How much slippage do you account for? In my post I allowed for 20%. (80%
efficient) I thought the general consensus was about 10%.


"IBNFSHN" wrote in message
news:c7pZc.75270$wo.17696@okepread06...
You are not accounting for slippage which all props do. A 20 pitch prop
will
move forward 20" in and ideal medium. Water is not an ideal medium.

--
Bill
Chesapeake, Va


"calhoun" wrote in message
...

"Josh Assing" wrote in message
...
1st I want to say thank you to all that have chimed in with advise &
input.
THANK YOU.

Here's the info I think everyone wants:

1990 AM210 Larson

According to Larson:
Came with a 3blade 19 pitch prop (alum)

According to Larson & Mercruiser:
200hp v8 5.0l engine

According to Mercruiser:
WOT RPM goal: 4200-4600

Currently has a 4 blade 20 pitch prop (alum)

Flat water:
I can, at WOT, achive 4600-4800 w/o trim @ about 30-34mph

Trimming the outdrive -- I get 5400 RPM at about 43-48mph

w/ waves -- I can't trim it that much -- as the prop gets too much
air -
and can
achieve 4800-5000 RPM @ 36-43 mph

so it seems I need to go up at least 1 in pitch -- maybe 2?

Any thoughts/input/ideas?
I've read that a 3 blade is more efficient; but more vibration is
felt.
And
that you need different pitches based on what you want.


I want a good "all around" prop -- I'm not planning on towing or water
skiing;
but I would like to plane quickly; and have good power/speed with "bow
down" in
waves.

What would my choices be for 3 vs 4 blade prop?

THANK YOU for your time & input....

Cheers
-josh

What are you determining speed with? 30mph at 4600rpm with a 20 pitch
prop
seems slow. (lots of slippage. If you know your final drive ratio you
can
calculate your max speed at any rpm with your prop. Just remember a 20
pitch
prop moves forward 20" in every revolution. With a 1.6 ratio (I don't
know
yours this is just an example) final drive your max speed would be about
55
at 4600rpm. Even at an inefficient 80 per cent you still should be doing
over 40.