Cooling Effect of Water Heater

[SV Sea Monkey]

A few months ago I was returning from a 4-day cruise on my (relatively) new-to-me boat. I did not have full confidence in the engine cooling (that's another story--see below) and the engine felt a bit warm to me. We were motoring with almost no wind, so sailing was not an option (needed to get home for work--part of the problem). To provide a bit of extra cooling in the home stretch, I ran the hot water tap in the galley--my thought process was that (1) we no longer needed the water for anything else (almost home), and (2) the heat exchanger for the water heater would give me a little bit of extra cooling for the engine.

This was just a precaution and was terribly wasteful of my fresh water, but it got me thinking. If I did shred an impeller and needed to continue motoring for a short time due to no wind, narrow channel, etc...how much cooling would I get out of cycling my fresh water through the water heater and back into the main tank? This would be very quick and easy by running a hose from the galley faucet back into the main tank--just a few seconds to rig with a 5-foot section of hose.

I suppose I could run an experiment to compare flow rates, heat exchanger efficiency, etc. but I have other more pressing projects right now. I thought I would go ahead and pitch the question here to see if anyone else has "run the numbers" on this.

Let me try to anticipate the initial flurry of criticism/feedback:

--Yes, I know it's a sailboat and the engine is just a backup. I am comfortable sailing on and off in the hook (did it earlier on the same trip and many times in the past). There are times, though, when it would be very inconvenient to have the engine overheat and have to shut it down.
--Yes, I know I need reliable engine cooling and that should be sufficient. I have increased my confidence in my engine, carry spare impellers, and know how to use them. I'm asking mostly because I'm curious. It's also very easy to do and wouldn't even waste fresh water--and is a lot quicker than changing an impeller if the anchorage is close and the channel narrow.
--Yes, I know the wet exhaust would be running dry. I wouldn't ever do this on purpose, but it might buy a few more minutes of motoring to get through the pass and drop the hook or tie up. How bad is it to run the exhaust dry for a while if the engine itself is not overheating? I really don't know that.

...Standing by for your thoughts.

[Oliver L.]

Hi, Sea monkey,

if you want to go that way, do a Tee valve off your kitchen drain into the wet exhaust. Then you can turn on the hot water, switch the valve to the exhaust and you will be "fresh water cooling".

To cool 5 kW (just as an example, I don't know the size of your engine) at 40 deg. C Delta-T (20 to 60) you would only need about 20 liters (4 gal) per hour. So you should have plenty of time to make it out of a tight situation.

Now, me, I would just shut down the offending engine and restart it for the last 30 seconds (parking a cat is no fun on one engine)...

Oliver

[Dockhead]

It's an interesting question, but I think the fatal flaw is the exhaust. A wet exhaust will burn up pretty quickly without water flow, and can cause a fire, flooding, all kinds of disastrous problems.

I think the conventional solution -- keep a spare impeller handy and change it while hove-to if necessary -- is a much better one.

[transmitterdan]

It's bad to not run water into the exhaust. A few minutes can melt the hoses and let exhaust gases into the boat. Fire is a possibility.

The efficiency of the hot water heater exchanger is not enough to cool an engine.

Fix any known cooling problems.

When motoring think about what you would do if the engine overheats or dies. Always be ready to raise a sail or drop an anchor.

[Blue Stocking]

IMHO, a prudent sailor always has innovative ideas of how to get home. Short version of story, I once had the mounting of a raw water pump fail. Plumbed the submersible bilge pump into the outlet of the raw water pump, flooded engine deep sump thru seacock, and let the bilge pump bring us home.

[SV Sea Monkey]

Thanks for the replies so far.

@Oliver: That would be an uphill run to get from my sink drain to the exhaust. The engine is a Yanmar 3HM35F. I'm not concerned enough to make a permanent change to the plumbing--just curious, mostly. I agree with shutting down the offending engine, and don't have a problem sailing into the dock if there's wind. I do have a tight S-curve to get through, though, which would be difficult if the wind is from the wrong direction. Docking a monohull with zero engines can be interesting, too. I had to do that frequently with my Westsail 32 many years ago.

@Dockhead: I figured the exhaust would be the problem. Oh, well.

@transmitterdan: Thanks. Known cooling problems have been fixed. I do like to consider my options; that's what prompted the question.

@BlueStocking: I like the way you think. Thanks for the idea.

[transmitterdan]

Using a bilge pump in place of the raw water pump can work if the impeller is removed. Even the fresh water pump could probably make enough water to cool the engine and exhaust until the fresh water ran out. At 3-4GPM a 50 gallon tank would last about 15 minutes which might be enough to avoid a serious problem.

Also, don't forget that coming into a marina you are never far from local help. Don't be afraid to get on the VHF and ask for help. Joshua Slocum used local tenders all the time in Spray so I am not embarrassed to do it. Just a couple of weeks ago we went out and towed in the local Fish and Game Warden who's engine failed coming into the harbor. He wasn't too embarrassed (well maybe a little) but now I figure we have earned at least one free pass on the next random safety inspection.

