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**skipperpete** · 27-08-2023, 15:54

There are a lot of posts on the forum about hydrolocked engines and associated damage from wet exhausts and I think it’s worthwhile to keep this subject alive for the benefit of “new to boating” CF members.
There are danger signs as well as forbidden practices regarding the piping and operation of raw water cooled engines and every system has its own quirks and deficiencies but without going into minute detail, here are a few things to look out for. This pic is a common Yanmar danger sign but if you see this corrosion line here on any mixer elbow it’s a red alert to take action. There should NEVER be corrosion at this sealing surface or this high up on the mixer elbow.
The damage is often not even visible until the mixer is removed and this Betamarine engine had a total running life of half an hour before being handed over to the owner who didn’t start it again for 3weeks and complained that he had a flat battery .... badly installed vent loop killed it and the engine was so badly damaged that it was un repairable.
Overcranking is a common cause of engine flooding and mostly not an issue for the average cruiser because quick venting and rapid starts after filter changes are the norm.... but if there is a major failure in the fuel system and the owner keeps trying and trying to get a start...... the exhaust will flood and allow seawater to enter whichever cylinders have open exhaust valves, the time this takes varies according to the size of the exhaust system and the size of the water pump">raw water pump.

markxengineerin · 27-08-2023, 16:25

Quote:

the exhaust will flood and allow seawater to enter whichever cylinders have open exhaust valves,

I've never understood this. The flow of air through the engine even during cranking should be net positive, from intake out exhaust. It is a positive displacement machine, meaning the pressure to force water out should easily be enough even without any combustion.

This can get a little messed up by having a lot of blowby leakage and unusual valve timing (I've seen unusual, high blowby engines that have a vacuum in the cylinder when exhaust port opens during cranking), but I just don't imagine this being the case with our boat engines. Has this really happened or was there always a factor like improper vent loop/some kind of syphoning action? In other words, can the root cause be "too much cranking?" I would argue No until shown otherwise.

joe_b · 27-08-2023, 16:37

I could have used this post a few weeks ago. Just last week I was motoring my new to me sailboat and had the exhaust fail.

Thank goodness I did not experience any backflow into the engine, but here are the pictures of the failed joint. Completely corroded through. When it failed I initially thought the engine was on fire because so much smoke and steam was coming out of the engine compartment.

Replaced it (fairly easily) with a stainless steel assembly.

rslifkin · 27-08-2023, 17:20

Quote:

Originally Posted by markxengineerin

I've never understood this. The flow of air through the engine even during cranking should be net positive, from intake out exhaust. It is a positive displacement machine, meaning the pressure to force water out should easily be enough even without any combustion.

This can get a little messed up by having a lot of blowby leakage and unusual valve timing (I've seen unusual, high blowby engines that have a vacuum in the cylinder when exhaust port opens during cranking), but I just don't imagine this being the case with our boat engines. Has this really happened or was there always a factor like improper vent loop/some kind of syphoning action? In other words, can the root cause be "too much cranking?" I would argue No until shown otherwise.

Some exhaust systems are susceptible to it, but not all. In some cases, the pressure while cranking isn't enough to push enough water out and the system can fill up enough to get water in the engine when you stop cranking. For other systems, it's a non issue and you can crank forever without flooding the engine.

Khaos13 · 27-08-2023, 20:36

It happens... often enough that it can't be ignored. If it doesn't start, close the seacock, open the drain on the muffler, or both. A great reminder skipperpete!

**skipperpete** · 27-08-2023, 21:24

The truth is that extensive cranking actually does flood engines (yanmar put a warning on the engine) and the cranking volume of exhaust gas fails to blow out the water… but I understand why its hard to sell that proposition because not all engines are equally susceptible . The air from dead cranking travels as big bubbles or pockets of air, not like if it was sealed behind a piston behind the water in the hose and even worse is that the back pressure increases to the point where even a healthy engine struggles to start. The point of this thread is to give forum members a sense of how quickly and silently a wet exhaust can damage an engine if the rules aren’t followed.

team karst · 28-08-2023, 05:07

Quote:

Originally Posted by markxengineerin

I've never understood this. The flow of air through the engine even during cranking should be net positive, from intake out exhaust. It is a positive displacement machine, meaning the pressure to force water out should easily be enough even without any combustion.

