Replumbing an Outboard for Heat

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

  1. gonzo
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    gonzo Senior Member

    Thermostats are installed to keep a constant temperature. The engine's temperature does not drop when it opens. As far as how much heat the exhaust water contains, we don't have enough data. What is needed, is the volume of water flowing through the block when it reaches it operating temperature, and the supply water is at 35F. Once the volume and temperature of the water is known, the rest of the calculations are easy.
     
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  2. DogCavalry
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    DogCavalry Senior Member

    Exactly. We know heat rejected to environment, and ambient water temperature. Although variation there is minor conpared to the quantity of heat it carries out.
    Hmm. Could we rig up an impellor, without power head, drive it at whatever rpm the engine gives, and measure flow? Although I'm sure a good pump engineer could probably calculate flow/rpm by measuring case volume, and offset of the impellor axis with respect to the case. Unfortunately I'm not an engineer, just a wood-butcher punching up.
     
  3. gonzo
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    gonzo Senior Member

    The issue is not the pump flow. We need to know what the flow through the block is. A large percentage of the water flow goes directly to the exhaust through the bypass.
     
  4. ondarvr
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    ondarvr Senior Member

    On Mercs the bypass (poppet valve) has changed a couple of times. Some dump the excess water into the exhaust, others into the water jacket, and some don't have them.

    Check what type yours is. If yours dumps it into the exhaust that works better for your project.
     
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  5. DogCavalry
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    DogCavalry Senior Member

    Okay. I was thinking that some remix the water from the block and the bipass before dumping it out, but it makes sense that some direct the bipass water directly to the exhaust.

    @ondarvr , I definitely have a vested interest here, but I also want to see generically how this might be done, and besides I don't know what engines I'll end up with.

    It seems there are different routes that different engines use, but I don't know nearly enough to generalize between manufacturers, models, power ranges.
    But they all-
    1 Take in water through ports on the front of the lower unit
    2 Pump it up the leg with the impellor
    3 The water passes by the exhaust, picking up heat
    4 A small stream of water, slightly warmed by the exhaust, squirts out the telltale
    5 Passes through the thermostats into the block if it's hot enough, or bypasses the block if not... and then there's uncertainty.
    When the thermostat(s) are wide open, does all the water go through the block, or only some?
    Does bipass water get mixed back in?
    In the engine I posted the diagram for, it's a v6. From one side of the block the water just dumps out. From the other side it goes into the exhaust.
    Do some engines direct all of the water into the exhaust?

    What am I forgetting to ask?
     
  6. ondarvr
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    ondarvr Senior Member

    There is just about every combination of cooling water routing you can imagine, some manufacturers use mostly one system, others change it depending on the exact motor.

    Older motors had outboard specific powerheads, newer motors frequently share powerheads with the automotive industry. Running too cold has been a problem at times with some of these, this resulted in gas dilution of the crankcase oil. I haven't tracked it closely in the last few years, so I'm not up to date on the latest motors.
     
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  7. DogCavalry
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    DogCavalry Senior Member

    Specific approach to specific engine then.
     
  8. gonzo
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    gonzo Senior Member

    Even when the thermostat is open, there is water flow on the bypass. However, in really cold water, it is most likely the thermostat will only be partially open.
     
  9. DogCavalry
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    DogCavalry Senior Member

    20220129_123118.jpg
    On the majority of engines I've checked, the waste water just pours out of these slots.
     
  10. powerabout
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    powerabout Senior Member

    What comes out of there is the water that overflows the water from the inner exhaust housing, its fills to keep it cool and dampen sound, its not that much.
    Its different on different models as some have an exhaust tube all the way to the gearbox hence there is no inner housing and the whole housing is full of water.
    Other with a short megaphone are inside an inner housing so water is filled between the outside you are looking at and the inner housing
     
  11. powerabout
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    powerabout Senior Member

    The thermostats are after the block. Typically for a 2 stroke, thermostats are on the heads and they dump down the heads and out into the midsection
    There has to be pressure relief to ensure there is always water flow when thermos are shut.
    That can be a separate valve or spring loaded thermostats seats.
     
  12. DogCavalry
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    DogCavalry Senior Member

    So Anne was driving us home the other night, and I was idling. That's bad. But there's a big splashing flow out the back of our OB like a tap filling a sink. I shoved my paw in the flow, and it's hot like a hot bath. The flow and temperature are enough to heat a house. The flow in question is near the center of the image. The arc under the lowerhead. Not the squirt from the telltale.
    IMG_20220817_080042415_HDR.jpg
     
  13. gonzo
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    gonzo Senior Member

    The water and exhaust gasses come out together. You would first have to find a way to separate them. Then, any backpressure would cause issues with the engine functioning properly. Backpressure would raise the water level on the leg and backflow into the engine block; particularly after stopping. In short, if you want to use the heat from an outboard, it will need to be a system that does not change the flow design of the water or exhaust, and then mixes them back in the leg.
     
  14. DogCavalry
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    DogCavalry Senior Member

    Not really. At least in this model I can just mount a basin to catch the water splashing out that overflow. Only some engines mix all the exhaust and cooling water. Some have part of the water going into the exhaust system and part just dumped into the leg to drop out the slots on the bottom above the anti ventilation plate. In that system some or all of the cooling water could be diverted with no harm to the engine. And as we established pages back, we only need a little. The waste heat from a 150 is around 60kW. That would heat a couple houses.
     

  15. gonzo
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    gonzo Senior Member

    Are you referring to the water that comes out of the two holes on the back? I suppose that if what you use to catch the water is open on the top, there would be no back pressure building up.
     
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