How high should an exhaust swan neck be?

Dipper

Dipper

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Some of the replies to Monfish24's post have made me think about my own exhaust swan neck.

At my last survey, the surveyor mentioned in passing that he thought that the swan neck in my boat's exhaust pipe just before it exits via the skin fitting was not high enough (but he didn't put it in his written report). I was just about to extend the height but I am now having second thoughts. A high swan neck would reduce the risk of water swamping the engine from the outside but increase the amount of water in the system when the engine is turned off. If the engine is then slow to start, it would flood quicker.

Is there an optimum height? The engine is a Volvo 2001 in a Seawolf 26.
 
Dipper said:
Some of the replies to Monfish24's post have made me think about my own exhaust swan neck.

At my last survey, the surveyor mentioned in passing that he thought that the swan neck in my boat's exhaust pipe just before it exits via the skin fitting was not high enough (but he didn't put it in his written report). I was just about to extend the height but I am now having second thoughts. A high swan neck would reduce the risk of water swamping the engine from the outside but increase the amount of water in the system when the engine is turned off. If the engine is then slow to start, it would flood quicker.

Is there an optimum height? The engine is a Volvo 2001 in a Seawolf 26.
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as high as possible with a water lock / pipe capable of holding any water that drains back after engine turned off + a vent to stop syphoning
 
Vetus recommend min 45cm max 150cm. Provided you have an adequate waterlock water should not be a problem.

Se the Vetus catalogue www.vetus.co.uk Page 168 for schematics and dimensions for water cooled exhaust systems.
 
The vetus ones are something over a foot from memory.
In terms of stopping water rushing up the exhaust when a wave hits, I think the tight right angle bend at the transom is as important as the height.
If you have a decent silencer and/or waterlock in the system, it takes a great deal of cranking to fill it. But one good (or bad) wave will do that in no time! I know people who have lost engines due to waves up the transom. If I were starting from scratch I might fit a transom fitting with a non-return flap.

If your engine is very low in the boat, you need to design the system to not retain too much water when running. I believe that too big a bore exhaust hose can be bad, but I wonder if anyone can suggest a definitive guide?
Cheers,
 
Dipper said:
Some of the replies to Monfish24's post have made me think about my own exhaust swan neck.

At my last survey, the surveyor mentioned in passing that he thought that the swan neck in my boat's exhaust pipe just before it exits via the skin fitting was not high enough (but he didn't put it in his written report). I was just about to extend the height but I am now having second thoughts. A high swan neck would reduce the risk of water swamping the engine from the outside but increase the amount of water in the system when the engine is turned off. If the engine is then slow to start, it would flood quicker.

Is there an optimum height? The engine is a Volvo 2001 in a Seawolf 26.
Click to expand...

There's more to it than just raising the swan's kneck "just in case." You definitely need a suitable waterlock - I fitted the recommended Vetus in my old Fulmar, and it worked just fine. Not a lot of money and DIY of course. A good move!

PWG
 
PeterGibbs said:
There's more to it than just raising the swan's kneck "just in case." You definitely need a suitable waterlock - I fitted the recommended Vetus in my old Fulmar, and it worked just fine. Not a lot of money and DIY of course. A good move!

PWG
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Sorry. I should have provided more details. There is a Vetus waterlock close to the engine, then quite a long exhaust hose along the bilges towards the stern before it rises up to the swan neck then down to the skin fitting which has a right angle bend in it. The skin fitting is on the side of the hull not on the transom.

Thanks for all the replies so far. Perhaps a proper Vetus gooseneck would be a better solution for next year although I haven't had any problems so far with water entering the exhaust system (as far as I know).
 
Dipper said:
Sorry. I should have provided more details. There is a Vetus waterlock close to the engine, then quite a long exhaust hose along the bilges towards the stern before it rises up to the swan neck then down to the skin fitting which has a right angle bend in it. The skin fitting is on the side of the hull not on the transom.

Thanks for all the replies so far. Perhaps a proper Vetus gooseneck would be a better solution for next year although I haven't had any problems so far with water entering the exhaust system (as far as I know).
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as long as you have a large loop above the wl thats ok.
if you add the Vetus gooseneck fitting you will be wasting wonga
 
lw395 said:
If your engine is very low in the boat, you need to design the system to not retain too much water when running. I believe that too big a bore exhaust hose can be bad, but I wonder if anyone can suggest a definitive guide?
Cheers,
Click to expand...

The diameter of the exhaust is determined by the diameter of the exhaust outlet from the engine. Not sure it is that critical. My Yanmar 1GM was 45mm, but when I re-engined I found the Beta was 50mm and the Nanni (which I chose) is 40mm, even thogh they are the same base engine. I stepped the 40mm up to 45mm at the waterlock and left the rest of the system the same. Clearly the larger the diameter the more water in the pipe, but also the longer the run the more the water. However, using the Vetus formula my waterlock has a safety factor of more than 2 over the minimum capacity.
 
I think you can get an effect where a small engine ticking over will 'blow bubbles' through the water in a big exhaust, rather than blowing the water out. Then when you use some revs, it all comes out in a lump!
My last boat would chuck out what looked like a gallon on start up, then nothing for about two minutes on tickover.
I suspect these things can be a bit subtle!
 
Yes, My 1GM did virtually nothing at tickover. The size of the exhaust is determined largely by the need to minimise backpressure which can cause poor starting, particularly on an engine like the Yanmar which does not have glow plugs so relies on combustion chamber pressure to get it going. Restrictions in the exhaust is a common cause of poor starting on 1GMs.
 
Exhaust on mine doesn't loop upwards at the outlet, nor does it have a right-angle outlet. And no waterlock. The exhaust manifold seems to be a dry riser type which loops up about 6 inches at the engine before water is injected. Do I have a potential problem?
 
halcyon23 said:
Exhaust on mine doesn't loop upwards at the outlet, nor does it have a right-angle outlet. And no waterlock. The exhaust manifold seems to be a dry riser type which loops up about 6 inches at the engine before water is injected. Do I have a potential problem?
Click to expand...

My Twister doesn't have a waterlock or a right angle bend either. The exhaust pipe leaves the engine and goes down about 8 - 10 inches then rises to to a silencer under the after deck before descending to the skin fitting, which is a few inches above the waterline.

Her present engine was fitted in 1982 and she has been to the Azores and back. During the 14 years I have owned her I have sometimes had the stern temporarily under water without destroying the engine. So the system obviously works despite not being as recommended (by manufacturers of waterlocks, that is. ;))
 
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I am in the process of changing my exhaust elbow as it has coroded over its life of 28 years, and i am thinking of fitting a silencer or waterlock, i already have a swanneck at the transom but wondering if i should be getting a silencer or waterlock, i primarily want to quiten the tone of the exhaust, and would a waterlock do the same as a silencer...
 
The waterlock on its own is an effective silencer. You might also consider the Halyard Marine units as well as the Vetus ones although they are not so flexible on installation.
 
Thanks for your info, i have also looked at the halyard system, but i dont have the space for them, a straight thru silencer would fit easily, or a waterlock could be fitted in, with a little bit of work...
 
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