Making a bubble hatch.

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Ric

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My boat has a Gebo hatch on the deck amidships. I'm toying with the idea of making a bubble-hatch so I can look out without opening the hatch. One option would be to have a new one-piece acrylic moulding made. But this would be expensive, and tricky as I would have to unbed the existing hatch and then reproduce its exact dimensions, cut-outs etc.

A much easier option would be to buy an off-the shelf hemisphere (such as https://www.decoupe-plexi-sur-mesure.com/6-demi-sphere-bulle-pmma-plexiglass), cut a matching hole in the existing hatch, and glue the hemisphere in place.

This then poses the question of how to cut the hole. And would a glue be strong enough to hold the hemisphere in place? Or would it be better to use screws (with the attendant risk of introducing leaks).
 
A friend of mine tried to make one of these by blowing hot perspex sheet in a mould. The process worked reasonably well but he found that the bubble would have to be absolutely huge in order to get one's head far enough inside it to see out properly. iirc it needed to be something like double the height he'd imagined to work at all but then the diameter would be so big it wouldn't even nearly fit any hatch. A hemisphere was not ideal, the shape required would be less bowl than bell/tall arch Viewing through a perspex bowl means you are looking through the material at maybe 45 degrees or so distortion, already large is magnified greatly. Sticking one's head right into a 15 inch bowl in a seaway is not very comfortable either, you'd need some very good handholds.
With eye height an inch or two above the deck vis upwind is zero (due to heel) ahead is similar due to deck clutter and sheer and only limited view on the downwind side. Distortion was very high even before water splashes and salt further limited vis to the point it was quite useless.

I know racing boats use them but unless you can get the right shape in aircraft-canopy true perspex I doubt it's practicality on a yacht and even then it will scratch/craze easily and fast too. Think old worn motorcycle visor in the wet. I'm sure one of those french bubbles might work for a while but I'd think long and carefully on the feasibility and practical aspects before cutting anything!

Good luck though, the fitting would make an interesting PBO article!
 
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A possibly more usable location might be the main hatch (companionway).
I once saw such a 'bubble hatch' on a French boat that had been sailed solo from Cherbourg down to Malta, all the way round via Gibraltar Straits. The visibility was quite good all round and he could poke his head up for a look-around while still comfortable inside the dry cabin. I think that it would be cheaper and simpler to make a temporary sliding 'bubble hatch' out of marine ply with the 'bubble' screwed and sealed around a suitable hole. This can be a substitute for the regular sliding hatch for when at sea on a passage - assuming that there is no 'garage' for the hatch.
 
A neat idea I saw on youtube (possibly SY Teleport) was a sort of pram hood that couod swivel 360deg and open or close as much as required. It looked a bit like the roof of an observatory. Could be an easier way of achieving the same thing as a dome?
 
France is definitely the place to look for such a dome. The old Glenans Mousquetaire had one over the nav table, but it was glassed in, rather than being part of a hatch. It tended to craze with age. Most Mousquetaires now in private hands have had it removed and replaced by a flat panel.
 
Thanks for all the suggestions, but I have already discounted all those options. The only option for my boat is a bubble over the central deck hatch. It would be in the perfect position and height for me to view outside in all weathers. I just want to have a better idea of best structural means to build it. I am just interested in whether anybody else has built a bubble into an existing hatch.
 
I once thought of doing this. I’d have used one of these
http://www.sunlightplastics.co.uk/acrylic-domes/
which are not expensive. My instinct would be to polish the existing flat Perspex ‘glass’, cut a circular hole in it with a jigsaw, and use acrylic glue to adhere the flange of this dome on top of - or beneath? - your existing plane. The joy of acrylic ‘glue’ is that it’s apparently not actually a glue but is molecularly the same as the acrylic (= Perspex) material of the sheets you’re gluing, so you get a strong and totally transparent bond. I used it to fashion a replacement companionway hatch. No, I am no chemist.

But they invite calls so I suspect they’d have a better idea - which could be to take the dimensions of your existing hatch and make the flange on their dome that size, so you’d effectively have exactly the dome hatch you dream of.

No connection; I have never spoken with them.
 
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In the best tradition of facile unhelpful replies... 'In defence it was when a respondent mentioned bell-like sections-
 
BelleSerene said:
I once thought of doing this. I’d have used one of these
http://www.sunlightplastics.co.uk/acrylic-domes/
which are not expensive. My instinct would be to polish the existing flat Perspex ‘glass’, cut a circular hole in it with a jigsaw, and use acrylic glue to adhere the flange of this dome on top of - or beneath? - your existing plane. The joy of acrylic ‘glue’ is that it’s apparently not actually a glue but is molecularly the same as the acrylic (= Perspex) material of the sheets you’re gluing, so you get a strong and totally transparent bond. I used it to fashion a replacement companionway hatch. No, I am no chemist.

But they invite calls so I suspect they’d have a better idea - which could be to take the dimensions of your existing hatch and make the flange on their dome that size, so you’d effectively have exactly the dome hatch you dream of.

No connection; I have never spoken with them.
Click to expand...

That is what I was thinking to do too. I'm just a bit worried how strong the join would be.
 
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BelleSerene said:
That is beautiful. I wonder how much...?

BTW, I might have added to the OP: another beauty of working with Perspex/ acrylic is that you can smooth its edges by hearing them. So once you’ve jigsawed a hole, by all means apply a file but then run a flame past the rough edge (swiftly!) and it magically becomes smooth and transparent.
Click to expand...

I suppose you'd have to ring them for a quote but some approximate prices -

http://www.sunlightplastics.co.uk/acrylic-domes/

http://www.hlnsupplies.co.uk/acrylic-domes-hemispheres

so a 900mm dome for about £120 ?
 
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