Re: hwsiii---Need proping advice
Cheapest way, IMO, to test a tach is to obtain a laser tach. Cheapos on Ebay for $15 to $20 shipped to your door. Put a small piece of reflective tape on the flywheel and point the tach at the spinning part, get an instant read.
Obviously this will only be safe to test while not running on the water...but at least can get you a good idea if your tach is accurate by simply running RPMS up to a reasonable speed while on muffs in the driveway.
Far be it for me to take something away from Walleyhead. But there is one part of his analysis which I may disagree with. "FACT - Known 17 Pitch prop." How was this "fact" nailed down?
Your boat is 16 years old, the prop is nicked up, you are not the original owner of boat or prop, and the lower unit has hit something and had some repairs done. So I find it entirely possible...frankly LIKELY...that your prop is NOT a "true" 17 pitch for many reasons: It could have actually been repitched (by a prop shop...or maybe by a rock laying at the bottom of a lake?), and it is entirely possible that it was NOT repitched, but was never accurately labeled to exacting standards when it was new.
I know on my boat I have absolute fact GPS speeds, fact outdrive ratio, fact tach reading (tested with THREE tachs, including one calibrated shop tach) and a brand new stainless prop that I purchased myself. When I run slip calculators for my setup, I get exactly 0% slip. Not possible, but simply the way the math works. If I add one inch of pitch for a "cupped" prop (did you do this? Is your prop cupped? do you know how much? How do I or anyone else know whether you or I should add 0.5", 0.8", 1.2", or 1.4" for the SPECIFIC AMOUNT of cupping my particular prop has?) and I get 6%. And this figure is STILL not truly possible for an I/O. To get a realistic slip figure on any prop calculator I have to actually call my 21 pitch prop a 23 pitch prop. And then the calculator suddenly works with a realistic number. But it's a number based on an exaggerated change of a mathematical constant...in other words...IT IS A GUESS. But I apply that guess in order to compare theoretical RPM and speed figures to other pitch props of the same brand and same style ONLY...but not if I plan to switch prop brands or styles.
My only possible conclusion given my known facts? My 21 Pitch brand new Solas prop is not an ACTUAL 21 pitch prop, despite what Solas says it is, and despite the fact that I actually had to exchange it thinking they made a labeling error. 2nd prop, same results. I beleive many manufacturers do not accuarely rate the pitch on their props...even when brand new.
So it's very important to not put too much stock in a prop calculator. It's only a guidline tool.
Another questionmark: Your first post says "lower unit was replaced by previous owner." And after that replacement you allude to the owner starting to do some prop experimentation. Now I'm not an Evinrude guy and I don't know interchangability of parts, but that throws up a red flag to me with respect to using prop calculators. Is there any chance this lower unit replacement could have resulted in a gear ratio change? Which send the previous owner on a blind prop search? That's yet another possible variable to consider when one obtains strange numbers out of a prop calculator...
But none of this changes the following: I agree 100% that you DO need to make sure your tach is accurate, that is absolutely CRITICAL for accurate prop tuning. So you're on the right path by wanting to check this first before wasting money on a prop that might take you in the wrong direction with your RPMS. I'm not an Evinrude guy at all, but I don't know of any other way to do this than to obtain another tach that is known to be accurate and hook it up...or as I mentioned try a laser tach or smilar shop/test tach that you can borrow. It might even be worth paying a marina to test it for you if they have a good calibrated test tach...they might just charge you an hour labor or less and then you would have a good answer.
Just bear in mind that you might actually find your tach to be accurate, and your current prop may NOT be a true 17 pitch prop...despite labeling and part number. And also your gear ratio may possibly not be 2:1 anymore? I'll let the Evinrude guys confirm or deny that possibility for me...
