Move the vented loop ?

[nveater]

Just reinstalled my 23 year old engine after having a rebuild -the connecting rod was bent fairly severely- the engine guy believes that water got into the cylinder from being syphoned back in when the boat was heeled over. As such he added an extension onto the mixing elbow he is also recommending moving the vented loop higher especially since there seems to be a big dip in the hose from the loop into the elbow (See pics). I can move the vent but it will involve some surgery so before doing that I am looking for other opinions and no I haven't yet checked that the current loop is working.

 

[TomY]

I'd raise the loop any way you can. If the back siphon was through the raw water intake(which I assume it was), only raising the vented loop will prevent it re-occurring. Get it as close to the middle of the boat as possible too.

If the sea water entered the engine from back flooding through the exhaust, the extended riser should solve that.

 

[Charles Erwin]

There are really two parts to this some of which your photo does not show.

We cannot see the path from the mixing elbow to the waterlock muffler nor from the waterlock to the exhaust hose outlet. The path from the waterlock to the exhaust hose outlet (overboard) fitting needs to rise as high as possible (inside the boat) before descending to the exhaust outlet. To protect even more you can put a flapped fitting on the exterior of the outlet fitting or even fit a seacock (ball valve) at the outlet fitting which you keep closed except when running the motor. These things prevent water from backfilling the exhaust hose, flooding the waterlock, and overflowing back via the mixing elbow and then to the exhaust ports of the cylinder head.

The other section - the path from the cylider head to the waterlock via the mixing elbow (the elbow is the top of the loop in this section) - rises about as high as possible according to your picture. The higher the elbow above the waterlock, the more water the waterlock can hold before back flowing via the mixing elbow back into the cylinder head exhaust ports.

The siphon-break loop protects the waterlock from overflow on cold water side of the circuit. So the same rules apply. The loop needs to be as high as possible such that the top of the loop is as high above the boat waterline as possible. But you can also close the cooling water intake valve (except when the motor is in use) and that will certainly defeat any threat of a siphon or even heel induced direct flow.

The very best arrangement - but not always possible - is to make the rearmost exhaust hose section rise less than the mixing elbow. That way if the waterlock became flooded (like when the siphon break air valve becomes clogged) the floodwater in the waterlock spills into the exhaust hose and overboard instead of back flowing into the motor.

If you are still conscious read this https://www.sbmar.com/articles/designing-a-marine-exhaust-system/

Charles

ps Put lagging on that riser - the steel pipe from the head to the mixing elbow. It is going to get way hot.

 

[JamesG161]

I may be wrong, but if the check valve on the so called "high point" vent is functioning, what I see will work. The vent should be above the static water line of your boat.

A siphoning back to the engine would be a rare event. Normally the high point is all that is needed.
The second there is negative pressure on the check valve, air is sucked in. No siphon.

I assume your water pump stops when the engine stops. and it is the standard flex vane pump.

I have never seen the reason for the vent break, if you have a seal loop in the muffler.

If I was in 10+ fore/aft seas I could see sloshing more that siphoning.
Jim...

 

[nveater]

Thanks very much for the interesting and thoughtful replies. I always assumed the water got into the cylinders from the exhaust side and not the raw water side which I always leave open during sailing but perhaps I should close ?? I will probably be raising the loop, but perhaps take and post a few more pics to show the entire system. Thanks again

 

[Charles Erwin]

Another cause of cooling water in the motor happens when the motor does not start. The user keeps on engaging the starter as this non start situation continues. Cooling water goes into the mixing elbow and drains into the water lock, fills the water lock and then overflows the mixing elbow back into the motor exhaust ports. Sometimes the starter motor abruptly stops turning because the combustion chamber gets water in it. Sometimes the user stops and the water just sits there ruining the valves and rings.

This is why it is good to have the mixing elbow slightly higher than the rest of the exhaust path. That way the cooling water accumulating in the water lock goes overboard instead of backing up. Obviously it is not always possible to route the exhaust tubes that way - but if so there are other ways to prevent this from happening.

Charles

 

[Stu Jackson]

but perhaps I should close ??

If your system is designed and installed correctly, you shouldn't have to close it. In addition, in extremes you need and want to be able to start the engine without having to go down below.

