Hatch landing on grp deck.

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I have got a hatch ready to fit but the deck is not quite flat.When i put a straight edge in the hatch position it rocks very slightly, maybe 2mm. What is the best way to beef up that area and make it perfectly flat.I'm guessing a marine ply frame fiberglassed into the existing deck? (Position in red line area).

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View attachment 41426
 
For 2mm I would possibly go down some 'plastic' washers between hatch and deck on the bolts and use a high quality sealant. Thickend epoxy is best but a lot of mess for a small gap.
 
It is really important to get it flat but a mm gap here or there does matter (the sealant will fix that). What you don't want to do is bend the frame because the acrylic may not seal too well. When I installed mine I overfilled any gaps with poly/talcum "bog" and then sanded it with a orbital sander so that it was close to perfect. My hatches are similar to yours but mine have 4 handles. If water seeps in around the handles they can be "nipped" up a bit and I found that helps
 
Seeing as the hatch is smaller than i originally planned i reckon i will get away with sealant straight onto the deck so which? I'm thinking one of the Sikaflex range?
 
No expert but I have recently fitted a new hatch on Rose. My yard manager saw me messing about with Sikaflex and stopped me before I had made too much mess. He gave me a reel of some sort of 'special' tape. It looks like self adhesive draught excluder.

It is called something like Gorilla tape.

You simply strip the backing of and fit on underside of hatch, then put hatch in place and make small holes through the tape with a pointy screwdriver and bolt it on. It works a bit like the self amalgamating tape, the ends bond together and the holes seal under pressure. It is about 4mm thick and takes up any imperfections. Brilliant, clean and quick to use. Yard manager, who is a boat builder and master shipwright, tells me that 'no uses sealing compounds anymore for windows and hatches'.
 
No expert but I have recently fitted a new hatch on Rose. My yard manager saw me messing about with Sikaflex and stopped me before I had made too much mess. He gave me a reel of some sort of 'special' tape. It looks like self adhesive draught excluder.

It is called something like Gorilla tape.

You simply strip the backing of and fit on underside of hatch, then put hatch in place and make small holes through the tape with a pointy screwdriver and bolt it on. It works a bit like the self amalgamating tape, the ends bond together and the holes seal under pressure. It is about 4mm thick and takes up any imperfections. Brilliant, clean and quick to use. Yard manager, who is a boat builder and master shipwright, tells me that 'no uses sealing compounds anymore for windows and hatches'.

Can anyone find a link for the Gorilla tape? I don't doubt Sophie as Gorilla do all that sort of thing but i can't find any product as described? It sounds ideal if i can get it.
 
I reckon sealant is no substitute for a proper, firm, flat base, whether wood, epoxy or whatever.

I suspect that either a mastic sealant or tape sealant will stop water getting in (at least initially), but would not provide the support across its whole width that the hatch frame will need. Eventually the hatch will be stood on, and you don't want it either pulling up on the opposite side to which it is being stood on, or worse still being bent over the curve in the deck.
 
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