Marine Cooling Systems and Heat Exchangers

Marine engines, as well as automotive engines are, cooled by circulating water thru the engine block. Marine engines are unique in that there are two different types of cooling systems. The standard raw water system, and the fresh water (commonly known as the closed) cooling system.

Raw Water Cooling Systems

Raw water cooling systems draw water from outside the boat (seawater or lake water). Water is pumped from the source to the engine block then the engine circulation pump forces the raw water thru the engine block and the water is expelled thru the exhaust. Raw water cooling systems are relatively simple and the standard cooling system on most Marine Engines. The raw water pump in most cases is inside the outdrive. On larger engines and inboard engines the raw water pump is located inside the boat and is driven by a v-belt or directly off of the crankshaft. There are hidden dangers that can accumulate over time causing you to spend big Dollars on repairs. The danger is using salt water as a coolant in your engine. Salt water can be highly corrosive. Running salt water through your engine block and exhaust manifolds will lead to destructive corrosion that is unseen until your engine or exhaust manifolds fail.
Generally speaking, marine engines cooled with raw water, especially ones that use salt water, have a shorter life span than marine engines cooled with a closed cooling system.

Fresh Water Systems with Heat Exchangers and Keel Cooled systems

Fresh water cooling systems, also known as a closed cooling systems, come in several varieties. The most common type utilizes a Heat Exchanger which functions similarly to the radiator in your car. Coolant (antifreeze) is circulated through one side of the heat exchanger where it is cooled by raw water that passes through the other side of the heat exchanger. The engine coolant is then circulated back into the engine. The raw water is expelled out of the boat thru the exhaust. Another common type of closed cooling systems is known as a Keel Cooler. This is done by eliminating the use of a heat exchanger. Instead of pumping raw water into the vessel’s heat exchanger where it cools the coolant, the coolant is pumped through pipes or aluminum extrusions on the outside of the hull where the surrounding water (lake or ocean water) cools the coolant before it is pumped back into the engine. The use of keel coolers removes the need for a heat exchanger, raw water pump and the other components necessary for pumping raw water into the heat exchanger to.

Fresh Water Cooling System

Fresh Water Cooling System

Fresh Water Cooling Systems using a Keel Cooler

Fresh Water Cooling Systems using a Keel Cooler

Closed cooling systems are more complex than raw water cooling systems, but have distinct advantages:

  1. Minimal internal engine corrosion.
  2. More effective at cooling the engine and allows you to run your engine at higher temperatures, resulting in better engine performance and fuel economy.

How A Heat Exchanger Functions

A Heat Exchanger transfers, or “exchanges,” heat from your boat engine’s coolant to raw water pumped from the water outside of your boat. The raw water is pumped through a bundle of small tubes in a chamber filled with the hot engine coolant. The tubes are cooled by the colder raw water the allowing the tubes to absorb the heat of the engine coolant.

To function correctly, a heat exchanger must be carefully matched to your boat’s engine. Go2marine carries over 100 different models of heat exchangers from San Juan Engineering , Seakamp Engineering and Sen-Dure . If you aren’t sure which heat exchanger is right for your boat, you can fill out Go2marine’s Heat Exchanger Request form and they will send you a quote for the replacement heat exchanger.

Expansion Tanks

Expansion tanks are an often overlooked but very important part of a closed cooling system. As the engine coolant gets hot it expands, increasing in volume. The expansion tank is a small tank that simply provides room for this increase in volume. In some systems, the heat exchanger is a piggy back style. This is when the expansion tank is built on top of the Heat Exchanger. On some Heat Exchanger systems, the expansion tank is a separate tank remotely mounted. Most heat Exchangers have a fitting built into the tank to install a Zink Anode. This sacrificial anode protects the Heat Exchanger form galvanic corrosion commonly known as Electrolysis. The Zinc should, be checked once a month to insure protection of the Heat Exchange and prevent pre-mature failures. Another important component to the Heat Exchanger is the cap on expansion tank similar to the cap on your cars radiator. It is an important component in, maintaining your cooling system’s pressure. It should, be checked regularly for leakage and corrosion. The boat engine parts specialists at Go2marine recommend that you replace the cap on your expansion tank every two years.

