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Troubleshooter: Take Your Trawler's Temperature

An Inexpensive Piece of Gear Can Diagnose an Engine’s Illness; Both cheap and easy to use, a handheld pyrometer could be one of the most useful tools in your collection.

We can often get a quick read on someone’s health by taking their temperature. The same can be said for a powerboat’s engine space. Temperature can be a reliable indicator of machinery condition—especially when compared to a known normal or baseline temperature. Fortunately, a commonly available tool can be purchased for this application: an infrared pyrometer. A decent pyrometer can be purchased for less than $50 and one should be kept aboard every cruising powerboat and incorporated into your regular ritual of engine room inspection. For the purposes of this column, all temperatures are given in Fahrenheit.

To use a pyrometer, simply aim the device’s laser beam at an object. The beam does not play a role in sensing temperature; it simply lets you know where you are aiming. Although it might seem like you are “shooting” the temperature, in fact, sensors inside the pyrometer are receiving and digitizing the energy it receives. Your engine room contains a rich supply of meaningful targets.

Let’s start by confirming the accuracy of the temperature gauge at the helm station. Note the temperature shown on the instrument panel and then aim the pyrometer at the top of the thermostat housing and record the temperature. Keep in mind that a pyrometer reads surface temperature radiation and it may take longer for the metal housing to reach the coolant temperature. It is not uncommon for the pyrometer to read slightly lower than the gauge, but a variance of more than about 5 percent indicates a problem with the gauge.


Heat in the engine’s freshwater cooling system is maintained by a heat exchanger that’s cooled by sea water. Over time, a heat exchanger can lose its efficiency due to accumulation of debris (internal zincs that break up, broken impeller vanes) or from a coating buildup on the internal copper tubes. The amount of heat exchanged varies depending upon the seawater temperature: colder water will more effectively remove heat from the engine. At a minimum, for most conditions you should see about a 10-degree difference between the temperature of the freshwater inlet on one end of the heat exchanger and the freshwater outlet on the other end.

The same principle applies to the transmission oil cooler (the temperature drop will occur between the oil inlet and outlet ports on the cooler). Since the oil temperature will vary according to the load on the transmission, testing under similar loads each time is important. If you make it a habit to perform this check you will gradually learn what to expect and will be more likely to notice a gradual reduction in performance.

The exhaust hose often fails where it attaches to the riser. The highest temperatures occur here as sea water is injected into the flow of exhaust gas, which might be as high as 700°F. The raw water disperses through small passages designed to create a controlled spray pattern to ensure there are no hot spots on the hose.

Over time these passages can become clogged, changing the cooling pattern and allowing an area of hose to overheat and fail. You can use your pyrometer to check all the way around the exhaust hose just abaft where it attaches to the riser. The allowable temperature varies according to the type of hose used, but in general the hose should not exceed 200°F.

Comparing cylinder temperatures can also provide valuable data. Finding a suitable target will vary from engine to engine—the goal is to hit the top of the cylinder. For most engines, the variation should not be more than about 10 percent from one cylinder to the next. Variations greater than 10 percent indicate a cylinder that is not fully combusting the fuel. It is also possible to detect a cylinder that runs at a higher temperature than the others, though this condition occurs more rarely.

Moving off the engine, we can turn our attention to a conventional shaft gland, or stuffing box. In an effort to reduce the drip rate, many cruisers overtighten the adjusting nuts, creating excessive friction and heat. The increased heat and friction will gradually wear grooves into the shaft, creating an area that will never seat properly with the packing even if new packing has been installed.

Water-injected boxes can also overheat if the cooling flow is reduced for some reason. When functioning properly, the metal housing temperature should not exceed the ambient seawater temperature by more than 20 degrees. Additionally, under normal conditions, the housing temperature should not exceed 120°F. Depending on the brand and type of packing material used, higher temperatures might melt the tallow and wax in the material, rendering the packing material useless even when new.

The infrared pyrometer provides valuable information, easily gathered, at a very low cost. Consistency is the key—perform the checks under comparable conditions (e.g., same cruising rpm). Some variables, such as seawater temperature, cannot be controlled and should be taken into consideration. Incorporating regular temperature checks into your cruising regimen will help you identify maintenance needs to be performed dockside before they become repair needs under way.

Steve Zimmerman is the president of Zimmerman Marine, which operates four boatyards in Maryland, Virginia and North Carolina. Zimmerman has been repairing and building boats for almost four decades.