Raw water cooling vs. fresh...basics, please

[Greenheart]

I'm a total novice here - my dealings with marine engines previously, didn't end happily or profitably. Please dumb-down any explanation you care to give.

As far as I understand it, fresh-water cooling means you use a reservoir of fresh water which will need some cooling unless it's big enough to be seriously burdensome...

...while raw-water cooling just sucks in whatever the boat is floating on, so it needs thorough filtration and the brine attacks the components it encounters, which must therefore be more particularly specified than if they dealt with fresh water...but the advantage is, no need for a cooling system or reservoir.

Is there much more to it than that?

Is one system generally found to be preferable over the other, costs aside?

Thanks.

 

[Vara]

I have always understood that the main benefit of fresh water cooling is that it allows the engine to run hotter which is good for it in terms of wear and efficiency.

Can you buy any new raw water cooled marine diesels nowadays?

 

[vyv_cox]

Most raw-water cooled engines are made from cast iron, so no special anti-corrosion measures. There have been a few marinised automotive engines that have aluminium heads, which do not last well.

Raw water cooled engines are simpler mechanically, only one pump rather than two, no heat exchanger. They tend to foul more, although this may never be a problem. Fitting a calorifier is not quite so straightforward. Modern manufacturing usually means that the additional complexity of an indirectly cooled engine does not increase unreliability.

 

[alahol2]

This page... http://www.tb-training.co.uk/15cool.htm#bmn53 should tell you just about all you need to know about cooling systems. About half way down the page there are diagrams of the three main types of water cooling.

I've had two of the three and couldn't, in all honesty, say one was better than the other from a practical point of view.

 

[wipe_out]

Simply all engines are cooled by "sucking" in sea water..

The difference is that a raw water cooled engine will pump that sea water around the engine and then out again while a fresh water cooled engine pumps the sea water through a heat exchanger (think radiator in your car with the air going through it) and the engine is cooled by the coolant flowing through the heat exchanger and around the engine (again like a car)..

 

[AntarcticPilot]

An air-cooled engine is the simplest, and of course least liable to corrosion problems. Some small boats in the 1960s were equipped with air-cooled Briggs and Stratton engines - essentially, the same as a lawn-mower uses. Draw-backs are no potential for water heating, and of course all the heat ends up in the engine compartment so the engine compartment must be well ventilated while the engine is running. Not practical for more powerful engines in a typical marine installation.

Next is raw-water cooling, which is simple and robust. My engine is 25 years old, is raw-water cooled and is still fine. Vyv's comments about materials are apposite; but an engine designed for raw-water cooling will have ample thickness of metal, and will have provision for an engine anode to protect the metal of the block. Problems are with build up of scale in the water channels, and as Vyv says, problems with corrosion in engines not designed for use as marine units.

Fresh-water cooling is the most complex, and means that the expensive engine has "fresh" water circulating round the expensive bits! The water usually has anti-freeze etc. in it, similar to a car's coolant. Snags are that you still use salt-water to cool the fresh water, so there are two water circulation systems with a heat exchanger between the two. Probably best for engine life, but you have the same number of holes in the hull as a raw-water cooled engine, and also additional complexity - two circulation systems instead of one.

You pays your money and takes your choice!

 

[prv]

Simply all engines are cooled by "sucking" in sea water..

Not quite all - a handful use closed-circuit cooling where the heat is transferred to the sea by conduction. Most common in steel boats, where a tank or series of channels on the inside of the hull exchange heat through the shell plating. Usually the tank is a hollow section of the keel, hence "keel cooling". I have also seen more than one old wooden boat with metal pipes protruding through the planking and running alongside the hull, with a gap of about an inch between hull and pipe.

I didn't like the look of the protruding pipes, but a keel cooler is something I'd seriously consider on a steel boat.

Pete

 

[maby]

I have always understood that the main benefit of fresh water cooling is that it allows the engine to run hotter which is good for it in terms of wear and efficiency.

Can you buy any new raw water cooled marine diesels nowadays?

Small Yanmars are still raw water cooled, I believe

 

[Greenheart]

Thanks for all these descriptions, I feel better acquainted already.

Wouldn't the requirement for raw-water cooling of fresh-water coolant, be better replaced by a much larger, closed-circuit fresh-water supply? Neither filtration of raw water, nor blockage of the raw water supply, nor raw-water pump failure, would ever be issues. How much stored fresh water would be needed, to maintain best operating temperature for a small marine diesel?

Is it a question of the cooling water reservoir warming up too much to cool down before it's needed again? I thought most yacht auxiliaries suffered because they don't usually run long enough for cooling to be a problem.

