Hull major soft spot repair

Hi guys,
Currently I’m trying to safe my beloved Shark24 from the chainsaw. She is a small sailboat of 2100 lbs. (950 kg) build in the early 90s. The hull is a 100% hand laid fiberglass laminate with a thickness of about 0.2’’ (5 mm). The deck and the inner shell are partially built with an unknown foam core.

The hull suffers from some large soft spots. The weakest area is located in front of the first bulkhead from astern. The main stringers are connected to this bulkhead. These stringers are made out of hollow fiberglass. I am concerned the stringers were not placed right by the boatyard, because there is a gap between stringer and hull. The only connection is some layers of fiberglass along the stringers. I was told usually the stringers were bedded in thickened resin so there shouldn’t be any gap.

People told me to cut my boat into pieces, but I want to safe her badly. I’ve put a lot of effort into fixing the soft spots at the bow and I want to teach my kids how to sail on this boat, as my grandfather taught me on her before. Some advice how to fix this would be greatly appreciated. I’m not in rush my son still needs three years to learn to swim so I don’t care if a repair takes a lot of time/effort.

A picture paints a thousand words, so here are some photographs, a small sketch of a cross section and parts of the construction plan of the boat from the early 60s. I have grinded away parts of the fiberglass along the stringer as you can see in the before-after-pictures. The hull laminate underneath the stringers is so soft, that I can push it from the outside against the stringer / inner shell without any pressure.
Please bear with me if my English isn’t perfect. I’m not a native speaker.


starboard connection of the stringer (before)


inner section of the stringer / inner shell (before)


overview starboard bulkhead and stringer (after)


starboard stringer / hull 0.4" gap (after)


starboard stringer / hull 4.7" depth (after)


cross section


plan #1


plan#2

 

Hi Fleix, and welcome, I suspect your yacht is plywood sheathed in fibreglass. If so it is probably best to buy another Shark that is 100% fibreglass and use your parts., because the rot will keep creeping up. If your boat is all fibreglass screw or bolt and resin the stringers back down, sand and clean the whole soft area and glass it up with a good quality woven mat, remove the screws and fill the holes with glass fibre and resin. And good luck.
PS. I just read Blueknarr,, .. so then relaminate inner skin to outer every corner of a 200mm grid cutting 50mm squares[or round] through the foam for the joins in the area affected is how I think I would do it,. cheers

 

Welcome

Bad news and fatal prognosis: you're emotionally attached to a sailboat.

The good news: its design has a proven great track record.

Solid wood stringers need to be fully bedded. Box stringers, like yours, are not bedded and utilize tabbing as part of their structure. The foam interior's sole purpose is to suspend the wet FG until the resin cures.

Why your vessel is plagued with delamination is a mystery that probably won't be solved. My guess is that a stack of core foam was contaminated.

Have you found and fixed the last of the delamination? I certainly hope so. Is your emotionally attachment to this boat fatal? Well we all must have our flaws.

You fixed the other delamination, your work so far on this one looks excellent. You know what needs doing. Take care and be proud of passing on a legacy that your children will enjoy.

 

Thank you for your response!

My shark is about 100% fiberglass. Only the bulkheads are made out of plywood. There is no foam either in the stringers so luckily there is no rot at all. Foam core was used exclusively in the deck and some parts of the inner shell but not at the stringers. The hull is build out of pure fiberglass and polyester.

From my point of view there never was a real connection between the hull and the stringer only this tiny strip of laminate which is marked orange in the cross section. So I don’t think bolting the stringers down would be the right way to repair this.

I’ve attached a picture of the inner shell of a newer shark. The shell is upside-down and you have a good view of the stringers. My boat was build similar to this version.

In the second picture I’ve marked the edges of the stringers. Between the red marked edges and the hull there is a gap of 10 mm. I was told by someone who refitted over 5 sharks that this gap shouldn’t be there. He said originally the stringers were bedded in thickened polyester and it appears that in my case the boatyard just forgot it / was too lazy. I have also marked the inner edges of the stringers green. I have no idea if or how they are bond to the hull.

In the original plans of the shark there wasn’t any gap at all between stringer edge and hull, so they were only connected by tabbing. Apparently they had troubles fitting the inner shell to the hull so they used thickened polyester to fill the gaps.

From my point of view the way my shark was build created some hard spots which led to the delamination. I’ve marked this in the cross section.

