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Yacht Charter Guide

The incredibly useful laser temperature gauge

By: Scott Fratcher - Marine Engineer/Captain

A lazar temp is a wonderful tool. Point the pistol like device at just about anything and it shows a digital real time temperature readout. This relatively inexpensive tool should be included in every yacht’s tool kit. The uses are seemingly endless. Here are just a few tricks that can be performed with a laser temperature gauge.


Diesel Engines

Check each cylinder temp while the engine is under load. They should all be equal. A cold cylinder means it’s not functioning, (IE the injector is not passing fuel into the cylinder) and a hot cylinder means it’s running too hard (IE it’s doing all the work cause another cylinder is not working).—Shoot the whole head and look for hot spots. Hot spots show where a head gasket is leaking, or a clogged coolant passage. If you have to use an engine with a hot spot draw temperature “topo” lines around the hot spot at a known load. Keep checking the hot spot “topo” lines. If they grow then the failure is progressing. Slow down to preserve what is left of the engine-Check the heat exchangers water in and out. Is it carrying off the heat? Under 80% load the raw water IN should be about eight degrees C cooler than the raw water OUT. The fresh water side of the cooling circuit should have about a ten degree drop—check the oil cooler and gearbox cooler the same way

If an engine is overheating you should be able to make a clear determination if the engine is producing too much heat (IE.-bad head gasket) or the cooling system is not removing enough heat (IE clogged heat exchanger). See this month “Yachtwork” question.

Check oil temp at the engine oil pan. Oil tends to be about five to ten degrees warmer than the coolant and 95C is about max.—Follow the whole cooling system path (shown in manual) with the gauge running. Can you follow heat flow from the engine heat production through the coolers and out the raw water into the exhaust?—check exhaust gas temp. This is one of the most important readouts on the whole engine system. All cylinders should be about the same. One cylinder running hot can indicate a faulty injector or a cylinder about to seize- If you are trouble shooting a piece of equipment that may go “catastrophic” (throw dangerous metal pieces out the side) then the lazar temp allows temperature samples to be take from a safe distance— Check for hot spots in gearboxes—look in the manual to see what is under the hot spot. Bearing going bad? Clutch pack beginning to slip? Without a turbo the exhaust temp should be under 500 (max) under full load, with a turbo it should be under 800 (max pre turbo), check the manual, the cooler the exhaust gas temperature the better your engine efficiency—The manual flax packing gland at the prop shaft should be no more than 60C



Make points with the cook by calibrating the galley oven and locating it’s hot spots— Save energy on the refrigeration by checking the temperature difference around refrigerator doors and sides for big losses showing wasted energy. Once found simply re-insulate or calk the small “cold leaks”-Trace the refrigeration system from the cooling pump, through heat exchangers, to the cold box and back to the pump. You don’t even have to know what the components do, just where the temperature transfer stops to make a basic diagnosis-Shoot the inside of the deepfreeze. Is the -60 really that cold? Where is the coldest beer in the fridge located? Check the HVAC cooling air for temperature at the outlet. This gives a baseline that can be checked again over time. Any change tells us the system is clogging or it’s time for service.


Electrical Systems

Check an alternator under full load. The case temp should not be over 100C, but the diode pack may be 150C. Much more and you know the alternator will overheat if used under continuous duty—Shoot electrical lugs for hot spots without having to touch live electrical leads. This way only problem lugs have to be improved—Check generators under full load for over heated windings—shoot breakers while fully loaded for heat—check electric motors on windlasses and pumps for overheating against the motor spec sheet—Look for one side of the motor heating up faster than the other, this indicates a failing windings—get a temperature of the finals of a SSB under full load then clean the antenna leads and check again. If the temp dropped you made progress.


Tip- When buying a lazar temp check the high and low temperature range. A range of a 600C should give you all the information you need. Some now read up to 1000C and this is even better. A reading of only 250C is not enough to effectively check exhaust gas temperatures but will diagnose overheating engines.

Tip: The better quality laser temps shoot a smaller diameter circle at a given distance. It’s worth paying the extra money to be able to shoot a smaller circle at a meter. The difference is the ability to isolate a hot spot to the exact location.

How to use a laser temp to troubleshoot an overheating engine

Q. Yachtwork-My diesel yacht engine overheats when I give full throttle. If I drive along slow all seems well. I have changed the impeller and cleaned the sea strainer but when I try to drive hard the engine temperature rises. How do I fix this problem?

