Bulkheads

[rajjes]

Hi all,

I'm replacing the bulkheads on my GRP folkboat as part of an interior refit and would like some help on the following:

1. What plywood thickness should I use for the structural B/H? (Best compromise between strength and weight)
2. What thickness for non-strucutral B/H (partitions) such as galley side?
3. What's the best material to use between plywood and hull to avoid hardspots? Or can I leave a slight gap (use wedges) and tab over?
4. How many layers of fiber (alternate roving/CSM) should I lay for the tabbing of the strucutural bulkheads? (I'm using polyester not epoxy$$)
5. Last one...Is there another way of removing the old tabbing apart from grinding...I'm still itching from last week

phew...quite a list...anyway, thanks in advance for your help!

 

[tr7v8]

A long discussion ran on this a few weeks/months ago. Suggest you search the forum for bulkheads.

Jim
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[30boat]

In my experience the best way to remove old fiberglass is by using a wood chisel and a heavy lump hammer.The chisel should have a plastic handle as these can take a lot of punishment and it should be of good quality or else it won't hold the edge for long.Old tabbing is particularly easy to remove with this method because it stands at right angles to the hull and if you wedge the blade under it it will peel off .
These days I never grind if I can avoid it and you'll find that once you get the technique right you can make very good progress.Be carefull not to dig into the hull though.

 

[LadyInBed]

Re: Removing old bulkhead

Get a Black and Decker electric saw (Alligator). It will cut through without having to drill pilot holes.

 

[snowleopard]

if you must...

use ply, 12mm for structural bulkheads, 6 or 9mm for furniture (6 where there's no risk of putting a knee through it in heavy weather.

whatever method you use to remove the old bonding, there's no getting away from the final grind!

the danger with ply & polyester is that the joint will fail in time if allowed to get damp. this can be reduced by painting the ply with cobalt accelerator before applying the bond, also by cutting 25mm dia holes close to the edge of the ply and running tows of glass through it to the hull side, thereby giving a strong mechanical bond.

a strip of 12mm airex (pvc) foam between the bulkhead and the hull, cut to give two 135 deg angles instead of one 90 deg should give all the shock absorbing you need.

for the bond on structural members, 300gsm CSM + 500 GSM biaxial cloth laid with the fibres at 45 deg so every fibre crosses the join. first pair of laminates 75mm either side of the join then the same again 100mm either side. for non-structural, omit the second pair.

personnally i would never use ply bulkheads again. i've recently built a 40ft cat with foam sandwich. the material is 25% of the weight of ply, a lot stronger and there are no wood-grp bonds to fail.

i won't bore everyone with details of the technique but pm me if you want to know more.