fisherman
Well-Known Member
ASAP do them, but the website's down at the mo.
Exhaust back-flooding
- Thread starter zoidberg
- Start date
Wing Mark
Well-Known Member
Is there a danger of creating some sort of woodwind instrument gone bad?
You can buy stainless back draught shutters to fit in ventilation systems, but the smallest looks like 100mm diameter....
zoidberg
Well-Known Member
This thread has ranged into circumstances and arrangements far and wide, and that's both instructive and appreciated. The repeated 'wave slap' on the transom of a fore-and-aft moored boat with a low system certainly is a concern I've not seen/heard explored elsewhere.
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 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.
coopec
N/A
I'm surprised by some of the comments made here as they seem suggest there is a lot of water mixed with the exhaust gases.
After the raw water has cooled the engine it is then injected into the mixer (water/exhaust gases) where most of the water becomes steam. Here is a shot of a powerful boat at idle.
After the raw water has cooled the engine it is then injected into the mixer (water/exhaust gases) where most of the water becomes steam. Here is a shot of a powerful boat at idle.
fisherman
Well-Known Member
I reckon the flat hose option with some additions, like the outlet from a woopee cushion, could be made very musical
coopec
N/A
If the exhaust/goose-neck is full of air how can water enter it?
fisherman
Well-Known Member
I'll amend my assertion: the exhaust system contains x% gas, y% water and z% steam....but so variable you still don't know how much, it's all guesswork. However, the scientist sitting opposite me tells me that water density is 1000 kg/m3, and steam is 0.6.
fisherman
Well-Known Member
By coming half way up the diameter.
bikedaft
Well-Known Member
There is about as much water in the exhaust of a small marine diesel as is pumped there by the impeller. The water is less than 100C when it enters the exhaust and less than that throughout the exhaust and exit. (I continuously measure exhaust temp just beyond mixing elbow, mostly to check exhaust and engine is still being cooled by said water...) If i cannot touch the exhaust by hand, it's running too hot.
Steam is 1000x the corresponding volume of water, please no explosions in my exhaust.
The vid you linked too clearly shows water (and exhaust gas) exiting the exhaust.
coopec
N/A
How many liters of raw water per minute of water does it take to cool (say) a Perkins 4-108 and how many CFM of exhaust gases in one minute?
coopec
N/A
So water will go up one side of the goose-neck and displace the air?
"What happens when you get an airlock in a pipe? If air gets trapped in your central heating or hot water system, this can stop hot water from circulating, causing radiators to stop heating up and hot water taps to stop running. Getting the hot water flowing again requires releasing the air out of the system."
Clearing an Airlock in Pipes: How can you do it yourself? - Help is here!
jwfrary
Well-Known Member
No really true at idle where theres a greater proportion of water than exhaust
Wing Mark
Well-Known Member
The engine pumps a lot more air than the seawater pump pumps water. My engine is 580cc or so and the pump move a cc or two per rev?
But that airflow is not 100% efficient at removing the water from the pipe work, it's not a uniform 1% of water, it's lumps of water sloshing around as air blows through.
I think water is mixed with the air as a spray, but hits the sides of the pipework and runs back towards the waterlock, then it builds up until the air throws out another gush.
The engine is idling at 800 rpm and the exhuast is throwing out spurts of water maybe once a second.
At high rpm, the airflow is faster and the water comes out the back in more of a steady stream..
zoidberg
Well-Known Member
ASAP do several of them. Here's a link to one.... exhaust-outlet-flap-40mm-51mm
At well under £20 to solve the problem, that's going into my 'Letter To Santa'.
pandos
Well-Known Member
That's a very interesting read...there was a thread a few months ago about damage to an engine in a yard, the contention at that time was that it was from power washing...seems his view that the yard may have dameged the engine was not too far fetched, but his view on how they may have done it was probably incorrect...
fisherman
Well-Known Member
The problem may be when the water is nearly over the top, and a slap, or othyer event which compresses the air but not the water, allows an overtopping.
If you think an airlock will keep your engine safe in these circs then lean into it.
coopec
N/A
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?)
coopec
N/A
That's exactly how my exhaust works. I made sure the water was injected and hit the other side of the "mixer" as a bit of a spray before it flowed into the waterlock.
I think you will be able to see the exhaust on my yacht being tested for the first time.
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Moodysailor
Well-Known Member
In this case (slow to start) - the recommended advice is to turn off the inlet seacock until the engine fires.
You are correct that there will not be enough pressure in the exhaust side to drive the water out under cranking alone.
My personal advice is to close the seacock if it hasn't fired after two good attempts. It's another reason to keep the engine in good order - a diesel should be able to fire quite readily if it is a healthy engine.
Moodysailor
Well-Known Member
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 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.
- 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
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.