We took our circular portholes out when relining the cabin. Getting them out is not too difficult - just drill out the rivets and lever off the two semi-circular backing rings inside the cabin and the main porthole fitting should come off after any silicone sealant is broken with a sharp knife. Remember to mark the backing rings so you know what position to put them back in if you want to use the same rivet holes, otherwise you can drill new holes later. Our perspex was fine, so we didn't replace them, but if you did, I would have thought that a local signmaker should be able to make new ones if you took the old ones in as a template.
On ours there was a fabric seal of some type between perspex and main porthole housing, which was pretty shredded and needed replacing, but I couldn't figure out how to get the perspex out without breaking it and so left it for another time. It doesn't seem to make any difference as far as watertightness is concerned, although there is a slight draught, but as a bit of ventilation is never a bad thing it doesn't bother us too much. The whole operation of getting them off and on is not too difficult, so if/when I get a bit of time, I might give it a bit of thought how to replace that bit of fabric.
Reassembling it is just all of the above in reverse with new rivets and silicone sealant, make sure that the little drain hole in the main housing is at the bottom on the outside so that any water that collects at the bottom of the porthole has a way of getting back out.
I would have preferred to replace ours entirely, but couldn't find anywhere that still sold that type - if you find somewhere that does, please post its whereabouts.
Actually this turned out to be quite a nice job and not too difficult. We used the method described in the post above.
We replaced the old opaque perspex with tinted plexiglass. everything got cleaned and lubricated. The old fabric seal was perished so we dug it out and replaced with window rubber. we discovered 4 screws holding the housing together. Once we had unscrewed these the whole thing came aprt for easy removal of th eold perspex.
Hubby only lost one finger and the swearing was kept to the minimum. Nice job done!
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There is NOTHING--absolutely nothing--half so much worth doing as simply messing about in boats.
Kenneth Grahame