[Alberg30] Prop pitch

[Scott Maury]

i have run a 10x6 three blade for three years on an A30-at4.... I can get
the engine to 2000rpm but really the max thrust is at 1850rpm... I find
minor pull to the left and much less plug fouling... Im happy with it
.scott

-----Original Message-----
From: public-list-bounces at alberg30.org
[mailto:public-list-bounces at alberg30.org]On Behalf Of Don Campbell
Sent: Tuesday, April 08, 2003 12:03 PM
To: public-list at alberg30.org
Subject: [Alberg30] Prop pitch


For Bill Blevins;
    I have just done some research on prop pitch for both A22 and A30
models and it is surprising how far out we have things on most boats. I
used the resources of Michigan Wheel (and others) and they use a couple
of things most of us forget, the first of which is that there should be
15% of the diameter of the prop slot left open. Thus the max prop they
suggest for an A30 is 10" diameter. The pitch depends on the motor
characteristics but for the Atomic 4 gas it was a 10 - 5   3 blade
sailer.
    Janski has done considerable with MB as well,  for Viva II and he
has purchased a MP-2  13 - 6   2 blade from Michigan Bronze to replace
the original 13 - 7 Columbia prop. There is about twice the surface area
on the blades of the MP-2 as on the Columbia. Unfortunately, he was
unable to get Viva II into the water last year to try this prop but has
plans to get her going this year  if we ever get rid of winter up here
in Canada.
    I have managed to find an Excel (TM) prop calculator thanks to Jose
Campione in Ottawa at (http://www.yosc.ukgateway.net/propcalc.xls) which
you might enjoy because you can simulate various situations without
having to go onto the water,  and get results for  for 2, 3 and 4 blade
props. This programme has a 55.94% slip built in to every calculation,
but also shows the theoretical pitch calculated before slippage is
considered.. I used increased displacement to simulate increased waves
and headwind.
    The other rules of thumb that most use for props are that 1" of
pitch = 200 rpm on the motor and 1" of prop diameter = 2.5" of pitch.
Motor performance is key to getting the correct prop.
    Net result:  Props are a compromise just like the rest of the boat.
Don Campbell, White Opal # 526

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