Exhaust back-flooding

[fisherman]

When I tested my exhaust system for the first time I fed water into the system via the garden hose (full bore) but the raw waterJabsco pump would have restricted the flow. (First time I've uploaded a video. Will it work?)

The easiest way is to put the hose in the top of the seacock, and close it until the water just keeps it full with the engine running.

 

[coopec]

Having a catchup this morning, there seems to be a little bit of thread drift on here causing some conflicting advice and possible misunderstanding (all very appreciable over 4 pages). I'll give my view (opinion as a time served and qualified marine engineer)

• A wet exhaust from engine back is used for RCD and class society compliance - there are maximum surface temperature requirements that have to be strictly adhered to and water injection is often easier for the engine manufacturers than heat shields or thermal blankets (it also eliminates the potential for liability defence if one were removed...)
• High rise exhaust elbow should be specified where the exhaust outlet at the engine is close to, or under the waterline
• A well designed exhaust system (in a perfect world) would have the following:
  • Water injection point well above the water-line
  • Anti-syphon valve higher again
  • A waterlock sized for the maximum expected* volume in the complete exhaust run, plus a safety factor of 2 (* this is not the same as the complete volume of the exhaust hose due to the air/water mix)
  • A gooseneck or loop at the transom that comes up well above worst case waterline
  • A downward facing exhaust outlet, alternatively a clamshell, side mounted or flap valve

The above is rare to find, as always with boats there is usually a compromise so it's a case of designing and installing as best as possible, then engineering out any problems. Individual cases (such as vessels that are hit by waves from astern whilst on their mooring) need to be looked at in their singularity as these problems are unique to that vessel, in that location. Individual considerations do not mean that every vessel needs to be over-designed.

A well-found vessel, with a decent engine installation and properly designed exhaust system will not back-flood when pooped or heeled. If any have then they are either extremely unlucky, or there is a flaw in that particular design or installation that needs to be addressed. These are not new concepts - IC engines have been around a long time and it is well known what is needed (whether or not it is actually done, or done properly is not my place to say...)

Finally, obviously a continuous assult by a raging sea with a series drogue out is different to heaving-to and riding out from the bow, so each case is different - but I would suggest that these conditions are an extremely low occurrence so again should be looked at individually. Most series drogues are retro-fitted equipment, so neither the vessel or engine builder would have thought to consider that during IC design and install. In these cases, I would suggest to series drogue installers to consider the full implications for use of this kit to all systems that may be affected - it may well be that an exhaust redesign is needed when fitting a SD.

Thanks for that.

You mention "anti siphon valve". I've installed a "tell tale" as I believe they are much more reliable than an anti siphon valve. Agree?
and
The exhaust on my yacht exits through the side of the hull (see comment 78 above). Should I glue a clam-shell over the exhaust exit?

 

[Moodysailor]

You mention "anti siphon valve". I've installed a "tell tale" as I believe they are much more reliable than an anti siphon valve. Agree?

Their function is the same (to break the syphon in the raw water injection when the engine is switched off), so all is OK. Some prefer the valve type, some prefer the open type for a tell tale, both are good provided the y work as intended.

The exhaust on my yacht exits through the side of the hull (see comment 78 above). Should I glue a clam-shell over the exhaust exit?

Not unless you have a specific need due to wave action or noise. The clamshell is there to prevent water being forced back in from waves hitting when the engine is off, it can also serve to reduce exhaust noise by redirecting the flow at the outlet. It wouldn't do anything in the case where an exhaust outlet is submerged (this is what the gooseneck/loop is for). It might actually be undesirable in your case as it's on the side of the hull (increased risk of damage from impact).


On a related note as you have a side exhaust - in this installation I would be inclined to use a loop rather than a gooseneck before the skin fitting. If space allowed, I would bring the loop inboard as close to the centre line as much as possible. This would mean that there is minimal impact from one tack to the other in terms of height above WL for the top of the loop.

