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Old 04-07-2021, 05:12   #1
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Engine siphon alarm or such?

I recently pulled a bone headed maneuver. In short I managed to block the vented loop on the engine anti-siphon. The engine flooded, to the brim. Luckily it was in fresh water and I seem to have recovered with no lasting ill effects except to my ego.

Having had the vital importance of the anti-siphon deeply impressed upon me I am now struck with how we install and forget this device. Surely these can eventually fail to gent, they are not fool proof and 100% reliable. Even if very highly reliable the consequence of failure is pretty nasty.

My long awaited question:

Is there some kind of alarm or other warning device to monitor their operation? Some way to assure they are working?

I am imaging a low pressure or pressure differential sensor, if this senses a low pressure (lower than atmospheric) it closes/opens a contact. Might be useful on head anti-siphons as well.
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Old 04-07-2021, 05:38   #2
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Re: Engine siphon alarm or such?

You’re introducing more failure than your removing.

The most failsafe anti siphon is a small tube to replace the one way valve. When the engine is running a tell tale water stream is ejected out of the small tube, when shut off the tube introduces air preventing a siphon.
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Old 04-07-2021, 05:55   #3
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Re: Engine siphon alarm or such?

There are 2 models of anti-siphon valves.
The one that puts out a continuously small stream of water, directly visible when looking overboard.
The other one leaks a small quantity of water, but is not continuously, that is the one in my boat. The original installation on my boat was that the small tube was pushed into a hole in the stringer next to the engine bilge. The water leaked was thus invisible because it ended up in the bilge.
I pulled this tube and pushed it in an empty transparent plastic water bottle. Now you can observe the functioning of the AS valve by looking at the level of water in the bottle. Empty when full. The bottle is visible when checking the engine oil level. Simple and effective.
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Old 04-07-2021, 17:21   #4
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Re: Engine siphon alarm or such?

I vote for the "no-valve" approach. Just a small bleed that piddles overboard. An excellent indicator of raw water flow. No piddle = plugged inlet, bad impellor, etc. Too much piddle, plugged exhaust nozzle? Just right, go sailin'.
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Old 04-07-2021, 17:41   #5
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Re: Engine siphon alarm or such?

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I vote for the "no-valve" approach. Just a small bleed that piddles overboard. An excellent indicator of raw water flow. No piddle = plugged inlet, bad impellor, etc. Too much piddle, plugged exhaust nozzle? Just right, go sailin'.

Ditto, though mine goes to the cockpit drain.
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Old 05-07-2021, 04:45   #6
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Re: Engine siphon alarm or such?

And what do you all do about your head/toilet anti-siphon?
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Old 05-07-2021, 04:49   #7
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Re: Engine siphon alarm or such?

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And what do you all do about your head/toilet anti-siphon?
Just a small piddle.
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Old 05-07-2021, 05:15   #8
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Re: Engine siphon alarm or such?

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And what do you all do about your head/toilet anti-siphon?


Don’t have one. The top of the bowl is just above the static waterline.
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Old 05-07-2021, 11:16   #9
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Re: Engine siphon alarm or such?

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Don’t have one. The top of the bowl is just above the static waterline.
At all angles of heel?
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Old 05-07-2021, 16:50   #10
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Re: Engine siphon alarm or such?

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At all angles of heel?


For most Normal sailing….yes. It will go below if we’re really pushing it, but in the thirteen years we’ve owned the boat there has never been water in the bowl unexpectedly. I do make it a habit of greasing the head valve o-rings annually so I’ve caught any impending failures before they’ve failed.
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