[MHml] outboard update

[Bill Gibbs]

As you may recall, I've got that ex-beach cat pusher boat project still
kicking around.  I finally found time to get a crew to go out and test a
variety of props in similar conditions yesterday.  By the time we hit the
water we had missed the no-wind hours of the morning and a nice sea breeze
came in that stayed 8-10 all day.  Unfortunately the previous day's front
had been blowing 30+ into the evening, leaving a lumpy 2-4 foot seas with an
occaisional bigger swell, and waves not angled with the current wind.  We
did runs roughly 90 degrees to the wind with no sails, but this resulted in
an "into the seas" leg, and a "with the seas leg".  Nothing like a little
surfing to screw up prop speed evaluations.  It was too rough to swap props
at sea, so we ducked back into the habor and docked at the pump out station
to swap props.
Afterburner weighs in at 10,050 pounds, not counting the pusher which has a
Suzuki 175.  We tested three props:
 
1. Alum 3 blade 16" dia, 10 pitch
2. Alum 3 blade 16" dia. 13 pitch
3, SS 4 blade 15.25" dia 19 pitch
The diameters produce about a 9% area difference.
 
Going in, expert recommendations said the SS was over pitched a lot and the
10 pitch s/be interesting.
Averaging all the data together produces:
 
Prop 1 max speed at 4,400 rpm of 8.9 knots.  Higher rpms produce speed loss,
and erratic rpms...  cavitation
Prop 2 max speed at 4,400 rpm of 12.1 knots. Higher rpms produce speed loss,
and erratic rpms...  cavitation
Prop 3 max speed at 5,000 rpm of 14.8 knots.  Motor maxes out at 5,000, a
bit below its spec'd WOT of 5500-6100
Perhaps similar to flatwater speeds, as into the waves was slower, and with
the waves was faster.
 
Looking ahead I may just drop the 4 blade pitch to 15", to get the WOT into
the spec range.  There is a 16" 4 blade and a 15.25 dia 5 blade to consider
as well.  If I can just test them without buying them.
 
After crashing into waves at 14+ knots for a while, I'm thinking that's
plenty fast for transiting.  It seems more tense and stressed than sailing
at these speeds. :-)
 
Bill
Afterburner
 
 
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