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Old 03-11-2015, 03:07 PM   #1
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Why Salt-water Cooling?

Or raw water, if you are in a lake?

All you salt-water cooled guys, why not replumb the coolers to run engine coolant through them? Salt water is colder but any boat that is keel-cooled runs its coolant through the coolers. My 6CTA is keel-cooled, it runs coolant through both coolers. You would have far less maintenance, the oils would never get hotter than the coolant.
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Old 03-11-2015, 03:18 PM   #2
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Keel cooled is good but like with everything else it has its pros and cons. Some boats are better suited than others for KC. The same sort of issues apply to dry stacking with pros and cons. For the engine dry stack keel cooled is great.
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Old 03-11-2015, 03:46 PM   #3
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My boat is an interesting mix, the engine is salt water cooled but the hydraulic pump and transmission are keel cooled. I'm the third owner of this custom boat so I really don't know the thinking that went into doing both systems.

I wonder how I can tell if my keel cooler would still be efficient if I added the engine to the mix. Is is determined by the capacity of the keel cooler? I'm pretty ignorant on this aspect of my boat (and other areas I'm sure).
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Old 03-11-2015, 03:48 PM   #4
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My boat is an interesting mix, the engine is salt water cooled ).
Don't you mean your engine HX is salt water cooled?
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Old 03-11-2015, 04:01 PM   #5
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I think the issue is of heat dissipation. If I re-plumb a raw water system into a tank and a pump and run coolant through the engine, then how am I dissipating the heat in the coolant before it recycles.

Cars and piston engine airplanes rely on airflow for the heat exchange which is not feasible on a boat. Especially if your end goal is to simplify the system.
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Old 03-11-2015, 04:04 PM   #6
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You could plumb your engine oil cooler and the transmission cooler to run on engine coolant. But if you have a sea water cooled after cooler, it will allow the inlet air to get much hotter if you plumb it for coolant.

Sea water is about 60- 80 deg F. Coolant coming out of the heat exchanger is maybe 160 deg F. That 80-100 deg difference will cause your inlet air to be 80-100 degrees hotter which will not be good for EGT and can cause aluminum pickup on the cylinder walls at high power loading. That is why Cummins used a sea water aftercooler on most of their high output engines.

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Old 03-11-2015, 04:05 PM   #7
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I'm not talking about the main heat exchanger, I'm talking about the two oil coolers. You still need salt water to cool your engine but you don't need salt water to cool the oils. Think how long your coolers will last without salt?
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Old 03-11-2015, 04:16 PM   #8
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The after-cooler is necessary because you can't squirt raw fuel in the cylinder like you can with gasoline. A turbocharged/supercharged aircraft engine (for example) uses the last 5+ inches of manifold vacuum to squirt raw gas in the cylinder to keep it cool and prevent detonation. That's why reduced take-off power is never approved on a supercharged radial (like Marin's Beaver). Diesel, however, would smoke like crazy with this system so some other way of dropping the temperature of the air charge, after-cooling, is used for high power settings on diesels. My engine doesn't have an after cooler as my version is rated at 250 HP yet the after-cooled ones run about 425+ hp.
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Old 03-11-2015, 04:16 PM   #9
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ok I think I understand what you're suggesting:

For example, my Cummins runs coolant through the engine, why not also through the oil coolers, and just leave the raw water running through the heat exchanger?

Is that what you're saying?
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Old 03-11-2015, 04:21 PM   #10
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Detroit diesel often cooled the gear oil with coolant. Most diesels above about 200hp also cool lube oil with coolant.

You could adapt gear cooler to engine coolant, should not be too hard. Just need some creative plumbing.
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Old 03-11-2015, 04:25 PM   #11
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No Mast, exactly!

There's another thread here where a Cummins engine transmission cooler failed and there is a milkshake in the trans. Why use salt water with its accelerated corrosion rate for oil coolers? I don't have my Cummins manual here but I'll bet if the salt water that goes through the coolers has already been through the heat exchanger and the after cooler I'll bet it's close to coolant temperature anyway?
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Old 03-11-2015, 05:03 PM   #12
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American Diesel told me they sell mods so the engine oil cooler could be coolant plumbed but strongly recommended against doing it to the gear cooler.

