Engine cooling pros & cons | Trawler Forum
Jun. 16, 2025
Engine cooling pros & cons | Trawler Forum
Hello fellow TFers,
Reading another post about keel cooling I was wondering what are the pros and cons about keel cooling and raw water cooling.
I am not a pro so information for my education will be welcomed.
At first sight I can see for keel cooling:
Pro: no raw (salty) water ingress so less corrosion
Cons: hot exhaust going through the whole boat. Noisier?
And for raw water cooling the exact opposite.
Anyone to jump in and elaborate a bit more on the subject?
L Well, conventional raw water circulating through a heat exchanger both cools the engine's coolant system as well as cools the hot exhaust gasses so they can be sent through a rubber hose out the transom.
Keel cooling uses the engine's coolant pump (usually) to pump coolant through an external heat exchanger, the keel cooler and then through the engine. But it doesn't do anything for the exhaust gasses. Most boats with keel coolers use a dry stack exhaust where the hot exhaust exits up though a chase with an air gap and out the top of the boat. These dry stack exhausts use a lot of interior room and can result in soot particles dropping back on the boat. Another disadvantage is the drag of the external cooler so it is only used on displacement hulls.
The major advantage is simplicity- no raw water pump and no circulating raw water to corrode exchangers, ruin impellers, etc. It is only really used on commercial boats.
Another way of dealing with the exhaust cooling on a keel cooled boat is to use a raw water pump to pump water through an exhaust mixer just like a raw water cooled engine. This scheme is sometimes combined with a hydraulic oil cooler or I suppose the raw water could be used for a lube oil and transmission cooler.
Here is a link to Nordhavn's rational for keel cooling with dry stack exhaust:http://www.nordhavn.com/models/fundmentals/exhaust/
David im going to go with a dry stack out the funnel, and the raw water that is used to cool the heat exchanger will be routed through a water jacketed exhaust manifold.......
i have thought about this a lot and figure the headache of building such manifold/expense of buying will far out weigh the possible scorch from an non cooled manifold (they can take you down to raw/cooked meat in a nano second under the right conditions)
not to mention the idea of engine room temps with non cooled exhaust......
im sure there are numerous "dry" methods of insulating the exhaust manifold, but i just cant see any of them coming remotely close to a water cooled exhaust..... but once past the manifold itself the cooling water will travel it's route, and the "dry stack" will be wrapped from that point up They are very common here in the "dirty water" fisheries where it's shallow and the streams dumping into the saltwater are muddy and gritty from glacier runoff or extreme tidal flows. Most of the gill net drift boats are keel cooled, most of the bow pickers are running inboard jets and I think they cool through the water flow through the jets. They fish really shallow water and really dirty water chasing the salmon into river deltas and through sand bar labyrinths at the mouths of the rivers. I've used some commercial boats w/keel coolers and dry stacks. They still had a water cooled exhaust manifold that was the last stop before the cooler. It does save having raw water pumps. One problem, when idling at the dock in warm water, the keel cooler eventually does a poor job of cooling is there isn't a flow, like a river or tide. It builds a pocket of warmer water around the cooler.
HD diesels I have run used coolant water, not raw for the aftercooler.
Steel commercial boats often use a an I beam as a keel, with baffles and boxed in. With rust inhibitor in the coolant they last the life of the boat. Obviously they have a self interested point of view, but these folks know all there is to know about keel coolers and other methods. https://www.fernstrum.com/
I personally wouldn't go to the bother of retrofitting a boat to use a keel cooler, but if a slow boat certainly would see it as a general positive in a boat I was considering for purchase. We see them here in the southeast on real trawlers, a lot, for the same reasons the guys in Alaska do.
Thank you guys for your comments.
Indeed I hardly see how to fit a keel cooling and dry exhaust system in a boat not plan for it as it would require severe modifications.
At the same time, limiting usage of salt water going through cooler and manifold looks like a good idea to me speaking of corrosion.
I am cruising in fresh water so this is less of a concern for me at this time but this is something I think I would look into when searching for a boat to cruise in salt water.
