The exhaust hose on a used boat I'm looking at runs out through the stern through-hull about a foot above the waterline. The hose runs from the stern about 8 feet inboard and mostly below the waterline to the rest of the raw-water exhaust system. I was pretty surprised that there's no siphon break here. Is this safe? Or do siphons breaks only need to be fitted on below-the-waterline through-hulls?
One foot above the static waterline is often underwater on heel so a siphon can easily occur. The exhaust should be looped sufficient to ensure the top of the loop, at which point the siphon break should be, is above the waterline under any condition.
You don't need a siphon break in the exhaustline. BUT you should run a large loop as far above the water line as is practical to avoid flooding from following seas. You DO need a siphon break in your water injection line attached to the exhaust elbow.
I suffered from a sticky plunger and spring in my break, which was otherwise to spec. It convinced me that a bulletproof method would be simply to clamp a length of hose onto where the plunger and vent device was, and to run that up to deck level or better. It's on this winter's to do list.
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