Trailing Exhaust extensions

I thought about that at the time, and even the engine cylinders are way above the waterline by about 8 inches which would be the lowest point in the system.
So no siphoning, it never has been a problem. It could be a problem on a sailboat where the engine is a lot lower then the waterline.
What could happen is shutdown and as it cools, it could draw some water into the big round waterlift muffler. But not enough to do me any harm.

I have backed up hard enough and never did those Black HDPE pipes fold up,
Nothing untoward has happened except for loosing one of them in rough water.
When backing they just bend away about 90* angle. They are not floppy in a short 3 foot length, but rather stiff.
I will put them back on but secure with a couple screws so they cant fall off again.
About 15 years ago, at the Atlantic Yacht Basin, I saw some old boats with 4 foot long thick rubber hoses screwed on to the exhausts. I have not seen any such thing since. I think what they were accomplishing was similar to mine own idea here.

What really happens on hard backing is they flop over at 90* or so and dont present an open mouth to the water, so water does not flow up into the pipe.

 

Thanks sdowney, I've been considering a design quite like what you've been experimenting with. I'm going to try something similar.