Article by: Michael Weller

85 thoughts on “Marine Cooling Systems and Heat Exchangers

  1. expansion tank located above my heat exchanger for the last 2 weeks was continuing to over flow with hot water with no temp rise in suspision was a leak in heat exchanger taking in raw water and circulating around engine thus never running out of water.
    took heat exchanger in to be cleaned and fixed and i was told the hoses were backwards,so now i am not sure what hose goes where.
    as it is permenantly sealed now i cant see what inlet is supposed to take fresh water and what one for raw water.
    from what i am reading salt water runs through the tiney tubes and back out again into ocean,and i assume somehow fresh water just fills the rest of the heat exchanger?
    i am a little confused as i always thought raw fresh water would of gone through tiny tubes and raw water surronnding tubes to cool water inside tubes?my heat exchanger has 2 inlets on the far ends of the device and another 2 outlets closer into the middle of heat exchanger.
    how can i tell which inlets are for what

  2. Thank you, Thank you, Thank you!

    I have been on The Internet for a week, trying to find a schematic diagram of the heat exchanger’s hose hook ups. (I removed my HX to replace engine mounts; unfortunately, all the numbers I marked the hoses with were erased by bilge water)! Don Casey, Nigel Calder, and EVERY OTHER website I searched DO NOT ADRESS this problem.

    I have added this site to my Favorites for future reference. One addition I would like to see is the inclusion of a Hot Water Heater Circuit. I assume one of this tank’s hoses connects to the thermostat’s nipple and the other to the Heat Exchanger’s Resevoir nipple. do you think this is correct?

  3. I have an 1995 moomba with a 350 and would like to run it in salt water what is it gonna cost to put in a cooling system in?

  4. Do we can use normal radiator for cooling marine engines
    Same question by other way: it’s possible go come normal radiator
    With marine engines from factory

    1. The challenge with using an automotive style radiator with a marine system is airflow. Car radiators are right up front and being pushed through the wind at some speed – they have plenty of ‘clean air’ flowing right through them.

      Boats do not usually use a radiator because of the limitations of proper ‘clean’ airflow. However, boats have plenty of cool water all around them – that is why marine motor applications use heat exchangers. Think of a heat exchanger as a water radiator.

      No current marine engine manufacturer makes a production marine engine with an air-cooled radiator.

  5. I have VT555 cummins, the old expansion tank cap had A #5 marked on the cover. I have new ones and they said there is nothing with #5 marked it would be a #4 or a #7 so i got number 7 will this be ok. Will the pressure be better to have #4 or #7

  6. I’ve replaced my original San Juan raw water filter (round bronze with a clear plastic cover) but would like to purchase a repair kit I understand is available for that unit. Supposedly it contains the gaskets and a new stud,

    If it is available, I would repair the San Juan and keep it aboard as a spare.

    My boat is a 1978 34′ CHB Tricab with a 120 hp Ford Lehman diesel.


  7. We have a 2001 Monterey Cabin Cruiser with an Ocean Cool System (aka river ac). The green primer switch for our river ac panel is broken. Now we are not getting power to fan and the primer is not starting the pump. How do we order a new panel with buttons/switches to try to repair? Is there someone we can cal to discuss?

    1. You may call us here at Go2marine 800-998-9508. If it a question about getting a substitute switch; we will have something from Blue Sea Systems or Paneltronics that will work for you.

  8. im a Automotive Mechanic and i just want to follow up regarding Muhammad inquiry about automotive radiator modified to marine application last April 12/11 …is it possible to modified auto radiator complete with fan ,4rows cooling fins, temp sensor switch and sufficient opening for air in front of the motor cover…im so interested in this idea because of the hussle of cleaning everytime i dock my boat whereas if it is converted and being cooled by anti freeze or automotive coolant my engine most pro’bly will last longer and dont need to flushed and clean the dirty water, mud sucked from outside most of the time…less hussle and time. …and mostly i am really considering the cost of heat exchangers versus fan/aircooled auto radiator.
    i’ve read your reply to Muhammad and it is a fact and greatly noted but i do hope that you can give me a hint ( YES or NO ) if it is really really possible then i will go for it.
    thanks in a million!!

