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Old 10-06-2019, 11:15   #1
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Vented Loop

Hi all,

I have a 2018 Leopard 40 with Yanmar 3YM30AE and SD25.

Somebody suggested I should install a Vented Loop like this one, to prevent water from coming into the engine from the exhaust:


al2o3 compound name

Frankly, I don't see the utility, since a cat doesn't heel and the exhaust is well over the waterline. I've crossed the South Atlantic Ocean with 5 meters waves (from the stern) and I've never had a problem.


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What do you guys think?

Thanks in advance!
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Old 10-06-2019, 11:27   #2
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Re: Vented Loop

The vented loop isn't for water coming in from the exhaust port, it is for preventing syphoning from occurring in the raw water cooling loop. The raw water intake has to be below the water line.
If your lift exhaust and emgine are above the waterline, you don't have a problem with your engine flooding in this manner, but that anti syphon loop isnt very expensive to get or hard to install, so it would add peace of mind.
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Old 10-06-2019, 11:45   #3
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Re: Vented Loop

Thanks for the clarification AndyEss.

I see there is already a loop on my engine, the problem is that is not vented?



Is it just as easy as replacing the existing pipe with a vented loop?
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Old 10-06-2019, 12:00   #4
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Re: Vented Loop

My leopard 40 2009 model looks exactly like yours and I have no issues. I have done over 10 000nm with her with no issues. Offshore ocean crossing and coastal cruising.
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Old 10-06-2019, 12:03   #5
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Re: Vented Loop

Are you on the Leopard forum on Yahoo ?.
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Old 10-06-2019, 12:15   #6
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Re: Vented Loop

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Originally Posted by MikeFergie View Post
Are you on the Leopard forum on Yahoo ?.
Yes, I'm in the Yahoo Group. I've done 5,600 nm on my cat without issues, but somebody I know had an issue and installed the vented loop.

I'm trying to understand what a vented loop does and if I need it. Hence the post in this forum.
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Old 10-06-2019, 12:21   #7
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Re: Vented Loop

I see your posts on Instagram . Iím based in Mozambique. Nice long coast like Brazil. Next year I will start cruising full time on the east side of Southern Africa..
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Old 10-06-2019, 12:30   #8
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Re: Vented Loop

The presence of a loop in the pipe does nothing to prevent a syphon from forming. The fact that someone else (I'm assuming with the same boat model/configuration like yours) had a problem, while you and MikeFergie haven't (yet?) suggests a marginal situation. Maybe the boat that had the problem was loaded with supplies, etc so heavily that what had been a marginally ok situation ended up as a potentially very major problem.
As it happens, I just added vented loops to my exhaust two weeks ago.
Yes, you need to buy marine water/exhaust rated hose in the proper diameter. It will probably have steel wire reinforcing in it. Use a hacksaw and then a steel cutting nipper to cut the hose to length. Obviously, get the vented loop.
You have good access to your engine. Most of the time I work on my engine exhaust in a small compartment accessed through my lazarette locker. I often am upside down, hanging from my toes, and contorted into positions Houdini never thought of.
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Old 10-06-2019, 13:50   #9
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Re: Vented Loop

Quote:
Originally Posted by AndyEss View Post
The presence of a loop in the pipe does nothing to prevent a syphon from forming. The fact that someone else (I'm assuming with the same boat model/configuration like yours) had a problem, while you and MikeFergie haven't (yet?) suggests a marginal situation. Maybe the boat that had the problem was loaded with supplies, etc so heavily that what had been a marginally ok situation ended up as a potentially very major problem.
As it happens, I just added vented loops to my exhaust two weeks ago.
Yes, you need to buy marine water/exhaust rated hose in the proper diameter. It will probably have steel wire reinforcing in it. Use a hacksaw and then a steel cutting nipper to cut the hose to length. Obviously, get the vented loop.
You have good access to your engine. Most of the time I work on my engine exhaust in a small compartment accessed through my lazarette locker. I often am upside down, hanging from my toes, and contorted into positions Houdini never thought of.
Ah ah ah! I hear you AndyEss!

"My friend says it happened to him from Bahamas to Bermuda, waves were coming on the stbd about 10/15 feet, winds were about 20/25 knots and they were making around 9 knots. The water came in the port exhaust into the oil via the heat exchanger."

I'm a bit confused here. The syphoning effect takes place when you shut down the engine, correct? It should have nothing to do with big waves getting into the exhaust outlet.

Do you agree?
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Old 10-06-2019, 14:38   #10
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Vented Loop

No, many boats get water into the engine from waves impacting the exhaust when the engine is off.
If you ask that question, I bet youíll get a few answers from people that itís happened to.
I donít think a vented loop will help in that account.
Of course a rubber flap on the exhaust may also prevent water from being pushed into the exhaust from a wave.
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Old 10-06-2019, 17:27   #11
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Re: Vented Loop

No, a vented raw water loop will do nothing for waves filling the exhaust port. The exhaust from the running engine pushes the raw cooling water up from the wet muffler and to the exhaust port on the transom.
After the engine is shut down, and with the raw water cooling thruhull still open, a syphon can develop pulling water from the raw water system and into the engine if the engine is lower than the water line. This is the situation the vented loop is intended to take care of.
As A64pilot says, a rubber flap valve on the transom exhaust port can help with following seas (or in your case, side seas) from flooding the engine.
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Old 10-06-2019, 19:26   #12
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Vented Loop

Or maybe sticking a tennis ball in the exhaust if itís accessible, or running the engine.
Yeah, I know itís a sin, but I know of at least one Mod here on the forum that had a motor hydro locked on them from a heavy boat and following seas, so it can happen.
I canít find a stupid flap to fit mine, I think Iím going to have to shim up my exhaust so that I can put a 3Ē flap on.
My exhaust is on the stern, but same thing
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Old 10-06-2019, 20:13   #13
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Re: Vented Loop

When (or if) the muffler gets full, it could backflow into the engine. Any open valves, and there will be one or two) will allow water into the piston area.

The water coming out of the engine is under pressure from the pump, so it closes the little valve in the AS loop. No pump pressure, it opens, preventing a siphon.

Whether that can happen to you should have no bearing, IMHO, on what others have experienced without putting in. I applaud their luck.

Might be appropriate to just read up on "boat engines below the waterline" and the basic system design. Sorry, I don't have a link right now.


Good luck.
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Old 10-06-2019, 20:27   #14
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Re: Vented Loop

Vented loops are only as good as the reliability of the vent valve and most commercial vents have a built in failure mechanism.

Read this article on a simple DIY method to keep moisture away from the vent valve. Hints and Projects

We had 4 vented loops on our boat for 14 years with zero maintenance and not a single failure!
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Old 16-06-2019, 13:26   #15
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Re: Vented Loop

My Leopard cat engines are not really below the water line. The sump is but mid block upwards is above the water line
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