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Old 07-04-2012, 06:57   #1
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Blisters above waterline

Hello everyone, my boat has a number of some small blisters (diameter 2-4mm) on the transom 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 the waterline. When I scraped the antifouling right below the transom, I found two blisters. I could pierce them with a knife and some brown liquid came out. However, I could not identify the smell of vinegar which, I have understood, should be quite obvious.

I am a little bit confused, whether I am dealing with osmosis or something else. The blisters above waterline look like osmotic blisters to me, but what is confusing that they are so far over the waterline (up to 55 cm or 1,8 ft). The boat is a 20 years old Hallberg-Rassy which I believe has a good track record of not having a osmosis. Otherwise the hull looks very good and there is blisters visible. I have also scraped antifouling in some parts of the hull, but the gelcoat looks perfect in those spots. I would be interested to hear your thoughts on blistering above waterline.
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Old 07-04-2012, 07:13   #2
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Re: Blisters above waterline

Greetings and welcome aboard the CF, anjolain.

See some earlier discussions ➥ http://www.google.com/cse?siteurl=ww...ers&gsc.page=1
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Old 07-04-2012, 08:24   #3
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Re: Blisters above waterline

Have you ever had the boat shrink wrapped? I've seen blisters develop when shrink wrap is allowed to adhere to the hull with water trapped in between. That's why you're supposed to put foam spacers between the wrap and the hull.
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Old 07-04-2012, 08:58   #4
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Re: Blisters above waterline

Have you recently moved from salt water to fresh/brackish? We experience a similar event whenever we spend a period of time in a river. Once we return to the saltwater the small blisters disappear within a couple of weeks...
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Old 07-04-2012, 14:47   #5
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Re: Blisters above waterline

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Originally Posted by Ziggy View Post
Have you ever had the boat shrink wrapped? I've seen blisters develop when shrink wrap is allowed to adhere to the hull with water trapped in between. That's why you're supposed to put foam spacers between the wrap and the hull.
I have owned the boat for two years now. I noticed the small blisters a year ago, but did not pay too much attention then. I am not sure if the boat has been shrink wrapped by her previous owners.

The boat has now spent the last 10 years in the Baltic Sea which has a low level of salinity.
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Old 07-04-2012, 15:05   #6
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It is it in the boot stripe? Sounds like hydrolysis like a paper towel the water wicks up if put in a glad of water. Newer boot stripe paint would have a tighter chemical structure. Like a screen below the water is a large mesh that boot stripe I'd a small mesh. Below the molecules pass freely above the screen won't allow the larger molecules to pass. Tension forms and then bluster. How deep is what you need to know. Best eY to find out is to grind out the laminate.
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Old 08-04-2012, 14:04   #7
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Re: Blisters above waterline

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Originally Posted by anjolain View Post
Hello everyone, my boat has a number of some small blisters (diameter 2-4mm) on the transom 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 the waterline. When I scraped the antifouling right below the transom, I found two blisters. I could pierce them with a knife and some brown liquid came out. However, I could not identify the smell of vinegar which, I have understood, should be quite obvious.

I am a little bit confused, whether I am dealing with osmosis or something else. The blisters above waterline look like osmotic blisters to me, but what is confusing that they are so far over the waterline (up to 55 cm or 1,8 ft). The boat is a 20 years old Hallberg-Rassy which I believe has a good track record of not having a osmosis. Otherwise the hull looks very good and there is blisters visible. I have also scraped antifouling in some parts of the hull, but the gelcoat looks perfect in those spots. I would be interested to hear your thoughts on blistering above waterline.


