Exhaust back-flooding

[Moodysailor]

Moody sailor,

thanks for such a comprehensive and clear response, I appreciate your time.

hopefully a quick question: would turning off the raw water inlet say a minute before stopping the engine also clear the exhaust? Or would you say this is a high risk strategy?

Hi,

I think you have your answer in the above, but yes - this is a high risk strategy. Most of the exhaust components have a relatively low operating temperature due to the cooling from the water so won't like being run dry. There is also the risk to the sea water pump impeller.

This is something you could do in an emergency if needed, but the risk of a further problem if something were to go wrong means I would advise it only for an emergency.

 

[DownWest]

May have missed a few bits here.. But the input to the water trap is from the top and the output from a tube down to the bottom of the box. Other wise it ain't going to work.

 

[Wing Mark]

....

As I said in one of my first posts, I've seen more damage from bad winterisation and boats being lifted incorrectly than I have from damage at sea. I'd be more worried in my boatyard than I would sailing around this island....

I've seen some dodgy and careless work in boatyards, but surely they'd have to excel themselves to make the water in the exhaust slosh around worse than sailing down a steep wave and hitting the wave in front?
I wonder if a lot of yachts are more marginal than people realise in this respect?

 

[Beneteau381]

A bad idea as the rubber hose will begin to overheat very quickly once the water stops, even a minute could cause damage and cumulatively over a season likely delaminate and block the exhaust.

I dont think so

 

[scottie]

I dont think so

I suspect you could be badly wrong!

 

[jwfrary]

I dont think so

Most hose is about 120 degrees rated. Exhaust gas is around 380-450 on an NA engine so wet exhaust will quickly fail without water. ( but not as quickly as a plastic water lock!)

 

[jwfrary]

I've seen some dodgy and careless work in boatyards, but surely they'd have to excel themselves to make the water in the exhaust slosh around worse than sailing down a steep wave and hitting the wave in front?
I wonder if a lot of yachts are more marginal than people realise in this respect?

I would tend to agree, accommodation often takes precedence to a fundamentally safe exhaust system.

 

[Moodysailor]

I've seen some dodgy and careless work in boatyards, but surely they'd have to excel themselves to make the water in the exhaust slosh around worse than sailing down a steep wave and hitting the wave in front?
I wonder if a lot of yachts are more marginal than people realise in this respect?

I think you are correct.

 

[Moodysailor]

Most hose is about 120 degrees rated. Exhaust gas is around 380-450 on an NA engine so wet exhaust will quickly fail without water. ( but not as quickly as a plastic water lock!)

Absolutely correct.

When the exhaust hose fails, it is usually delamination that occurs. The most common sign before hose failure is reduced power, as the hose blisters and delaminates it reduces the internal diameter causing a restriction in the exhaust flow. No air flow, no engine power.

 

[fisherman]

I wonder if a lot of yachts are more marginal than people realise in this respect?

This is what I suspect from the transom slap event after twenty years, must have been close all the time. Being fairly paranoid I think I would run and shut down as normal, then detach the exhaust hose at the engine end and run a dipstick down it. (or flexi pipe, blow bubbles) It might set your mind at rest, or scare the crap out of you.

 

[jwfrary]

Absolutely correct.

When the exhaust hose fails, it is usually delamination that occurs. The most common sign before hose failure is reduced power, as the hose blisters and delaminates it reduces the internal diameter causing a restriction in the exhaust flow. No air flow, no engine power.

I should add there are some specialist hose around that will take more but I have rarely seen it fitted, and only ever on high speed workboats and motor yachts where the systems fairly special.

In my experience the hose often looks fine on the outside but the inside its folded back and knackered blocking the flow.

 

[penfold]

The inside generally ends up looking like Nora Batty's stockings.

 

[zoidberg]

The inside generally ends up looking like Nora Batty's stockings.

Now there's 'specialist knowledge' that like as not will not survive the Online Harms Bill.....

 

[differentroads]

I've no great knowledge of marine diesel exhaust installation, but more than enough experience of when its not right. I've had seawater in my cylinders three times at sea. Without disasterous consequences so far, though oil consumption has gone up, so I suspect not consequence free either.

The first time was on starting the engine after a day of sailing in big following seas. I put it down to waves slapping up the exhaust outlet which was under the counter stern only 6'' above the water line and facing downwards, an obvious bad design in my judgement. That winter I relocated the outlet onto the transom, fitted a flap valve and raised the swan neck by a couple of inches. I also replaced the waterlock and exhaust hose too, for good measure.

The second time was the next year after sailing for something like 8 hours in rough, confused following seas. The following winter I fitted a second water lock at the lowest point in the hose between the mixing elbow and the original waterlock, thinking that seawater was climbing back up into the engine from this metre long low point.

This summer it happened after beam reaching for 8 hours in rough seas. I think that I've eliminated the exhaust system after the mixing elbow as the source of my woes. I've started to look at the ''marginal'' aspects of my exhaust system design. I suspect it'll be something to do with the depth of the mixing point below the waterline and the (minimal) height of the antisiphon valve. And maybe the height of the seawater strainer, too. I'll start a new post about that to seek advice once I've measured it all up.