 

[Poey50]

Their function is the same (to break the syphon in the raw water injection when the engine is switched off), so all is OK. Some prefer the valve type, some prefer the open type for a tell tale, both are good provided the y work as intended.

Just a small tangential note. I have the Vetus valve-type anti-syphon. The downside compared to the tell-tale is that there is no easy way to check whether the valve is working or whether it is crudded-up with salt. Obviously it is a job for preventative maintenance but I've now added a quick check to my engine start pre-checks. I have a vent tube that hangs down loose about 30 cms and a gentle blow and suck on that tells me how the valve is doing.

 

[Moodysailor]

Just a small tangential note. I have the Vetus valve-type anti-syphon. The downside compared to the tell-tale is that there is no easy way to check whether the valve is working or whether it is crudded-up with salt. Obviously it is a job for preventative maintenance but I've now added a quick check to my engine start pre-checks. I have a vent tube that hangs down loose about 30 cms and a gentle blow and suck on that tells me how the valve is doing.

Yes, fully agreed that this is an additional requirement with the valve type and well worth incorporating into routine checks as you do. I have also seen drain hoses blocked on the non-valve type as well, so it's not to say that these are fool-proof.

 

[zoidberg]

....I'll give my view (opinion as a time served and qualified marine engineer)

• High rise exhaust elbow should be specified where the exhaust outlet at the engine is close to, or under the waterline
• A well designed exhaust system (in a perfect world) would have the following:
  • Water injection point well above the water-line
  • Anti-syphon valve higher again
  • A waterlock sized for the maximum expected* volume in the complete exhaust run, plus a safety factor of 2 (* this is not the same as the complete volume of the exhaust hose due to the air/water mix)
  • A gooseneck or loop at the transom that comes up well above worst case waterline
  • A downward facing exhaust outlet, alternatively a clamshell, side mounted or flap valve

Individual cases (such as vessels that are hit by waves from astern whilst on their mooring) need to be looked at in their singularity .
Finally, obviously a continuous assult by a raging sea with a series drogue out is different to heaving-to and riding out from the bow..... Most series drogues are retro-fitted equipment. In these cases, I would suggest to series drogue installers to consider the full implications for use of this kit.....

I much appreciate this input and, with reference to my Original Post, it is most relevant. I AM considering the full implications, hence this post...

I do have a suitable waterlock and a gooseneck ( both Vetus ).
I do have a 'high rise' raw cooling water tube with a 'piddle' telltale tube, acting as an antisiphon device.
I am unable to accommodate the desired 'high rise exhaust elbow' as recommended, due to insufficient undercockpit headroom.

There is what seems to be clear and excellent advice in the BetaMarine Installation Guide, pages 24-28.
OM 221 20031 HE REV28 1121

I'm still intent on fitting a 'flapvalve' of some sort.

 

[Moodysailor]

I much appreciate this input and, with reference to my Original Post, it is most relevant. I AM considering the full implications, hence this post...

I do have a suitable waterlock and a gooseneck ( both Vetus ).
I do have a 'high rise' cooling water tube with a 'piddle' telltale tube, acting as an antisiphon device.
I am unable to accommodate the desired 'high rise exhaust elbow' as recommended, due to insufficient undercockpit headroom.

There is what seems to be clear and excellent advice in the BetaMarine Installation Guide, pages 24-28.
OM 221 20031 HE REV28 1121

I'm still intent on fitting a 'flapvalve' of some sort.

In your case, it falls under the engineering requirement I mentioned.

Without seeing your particular installation these are just blind comments, but as the goal is to keep water out of the engine, it might be possible in your case to use a water separator, or a cascade waterlock design - with one waterlock acting as an engine preventer, and one to catch anything in the exhaust line. But the reality and practicality of this may make it too difficult to achieve. A water separator is good as it reduces or removes the need for a continual downward run from the engine back.

On our previous boat we had a similar issue and the eventual solution used by the builder was to have a dry exhaust from the engine up to a high central point, with a custom mixing elbow here and a water cooled exhaust from this point aft. I was never enamored with it as there was a big dry section that needed lagging, but it worked perfectly.