I haven’t fixed the delamination of the stern yet. I have to open the inner shell first to reach the hull. I want to do two cuts along the cross section. One cut close to the bulkhead and the other one as close to the front as possible, which is about 100 cm of the bulkhead. I hope to get this part of the inner shell out in one piece. Afterwards I hope to get a good overview of the mess and have enough space to grind down the damaged laminate and build up a new one. Following this, I’m planning to fill all the gaps between the edges of the stringers and the hull and finally get back the cutout, place it in thickened epoxy and laminate it back in.

Yes I’m emotionally attached to this boat but I’m not completely naive. I’m giving my best to fix it. If that’s not enough I will keep on sailing her until she falls apart. The boat is rock solid around the keel, the mast and the chain plates. If this wouldn’t be the case I wouldn’t bother fixing it.

Inner shell / stringers




cross section: left current situation, possible hard spots. right possibility to fix it?

 

Too bad the warranty has long sense expired.

How did you get that incert out in one piece?!?

I agree that the incert was not installed correctly. It didn't prevent the hull from flexing. As the hull flexed it tugged on the inner skin where or was semi attached and caused the inner skin to delaminate.

If you also have hard spot issues; they will be manifested as cracks in the outer skin at and parallel to the stringers.

Your solution is mostly sound. I would be hesitant to reuse old inner skin. It would be easy to insufficiently tie it to the undisturbed skin and have voids under it.

Keep up the good work

 

Thank you for you encouragement!
I didn't get the inner shell out. It's a picture of another shark being build in Poland at the new home of the shark mould.
The hull suffered from many cracks in the bow area along the stringers because they weren't bond properly to the hull. This boat must have been put together on a Friday evening... The boatyard produced only a few sharks before the mould switched places again.

 

Friday evening ... or Monday morning after a three-day week end.

 

So is the flexing due to the hull skin becoming detached from the stringers? Or is is a bigger problem, like the hull laminate sheering in two. Resulting in two thin laminates next to each other instead of one thick laminate.
To find out, you kind of have to cut a piece out and see if the sample is one or two pieces.
SHC

 

I am a little confused. Perhaps I am missing something.

In the picture of your cross section, you clearly show the soft section, but you don't show a repair going from the soft section to 'hull intact' on the sides.

Perhaps your drawing is not showing the verbage, but I don't understand how you expect to succeed only fixing the bottom if the soft section extends up to the sides.

And it is late and I am a bit tired.

 

Not sure I understand if the hull is soft on the inside (because of the inner liner), or soft on the outside (because the hull itself is too thin).

 

My take is that the bottom was poorly built and that weakened the fiberglass.

The problem with the repair is that at least in the cross section; he does not appear to be addressing the soft area adjacent to the intact hull.

Unless it is implied or I missed it, he must glass all the way to intact hull sides.

The soft hull area cannot magically become stiff by the bottom work if it has mechanically fatigued.

And as drawn, he can make the bottom stiff, but the loads will transfer to the two weakened sides.

The only way to properly fix would be to add new glass of sufficient strength well into the 'hull intact' areas. Personally, I would assume both inside and out, although inside may suffice. And then the issue is always peanut butter and jelly rule. He would be repairing soft material with harder material. And this can be done, but only with sufficient glass to overcome the existing weakness. (Soft material as in damaged and weakened glass)

 

Tanks for your replies.

From my point of view the hull laminate flexes because it was poorly attached to the stringers. I can't hear any cracking sounds when I'm bending the laminate. I'll take a closer look at the soft spots after I've opened the inner liner.

Well the hull is 5 mm of solid fibreglass. This laminate is entirely soft. The inner liner is still good.

Sorry for the confusion. It's crystal clear to me, that the soft spots have to be fixed first. I didn't point that out in the cross section but mentioned above, that I'll have to open the inner liner first. Afterwards I'll grind down the damaged laminate and rebuild it.

I hope I'm getting away with fixing this from the inside. At the bow I had to work from both ways and it's a big pain in the behind to get the fairing of the outside hull done properly. Grinding overhead is one of my least favoured jobs... If I have to fix the stern laminate from the outside as well I'll have to get something light like a Mirka because my Rotex 150 is killing me.

 

I'm thinking that if you had 5 mm (0.2") of good, "solid" fiberglass, then it should not feel soft. Just wondering what you will find as you start grinding into the hull from the inside ( areas of dry fiberglass maybe ?)

If I were doing the work, I'd want to know more about why that area of the outer hull is soft to begin with.

Do you have any pictures to share of the work that you did on the bow section?