Replumbing an Outboard for Heat

Discussion in 'Outboards' started by DogCavalry, Nov 24, 2021.

  1. fallguy
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    fallguy Senior Member

    Why? Because I suggested a heat calc against ondarvrs 140F comment?
     
  2. Mr Efficiency
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    Mr Efficiency Senior Member

    Shoot the messenger ? Seriously, I would say any reputable outboard mechanic would tell you to forget about the idea, these things are highly loaded engines with little room for being compromised by tinkering with the workings, and most decidedly they have not been designed with this idea in mind.
     
  3. ondarvr
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    ondarvr Senior Member

    The 140 temp is at idle, there are bypass valves that open as RPMs increase.

    If the thermostat fails there is still plenty of cooling water flowing at high rpms.

    So it potentially runs cooler at higher speeds.
     
  4. fallguy
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    fallguy Senior Member

    John can be mad at us for doubling down. But there are realities, and he ought to recognize who is looking out for him.

    My wife has a 2018 Teslas S. We won't delve into the price. When the temps here drop to 20F; the car has a problem. The same one John may encounter. There is insufficient heat to keep my feet warm and keep the windows from fogging. A damn shame for the price.

    DC will probably run into some fogging issues in Serenity. I doubt very much a hose from the ob will be enough; even if he doesn't mind the feet getting cold. But a decently high cfm diesel air heater would probably be enough and operate if he wanted to sleep onboard w/o engines. Yeah, dream killing, but real. Yes, costly to do well.

    My guess is he will need some way to reduce rh in the cabin..
     
  5. DogCavalry
    Joined: Sep 2019
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Not the messenger. @Mr Efficiency you seem to spend you time trying to prevent anyone from doing anything, mostly by wringing your hands. You'll endlessly lecture and caution people on the dangers of something you know literally - and I'm using the word correctly- nothing about. Ten years ago you were arguing with PAR about sea sleds. He owned several, and had ridden in many others. He was a respected designer and builder. You've still, ten years later, never so much as seen one, or done anything else to correct your ignorance. But still you yammer and bleat.
    I occasionally disagree with some heavy hitters. @baeckmo , @jehardiman , @Ad Hoc , but before I presume to do so, I spend hours reading research papers from institutes around the world. If I am lucky enough to have someone of their caliber, or @bajansailor or @fallguy 's incredible generosity of spirit helping me with my adventure, it would feel disrespectful of their time to not try to inform myself.

    So, to this particular issue:
    I have spent hours looking at the little I could find online for free. But, the telltale pours water from before the block, but after the exhaust manifold, so the water is never going to be more than warm.
    The thermostat or thermostats don't open until the water hits the design temp for that engine. So a late model honda, for example, pours out water at 200°. From the bottom of the leg, just above the prop. Part of the stream is diverted into the exhaust just below the power head. Everything after that is dumped out.
    I've crossed the Rockies in winter many times. The waste heat from my V6 vortec kept the vehicle fairly warm, and that's when it was only making around 75hp, at well bdlow freezing. A pair of 150's when its a well above freezing will give more heat than I could possibly use.
    As far as temperature goes, I've installed plenty of residential heat pumps, and I can insulate lines. The water used for residential under floor hot water heating comes from the hot water tank, in a combined system, or a separate tank in a less sophisticated system. It ranges in temp from 35°, to 55°. Or 95° to 135°F, so even water at 140° is hot enough to burn a person, and definitely hot enough to do work going through a salvaged heater core. But 140° is lower than an outboard wants to run.

    As far as voiding the warranty goes, that's a boogyman for scaring children. What percentage of working boats have engines still under warranty? Certainly not mine. And why would it matter? Collecting water falling out the bottom of a leg isn't going to do anything.

    I welcome, with gratitude, all considered opinions, and informed challenges to my assumptions. But not the same uninformed notion repeated endlessly. I spend a lot of time researching before I ask a question, and pay respectful attention to the answer, even if I don't like it. Anyway, sorry I was grumpy. I'm under a lot of pressure right now.
     
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  6. Mr Efficiency
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    Mr Efficiency Senior Member

    I would say the idea of actually plumbing into the passageways, is crazy, you might get away with a spiral coiled copper tube in the leg after the exhaust and cooling water are freed from the block, on a closed circuit with an in-line pump, with its own water, but what might happen if you cease to circulate that water, could be a problem
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    Oh dear, let me tell you this, I have seen and been in many boats, and it is a fact that stuff that does not do the job, quickly gets culled from the herd, in this country, like few others, so I make no apology for being more than a little sceptical of sea sleds and their close relation, cathedral hulls.
     
  8. baeckmo
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    baeckmo Hydrodynamics

    Well frankly, I can understand Johns frustration here, since a number of the statements given regarding heating power are not based on facts. Living in high in the northern hemisphere forces one to gain some insight into the “keeping alive and comfortable in the cold”-issues! Wasting "free" (as in "already paid for") heating is an engineering insult and a shame.

    Roughly speaking, the shaft power of the average combustion engine is about 30% of the input fuel chemical power. That means that the rest falls as losses; initially about equal proportions to cooling and exhaust. But in a marine engine, the exhaust manifold is watercooled, so the available heating power from the water is about 40% of the input fuel power, or about 1.3 times the shaft output.