Sounds like a pretty common issue of a centerline seams fairing compound blistering. I have seen this many times. Often boats are built in a split mold because the shapes involved (tumblehome and flair) create a negative release, ie you cant build the hull in a single mold but must use a split mold. Often the seam where the split mold seals causes there to be an imperfection or void in the original gelcoat right up the centerline of the whole boat. Many times this void is simply filled and faired without any external glass as it is not strucural. Water can then get into the seam below the waterline and be wicked up the seam above the waterline in the fairing compound. Often the fairing compound will blister, resulting in a perfect vertical line of small blisters where the split mold seam was. Obviously you can only find out if this is the actual problem by doing some exploratory grinding, but if what you are seeing is as I describe it is almost certainly an issue with a faired seam. It is really quite common. These kinds of blisters are not a structural problem, but the usual fix is to grind the whole external seam and lay a couple plies of glass on it, then fair and recoat. Obviously a big job for a problem that is simply cosmetic, and probably not worth doing unless the boat is already being painted or refinished. Of course it is easy to spot repair a small problem area of the seam, but that doesn't guarantee that you won't experience the same problem later on another part of the seam, which runs full length. Good luck!
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Old 08-04-2012, 14:33   #8
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Re: Blisters above waterline

I learn something new everyday...

Thanks Minaret!


from How a Hallberg-Rassy hull is built
Quote:
This is what the mold to a hull looks like. The outside is reinforced with strong metal stringers. The inside has a high glossy finish. The mold is made in two parts, which allows us to build the hulls with an integrated rubbing strake on the hull and a deep bilge. This would not be possible if the mold was built in one part.
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Old 08-04-2012, 15:00   #9
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Re: Blisters above waterline

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I learn something new everyday...

Thanks Minaret!


from How a Hallberg-Rassy hull is built
Dirk

Anytime! Like I say this is quite common, though probably little known outside the pro field. An integral rubrail would create a negative release, aside from hull shape. Most quality boats are built in a split mold, a designer going out of his way to create a hull design with a positive release is a sure sign of economy in a production run from start to finish. Still not that uncommon in production boats with large numbers built. A single mold saves a lot of money for obvious reasons. I have seen split molds with as many as four parts for the hull, and more for decks. Having built quite a few split molds myself I'm very familiar with the pitfalls involved. Have fun fixing it and good luck! Oh, and next time pics would help, it's more difficult to guess from a text description. A pic and someone would have nailed it right away.
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Old 09-04-2012, 02:21   #10
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Re: Blisters above waterline

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Anytime! Like I say this is quite common, though probably little known outside the pro field. An integral rubrail would create a negative release, aside from hull shape. Most quality boats are built in a split mold, a designer going out of his way to create a hull design with a positive release is a sure sign of economy in a production run from start to finish. Still not that uncommon in production boats with large numbers built. A single mold saves a lot of money for obvious reasons. I have seen split molds with as many as four parts for the hull, and more for decks. Having built quite a few split molds myself I'm very familiar with the pitfalls involved. Have fun fixing it and good luck! Oh, and next time pics would help, it's more difficult to guess from a text description. A pic and someone would have nailed it right away.
Many thanks Minaret! This seems like the most reasonable explanation I have heard of. I would like to just clarify that when you talk about blisters in fairing compound, you mean that they are osmotic and of same kind that can be found in GRP? Like I wrote earlier, I could burst couple of them located under the waterline, but could not identify the smell of vinegar. However, those that are over the waterline, are hard and could not be bursted same way (I have not tried drilling).

Mostly blisters are formed in the shape of lines, but there are also quite many around them. I try to take a photo of the transom next time I am at the boat. I am not planning to do much before the coming season, since before the next winter lay-up I am planning to have the bottom soda blasted and add new primers. So this could be the good time to work with the seam also. However, for the coming season, would you recommend that the underwater part of the transom should be covered with epoxy primer to prevent the moisture wicking further above the waterline, or will epoxy primer make matters just worse?
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Old 09-04-2012, 08:55   #11
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Re: Blisters above waterline

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Originally Posted by anjolain View Post
Many thanks Minaret! This seems like the most reasonable explanation I have heard of. I would like to just clarify that when you talk about blisters in fairing compound, you mean that they are osmotic and of same kind that can be found in GRP? Like I wrote earlier, I could burst couple of them located under the waterline, but could not identify the smell of vinegar. However, those that are over the waterline, are hard and could not be bursted same way (I have not tried drilling).

