Thanks Commodore-H for sharing this item.
In fact it can be very useful to know the number of revolutions in different working conditions.
It could be useful to check the effectiveness of the applied propeller or otherwise.
Are you planning on doing any specific tests?
If you run any tests I would be happy to see them. If you can, make videos.

I have one I bought from Harbor Freight several yeas ago. Works fine. It works by putting a narrow piece of tape (supplied) on the shaft and point the laser at it. If you want to check it under load and you can see the shaft/coupling (structure or deck removed) you can put the tape on that and run it in a test tank.

Lew
Florida ⛱️, USA 🇺🇸

I noticed on Amazon that you can purchase a touchless laser tachometer at very reasonable cost $20 USD. I have a box of motors that I have accumulated over the years that I would like to test and compare and I could see other applications for this so I thought I would give it try. For example you could test how fast a motor was with different battery voltages, or you could see how many rpms are lost when a motor is attached to a prop shaft in a stuffing box with prop.
I know this only tells you the unloaded rpm, not the torque or current draw under load. Still a nice piece of data to have.

Frankly, the gadget looked cheap and the reviews were not all positive, but for $20 I thought I'd try it anyhow. To make it work you attach a piece of reflective tape (included) to the object you want to determine the rpm of. If you do purchase one of these, note that you need some distance, around 1 foot from the rotating disk or it doesn't work too well .

To test the uninstalled motors I made up a brass disk about 2" in diameter, then soldered a collar with a grub screw to the center to secure it to the motor shaft. I painted it with just gray primer so I would be sure just the tape was the reflected part. It's easy to cut thin brass if you secure to a piece of wood with a little CA or double stick tape first then cut and separate it later.

Then I made up a little fixture where I could mount the motor. I added a switch, alligator clips and a battery connector.

The test was a success, the little tachometer consistently showed the rpm of the motors tested and I played around with it a little and worked OK in every case. I am sure it will be useful in the future