This is pretty long but hopefully worth the read.==========
We had a similar experience way back in 2002 crossing Lake Michigan aboard Peg Klienert's boat Pegasus overnight in a thunderstorm on our way from her home port of Muskegon MI to Chicago for the 2002 C320 Regatta.
The plan was to leave Thursday evening and sail overnight to arrive in Chicago sometime Friday morning. We left around 6:30 and the forecast was for possible thunderstorms. Sure enough around midnight the storms came. Wind clocked around to be on the nose so we fired up the engine and quickly doused the sails. Waves kicked up and we were pounding. They were pretty steep and I remember the nose burying into the water in each trough - sometimes up to the forward hatch. Not fun. We'd been doing this for I don't know how long but quite a while. The storms were predicted to blow through eventually. I went below to use the head and saw the bilge access floorboard FLOATING! The pump was running. We hadn't heard that on deck because of the engine noise and the noise of the storm. I alerted the other two and went looking for the source of the water. Meanwhile Peg reported that no discharge was coming out of the bilge output. I realized the water level was rising and shut off the bilge pump. Peg began pumping using the manual Whale pump.
At some point I noticed that the water was warm. Warmer than Lake Michigan. Very odd. It is hard to think clearly in that situation but eventually it dawned on me that the only explanation was engine exhaust water. I went to the aft berth and, sure enough, the water lift muffler was cracked and leaking. I reported this to the others and our mutual angst abated some (finally!) knowing that we could stop the boat from flooding by shutting down the engine. We needed the engine to avoid being kicked around by the storm but could throttle down to the point of maintaining steerage to reduce the flooding rate.
I came on deck to relieve Peg pumping. From that point on we alternated 10 minute pumping shifts while her friend manned the helm. I'd pump for 10 minutes while she huddled under the dodger to warm up and then we'd switch so I could huddle under the dodger to warm up. We did that for about 2-3 hours and then finally the storm let up. We raised sail about 3 in the morning and shut off the engine.
We had been roughly keeping up with the influx of water. Once the engine was off, the pumping made headway and the water level dropped. When I went below to check this, I saw that there was all kinds of crud floating in the bilge water - wood chips, spade terminals, electrical wire. We later concluded that a piece of the crud must have blocked the pump diaphragm from closing preventing proper pumping. We also concluded that the pounding on the waves likely cracked the muffler. And we concluded that the crud must have been construction debris that went floating once the bilge water level was high enough to float it - just like Graeme concluded.
At some point while we were in Chicago, Peg's friend disassembled the diaphragm pump and sure enough the diaphragm was indeed blocked open by a piece of crud. He removed that and reassembled. Fortunately it was a great Regatta and the three of us had a wonderful time Friday and Saturday. Peg decided to leave Sunday morning to give us plenty of time to get back to Muskegon. As it was I remember we got back about 1am. Peg's daughter and friend joined us for the motor back across Lake Michigan (no wind). At some point over the weekend, one of us made a trip to West Marine to buy a Beckson hand pump with a long hose. Keeping the RPM moderate, a functioning electric bilge pump and now 4 crew instead of 2 to man the Whale and the Beckson, we kept up with the muffler flooding and only had to pump occasionally instead of continuously.
A lesson from all of this. While you can't prevent your muffler from cracking in pounding seas, I recommend that sometime at the dock you purposely flood your bilge up the the floorboard to float your construction crud so you proactively can remove it all.
Thanks Graeme for sharing your story to prompt me to share mine.
Bill CulbertsonC320IA Secretary/TreasurerHarmony #859