• How Boat Cooling Systems Work!

    Just like your car a boat's engine produces heat. On your car this heat is removed by the airflow circulating around the engine and the radiator. On a boat the principle is very similar with a few minor differences.

    There are two general types of cooling systems found on most
    boats: open cooling or closed cooling systems. Both systems use the water that the boat is sitting in for cooling purposes, the difference lies in where that water flows and what components are cooled.

    Both Systems

    On both systems the water flow is usually the same up to the oil cooler with some minor differences between inboard boats versus inboard/outboard boats.

    The water is normally taken from the lake through a thru hull using a raw water pump on inboard boats.

    Attachment 1055

    Attachment 1056

    From the raw water pump the water goes to the oil cooler to help remove heat from the engine oil.


    On inboard/outboard boats the water is pumped in through ports on the sterndrive using a pump located in the lower unit.

    Attachment 1057
    From the sterndrive water pump the water may pass through a transmission cooler on its way to the oil cooler.

    Attachment 1058
    On most engines when the water leaves the oil cooler is where the differences begin.

    Open Cooling System (Seawater or Raw Water Cooled)

    If the engine is cold, the thermostat assembly routes the water to the exhaust risers where it is passed overboard through the exhaust. The water contained in the engine remains in essentially a closed circuit moving from the engine cooling passages to the thermostat assembly to the engine circulating pump then back to the engine cooling passages. This closed circulation allows for a quick warmup.

    As the temperature of the water passing through the thermostat assembly increases, the thermostat begins to open allowing the hot water to be sent to the exhaust manifolds where it is discharged overboard. The thermostat assembly also allows the incoming cold water to mix with the hot engine discharge water, this cooler water is now routed to the engine circulating pump then to the engines cooling passages.

    This system uses the thermostat to maintain the engine within operating parameters. As the water temperature fluctuates the amount of cold water added to the engines circulating system is controlled by adjusting the discharge path of the water. If the engine is too hot, water is allowed to discharge via the exhaust manifolds and more cold water is introduced into the engines circulating system. If the temperature is too cold, the cold water bypasses the engines circulating system and is discharged overboard through the exhaust risers. Click the Thermostat Closed button to see the differences in waterflow.

    Open Cooling

    In the example above notice with the thermostat closed how the orange arrows show the closed circuit flow of the water from the engine through the thermostat assembly to the engine circulating pump and back to the engine. The cold water from the water inlet (blue arrows) flows to the thermostat assembly and is routed to the exhaust risers where it is discharged overboard.

    Select the thermostat closed button to open the thermostat. Now notice how the engine flow (orange arrows) passes through the thermostat assembly where cool water (blue arrow) is mixed with it and sent to the engine circulating pump then to the engine. Since the thermostat is open it directs some of the hot water (red arrows) to the exhaust manifolds where it is discharged overboard.

    Closed Cooling System (Freshwater Cooled)

    The closed cooling system is composed of two separate subsystems. A closed cooling system (freshwater system) and seawater system. The system operates similar to the cooling systems on automobiles. However, instead of a radiator boats utilize a heat exchanger.

    On a car the engines hot water is pumped through the fins of the radiator while cool air passes over the fins absorbing the heat and cooling the water. On a boat the cool water (seawater) is pumped through a flowtube inside the heat exchanger removing heat while the engines hot water (freshwater) circulates around it the tube.

    Seawater (Raw Water) Subsystem
    Cold seawater is pumped into one end of the heat exchanger where it circulates around the flow tube before exiting out the other end of the heat exchanger. The seawater then exits the discharge end of the heat exchanger and is routed to the exhaust risers where it is discharged overboard.

    Closed Cooling (Freshwater) Subsystem
    Freshwater is routed from the engine to the thermostat assembly. From there the freshwater goes to the exhaust manifolds cooling passages then to the heat exchanger. The freshwater passes through the flowtube transferring its heat to the seawater. When the freshwater exits the heat exchanger it is routed to the engine circulating pump and pumped through the engine's cooling passages to return to the thermostat assembly.

