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Old 16-06-2019, 13:57   #16
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Re: Vented Loop

Most flexible vane after pumps form a pretty effective seal against raw water intrusion when the engine is stopped.
The system does retain a good slug of raw water that theoretically could flood the exhaust system to the point of backflowing through open exhaust valves, but with careful engineering of the exhaust system, utilizing a dry riser to a point 12” above the waterline will eliminate the possibility.
The vented loop in the wet elbow supply line does not insure against flooding, and the taller it is, the more capacity it has to flood the wet exhaust after the engine is shut off.
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Old 16-06-2019, 14:19   #17
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Re: Vented Loop

Would not use a loop with an anti-syphon valve. Have had the valves fail to open, flood the engine and necessitate a rebuild. If you want a vented loop remove the anti-syphon valve on the vented loop and install a 1/4" or smaller hose bib, attach a hose and run it overboard either into the cockpit or overboard. Loop the vent line as high up as possible before it dumps overboard.
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Old 16-06-2019, 15:13   #18
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Re: Vented Loop

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Originally Posted by MikeFergie View Post
My Leopard cat engines are not really below the water line. The sump is but mid block upwards is above the water line


Same for my IP, and apparently it got lucky for 30 yrs, cause it never had an anti siphon loop.
I installed one, cause I hate relying on luck.
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Old 18-06-2019, 08:15   #19
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Re: Vented Loop

Your gambling that your vessel will never be sailing in sea conditions that would momentarily bring the waterline up over the top of the engine and thus have the potential to create a siphon through the raw water pump (which it can) and fill your exhaust and subsequently your engine (s).

At a minimum, water in the engine will be an expensive matter. In the right sea conditions or circumstances, having an engine or two down could be catastrophic.

A decent vented loop, hoses and clamps would cost about $120.00 per engine.
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Old 18-06-2019, 12:09   #20
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Re: Vented Loop

Vented loops made of marlon are good to 180'F which is way hotter than your raw water exhaust will ever get and are cheap insurance to prevent a siphon or water lock.
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Old 18-06-2019, 15:25   #21
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Re: Vented Loop

Well, mine is Bronze, a Groco. Not sure how much heat bronze will take, but think itís a lot
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Old 19-06-2019, 04:49   #22
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Re: Vented Loop

Just to make sure we're all on the same page the water line on my L40 is about at the same height of the top of the muffler. Based on this, and also considering it's a catamaran, do I really need a vented loop?


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Old 19-06-2019, 11:52   #23
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Re: Vented Loop

I donít have it and donít see the need of this in our cats. I have run downwind for 6 to 7 hours in 4-5 m seas in 35 knots of apparent wind at about 120-130 degrees and my motors started on the first turn.
Ask on the Leopard forum if any other Leopards have fitted anything to theirs.
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Old 19-06-2019, 14:38   #24
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Re: Vented Loop

Vented loops can fail. If it does it will be spraying hot water all over your engine and engine room. You need to run a tube from that vent to a safe spot. They also need to be pulled apart and checked yearly. It's simple to open and clean them, but I'm not sure I would install one just for fun.

I did add one to my 2008 L40 but for the heads. The intake always had one, but the discharge didn't. Now I have them on both heads for both intake and discharge. It was a lot tighter fit that the engine rooms.

I'll double check my Leopard 40 to see if I think they would be helpful on the exhaust.
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Old 19-06-2019, 15:11   #25
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Re: Vented Loop

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... create a siphon through the raw water pump (which it can) and fill your exhaust and subsequently your engine (s)...
For most of us with rubber impeller raw water pumps a siphon can't be created through the pump unless the impeller is damaged. The vented loop is backup security in the event you don't inspect your impeller often enough.

The impeller pumps are positive displacement, in normal operation there should be at least two full vanes (6-vane impeller, more for greater number of vanes on the impeller) between the saltwater inlet and the pump outlet. The front and back of the impeller seal to the pump body and the cover plate. If these seals are not good pump flow decreases. If everything is in working order the sealed impeller faces and the vanes prevent any siphon from forming. Of course, if your impeller or pump faces are damaged then you can siphon past the damaged areas.

If you do install a vented loop then it is pretty easy to remove any valve at the vent and route a very small diameter tube from the vent to a discharge outside the boat. This is very much like the telltale/peehole on an outboard. The amount of water that comes out the telltale reduces total flow through the cooling system, so you use a pretty small tube to restrict flow to a small amount. Then you can use the telltale to let you know if anything is clogged, or if there's a problem with raw water flow.
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