Exhaust back-flooding

[differentroads]

This is a rough and not well-proportioned sketch of the exhaust system on my Sadler 32 which I'm preparing with offshore passages in mind. It will be fitted with a Jordan Series Drogue. There is a healthy-looking high gooseneck (together with an almost vertical muffler) but the height of the injection point of raw water into the exhaust elbow is a meagre 3-4 inches, nowhere near the suggested 10-12 inches vertical height. Because of this the hose from the elbow dips well below the drain point for the 4.5 litre waterlock. 'A' is the estimated height of water when the waterlock is full and 'B' is the height drained. (Some undrained water will always be present.)

I'm thinking that my best strategy is to drain the waterlock before a lift and with any sea that can seriously hobby-horse the boat. The spigot on the waterlock will allow a hose to be fitted to make an accessible drain point. I'm still contemplating whether to fit a ball valve above the muffler as an extra safeguard especially if ever deploying the JSD. it would have to be a 38mm ball valve with 45mm hosetails but I'm not sure if that extra constriction would have a bad effect on back pressure. I should add that in six years I've never had any issues in strong conditions although nothing to compare with the specific problems with JSD deployment which is where this thread started. Thoughts?

My engine/exhaust layout is very similar to yours. I've had seawater in the engine three times in F6/7 conditions and think I have eliminated the wet exhaust side as the cause of my own problems by moving the outlet onto the transom rather than under it, adding a flap valve to the transom outlet, raising the swan neck and even adding a second 3L waterlock at my exhaust low point exactly where you have identified a risk. And still I got seawater in the cylinders in rough conditions.

I'm not saying those precautions are not worthwhile. I'm happier (but poorer) improving what I think was a marginal installation. Just that they haven't cured my own non-hypothetical problem. The height of the mixing point, the anti-siphon valve and the strainer are other risk areas relevant to the question posed, I think

 

[fisherman]

I have made this point so many times I begin to think I must be wrong.

It makes no difference what the capacity/length of the system is, the % water stays the same, and that water stubbornly seeks its own level. What does (make a difference) is the overall inclination between engine and high point.
If the sketch above was accurate I would say that's a flood on shutdown situation, due to the capacity of the 45mm aft section being so much greater than the forward engine/muffler section. OK, not accurate, thankfully.
The 'healthy looking high gooseneck' can hold water in the system which might drain back to the engine in benign or less benign conditions. Take it to the extreme, make the transom gooseneck 20 feet high, engine flooding on shutdown inevitable. Reduce the height, at some point is that at which the water will just reach the engine, then you need a safety margin for weather/lifting/pump vagaries.

The luxury most of you don't have is enough height in the engine room for a decent gooseneck at the dry end.

 

[ianat182]

I wonder if using a form of manometer tubing might help to at least reduce the likelyhood of overcoming the u-bend and outlet,; by a second higher U-bend on the inboard to engine part of the exhaust tubing.

ianat182

 

[Wing Mark]

I have made this point so many times I begin to think I must be wrong.

It makes no difference what the capacity/length of the system is, the % water stays the same, and that water stubbornly seeks its own level. What does is the overall inclination between engine and high point.
......

Seems to me, the waterlock and the hose between engine and water lock can hold x-amount of water.

How much water is in he section from water lock to top of transom gooseneck at shutdown will depend on many things.
Obviously the volume of it is a hard limit. But idling at say 700 rpm, the engine will blow some water out . How much will vary with many factors, size and slop of pipe, how straight it is, internal features which mix the gas and water etc etc. Some of those factors might change if the boat is heeling one way or the other.

A bigger hose not only holds more air and water, but also lowers the air speed so the water is blown out less effectively.
I'm sure firms like Vetus know a bit about this and design their parts to work well in a typical installation.
But people seem to install stuff in a yacht vaguely copying 'normal practice'?

How many people have every checked the water content of the exhaust after installing an engine?