    This system behaves somewhat differently in how it maintains the operating temperature within the optimum range. The thermostat opens allowing a higher volume of waterflow through the heat exchanger as the temperature increases, the reverse is true if the temperature is too low.

    Closed Cooling

    In the above example with the thermostat closed notice the small yellow arrows leaving the thermostat going to the exhaust manifolds then to the heat exchanger. This signifies the reduced waterflow with the thermostat closed. Select the button to open the thermostat and notice the waterflow increases signified by the arrows getting larger and changing color. Also note the seawater flow pattern from the pickup pump to the heat exchanger then discharging overboard via the exhaust risers, while the freshwater flow is a closed system.

    There are many variations to the above examples, consult your service manual and keep in mind the systems primary function is to provide a quick warmup and maintain a proper temperature environment. With that thought the flow patterns should begin to make sense.

    Here is a merc service bulletin for troubleshooting cooling problems on the MCM 470, 485, 488 engines


    Comments 27 Comments
    1. limacina's Avatar
      limacina -
      Just an example of variation.
      I've seen in many of the seadoo motors where the water goes through the exhaust cooler first, then on to the engine block waterjacket and thru the wet exhaust. I'm thinking this is their thermostat/preheat system. Also they use the high pressure stage of the jet pump as the water pump to feed the engine cooling system.
      Pretty neat, unique design.
    1. TimG's Avatar
      TimG -
      I haven't had the opportunity to see that, next time I am around one I will have to pay closer attention.
    1. joeseawhales's Avatar
      joeseawhales -
      Do I need an oil cooler for my Lister Petter 200hp inboard diesel engine?
    1. TimG's Avatar
      TimG -
      Joesea are you re-powering? I would think yes you would but honestly the folks at Lister Petter are much better equipped to answer that question, you can call them toll free here in the US at 1-800-888-3512, or send them an email at sales@lister-petter.com they can forward it to the techs if you email them.
    1. clyde's Avatar
      clyde -
      Hello everyone. Im a new boat owner, and lol, already have problems. My wife and I bought a 1992 Renken , outfitted with a Yamaha 5.0 inboard/outboard. It has a cooling problem . Question, does the Yamaha stern drive have an impeller ? And if so, where is it located ? Ive had the foot off and couldn't find any thing that would even resemble an impeller. I also rebuilt the Sherwood raw water pump. Still wont pick up water.
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      If it has a raw water pump and a 'suction' line anywhere, check that suction line. You reminded me of a problem I found in the carver- port engine would NOT pick up water- turns out, the intake line was warn through where it passed under the engine. Little "vacuum" leak prevented it from drawing water up high enough to get into the pump. Doesn't take a very big hole.
    1. clyde's Avatar
      clyde -
      Thank you Cap'n Ray. Ive checked for leaks, and back flushed the entire system. So far everything appears fine. It has me a bit baffled . The pick up runs from the stern drive. Would I be able to change it to a sea cock ? I rigged up a device to hook a garden hose up to the pick up line and it ran just fine. Just disconnected the pick up from the stern drive, hooked up the hose and all was good. Couldn't rev too high it sucked the hose down flat, so I know the pump is working. I also put the hose in the connection to the stern drive to back flush it. There didn't seem to be any blockage there.
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      It would be possible to have a one-way blockage- water will flow down well but not up; likely with something drawn into an intake in fact. I would try flushing water in from the bottom if you can- See what happens. Might be time to pull the lower unit apart for a thorough cleaning. Let us know what you find.
    1. clyde's Avatar
      clyde -
      Thanks again Cap'n. I'll let ya know what I find.
    1. gregross.us's Avatar
      gregross.us -