[sailorchic34]

What an interesting question.

So roughly you could get up to oh 1080 btus' per gallon of water (180-50)*8.33. So water heaters have temperature controls on the engine water while will prevent the water from getting too hot.

A 50 gallon water tank in theory would provide about 54MBH of heat rejection, not counting radiant losses thru tank walls. So if you had a supply and return to the tank running the thru the raw water side, a 5kw engine would run over 3 hours without over heating. Of course you would need to hook the bilge pump up to the discharge elbow and have a means of adding water to the bilge.

As someone else indicated, the heat exchanger in the typical water heater does not have enough surface area to reject more then, again roughly, 4000 btu's per hour, or around one KW. So at idle or maybe a bit above, you could run the engine quite a long time, running about 6 gallons through the sink every 1/2 hour or so. This assumes you have raw water flow of course.

[SV Sea Monkey]

Good points all, thank you.

I like the idea of running the bilge pump through the cooling loop--this seems just as easy to rig in a hurry, and still uses the more effective primary heat exchanger. I have a good strainer on my bilge pump, and could pretty easily keep up with clearing that if needed. That option also takes care of the wet exhaust. Getting water into the bilge would be easy.

If the heat exchanger gets clogged with impeller bits, the water heater option remains a viable option, and can be rigged quickly--but I would need to also find a way to inject some water into the exhaust if running more than a minute or two.

This has been an interesting thought experiment at least. I like knowing my options for those times when things go wrong, and I appreciate all the input.

[SV Sea Monkey]

Originally Posted by sailorchic34

What an interesting question.

So roughly you could get up to oh 1080 btus' per gallon of water (180-50)*8.33. So water heaters have temperature controls on the engine water while will prevent the water from getting too hot.

A 50 gallon water tank in theory would provide about 54MBH of heat rejection, not counting radiant losses thru tank walls. So if you had a supply and return to the tank running the thru the raw water side, a 5kw engine would run over 3 hours without over heating. Of course you would need to hook the bilge pump up to the discharge elbow and have a means of adding water to the bilge.

As someone else indicated, the heat exchanger in the typical water heater does not have enough surface area to reject more then, again roughly, 4000 btu's per hour, or around one KW. So at idle or maybe a bit above, you could run the engine quite a long time, running about 6 gallons through the sink every 1/2 hour or so. This assumes you have raw water flow of course.

Sailorchic,

I had envisioned running the galley sink constantly--just cycling the water back into the main tank--to maximize the temp difference between the engine cooling water and the fresh water in the water heater, and thus suck as much heat out of the engine as possible. I was just curious about this, now I'm really curious. If I ever get my other projects done (yeah, right) this might be an interesting experiment. I'll have to rig the exhaust water first, and monitor temps closely to avoid potential damage, but it would be interesting to know if I can operate at idle indefinitely this way.

Of course, if I had a pink engine it would work better.

[sailorchic34]

Originally Posted by SV Sea Monkey

Sailorchic,

I had envisioned running the galley sink constantly--just cycling the water back into the main tank--to maximize the temp difference between the engine cooling water and the fresh water in the water heater, and thus suck as much heat out of the engine as possible. I was just curious about this, now I'm really curious. If I ever get my other projects done (yeah, right) this might be an interesting experiment. I'll have to rig the exhaust water first, and monitor temps closely to avoid potential damage, but it would be interesting to know if I can operate at idle indefinitely this way.

Of course, if I had a pink engine it would work better.

Another thing you could do, say if your fresh water pump died is connect the pressurized discharge fresh water to the engine block fresh water inlet and run a return line from the engine block return to the water tank. Drain any anti freeze from the engine first and flush.

Put a U bend in the return line, rising above the engine head so that the engine water jacket stays full. Then you can use your fresh water pump to recirculate fresh water thru the engine jacket back to the tank using the electric fresh water pump as a engine cooling pump.

Of course you would need to completely clean the tank and water lines afterwards. Plus start with a cold engine, but that would give you a few hours running.

You could also use the electric fresh water pump, or spare electric pump as a raw water pump if your raw pump died without spares.

Of course your idea of using the hot water heater should work pretty well as long as you just run the engine at idle speeds in gear. Should be enough heat transfer to make that work for a while.

Actually Purple engines with Pink Injector lines seem to run the best

[sailr69]

reading all of this thread has my brain in a wrestling match. I give up

[atoll]

if you take out the thermostat and remove the fanbelt.

,many marine engines will thermosiphon if the system is pressurised,as long as run under low load,and the raw water pump is working.

worth bearing in mind if the circulation waterpump packs up,

Thermosiphon - Wikipedia, the free encyclopedia

[Pete7]

Don't forget many gearboxes are also water cooled.

Pete

[SV Sea Monkey]

Thanks all for the great discussion and food for thought. I've got to focus on my last few projects before pulling out for the winter, then shift focus to hauling out and winter projects. I hope to get back to this next year before heading out. It really would be an interesting experiment to play around with different workarounds for various casualties. Obviously best just to keep things in working order, though.