[thedoc (OP)]

keith jones asked the Naked Scientists:
   
I sailed my yacht up to a glacier in patagonia. I sailed through the canals (salt water) to get into the lagoon directly in front of the glacier - which was probably mostly fresh water and full of icebergs.

The temperature variation cracked the head of my water cooled engine.

I cannot figure out the temperature gradient and if I cracked it on the way in or the way out ?

What do you think?

[CliffordK]

Were you sailing, or running with the engine?

Did you have diurnal temperature variation?  Could you have frozen the engine at night?  Especially if you had gotten freshwater into the cooling system.

[graham.d]

Does the engine use primary or secondary cooling? By that I mean is the engine cooled by the seawater circulating around the engine or does it use a heat exchanger such that the seawater cools a coolant liquid which then, in turn, cools the engine? In the latter system (which is very much more the common system nowadays) the coolant has a large amount of anti-freeze so should not be a problem. I am guessing that you have a primary cooled system and, as Clifford suggested, water in the system froze overnight.

[CliffordK]

It may be that the extreme temperature shifts from several hundred degrees to arctic water temperatures of around 0°C would be hard on engines. 

Thus, the heat exchanger might not only allow you to keep antifreeze in the engine, but also buffer the engine from temperature extremes.  And, also buffer the engine from salt.

[graham.d]

The main idea is to allow antifreeze usage so that no ice forms in the engine when not in use and to protect the engine from the corrosive effects of seawater. I don't think the effects of coolant at 0C as opposed to (say) 15C on an engine would cause the head to crack. In any case, if the coolant was circulating there would not be any "shock" of water on an overheated engine. I suppose ice could have blocked the water intake though.

[CliffordK]

With my old AMC (car), I rusted out the freeze plugs one winter.  And, of course they were the ones on the back of the head, inaccessible without pulling the head.  So, I was running water in the engine through the winter.  I found that I could fill the radiator with warm water at home.  Then about 5 miles down the road, the radiator would freeze up from the wind blowing through the radiator, at a point where the temperature in the engine was just getting hot enough to open the thermostat.

Anyway, I'm trying to think of how a similar mechanism might work in the boat.

Perhaps Graham has the answer.  You could be sucking ice in through the intake.  With a heat exchanger, perhaps there is a point where the ice clogs the heat exchanger, before the engine gets hot enough to melt it out, or, before the thermostat opens. 

This could cause the engine to run hot for a short period, then get doused with very cold coolant. 

However, I think heat tends to warp heads, rather than cracking them.  So, from heat, you're more likely to have a warped head and blown head gasket. 

Cracks are most often due to ice.

Maybe one should ask where the crack is.  If it is in the water jacket where it is likely to experience a pressure buildup, then ice would be a good candidate.  Can you get cracks elsewhere?

[grizelda]

I imagine most engine cracks are caused as a result of loss of lubrication. Perhaps insurance companies are reluctant to cover this as it is possible to prevent damage by shutting down the engine in time.

[SeanB]

Temperature stress because of cold water can crack engines. Ask any owner of a car who put cold water in when hot and low of water and cracked the head. The stress of the cold water contracting a section of the metal will crack it, and you will not see the crack until you undo the bolts, when it will come off in large chunks. Only symptom is water loss either into the oil or the cylinders.

[graham.d]

I agree Sean, but the issue is how the engine was allowed to get very hot and then suddenly cooled. Maybe if ice was blocking the intake and then was, in a sudden way, dispersed then this could have such an effect. But using an engine continuously with water at 0C would not harm an engine any more than using it with water at 15C or 20C. As you say, it is the sudden stress induced by rapid cooling that can crack the engine. I can't see how this could happen with a secondary cooled engine. Also there are usually temperature sensors that tell you if the coolant is overheating. It is key to know if the engine was cooled with a primary or secondary system.