 

[zoidberg]

In your case, it falls under the engineering requirement I mentioned.

Yes.

Marine engineers are thin on the ground in my neighbourhood, as are marine 'leccys. I have to make-do with 'online consultancy'.....

BetaMarine's excellent people at Gloucester considered my installation to be sound, but the JSD/storm issue was, as indicated, 'above and beyond'.
ASAP's people have suggested this Inline Check Valve. It's ten times the cost of a flapvalve.

What's the view/experience of such devices?

 

[scottie]

Yes.

Marine engineers are thin on the ground in my neighbourhood, as are marine 'leccys. I have to make-do with 'online consultancy'.....

BetaMarine's excellent people at Gloucester considered my installation to be sound, but the JSD/storm issue was, as indicated, 'above and beyond'.
ASAP's people have suggested this Inline Check Valve. It's ten times the cost of a flapvalve.

What's the view/experience of such devices?

There used to be and may still be a rubber version that used an internal cone with slits that acted as a nonreturn valve but too many failed when the cone lost its flexibility

 

[coopec]

Yes.

Marine engineers are thin on the ground in my neighbourhood, as are marine 'leccys. I have to make-do with 'online consultancy'.....

BetaMarine's excellent people at Gloucester considered my installation to be sound, but the JSD/storm issue was, as indicated, 'above and beyond'.
ASAP's people have suggested this Inline Check Valve. It's ten times the cost of a flapvalve.

What's the view/experience of such devices?

There's a few threads here that may be worth reading?

 

[Moodysailor]

Yes.

Marine engineers are thin on the ground in my neighbourhood, as are marine 'leccys. I have to make-do with 'online consultancy'.....

BetaMarine's excellent people at Gloucester considered my installation to be sound, but the JSD/storm issue was, as indicated, 'above and beyond'.
ASAP's people have suggested this Inline Check Valve. It's ten times the cost of a flapvalve.

What's the view/experience of such devices?

Hi, sorry if I cam across bit brash - the curse of the lack of context with the typed word sometimes...

Centex stuff is good quality, the US equivalent of Halyard, so I'd have no issue fitting their equipment. I haven't fitted any of these personally, so my advice is to consider two things:

• Ask yourself if this is a "solution looking for a problem" - i.e, is there something else you can do first?
• Only fit a non-return valve in an exhaust line if it has no moving parts. I had a look on Centex's website and couldn't see how it is constructed internally so definitely check with ASAP before purchasing. If it has a moving part (likely an internal hinged flap if anything) then it will eventually fail - it's a when, not if. Hot, corrosive marine exhaust is not a pleasant place for anything complex to exists, so plan with that in mind.

 

[zoidberg]

Not at all. No 'huff' taken or given. You're right, and I'll check/ensure the construction isn't adding more problem than it solves.

 

[fisherman]

The more I read here the more I think the flat hose tail is the best option. Even a weighted counterbalanced flap might be forced upwards with the right splosh.
I toyed with having a wet exhaust because the dry one kept needing replacement, and dismantling one bit often wrecked a lot of it. Eventually I sourced better parts, S/S and from the local car factor's customising department.
I didn't want wet because the outlet would be below deck and I frequently loaded the boat enough to drown it, sometimes left overnight and drying out, so change of attitude thrown in. Lucky escape I suspect.

 

[zoidberg]

The more I read here the more I think the flat hose tail is the best option. Even a weighted counterbalanced flap might be forced upwards with the right splosh.

What d'you think would be appropriate material?

 

[Poey50]

I certainly have no wish to constrain or steer this topic, but reflect on the original source of concern - that of a 'Jesterer' dragging a Jordan Series Drogue downwind in a developed storm, where the boat is progressing at about 2 knots, while the seas breaking against her transom ( and exhaust outlet ) are travelling at around 20 knots ( ~10m/sec )..... hour after hour.

