Salt or fresh water cooled?

[Equinox]

Can anybody please explain to me what the advantages and disadvantages are between salt water cooled and fresh water cooled diesel engines. Thank you.

 

[Guest]

There are no disadvantages to having a fresh water cooled engine, except that the possibility that they may cost slightly more than raw water cooled.

Raw water cooled - salt water cooled engines have a shorter life span, require more maintenance, more parts replaced and in general don't last nearly as long and ultimately end up costing far more to maintain and keep running.

Salt water begins corroding the interior of the engine the first time it is run. With a fresh water cooled engine..probably the one part that will need repair or replacement will be the impeller in the raw water intake pump and of course the heat exchanger.

If you have the choice...buy fresh water cooled. My Perkins 4-108 is 27 years old, has been in salt water all its life and has had no significant repairs done to it.

 

[kgi]

freshwater cooled engines are far superior, yes they cost slightly more but they are a lot less trouble, as the previous post said rwc engines start rotting from the inside out from day one you will have trouble with the thermostats sticking because of the rust and scale build up, they can be a real nightmare in the tropics where the water temp is 80 degrees and salt builds up in the cooling passages at an alarming rate, watercooled elbows tend to corrode away quicker as well, I've had both types in the same boat and fwc wins hands down.....keith

 

[dickh]

Another advantage of fresh water cooled engines is that they are running at a more optimum temperature - circa 80/85ºC and therefore more efficient and less polluting - also you can run a calorifier off them for hot water. The fresh water is usually a 50/50 water & antifreeze mixture so you don't have to do anything over winter apart from changing the mixture every few years.
Swc engines only run at max 60ºC as they would block up completely if they ran hotter. There are very few swc cooled engines about nowadays as most manufacturers realise the advantages of fwc(the only ones are Bukh and also the small Yanmars can be swc).
Go for the fwc, you won't regret it - what engine were you thinking of fitting?

dickh
I'd rather be sailing...

 

[yoda]

Beware, the fresh water cooled version of the Yanmar 2 GM 20 does not have a hand start option, the SWC version does. Just another little thing to consider.

Yoda

 

[ArthurWood]

I think you will find that in both swc and fwc engines the exhaust elbows/risers are exposed to exhaust and salt water.

 

[ccscott49]

Quite right, Arthur. But you would go with fresh water coolilng aswell wouldn't you? But apparently theres still quite a few engines around still use raw water coolling, apart from outboards.

 

[ArthurWood]

Colin - I have 2X7.4l Mercs which are RWC. The boat has been in salt water (warm variety) for 5 years and I check/replace elbow gaskets annually. Never had to replace elbows as they are SS. After 1100 hr had the heads overhauled. No other probs. Boat still runs like it did when new. Was told by Sea Ray that they don't install FWC on Mercs, but certainly all diesels I've seen have FWC. Had I thought about it before I put my boat in salt water, I probably would have installed FWC.

 

[ccscott49]

Most of the mercs over here seem to be RWC aswell, don't know why.

 

[Equinox]

Many thanks for all the responses. At present I'm looking at buying a yacht and the one I'm viewing tomorrow has a FW cooled engine. As its my first boat my knowledge of diesels isin't fantastic. I appreciate now that a FW diesel engine is better. I know it's probably a very silly question but where does a FW diesel get its fresh water from. I can only asume that it draws it from a tank onboard and so that imposes limitations on the cruising range due to the size of the water tank that may be onboard.

 

[ccscott49]

The fresh water is contained within the engine, as in a car, but the radiator, becomes a heat exchanger, with sea water pumped through it, to cool a closed loop system of pipes within the heat exchanger, this in turn cools the engine, the warm sea water, is then injected into the exhaust system at an elbow and goes out over the side, whilst coolling the exhaust gases. So the fresh water side of the system is sealed, with a radiator cap, as in a car. I hope this makes sense, as I know whjat I mean, it's a bit difficult explaining it to a novice, but I think it makes sense.

 

[kgi]

yes, your absolutely right, they are both saltwater cooled at the injection point, but the rwc exhaust elbows just don't seem to last as long. (maybe because most of them are cast iron).......keith

 

[Strathglass]

To answer your question. The fresh water is equivalent to the water in a car cooling system. How often do you top up your car radiator?
The heat exchanger in a boat can be considered equivalent to the radiator in a car. In a car it is air that cools the radiator and in a boat sea water is pumped through the heat exchanger to cool the fresh water in the engine.
Does that answer question?

Regards

Iain

 

[Equinox]

Aaahhhhh, now it makes sense ! Many thanks chaps