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 Soy Soylent Green: I can't believe it's not people

    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 Soy Soylent Green: I can't believe it's not people

    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 Soy Soylent Green: I can't believe it's not people

    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 Soy Soylent Green: I can't believe it's not people

    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.
     
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