However I do not see this very common in recreational trawler.
Is fouling a concern on the keel heat exchanger?
Keep the comments coming, it is a very interesting subject.
L
Some ships have a raw water system with heating, back wach system or two separate raw water lines so that it can be serviced when another is used if there is no keel freezing.
Litle video Finland ice cruising(On ships also, ice gain propellers and shaft rated to withstand the propeller hack for ice cubes.)
https://youtu.be/2nbDIDG30TY
NBs Modern materials make the exhaust a simple job.
By running the exhaust pipe thru a quality SS fire place chimney pipe the diameter can be held down to about 10 inches.
Although this choice makes a drying locker for foul weather gear harder to build in.
The use of a "stack" outside can easily hide a "hospital critical" muffler that reduces the exhaust noise to auto levels.
On metal boats the keel cooling can be flush , all inside the hull for no drag.
Both PM and the need for repairs suck for cruisers .
Dry stack and keel cooling get rid of lots of items that are a PIA ,
seacocks , water inlets and filters, sea water pumps ,exhaust mixers or lift mufflers,, plus the exhaust in a following breeze is less likely to kill.
DSKK , gets my vote after living with both style systems. I would not jacket a dry exhaust pipe with sea water. It will be too cold and cause the moisture in the exhaust gas to condense and drain back into the engine. This condensation could also lead to corrosion in the exhaust pipe, and if it developed leak the cooling water would end up in the engine. It is not that big a deal to have the exhaust insulated to a level that the outside temp is no problem. Libra has a keel cooler and dry stack. The stack chase is on the end of a cabinet that separates the helm from the settee upstairs that you hardly notice and the coolers are inside these 10mm steel bilge keels that seem to take the straps just fine. With this 60 ton beast I doubt the drag from these keels is a material factor and I expect they dampen roll a little.
Reading another post about keel cooling I was wondering what are the pros and cons about keel cooling and raw water cooling.
I am not a pro so information for my education will be welcomed.
At first sight I can see for keel cooling:
Pro: no raw (salty) water ingress so less corrosion
Cons: hot exhaust going through the whole boat. Noisier?
And for raw water cooling the exact opposite.
Anyone to jump in and elaborate a bit more on the subject?
L Well, conventional raw water circulating through a heat exchanger both cools the engine's coolant system as well as cools the hot exhaust gasses so they can be sent through a rubber hose out the transom.
Keel cooling uses the engine's coolant pump (usually) to pump coolant through an external heat exchanger, the keel cooler and then through the engine. But it doesn't do anything for the exhaust gasses. Most boats with keel coolers use a dry stack exhaust where the hot exhaust exits up though a chase with an air gap and out the top of the boat. These dry stack exhausts use a lot of interior room and can result in soot particles dropping back on the boat. Another disadvantage is the drag of the external cooler so it is only used on displacement hulls.
The major advantage is simplicity- no raw water pump and no circulating raw water to corrode exchangers, ruin impellers, etc. It is only really used on commercial boats.
Another way of dealing with the exhaust cooling on a keel cooled boat is to use a raw water pump to pump water through an exhaust mixer just like a raw water cooled engine. This scheme is sometimes combined with a hydraulic oil cooler or I suppose the raw water could be used for a lube oil and transmission cooler.
Here is a link to Nordhavn's rational for keel cooling with dry stack exhaust:http://www.nordhavn.com/models/fundmentals/exhaust/
David im going to go with a dry stack out the funnel, and the raw water that is used to cool the heat exchanger will be routed through a water jacketed exhaust manifold.......
i have thought about this a lot and figure the headache of building such manifold/expense of buying will far out weigh the possible scorch from an non cooled manifold (they can take you down to raw/cooked meat in a nano second under the right conditions)
not to mention the idea of engine room temps with non cooled exhaust......