'fisherman' has suggested an answer I can readily retrofit.... and shall.

I'm also pondering, in such rare circumstances, the merits of running the engine at idle/neutral for the duration, to maintain a positive pressure of exhaust gas.

I've got similar ambitions and the same concerns about the specific problems of engine flooding when deploying a JSD. The combination of the exhaust through-hull being pulled downward at the same time as being hit hard by a wave whilst the boat speed is restrained and this being repeated over and over for maybe 2 to 3 days certainly suggests to me that extra thought is required even for otherwise well-designed systems. I found this article helpful and it specifically mentions a boat slowed by drogue. Cool, Quiet, and Trouble Free Exhaust - Good Old Boat

Personally I'm just going to straight-forwardly follow Nigel Calder's advice and fit a shut-off valve near the transom. I can see flaps getting stuck or blasted-off or potential back-pressure problems if something sticks. A valve can be fitted reasonably in reach on my boat and I think it possible to set up safe reminders of the open / closed status. It would be rarely deployed (closed) - perhaps never - just like the JSD. I think my hose is 50mm so the largest TruDesign should do it. It is rated to 120 degrees C, higher than the hose.

 

[fisherman]

What d'you think would be appropriate material?

Fire Hose | Layflat Hose | PAR Group

might do it. Last time I got some from the local salvage yard. Trouble is a lot of skin fittings are flush, no spigot

 

[Moodysailor]

I've got similar ambitions and the same concerns about the specific problems of engine flooding when deploying a JSD. The combination of the exhaust through-hull being pulled downward at the same time as being hit hard by a wave whilst the boat speed is restrained and this being repeated over and over for maybe 2 to 3 days certainly suggests to me that extra thought is required even for otherwise well-designed systems. I found this article helpful and it specifically mentions a boat slowed by drogue. Cool, Quiet, and Trouble Free Exhaust - Good Old Boat

Personally I'm just going to straight-forwardly follow Nigel Calder's advice and fit a shut-off valve near the transom. I can see flaps getting stuck or blasted-off or potential back-pressure problems if something sticks. A valve can be fitted reasonably in reach on my boat and I think it possible to set up safe reminders of the open / closed status. It would be rarely deployed (closed) - perhaps never - just like the JSD. I think my hose is 50mm so the largest TruDesign should do it. It is rated to 120 degrees C, higher than the hose.

I'm in the same frame of mind as the writer of the article you linked regarding valves in the exhaust line - they should be fitted only when there is no other option "For these reasons, we consider the valve as an option of last resort to be used only if the geometry of the boat does not allow a layout that can function properly without it. "

If you do need to fit a valve, it might be worth considering also fitting a switch of some kind to lock out the ignition - or in the simplest form, drill a hole in the handle and clip the engine start key to it with a karabiner! That way, once all is over it will be near impossible to start the engine with the valve closed.

Thinking on this last night, I'm of the view that the best combination for a vessel with a JSD setup (if space and layout allows) would be to have a dry exhaust section from the engine to a much higher point, as close to the centreline as possible. Site the water injection point here, and a conventional exhaust arrangement from here back. After working through this great thread, it is now my view that a combination of high pitch angles, wave motion and stern coming under water could overcome most conventional wet exhaust systems with a JSD deployed.

 

[Moodysailor]

Fire Hose | Layflat Hose | PAR Group

might do it. Last time I got some from the local salvage yard. Trouble is a lot of skin fittings are flush, no spigot

On a slightly related note, we use this setup for the transom drain on our dinghy as the factory one was rubbish. Works brilliantly for that too..

 

[Poey50]

... or in the simplest form, drill a hole in the handle and clip the engine start key to it with a karabiner! That way, once all is over it will be near impossible to start the engine with the valve closed.

That's the one for me. Thanks.

 

[fisherman]

On a slightly related note, we use this setup for the transom drain on our dinghy as the factory one was rubbish. Works brilliantly for that too..

Yes, one local boat has an old rubber glove clipped on with the finger ends cut off