We took out an old port seal today to test whether our replacements will work. The old seal was very perished, just touching it resulted in black bits dropping off. It came out from the recess easily enough with a gentle pull so as not to break it. There were patches of black rubber that remained stuck to the frame of the porthole and in the recess; we cleaned these out with by running a chisel along, followed by wire brush, then a final clean with EvoStik adhesive cleaner.

We measured the length of new seal to be 30" (our boat was built in the days of imperial measurements), cut the required length from the new Perko weatherstrip we'd bought and superglued the ends together to make a continuous round.

Then EvoStik contact adhesive painted onto the rubber seal and the bronze port, wait for it to go tacky (which was as long as it took me to do the painting on which was quite a messy job as the glue kept going stringy) and then a two person job to fit the seal into the recess.

The porthole was carefully closed so as not to knock the weatherstrip out and fastened closed. We'll now leave it a week for the weatherstrip to form into the shape of the recess correctly.

And if all looks well after that we have another 13 rectangular ports and 1 circular port to do!