Syphon break.

[ccscott49]

Re: Sorry if you thought...

Believe me it must produce a seal, or it won't pump. I didn't take it badly, even if I did bite your head off, sorry!

 

[vyv_cox]

Re: Sorry if you thought...

Positive displacement pumps rely on close or zero clearance between the rotor(s) and casing for their function. These are self-priming pumps that rely on the fact that they are air-tight. I suggest you try blowing into the suction side of your Yanmar pump, or sucking the discharge. You should be unable to do either.

Jabsco's website says that their pumps should not be operated dry for more than 30 seconds.

 

[ccscott49]

Re: Sorry if you thought...

Only 30 seconds! Mmmmmm. I'll take note of that!

 

[Guest]

Interesting thread! Fact is that any impeller pump can let water back-flow through it under certain conditions, e.g. partially worn face plate. It may still pump adequately, but a back-syphon can develop.

Anti -syphon valves can work, but not necessarily for that long. Best course in my opinion is to simply fit a swan neck in the cooling system between the raw water pump and the engine. At the highest point of this swan neck fit a tee piece and a small (say 6mm) bore tell-tale hose led overboard at an obvious point high up in the topsides. The tell tale will show that water is circulating. If it is not then you either have an intake blockage, or a faulty pump, or the tell-tale line is itself blocked: all obvious!

 

[petery]

I called up a well respected local chandler asking whether he had an anti-siphon loop for my engine cooling system in stock. He didn't, but suggested I made my own and saved money by just putting a continuous loop of hose in place of the faulty anti-siphon loop. Oh dear!

 

[Guest]

The system on our boat is set up exactly as you describe ; our water drains into the cockpit and I can see the bypass working all the time!
Gene

Keep on Learning!!