I installed a seawater pump for a new AC unit to an existing 3/4 inch thru hull valve that goes to the head.I noticed a slight amount of water seeping around the wood backing plate and was wondering if there is a way to temporarily seal this(until next outhall).Is there a compound that could be placed around thru hull that could be used underwater. If anyone has tried this I like to here how it worked.
This is the big problem with trying to replace a valve on a thru-hull, which is not an ideal situation anyway. When tightening the new valve, with mismatched threads, you must be 100% certain you do not allow the thru-hull fitting to move. If it does, you break the seal. The other issues with older Oday boats is that they very often used unsealed plywood as a backing plate. This eventually gets wet and can no longer support the thru-hull the way it needs to to keep water out. I have an Oday 30 with your same situation right now and over the winter she is getting all new "proper" flanged base seacocks with solid glass backing plates.
Your situation is not unique and at the age of your boat can be somewhat expected.
You can try and band-aid it with a mess of polyurethane sealant, if you can get it dry enough for it to stick, or haul out and fix it..