Our friend and longtime OMC Tech, Joe Reeves, has posted thousands of answers to technical questions about Johnson and Evinrude motors on our Boat Repair Forum. He started contributing to the forum in 2001 and is still at it! He has helped countless visitors solve their engine problems. Here are his thoughts on the Purpose Of The Tachometer
Purpose Of The Tachometer
An engine in ideal condition should obtain an rpm (revolutions per minute) as close as possible to the highest rated rpm specified for the particular engine one might have. To pick a specific engine for example purposes, a 1979 200hp Evinrude or Johnson has a top rated rpm of 5750 rpms. Yes, I know it’s old…. so am I.
On whatever boat the above engine is installed, the desired top rpm of the engine when running at full throttle would be somewhere in the range of 5600 or so, possibly slightly higher BUT not to exceed that 5750 top rated rpm limit.
Some boaters have their engines on, say, a 17′ runabout and all is well pertaining to speed and the tachometer rpm reading. At some later date they upgrade to a 20′ boat of some kind, perhaps a different model, and simply have their engine switched over to it. Refreshing your memory, this engine is still in tip top condition but suddenly the engine acts like it has lost half its horsepower and the tachometer reading has dropped considerably. What has been overlooked is the propeller.
The horsepower rating and the boat size demand a matched propeller. The exact size of the propeller required can be determined by one’s past experience or the knowledge of other boaters with like engines and boats, or by a chart or online Propeller Selector.
Regardless of whatever engine is teamed up with whatever boat, when all is well and the tachometer reading is where it belongs, that brings forth a sense that no problems exist. And should those rpms suddenly drop, especially when underway, that indicates that a problem does exist and the engine should be checked out as soon as possible.
I state “especially when underway” above for the following reason. It can’t be helped that our boats simply sit in storage at times for months on end. This usually results in having the carburetor(s) foul, gum up, and become somewhat clogged. Obviously this results in a fuel restriction which in turn causes the engine to run lean. With this flaw, one takes their rig out of storage and on the very first run they encounter a lack of power and a much lower tachometer reading than when it was last run. This problem might have been overlooked if not for the tachometer, and if overlooked (running lean), it would lead to a blown powerhead.
A side purpose to the tachometer is to have it coupled with a speedometer. Now, when all is well, that is the boat hull is clean, the engine is in tip top condition, and the propeller is matched to the boat so that the top full throttle rpm is close to the engine’s top rated rpm, the thought is to think about fuel consumption and hull resistance. Picking a figure out of the air here….. say the engine I’ve picked here for an example tops out on the tachometer at 5600 rpm. Now, let’s say that the top speed of the boat is 38 mph and this speed is obtained at 4800 rpm, and the speed does not increase at any rpm above 4800 rpm. This indicates that at any rpm setting between 4800 rpm to 5600 rpm, you are simply wasting fuel and money.
Also, running the engine at no improved speed in the above dead area of 800 rpm area results in an engine condition we call “lugging”, but that’s another story I’ll get into later. Suffice it to say it causes a huge waxy gray substance to quickly build up on top of the pistons and you do not want to encounter that.