Dean,
In general the idea is to use a large
diameter prop turning at a slow speed, it'll produce more thrust and be much
more efficient. According to Dave Gerr in "The propeller handbook", the
ideal is for the prop diameter to be one third the beam of the vessel. To give you an idea how much of a difference
diameter makes, I'll quantify with an example: 1 HP at the shaft could
produce 39.5 lbs of thrust with a 6" prop, or 62.7 lbs with a 12" prop.
Does it really have to be direct drive, or could you use say belt
drive to slow down the rpms?
Just to recap the design steps I went
through to answer this same question on my own project, they
were:
1) Determine largest diameter prop I can
fit. In my case the limiting factors were interference with the surface,
and the desired draft. My prop ended up being 16".
2) Next, calculate the most efficient
RPM. You can get it from:
rpm^0.6 = (632.7 * SHP^0.2)/D
where
SHP = shaft horse power
D = prop diameter
In my case, I estimated SHP to be 90% of
the motor's rated continuous power, which led to 652 rpm.
3) Estimate propeller slip. Your prop
isn't slicing through a solid, so it'll slip. How much depends largely on
how streamlined the boat is. The normal practice in ship design is to base
this number on comparison to similar vessels -- not easy to do for a sub,
where I had no data at all. For a displacement hull surface vessel a
normal value might be 45%. I increased that to 60% slip, because a PSUB
is far less hydrodynamic than your typical boat. However my value is a total
guesstimate.
4) Estimate the vessel's theoretical
speed.
Speed in knots = [10.665 /
(Displacement in lbs / SHP)^0.3]*(waterline length in feet)^0.5
Translate to mph multiplying by
1.15
In my example, I get 5.68
mph
5) Calculate the ideal propeller
pitch
Pitch in inches = mph * 1056 / (rpm *
(100 - % slip)/100)
In my case, pitch = 22.9"
Now, because I'm planning to use a Kort
nozzle, I reduced the pitch quite significantly, to 18. Then I went on eBay,
and found a Michigan Prop 16 x 18. I can't remember how much it cost me, I
got it a while back -- but it was probably small change compared to what
they cost new. Another detail, it's what they call a "cupped" prop which
refers to the shape of the blades. Cupped are most efficient for slow
speeds, so that's an added bonus.
I realize
several parameters are estimates. My objective is to get into the
ballpark of the correct values, and I do fully expect to play around
with different pulley combinations to fine tune shaft speed.
Hope that helps.
Alec
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From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of
Recon1st@aol.com
Sent: Sunday, February 08, 2009
14:08
To: personal_submersibles@psubs.org
Subject:
[PSUBS-MAILIST] buying a propeller
I am in need of choosing a propeller.
Any suggestions on what to consider? cost is for sure a
factor.
I am not gear reducing at this time so I am going to be in
the
1500 rpm range. I will be kicking up to 12hp and should have a
range
between 1 and 2000 rpm.
My overall dry weight should be about 6000lb
Any help greatly appreciated.
Dean
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