Sealing a boat hatch

[Chris Green]

On our boat Odin there's a hatch on the bathing platform which currently lets water in (to the inside of the bathing platform). This isn't disastrous in any way because the bathing platform was added by the previous owner and thus its 'inside' is outside the hull of the boat proper. However it is rather annoying and (as we discovered recently) a *lot* of water in the bathing platform does actually upset the trim of the boat slightly.

So I'm thinking of the best way to seal the hatch. The difficulty is that it's not a proper hatch with any sort of rim, it's just a flat plate bolted down onto the surface of the bathing platform. There is a slight recess in the bathing platform, presumably with the intention of making the whole thing flat (i.e. the recess is 4mm or so and the plate is 4mm thick).

As I said the plate is (about) 4mm thick and the bathing platform 'body' is also 4mm thick. The plate is 500mm square and there are 8mm bolts all the way round at about 75mm intervals.

My thought is to get a sheet of neoprene or similar that's 500mm square and stick it to the bottom of the plate, make holes for the bolts and hope it will seal. I'm thinking of 3mm thick.

The question is whether a full sheet of neoprene is necessary. Would I get away with a strip (say 25mm wide) with joins at the corners, can one seal joins like that satisfactorily? Or is there some other sort of sealing strip that might be appropriate?

Any/all ideas would be most welcome.

[Bob Minchin]

How about laying down a thick continuous bead of neutral cure silicone on the deck portion of the seal, grease the mating surface of the plate and fit the plate but don't tighten the nuts much just enough to squeeze out a little sealant. Clean up excess inside and out. set aside a blob of the excess so you can tell when it had fully gone off (neutral cure is often slower than the common acetic acid stuff). Remove the hatch, clean off any grease and re fit tightening down the nuts this time.

[Nightjar]

If it is an entirely separate structure, does it really need to be watertight? If you drilled a few 4" diameter holes in the bottom, would it affect the boat in any way?

[Chris Green]

Well no, except that the back end of the boat would sit a couple of inches lower in the water. As I said it doesn't *matter* if it's got water in it but I'd prefer that it doesn't.

[newshound]

Or wrap the plate in a couple of layers of cling film, or kitchen foil, or greaseproof paper.

[Brian Gaff]

I'm a bit confused why the hatch exists in the first place. if this structure is hollow why did they not put the sheet of whatever its made of over the whole platform? Brian

[Bill]

In message , Chris Green writes

Our boat has a full-width cabin (ie no side decks) and we have guard rail stanchions mounted on sort of inverted L-shaped plates made from scrapyard stainless steel. These are bolted through.

I used the strip stuff sold by Aldi as garage wall car door protection between the plates and deck, and it seems to have lasted and stopped leaks through the bolt holes. I don't see why it should not compress enough to seal at the joints, especially if they were not exactly at the corners and with perhaps a bit of gunge.

[Chris Green]

It's about 3 metres by 1 metre so a sheet that big would be rather clumsy and, if made of 4mm steel, rather heavy too I suspect.

[Chris Green]

OK, thanks, that sounds a possibility.