engine exhaust expert

Lesley & Derek Mercer
Tue 3 Sep 2013 12:41
Hello from Sant Antioco.

I have spent my time here becoming an absolute expert on diesel engine exhausts. ( I know, I should get a life!).

It turns out the problem is that when Fortuna Catamarans fitted the engines, they had no experience of Volvo engines.  I had asked for these to be fitted, as spares are so much easier to obtain in Europe than the Japanese Yanmar engines they normally used.  The way a diesel exhaust works is slightly complicated.  On a car, the hot exhaust gases are simply piped to the outside world via steel pipes and a silencer and catalytic converter.  The gases are VERY hot, but everything is steel, so no problem.  You can have a similar system on a boat, but there are big issues about not setting fire to the surrounding bits and pieces, e.g. fibreglass hull, partitions the exhaust goes through etc.... and also, as the engine is under the bunks in the rear cabins, it would tend to make them very very hot.  

Most pleasure boats, therefore have a "wet" exhaust.  The engine is cooled with fresh water, as in a car, but this is circulated through a heat exchanger where the heat is transferred to sea water (In a car, the heat is then sent out into the air through the radiator). With a heat exchanger, the sea water takes the place of the air in a car radiator, but it needs to be pumped in, pass through the heat exchanger, and then sent overboard.  Some bright spark discovered that if you inject it into the hot exhaust gases, it turns to steam and gets spewed out with the exhaust.  It has the advantage of getting rid of the water and heat, and also allows you to use cheaper rubber hosing, rather than steel exhaust pipes with lots of insulation to protect everything around it, as the exhaust gases are now much cooler.  Almost all leisure boats use this system.

Problems arise when you switch the engine off.  There is always some water in the exhaust, and this can run back into the engine. If it gets into the pistons, when you next turn the engine to try and start it, you have completely incompressible water inside the cylinder, rather than diesel fumes and air.  Net result is everything gets bent, twisted, distorted.  You now have a complete engine rebuild on your hands.  

It turns out the builders miscalculated the volume of water left in the exhaust pipe when the engine get switched off.  There is a "water trap" present, but it only had 5 litre capacity.  About half of this is on the engine side, and half on the exhaust side.  My engineers have calculated the capacity of the exhaust tubing to be 2 litres, i.e. almost no margin for error.  We are therefore now going to fit new water traps with a 17 litre capacity, and automatic flap valves.  The traps are not expensive, but the labour to fit them will be.  It is going to take a further 3 days to get them delivered from Holland (where Vetus is based), as there are none in Italy!

The only saving grace is that the winds are all wrong for our planned trip to Sicily for the next 4 days, so lets make the most of it!