    As mentioned by Ondarvr, the thermostats are set to about 60 degrees C. At this temperature, the cooling water is redirected from by-pass flow (in old 2-stroke engines through the cylinder head) and more through block and head. There is a slight overcapacity in the pumped flow (veeery slight in some engines….), which will “dilute” the 60-degree outflow to something like 50 degrees (depending on the sea water temperature).

    So, even when using a fraction of the available engine power, say 60 kw (~80 hp) from a 100 kw engine, there is about 75 kw heat at 50 degrees available (yes, depending on the temperature difference at hand....) in the cooling water, provided it can be tapped off without negative consequenses for the engine.

    As a comparison, I live at the western coast of Scandinavia, roughly at the latitude N 57 degrees. Right now we have the first snow, 10 to 20 cm’s, outdoor temperature minus 5 centigrade and an NE wind of 11 m/s. The Skagerrack sea is still open with 8 to 9 degrees; ie we are still boating. But just everybody is using waste heat from inboards to get some comfort. My domestic house (~150 m2) is right now using an average of about 4.5 to 5.5 kW for heating; 5 radiators with a surface temperature of 58 C give an indoor temperature of +20 C. Compared to that, the surplus heat from the outboard is huge.

    Back to the boat then; even the possibility to swap a pair of wet and cold working gloves for a dry and cosy pair hanging from the heating pipe is a blessing a day like today. And any inboard temperature above +5 C feels like a heat wave.

    The problem is not the heating power available, but harvesting it from an outboard. This is why I suggested an inboard for the “Serenity”; there you always have a hot water connection available. Modern OB’s are designed with “zero” margins for just about everything mechanical to save the final cent of profit. The cooling water in critical zones of the engine is carefully directed by nozzles or apertures to use the water as efficiently as possible (-smaller pump, lower cost…) aso.

    The hot water must be taken from the ducting down the rig, without changing the total pressure difference over the engine, and it must be returned to the rig in total in order to keep the transmission components at a steady temperature (fi not "overcooled" in low temperature waters, or overheated in warmth), that is the engineering challenge here. And remember, engine makers have different solutions to the thermostat placement and the shunting schemes; you cannot come up with a one-for-all solution to the heating supply.
     
    Last edited: Dec 1, 2021
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  9. gonzo
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    gonzo Senior Member

    I think the only way to get a lot of heat from an engine is to run a closed cooling system. In short, add a heat exchanger.
     
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  10. fallguy
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    fallguy Senior Member

    Don't forget the goal. It isn't simply the tap side. Let's agree he has 75kw available. Plenty for cabin heat.

    The main issue on the cabin side will be fogging all the glass in the cabin and an inside helm. The best way to deal with fogging glass is hy moving air across the glass and lots of it. This results in even higher heat losses to the pipes because in order to achieve high cfm; you use some good sized fans.

    Ebay has Webasto diesel 2kw (8000btu) air heaters (not sure how) for $800. A little wire and a small tank and DC can even sleep inside the boat if he likes without running engines all nite. Placing a heater output with a duct towards his helm station would manage all the fogging.

    Does it seem possible or likely that DC, who I have great respect for his efforts on his build will spend $1000 tapping his ob? He needs a heat exchanger; he needs bronze fittings for the ob. He needs a circ pump or two. He needs boiler lines. He needs a heater coil. He needs coolant. He needs an expansion tank. He needs a fan to run over the coil. He needs to wire the fan. After all this effort; let's say he is in for $500. Does that system even clear his windows? Probably not, so he needs a better fan and suddenly he is at $600 for a hodge podge system of tubes at his ob and all the risk to his engine.

    I had the precisely exact same curiosity on this issue. I have two outboards and a need for cabin heat. It came down to warranty concerns for me.

    So, today DC doesn't care about warranty. But someday he may get a new engine for the rig or even 2. Then what? Well, then he has to go get an independent heating system because he'll also not want to void the warranty. And he'll need a new heating system.

    And he is in relatively mild climate, not like yours or mine. But running in winter will be a bear without cabin heat.

    The cheap Chinese knockoff heaters are basically a couple hundred bucks all in. He'd have it installed in less time than tapping the engine and cutting holes in the cowling and making chafing protectors that don't fall out.

    Sometimes your friends tell you to not marry the girl. They are rewarded by you not liking them. Years later, she leaves and you look back and say her and I had some good times and I am still paying support. The friends are gone and you may forget they told you, but they did. I may not have the engineering knowledge to tap into the ob and grab some heat, but I can tell you it ain't a great idea because the costs are too high and you only get heat running your ob. All night. And you'd ditch the system for new engines and go back to needing a diesel tank somewhere, etc.

    Engineering decisions must follow cost/benefit. It isn't just whether it can be achieved.
     
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  11. bajansailor
    Joined: Oct 2007
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    bajansailor Marine Surveyor

    I do not have anything to contribute here, as I know nothing about the topic being discussed, but I am following with interest (and I want to receive email updates re future posts).
     
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  12. DogCavalry
    Joined: Sep 2019
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Well, I kind of went off the leash there. Sorry for the bites.
     
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  13. BlueBell
    Joined: May 2017
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    BlueBell . . . . .

    This seems to have become another distraction for you John.
    Is it taking away from progress on the boat?
    Is it worth it?
     
  14. DogCavalry
    Joined: Sep 2019
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    You guys are right. I'll put it down.
     

  15. DogCavalry
    Joined: Sep 2019
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    DogCavalry Soy Soylent Green: I can't believe it's not people

    Squirrel!
    I do not like the cone of shame.
     
    Last edited: Dec 1, 2021
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