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hanschristian.org • View topic - Near Engine Disaster...Your Vented Loop?

Near Engine Disaster...Your Vented Loop?

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Near Engine Disaster...Your Vented Loop?

Postby SVTambadil » Thu Apr 19, 2007 1:11 pm

A week ago we went to start the engine, only to be met by a very unpleasant "clunk". Fearing the worst we hired a "real" diesel mechanic to come, and he confirmed our fears, that the engine was full of seawater. Luckily, we were able to work together to get it all drained out and save the engine. However, the problem, as it appears to the mechanic and I, seems to be very simple, and very big (so I hope we are missing something!). On the port side of the engine compaprtment is the water intake for the engine. It then comes up and makes a loop just under the sink, right there at the engine. That would be...just under the sink and under the waterline! In this case you dont even need any siphon action to fill the engine with water, it just comes into the engine at a steady trickly by good old gravity.

Or...am I missing something simple? The mechanic says that at the least I am going to need to re-route those hoses back to my genset compartment to be able to get enough height to get above the waterline (he suggests at least a foot above).

But why hasnt this happened before with this boat? We guess that by lowering the boat an inch or two with our cruising gear, that since it was already right at the waterline, it just needed a tiny bit to start trickling in.

Are we missing something? Or do most of you have that vented loop under their sink/in their engine compartment area, and thus right around waterline height? IF this is a bad design, I am wondering why my regular or specific "engine" surveys did not pick up this dangerous problem?

Chad
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Postby Bill » Thu Apr 19, 2007 2:02 pm

Chad,

I believe you are talking about the "return" of your raw water not the actual intake. Your intake needs to be below the water line and without an anti siphon as it feeds directly to the raw water pump.

The raw water pump should lead directly to your engine heat exchangers, transmission heat exchanger and then up to the anti siphon valve before being injected into the exhaust.

Our anti-siphon is also directly under the galley counter, not the sink. If it was as low as the bottom of the sink it would certainly be level with the outside water level if not below the surface with loaded with weekend accoutrements. Have you checked to see if perhaps the screw that holds the anti-siphon loop could have come out dropping the loop below the water level?

One additional thought, are all your exhaust ports above water?
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Postby Bill » Thu Apr 19, 2007 2:06 pm

Actually I assumed your engine is cooled by both a closed system (anti-freeze) and a seperate open (raw water/heat exchanger) system. Is this correct?
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Postby SVTambadil » Thu Apr 19, 2007 2:31 pm

Yep, exhaust ports are above water. Right, it is the return, rather than the intake. And yes to both coolant and heat exchanger arrangement.

The loop is solidly in place, but comes up to just the bottom of the sink. My idea was to just move it over 10 inches or so to port so that it can come up alongside the sink about 6-8 inches higher than present. My diesel guy says a foot higher, as a basic standard, and the nearest place for this would be the genset compartment. But not if I have another option.

But you do agree then, that if that loop is at/below the waterline, that this could be the cause? I just posted the same scenario in SSCA, and got a flurry of responses. Here is one..

______

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PostPosted: Thu Apr 19, 2007 1:56 pm Post subject: Reply with quote
Chad,

I think the only way that seawater could get into the inside of the engine itself (e.g. cylinders and crankcase) would be in through the exhaust. Thus there are two possible siphons -- the raw water part of the cooling system, as it dumps into the exhaust, and the exhaust plumbing itself. Normally, the exhaust would be dumped out of the boat above the waterline, so that shouldn't siphon water, but it could if you've loaded the boat significantly, or in certain sea conditions.

For the raw water siphon, the impeller vanes in the pump would normally stop the flow of water when the engine isn't running. But if one of the vanes is broken or there's some other failure of the impeller, that doesn't help. So you should have an above-the-waterline vented loop, but that loop should be AFTER the pump, and probably after the heat exchanger, etc..., but right before the raw water goes into the exhaust mixing elbow. If you have a vented loop before the pump, it will never prime and draw water.

Finally the other way that the exhaust could fill up with water is if you crank the engine for a while (and thus the raw water pump), but the engine doesn't fire so it doesn't generate enough exhaust pressure to push the water out of the muffler.


_____________

And...although I would never do this...the simple quick fix for this is just to turn the water intake shut-off off/on whenever you stop/start the engine. Foolish and risky.

The problem just happened while sitting there at the slip, it just trickled in till the enigine was full of water.

Chad
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Postby Bill » Thu Apr 19, 2007 3:02 pm

You may want to get other opinions on the placement of your "loop", but it certainly should not stay were it is (beneath the water line). Running it back to the genset, which is most likely under the companion way or in the lazerette seems a long way to go for the exhaust water, but better than having your engine pickled with salt water again. Have you looked at other options, running up the inside of a cabinet or if your engine room is vented to a dorade, going up the exhaust tube for the engine room.

I hadn't given thought to the "impellor theory" but while you are messy about getting things right, you may as well replace the impellor.

I think that most of our HC's are living on the edge with concern about the vented loops under the galley, for sure we are nibbling at the edges of safety in comparing the Vented loop height to the waterline.
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Postby toddhuss » Fri Apr 20, 2007 5:09 pm

In the exhaust system it seems to me that the most important factors are the height of the vented loop relative to the waterlift muffler and the capacity of the waterlift muffler. The waterline shouldn't play any factor because the exhaust output is above the waterline.

The only way you should be able to get water in your engine is if your waterlift muffler can't handle the amount of water still in the exhaust line that flows back into it when you shutdown the engine.

Certainly the higher the better on the vented loop so I would look if you can raise it up to just underneath the countertop instead of under the sink. Next I would look at the capacity of your waterlift muffler. It needs to be quite large.

-Todd

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Re: Near Engine Disaster...Your Vented Loop?

Postby warmrain » Fri Apr 20, 2007 8:09 pm

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Postby chuck » Fri Apr 20, 2007 11:15 pm

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Postby SVTambadil » Sat Apr 21, 2007 2:56 pm

Thanks so much guys. Great lesson and tips. I will have my engine guy check this out as well!

Chad

PS for those of you who have experienced having your engine flooded with seawater, what process did you use for flushing the engine (multipple oil/filter changed, etc) to prevent corrosion?
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Postby johnrobinson » Mon Apr 23, 2007 9:09 pm

Is this a picture of the space under the starboard saloon seat ?? Is this a discharge line or intake?? Your post mentions two pictures but only one seemed to show up. I have a 43T Cutter and am interested in solutions to this problem as well.

Regards, John Robinson, Crossing, Seattle 43T
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Postby SVTambadil » Mon Apr 23, 2007 9:16 pm

ditto. Let us know. Im going to get greasy this week and work on our solution. Still have to trace the black hose that comes from my anti-siphon loop and goes down and aft to...?? My mechanic thought it might be going to the stuffing box or something, but I have not traced it yet. Anyone have this same "loop with a (third) hose going aft" arrangement?

Chad
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Postby johnrobinson » Tue Apr 24, 2007 5:00 pm

Chad; I also had the "blask hose that runs aft" . . . its a cooling water discharge bypass that connects to the antisiphon on the engine exhaust (see pic) before it exits the through-hull. When I installed my Cape Horn wind vane I had to move the antisiphon can (see pic next post) and decided just to replace it with a loop of exhaust pipe that went high up into the combing . . . reinstalling the bypass was not convient and after talking to two engine mechanics I removed it. Engine seems to run fine without it (no backpressure).

Regards, John Robinson, s/v Crossings 43T
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