im sure there are numerous "dry" methods of insulating the exhaust manifold, but i just cant see any of them coming remotely close to a water cooled exhaust..... but once past the manifold itself the cooling water will travel it's route, and the "dry stack" will be wrapped from that point up They are very common here in the "dirty water" fisheries where it's shallow and the streams dumping into the saltwater are muddy and gritty from glacier runoff or extreme tidal flows. Most of the gill net drift boats are keel cooled, most of the bow pickers are running inboard jets and I think they cool through the water flow through the jets. They fish really shallow water and really dirty water chasing the salmon into river deltas and through sand bar labyrinths at the mouths of the rivers. I've used some commercial boats w/keel coolers and dry stacks. They still had a water cooled exhaust manifold that was the last stop before the cooler. It does save having raw water pumps. One problem, when idling at the dock in warm water, the keel cooler eventually does a poor job of cooling is there isn't a flow, like a river or tide. It builds a pocket of warmer water around the cooler.
HD diesels I have run used coolant water, not raw for the aftercooler.
Steel commercial boats often use a an I beam as a keel, with baffles and boxed in. With rust inhibitor in the coolant they last the life of the boat. Obviously they have a self interested point of view, but these folks know all there is to know about keel coolers and other methods. https://www.fernstrum.com/
I personally wouldn't go to the bother of retrofitting a boat to use a keel cooler, but if a slow boat certainly would see it as a general positive in a boat I was considering for purchase. We see them here in the southeast on real trawlers, a lot, for the same reasons the guys in Alaska do.
Thank you guys for your comments.
Indeed I hardly see how to fit a keel cooling and dry exhaust system in a boat not plan for it as it would require severe modifications.
At the same time, limiting usage of salt water going through cooler and manifold looks like a good idea to me speaking of corrosion.
I am cruising in fresh water so this is less of a concern for me at this time but this is something I think I would look into when searching for a boat to cruise in salt water.
However I do not see this very common in recreational trawler.
Is fouling a concern on the keel heat exchanger?
Keep the comments coming, it is a very interesting subject.
L
Hello fellow TFers,
Reading another post about keel cooling I was wondering what are the pros and cons about keel cooling and raw water cooling.
I am not a pro so information for my education will be welcomed.
At first sight I can see for keel cooling:
Pro: no raw (salty) water ingress so less corrosion
Cons: hot exhaust going through the whole boat. Noisier?
And for raw water cooling the exact opposite.
Anyone to jump in and elaborate a bit more on the subject?
L
Some ships have a raw water system with heating, back wach system or two separate raw water lines so that it can be serviced when another is used if there is no keel freezing.
Litle video Finland ice cruising(On ships also, ice gain propellers and shaft rated to withstand the propeller hack for ice cubes.)
https://youtu.be/2nbDIDG30TY
NBs Modern materials make the exhaust a simple job.
By running the exhaust pipe thru a quality SS fire place chimney pipe the diameter can be held down to about 10 inches.
Although this choice makes a drying locker for foul weather gear harder to build in.
The use of a "stack" outside can easily hide a "hospital critical" muffler that reduces the exhaust noise to auto levels.
On metal boats the keel cooling can be flush , all inside the hull for no drag.
Both PM and the need for repairs suck for cruisers .
Dry stack and keel cooling get rid of lots of items that are a PIA ,
seacocks , water inlets and filters, sea water pumps ,exhaust mixers or lift mufflers,, plus the exhaust in a following breeze is less likely to kill.
DSKK , gets my vote after living with both style systems. I would not jacket a dry exhaust pipe with sea water. It will be too cold and cause the moisture in the exhaust gas to condense and drain back into the engine. This condensation could also lead to corrosion in the exhaust pipe, and if it developed leak the cooling water would end up in the engine. It is not that big a deal to have the exhaust insulated to a level that the outside temp is no problem. Libra has a keel cooler and dry stack. The stack chase is on the end of a cabinet that separates the helm from the settee upstairs that you hardly notice and the coolers are inside these 10mm steel bilge keels that seem to take the straps just fine. With this 60 ton beast I doubt the drag from these keels is a material factor and I expect they dampen roll a little.