaning.

could it be barnacle grooves? or am I deluding my self.

scale of the circles was approx 1- 1 and 1/2 inch in diameter

any views welcome
Thanks
Emo


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[sailorman]

looks ok 2 me, osmotic blisters would be more conical / raised

 

[NorthUp]

Too big for barnacles, but maybe limpets if it was a long time without cleaning.
I think you will have to scrape off the antifouling paint to reveal the gelcoat, and post new photos of what lies underneath.

 

[nimbusgb]

If when you scrape it right back you pop a small blister and brown evil smelling liquid pops out its osmosis. What you have does not look look osmosis at all!

 

[Twister_Ken]

Looks like there's a lot of dead a/f anyway, so a scrape or blast would be in order in any case. That would reveal what's going on (if anything). They do look suspiciously limpet shaped, though.

 

[johnlilley]

From what I can see, not typical osmosis, a slight possibility of a blister visible at 1200 on the rudder photo, but not very large even if it is osmosis. Sometimes you have to remove the antifoul to see the smaller osmosis type blisters. Look for raised blisters anything from 5mm dia to in excess of 13mm. The larger domes are easily visible beneath antifoul.
John Lilley

 

[emo]

thanks for all the replies, looks like I first taught
'Limpet' grooves or as I wrongly named them - Barnacles!

thanks again

emo

 

[vyv_cox]

Read all about it here.

The situation may be rather worse than others have said. Must admit they don't look like osmotic blisters I have seen before, but Page 8 says:
Small, or well formed blisters usually indicate that the gelcoat is adhering well to the laminate, and that the laminate itself has good interlaminar adhesion.
By contrast, large, shallow and irregularly shaped blisters are usually formed where adhesion between the gelcoat and structural laminate is poor. In some instances, two or more blisters will merge together to form larger blisters, again indicating poor adhesion between the gelcoat and laminate.
Very large or shallow blisters, and unevenness in the hull surface generally indicate de-lamination.

Scraping a few off should reveal the truth beneath.

 

[Tradewinds]

Let's hope they are just limpet remains as they don't seem swollen in the pics. I think you would be best to investigate one of the "circles" if only for peace of mind.

I saw a yacht that had large blisters 38-50mm across. As suggested in Vyv's Clegg PDF there was delamination underneath. The owner had to deal with a dozen or so of these, by then, quite deep holes which he had ground out. This involved re-laminating & fairing. IIRC they only appeared on one side of the yacht - perhaps a case of an operative having an "off" day. After he'd finished you would never had known there had been a problem.

 

[Lakesailor]

They look too irregular to be blisters.

 

[Tradewinds]

I tend to agree with you - just looking at the worse case scenario /forums/images/graemlins/ooo.gif

 

[vyv_cox]

See my post above - 'large, shallow and irregularly shaped blisters'

 

[Tradewinds]

In my example, I think the delamination was not actually between gel coat & laminate but between two layers of laminate immediately below the gel. The hole was quite deep but a constant depth over the area.

 

[Phoenix of Hamble]

i'll put my money on that they are where a limpet has previously lived, and that the hull hasn't been cleaned/sanded properly before a new coat of antifoul has been applied...

One pic shows the marks in what is almost a ring.... this is typical IME of limpets.... whereeas Osmotic blisters tend to be more random in location....

 

[Lakesailor]

If you read that in the context of the document the large irregular blisters are a sign of delamination. He says that osmosis will create those blisters due to osmotic pressure where the gel coat is already delaminated. However if there was osmotic pressure in the grp you would expect to see many more regular osmosis blisters in addition.
All the pictures in the document are as you expect to see osmosis blisters, round and fairly evenly spaced. I've seen them grouped around problem areas like where bulkheads are bonded to the hull, but not those great mollusc foot-like shapes.

 

[RAI]

My first reaction was that they used to be blisters that have been popped and epoxyed, then painted over.

 

[theoldplucker]

Difficult to tell the size from photo. If small flat discs which pop out it could well be dry blisters. No weeping of any fluid could confirm this. I believe there are over 200 types or reasons for blisters not always osmotic. Lots of information here http://www.yachtsurvey.com/blisters.htm
Suggest removing antifouling, pressure washing or sandlblasting to remove discs, check on moisture levels, fillinng voids with epoxy, sanding whole gel coat, a few coats of Bakes epoxy or Gelshield.