Exhaust back-flooding

[fisherman]

a combination of high pitch angles, wave motion and stern coming under water could overcome most conventional wet exhaust systems with a JSD deployed.

Worst case scenario, a knockdown and your whole exhaust system is laid on its side, nothing is going to help then except a shut off valve, and even so there will be residual water in the system.......and there lies the nub. Even though you have shut an outboard valve after running the engine the water is still in the system, and can in extremis overtop the gooseneck at the engine end......if you have one. This is getting worse all the time, now you need to drain it as well.

 

[Moodysailor]

Worst case scenario, a knockdown and your whole exhaust system is laid on its side, nothing is going to help then except a shut off valve, and even so there will be residual water in the system.......and there lies the nub. Even though you have shut an outboard valve after running the engine the water is still in the system, and can in extremis overtop the gooseneck at the engine end......if you have one. This is getting worse all the time, now you need to drain it as well.

For me, a knockdown would also introduce a lot of other issues - water in the exhaust is only one factor.

The valve is only there to prevent water being forced back up the exhaust, it is not going to prevent anything further upstream running back. Now the valve could be fitted closer to the engine......

 

[Poey50]

Worst case scenario, a knockdown and your whole exhaust system is laid on its side, nothing is going to help then except a shut off valve, and even so there will be residual water in the system.......and there lies the nub. Even though you have shut an outboard valve after running the engine the water is still in the system, and can in extremis overtop the gooseneck at the engine end......if you have one. This is getting worse all the time, now you need to drain it as well.

Interesting point. I'm going to add 'draining the water-lock' to my preparing for heavy weather checklist.

 

[fisherman]

For me, a knockdown would also introduce a lot of other issues - water in the exhaust is only one factor.

The valve is only there to prevent water being forced back up the exhaust, it is not going to prevent anything further upstream running back. Now the valve could be fitted closer to the engine......

It would be a nice probvlem not to have though, with maybe a compromised rig.
The point I keep coming back to is we can't know how much water is present after shutdown. It may be within one foot or one inch of flooding the engine.

 

[bikedaft]

I do drain the waterlock, at the low point of the U after engine/mixing elbow, and shut the seacock if going to be turning engine off in heavy weather. Now you've got me wondering if i need to drain a bit of the exhaust before the water lock/whick part of the waterlock is the drain sited on? Ho hum ☺

 

[Moodysailor]

It would be a nice probvlem not to have though, with maybe a compromised rig.
The point I keep coming back to is we can't know how much water is present after shutdown. It may be within one foot or one inch of flooding the engine.

The maximum water volume that can be contained in the exhaust system is easy enough to calculate - it needs to be done for waterlock sizing for example.

If the valve is close to the exhaust outlet, then the amount of water remaining in the system between engine and valve will correspond to this volume. If knockdown is a risk, this should be drained.

Alternatively, the shutoff valve could be located between the engine and waterlock - this would negate the need to drain the waterlock prior to heavy weather. The decision to drain or not could be taken after the weather has passed depending on the severity of the storm and what happened to the vessel.

 

[zoidberg]

I'm advised by those good folks at ASAP that the Check Valve above has an internal rubber flap.....

'Fisherman's suggestion appeals to my uncivilised sense of humour. I see a large yellow 'Marigold' with two fingers remaining and inflated, the rest cut off.

 

[zoidberg]

Here's a further question from mulling the thought that. in the aftermath of a knockdown, some water residual in the waterlock might have made its way back to the cylinders' tops..... thereby risking a bent rod on engine start.

I have a facility to turn over the engine, slowly, using a socket and drive on the lower pulley nut. That wouldn't be/isn't for starting. I'm querying whether one could exhaust most of any water sitting on the piston-heads, and feel if there's 'hydralock' or otherwise....

 

[fisherman]

I'm advised by those good folks at ASAP that the Check Valve above has an internal rubber flap.....

'Fisherman's suggestion appeals to my uncivilised sense of humour. I see a large yellow 'Marigold' with two fingers remaining and inflated, the rest cut off.

...or a big horrible willy

 

[Moodysailor]

Here's a further question from mulling the thought that. in the aftermath of a knockdown, some water residual in the waterlock might have made its way back to the cylinders' tops..... thereby risking a bent rod on engine start.

