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

    Currently, the highest point is the expansion box. What if I put a small bleeder (like a rad bleeder) in the rubber hose going into the expansion box. Then when filling it up, I'll open it and let the air out.
     
  2. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    Holy grail, how often do we have to tell people NOT to take the thermostat out of the engine!!!!????? When there is no thermostat, the shortest loop for the water is directly back to the engine through the internal shunt; ergo NO COOLING!!!!

    Drill the leak hole as I told you, put the darn thing back where it belongs and off you go......
     
    Last edited: Aug 26, 2009
  3. baeckmo
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    baeckmo Hydrodynamics

    Normally the cooling circuit in the engine is as follows:

    1) Water from cooler coming in at pump suction side.
    2) After the pump, water is distributed trough engine block and top, ending up at thermostat housing.
    3) When the engine is too cold, there are two methods;
    3a) the thermostat is directing all water straight back to the pump inlet via a small bore pipe "internal shunt". Often this channel is hidden in the casting.
    3b) older engines without shunt stop the flow totally. This method has been obsolete for ages.
    4) When block and top has normal operating temp, the thermostat is balancing the flow to cooler/shunt to keep temp constant.

    The shunting allows full flow through the engine all the time, preventing local overheating due to pockets of low flow. It also lets the centrifugal pump work close to its design point, preventing cavitation and aeration.

    So, removing the thermostat means that you have no balancing restricion that forces cooling water via your cooler when the engine has reached working temperature, neither is there any water circulating through your manifold, which is quickly boiling the slug of water standing there.

    And: The water must NOT circulate through the tank, because you will most certainly have a free surface there. With the turbulence of the moving fluid, air will be drawn into the cirquit, causing expansion troubles plus increased corrosion due to the free oxygen available! The tank should be a "dead end", connected to the pump inlet, and with a vent from the highest hot spot, as mentioned before! The ideal is to have the water degassed as completely as possible, so any contact with a free surface is to be avoided!
     
  4. baeckmo
    Joined: Jun 2009
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    baeckmo Hydrodynamics

    For a better understanding, see attachment! Top sketch shows "as built". If you remove the thermostat valve, the water will take the easy way = the shortest way, hence no cooling, neither to engine nor to manifold/oilcooler.

    Without the shunt, as in older engines, the thermostat (if fitted) would be closed during warmup; still no cooling to the manifold!

    Lower sketch shows a reasonable compromize, note the connection of tank with vent. This arrangement needs the leak hole in the thermostat, otherwise no manifold cooling during warmup.

    Normally in marine engines, the cooling water to the xh manifold is taken from block before the thermostat (bypassing it), and then connected to the thermostat outlet to cooler via a calibrated restriction.

    And,finally: even with this corrected and with a lower mix of antifreeze, your cooling surface is still not enough for more than ~50 hp!!
     

    Attached Files:

  5. kapnD
    Joined: Jan 2003
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    kapnD Senior Member

    Dont run it without a thermostat! If you're still getting air after you install the thermostat, you could have a leaking head or turbo gasket.
    Drilling the hole in the thermostat flange sounds like a good trick, as the manifold does get real hot fast.
    Some marine engines have a bleed hose off the top of the manifold to the tank.
    Some motors need to have the coolant filled very slowly to avoid trapping air pockets, use a funnel with a small opening to regulate fill rate.
    Good luck!
     
  6. Brent Swain
    Joined: Mar 2002
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    Location: British Columbia

    Brent Swain Member

    I have always used 1.5 sq ft per 10 hp. No problems. I prefer to run the water from the engine to the cooler, and would l cool the tranny, then the exhuast water jacket after the tranny. That way you are not warming the water before it hits the engine.
    I set up my header tank so I can jam a plastic hose barb in the bottom, outlet to the cooler, and attach it to a presure tank (ss pop can ) filled with antifreeze and water, Then I pressurise the tank and force the coolant into the system to force any air bubbles out.
    This also lets you flush the sytem by hooking it up to a dock hose and letting the hose flush it out.
    Brent
     
  7. Lawrencemd
    Joined: May 2008
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    Lawrencemd Junior Member

    OK, today we rearranged the coolant flow, matching it up to another fishing boat here, which had the same engine transmission combination. After very slowly filling it with coolant/water, the temp stayed down at 185 for 35 mins at idle. Much better than yesterday.

    I was puzzled that this worked, and was initially thinking that there had to be air in the system yesterday. However, after thinking about the new flow route, we are now moving coolant through the water cooled manifold, in the opposite direction from yesterday.

    Yesterday, we were moving liquid through the manifold, from front to back. Today it is running back to front (which is the opposite of what I would think). The other fishing boat, and another boat online here, are setup the same way.

    I am thinking that maybe running coolant through the wrong direction was causing turbulance, and making it much harder to get the air out of the coolant lines? nd yes we are installing a thermostat.

    Does this sound plausible?
     
  8. baeckmo
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    baeckmo Hydrodynamics

    .....that equals 72 hp per square m; ie slightly more conservative than my 85 hp/m2. That would allow a power of 41 hp with the cooling area Lawrencemd has built in. So, back to basics as I have explained!
     

  9. hartley
    Joined: Feb 2006
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    Location: australia

    hartley Junior Member

    Overheating

    If your cooling system is in order perhaps it is time to look further, is your engine overloaded,what gear ratio,what size prop? .Remember props move boats not engines ......cheers Hartley
     
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