    1. Once again, getting enough air moving through a radiator is really the problem – and that problem becomes significant with larger engines. Take a look at Airboats to understand how much room you need and the type of engine required to be cooled by air or air through a radiator. You could get away with an air cooled engine, if it were under 20HP. You might get away with a radiator /air cooled engine if it were under 50HP, with LOTS of air movement, maybe. You would truly be challanged to use a radiator to cool a V8 engine in a boat – it is not a question of radiator size or how many rows of cooling fins, it is a question of air movement past the radiator

  9. My boat is powered by a Perkins P6 diesel motor. It is not fitted with a thermostat, It eventually runs at 60 deg. centregrade though the keel cooling system.

    Can a extenal thermostat be fitted.
    Would it be better to manually control the temperature with a three way valve.
    What temp. should this very old motor run at.

    1. First – Follow the rule of thumb – if it is not broken, don’t fix it. It sounds like the engine has been run like this for years. I would not fit a valve, unless you really think that you can monitor it every minute. Lastly, fitting with an external thermostat and system would be complex – there is no ‘simple kit’ for this.
      About the temperature – 60C / 140F is not out of order for a slower, older motor. You have one of the best historical engines ever built – don’t mess with it.
      For those who want to hear one of those old Perkins P6 Diesel running.

  10. Well I have to say I am new to the inboard outboard boat systems or should I say Stearn Drive. I bought a boat motor and trailer for 500. bucks and after doing research on the Merc 170 engine I was amazed of how much problems these engines have and the price of parts for this thing is just crazy. I am also interested in giving this boat a transplant to a Chevy small block and I am going to radiator cool it. I be leave I can do this without having an issue with cooling. I would have to say that you will need to use a good 4 core radiator, a high rpm electric cooling fan, and you may have to leave the engine cover off or have a grate type cover for the engine. Also a low temp thermostat and a good shroud to direct air flow through the radiator would be needed. As far as the exhaust you may need to run headers over the top as the camshaft in car engine would suck water in the engine and probably hydro lock the cylinders They do make short mufflers that fit on the end of the headers, but will still be very loud. I am going to give this a try. I could do this for as much as the Merc 170 parts would cost just to do a small amount of repair and the thing would still be unreliable. There is nothing good said about the Mercusier 170 type engines that would make me want to put money into it. As far as the weight factor I am sure that this boat will handle it. So far I am not to impressed with Stearn Drives. I love the old Merc 175 outboards and did nothing but fog the engine to store it in the winter and gas her up and go next spring. That thing was problem free and the best boat I ever owned. Its to bad I had to sell it for cash for my driveway,but thats the way life goes.

  11. I am looking for a port side manifold for a i980 Chevrolet 305 cid converted by Mercury marine for a Century Arabian ‘180’. Engine was overheating & found the manifold to be full of rust & scale. I need to find a manifold with full set of gaskets, including exhaust, end seals & riser elbow.Any ideas?

  12. im a comercial fisherman running nanni 85 horse engines,had to replace heat exchanger due to salt corrosion very expensive,as i have a catamaran,i would like to fit keel coolers to the inside of each hull,is this possible with these engines/? also to make it easier should i leave raw water pump to cool exhaust.

    1. The heat exchanger is not really that expensive – your Alternator likely costs as much. Replace it with a Copper Nickle system and it will last longer; 15% increase in cost with 30%+ longer life.

    2. Keel coolers, on the other hand, are quite expensive. Yes, either way, you will need to have raw water cooling your exhaust.

  13. I have a fresh water cooled marine engine which is also fitted with a calorifier. For no apparent reason, the coolant level in the expansion tank sometimes INCREASES! I can only assume that I have a very small leak between the fresh water and sea water parts of the heat exchanger – OR – between the coil and the tank of the calorifier. Are there any other possibilities? Is there an easy way to check for salt water in the fresh water coolant?
    I would appreciate any suggestions or comments.

    1. The quickest way is to do a pressure test on the system and see if the pressure bleeds down. You can purchase (or borrow) a pressure tester to apply pressure through the cap or a hose.

