Read it and learn:

http://www.practical-sailor.com/news...iftmuffler.pdf
(note the bit about the anti-spihon valve)

Cheers

JAXAshby wrote:

It is incredible what one can learn on a usenet group. Why, just today, I
learned wet exhausts need an anti-siphon valve installed. I bet tomorrow I
learn that sails move a boat forward "because they are curved on one side".