One my 21 Scarab which had a 350 Chevy engine was purchased with a 14 x 21
aluminum prop which maxed out at 5000rpm which was too much. I installed a
Merc SS 14-3/8 x 21 and rpm dropped to 4700rpm and MPH went up by 4. Do you
have any friends with SS props who will let you try them even if they are
running more pitch than you need it gives a base line on which way to go in
pitch?


"Steve Lomax" wrote in message
...
I am thinking in terms of buying a SS prop but I would like to be sure of
getting the right size. As the engine will conbsiderably over-rev, I would
like to use this power to get a prop that will give me WOT at 4800 RMP. At
the moment the best I can get is 3/4WOT. It seems a shame to have the
power
and not use it. I agree that an extra 5mph is probably neither here or
there
but I feel that there is enough unused 'grunt' to get at least 10mph (25%)
,
or more significantly, better fuel efficency - it costs nearly 100 UKP
to
fill the tank. (Yes.... I know, the deeper the draught, the bigger the
overdraft!). I'm getting quite adept at prop changing in the water, and
dropping to the 19" for skiing is not so much of a problem, so getting
speed
and economy for cruising is probably the order of the day.

Steve

"Woodchuck" wrote in message
...
The 22 is one prop I would toss in the can! But the question is: "What
are
you trying to achieve"? Consider this -going with a SS prop may net as
much
as 5mph...


"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