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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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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