I have a facility to turn over the engine, slowly, using a socket and drive on the lower pulley nut. That wouldn't be/isn't for starting. I'm querying whether one could exhaust most of any water sitting on the piston-heads, and feel if there's 'hydralock' or otherwise....

In that case, it is unlikely that the water will be evacuated from the cylinder (remotely possible, but unlikely), and you will feel if there is a hydraulic lock as the fluid is incompressible. The cylinder is designed to trap air inside and compress that, so by design will not want to let anything (gas or fluid) out. Even if you were lucky enough to turn it over this way, some water will force it's way into the crankcase which will contaminate the oil also. Not necessarily a show-stopper in an emergency, but will need to be addressed PDQ.

In the case you mention, depending on the engine the easiest/best option will be to remove the glow plugs or injector and evacuate the water from the engine that way. I've seen someone drain a Lister-Petter from water using the decompression lever in the past, but those devices are long gone......

The attention should definitely be on keeping water out. Once water gets into an engine, unless it is properly removed it can cause internal problems that go unnoticed - until one day!

 

[fisherman]

The maximum water volume that can be contained in the exhaust system is easy enough to calculate - it needs to be done for waterlock sizing for example.

If the valve is close to the exhaust outlet, then the amount of water remaining in the system between engine and valve will correspond to this volume. If knockdown is a risk, this should be drained.

Hang on-, yes we know the volume of the system but I keep banging on, we don't know the gas/water ratio in that volume with so many variables, raw water pump flow, gas flow, pressure and speed, and how tortuous the pipework is. A high transom gooseneck is going to keep a lot of water back.

 

[Moodysailor]

Hang on-, yes we know the volume of the system but I keep banging on, we don't know the gas/water ratio in that volume with so many variables, raw water pump flow, gas flow, pressure and speed

With the engine data it is possible to calculate quite accurately across several operating modes, but this isn't necessary. FWIW, we used to size waterlocks (WL) off 40% of total volume given normal use, with a added margin of safety. The hose is supposed to have a downwards run from the engine back to the gooseneck, so the volume contained in the WL only needs to hold a small proportion of the total volume. Note, this is thread drift as this is for normal calculations, not for engineering of unique situations (as use of JSD would be, for example).

You can size for 100% of hose volume if you feel it is necessary as the amount of water in the exhaust system will always be less than 100% of the hose volume under normal and extreme use. In fact, this is what we would do if there was an upwards run from the WL to the transom (more common than expected). This calculation will suffice for 80-90% of installs as the vast majority do not experience water ingress via the exhaust outlet.

Up to the point of practicality (size, space & weight), it does not matter to have more available volume in the WL, so in the absence of any specific requirement, simply getting the biggest possible one will probably be more than enough (by rule of thumb).

I have done a lot of generator and engine installs over my time, the majority do not need to be over-thought, or over-engineered. However, in the cases where we have had the requirement to make the calculations, needed to reverse-engineer or understand a failure or something then the honest fact usually comes up that there are more things to consider (multiple smoking guns...). When we have looked at "failsafe" installs, we usually went with a multi-step approach (water-lift silencer/WL, water separator, custom exhaust risers, custom outlets with gooseneck, etc). All using high quality equipment.

I would add that most small vessels have adequately sized WL installed, but more often than not these are just not put in the best place, so the engine is still vulnerable.

 

[Poey50]

I have a Vetus plastic muffler fitted on quite a steep downslope between the top of the gooseneck and the transom outlet. I'm just wondering what contribution that might make (in addition to the gooseneck) in resisting back-flooding. Vetus don't make any claim for this potential benefit - or none that I could find.

Vetus Exhaust Muffler | Force 4 Chandlery

 

[Moodysailor]

I have a Vetus plastic muffler fitted on quite a steep downslope between the top of the gooseneck and the transom outlet. I'm just wondering what contribution that might make (in addition to the gooseneck) in resisting back-flooding. Vetus don't make any claim for this potential benefit - or none that I could find.

Vetus Exhaust Muffler | Force 4 Chandlery

It adds overall volume to the system, which is good in one hand. But it won't prevent or help anything that could overcome the gooseneck due to it's location in the system - one would need a waterlock further upstream for that.

