Mike Longcor (SV Trilogy)
My understanding of the anti-siphon on the raw water line (between the exit from heat exchanger and input to exhaust elbow), is to prevent continuous water flow (after engine shutdown) through the intake line should something in the line break/leak/disconnect/etc. The design of most raw water pumps prevents this continuous flow as long as the vanes of the impeller are intact. The flow of water out of the anti-siphon while the engine is running provides a good visual check that 1. your anti-siphon is working and not clogged and 2. that your engine is getting a good supply of raw water for cooling. I'm not sure I follow Oliver's comment about water standing above the engine for long periods because I'm pretty sure the water would find it's way down to the muffler with or without a siphon break at the top. Again, this assumes your raw water pump/impeller is fully functional, and those certainly fail from time to time. So a working anti-siphon is a good way to ensure your engine doesn't flood and/or your boat doesn't sink.
I may be wrong on some of this so if anyone has any other input please share.
Also... I'm still searching for any information on the rubber anti-return flap at the end of the exhaust line near the through hull. Has anyone bought, replaced, or fabricated something like this for their Amel?
SV Trilogy - SM23