How high above the water should my exhaust be?

I will be installing my wet exhaust shortly in a Mauritius 43 yacht so all your views are very important to me. I do have a Vetus Water-lock and Goose-neck so I will certainly consider Vetus recommendations.

Because of the design of the hull I have concluded I can't take the exhaust out through the transom as the transom would be (slightly) higher than the motor so I will take it out the side of the yacht alongside the center cockpit. Because of back-pressure considerations I will try to keep the outlet 5+ in above WL (as recommended) but of course that won't stop the outlet being submerged a lot of the time. But as water pressure increases with depth I suppose >5 inches above WL is better than 5 in below WL.
 
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NormanS said:
I generally find that engine cooling systems, toilets etc, don't work well if the seacocks are closed. I expect it is the same with your ball valves.
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Yes Norman

I find the thread terribly confusing. Actually I agree with you and Claymore. You turn the sea-cocks off when you aren't on the boat.

I want to keep the exhaust >5 inch above the WL but I guess that would not be much help (even in a pen) if the skin fitting gave away. (hopefully you'd have a bit of warning) I'll make mine readily accessible for inspection purposes
 
coopec said:
Because of the design of the hull I have concluded I can't take the exhaust out through the transom as the transom would be (slightly) higher than the motor so I will take it out the side of the yacht alongside the center cockpit.
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I don't think it matters if your exhaust outlet skin fitting is above the level of the engine - this will be the case on many, if not most, boats. You will need to get part of the run of the exhaust well above the outlet skin fitting to avoid swell etc. washing back up the pipe to the engine, so there will be a significant fall in the exhaust down to the skin outlet, even if the outlet is a little above the engine.

The critical thing is, as has already been said, that the waterlock is capable of holding the content of the run of pipe (and any other fittings) between the water lock and the highest point of the system. The Vetus site has a formula for calculating the capacity of that section of pipe (andf hence size of the waterlock), and minimum dimesions for the length of fall from engine to the water lock.

So the exhaust goes: from engine, fall to waterlock (dimensions of fall critical), waterlock, rise to highest point (relative capacity of waterlock critical), fall to skin outlet (fall dimension not precisely critical, but has to be enough to stop water flowing backwards into rest of system).

(Happy to be corrected if I'm mistaken.)
 
LittleSister said:
I don't think it matters if your exhaust outlet skin fitting is above the level of the engine - this will be the case on many, if not most, boats. You will need to get part of the run of the exhaust well above the outlet skin fitting to avoid swell etc. washing back up the pipe to the engine, so there will be a significant fall in the exhaust down to the skin outlet, even if the outlet is a little above the engine.

The critical thing is, as has already been said, that the waterlock is capable of holding the content of the run of pipe (and any other fittings) between the water lock and the highest point of the system. The Vetus site has a formula for calculating the capacity of that section of pipe (andf hence size of the waterlock), and minimum dimesions for the length of fall from engine to the water lock.

So the exhaust goes: from engine, fall to waterlock (dimensions of fall critical), waterlock, rise to highest point (relative capacity of waterlock critical), fall to skin outlet (fall dimension not precisely critical, but has to be enough to stop water flowing backwards into rest of system).

(Happy to be corrected if I'm mistaken.)
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correct & all diagrams are in the Vetus website
some of those noisy Westerly exhausts are the 1/2 way up the transom
 
claymore said:
The advantages of having an ageing, slack engine are many and varied!
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I think an engine with poor compression is actually far more likely to fail to start due to back pressure.
A decent engine in good nick with a normal exhaust capacity will fire before the exhaust pressurises, then a few cm head of water is of no consequence against a diesel exhaust.

I have known several people lose engines due to following seas filling the exhaust, at least one of those had a standard setup with waterlock, silencer and moulded swan-neck. He was sailing properly offshore for a couple of days without running the motor though.
If I was installing or changing an exhaust, I'd be tempted t fit some sort of flap valve at the transom.
 
LittleSister said:
...
The critical thing is, as has already been said, that the waterlock is capable of holding the content of the run of pipe (and any other fittings) between the water lock and the highest point of the system. The Vetus site has a formula for calculating the capacity of that section of pipe (andf hence size of the waterlock), and minimum dimesions for the length of fall from engine to the water lock.

...

(Happy to be corrected if I'm mistaken.)
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That should not be necessary, the exhaust should never contain much water, it gets blown out even at tickover.
Each turn of the crank is say 500cc of air and perhaps 5cc of water?
It's a good idea to see what level the water stands at when stopped after extended ticking over though.
I suspect that an exhaust of too big a bore can allow the water to run back instead of being blown out?
 
LittleSister said:
I don't think it matters if your exhaust outlet skin fitting is above the level of the engine - this will be the case on many, if not most, boats. You will need to get part of the run of the exhaust well above the outlet skin fitting to avoid swell etc. washing back up the pipe to the engine, so there will be a significant fall in the exhaust down to the skin outlet, even if the outlet is a little above the engine.

The critical thing is, as has already been said, that the waterlock is capable of holding the content of the run of pipe (and any other fittings) between the water lock and the highest point of the system. The Vetus site has a formula for calculating the capacity of that section of pipe (andf hence size of the waterlock), and minimum dimesions for the length of fall from engine to the water lock.

So the exhaust goes: from engine, fall to waterlock (dimensions of fall critical), waterlock, rise to highest point (relative capacity of waterlock critical), fall to skin outlet (fall dimension not precisely critical, but has to be enough to stop water flowing backwards into rest of system).

(Happy to be corrected if I'm mistaken.)
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Thank you for your suggestions.

Even putting the goose-neck as high as I could in the engine compartment I realised I could not get fall to the transom.

If I raised to goose-neck as high as I could in the engine compartment the dimensions would have been outside those recommended by Vetus.

While admiring photos of beautiful amateur boats in a magazine the other day I realised the builder of a Mauritius 43 had done exactly the same with his exhaust as I intend doing.

I think when you look at the line drawing you will realise the problem

I always find it good to run my ideas past the experts on this forum!! Thanks

View attachment 40712
 
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