A. Good job on starting your repair with two of the most common reasons marine engines overheat, worn out impellers and clogged sea strainers. Now it’s time to put on our “Marine Engineer” hats and begin some real trouble shooting. The purpose of this trouble shooting is to find the exact cause of the overheating engine by performing some simple tests. This way we locate and fix the problem without changing extra parts.

It should be noted even if your engine is in perfect condition you can run this same series of tests noting the results in your log. At any time in the future if your engine were to begin to overheat simply compare the old number to the new to find the source of the problem.

To perform these tests we are going to use a laser temperature gauge to check if each component in the cooling circuit is performing to standard. Before we begin we must first ensure the salt water pump is moving enough water. With the engine in neutral run the engine at 80% of it’s maximum RPM’s. You might set typical 3600 RPM yacht engine at 3000 rpm’s. Take a bucket to the exhaust outlet and time how long it takes to fill with exhaust cooling water. Use this measurement to figure how many liters per minute the cooling system is passing. Compare this number with your manual.

If you don’t have a manual you might be in the ballpark if your 12 hp engine is pumping about 15 liters per minute and a 50 hp engine might move something like 30 liters per minute.

If your engine has flow restriction you can locate this by breaking the salt water loop in various sections of the rout and doing a quick flow check till you find the clog. Remember the exhaust side of the loop may want to blow back so put a plug in the downstream side of the break and only run the engine for a few seconds jut to check flow rate.

Now the real testing can begin. We want to know if the engine is overheating because it is producing too much heat (IE-bad head gasket) or if a cooling component has failed (IE-clogged fresh water heat exchanger).

We need to run the enigne hard. Small yachts might be able to stay tied to the dock while boats with more horse power should do this test in open water. Either way we need to test the diesel under load.

For small yacht be sure you boat is well tied to a strong dock and pilings. Double your stern lines if necessary because were going to pull against them while trying to make the engine overheat.

Put the engine in gear and slowly increase the rpm’s till your at about 80% load. Check all your dock lines, cleats, and chafe points one more time. If everything looks secure take your laser temp and head to the engine room.

Wait till the engine begins to overheat and test. Start at the sea water inlet and follow the salt water through the cooling circuit testing and noting temperature as you go.

The salt water will probably pass the sea strainer, and on to the gearbox heat exchanger. You can expect a one degree rise in sea water temperature. A larger temperature rise indicates the gearbox is producing too much heat. (IE-slipping clutch), the seawater should pass through the salt water pump and on to the main engine heat exchanger. Expect about a five degree rise in sea water temperature as the salt water passes through the exchanger . More indicates the engine is producing too much heat.

At this time also check the fresh water in and out at the heat exchanger. The temperature change should be about ten to fifteen degrees between the cooling water leaving the engine and the cooled water returning to the engine. If the water re-entering the engine is still hot the problem is in the heat exchanger. (IE-clogged with heat insulating gunk), but if the water re-entering the engine has been properly cooled the problem is in the engine (IE-no cooling water flow, or blown head gasket).

Continue this test till the salt water leaves the boat. You might come across a couple more heat exchangers like maybe a hydraulic system cooler or HVAC or refrigeration system. Test them the same way. A one degree rise in sea water temperature should be about maximum for any of these additional cooling systems.

If you found the engine is producing too much heat continue testing with the laser temperature gauge. Begin at the head and look for hot spots. Follow the cooling water circuit looking for the sudden increase in temperature. Check the oil temperature and oil heat exchanger the same way looking for oil heating due to thinning (IE-fuel pump with leaking seals bleeding diesel into the liberating oil thus diluting it).

With this series of tests and a little reasoning you should find the problem and only have to fix that single item. This will save time and expense in the long run.


Laser temp gauge and overheating alternator

Q. Yachtwork-I read your dual alternator article and noticed you mention reverse mounting an alternator. I have this exact system on my yacht, but after fifteen minutes of running the Balmar temperature sensor reduces the alternator output as it got too hot. Why is this?

Answer-The problem is probably because the original cooling fan was left on the alternator when it was reversed. Cooing fans are made to turn left, right or unidirectional. You can tell one from another because the cooling blades should be angled back away from the direction of rotation. Change the angled cooling fan to one with straight blades and the alternator should run cool again


Author profile-Scott Fratcher has served as Senior Technician for Yanmar Engineering Services in Auckland where he used many of the tricks shown in this article. More information is available, or see “How To Make Money With Boats”


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