  14. 21/11/11

    Dear Sr/Madam
    I have got a CAT SER NO: MBD07777879 Engine Model C9DI I would like to marinase this engine I am looking for the following itams: -Expansion tank and Heat Exchanger wich could be suitable for this engine.
    Thank you very much for your help
    Best regards

    K. Mwanza
    115 Green Lane Thornton Heath cr7 8bl

  15. Looking at buying a 1993 Thompson 3100 Santa Cruz with closed cooling system. The starboard engine is running hot. Which side of the heat xchanger – raw water or antifreeze – is the best place to start eliminating potential causes. My previous boats did not have closed loop system. Thanks

    1. When you say ‘running hot’, do you mean overall or at a freshwater temperature sender? Likely causes are the Heat Exchanger itself, water pump to heat exchanger, water pump on freshwater side or issues with the manifolds themselves.

  16. I am building a aluminum hull boat for fresh water with a small block Chevy, this boat is designed with a surface / mud drive . I would like to build a closed coolant system using the bottom and side of the boat with 2 heat exchangers / Keel Cool / built inboard enclosing 2.5 h x 14” x 60” L that would equate to 9.09 gals. per side . My idea is that the L shaped cap would be welded to the side of the hull 2.5” h x 14” w on the bottom = 14.58 cu. ft. per side .
    My question is ? with the good heat exchange properties of Aluminum would that be too much cooling or not enough ? What is the proper formula to check my requirements for this application . Thanks Swamp Shark

    1. You need to get in touch with a company that builds keel coolers. My concern would be the ambient water temperature for the area in which the boat will operate. They have all of the tables and can give you the surface area you need for the heat transfer.

  17. What I should have said was this L shaped 2.5in. h x 14in w x 60 in. L / keel cool / would be welded on the inside of the boat using the bottom 14in w x 60in L on each side of the boat .

  18. I have two 350 sbc in my cris craft and am having trouble w/ belts flipping off the raw water pumps– I’m looking for some way to run the jabscos( actually sherwoods) with a direct drive or, an electric(12v) pump– the fresh water system works great, just the exhaust manifold heat is the problem—-thanksbruce

  19. Old inboard Holden grey motor with salt water cooling! Water is being ejected thru the exhaust, but does not seem to be going thru the water jacket! Can I find a diagram/ lay out to try & work the oroblem out?

    Tanks, Ian

    1. Hello Ian, do a Google search for “Holden Grey owner forums.” They can probably connect you with someone that can help you. Good luck!

  20. Hi Guys. My Isuzu 240c Marine engine’s exhaust manifold is overheating but I am getting what appears to be normal flow of COLD water out from the exhaust. I would appreciate any help

    1. Hello Alan, if you are getting plenty of cooling water to the manifold there is a restriction in the manifold itself. When this happens, the water can’t pass thru the manifold fast enough, causing the engine to over heat. You can try boiling out the manifold. If that doesn’t take care of the problem you will need to replace the manifold.


  22. Hi
    I have a Broom speedboat with a mercruiser 470 (3.7ltr) engine.
    I have a 2.1 turbo diesel engine which I hope to fit,my question is what
    to do with the intercooling system and will running the exhaust through a ss
    water jacket cool it enough.
    It will have a heat exchanger but the manifolds are dry also thinking of
    putting a ss jacket on top of the manafolds with raw water running through it
    and thin into the Exhaust on the way out.
    Anyone with a better plan/

    1. You will need a good 3/4″ sea water pump. Run the water into the cooler and out to the exhaust elbow. You will need to wrap the manifold if it is dry. This is not a great way rig the exhaust and would be better off with a wet manifold.

  23. I have an overheating problem at higher speeds with my 1980 SeaRay 270 sedan. Have no issue at Idle speeds. Sealed system (anti freeze) has been cleaned and checked for leaks, also thermostat , impeller, water pump. I am now being told there is an inner water pump that could be the problem and or a bad head gasket. I also had all the hoses and connections replaced. Any ideas or opinions before I have to spend thousands more to correct what MIGHT be the problem. Just had lower outdrive replaced and prop.