This can get a little messed up by having a lot of blowby leakage and unusual valve timing (I've seen unusual, high blowby engines that have a vacuum in the cylinder when exhaust port opens during cranking), but I just don't imagine this being the case with our boat engines. Has this really happened or was there always a factor like improper vent loop/some kind of syphoning action? In other words, can the root cause be "too much cranking?" I would argue No until shown otherwise.

Cranking just non-ignited air thru the motor is far less volume than ignited exhaust right? I would think several orders of magnitude. Hold your hand at the exhaust output and estimate. [emoji4]

rslifkin · 28-08-2023, 05:23

Keep in mind that the over-crank flooding issue is a symptom of a poorly designed wet exhaust, not a natural limitation of wet exhausts in general.

In a properly designed system, the spillover point of the water injection elbow should be higher than the spillover after the muffler. In that situation, it takes very little force to push water out the exhaust, and even if the muffler is full, it won't overflow back to the engine.

Many systems are poorly designed due to space constraints, and many people just accept that as ok.

team karst · 28-08-2023, 05:34

Quote:

Originally Posted by rslifkin

Keep in mind that the over-crank flooding issue is a symptom of a poorly designed wet exhaust, not a natural limitation of wet exhausts in general.

In a properly designed system, the spillover point of the water injection elbow should be higher than the spillover after the muffler. In that situation, it takes very little force to push water out the exhaust, and even if the muffler is full, it won't overflow back to the engine.

Many systems are poorly designed due to space constraints, and many people just accept that as ok.

One also needs to consider forces of the sea trying to push seawater into the system. Thus the use of higher loops past the muffler. For whatever reason, i see power boats tend to have exit flaps, sailing boats not. Not sure why. Sailboats tend to sail with engine off.

rslifkin · 28-08-2023, 05:49

Quote:

Originally Posted by team karst

One also needs to consider forces of the sea trying to push seawater into the system. Thus the use of higher loops past the muffler. For whatever reason, i see power boats tend to have exit flaps, sailing boats not. Not sure why. Sailboats tend to sail with engine off.

I see flaps on both periodically. And yes, you do want a tall loop after the muffler for that reason. But you want an even taller one between the engine and muffler. That way if the muffler fills up (from overcranking or from seawater backflow) it'll generally flow back out the exhaust rather than into the engine.

Another option for avoiding back-flow in rough sea states is the "north sea" exhaust. You run the output of the muffler loop into a tee with a pipe going out each side of the boat. So most water that comes in just goes right out the other side and it's much harder to slosh water up over the loop and into the muffler.

Bill O · 28-08-2023, 05:56

Quote:

Originally Posted by team karst

One also needs to consider forces of the sea trying to push seawater into the system. Thus the use of higher loops past the muffler. For whatever reason, i see power boats tend to have exit flaps, sailing boats not. Not sure why. Sailboats tend to sail with engine off.

We had an exhaust flap on our last sailboat and agree it helped w/following seas. Very simple device.

Would have put a flap on this boat except it wouldn't work correctly due to the orientation of the exhaust flange.

team karst · 28-08-2023, 06:16

Quote:

Originally Posted by rslifkin

I see flaps on both periodically. And yes, you do want a tall loop after the muffler for that reason. But you want an even taller one between the engine and muffler. That way …. .

Ouch. That way starting/cranking starts filling that loop immediately. I would think cranking air would not be enough to clear it either. I submit hose from engine heads down to muffler with no loop on the way.

rslifkin · 28-08-2023, 07:52

Quote:

Originally Posted by team karst

Ouch. That way starting/cranking starts filling that loop immediately. I would think cranking air would not be enough to clear it either. I submit hose from engine heads down to muffler with no loop on the way.

Yes, it should lead directly down from the injection elbow to the muffler. The injection elbow is at the top of the "loop", so the uphill part is the dry part of the system before the injection elbow. Then the water is added on the downhill side.