      Hey guys, confused new (old) boat owner here. I bought an 18 yr old center console with a Force 75 that had been sitting for a couple years. When I first started it, no water from the tell tale, so quit shut down, new pump impeller, gaskets etc. Restarted and water now flowing from tell tale. But water also flowing out if the tower shaft housing near the lower pivot point. At idle, not in gear. Is this normal?
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      Hope one of the OB guys can answer this one. I'm not sure.
    1. TimG's Avatar
      TimG -
      I'm not sure either but it doesn't sound right.
    1. dcducks1's Avatar
      dcducks1 -
      I have a 90hp Mercury and was having a similar problem. Nothing was coming from the tell tale and lots of water was flooding out from other areas of the lower unit. I dropped the lower unit and found a chunk of hard plastic laying on the lower unit. I couldn't figure out where it went to and started studying schematics until I found something that looked like it could be the culprit. I had to pull the power head off, remove the midleg, disassemble the midleg to replace a $1.50 piece and $2.00 grommet at the top of the water tube. Works better that it ever has. Not difficult but it does take some time and some "sweat equity" to fix it. If your tell tale seems weaker than normal, you might have some leak issues at the top of the water tube.
      Quote Originally Posted by gregross.us View Post
      Hey guys, confused new (old) boat owner here. I bought an 18 yr old center console with a Force 75 that had been sitting for a couple years. When I first started it, no water from the tell tale, so quit shut down, new pump impeller, gaskets etc. Restarted and water now flowing from tell tale. But water also flowing out if the tower shaft housing near the lower pivot point. At idle, not in gear. Is this normal?
    1. Terry Reed's Avatar
      Terry Reed -
      help!!! I have a2004 28 sunsation i just bought w a 525 efi merc. took if out for first time and it ran very cold...you could touch the headers it was so cold. this boat would not get up to 69 with a new 28p prop. the first owner said it came w 26p prop but he could not get it pass 74.. this is my 2nd sunsation and this motor should have mush more power. my question is. is there something that protects this motor to only prduce 90%..sensor? i have looked at the thermostat and it seems to be fine. but i dont think this boat has ever preformed the way it should..tthis boat should run in low 80.. any input. it is not heating up to temp 135-140 and rpm 4800. burning very rich. black.. restrictor plug?
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      Is it fuel injected? Could be that the thermostat is not CLOSING enough and the engine isn't allowed to warm sufficiently for the computer to get out of "warm up" mode (cold, rich and lacking power). If it's carbureted- check your choke; stuck choke means rich mix, cold running and poor performance. Let us know what you find! Good luck.
    1. Terry Reed's Avatar
      Terry Reed -
      It's fuel injected. Replaced thermostat factory 160. We ran it hard Saturday and engine will not reach over 150 deg. Will will excellerate over 4800 rpm. Still burning rich...
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      Two more trouble shooting techniques- make sure the boat has the correct pitch prop- "over propping" will bog the engine down resulting in low WOT rpm and likely rich burn. Also, be certain your flame arrestor is not restricted- good airflow is critical to efficient combustion. If the engine is running rich, it probably won't reach normal temperature but that is not an absolute certainty. Fuel injected means computer controlled which quickly goes beyond my skill set. My suggestion would be to bite the bullet and have a shop run diagnostics on it if nothing above yields any good results.
    1. Terry Reed's Avatar
      Terry Reed -
      Thanks, sounds great
    1. clifdwlr's Avatar
      clifdwlr -
      Hello. We have an older Carver Santa Cruz with an open cooling system that has cooling system problems. We have tested the exhaust manifold for leaks and it was ok and purchased a new riser. After we get it all assembled and put water to the system we still get water into the motor. Any ideas?
    1. Cap'n Ray's Avatar
      Cap'n Ray -
      Look for a cracked block, intake manifold, or head. Damaged gasket on the intake manifold could do it too. Regardless of the cause, water entering parts of the engine it's not supposed to be in isn't a good symptom. Good luck- let us know what you find.
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