My Hobie 18 has what I can only describe as 'acne' on the outside of the port hull. The inside of the hull, and the complete stbd hull, are nice and smooth. But if you sight down the outside of the port hull, it looks and feels somewhat like a relief map, with 'mountain ranges' here and there. I know there must be something written about the cause of this since I've seen it on other boats (including a Prindle 16 I once had). But can't get the right words in any search engine to do my homework. Can anyone help? Just wondering what the exact cause. Seems as though it would be way too much work (and added weight) to try to fair it smooth again.
Thanks much!

http://www.zahnisers.com/…air/blister/blister1.htm
this should help it is long but informative you basically got water between the lamination if the fiberglass.

OK, I have the same problem that developed when I kept the boat in the water for 2 weeks. So it looks like the only solution is sand, coat with fiberglass resin and re-gelcoat. Or live with it... which is probably what I am going to do considering the cost and time involved.

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Dave Bonin
1981 Nacra 5.2 "Lucile"
1986 Nacra 5.7 "Belle"
Winnipeg, Manitoba, Canada
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osmosis is the technical term i think

otherwise known as bubbles that form on gelcoat when it remains wet for too long

Thanks for the replies so far, and the reference to the blister article. I have been around (big) boats most of my life and am very familiar with the blister issue in fiberglass hulls. But this is not that. This 28-year-old boat has literally been in the water maybe a dozen times in its entire life, and was always pulled out and covered. If I'm to believe the former owners, it never even got wet from rain until the last few years when it was stored outside. Boats with blisters develop them over many many years of continuous immersion.

Again, I'm at a loss for words to describe this phenomenon properly. The relief map analogy is about as close as I can get. There are 'rough' areas along the hull. You can see then when you sight down it, and feel them. The roughness formed on the inside. It's almost as though the resin 'went off' too fast, or it curdled or melted the foam and re-hardened ? or something. For what it's worth, the boat was stored in the high desert and the affected hull would have gotten more 'cooked' in the sun than the rest of the boat.

Also again, pretty sure the only fix is a new hull, so I'm probably just going to live with it. I'd just like to know why some hulls do it ? and some sides of hulls ? while the rest of the hull remains okay.

well i don't know much about mono construction, but it is well documented that beach cats don't do well on the water for extended periods.. (weeks, not years).

It could be moisture from the inside. rain water can get in a cat pretty easy (through cracks/tramp tracks/etc).

also, it may simply be a bad repair job.