Prop selection 20' MA 175 Mariner

[Bushwacker]

Originally Posted by Chaser

. . . I don't know if the tach is accurate or not. Its a new analog unit, but I don't know if its possible to calibrate it. How do I check the actual RPM without the tach? Is there a way to put a timing light on it? Maybe I should read more in my manual...

Sounds like your motor height is about right. It's amazing how many dealers mount them too low!

I've had new factory tachs that were off by 300 rpm, so just because it's new doesn't mean it's accurate. I'd see a dealer about checking your tach. There are universal strobe tachs that work somewhat like a timing light, where you turn a dial that changes the strobe light frequency until a timing mark stands still, then just read the frequency/rpm from the dial. Some dealers also have a dyno they can attach to the prop shaft and run the motor up to max rpm using a known good tach while using a flushing attachment. Need to check it over the whole rpm range, as some tachs are ok at low speed and only off at high speed.

One crude check is to plot MPH (from GPS) vs RPM. It should be pretty linear once on plane. If the plot isn't a straight line, that's a clue that the tach may be off.