I would bet your re pitched prop has an inefficient shape.


"Steve Lomax" wrote in message
...
I took 3 Aluminium props on the lake today and compared them. No special
hi-tech gear other than a stopwatch and the standard helm instruments.
FWIW,
I Thaught I would share the results:

Boat: 1996 Bayliner 2052 (20' cuddy) 2 adults, 2 children, full fuel
tank etc
Engine: 4.3 LX mercruiser with Alpha 1 gen II outdrive.
Lake: Windemere (fresh water)
Weather: Warm, dry, steady breeze

Prop 1: 14 1/4" X 22" (originally 21" re-pitched to 22") re-conditioned
Prop 2: 14 1/4" X 21" Tiny scrape on tip of one blade. 1/4" sq missing
Prop 3: 14 1/4" X 19" good (but not perfect) condition. re-conditioned

Engine 22" Prop 21" Prop 19" Prop
RPM MPH MPH MPH
4.5K 39 42 38
4.0K 33 38 32
3.5K 30 30
3.3K 22*
3.0K 22* 22* Not Planing

* just on plane

From rest 7.8 secs 6.7 secs 3.9 secs
to plane
(approx 22mph)


The recommended WOT max RPM is 4800. At WOT the engine will considerably
exceed 4800 (max RPM) if allowed to with all props. The 22" is not as fast
as the 21". Could this be due to 'slip' or cavitation? There is not much
difference between the speeds of the 19 and 22 but the 19 is on the plane
in 1/2 the time. Again is this the 22 'slipping'? I cannot percieve any
evidence of, or can feel cavitation from the helm. Would a 4 blade prop be
better? Is any improvement likely to be only marginal?
The speed for a given RPM was 3 MPH lower when travelling into the wind.
the
above figures were the average runs into and with the wind..

I understand that the boat is supplied with a 19" as standard.


Steve