Maybe not enough cooling exchange, he didn't go into it...but that is what I thought he was inferring.
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Old 03-11-2015, 05:16 PM   #13
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N... I don't have my Cummins manual here but I'll bet if the salt water that goes through the coolers has already been through the heat exchanger and the after cooler I'll bet it's close to coolant temperature anyway?
Isn't that a good thing, having the temperature up high which drives off the moisture (>165 degrees)?

On our BG Velvet Drive, the transmission capacity is ~2.5 quarts. I'm not sure how much temperature difference it takes to drop the temperature of the fluid to be in a safe operating zone (<200 degrees).

Ski?
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Old 03-11-2015, 06:09 PM   #14
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Our salt water here is 50-something degrees. Our coolant temperature is 180 degrees. The cooler a transmission runs the happier it tends to be. At cruise rpm I can put my hand on one of our transmissions and keep it there very comfortably, even moreso after we switched our raw water pumps to higher flow rate Johnsons. The engines' coolant header tanks, on the other hand, are in the neighborhood of 180 degrees. If that was what was running through the transmission coolers, the transmissions would running very hot indeed, I thnk.
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Old 03-11-2015, 08:04 PM   #15
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The Twin Disc trans I looked at specifies a working oil pressure measured at 180 degrees F.

The Velvet Drive is 140 - 190 degrees F at about 40 psi.

That would be right in the ballpark for coolant cooling. I might suggest that your transmissions are over-cooled, Marin. I don't have a Velvet Drive manual but I would check the suggested operating temperature and see if yours are too cold?
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Old 03-11-2015, 08:06 PM   #16
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The Twin Disc trans I looked at specifies a working oil pressure measured at 180 degrees F.
Okay. Our transmissions are BW Velvet drives.
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Old 03-11-2015, 08:20 PM   #17
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BW gears were used on some smaller detroits like 453, 653 and used engine coolant to cool the gear. Those gears would easily run 190F on the case, but seemed completely happy there. I like gears to be coolant cooled as the higher temp tends to keep moisture out of the oil. And of course coolant does not eat up coolers like salt water.

High temp does kill trannies, but 190-200F is nowhere near hot enough to cause trouble.

And you Lehman guys, if you could set up an engine oil cooler to run off coolant, I think that would be a big plus. Lehman, Perkins, Volvo, Yanmar with their salt water engine oil coolers always baffled me. Cold oil at low load is not good. DD, Cat and Cummins all use engine coolant to cool engine oil.
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Old 03-11-2015, 08:50 PM   #18
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While you may have a point about oil temperatures, given the fact that Lehman 120s are typcially considered a 12,000 to 14,000 hour engine in recreational boat service assuming proper operation, service and maintenance, and some have gone in excess of 25,000 hours in commercial service (Washintgon State Ferry system) before needing an overhaul I'm guessing that the way they're set up with raw water oil cooling works just fine.
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Old 03-11-2015, 09:34 PM   #19
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I'm sure they work fine, the ferries only shut off for a couple of hours if they ever stop. Besides, I doubt if a ferry is using Velvet Drives. Our coolers are not used for long periods of time and if they fail, catastrophic to the gears/engine. They will never fail with coolant.

I think yours are over cooled, Marin. 140 - 190 is the working range for a Velvet drive. Check your manual?
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Old 03-11-2015, 10:07 PM   #20
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The manual says the absolute max operating temp for a Velvet drive is 190 degrees. Anything over that and the fluid must be changed immediately. No mention of a minimum temp.

We go by the advice of several very experienced marine gear folks we've come to know over the years who've told us that while you don't want to run a transmission cold, a cooler transmission, at least of this type, is a whole lot happier than a hotter transmission.

Nothing to say they won't fail tomorrow, but we've had the boat 17 years now and the transmissions shift as smoothly and silently and run as quietly (as much as can be determined over the clatter of the engines) as they did the day we sea trialed the boat when we flew down to California to inspect and survey it. We're not ones to mess around with success.

Years ago Bob Smith told me that one of the benefits of changing our engines to the larger Johnson raw water pump (as opposed to the one normally installed on an FL120) is that the transmissions would run cooler. So he seems to think along the same lines as the marine gear folks we've talked to on the subject.
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