Hull blisters ?

[75R20]

OK, I searched and found very little to help.
While diving on my boat swimming, and scrubing the hull, I believe I found some blistering on the hull. They appear to be ~ 1/4" in size and not uniform in placement or numbers. Questions.....
How ?
Why/What causes this ?
Is it serious ?
Should it be repaired ?
The boat was built in 1975, and has been in the water at a moorage in fresh water for ~ 8 months. It has been trailered most all of it life, and is a fresh water boat, seeing salt for only a week or two at a time. Hull/boat is in excellant condition, buffed, polished, and waxed.
The boat is in the Seattle area.
Thanks
Kary
S/V Mariah
#49080

[SlowButSteady]

Originally Posted by 75R20

OK, I searched and found very little to help.
While diving on my boat swimming, and scrubing the hull, I believe I found some blistering on the hull. They appear to be ~ 1/4" in size and not uniform in placement or numbers. Questions.....
How ?
Why/What causes this ?
Is it serious ?
Should it be repaired ?
The boat was built in 1975, and has been in the water at a moorage in fresh water for ~ 8 months. It has been trailered most all of it life, and is a fresh water boat, seeing salt for only a week or two at a time. Hull/boat is in excellant condition, buffed, polished, and waxed.
The boat is in the Seattle area.
Thanks
Kary
S/V Mariah
#49080

Tiny blisters like that are often just paint blisters. Try pushing on them with your fingernail; if they "give" then they aren't in the gel coat. Not much to worry about. But, if they are just paint blisters, part or all of the problem could be an incompatibility between new and older layers of bottom paint. You may need to strip off all the old paint the next time you do a bottom job.

If you start seeing larger blisters that feel as hard as the surrounding surface, then they are probably in the gel coat and MAY need to be addressed at some point. In any case, the cause is osmotic pressure from the water trying to migrate under the paint, or into the fiberglass. It's a little complicated (and its not completely understood why some boats are more prone to the problem than others), but you should be able to find lots of info on the internet if you search for "osmotic blisters fiberglass".

[CasparB]

Originally Posted by 75R20

Hull/boat is in excellant condition, buffed, polished, and waxed. #49080

If that is your only treatment of your boat below the waterline there is a risk of osmosis. Next time your boat is on dry land, try to poke one of the blisters with something sharp. If the blister leaks a vinegar smelling liquid there is no doubt you have osmosis. Or as they say in Norway; "plastic plague"...

[DwayneSpeer]

A few small blisters aren't a worry. If they get big ( >1/2 inch dia) they need to be ground out and repaired. If your boat bottom is covered with blisters, either small or big then you have a good sized repair bill coming. The absolute worst is having the hull peeled, dried for months in a tent or heat dried, and replaced but that is only necessary if the hull below the waterline is WET using a moisture meter after a few days on the hard.