Prop dia and pitch

From: Bill Blohm (bblohm@hpbs1686.boi.hp.com)
Date: Mon Mar 13 2000 - 10:48:01 PST


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Strictly FWIW. If you're not interested in a neophyte non-designer
possibly in over his head trying to figure out the designed prop for a 4
HP on a P-19, skip this.

Not to continue a dead thread (I think :-), but merely as a point of
possible interest on these two factors of prop pitch and diameter....

Over the weekend, I dug out my Skene's Elements of Yacht Design and was
reading in there about props. There were a couple of interesting items
I'd like to share, for whatever they're worth. (Probably nothing. ;-)

One was that supposedly a small change in prop diameter will do more
good than a large change in prop pitch will.

The other was an attempt to design my own prop for the P-19 with a 4 HP
Suzuki. I used the included charts, as well as I could since they don't
go down to the necessary small and slow values, and attempted to work
out the design prop for the P-19. It was an interesting result I got.

With a Suzuki 4 HP, I currently have a 7.5" x 6.5" (dia x pitch)
3-bladed prop. Working it out using the material in Skenes, I should
have a 4.9" x 3.26" three-bladed prop!

So much for the math! I had done this with the prop clear value of .9
for computing the speed of water past the prop, no bearing loss, 1:1
reduction gearing, and assuing shaft RPM was the same as engine RPM.
Those of you that might be interested, small boats are supposed to use
.46 (I think) instead of .90 (I found out later I'd missed this). Using
.90 gave me a pitch ratio of .66, efficiency of .36, and a delta of 425.
I don't know just what all these mean, exactly, they're just here for
anyone that might be interested in the numbers used. Oh, and my
manufacturer's recommended full throttle RPM is 4500 - 5000 RPM, so I
used 4750 RPM. Skene's lists the hull speed as 1.25 * sqrt(DWL) which
worked out to 5.1159.

I wanted to try to do it again, this time using the .46 (I think it was)
indicated for small ships instead of the .90 for determining the speed
of the water past the prop. I'm afraid, though, that the chart won't go
down far enough for the speed of water at prop to 2.5 power. With the
.90 I'd had a speed of 4.6 knots and the chart starts at 5.0! The .46
(it was somewhere down there, possibly even lower) gives me a speed of
water at prop of 2.3552 knots and the chart isn't linear, at least not
that I can tell, so I can't extrapolate this low.

I enjoyed working the math, it was fun, and I'd have liked to have the
formulae for the charts instead of having to use the charts. At any
rate, I think I'm going to toss that 4.9" x 3.26" prop idea out. :-)
Skene's does mention that you still have to adjust the designed prop
specs based on experience and a couple other factors, anyway. :-)

Bill B.
P-19 #454, "Dream Catcher"
Nampa, ID



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