The exhaust silencer located under one of the aft cabin bunks could be leaking water. On a boat the age of yours its odds on that the stainless steel cap at the lower end of the silencer has corroded through. The original volvo exhaust silencers from that era were constructed from a large diameter length of rubber hose into which a stainless steel cap is fitted at each end and are held in place with large jubilee clips. Because the silencer is usually angled downwards at the aft end, sea water accumulates there when your engine is turned off, and stays there until the engine is next run. For stainless steel to remain stainless and not rust through, it needs oxygen to enable the stainless to retain its protective coating. While your engine is stopped, the oxygen within the seawater gets used up, the atmosphere within the silencer consists of exhaust gas which has had all of its oxygen used up during combustion within the engine, and so during the days or weeks between the engine last being run and your next starting it, the stainless end is starved of the oxygen it needs and corrosion takes place where it rusts through from the inside.
Lift the aft cabin bunk mattresses, then lift the portable boards and locate the silencer. Make sure before you run the engine that the inside of the hull below the silencer is dry, then lay some dry toilette tissue under the silencer. Also open the port lights to that cabin to allow any leaking exhaust gasses to dissipate. Run the engine for a few minutes, but dont stand in that cabin to watch what happens as any gas escaping from a leaking silencer will contain carbon monoxide, “the silent killer.” After running the engine for a few minutes, turn the engine off, allow the cabin to ventilate, then go in and check the toilett tissue under the silencer. If its wet, then you have found your leak.
If it is the exhaust silencer that leaks, dont waste your money getting it welded up unless that really is the last resort. Much better to get a plastic silencer, (they dont rust and are very reliable) to replace the old Volvo one, and plastic ones are available at prices from around £70 upwards. A new volvo silencer = £500 or so, and will rust through again!!. At a push you could slap some glass fibre tape over the leaking end and smother it in resin, but if you do that also fit a carbon monoxide detector in that cabin, preferably just below the bunk support board.
Good luck with your investigations.
