Overheating Issue on maiden voyage.pics

Discussion in 'Boat Design' started by Lawrencemd, Aug 25, 2009.

  1. Lawrencemd
    Joined: May 2008
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    Lawrencemd Junior Member

    We just launched our 28 ft home built cabin cruiser and are having some overheating issues.
    To start off, I will outline the the cooling system route. It is an enclosed system using 50/50 water and antifreeze. There is approx. 25 ft of 1" cooling pipes under the boat. The cooled water runs from the cooling pipes, through the transmission cooler, into motor (250 chev 6 cyl), out of motor into cooling tank, then on to marine exhaust manifold, then back out to cooling pipes.

    My question is whether this is the correct order in which to cool the different systems. Should the motor be cooled before the transmission cooler??Should the exhaust manifold be cooled first? What order should the systems be in the flow of cooled water from the pipes?

    Thanks
     

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  2. mark775

    mark775 Guest

    Your order is fine. What are your coolers made of? What pump do you have?
     
  3. baeckmo
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    baeckmo Hydrodynamics

    Sorry, have to correct you there!

    A) During "heating-up" period, there is no coolant flow through the xh manifolds with your setup, resulting in a massive heat build-up. The thermostat is in closed position. In most engines today, this means that the flow is recirculated thru' an internal shunt; nothing trough manifold and cooler.

    B) On top of that, you have about 0.5 m2 of cooling area exposed to the sea, which may cater for the cooling of ~50 hp, depending on flow velocities outside and inside the pipes! Your engine would produce more than that, unless throttled. A working rule of thumb is 100 hp/m2 for diesels and ~85 hp/m2 for gassers (lower thermal efficiency > more heat to reject)!

    Manifolds should have a parallell cirquit, but you may use "fiddlers recipy"; drill a leak hole, dia ~4mm in the thermostat valve. Heating period will be longer, and the thermostat will often allow wider temp variations in the block, but try it, you may live with it! Don't forget to install a vent pipe (~5 mm) from highest point in the "hot branch", probably the manifold, directly to tank, in order to dissipate gas and vapour. This prevents vapour locks that choke the centrifugal circulating pump!

    In order to prevent cavitation in the pump, it should get the coldest water, so from cooler first into engine water inlet! Then from engine to oil cooler (transmissions often run too cold!).
     
    Last edited: Aug 26, 2009
  4. tkk
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    tkk Junior Member

    Isn´t the antifreeze ratio a little extreme? Water is better than antifreeze when it comes to heat transfer.

    Unless you are operating an ice-breaker in arctic conditions, you would need antifreeze only for corrosion-prevention and 20% should be enough.

    We use 50-50 in our cars, that gives you freezing point around -35C.
     
  5. Lawrencemd
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    Lawrencemd Junior Member

    I should have also mentioned that there currently is no thermostat in the motor, in other words coolant is flowing all the time.

    The coolant is pumped through the system with the original engine water pump.


    Im very interested in your statement, "Don't forget to install a vent pipe (~5 mm) from highest point in the "hot branch", probably the manifold, directly to tank, in order to dissipate gas and vapour. This prevents vapour locks that choke the centrifugal circulating pump!". Do you mean another hose from manifold to tank? This is what we were worried about and had trouble with, vapor locks in the system.

    I am thinking of changing the flow to the following; Cooled water comes from keel coolers directly into engine (water pump housing), out of engine down to trans cooler, out of trans cooler into exhaust manifold, out of exhaust manifold and into expansion box (highest point in "hot branch") and then out of expansion box and back out cooling pipes. What do you think?
     
  6. mark775

    mark775 Guest

    Baeckmo is right. I missed the part about the cooling tank. And transmissions often run cold, as he said. But your routing is obviously not the cause of the overheating. Yes, a smaller antifreeze/water ratio would transfer heat better but we're still not finding the overheating culprit. The pump should pump water first, before the cooler, to have to draw less. There's a big fiddler's recipe now!
     
  7. tkk
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    tkk Junior Member

    What kind of temp metering do you have?
    Can you measure the temp of coolant coming from the cooler pipes?

    That would help determine if the problem is in circulation or cooling capacity of your cooler.
     
  8. wardd
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    wardd Senior Member

    get a infrared thermometer to check various points in the circuit, i bought one for about $50 from pep boys auto store and it seems fairly accurate, accurate enough for what you need

    by the way, cool get out on the water thing you've built
     
  9. Lawrencemd
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    Lawrencemd Junior Member

    We have an electronic water temp gauge connected to the motor. I will see if we can record the temp leaving the pipes.

    What I found was interesting, is that after it overheated, and we let the temp come back down to around 180, we would start the motor and chug along for quite a long time before the temp would start to rise again. You would think the temp would start to rise gradually, but no, after 20 mins of motoring between the 180-190 degree temps, all of a sudden it would start to rise very quickly. We would then shutdown and let it cool.

    By the way, thanks for your quick resonses.
     
  10. tkk
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    tkk Junior Member

    Is your system properly pressurized? Maybe your engine boils the high-antifreeze coolant and you get gas bubbles in the system that prevent proper circulation
     
  11. Lawrencemd
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    Lawrencemd Junior Member

    It was properly pressurized on land but that is what we are going to test today.
     
  12. tkk
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    tkk Junior Member

    Is it possible that some air was trapped in the system all the time?

    I think someone already mentioned a small pipe from some high spot, preferabley above cyl head into the expansion tank. That should take care of both.
     
  13. Lawrencemd
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    Lawrencemd Junior Member

    That is what we were thinking, that there was air in system. When we first set it up, we had a lot of trouble getting the air out.
     
  14. wardd
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    wardd Senior Member

    if any part of the circuit goes up then down that may be it's own high spot and may need a vent that can be opened, it would act as a trap for air much as a trap under a sink acts as a trap, for air the u would be up and for water the u would be down
     

  15. tkk
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    tkk Junior Member

    You might need to open some hose connections slightly to let the air out while filling up the system. Once filled, the small pipe from the big hose coming from the cyl head should be enough
     
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