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Old 20-08-2015, 03:47   #1
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KBW21 gearbox - why salt water cooling?

Our KBW21 gearbox has an external cooler connected to sea water. Our engine (4JH3-HTE) has a freshwater-cooled water jacket, cooled by sea water through the heat exchanger..

I asked a mechanic why the gearbox was cooled with salt water and he had no answer other than to say that this was always the way it was done. Anybody have any thoughts?

I am considering rerouting some hoses to use the engine freshwater coolant for cooling the gearbox.

Cheers!

Steve
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Old 20-08-2015, 06:36   #2
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Re: KBW21 gearbox - why salt water cooling?

The saltwater is always much cooler than the engine coolant. Your gearbox won't appreciate much running hotter than 200f.


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Old 20-08-2015, 07:14   #3
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Re: KBW21 gearbox - why salt water cooling?

One can readily see that it might be necessary to remove heat from the transmission oil, particularly on those transmission designs that employ hydraulic pressure to activate the clutch packs as opposed to mechanically activated clutches. The question is why would a designer choose to use sea water cooling as opposed to freshwater cooling to remove that heat?

One of the most important factors in heat transfer is the "delta T" of the two fluids, that is, the difference in temperature between the fluids. The greater the temp difference, the greater the heat transfer rate.

An engine designer usually uses a maximum sea water temperature of about 90 degrees F. for heat transfer calculations, and then builds some additional safety factor into the mix to account for fouled tubes, thruhulls, etc. His calculation might use a delta T where the transmission fluid is running at 220 degrees F and the sea water used to cool it is 90 degrees F. Using these numbers he comes up with a cooler of a certain size and cools his oil temp down to 200F.

Contrast this to a calculation where the transmission fluid is the same 220 F but the antifreeze flowing to the cooler has just come out of the engine heat exchanger at 160F. You can see that the size of the transmission cooler required is going to be much larger than in the case of the sea water cooled example.

So the answer to your question is that you can change your cooling medium, but probably you will have to increase the size of the gear cooler that you have inorder to adequately cool the transmission.
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Old 20-08-2015, 07:30   #4
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Re: KBW21 gearbox - why salt water cooling?

I never thought about this before, but what is the temperature of the water coming out of the cold side of the heat exchanger? Send that to the tranny. On freshwater systems there is always water flow, it just is bypassed to not go through the block when the block is cold.

Of course then you have to know that the heat exchanger has the capacity to deal with the additional heat of the tranny. Seems like some of the stories here of old Perkins indicate they don't have enough capacity just for the engine running at high rpms.
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Old 20-08-2015, 08:31   #5
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Re: KBW21 gearbox - why salt water cooling?

A few minutes after an engine is started the entire freshwater side of the cooling system hits equilibrium, or very close to it anyway, the water going into the engine is only a few degrees cooler than the water exiting it.
In other words you would be trying to cool your tranny with some pretty hot water.
Might help if we knew what the ideal operating temp for the transmission is, if it is 200F, then you might could cool it with the engine fresh water side, assuming of course there is enough reserve capacity in the cooling system.

But, I'm going to bet your going to find out that it runs quite cool, like maybe 150F, tops?
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Old 20-08-2015, 18:34   #6
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Re: KBW21 gearbox - why salt water cooling?

As I remember when you double the temperature difference you get 4 X the cooling effect.
Basically it's non linear
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Old 22-08-2015, 04:30   #7
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Re: KBW21 gearbox - why salt water cooling?

Thanks for the input, everyone. I confess I was looking at this more from a corrosion issue than a heat transfer problem. Since I have no idea what the water temperature is out of the heat exchanger, nor the operating temperature of the gearbox, it's hard to make an informed decision.

I'll probably leave it the way it is. Thanks again.

Steve
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Old 03-11-2015, 13:39   #8
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Re: KBW21 gearbox - why salt water cooling?

I am trying to find a thread that discusses the pros and cons of raw water versus fresh water cooling on cruiser engines, i am sure there is one already running, but buggered if l can find it! (New to forum)
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Old 03-11-2015, 13:59   #9
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Re: KBW21 gearbox - why salt water cooling?

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I am trying to find a thread that discusses the pros and cons of raw water versus fresh water cooling on cruiser engines, i am sure there is one already running, but buggered if l can find it! (New to forum)

For the record I can't think of a single pro to seawater cooling an engine.....ok, one, but it's not weighted enough to consider.


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Old 03-11-2015, 22:05   #10
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Re: KBW21 gearbox - why salt water cooling?

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For the record I can't think of a single pro to seawater cooling an engine.....ok, one, but it's not weighted enough to consider.


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Outboard motors can be either salt water cooled or fresh water cooled depending on whether you are at sea or in a lake. Seem to work OK.
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Old 06-11-2015, 05:38   #11
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Re: KBW21 gearbox - why salt water cooling?

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Outboard motors can be either salt water cooled or fresh water cooled depending on whether you are at sea or in a lake. Seem to work OK.

And when you pull an outboard out of seawater you typically flush it with fresh. But these are also engines designed from the ground up for raw water cooling. I don't believe there are any purpose built raw water cooled inboard marine engines anymore.


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Old 06-11-2015, 12:12   #12
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Re: KBW21 gearbox - why salt water cooling?

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And when you pull an outboard out of seawater you typically flush it with fresh. But these are also engines designed from the ground up for raw water cooling. I don't believe there are any purpose built raw water cooled inboard marine engines anymore.


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I still have a 17 year old salt water cooled Yanmar. I think you are correct about them all being "fresh water" cooled now; if I were to replace it. Salt water cooled diesels have their thermostats set to a lower temperature than fresh water cooled, to prevent salt deposits. This theoretically increases wear over fresh cooled motors. Also of course the fresh cooled motors should use an antifreeze like, coolant with a corrosion inhibiter. They can be pressurised and able to run at an even higher more satisfactory temperature.

There is still the heat exchanger maintenance to consider but I would rather have that than salt water through my motor. It still goes fine with salt water cooling though.
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