Cooling system question boiling temp of water

[agallant80]

So I was talking with my mechanic neighbor last night and he brought up a good point. The cooling system in a car is pressurised because water boils at a higher temp when under pressure and the block runs at a temp above boiling with no pressure. So how does this work on a fresh water boat. If water boils at 212F and the motor runs at 220 and there is no closed pressurised system how does the water not boil?

 

[hivoltg]

Re: Cooling system question boiling temp of water

Antifreeze in a closed system raises the boiling point.

An open system at sealevel...water boils at 212F.

 

[agallant80]

Re: Cooling system question boiling temp of water

Yeah so if water boils at 212 and the motor runs at 220 then the water will boil unless it has antifreeze in it (which won't help because its really bad at conducting heat away) or it will need to be under pressure to raise the boil point above 212. So how does the cooling system in the boat work? Does it hold pressure some place?

I miss DonS, he would know.....

 

[hivoltg]

Re: Cooling system question boiling temp of water

Boats closed cooling systems hold pressure and have a "radiator cap" off the heat exchanger. The heat exchanger is the radiator on a boat. Instead of air cooling the antifreeze system, you have non-pressured raw water flowing around it from the impeller and dumping out the risers. The heat exchanger is a radiator stuck in a can.

It was a requirement that boat builders had to do. None of the engine makers where staying with carb'd motors and emission laws really forced fuel injection and computer controlled motors into boats. Both require higher and constant temp.

 

[agallant80]

Re: Cooling system question boiling temp of water

I get what you are saying but I don't have a heat exchange system. I have raw water cooled. So im wondering if there is some way that the cooling system on the boat builds pressure or it it just runs water through the block and the thermostat is always opening and closing to keep the block at the proper temp.

 

[hivoltg]

Re: Cooling system question boiling temp of water

I dont have a heat exchanged system either. You are correct on the thermostat regulating temp but its not very good at it. So on our open systems water will boil over 212f. There might be enough pressure via the water pump to raise it some but maybe only 10F at most.

 

[skydiveD30571]

Re: Cooling system question boiling temp of water

Boats are not designed to run anywhere near 220?. If yours is, you have overheating problems.

 

[hivoltg]

Re: Cooling system question boiling temp of water

Mine is at around 140-160. When mine hit 175 one day I changed out the manifolds and risers, but on a closed system they will be more towards 190 at normal operating temp.

 

[agallant80]

Re: Cooling system question boiling temp of water

Boats are not designed to run anywhere near 220?. If yours is, you have overheating problems.

I guess that answers the question and why my mechanic neighbor was wondering. Cars run 210-240 and need a pressurised system to not have the coolent boil. Boats run more around 160. Didn't think of that.

 

[Lou C]

Re: Cooling system question boiling temp of water

Raw water cooled boats are set up to run cool purposely to avoid localized boiling in the hotter parts of the engine and that is why you have to run either a 145 or 160 stat vs 195 in auto and truck applications. A raw water cooled boat motor can run from a low of 140 to a high of 175 180 tops. You don't want it hotter or then you can get localized hot spots. Since the system isn't pressurized it relies on rapid flow of the raw water in and out. That is why any restriction like clogged risers can cause an overheat. In an open system if your temp gauge reads 200* (keep in mind that is the temp of the water in the intake manifold usually) the water temp in the heads could be hotter and it could be well beyond boiling near the exhaust ports, etc.
Outboards like raw water cooled I/Os are also set up to run cool with 140* stats.
Most of the young'uns won't recall but back in the old days like the 50s cars ran 160 stats and 7 psi pressure caps on the rad because the technology of high pressure rads had not really hit the auto industry. Sometime in the '60s they realized that an engine would run more efficiently at 190* or so but to do that the cooling system had to prevent water from boiling till about 265* to give a margin of safety. That had to wait till they came up with components that could take the increased stress of 15 psi caps, better rads, heater cores and hoses. Also, better ethylene glycol based antifreezes instead of the alcohol based ones used in the old days.

 

[HT32BSX115]

Re: Cooling system question boiling temp of water

Raw water cooled boats are set up to run cool purposely to avoid localized boiling in the hotter parts of the engine and that is why you have to run either a 145 or 160 stat vs 195 in auto and truck applications. .....................................Outboards like raw water cooled I/Os are also set up to run cool with 140* stats.

There's another more important reason also......running higher temps also causes precipitation of minerals and salt (in salt-raw water applications) that will collect inside water jackets, manifolds and risers. It's really bad juju!



Cheers,


Rick

 

[Lou C]

Re: Cooling system question boiling temp of water

Yep I have heard that one Rick many times...and....my 25 year old OMC has a 160 stat, OE as do nearly all of them and most Volvos too. My operating temps are usually between 160-175, and have not found that cooling passages have clogged up because of salt build up. It might depend on just how salty the water is, but the water here (Long Island Sound region) is pretty salty, not as much as Fla and the Gulf Coast but salty enough that you have to do the every 5-7 year manifold check/replacement likely. And painted trailers do not last around here at all. I would have gone to a 145 stat but I could never find one for that OMC thermo housing that many Volvos (the one with the rubber ring that holds in the thermostat) also use. I know that later model Mercs esp with the MPI system seem to use 160 stats too....