Fixing a new bulkhead

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beerbiker21

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Hi

I have the unenviable task of replacing a rotten bulkhead. It is under and supports the mast The question is..."how to I seal it to the deck above"? I will be using polyester (plus G4) but research has offered a number of ways of sealing the gap between the ply and the deck. I have seen options such as a strip of wood (trapezoidal!), seal with polyester putty, seal with closed cell foam and others. Does anyone really know the best way to ensure the compression forces are transmitted down whilst avoiding "hard spots". I feel that using polyester is the best way as it is more flexible and compatible but have been advised by a professional builder to avoid using both a wood fillet and foam together but would like other opinions. :confused:
 
You are supporting the weight and loading of the mast, so the deck needs to be fully supported. That means it must be solid, so do not use foam. Timber is probably your best material as it has slightly move give than fibreglass. That does not mean you cannot glass the bulkhead to the deck, but you do not want any gap between the deck and the bulkhead. If bonded in correctly, then there should be no problem of hard spots. I would recommend having a plywood pad that is bonded to the deck to spread the load and make sure you have a tight fit on the bulkhead under the mast area.
 
Is the deck solid or sandwich in the way of the mast step?There will be one hard spot which is directly under the mast.The connection should be as hard and direct as possible.I would glue a wide ish strip of hardwood about 20mm thick to the underdeck with epoxy putty making sure the voids are all filled by the mix and then laminate over.Then fit the bulkhead under it and hold everything in place with a rovings strip and epoxy, The strip of wood,plank if you will, will spread the loads to the bulkhead.
Edit. Sort of what's said above.
 
Banana...ooops..balsa sandwich! It has a large ply pad under the mast already which sits over the b/head and the the ply sections for'ad which make a vertical box section support
 
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beerbiker21 said:
Banana...ooops..balsa sandwich!
Click to expand...
In that case check that there's a hardwood insert to take the compression forces.If not there remove a square bit of the lower skin of the deck , scoop the balsa out and bed in a hardwood pad with epoxy putty or thickened epoxy.
 
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2 sheets of ply with any reasonable core provide pretty impressive compressive strength. It can easily be beefed up with with some timber in the appropriate places to carry load on through to the keel. Sealing to the deck should be easy and can be done with a bit of wood the same thickness as the core simply being glued with epoxy. Alternatively a fillet of thickened epoxy can be used and then a small strengthening strip of glass fibre in epoxy or both for overkill. You don't want any flex.
 
Beerbiker21

I think you have answered your own question now. There is a ply pad to spread the load. All you need to do is replace the bulkhead. Just out of interest what is the type of boat.
 
Well, the combined wisdom plus questions has provided me an answer and they have enabled me to comprehend the set-up. Reading and thinking only get you part way.

Cheers and thank you all

P.s. It is an Ocean Winds catamaran, the bigger sister to the Heavenly Twin designed by Pat Patterson
 
A catamaran will next extra good support under the mast to transmit loads of the mast to the keels and also to the chain plates. I don't know what thickness of ply you have in mind but thickness even 2 layers of ply with a foam or similar core will combat the tendency to buckle under load.
If you have a robust cross beam under the cabin floor then the ply can be thickened in the straight line mast base to beam. As said it is only the area under the mast base that will take the real load.
Polyester resin is not the best to lay on wood. Epoxy is far better for adhesion. Polyester can be OK if it is built up very thick so that rather than describe it as a plywood bulkhead with GRP skin it is better if you can describe it as a GRP bulkhead on plywood male mold left in place.
So it may be easier and more practical to use the existing rotten bulkhead as a mold to lay up in place the GRP bulkhead over it. In this case polyestr may be OK as adhesion is not so critical. But even though more expensive epoxy is a better adhesive.
You might also consider carbon fibre cloth in lieu of glass. (epoxy or polyester) With much greater stiffness. But still needs thickness for the geometric stiffness.
Perhaps I am just waffling on but do not under estimate the mast down load of a cat at the point of lifting a hull in strong winds. good luck olewill
 
That's a great insight, thanks. I have been weighing up the options re. polyester or epoxy and and aware of their differing problems/properties. I have seen some ideas on putting holes or slats through the ply and lay polyester thro' those to provide extra bonding/strength before doing the main glassing. I will look again at the boat structure when I am next on board with your comments in mind. There is a crossbeam but this consists of two extra lengths of 1" ply giving a crossbeam width of 3". Replacing those does demands the use of epoxy there. Certainly your comments on load transmission give an helpful insight..not least as I m having to do some work on the chainplates as well! Big ty.

Ultimately it will be do it well...do it once!

The cost will reflect itself if and when I come to sell in a good few years

P.s I will be using G4 between the poly and the wood
 
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Re the epoxy, West have a great document here which you may already have found:

http://www.westsystem.com/ss/assets/HowTo-Publications/Fiberglass-Boat-Repair-and-Maintenance.pdf

Very good documentation full of useful tips. I'd go Epoxy throughout to reinforce and glue in the bulkhead, it far stronger and has far better secondary bonding properties.

FYI a good rule of thumb for the compressive loads of the rig is the displacement of the boat. Roughly 3 tonnes on my Ballad for instance, so work your material sciences based on that. Good luck!
 
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