In the meantime I've taken to turning off the seawater valve before closing down the engine in order to evacuate water. It takes only 10 seconds for water to stop coming out of the transom outlet. I fitted an exhaust temperature monitor and can say that it takes well over a minute without seawater for the temperature to rise above its normal operating maximum (which is at tickover or thrashing it at near full revs for 5 minutes or more. Its a freshwater cooled Yanmar btw.) The hose and impeller look like they are suffering no damage, though I accept that continual and prolonged use of this method could cause damage. I'm bound to delay turning the engine off one day, or something equally stupid

 

[pandos]

I've no great knowledge of marine diesel exhaust installation, but more than enough experience of when its not right. I've had seawater in my cylinders three times at sea. Without disasterous consequences so far, though oil consumption has gone up, so I suspect not consequence free either.

The first time was on starting the engine after a day of sailing in big following seas. I put it down to waves slapping up the exhaust outlet which was under the counter stern only 6'' above the water line and facing downwards, an obvious bad design in my judgement. That winter I relocated the outlet onto the transom, fitted a flap valve and raised the swan neck by a couple of inches. I also replaced the waterlock and exhaust hose too, for good measure.

The second time was the next year after sailing for something like 8 hours in rough, confused following seas. The following winter I fitted a second water lock at the lowest point in the hose between the mixing elbow and the original waterlock, thinking that seawater was climbing back up into the engine from this metre long low point.

This summer it happened after beam reaching for 8 hours in rough seas. I think that I've eliminated the exhaust system after the mixing elbow as the source of my woes. I've started to look at the ''marginal'' aspects of my exhaust system design. I suspect it'll be something to do with the depth of the mixing point below the waterline and the (minimal) height of the antisiphon valve. And maybe the height of the seawater strainer, too. I'll start a new post about that to seek advice once I've measured it all up.

In the meantime I've taken to turning off the seawater valve before closing down the engine in order to evacuate water. It takes only 10 seconds for water to stop coming out of the transom outlet. I fitted an exhaust temperature monitor and can say that it takes well over a minute without seawater for the temperature to rise above its normal operating maximum (which is at tickover or thrashing it at near full revs for 5 minutes or more. Its a freshwater cooled Yanmar btw.) The hose and impeller look like they are suffering no damage, though I accept that continual and prolonged use of this method could cause damage. I'm bound to delay turning the engine off one day, or something equally stupid

Wouldn't a better solution be to fit a small drain at the low point and leave it open when engine not in use. You could even fit a solenoid valve tied to the ignition?

 

[differentroads]

Wouldn't a better solution be to fit a small drain at the low point and leave it open when engine not in use. You could even fit a solenoid valve tied to the ignition?

Worth me looking into. I had fitted a drain to the original waterlock. I don't want to hijack this thread too much with my own problem, but I no longer think its the wet exhaust.

I mainly wanted to make a contribution about exhaust temperature when turning off the seawater valve and reinforce Wing Mark's point about some exhaust system/ engine installations being marginal. It appears that the margin of mine is short period waves of 2m or more on the stern or port beam if any seawater is left in the system! I'm lucky that it hasn't killed my engine yet. And even luckier to have a wife that'll tolerate such conditions as long as a)its no more than three times a year and b)I pretend to not enjoy it ?

 

[Poey50]

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?

 

[Wing Mark]

I think there are perhaps 3 parts to the 'question':
1) How much water is needed in the waterlock section to put the engine at risk?
2) How much water is in that area after a normal engine shutdown?
3) What conditions will cause more water to get there and how quickly?

If sea conditions can increase the water content fairly quickly, then agonising over what a normal engine shutdown leaves behind is missing the point.
If the engine normally leaves a nearly-dangerous amount of water in the system, then you need to address that before worrying about sea conditions.

 

[Wing Mark]

....

With this situation, I would be concerned from two directions:
1) it looks like there could be a fair amount of water between engine and waterlock, and in the 45 hose aft of the waterlock.
Put the boat nose down and then decelerated it sharply, will anything reach the engine?
2) waves slapping the stern might over come the gooseneck in extreme conditions? It's hard to place limits on what the sea might do if you're thinking about 'drogue conditions'

 

[Poey50]

With this situation, I would be concerned from two directions:
1) it looks like there could be a fair amount of water between engine and waterlock, and in the 45 hose aft of the waterlock.
Put the boat nose down and then decelerated it sharply, will anything reach the engine?
2) waves slapping the stern might over come the gooseneck in extreme conditions? It's hard to place limits on what the sea might do if you're thinking about 'drogue conditions'

Exactly. My thinking is that draining the waterlock at least restores the 12 inches minimum height from the water left undrained between waterlock and exhaust elbow. The drop from the elbow is near vertical so should help with the effects of sharp deceleration.