 

[vyv_cox]

Not quite all - a handful use closed-circuit cooling where the heat is transferred to the sea by conduction. Most common in steel boats, where a tank or series of channels on the inside of the hull exchange heat through the shell plating. Usually the tank is a hollow section of the keel, hence "keel cooling". I have also seen more than one old wooden boat with metal pipes protruding through the planking and running alongside the hull, with a gap of about an inch between hull and pipe.

I didn't like the look of the protruding pipes, but a keel cooler is something I'd seriously consider on a steel boat.

Pete

Bukh still make a keel cooler, described in the workshop manual. They are fitted to lifeboats, for which Bukh almost seem to have a monopoly. The intention is that debris and pollutants are then kept out of the engine.

 

[Dave_Seager]

Thanks for all these descriptions, I feel better acquainted already.

Wouldn't the requirement for raw-water cooling of fresh-water coolant, be better replaced by a much larger, closed-circuit fresh-water supply? Neither filtration of raw water, nor blockage of the raw water supply, nor raw-water pump failure, would ever be issues. How much stored fresh water would be needed, to maintain best operating temperature for a small marine diesel?

Is it a question of the cooling water reservoir warming up too much to cool down before it's needed again? I thought most yacht auxiliaries suffered because they don't usually run long enough for cooling to be a problem.

Remember that marine engines can be running for hours at a time. Once the system has warmed up you have to dispose of the waste heat from the engine as fast as it is produced. This requires some form of heat exchange for your closed system; either to raw water through a heat exchanger or to the air through a radiator, probably fitted with a fan.

 

[Tranona]

Thanks for all these descriptions, I feel better acquainted already.

Wouldn't the requirement for raw-water cooling of fresh-water coolant, be better replaced by a much larger, closed-circuit fresh-water supply? Neither filtration of raw water, nor blockage of the raw water supply, nor raw-water pump failure, would ever be issues. How much stored fresh water would be needed, to maintain best operating temperature for a small marine diesel?

Is it a question of the cooling water reservoir warming up too much to cool down before it's needed again? I thought most yacht auxiliaries suffered because they don't usually run long enough for cooling to be a problem.

That is essentially what Pete describes where the water is in the keel of a steel boat and heat transfer is through the plating to the water outside. However it takes up a lot of space and is really only feasible with steel hulls. You are right about warming up, but this is perhaps more of an issue with saltwater cooled engines as they generally run cooler than fresh to minimise the corrosion from salt. You could have an air cooled heat exchanger like a car radiator, but it would have to be massive and exposed to ensure good flow of air. For all its perceived drawbacks it does make sense to use the water surrounding the boat as a cooling medium as the alternatives have even bigger drawbacks.

 

[vyv_cox]

Thanks for all these descriptions, I feel better acquainted already.

Wouldn't the requirement for raw-water cooling of fresh-water coolant, be better replaced by a much larger, closed-circuit fresh-water supply? Neither filtration of raw water, nor blockage of the raw water supply, nor raw-water pump failure, would ever be issues. How much stored fresh water would be needed, to maintain best operating temperature for a small marine diesel?

Is it a question of the cooling water reservoir warming up too much to cool down before it's needed again? I thought most yacht auxiliaries suffered because they don't usually run long enough for cooling to be a problem.

I doubt if there is any sort of machine anywhere that is designed in this way. Engines need to achieve their operating temperature as quickly as possible and operate close to it. Cooling using a big bucket would be remarkably inefficient and the water would inevitably boil unless its volume was impossibly large.

 

[prv]

Small Yanmars are still raw water cooled, I believe

My old 2GM20 was, but I think the replacement model is freshwater only, leaving the venerable 1GM10 as their only raw-water design.

Wouldn't the requirement for raw-water cooling of fresh-water coolant, be better replaced by a much larger, closed-circuit fresh-water supply? Neither filtration of raw water, nor blockage of the raw water supply, nor raw-water pump failure, would ever be issues.

You have pretty much just described keel cooling, except that at least one side of the tank (preferably more) should be part of the hull so that the heat can escape through it.

Is it a question of the cooling water reservoir warming up too much to cool down before it's needed again?

Exactly. Although most runs might be short and would work ok with that system, what happens when you want to cross the Channel on a windless day?

Bukh still make a keel cooler, described in the workshop manual. They are fitted to lifeboats, for which Bukh almost seem to have a monopoly. The intention is that debris and pollutants are then kept out of the engine.

Plenty of non-Bukh keel coolers in use, but normally built as an integral part of a metal boat so no need for a manufacturer's option. I guess Bukh's is the tube type, for fitting on GRP lifeboats? I think I've seen them on ferries.