Mostly blisters are formed in the shape of lines, but there are also quite many around them. I try to take a photo of the transom next time I am at the boat. I am not planning to do much before the coming season, since before the next winter lay-up I am planning to have the bottom soda blasted and add new primers. So this could be the good time to work with the seam also. However, for the coming season, would you recommend that the underwater part of the transom should be covered with epoxy primer to prevent the moisture wicking further above the waterline, or will epoxy primer make matters just worse?

Post a pic so I'm not speculating. Otherwise it's just guesswork. When you blast the bottom it will immediately become clear if this is a problem with seam fairing or not. I wouldn't prime it until you have a chance to dry it out, otherwise you are just sealing moisture in.
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Old 09-04-2012, 15:30   #12
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Re: Blisters above waterline

Here are two pics. Hope that they work...lighting could have been better since the boat is still covered. The windvane marks the centerline so there are two trails of blisters to right and left of the centerline and some blister formations in between. The trails are not totally straight as you can see in the pics. Hope that this clarifies the situation.

[IMG][/IMG]

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Old 09-04-2012, 17:11   #13
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Re: Blisters above waterline

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Originally Posted by anjolain View Post
Here are two pics. Hope that they work...lighting could have been better since the boat is still covered. The windvane marks the centerline so there are two trails of blisters to right and left of the centerline and some blister formations in between. The trails are not totally straight as you can see in the pics. Hope that this clarifies the situation.

[IMG][/IMG]


Those are truly teeny blisters, I had to blow up the pics just to see them! Looks more like a failing repair than anything else, the lines are too wobbly to be a seam fairing problem, that would be dead straight or close to it. Only way to find out for sure is to take a grinder to it. Are you sure that's the original gelcoat and not paint? Those blisters are small enough to just be blisters in the paint due to contamination or something else, if the boat has been painted. If it's a poor repair where they used lots of fairing compound which extends below the waterline without barrier coating, then the fairing can easily become saturated and "rot" both above and below the waterline. This is why all of my repairs below the waterline are done in solid glass with no fairing compound, seen too many failures in epoxy fairing compounds.
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Old 09-04-2012, 17:23   #14
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Re: Blisters above waterline

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seen too many failures in epoxy fairing compounds.
On many boats you will see this on a skeg or keel hung rudder where the bronze rudder gudgeon are plastered over with just compound. When I surveyed my current vessel, I had to grind it all out to see if the bronze was in good condition. Luckily it was. The compound was clearly failing.
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Old 10-04-2012, 12:57   #15
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Re: Blisters above waterline

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Those are truly teeny blisters, I had to blow up the pics just to see them! Looks more like a failing repair than anything else, the lines are too wobbly to be a seam fairing problem, that would be dead straight or close to it. Only way to find out for sure is to take a grinder to it. Are you sure that's the original gelcoat and not paint? Those blisters are small enough to just be blisters in the paint due to contamination or something else, if the boat has been painted. If it's a poor repair where they used lots of fairing compound which extends below the waterline without barrier coating, then the fairing can easily become saturated and "rot" both above and below the waterline. This is why all of my repairs below the waterline are done in solid glass with no fairing compound, seen too many failures in epoxy fairing compounds.
Thanks for your contribution!

I have had the boat for two years now, so I do not know if the transom has been repaired or painted by one of her previous owner. However, I do recall that the transom might have a slightly different colour at some places which would imply that it has been painted. So that would indeed seem to be a plausible cause for blistering. However, the surface is otherwise similar to the gelcoat on the topsides, so I have not thought before that it could have been painted.

If it is not the seam that is failing it must be something else. I have not heard before that moisture could be wicked up over 0,5 meters over the waterline. Well it might be the case here, but I am hoping that it is something else.
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