Anti Syphon Valves and Engin Backflooding

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Old Bumbulum

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MY 3GM30 is somw way below the waterline and has suffered backflooding via the exhaust despite the cooling circuit having a (Maestrini brass type) anti syphon valve.

Question is - why & how?

The valve is between the engine output and the exhaust elbow. I'd expect it to normally be on the inlet side of the engine. Is that right, but does it really matter?

It is also only about 30cm above the waterline which is all that can be achieved in the engine space. Is that enough?

Ideas? - apart from not forgetting to turn off the seacock which sooner or later is inevitable.
 
Old Bumbulum said:
MY 3GM30 is somw way below the waterline and has suffered backflooding via the exhaust despite the cooling circuit having a (Maestrini brass type) anti syphon valve.

Question is - why & how?

The valve is between the engine output and the exhaust elbow. I'd expect it to normally be on the inlet side of the engine. Is that right, but does it really matter?
Click to expand...

Should be fitted just after the water pump outlet.

It is also only about 30cm above the waterline which is all that can be achieved in the engine space. Is that enough?
Click to expand...

Should be enough height.

Ideas? - apart from not forgetting to turn off the seacock which sooner or later is inevitable.
Click to expand...

Make sure the valve is working. Another option would be to remove the valve completely and run a small bore hose to the cockpit coaming and have a pee pipe, This allows a small jet of water to flow overboard, but allows air back in when the engine stops, breaking the syphon.

Are you sure the water ingress is from the cooling circuit and not coming back in from the exhaust ?
 
Old Bumbulum said:
MY 3GM30 is somw way below the waterline and has suffered backflooding via the exhaust despite the cooling circuit having a (Maestrini brass type) anti syphon valve.

Question is - why & how?

The valve is between the engine output and the exhaust elbow. I'd expect it to normally be on the inlet side of the engine. Is that right, but does it really matter?

It is also only about 30cm above the waterline which is all that can be achieved in the engine space. Is that enough?

Ideas? - apart from not forgetting to turn off the seacock which sooner or later is inevitable.
Click to expand...

Between the engine or heat exchanger water outlet and the exhaust elbow is a common location for the anti-siphon loop and valve. On some engines, esp when this is not possible, it may be between the water pump discharge and the inlet to the engine or HE.
Either location is acceptable.

Vetus recommend 40cm above the water line, some engine manufacturers recommend more ( Beta say 50 cm) but if 30cm is the best you can manage then that's the best you can do.

It is essential to regularly check the the anti syphon valve is functioning correctly and not stuck shut or the vent blocked .... It must allow air the enter when the engine is stopped. I suspect yours is not functioning and is not allowing air to enter ( you don't have a length of tubing on it going down well below the W/L I hope )

Fitting a non valved type that pees into a cockpit drain or overboard ( or modifying the existing one) may be a solution
 
is the anti syphon with or without a tell tale pipe. if with, ensure there is definitely no uphill sections in the pipe, if not; clean the spring valve in the anti syphon and spray with ptfe lubricant.
 
had this with a 3gm30f - turns out (on ours) there should be a high riser exhaust water/exhaust mixer fitted - but no space. i now shut the exhaust seacock when the engine is off, and drain the water from the exhaust at the antisyphon plastic box thingy (sorry!)

also shut this seacock at anchor or mooring if any waves - they seem to overfill the exhaust, over time, which ends up going back to the exhaust manifold and in any open exhaust valves
 
I reckon this guy really knows his business.

Designing a Marine Exhaust System
Also, and another all too common error in exhaust design, is the use of an anti-siphon valve as a “fix-all” for an otherwise poor and sure to fail design. It’s not that an anti-siphon valve is not needed in many applications, but “Average Joe” has no clue as to what they can do and more important, what they CANNOT do, and how small changes in the basic design of an anti-siphon valve can greatly enhance its effectiveness. More on “Anti-Siphon” valves and their shortcomings below.
https://www.sbmar.com/articles/designing-a-marine-exhaust-system/
 
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