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Old 06-06-2013
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Very Large Hull Indentation

Hi everyone! Many a google search have led me here (because you guys really know what you're talking about!) but for once, I just can't find an answer because no one has asked an identical question... so here I am!

I have a 1984 US Yachts 25. It's always been good to me and has been rock solid. However, I skipped a sailing season last year, so it has spent two consecutive winters on its trailer (a flatbed trailer with a cradle attached to it, basically).

Now, there are two large 'dents' in the hull where the rear supports are. This probably formed last winter when the cockpit became flooded with water and froze, creating a huge weight in the stern area. I didn't notice the dents until a week ago, while tearing out the cabin which I intend to redo. There is only one bulkhead near the dents (which are around 18 inches in diameter, the port side being more prominent) and it isn't even tabbed except in two small spots, and I can now clearly see it doesn't fit the hull shape anymore. I should mention now that the hull is *solid fiberglass*, so this shouldn't be a matter of delamination. It is also bone dry, and there are no cracks that I can see.

I've heard it numerous times that this 'oil canning' effect is common and not a problem, as the fiberglass either return to its original shape or will stay that way.

However, what I haven't found online yet is someone explicitly mentioning that the area is now *soft*. I jacked the boat up by the rudder so it was just off the supports. The hull in general is completely stiff, but these indented areas are around as flexible as say, a car hood. Not spongy or anything, but you can visually see the are bend in if you put pressure on it.

What does this mean? I'm worried sick

Last edited by superhornet59; 06-06-2013 at 04:11 PM.
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Old 06-06-2013
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Re: Very Large Hull Indentation

A picture would help BUT if you can flex it that easy compared to other nearby areas of the hull I would think you have some localized delamination

It would not be that big a deal to grind it out form inside and apply some new fiberglass and fill and fair it from the outside
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Old 06-06-2013
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Re: Very Large Hull Indentation

It is very possible that under the extreme weight of water and/or ice, this/these area(s) gave up the ghost and that you may have some delamination in glass layers going on.

you do have several layers, your gel-coat, a matt, a roving, followed by the chopper gun, followed by a roving, then a matt. The worst part is/was the additives added to the resin ( for fast set ) made for some brittle resin batches .

I know that's not what you want to hear
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Old 06-06-2013
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Re: Very Large Hull Indentation

Welcome aboard. Pictures are worth somewhere around a thousand words. It would be helpful if you would post some, maybe with a ruler or pencil in the shot to give perspective.

That said, if your all-fiberglass hull is soft then I don't see any other explanation than it has failed within the layers by delaminating. Doesn't sound like that is really the case though from your explanation. Sounds more like typical oil canning from prolonged sitting on the trailer with a large amount of weight in it from the water.

Is your trailer a factory one or after market and are you certain that the supports are in the correct position for your boat?

You probably shouldn't jack your boat up from the rudder as you are asking for a problem of a whole different kind, maybe lifting from the keel would work?

I wouldn't lose too much sleep over what you have described. If it appears intact and doesn't leak, go sailing and see how the hull shape responds to being in the water for a while. If it keeps you up at night worrying then cut out the bad areas, feather the edges out at 12:1 and patch in new fiberglass.

Get some photos up when you can, it will help considerably with the advice you get.
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Old 06-06-2013
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Re: Very Large Hull Indentation

You already did the first part.. got the weight off. get some heat lamps try and warm the inside and out, see it it's finds it's "memory" If not get it even warmer and try to push it back into the curve it once had. If it doesn't want to stay.. cut some 1/4- or 3/8 strips of wood press them inside the curve, and use sticks to hold them and the hull back in place from the inside until it cools. Just sticks may puncture
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Re: Very Large Hull Indentation

Hi all, I've uploaded some images of the indentations, and I believe they speak for themselves.

I didn't know non-cored fiberglass could become delaminated (I thought either the fibers or resin would snap/fracture first) but that wouldn't be so bad because then I could just perform a resin injection + vacuum repair, which I have the equipment for. I've also considered adding some reinforcing fiberglass (possibly with a core in between) to stiffen the area, but I worry about creating high stress areas around the perimeter.

In the interior you can see there is a fiberglass covered plywood sole, but there used to be another plywood 'floor' overtop of that which rotted. You can see the perimeter of it where I cut the tabbing flush with the hull. I don't believe it was structural, but a part of me is still not particularly comfortable with launching and sailing the boat without a new floor. I don't want to install a new floor, only to find that the hull deforms again after launch and messes everything up, or that I have to tear it off to perform a hull repair I could do now.


I've done some tapping with various objects to try and 'hear' delamination (would expect a dull, hollow sound) but I haven't noticed anything. Mind you this is while it is still sitting on the support, the rudder-lift was just a quick test to see if the glass would bounce back (it didn't). Maybe if pressure was taken off, the layers would indeed 'inflate'. In any case, I don't know what failed here. There is no core, therefore there is no rot. I'm essentially dismissing the possibility of water-related damage. There are no cracks, fractures, tears, etc... however it is soft. What on earth can create such symptoms (or lack thereof)...?


Edit: looking at those interior photos I really have to think to myself.. why in the world did they NOT tab that entire rear bulkhead to the hull, geeeez!
Attached Thumbnails
Very Large Hull Indentation-indentation-exterior-port-1.jpg   Very Large Hull Indentation-indentation-exterior-port-2.jpg   Very Large Hull Indentation-indentation-exterior-starboard.jpg   Very Large Hull Indentation-indentation-interior-port.jpg   Very Large Hull Indentation-indentation-interior-starboard.jpg  


Last edited by superhornet59; 06-06-2013 at 09:51 PM.
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Old 06-06-2013
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Re: Very Large Hull Indentation

The bulkhead may not be tabbed because it would create a hard spot with no "give" The hull really doesn't look bad at all. I say, go sailing!
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Old 06-06-2013
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Re: Very Large Hull Indentation

I seem to recall reading this is fairly common on some lighter weight hulls. Most solids are not really solids, the resins in glassfibre qualify and will creep slowly over time.

I also agree that this does not look bad. As long as the glass is not cracked or spiderwebbing, I would not worry too much about it
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Old 06-07-2013
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Re: Very Large Hull Indentation

Go Sailing...
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Old 06-07-2013
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Re: Very Large Hull Indentation

So this is what our hulls look like without the interior... seriously we have solid fiberglass and no core, I would not be so worried. Fiberglass will do this under extreme weight and in time will flex back... I would apply reinforcement in the localized areas where it flexed to 'push' it back into it's original shape. Other than that enjoy the boat.
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