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.

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.

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).

Rsysol said:

My boat is an interesting mix, the engine is salt water cooled ).

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Don't you mean your engine HX is salt water cooled?

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.

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.

David

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?

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.

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?

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.

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?

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.

Xsbank said:

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?

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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?

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.

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?

Xsbank said:

The Twin Disc trans I looked at specifies a working oil pressure measured at 180 degrees F.

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Okay. Our transmissions are BW Velvet drives.

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.

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.

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?

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.

Marin,
Mike at harbor Marine is the expert in the BW department.
Call him.

I always thought cooling tx fluids w engine cooland was stupid as they run the same temperature. How much heat are you going to exchange? Noticed that years ago on cars cooling tx oil w engine coolant in the radiator. Seemed stupid to me then too but there must be a reason for it. David?

My salt water cooled volvo MS17D engine and MS-2 gearbox recently were recently replaced after I found water in the engine oil. I didn't even bother chasing down the source of the problem. It gave 33 years of good service, but I think one can not expect these components to last much longer than this.

I considered keel cooling when I first bought the boat, but I felt since the engine already had 30 years of contact with salt water, it would have been a big investment when the damage had already been done.

The Volvo MD17 was one of the few engines that were purpose built marine engines with salt water cooling in marine. They were heavy walled with plenty of corrosion allowance. I doubt if most modern day engines would last 33 years with salt water flowing through them.

Cooler is not necessarily better. You want the lube oil temp in a certain range, and usually the tighter the range, the better. The advantage of cooling gear or engine oil with coolant is regardless what the load or sea temp is, the fluid stays in a fairly narrow temp band. Cooling with sea water means at low load and cold sea, the lube temp gets very low.

Gears do not make much heat, so even a modest cooler with 180F coolant will keep it happy. And I don't think that 190F number is a hard limit. The BW uses innards basically the same as an automatic tranny that can easily run at 250F.

One BW behind a Detroit 453 has been shrimping and oystering here for over 45years. It runs about 200F as the keel cooler is a bit undersized. As far as any of us can tell, the gear has never had any work done on it. No hour meter.

My Perkins Sabres have coolant cooled oil and raw water cooled transmission fluid. If there would be a problem with too cool a transmission that would be a first in my memory. But, maybe someone has a story to tell.

So I'm in Marin's corner, we both boat in cold water and a 115F transmission is normal for me at a 30% engine load. Still plenty hot enough for ATF to be flowing easily. BTW, stock coolers on my transmissions are designed for warm Middle East water to keep the transmissions below 160F or so at full load.

Borg Warner manual; Page 16; Series 70, 71, 72, 73 transmissions:

"Better efficiency and extended gear life will result when the transmission sump temperature is maintained between 140F and 190F or 60C and 88C. Transmission pressures are dependent upon cooler flow. It is important to select a cooler which has suitable flow characteristics as well as proper cooling capacity. Cooler back pressure affects line and cooler pressure. low cooler pressure after an extended period of hard running indicates the need for a cooler which has more cooling capacity and possibly more back pressure. High cooler pressure after an extended period of hard running indicates the need for a cooler which has less back pressure."


When in doubt follow the manufactures directions!!

I`ve seen Lehman 4cyl 80s that were salt water cooled, no exchanger. Lots of shell came out when the broker started them, he told me it was running hot and needed cleaning out. How would that be done?

It's strictly a matter of economics. It wasn't too long ago that straight salt water was used for engine cooling (directly through the blocks) and reduction gear (transmission) cooling was as cheap as possible. Now with other priorities included it makes more sense (since you have more cents) to install a heat exchanger and/or a keel cooler. Not everyone has the cash to do the 'right thing'. Heck... It wasn't too long ago that stainless was not an option for anchors too. Just depends on the finances.

I have debated that exact subject on a few other forums. What if,,,,,I raw water cooled a Cummins 6b 120 hp engine. How long would it last. Of course I would need a 140 thermostat. The 6B is a parent bore block (AKA a throw away) so no liners to be concerned about. Chevy small blocks were RW cooled and stood the time test very well, if you kept up with exhaust risers. The really interesting thing is that all the guys in the "know" are appauled at the mention of the idea and all say it wont work. And then have absolutely no knowledge, first hand or otherwise, as to why. If a 6B cummins would last 10 years with raw water cooling in salt water that would not be to bad. I can buy a rebuilt long block for about $2500. It would be VERY easy to cool a 6BTA, even at 370 hp, with raw water. In higher hp the problem has always been cooling related on the 6B series. The 6BTA is capable of 450 hp sustained but its cooling system is not even close to that. Raw water cooling is. Someday I'm gonna do it. I have a pair of 6Bs and a 40 foot hull that I would love to turn into a workboat/fishing boat type. I need more time !!!

One of our good friends still has and uses the boat his father bought new over 50 years go. It is a 28' Richardson. Two Graymarine gas engines, raw (salt) water cooled. Both engines are still running great.

BTW, the BW Velvet Drive manual that came with our 1973 Grand Banks sets a hard limit of 190 degrees maximum operating temperature (anything over may cause damage and the ATF must be immediately replaced, according to the operating instructions) and gives no temperature envelope or minimum operating temperature.

So like the FL120 itself, the do-this, don't-do-that instructions in Velvet Drive manuals seem to vary with whoever was writing the manual on any given day.

Since we have all the original operating, service, shop and parts manuals that were delivered with the boat in 1973, we have elected to follow the manufacturers instructions or specifications as called out in those manuals.

I've never had an engine or transmission specialist tell me I will damage an engine or a transmission by following the manufacturer's operating and service instructions for that engine or transmission, so we figure if we do that we'll get the maximum service life out of the thing that it was intended to have.

So far, that seems to be the way it's panned out be it vehicles, boats, or planes.

You can't run salt water in a diesel. You must use coolant (in good condition). The cylinder walls expand when the piston sweeps the cylinder and contract once it has gone by. If the coolant is old or salt water, there is cavitation against the cylinder, causing overheating. Scored cylinders, broken rings etc.

Anybody ever shot the temperature of the salt water as it leaves the coolant daisy train and is injected into the exhaust?