While myriad of circumstances can cause an engine to overheat, it is highly preventable in most cases, starting with ample water drawn into the intakes. Inboard-powered boats typically have grillage guarding the intake passages to prevent eel grass and other debris from being sucked in. Problems occur when the grill slits or holes are covered with marine growth or too much bottom paint, as the flow to the water pump can be restricted. This can be further compounded when the intake through-hull fittings serve as home to barnacles, which create more constriction.
If you rely on the yard to bottom paint your boat, insist they remove the screens, clean out the throughhulls and dab some paint on the insides. Once marine growth calls these surfaces home, you own it, alive or dead, and it crowds out the water available to your engine and other water-dependent accessories.
Some boats also have internal engine and accessory strainers. Marine foliage can wick its way inside the metal or nylon baskets, and I’ve found shrimp living in mine between regular cleanings. But the best find was aboard a boat in Killarney, Ontario. Most internal raw-water strainers use a Lexan globe that makes it easy to see when it needs cleaning. This strainer, however, had a stainless steel case that required disassembly to reveal what it contained. In addition to Georgian Bay weeds, wedged tightly inside was a hapless fingerling walleye.
Obviously, this foreign-built boat never had, or, had lost its exterior grill on the hull bottom. Confirm any exterior intake protection is replaced properly before launching your boat. On my boat I always use new fasteners each spring.
Churned up sand or mud from the prop wash that occurs while operating in shallow water will also find its way into the strainers. To save wear and tear on the water pumps, my rule is to shut off the generator, air conditioner, and live well when backing into the slip if such conditions exist. Similarly, if it’s low tide when I leave, I will not start up these systems until I am in good water.
Overheating can also be caused by a worn or damaged water pump impeller. If you don’t recall the last time you replaced the impeller for your engine or other accessories, consider adding this task to your list of chores, and don’t forget to keep spares in your tool box. If the impeller is damaged and is missing a vane, make sure any displaced pieces are accounted for and removed. Any debris left in the water passages may reduce flow, which leads to persistent overheating.
When a new impeller is installed, run the engine or related system as soon as possible to ensure proper operation, as I learned from experience.
The engine folks who replaced the impeller on my starboard diesel insisted all of the old pieces had been found and the boat was ready to go. The following weekend, when I had guests aboard and powered up to cruising speed, the engine alarm suddenly went off again. I pulled back the throttle and the alarm silenced. With both engines in neutral I checked the water coming out the exhausts and noticed plenty of flow on each side. After limping back to the dock with no alarms ringing, I climbed in the engine room. After uncoupling the raw-water hoses at the heat exchanger I found a handful of loose particles from the old impeller clogging the bundle. Once all the debris was cleared and the heat exchanger hoses reassembled, the engine ran fine once again.