Boat motors use the water they’re floating in to cool them. Some, like most outboards, have what’s called a “raw water” system. The water the boat is floating in circulates through the engine block, then exits with the exhaust. Most modern inboards have a “fresh water” or “indirect” cooling system. In this system, coolant like that in your automobile is pumped through a heat exchanger. The coolant is circulated through the engine block rather than the raw water itself.
The heat exchanger is analogous to the radiator in your car. In your car, air blows past cooling fins to take the heat from the coolant (antifreeze). In a boat, the raw water is circulated past the coolant in the heat exchanger. The raw water flows through a system of copper tubes, surrounded by the coolant.
Both use a pump to pull the water in through an opening in the bottom of the boat. A fresh water system also employs a circulation pump to keep the coolant moving.
That’s the bare bones explanation. In practice, both systems usually have an oil cooler (like a mini heat exchanger that cools the engine oil) and maybe a transmission oil cooler as well.
Notice that I said above, most modern inboards have a fresh water cooling system. But most of the old wooden runabouts used raw water to cool the engine. This is fine for a lake boat. But bring the boat to the coast, and you might feel it’s not a great idea to run salt water through a 65 year old cast iron block to cool it.
This owner got tired of avoiding the stumps in nearby Lake Marion and asked if I could convert his boat to fresh water cooling. It seemed like a fun project, so we got right into it.
This engine has a displacement intake impeller pump, then an oil cooler, then the water enters the block, exits the cylinder head, and goes to the exhaust manifold. Probably one of the more difficult aspects of this job, is deciding where to mount the various components: expansion tank, heat exchanger, and circulation pump. You’ve got to either fabricate or scavenge brackets to hold these things. I was lucky that this owner works at a local metal fabricator and said he could make any bracket I designed.
I made a new friend at Seakamp Engineering who helped with the design of the heat exchanger and expansion tank. And I was able to source a good Groco electric circulation pump. Then, with a lot of hose and clamps, we got it done. You can see in the picture at left we added a port to flush the heat exchanger and exhaust manifold with water from a garden hose.
Now Eric can boat confidently in the salt water areas around Charleston.