  24. 1964 Johnson inboard using a Buick v6 engine. how is the water forced into the engine? Where is the entry port for the water? Is the water coming in from somewhere on the stern drive?If so, where.

  25. I have a 2000 Mastercraft ski boat with a 5.7 liter Vortek engine. I am getting oil in the crankcase oil. Is it possible for the exhaust port that mixes exhaust gases and cooling water to malfunction and cause water to back fill into the combustion chamber?
    I have taken off the intake manifolds to check the gaskets. One seems slighlty worn but doesn’t seem enough to cause the milky oil problem. I am just trying to figure out if there could be another way the water can get into crankcase before putting it all back together.

    1. After/intercoolers work by cooling the air from turbochargers, thus making it denser, which means more air can fit inside the cylinder.

      Turbochargers typically heat the air when they compress it. If an after/intercooler is used, the volume of the cylinder can contain a larger amount of air (therefore oxygen). This produces extra engine power.

      After/intercoolers are essentially a type of radiator tuned for high flow rates.

    1. That depends on whether the boat sits in the water for long periods of time. Typically on a trailer boat you will get eight to ten years. If the boat sits in the water and plugged into shore power it could be three to four years depending on how well you keep up in the zincs.

  26. Hi, How long will a 4.3.Mercruiser w/ alpha1 outdrive last in salt water? Its is used 1 or 2 times a week. It is not a closed cooling system. It will be serviced regularly. Thanks,Ron

    1. If you flush the engine after each use and the boat does not sit in the water for weeks at a time, it will last longer than you will most likely own the boat.

  27. I have a freshwater cooled 350 indmar. It has been heating up to between 180 and 210 degrees. I have replaced the 160 degree thermostat, temp sending unit, temp guage, and the impeller on the raw water pump. When I shoot the engine with a temp gun the starboard side is consistently warmer by 20 or so degrees. The boat did sit in the water for a year and a half only being started once a month and run only a couple of times. And yes I had fuel problems too!

  28. My 5.7L has a closed circut cooling system and after a severe freeze I ran it and now have anti freeze coming out the exaust mixxed with raw water instead of just raw water. Coulkd this be the inter cooler or exaust maifold. The motor runs smooth so I doubt it is a cracked block.

  29. I have a Mericruiser 7.4 with a Bravo 3 outdrive. It has a partial closed cooling system (engine only). What steps will need to be taken to winterize the boat? If I installed a completely closed cooling system to include risers and manifolds, would I still need to winterize the boat?

    1. You need to drain the manifolds and any oil coolers. If you do a full system you still need to drain any oil coolers that have raw water going thru them.

  30. I have a 1976 165 cevy mercruise motor with fresh water cool. Salt water is blocked off at elbow and goes thru hoses. I’m trying to fined out what tempature thermostat should I run in it

  31. I recently purchased a 2007 Bayliner 192 Explorer with 50 hours total time. It has a 3.0 L Mercury marine engine and It has never been in salt water. I will be using it in San Juan Islands so I am needing a heat exchanger.
    What kind of heat exchanger do you recommend and approx. installed cost?
    Can a cabin heater also be installed at same time?

  32. I have recently purchased a 2004 glastron with a volvo 5.7 litre motor. Upon sea trials, the engine overheats every time out. I changed the impeller, and the thermostat. The rad cap looks a little rusty, but the spring seems to work fine. I have checked it with a heat gun, and it corresponds with the gauge and the audio alarm. I’m not sure where to go, or what to do next, any suggestions will be appreciated. Thanks in advance, John B

  33. Have a 2001 formula boat with a mercury 454 motor running in salt water, first time owning a boat like this and some what of a rookie. Temp gauge is running about 180 to 210 without running full throttle, is this a concern and does the cooling system use the salt water to cool the engine or is it a closed system like a car? Please help just purchased this boat.

  34. I have a 454 7.4 litre engine L29 with 6 inch risers, I purchased a closed cooling from San Juan engr. But the instructions don’t tell how to install plumbing to 6″ blocks.
    Please help.

Leave a Reply

Your email address will not be published.

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <s> <strike> <strong>