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How to trouble shoot yacht problems with a laser temp gauge DIY

Trouble shooting with a laser temp gauge DIY
How to trouble shoot yacht problems with a laser temp gauge

This a DIY article for using a laser temperature gauge to trouble shoot yacht problems such as an overheating engine or hot breaker. Cheap laser temp gauges cost about 20 bucks and a good one about 150. I find this inexpensive handy tool to be very useful.

The questions about how to trouble shoot an overheating engine about two thirds of the way through the article is well worth the read.

This article was original published in the book "How to make money with boats" then Tradeaboat in NZ, finally I posted the article here-

Laser temp gauge use

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) ...

The rest of the article can be seen here-

Laser temp gauge use

At the bottom of the article is a link back to this forum.

Disclosure: Yachtwork is a marine vendor
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