craze1cars
Lieutenant Commander
- Joined
- Dec 26, 2004
- Messages
- 1,822
I was just playing with some prop calculators, slip etc for fun and education...and I can't figure this out. My boat: 1992 Rinker 181, 18 foot bowrider runabout, Smart Tabs, well tuned, with stock 3.0L Mercruiser Alpha 1 Gen II (the smaller 115hp version with 2.5L head).
My test numbers:
1. Top speed fluctuates between 36 and 37 per GPS on inland lake in good conditions (no current, no wind). Boat speedo says 39, but I just throw that figure out since GPS is more accurate.
2. Peak RPM at this speed was 4,500 to 4,550. Call it 4,525. This is verified by both my onboard tach which was replaced brand new just last year, AND a digital hand-held mechanic's tach that I ran side-by-side during my last test run trying to figure out the reason for my anomoly. Both tachs were always reading within 50 rpm of each other from idle to peak.
3. Prop is a 17 pitch Turning Point Aluminum Hustler, unmodified and undamaged, purchased brand new here on Iboats.
4. I presume I am running a 1.98 or 2.0 gear ratio on my stock outdrive? Am I mistaken on that? My outdrive serial numbers are all numbers matching for 1992, nothing appears to have ever been changed and last time I had the drive off for annual maintenance, I spun the prop and counted the turns of the shaft to confirm it is indeed approx 2:1.
So when I put these numbers into the formula (using 1056 as the constant on the bottom line), I get a theoretical top speed of 36.58 to 36.99, based on 4,500 to 4,550 RPM. This almost exactly matches my GPS which means I have zero slip. And yes, I know this is not possible.
As I've already addressed the possibilities of inaccurate speedo and/or tach...by utilizing GPS and a calibrated digital test tach. So the only thing I can come up with is that Hustler props are not necessarily a true 17 pitch prop, as stamped on the side? I base this on the fact that if I change my pitch number to 18, I calculate a much more realistic figure of about 8 or 9 % slip....
Or does the aggressive rake/cupping of the Hustler prop compared to some other props possibly make a difference? What could I be missing here? It's not that important as I'm well-propped and very happy with my performance, but I'm just curious and working on educating myself on some things here. Thanks!
My test numbers:
1. Top speed fluctuates between 36 and 37 per GPS on inland lake in good conditions (no current, no wind). Boat speedo says 39, but I just throw that figure out since GPS is more accurate.
2. Peak RPM at this speed was 4,500 to 4,550. Call it 4,525. This is verified by both my onboard tach which was replaced brand new just last year, AND a digital hand-held mechanic's tach that I ran side-by-side during my last test run trying to figure out the reason for my anomoly. Both tachs were always reading within 50 rpm of each other from idle to peak.
3. Prop is a 17 pitch Turning Point Aluminum Hustler, unmodified and undamaged, purchased brand new here on Iboats.
4. I presume I am running a 1.98 or 2.0 gear ratio on my stock outdrive? Am I mistaken on that? My outdrive serial numbers are all numbers matching for 1992, nothing appears to have ever been changed and last time I had the drive off for annual maintenance, I spun the prop and counted the turns of the shaft to confirm it is indeed approx 2:1.
So when I put these numbers into the formula (using 1056 as the constant on the bottom line), I get a theoretical top speed of 36.58 to 36.99, based on 4,500 to 4,550 RPM. This almost exactly matches my GPS which means I have zero slip. And yes, I know this is not possible.
As I've already addressed the possibilities of inaccurate speedo and/or tach...by utilizing GPS and a calibrated digital test tach. So the only thing I can come up with is that Hustler props are not necessarily a true 17 pitch prop, as stamped on the side? I base this on the fact that if I change my pitch number to 18, I calculate a much more realistic figure of about 8 or 9 % slip....
Or does the aggressive rake/cupping of the Hustler prop compared to some other props possibly make a difference? What could I be missing here? It's not that important as I'm well-propped and very happy with my performance, but I'm just curious and working on educating myself on some things here. Thanks!