My boat has a fairly long run in wide pipe from waterlock to the transom gooseneck, but there's a muffler in it, which might be mixing the air and water, ensuring that more water gets blown over the gooseneck?
I'd expect a waterlock to be designed ot mix the air and water, so the max amount of water is blown out, but I'd also expect a long pipe to tend to separate the water, Water which touches the sides will tend to run back, you can imagine a long smooth section of pipe with water in the bottom going one wat and air in the top going the other until the water builds up too much/

Just looking at an idling engine running at 700rpm and the pulses of water coming out the exhaust every couple of seconds, it's clearly not anything like a steady stream of air mixed with water in a constant %.
Quite different at full chat though.

 

[Poey50]

The Vetus formula for calculating the size of the waterlock muffler is interesting for the assumption it makes. Namely that 25% of the hose from waterlock to gooseneck is filled with water. It also doubles the available volume of the waterlock as a safety margin. That may be one reason why I've never had a problem - so far - in the 6 seasons since I've owned the boat despite the apparent problems of a low height difference between top of waterlock and raw water injection point. Still I'm not complacent and the sudden halting of forward motion followed by tilting sounds very much like the bungee cord-type arrest from a series drogue. I haven't heard any argument against draining the waterlock with the possible addition of closing the exhaust near the transom as tactics.

 

[NormanS]

There's something about this that I'm not getting. Are the proponents of fitting a drain to the exhaust, because of the possibility of water coming in from outside, considering the fact that this water coming from outside is now going in to the bottom of the boat? Is this a good plan?

 

[DownWest]

There's something about this that I'm not getting. Are the proponents of fitting a drain to the exhaust, because of the possibility of water coming in from outside, considering the fact that this water coming from outside is now going in to the bottom of the boat? Is this a good plan?

Rather let the bilge pump supp it up than the engine?

 

[Wing Mark]

There's something about this that I'm not getting. Are the proponents of fitting a drain to the exhaust, because of the possibility of water coming in from outside, considering the fact that this water coming from outside is now going in to the bottom of the boat? Is this a good plan?

A better plan than having it in the engine I'd say.
But not great, unless you can drain it into a container.
It will be dirty oily smelly water.
On my boat I suspect it would be easier to disconnect the exhaust from the engine and shove a wooden plug in the hose.

 

[Poey50]

What I have in mind are two possible interventions - draining the waterlock muffler and closing the exhaust at the transom gooseneck. How these might be used is as follows.

1. Boat going to be lifted - drain the waterlock muffler.

2. Boat sailing in following seas - risk coming from the motion of the boat tipping the waterlock water forward into the engine - drain the waterlock muffler into the bilge (around 2-3 litres) then close drain.

3. Boat with series drogue deployed but with waterlock muffler already drained as above. Risk comes from water forced into the exhaust - close the exhaust to prevent more water reaching the previously drained waterlock muffler.

In heavy seas there is another possible option between 2 and 3 which is is to temporarily open the drain to see if more water has got into the waterlock muffler from the outlet. If so then closing the exhaust is sensible. In none of these options does the waterlock muffler drain need to be left open.

Another option without fitting an exhaust valve would be to simply keep the drain open and have the bilge pump clear it. But that. isn't my plan.

 

[DownWest]

On Poey's sketch, the dip from the injection point before the water lock is wrong (IMHO) Too much water sitting before the lock. In the example I quoted near the top of this thread, the drop from the engine to the lock was maybe 30cm and the lock was not well designed to avoid surges (VP) Finding some space lower down with a custom GRP lock with much more capacity sorted the problem.

On the recent re-engine of a Jeanneau, the injection point was about 50cm above the Vetus box and the hull fitting was on the stb quarter.Not a worry.

 

[DownWest]

What I have in mind are two possible interventions - draining the waterlock muffler and closing the exhaust at the transom gooseneck. How these might be used is as follows.

1. Boat going to be lifted - drain the waterlock muffler.

2. Boat sailing in following seas - risk coming from the motion of the boat tipping the waterlock water forward into the engine - drain the waterlock muffler into the bilge (around 2-3 litres) then close drain.

3. Boat with series drogue deployed but with waterlock muffler already drained as above. Risk comes from water forced into the exhaust - close the exhaust to prevent more water reaching the waterlock muffler.

