Fitting stringers and bulkheads to GRP hull

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Petel
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Fitting stringers and bulkheads to GRP hull

Post by Petel » Thu Jun 07, 2018 9:52 pm

Hello all

First post from a new steam boater in the UK – be gentle with me! I am currently refurbishing my (new to me) boat after having discovered wet and rotten stringers and bulkheads under the floors.

It’s a 16’ GRP / fibreglass hull. My plan is to glue new stingers to the hull with thickened epoxy, add an epoxy fillet either side and then run some glass tape / epoxy along the joints. In order to help keep everything dry I’m also planning to epoxy coat the timber before fitting.

I then read lots of things on the ‘net about ‘tabbing’ or leaving the stringers and bulkheads clear of the hull on foam strips and just fixing them with glass tape and epoxy on either side. Apparently this is the correct way to do the job as it prevents hard spots on the hull which are liable to cracking….I’m now confused! Is this really necessary on a small steam launch?

A couple of questions:-

1 - What timber would be best for the stringers and bulkheads? I understand Douglas Fir to be good but maybe also one of the Mahogany family? Would I be better off with marine ply rather than solid timber? I’m looking for good durability I guess.

2 - Do I stay with my original ‘direct fixing’ plan or should I follow the ‘tabbing’ method?

Any advice on fitting stringers and bulkheads to GRP much appreciated, thanks!

Pete
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Re: Fitting stringers and bulkheads to GRP hull

Post by barts » Fri Jun 08, 2018 3:30 am

Check this series out: https://www.youtube.com/watch?v=Drc6zdlLGY0

Note that they do as much as they can in the same day for better adhesion.

- Bart
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Re: Fitting stringers and bulkheads to GRP hull

Post by PeteThePen1 » Fri Jun 08, 2018 8:21 pm

Hi Pete

I don't know if this will help but here are a few thoughts based on my experience of fitting out a glassfibre hull. I had no experience of glassfibre hulls before starting the project so I treated it as I would have done a plywood hull of which I had built three.

Bulkheads - I used plywood and did what you suggested. Cut a cardboard template then cut them carefully to match and bedded them on thickened Wests Epoxy. Once set I then ran glass tape up the join on both sides and wetted it down with resin. Happily the bulkheads have not moved or come unstuck, but some lockers that I fitted in the same way did. I think that was possibly down to not having 'clean' glassfibre where I tried to secure them. So scraping or wire brushing the hull before sticking may be useful.

Engine bearers - These were green oak about 3" X 4" which I shaped fairly early on but they were well past being green by the time I got around to fitting them. The same method was used but the wood, although shaped was not a perfect match to the shape of the hull. The beams were coated with resin, but perhaps not perfectly. Anyway, fixing was on thickened epoxy with a follow up of glass tape on both sides. The same was used for the cross members (not quite ribs). In use the boat got a lot of water in the bilges for one reason and another. The bearers soaked it up and swelled pushing up the glass tape on the outside edges. The inside edges held because the cross members did not allow movement on that side.

As for stringers I can recommend Douglas Fir. If you cannot get it in long enough lengths do not be afraid of splicing (scarfing) it with epoxy as your jointing glue. It works well.

'Mahogany' or more likely red hardwood of tropical origin will do nicely for fancy trim work. Also for decking you might consider mahogany veneer in 3mm or a bit thicker (see what is available near to you) but stuck down onto a plywood base. If I was doing the job again I would avoid coating the fancy trim areas with resin. I thought it would be good and hard wearing. I guess that I may have got the mix wrong or something, but the resin has degraded under the varnish and I have had to scrape a lot of it off. Very expensive given the cost of a can of resin.

Hope this helps.

Regards

Pete
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Re: Fitting stringers and bulkheads to GRP hull

Post by Lopez Mike » Fri Jun 08, 2018 11:41 pm

All good advice.

My experience has been that having foam strips between bulkheads and the hull is only important in a sailboat where you might be pounding in a seaway with serious loads. Especially a bulkhead that takes the loads from the rigging.

For the rest of the hull, I do some serious work with a disk sander maybe four inches wide on the hull and I precoat the bulkhead with epoxy (and cloth) and let it cure. Then I use wood flour to thicken the epoxy until it is what is called Thixotropic which means that it hangs on the stirring stick like mayonnaise rather than like peanut butter. It saves on epoxy if your bulkhead is a decent fit. The fillet should have a radius about that of your thumb.