 

[Charles Erwin]

Stu's drawing is an example of an inherently dangerous design. There should be a riser at the mixing elbow elevating the mixing elbow as high as possible - just exactly the way nveater has already done.

Charles

 

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[TomY]

Thanks very much for the interesting and thoughtful replies. I always assumed the water got into the cylinders from the exhaust side and not the raw water side which I always leave open during sailing but perhaps I should close ?? I will probably be raising the loop, but perhaps take and post a few more pics to show the entire system. Thanks again

When I read 'siphoned back in', I assumed you meant raw water siphoned from the raw water intake. If the seawater back flooded through the exhaust, then raising the vented loop won't help.

Perhaps the loop is high enough above the waterline, already?

And raising the riser would help the back flooding problem, but only to a point(adding volume capacity). You should eliminate that problem if it exists.

Your prevention from back flooding should be in the height of the exhaust outlet in the stern and a high loop just before it exits. Can you explain how the exhaust outlet and hose is set up? Obviously, you want to prevent a re-occurrence.

This is a good article on marine exhaust design. There are few boats were the perfect set up works, but this explains the various critical areas.

http://www.goodoldboat.com/reader_services/articles/Marine_exhaust.php

 

[kloudie1]

It is crucial that the air vent in the vented loop be checked to make sure it is not plugged.. In my opinion, the raised mixing ell makes the possibility of back flooding worse because a small amount of heel will cause the water to flow from the heat exchanger, over the loop, and into the exhaust hose, filling the muffler then backing into the engine.. A bunch of 335's work fine with no riser on the mixing ell.. I would kinda put that back to original condition and keep the vented loop as high as possible and make sure the exhaust hose has a high loop back at the transom. From the drawings submitted, you can see that the location of the mixing ell can (and should) be a good bit lower than the vented loop.
The mechanic also should have insulated the riser since it will be very hot and could cause some damage.. If ya look at the level of water.. The water in the mixing ell will be just above the exhaust hose clamps (internal casting wall) which is only an inch or two below the high spot on the little vented loop.. With the boat heeled over it wouldn't take a lot of pressure from forward movement to have the water go into the exhaust and back-fill into the cylinders..

 

[JamesG161]

If you all will note, there is NO Elevation view (side view of his boat). To design and give firm advice, the boat's static water level is needed, relative to the pictures.

This is from my Hunter manual which includes the PLAN view (looking down) and the ELEVATION view.

@kloudie1 note the elevation view "Loop" prevents healed forward motion "water pressure" blow back, on my boat. That loop also provides the seal loop for the exhaust and causes the "burping" effect of engine exhaust, as the engine exhaust pressure "burbs out" the riser quench water.

@nveater

the engine guy believes that water got into the cylinder from being syphoned

I suspect you have a restricted muffler/exhaust system. Which might have shown a "tough or long" cold start.
______
A siphon is a "dynamic" event. If there is no flow... no siphon. It is a Reverse flow normally.
_______
IMHO there is only one reason for the vented high loop....

Prevent siphoning from the sea water pump.

This siphon can happen with a worn pump impeller, if it was properly installed.
If that is the possible water source, then simply shut the sea cock or make sure your vent works. The elevation of that vent must be above your water pump (which it is) and above the static water line.
__________
Since there is water in the seal loop of the muffler system, I have always worried about high sea's SLOSHING of that water backwards. I will aways run my engine, if in high "fore and aft" seas
Jim...

PS: I got the static water level, on my boat, buy using a tap measure and seeing the tape through a port light. Then moving the from port light reference to my engine compartment.

 

[nveater]

My exhaust system is as shown in Charles' elevation and plan view although I can't confirm the final loop in system before exiting boat as I am currently 1500 miles away from the dock. I am going to see if I can determine where my static waterline is as compared to the engine and attachments and provide some more data. Any thoughts on the type of pipe insulation for the extension piece?

 

[nveater]

So I have raised the vented loop- its now at the top of the companionway stairs about 33" above the static water line. I replaced the old vent with a new one- I did find the old one blocked after I took it apart- and I confirmed that my system looks like the Hunter plan shown in JamesG's post- Thanks for all the input that's why

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