Keeping debris out of the engine, together with fewer holes in the boat and ways of flooding the engine, are why my fantasy long-distance design has both keel cooling and a dry-stack exhaust. The latter of course is only really practical in a boat designed that way from the start.

Pete

 

[prv]

I doubt if there is any sort of machine anywhere that is designed in this way. Engines need to achieve their operating temperature as quickly as possible and operate close to it. Cooling using a big bucket would be remarkably inefficient and the water would inevitably boil unless its volume was impossibly large.

You could easily solve the warm-up problem with a thermostat just like a conventional engine - just consider the sea as a very large bucket

As for the water boiling, I believe the Lister D series engines worked this way - an open trough of water around the cylinder head and you would pour buckets of water into it while running to keep the level up as it boiled off! There's a brass tap at the bottom, presumably to empty the remaining water at the end of a run but perhaps you could pour it straight into a teapot while running...

"Cooling using a big bucket" is exactly what the bigger Lister engines (eg CS series diesels) had. A great big tank, hundreds of litres, acting as a kind of crude radiator.

Pete

 

[Greenheart]

Hmm. In spite of my professed ignorance, I'm fairly quickly making sense of all your explanations, thanks.

It seems odd - we're often challenged by the question of adequate cabin-heating aboard boats, when there's a device aboard that runs so jolly hot it creates the opposite problem.

If a diesel auxiliary can boil large quantities of water and keep them hot, shouldn't that heat be storable on board in a large insulated chamber for use in radiators later?

I've heard that calorifiers really aren't as effective as the concept seems (as I understand it). From the cooling issues described here, I'd imagine it wouldn't be hard to keep a yacht's large domestic water supply very warm, if required on the same day the engine was used for more than a few minutes.

Have we already reached the top, as regards efficient cooling and re-use of heat produced? Not trying to drift the subject, but it sounds as if there's lots of room for improvement.

 

[Tranona]

Limited use for heating as that is usually required at times when the engine is not in use and therefore not producing waste heat. Running waterheated radiators in a small boat is a nightmare, although possible in larger boats. Remember diesel fired hot air heaters are often installed in motor vehicles because of the limitations of using waste heat from the engine. A calorifier is the best way of storing the waste heat, but in many smaller boats there is simply nowhere to keep large quantities of hot water and 40l is the typical size, which would last no time at all if used for heating, although usually adequate for typical overnight usage for washing up and showering.

 

[ghostlymoron]

My experience of Calorifiers with a Yanmar 2GM20F is that they are brilliant! Having hot water available transforms boat life and the water stays hot overnight for a shave in the morning.

 

[prv]

If a diesel auxiliary can boil large quantities of water and keep them hot, shouldn't that heat be storable on board in a large insulated chamber

Yep, that's a calorifier

(OK, I know you were thinking of space heating, but it did make me smile to watch you apparently re-inventing the standard hot-water system )

I've heard that calorifiers really aren't as effective as the concept seems (as I understand it).

Don't know why you think that. I can't remember the last time I turned on the hot tap at Ariam's galley sink and didn't get scalding-hot water out of it - hell of a luxury compared to boiling the kettle on Kindred Spirit!

It heats up really quickly, within minutes of starting the engine, and stays hot seemingly for ever. I went down to the boat to do a quick engine job a couple of days after my parents had used her, turned on the hot tap without thinking to wash some diesel off my hands, and the water still came out pleasantly warm. We don't have shore power plugged in on our home berth, it was just residual heat from them motoring up the river.

Pete

 

[mtb]

My colvic had a petter twin but now has a yanmar 2qm15 raw water cooled which has a thermostat the engine warms up quickly which goes some way to help the over cooling problems re mineral build up and engine wear, raw water cooled engines can suffer from
My all time fav set up was in my old narrow boat which had a bmc 1.5, as it was a falkener hull it wasn't flat bottomed and had water tanks under the floor. This boat had a toothed belt from the gear box out put to the shaft with smiths taperloc's.
The gear box faced forwards. I didn't like the set up because if you really opened it up the belt could jump.
The whole engine bay could be slid out and another put straight in which was a very good idea re boat hire turn around maintainance. To sort the belt issue I welded the gaps around the engine bay cut the rest out and turned the engine around lowering it in line with the shaft. Then used one of the water tanks for cooling, job done no more issues re sucking silt or weeds up or that worry that if you opened it up the belt might come off just at the wrong time
On a tug I had there were keel pipes along the chine it worked fine but I was never happy taking the ground
These days most smaller engine's are cheaply made, I'm one of the old school, old engine design over engineered suit's me with no electronics
cheers
mick