Marine Engine Heating and Cooling Problems

We have been salt water sailing and cruising for many years and have had a variety of issues connected with the heating and cooling of our inboard (and outboard) boat engines. This article deals primarily with inboard boat engines though most of the same issues apply to outboards.

Cooling System Basics

Inboard marine engines have two basic cooling system configurations: Fresh Water with heat exchanger, or direct raw water cooling. The primary difference between fresh and raw water cooling is that a freshwater cooled engine incorporates a heat exchanger, similar to an air-cooled radiator, to transfer engine heat from a freshwater (antifreeze) medium to a raw or salt water bath. The advantage of this method is that hot salt water, which is very corrosive, never makes physical contact with the interior workings of your motor. A straight seawater cooling system pumps raw water directly through to cooling water channels in the engine thus allowing direct contact between the engine metals and the corrosive salt coolant.

A freshwater cooling system will provide greater engine protection without the need to worry about pencil zinks in the engine itself. It also provides greater latitude in configuring a shower or other freshwater heating supply.

Common Components

No matter what cooling configuration you have, there are many common components that can create problems. I think that we have experienced most of them! On any marine engine, ensuring that there is a steady flow of cooling water is essential. Look for regular (or steady) flow of water through the discharge port(s) or exhaust. Look for steam coming out of these same ports. Excess steam or lack of water flow are strong indications of a cooling problem.

In either system, you have a seawater intake valve or fitting. This is basically a hole through the bottom of your boat (in an outboard sterndrive, through the lower unit). On outboards or sterndrives, this will lead directly to the water pump. On inboard engines, there should be a through-hull fitting with a shut-off valve and then a seawater strainer. The seawater strainer is essentially a screen designed to keep out the larger particles of sea grass and other ocean crud that can plug up your water flow. It needs to be checked and cleaned regularly.

If you are having water flow issues, the first thing to check and clean, is the seawater strainer! Having done so, make sure your seals are firmly seated. An air gap here can result in drawing air rather than water and can crate a lot of problems down the line.

After the strainer, is the raw water pump. The raw water pump is usually driven by a belt off of your engine. They are not particularly large and are fairly simple to service. The most usual failure is from the impeller which is typically a 6 finned rubber star-like contraption. If you run dry due to a blockage in the intake or sea-strainer, expect a failure of the impeller. I always keep one or two spares.

On a fresh water cooled engine, there will be a secondary water pump, driven by a belt from the drive wheel, feeding a water/antifreeze system similar to that on your car. I have seldom had problems with this part of the system though frequently checking fluid levels is highly recommended. Again, checking belts, fluid levels and overall condition is very important. We were out a few weeks ago and discovered that 3 out of 4 of the bolts holding the water pump pulley on, had sheared off. We only had about 250 hrs on the engine and I would never have expected it.

If you have checked and cleaned your strainer, the impeller, belts and pulleys, and coolant levels; and are still having water flow problems, further investigation is called for.

We recently had an on-going issue where we were not able to get a good coolant water flow. I checked all of the above without finding any issues. I finally pulled the hose from the through-hull fitting and found that, where there should have been a flood, there was only a trickle. I tried to run wires and probes down from the to, to no avail. I donned a wetsuit and went under to try to clear the through-hull, to no avail.

In the process of all of this, I did find some smallish pieces of kelp in the system. It was a bulb, or “popper” type kelp common along our waters and which we gather on the beach to pop, just as you would do with bubble-wrap. No matter what I probed the opening with or how much I blew through it, I could not get it out of the intake pipe. It just floated back up and got stuck at a reducer in the line.

On the eve of hiring a diver, I determined to try one last solution; to hook up a pressurized freshwater supply to the outlet of the input through-hull. Where air pressure wasn’t sufficient to push the kelp out of the tube, water pressure did the trick. After flushing the through-hull for a few minutes, we had great cooling water supply and were able to continue our cruise with no further issues. What could have cost hundreds or thousands of dollars was resolved with a bit of ingenuity and a few pieces of hose.

As cost conscience cruisers, we try to be as self-sufficient as possible. We look to find solutions to common problems and to share those solutions with others on the water.

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One Response to “Marine Engine Heating and Cooling Problems”
  1. Alex Trodder says:

    I went sailing around the San Juan Islands for a week once on a forty foot Yawl while I was a teenager. It was primarily powered by sails, but it did have an inboard motor on it. If I remember right, it had a freshwater cooling system. Every time we dropped anchor for the night someone would check the engine to make sure there weren’t any problems. I hear it’s really important to care for your inboard motors in saltwater environments. Especially since you might be further away from land.

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