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Northeaster 04-26-2007 10:34 AM

Connecting raw water cooling to water heater
My new (30 year old) sailboat has a 1982 2GM (raw water cooled), that was previously hooked up to a Rartitan water heater - that went bad, and was disconnected. I recently removed the Raritan, and am considering replacing it with a Force 10 (as they are half the price). The previous owner had disconnected the faulty old heater, and put the engine hoses back to original, so I am not sure where to come off the engine raw water cooling. I follow the raw water, from seacock to raw water pump, into the engine, but then there is a short hose, that connects to the engine again, and then the hose going to the water exhaust pipe. Just want to make sue this is feasible, and any help on the best way to do it would be appreciated.

The boat was in salt water, during the engines 25 year life, but will now be in fresh water 95% of the time. I realize that I could add a fresh water kit, but as the boat will be in fresh water anyway, I would like to avoid that expense (as presumably one of the biggest benefits would be to avoid salt water to the engine)
I would like to hear form anyone who has experience using a raw water system to heat the hot water, or can tell me how much efficiency difference there would be this way.

Northeaster 04-26-2007 06:35 PM


sailingdog 04-26-2007 07:02 PM

The output hose going to the exhaust system from the raw water system should be the one that goes into the hot water heater. This is where the raw water is the hottest, and will be the most efficient at heating the water in the calorifier... Hot water heated by the raw water cooling system gets very hot, so be careful not to scald yourself.

Northeaster 04-26-2007 09:07 PM

I am not sure how often a thermostat would open - or stay open - when a diesel engine is running. Do you think a raw water cooled engine would produce near as much heat as a fresh water cooled engine (which has a closed coolant loop)? I would think that the coolant would continue to be fairly warm, thus giving off more heat, even when the thermostat is closed. I have no idea what percentage of running time, a diesel would have it's thermostat open, thereforre giving off heat to the hot water heater!

sailingdog 04-26-2007 09:52 PM

The thermostat is designed to open when the engine coolant reaches an operating temperature of about 180˚ F. Once, the raw water has passed through the heat exchanger, it is likely to be well over 100˚ F. Home water heaters are usually set around 140˚ or so... anything hotter will risk scalding. BTW, diesels run pretty hot, due to the ignition temperature of diesel fuel... so if the engine is running under any sort of load, the raw water side will be pretty hot.

hellosailor 04-27-2007 12:08 AM

Two caveats. In a closed water cooling system, or a fresh water cooling system, you can use a 160-180F thermostat and that gives you better engine performance, and hotter hot water.
In a raw water cooled engine, a 140F thermostat should have been used to prevent salt crystals building up in the engine. So, check the thermostat, you should be running the hotter one spec'd for your engine.

Automobile thermostats usually are simple open/close valves. Boat thermostats are usually (not always) "Y" or "T" valves, that allow the raw water to be dumped into the exhaust when the engine is cold, because that water is being pumped all the time, and the exhaust is a convenient way to dump it. As the engine heats up, the flow is diverted through the engine block to cool it--and then routed to the exhaust to cool it on the way out.

So how your thermostat works depends on how your cooling system is set up. If you have the diverter setup, make sure you are not splicing into the cold "dump" side for your hot water tank, it wouldn't work at all.<G>

Wherever your water comes directly OUT of the engine block and then goes INTO the exhaust, that should be the final stop where the heated water can be tapped and sent into the hot water tank, and then back out to the exhaust. Route those hoses carefully, and carry an extra piece of hose so that if the water heater fails, you can pull the hoses and restore the direct flow into the exhaust. You may also find the hot water tank uses a sacrificial anode (a zinc) to protect the coils in it. If it does--buy a couple of spares now, and make sure to keep up with them, as well as any similar zinc that goes in the engine block. (Some have a zinc in the block, others don't.) Running raw water through it will be harsher on the heating tubes than a closed water system would have been. Make sure the manufacturer intends it for this purpose.

CapnHand 04-27-2007 12:24 AM


Originally Posted by sailingdog
Home water heaters are usually set around 104˚ or so... anything hotter will risk scalding.

A temperature of 104˚ is a high fever, not scalding water. You meant 140˚, that's a typo, right?

Northeaster 04-27-2007 07:11 AM

SD- I would assume, that, although the hot water from the engine is "very" hot, it will / may not heat the actual water inside the tank to the same degree, as you would have loss to the exchange unit, etc, and it will not be giving off all of it's heat as it passes through. I will keep an eye on the temporature, however. Thanks for the word of caution.

HS - That was a great description, of the y / t valve. That is the setup on my engine. The short hose I described earlier connects the engine to a "t" fitting thermostat. I will have to look at it again, and will make sure that I am on the final "hot" side of the engine. It was this "t" fitting with teh short hose going bto teh engine that confused me. Didn't like not understanding why it was there / the flow of things!
Good advice on spare hose and anode.

sailingdog 04-27-2007 07:22 AM

Capn Hand,

Yes, it was a typo... I did mean 140˚. Ideally, hot water temps should be at to 120-125˚ to prevent burns... Water at 125˚ can scald, but is far less likely to do so... temps above 125˚ are correspondingly more likely to scald and do so more quickly.

BTW, the hot water on some boats comes out at well over temperatures that can scald almost instantly... I've had to treat such injuries, and its not much fun.

hellosailor 04-27-2007 01:25 PM

"BTW, the hot water on some boats comes out at well over temperatures that can scald almost instantly..."
Even on boats, there's little reason not to install a $10 anti-scald valve directly on the plumbing. Sink, showerhead, whatever. (I'd say the extra weight slows you down if you're racing, but racers don't need hot water systems anyway, right?<G>)

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