Water lock capacity

contessaman

contessaman

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Can anyone remember the rule of thumb for the capacity of an engine water lock?

The existing exhaust system in my boat has a Volvo water trap / muffler. Its made from stainless end caps and soft rubber tube body. Engine is well below waterline. A short run of wet exhaust down to the water lock. Then what I measure to be circa 3 metre run of 60mm I.d. hose uphill all the way to the top of the swan neck at the transom. I bit of calculation gives me a FULL capacity of that hose as being about 8.5 litres and the full capacity of the waterlock being 8.7 litres. So I will just about hold it but then the hose is never full of water with the engine running is it? What's the rule of thumb? Half? A third?


Grateful for any advice from anyone in the know. I know I need to fit anti syphon loops etc but just want to know the basics are right. Given the location of the water lock ( under floorboards in aft cabin) there's not much scope to fit anything bigger in there.

Never saw the original engine run in the boat before anyone asks the simple question of was it okay before!

Ta
 
The calculation is on the introductory page to Exhaust sytems in the Vetus catalogue. P 84 in my 2015-6 edition, but also on the website.
 
I solved a similar situation on a previous boat (Centaur) by removing the swan-neck from the transom and re-siting it to the upper regions of a cockpit locker, much closer to the water trap. This meant that the maximum volume of water that could run back to the trap was drastically reduced. IIRC this amounted to less than half of the trap's capacity.
 
When I re-engined I spoke to Vetus who reckoned to allow 25% of the exhaust hose capacity, if full, for the water trap.
 
pvb said:
Here's the calculation...

View attachment 63051
Click to expand...

Thanks buddy. I'd forgotten the hose length between engine and water trap but even a pessimistic total of 4 metres gives a 25% wet volume of 2.8 litres with the x2 safety factor taking it to 5.6 litres and my watertap can hold 8.8 so that should be fine then. I definately need the anti syphon loop....
 
My Impala used to have an awful lot of water in the hose at times.
Particularly f you idled the motor for a long time.
60mm bore seems quite big?
A smaller hose will be more effectively cleared of water by the gas flow.
 
pvb said:
Isn't the bore normally specified by the engine manufacturer?
Click to expand...

Yes
My Nanni 1500cc stated 63mm, the 4108 it replaced 1750cc was 50mm, i kept with 50mm, the raw water pump was the same pump

The 63mm was the same for three engines & the 1500 was the smallest, they all used the same exhaust elbow & that was the factor that governed the hose size
 
lw395 said:
Sometimes I think it's specified by what's lying around.
I've seen more than one waterlock with different in/out.
Click to expand...

But the manufacturer will normally specify a minimum diameter for the exhaust hose - indeed it's usually indicated by the size of the exhaust outlet on the engine.
 
lw395 said:
Sometimes I think it's specified by what's lying around.
I've seen more than one waterlock with different in/out.
Click to expand...
I think itt wise to follow engine manufacturers specification.
Yanmar require 76mm hose for the new engine we are installing, the old Perkins had 50mm.
It's possible to reduce size but that requires measuring the pressure in exhaust system to not loose warranty.
 
knuterikt said:
It's possible to reduce size but that requires measuring the pressure in exhaust system to not loose warranty.
Click to expand...

Injecting water into the mixing elbow reduces the temperature of the exhaust gases and therefore also their volume, thereby also the pressure. Effectively, the maximum pressure in the exhaust line is governed by the head of water that is down the line from the water trap.
 
PuffTheMagicDragon said:
Injecting water into the mixing elbow reduces the temperature of the exhaust gases and therefore also their volume, thereby also the pressure. Effectively, the maximum pressure in the exhaust line is governed by the head of water that is down the line from the water trap.
Click to expand...
If you say so.
I am not planning on loosing the warranty on my new engine based on your advice.
 
lw395 said:
My Impala used to have an awful lot of water in the hose at times.
Particularly f you idled the motor for a long time.
60mm bore seems quite big?
A smaller hose will be more effectively cleared of water by the gas flow.
Click to expand...

60mm not big for my 60bhp engine?
 
contessaman said:
60mm not big for my 60bhp engine?
Click to expand...

60BHP is a fairly big engine.
You're probably not contessa32man then?
TBH the only problems I have known with water in exhausts have been from following seas overcoming transom swan-necks little by little.
But, I have doubts about airily assuming 'the long run of exhaust hose will be 25% water'.
Certainly my Impala used to hold a lot of water in the exhaust at tickover, applying some revs would shove gallons out the back. But the whole run of exhaust was well below the engine, so no problem.
 
lw395 said:
60BHP is a fairly big engine.
You're probably not contessa32man then?
TBH the only problems I have known with water in exhausts have been from following seas overcoming transom swan-necks little by little.
But, I have doubts about airily assuming 'the long run of exhaust hose will be 25% water'.
Certainly my Impala used to hold a lot of water in the exhaust at tickover, applying some revs would shove gallons out the back. But the whole run of exhaust was well below the engine, so no problem.
Click to expand...

No that boat was a long time ago.. Rassy38man doesn't have the same ring to it. Agree that the carpet bomb 25% approach is probably not always going to be the case. I guess that's what the x2 safety factor is for. I've calculated mine is 3x the 25% capacity so I'd like to think it will be okay.. But the engine is so low in the boat its uphill all the way from the water lock to the summit of the swan neck (actually a big loop in the hose).
Perhaps we should all be giving the engine a rev before shutting it down to purge excess water. Unless its a turbo charged engine of course.
 
contessaman said:
No that boat was a long time ago.. Rassy38man doesn't have the same ring to it. Agree that the carpet bomb 25% approach is probably not always going to be the case. I guess that's what the x2 safety factor is for. I've calculated mine is 3x the 25% capacity so I'd like to think it will be okay.. But the engine is so low in the boat its uphill all the way from the water lock to the summit of the swan neck (actually a big loop in the hose).
Perhaps we should all be giving the engine a rev before shutting it down to purge excess water. Unless its a turbo charged engine of course.
Click to expand...
Don't know if reving the engine will help much, increasing the revolutions will increase the water from the pump.

I know a boat where they had to install a pump to empty the water lock to prevent engine from flooding.
The pump run when the stop button is pushed in, just hold the button until the water stop running.
Problem was long hose run and limited space for a the water lock.
 
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