nowakezone
New Member
It is generally accepted that planing cruisers typically experience 25%-35% prop slippage, pathetic - but it is what it is.
Please correct me if I'm mistaken, I have the gear ratio for the Horizon 8.1S 370 hp with V-drives as 1.2:1. The props are the stock 4 blade NiBrAl 18x21's. Our boat runs pretty close to the Boattest numbers so I used those, adjusting speed 1-2 mph downward to more accurately reflect my boat's performance.
Using the Prop Slip Calculator found on Go-Fast.com, the same calculator I used on previous boats...here are some startling numbers:
RPM MPH Theoretical Speed Slippage% Speed at 30% Slippage
3000 21(22.8) 50 58(54) 35
3500 26(27.3) 58 55(53) 41
Edit: this didn't come out as I typed it. Sorry about that
The numbers in parentheses are Boattest results. Theoretical Speed represents 0% slippage - not achievable. I chose 30% slippage as a reasonable goal only because it's midway between the acceptable range so I'm not asking too much nor too little.
The point of this post isn't that I want to go faster at the same rpm. The point is that given the numbers as they are above, our 340's could be running a lot more efficiently which translates into greater engine longevity and lower fuel costs. For example, say you enjoy cruising at 23 mph. Presently you have to run about 3000-3200 rpm's to get there. At 30% slippage, you'd need just less than 2000 rpm's to get 23 mph. That speaks for itself.
Real world example: my 2000 Proline 251 has a single Optimax 225. It ran like a pig. My WOT rpm barely made it into the range and it should be within 100 rpm of max. After repropping, cruise at 25 rpm's dropped almost 500. Mpg at 25 went from 1.7 to 2.5. Think that's small? It's almost 50%. Top speed went from 31 to 39.
I have a call in to my prop guy. He'll know what to do. My WOT rpm's for each engine is funky enough anyway. The port engine goes way over 4600, and the starboard engine is 300 under 4600. This is on freshly tuned engines with no prop or shaft damage. Weird.
Please correct me if I'm mistaken, I have the gear ratio for the Horizon 8.1S 370 hp with V-drives as 1.2:1. The props are the stock 4 blade NiBrAl 18x21's. Our boat runs pretty close to the Boattest numbers so I used those, adjusting speed 1-2 mph downward to more accurately reflect my boat's performance.
Using the Prop Slip Calculator found on Go-Fast.com, the same calculator I used on previous boats...here are some startling numbers:
RPM MPH Theoretical Speed Slippage% Speed at 30% Slippage
3000 21(22.8) 50 58(54) 35
3500 26(27.3) 58 55(53) 41
Edit: this didn't come out as I typed it. Sorry about that
The numbers in parentheses are Boattest results. Theoretical Speed represents 0% slippage - not achievable. I chose 30% slippage as a reasonable goal only because it's midway between the acceptable range so I'm not asking too much nor too little.
The point of this post isn't that I want to go faster at the same rpm. The point is that given the numbers as they are above, our 340's could be running a lot more efficiently which translates into greater engine longevity and lower fuel costs. For example, say you enjoy cruising at 23 mph. Presently you have to run about 3000-3200 rpm's to get there. At 30% slippage, you'd need just less than 2000 rpm's to get 23 mph. That speaks for itself.
Real world example: my 2000 Proline 251 has a single Optimax 225. It ran like a pig. My WOT rpm barely made it into the range and it should be within 100 rpm of max. After repropping, cruise at 25 rpm's dropped almost 500. Mpg at 25 went from 1.7 to 2.5. Think that's small? It's almost 50%. Top speed went from 31 to 39.
I have a call in to my prop guy. He'll know what to do. My WOT rpm's for each engine is funky enough anyway. The port engine goes way over 4600, and the starboard engine is 300 under 4600. This is on freshly tuned engines with no prop or shaft damage. Weird.
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