In heavy seas there is another possible option between 2 and 3 which is is to temporarily open the drain to see if more water has got into the waterlock muffler from the outlet. If so then closing the exhaust is sensible. In none of these options does the waterlock muffler drain need to be left open.

This supposes an attention to detail which one would hope a long distance sailer would apply. But, it would be a good idea if it wasn't needed. Still, check lists on an approaching blow would help. Plenty of other things to do , along with the drogue.

 

[Poey50]

This supposes an attention to detail which one would hope a long distance sailer would apply. But, it would be a good idea if it wasn't needed. Still, check lists on an approaching blow would help. Plenty of other things to do , along with the drogue.

Check-list is already drafted.

 

[Poey50]

On Poey's sketch, the dip from the injection point before the water lock is wrong (IMHO) Too much water sitting before the lock. In the example I quoted near the top of this thread, the drop from the engine to the lock was maybe 30cm and the lock was not well designed to avoid surges (VP) Finding some space lower down with a custom GRP lock with much more capacity sorted the problem.

Yes, that is the main weakness of my system as far as I can judge. However it has never proved a problem ... so far. But in riskier conditions draining the waterlock lowers the level of this residual water. It then has a near-vertical hose to climb about 13-14 inches to reach the exhaust elbow and cause trouble. Draining then introduces the 12 inch minimum which exhaust designers recommend as safe. The exception is if the boat is laid on it's side but then it will have to compete with other problems.

 

[NormanS]

A better plan than having it in the engine I'd say.
But not great, unless you can drain it into a container.
It will be dirty oily smelly water.
On my boat I suspect it would be easier to disconnect the exhaust from the engine and shove a wooden plug in the hose.

Disconnecting the exhaust from the engine, isn't a five minute job on my boat anyway. Closing a valve, if one was fitted, would seem much easier. That's what I used to do in a previous boat, as mentioned way back in this thread.

 

[fisherman]

Seems to me some of this is 'think of a number and double it' country, given all the variables. I'm glad I didn't go wet, and now if I did I would certainly disconnect and find the actual water level at shutdown. (Unless I had a supersafe high engine room dry gooseneck)

 

[Poey50]

Seems to me some of this is 'think of a number and double it' country, given all the variables. I'm glad I didn't go wet, and now if I did I would certainly disconnect and find the actual water level at shutdown.

I'm definitely going to do this.

 

[BabaYaga]

Yes, that is the main weakness of my system as far as I can judge. However it has never proved a problem ... so far. But in riskier conditions draining the waterlock lowers the level of this residual water. It then has a near-vertical hose to climb about 13-14 inches to reach the exhaust elbow and cause trouble. Draining then introduces the 12 inch minimum which exhaust designers recommend as safe. The exception is if the boat is laid on it's side but then it will have to compete with other problems.

With regard to your sketch, the main weakness remains IMHO, whether you drain the waterlock or not,
It's the water standing in the dipped hose between engine exhaust bend and waterlock that is the problem, I think.
Could you not move the waterlock closer to the engine, so that you get a strait down hill hose run into the waterlock?
That would make the system more in line with the recommendations.

 

[Poey50]

With regard to your sketch, the main weakness remains IMHO, whether you drain the waterlock or not,

I don't see the logic of draining making no difference, if that is what you are suggesting, (other than if the boat is flattened).

Could you not move the waterlock closer to the engine, so that you get a strait down hill hose run into the waterlock?
That would make the system more in line with the recommendations.

That would certainly be ideal and I have looked for options that could fit in the limited available space.

 

[DownWest]

Not relevent..

 

[BabaYaga]

I don't see the logic of draining making no difference, if that is what you are suggesting, (other than if the boat is flattened).

My assumption is that it is the small amount of water standing in the dipping section of hose upstream of the waterlock that is the main danger to the engine.
This water will be uneffected by the waterlock being drained.

That would certainly be ideal and I have looked for options that could fit in the limited available space.

One other option could be to install a high rise exhaust bend on the engine. This will make more space available for the waterlock and create a higher threshold between exhaust system and engine, making back flow less likely.