It has the primary purpose of reducing noise, but the secondary benefit of adding some volume just before the transom fitting is handy.

 

[Wing Mark]

I have a Vetus plastic muffler fitted on quite a steep downslope between the top of the gooseneck and the transom outlet. I'm just wondering what contribution that might make (in addition to the gooseneck) in resisting back-flooding. Vetus don't make any claim for this potential benefit - or none that I could find.

Vetus Exhaust Muffler | Force 4 Chandlery

I'd guess one of those mufflers on a steep down slope would do very little, Vetus saythey work by mixing the gases and water, which won't happen much if the water is running straight out?

I've looked at a few boats over the past few months, AFAIK, every one of them had the exhaust hose going uphill from waterlock to gooseneck at the transom, following the slope of the hull.
Mine has two mufflers in that line.
That's a lot of volume of water/gas mix to run back to the waterlock.

But that only affects how many cupfuls of water need to make it over the swan neck, to have problems in a following sea.

If these were 21st century cars, there would be a sensor to detect too much water in the waterlock.

 

[Moodysailor]

If these were 21st century cars, there would be a sensor to detect too much water in the waterlock.

I have seen a lot of installations as you describe too. It's important to remember that the vast majority of boats operate just fine like this, the gooseneck / loop does a very good job in reality. A lot of this thread has been about extreme / unique cases, or poorly designed/engineered solutions, so it's important for us to not throw the baby out with the bath water.

A water level sensor in the waterlock would not be a practical solution unfortunately as it would have to be disabled when the engine is running. I think a simple prevention is better than cure philosophy is applicable here.

I am still an advocate of a water separator close to the outlet if there is space to fit one. This, and a good sized waterlock/lift silencer would solve pretty much all the problem with back-flooding (Example water separator). It creates an additional problem of how to tell your engine is pumping water though

 

[fisherman]

My contention is that it makes no difference how big the muffler, how long the pipe, it will still contain the same % mix of water and gas, so the system will contain x% water. BUT, it seems very likely that most systems are well over spec in terms of holding water, otherwise there would be more incidents and we would be relating many more experiences of flooded engines.
Both wet exhausts I had were just a swan neck with water injection bend, and a 3m or so hose, in a gentle catenary down and up to the outlet, which was not much lower than the swan neck. I remember being aware of a possible problem when my crew shut the engine down when were loaded and I yelled at him to start it again quick, and no problem. That boat is still fishing and I have spoken to successive owners who have had no trouble. The other one had the slap up flooding, but so did other boats the same night. Its outlet was very high off the water at rest, but the boat sat head down. A flap would have saved it, £8k.

 

[jamie N]

I've not got a picture to hand, but if one had a look at an RNLI boat, they've simply got a flap on a hinge, covering the exhaust outlet.
I reckon that they've a clue or two about this stuff, and might just be on to a good thing with it.

 

[zoidberg]

[QUOTE="Moodysailor, post: 7842682, member: 1818]...... and you will feel if there is a hydraulic lock as the fluid is incompressible.
In the case you mention, depending on the engine the easiest/best option will be to remove the glow plugs or injector and evacuate the water from the engine that way.....
[/QUOTE]
I agree. One needs to take a balanced view of the risks and what one is prepared to do to mitigate them. I like the elbow-length Marigold glove clamped on to the transom outlet, ( which colour does the team suggest? ) and I will explore fitting a shutoff valve somewhere accessible.

I know it is perfectly possible to suffer a severe knockdown even when one hasn't ( yet ) deployed a JSD, for a breaking sea just ~60% of boat length is likely to do that, if taken beam-on. ( Wolfson Unit research ). If I still have even half a mast after that, I can sail the boat to harbour and put her alongside/onto a mooring.

Provided I can find the tools to turn the engine over manually, feeling for 'hydrolock', and if I can pull glowplugs/injectors to 'mop up' whatever is in there, then I can live with emulsified oil, etc. until I can find my way to a diesel engineer.

 

[fisherman]

I've not got a picture to hand, but if one had a look at an RNLI boat, they've simply got a flap on a hinge, covering the exhaust outlet.
I reckon that they've a clue or two about this stuff, and might just be on to a good thing with it.

And you don't think there might be other measures in a self righting boat?