[Alberg30] dyno and HP with RPMs

[Don Campbell]

George;
    I agree with you completely from a practical point of view and have
used your figures for torque and HP in
Propcalc.xls.(http://www.yosc.ukgateway.net/propcalc.xls)  In order to
obtain torque of 43 Ft-lbs  with HP at 19, required an adjustment to
RPMs to 2300. I used 9600 for the weight and a waterline of 23 feet,
with a desired speed of 6.34 knots, although they suggest it will be
over 7 knots. That used almost the full shaft HP available as the lbs
per required shaft HP were 504 lbs and the lbs per available shaft HP
were 514 lbs. The props that the programme suggested were 6 pitch.
The 2 blade was a 12 - 6
The 3 blade was an 11 - 6. In the alternative, they suggest a 10 - 9
The 4 blade was a 10 - 6
    This programme adjusts things to whole numbers quite quickly, and
from their rule of thumb, the 10 - 9 should probably be 10 - 8.5.
    That means that you have little reserve with the current prop (and I
presume the 6.2 k was 6.2" of pitch but you did not spec the diameter),
if you do  not go above 2300 rpms which is a most reasonable stop for
the RPMs. The 36 HP is the spec from the manual. So after that you would
either adjust the diameter of the prop to reduce RPMs if you can, (one
inch = 450 -500  rpms) or use the mainsail to motor sail.
   I  re-ran the programme using desired speed of 5 knots, and that
changed the pitch to 5" and used only 9 of the 19 HP, but did not change
the prop diameters. Dropping the speed to 6.00 knots dropped the HP used
to 16 for the 6 pitch props.
Don Campbell

 +---------------------------------------------------------------+
 |                This Old Boat by Don Casey                     |
 | http://www.amazon.com/exec/obidos/ASIN/0071579931/alberg30-20 |
 +---------------------------------------------------------------+

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