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Author Topic:   slippage under 2%?

Ector posted 07-07-2004 07:46 AM ET (US)
yesterday i raised the engines (they were too low making a very big spray and needing to be always trimmed out very much), so today, quite calm sea and a little bit of free time from my job, i had a ride to test the new arrangement...
I was alone on board, with 50 gal of gas.

I hit a 43,6 knots (49,5 miles) speed at WOT, with 19 props and 5.300 rpm.
the speed data is from a garmin gps, the rpm data is from the smart craft gauge of my 150 opti's...
putting all the data in the prop calculator this should put the slip value at 1,61% (gear ratio 1.87)...
at 3700 rpm, the speed was 30knots (----> slip 3%)

i assume this could depend from:
1) the laser II props, running just under water surface, are actually a little bit more pitched of 19
2) the smart craft digital gauges are not so accurate (both them?)
3) this morning i found the perfect engine-boat-trim-weight combination!
what do u think about?
Ector
Marlin posted 07-07-2004 09:19 AM ET (US)
Ector,
It seems pretty common for stainless props to show very little slippage when using the prop calculator. My 16" Vengeance gets as low as 0% at some speeds.
Of course, that doesn't really mean the prop has no slippage, it just means that what I'll call the "effective pitch" of the prop is somewhat greater than the listed pitch, perhaps due to the cupping, blade shape/twist, or other factors.
-Bob

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Author	Topic: slippage under 2%?
Ector	posted 07-07-2004 07:46 AM ET (US) yesterday i raised the engines (they were too low making a very big spray and needing to be always trimmed out very much), so today, quite calm sea and a little bit of free time from my job, i had a ride to test the new arrangement... I was alone on board, with 50 gal of gas. I hit a 43,6 knots (49,5 miles) speed at WOT, with 19 props and 5.300 rpm. the speed data is from a garmin gps, the rpm data is from the smart craft gauge of my 150 opti's... putting all the data in the prop calculator this should put the slip value at 1,61% (gear ratio 1.87)... at 3700 rpm, the speed was 30knots (----> slip 3%) i assume this could depend from: 1) the laser II props, running just under water surface, are actually a little bit more pitched of 19 2) the smart craft digital gauges are not so accurate (both them?) 3) this morning i found the perfect engine-boat-trim-weight combination! what do u think about? Ector
Marlin	posted 07-07-2004 09:19 AM ET (US) Ector, It seems pretty common for stainless props to show very little slippage when using the prop calculator. My 16" Vengeance gets as low as 0% at some speeds. Of course, that doesn't really mean the prop has no slippage, it just means that what I'll call the "effective pitch" of the prop is somewhat greater than the listed pitch, perhaps due to the cupping, blade shape/twist, or other factors. -Bob