Exaust Systems what size waterlock.

[10oxometer]

Hi all, How much water is left in the exaust system after the engine is switched off? I am told i need to change the water lock to a different type to enable all the water between engine and water lock to drain into said unit. existing pipe bends lower than the waterlock inlet leaving a potential puddle in the pipe.There is a possable volume of 2.7mts x 50mm internal dia plus waterlock an muffler between the last top bend and engine. Many thank in advance.

 

[Tranona]

The size of your waterlock depends on the size of your engine and the diameter and length of your exhaust system downstream of the waterlock. You will find guidance in the Vetus catalogue on sizing. Generally, small engines up to about 25hp with"normal" hose lengths and up to 50mm diameter use the common approx 4.5 litre capacity units. Next size up is double the size and is for hose sizes up to 90mm. An alternative supplier is Halyard Marine who do similar sizes but made out of GRP. Either supplier will give you technical advice if you ask.

 

[vyv_cox]

Just a warning of the need to get it right. A very well known holiday charter company purchased a fleet of new boats for one of their bases in Greece. Only after four engines had been scrapped and replaced was it discovered that the trap was too small for the volume of water contained in the exhaust hose and silencer. Backed-up water was entering the engine via exhaust valves. The problem was solved by fitting larger traps.

 

[Mistroma]

Sound advice. One of Nigel Calder's books states that the water muffler should have a volume at least as great as the volume of the vertical section of the exhaust pipe exiting the muffler. The fall in the pipe after that point will cause it to drain overboard (unless following seas drive water back into the exhaust).

So you could measure the length of pipe from trap to highest point downstream and calculate the internal volume. That's pretty trivial and will give a minimum volume for the trap. However, I imagine that some water from the injection elbow will still drain down from that side. So might be sensible to add a safety margin. I'd guess something like 25% of the volume of that section of pipe (again, easy to calculate). Someone else might come up with a better figure.

Knowing the volume of each section of the exhaust pipe should give confidence in size of trap you buy. Engine size would only be a rough guide as installations vary.

We met a couple doing the ARC last year and followed their blog. They met a Dutch guy who'd had a new engine fitted after leaving Holland. It died within a few weeks due to water in the cylinders. We read his blog (tricky as it was in Dutch) and worked out that he got a replacement under warranty as something was wrong with the installation. He then arrived in the Canaries with a damaged engine, water in the cylinders again and another new engine. So he's on his 4th engine in under a year and failures all seem related to water finding a route into the cylinders. Really important to get a properly run exhaust with correct size of water trap.

 

[Tranona]

The Vetus catalogue has some rules of thumb. For exhaust lengths of up to 3m to the top of the swan neck the water trap is sized for the hose diameter. So up to 50mm is c4.5l. If the length is greater than that, you can either go up to the 10l size if you have 50mm hose or a 7.5l for 40 or 45mm. For 60mm to 90mm the extra large ones are 16 or 17l depending on configuration of spigots. Most of the sub 30hp engines have exhausts less than 50mm so the determining factor of trap size is the length of hose. Many engines are under the bridgedeck and 3m hose downstream of the trap is more than adequate to the top of the swan neck.