distance between wooden fittings and hull in GRP boats

[MarkGrubb]

Hi all,

I'm refitting the galley and nav table in my boat. I've made a new top of the nav table and a new galley top, as well as shelves underneath. All out of 12mm exterior ply.

All tops and shelves are supported by battens on three sides, which are screwed into the surrounding bulkheads.

The boat's fibreglass hull is on the fourth side. I've made the tops a very snug fit, so there is very little distance between them and the hull.

However, I was reading that when fitting wooden bulkheads in a fibreglass boat, there should be some clearance (around 10mm) between the bulkhead and the hull, otherwise their will be a hard spot, and stress marks, cracks, or gel coat crazing may develop because the hull can't flex properly.

My question is should this rule be applied to all fittings that butt up against the hull? Should I cut 10mm off the depth of my table tops to allow clearance for the hull to flex?

The woodwork is currently in my shed for sanding down and varnishing, so alterations can be easily made.

Advice and comments appreciated.

Thanks,

Mark.

 

[allangray3]

Recently carried out the same refurb to my boat, and under instructiom from old boatbuilder left 3/8 th gap between hull and fittings to allow forflex but told to use uPVC window backing rod in gap to provide flexible foundation to stop the inevitable creeks. Backing rod held to fittings by silicone mastic or similar. Hope this helps.

 

[oldsaltoz]

G'day Mark,

The gap left around most bulkheads when installing is to ensure access to fully seal and fill them, more glass is added to stiffen the area thus avoiding stress cracks.

Installing cabinets made from 12mm ply could cause stress cracks if fixed hard against the hull, particularly just above the water line. Even some of the older 'Built like a brick' glass boats have suffered stress cracking from this type of fit out.

A gap filled with a foam rod will work, but you have no real seal of any strength.

When the new storage area has to be fully sealed, the hull to item area is treated much the same as a bulkhead, though some prefer to just lay extra glass along the hull with a 2 to 3 inch overlap between layers to spread the load.

Hope this helps.

Avagoodweekend......

 

[captainslarty]

Hi
Just done very similar work to Ruddles - new seating and centre cabin rework.
5 to 10 mm is fine - we just fixed the base of the uprights to the floor, made sure we had flex clearance, then glassed the bulheads to hull with 3 layers of 300 gsm CS mat.
BUT !!!.. it is good to know the flex in your hull.. for instance, we get some flex around the pilot house area and side decks.. the original design does not include and tabbing to these underdeck areas.. just clearance. Thats what we continued with.. all areas underdeck are non fixed allowing the main flex.
Look at the originals.. dont attach to anywhere where the originals were obviously left floating.

Hope that helps

Joe n Jayne

 

[MarkGrubb]

Hi all,

Thank you for your comments. All very helpful. I'll allow some clearance and fit the same way as the originals (very useful point Joe n Jayne)

Mark.