After it cures, start with a narrow strip of tape or cloth maybe 2" and then one 4" wide. These last two steps can be done from the same batch of epoxy. Unless it's really cold I use slow curing hardener.

To make things clear, any work you can do such as coating and glassing bits and pieces before you glass then in place will save you an incredible about of time later. If I have a large sheet for something like a deck or a large bulkhead or even smaller bits, I cut it out, fit it and then glass it before installing it. Fussing about in tight quarters with wet cloth is just dumb. Also lay the cloth on dry over a freshly cured surface and then apply the epoxy.

Thee is a small trick to having your fillets smooth. There is a critical point as the epoxy cures where running your gloved thumb down the fillet will smooth it nicely. Too soon and you are just moving the stuff around and too late and it sort of tears the surface.

Also, disposable gloves are totally necessary! Once you get sensitized to epoxy you can barely be in the room with it. Maybe a mask and safety glasses though I have been careless about them.

I just went though building a hull working with a first class builder, Scott Hauser, and all of this is fresh in my mind. He worked for Sam Devlin, the stitch and glue guru, and invented many of the tricks in Sam's book.

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Re: Fitting stringers and bulkheads to GRP hull

Post by Mike Cole » Sat Jun 09, 2018 8:53 am

Hello Pete

For a boat in our size, just go with the direct fixing forget about tabs. I have a 17 foot west system plywood boat and the hull is totally rigid.

Mike
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Re: Fitting stringers and bulkheads to GRP hull

Post by Lopez Mike » Sat Jun 09, 2018 11:01 am

Not clear what you mean. No fiberglass tape at the joints? It might work but the tape sure adds strength in tension. I dunno how strong the filled epoxy is.
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Re: Fitting stringers and bulkheads to GRP hull

Post by DetroiTug » Sat Jun 09, 2018 5:41 pm

T'were me, I would mark out where the bulkhead or any layup on an existing Fiberglass or epoxyglass hull is going to be and prepare the surface in this manner:

Lay out the area, then degrease it with Acetone or Lacquer thinner (Cellulose thinners) and once dry, I would take an angle grinder with a cutoff wheel and score the surface about 1/16" deep diagonally about 1/8" apart all the way across, then I would score at 90 degrees the same all the way across the other way and make a cross-hatch pattern, this would only take a few minutes. This would give the resin some "tooth" and provide much more surface area for adhesion. Laying resin on an impermeable smooth surface is like trying to glue two pieces of steel together, there is nothing for it to hang on to. New resin isn't going to react and form any sort of chemical bond with the old cured resin, it just sees it as a smooth impermeable surface, it may stick and it may not.

That is why wood to wood joints are so strong, the resin can soak in to the wood, making the joint stronger than the wood itself. New resin won't soak in to cured resin.

Or is that what "tabbing" is? :)

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Re: Fitting stringers and bulkheads to GRP hull

Post by Lopez Mike » Sat Jun 09, 2018 7:06 pm

It's true. Old cured polyester resin isn't a good thing to try to bond with without some mechanical work. Certainly never depend on polyester resin for any sort of secondary bond.

I rebonded some bulkheads in my sailboat. The hull is polyester, of course, and dates from 1971. I used an angle grinder and took the finish down to where the weave of the old cloth didn't show any more. Then the wipedown with lacquer thinner, a fillet of wood powder filled epoxy to form a nice radius and, in this case, three layers of cloth. In order, 2", 4" and 6". These bulkheads take the loads from the standing rigging which in a knockdown have to carry loads that are a significant percentage of the seven ton boat weight. I don't know the righting moment of the boat but with 3 tons of lead centered down maybe 5 feet below the water line and the chain plates only five feet off of the center line, the forces must be significant at a 45 degree angle of heel.

The very next time out I was carrying a spinnaker in way too much breeze and took a slam bang knockdown. AFTER I had everything under control including my mind, I noticed that the rig was still standing.

I think you can probably get away with murder on a small launch. I've seen modern charter sailboats with some of the joinery bulkheads stuck in with caulk!
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