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Re: Chasing down a persistent Porthole Leak
The porthole may not be leaking at all.
Before the next rain, cover it completely on the outside with plastic and make sure that's well taped down.
Now, if the water is coming from someplace on the deck, and simply running down to the place where the porthole allows water in the deck to exit into the boat...you'll find out.
But there's often only one way to really fix a porthole leak if it isn't the gasket. Remove, restore, rebed the entire thing and make sure the core around it is also solid and dry.
You might want to pressure test the entire hull to see if there are any leaks, portholes or deck included. Close up all the openings, put a piece of plywood or corrugated cardboard in the companionway. Stick the back of a shopvac or a leaf blower through that cardboard and let it run. It will build up a little positive pressure in the hull. Now you throw soapy water all over the deck (and that will make it dangerously slippery, be warned) and you look carefully for any spots that are blowing bubbles. Could be a cleat six feet away from your porthole is where the water is getting in. You'll see the bubbling that confirms it.
There are ALWAYS leaks that you never suspected, so this is not a waste of time.