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Old 17-02-2008, 01:59   #1
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Antisyphon & anti backflow positioning ?

Today I was pondering the water supply to the motor we have just dropped into the vessel. The engine is mostly below the water line, so I was going to put in an anti syphon valve. Now should I come from the seacock up to the AS Valve then to the strainer (or visaversa), then off to the engine and other items that will draw off that through hull in the future (watermaker, head, etc). Should I use antibackflow valves in the lines at strategic points considering numerous items will feed off the intake ?

I am trying to minimise through hull flanges where possible.
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Old 17-02-2008, 10:46   #2
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I'm presuming that you have a freshwater cooled engine with a heat exchanger that is cooled by the seawater.

You should have some sort of anti-siphon arrangement on the OUTFLOW of seawater from your engine. None would be needed on the INFLOW: where would it go? You have (in order): intake, strainer, pump, exchanger, mixer, exhaust muffler can, exit hosing [insert anti-siphon loop here] and thru-hull exit (hopefully ABOVE the normal waterline).
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Old 17-02-2008, 11:10   #3
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You don't want an anti-siphon valve on the intake side of your raw water cooling system because you will lose your prime and burn up your raw water impeller.

On the exhaust side, the exhaust system is supposed to have a flapper (a large check valve) that reduces the chance of water backing up into the exhaust manifold and into the engine cylinders. What IS critical is the height of your exhaust riser. The bottom bend of the top part of the riser must be above the boats waterline. Your riser acts as as the anti-siphon valve that you don't need. Remember, the engine side of your riser is open to atmospheric pressure. Engines are not a perfect vacuum seal. Engine rings and valve seats are not perfect seals.
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Old 17-02-2008, 12:30   #4
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OK, I think the brain is starting to get the picture. No antisyphon on the intake.

As for the wet exhaust, the waterlock muffler is installed just below the motor at the lowest point, then the hose climbs past another muffler (non waterlock) then up through the waterline again to a Vetus inverted U shaped thing that sits just inside the transom (all of it above the waterline), no flap valve, just straight out the back.
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Old 17-02-2008, 12:40   #5
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If your exhaust outlet is above the waterline you will not need a syphon break. There is a thread on here that shows 3 different scenarios of an exhaust layout depending on whether your engine is above / on / or below the waterline, I believe it is based on a Yanmar setup, search exhaust and you should find it. IF you can't locate it Gord will probably repost it.
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Old 17-02-2008, 12:44   #6
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Quote:
Originally Posted by Steve Pope View Post
If your exhaust outlet is above the waterline you will not need a syphon break. There is a thread on here that shows 3 different scenarios of an exhaust layout depending on whether your engine is above / on / or below the waterline, I believe it is based on a Yanmar setup, search exhaust and you should find it. IF you can't locate it Gord will probably repost it.
The location of the exhaust outlet will not determine whether you need a anti-stphon valve but the location of your raw water discharge into the exhaust mixing elbow will. If the engine and mixing elbow is below the the waterline or can be under sail and healed over than you most certainly will need the valve.
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Old 17-02-2008, 12:56   #7
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I found the other thread, more good info !

By the way, the engine is freshwater cooled.

The waterline under sail, now that is a question that I can not answer as we have not sailed it, yet. I think that the top of the engine is just above the waterline, where as the lower part is definitly below.
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Old 17-02-2008, 13:19   #8
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I had a dig around some old literature for a similar Nanni motor (latter model) and there is a diagram in there for install options. It shows that a motor installed below the waterline needs an antisyphon valve installed between the raw water pump and the heat exchanger (it notes it must not be placed before the pump). As there is a flexible hose between the pump and exchanger it should be a simple job to divert the water up to a AS valve. What do you all recon ?
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Old 17-02-2008, 14:00   #9
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anti siphon

Ribbony: you're on the right track, and, as others have said, you do need an anti siphon valve well above the water line in the line you're talking about. I went one step further, and took a tee off the valve with a 3/8" hose and directed a small stream of water right into my port side cockpit drain. I can stand at the helm and look down the drain and know immediately whether or not I've got raw water flow, which is nice information to have. I can also just barely hear it, and the one time I started the engine with the sea cock closed, I knew instantly from the sound (or rather, lack of sound) that something was wrong. It was a pretty simple system to rig, and I think well worth the time. Best, Bob S/V Restless
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Old 17-02-2008, 14:22   #10
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Bob - what a great idea ! I guess it works like the telltail water spout on the outboards that lets you know your not cooking the motor.

If you have a small hose feeding off to the drain, then is a anti syphon valve necessary or will the small hose act as a syphon break ?
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Old 17-02-2008, 14:52   #11
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siphon break

The small hose is the siphon break, in effect. I just welded up a 316 stainless u shape, with a tee in the middle. From either side of the tee, a 90 degree elbow facing down, with a hose barb welded to that. At the top of the tee, another 90 pointing sideways, and the 3/8 hose barb off that, then led that hose to the cockpit drain. Important that there is a constant slope down to the cockpit drain, so there's no way for water to hang in a loop and block the anti siphoning action you're looking for. There's a picture of it (partially done) under gallery-woodwork-components on our web site. Hope this helps. Bob S/V Restless
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Old 18-02-2008, 03:39   #12
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Bob, what a magnificant yacht you built ! We enjoyed looking at some of the photos of launch day. The gloss on the paintwork looks so mirror like, awesome.

We like the approach to the antisyphon that you used in SV Restless, it is on our drawing board, thanks.
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Old 18-02-2008, 08:59   #13
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ribbony, thanks, glad you like it. Both the boat and the web site were a lot of work, but it was enjoyable work which kept me out of the bars (well, most of the time).
You mentioned above that you were trying to minimize thru hulls, and I did the same thing... I have only 2 below the water line, main sea chest in (with 4 valved branches off of it) and head out, that's it, and the system works well. Gray water (head sink, galley sink, and shower) goes to a sump tank, and then is pumped over board via a cockpit drain, above water line. I can also open a small 3/8" valve and empty the water lift muffler into the sump tank, if I ever feel I've cranked the engine too long without starting and may be filling the water lift. Have been toying with the idea of putting an electric solenoid valve in that position, so that the water lift is drained on every shut down, but it hasn't seemed necessary yet and it's one more thing to break at the wrong time. I think it would take more than 90 seconds of cranking to fill the water lift enough to worry about water getting back into the exhaust manifold, and then the valves, and I assume that if that ever happens I'll remember to stop, drain the water lift, and then figure out why the engine won't start. Best, Bob S/V Restless
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