# AUV propeller question

Discussion in 'Props' started by Rocket888888, Mar 2, 2018.

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1. Joined: Mar 2018
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### Rocket888888New Member

Hello. I'm trying to generate 10kgf at 4kts from 300W shaft power (it is to power a smallish AUV). Max torque is *probably* about 3Nm and diameter <300mm.

Don't really have any practical experience of propulsion though apart from what I have devoured from Gerr and Carlton textbooks so please bear with me!

I have some series data for things like the B-series and ducted Ka (and a very helpful manufacturer willing to make whatever I request!) and have identified potential propellers from both those series but I'm kind of making it up as I go along and wondered if there was either a) a more comprehensive database somewhere (either for sale or open source) of Kt/Kq/J or simply b) a propeller that wiser people here would recommend I look at.

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### jehardimanSenior Member

You have Bp-sigma diagrams?
Lets use them.
10 kgf = 98.1 N
4 kts = 2.05 m/s
98.1 * 2.06 = 201 Nm/s = 201 kW = effective Horsepower desired
300W = 0.402 hp = delivered Horsepower
minimum eta = 201/300 = 0.66 which is reasonable for a open B-series but a little high
for a ducted design point (Bp > 30).
Let's assume you want high efficiency so we will start with a B-series 2.38 with assumed Bp = 5
Bp = 5= n (0.402)^0.5/(4)^2.5
n = 5(4)^2.5/(0.402)^0.5 = 160/0.634 = 252 rpm
D = 300mm = 0.984 ft
sigma = 252(0.984)/4 = 61.9 which gives a P/D > 1.4 so wee need to increase Bp
Quickly optimizing we get
n = 755 rpm
Bp =14.95
sigma = 185
eta = 0.68
P/D = 0.8
So it is possible to use a 300mm dia B2.38 with a P/D of 0.8 to power the AUV. Your problem now becomes gearing the motor to deliver 3.79 Nm @ 79.1 rads/sec with 0.98% efficiency.
Looking at a B3.50, eta is approximately the same but P/D increases to 0.82 while n drops to 650
A B4.40 seems to offer the best solution with a P/D =0.97, n= 550
Improving the aspect ratio of the propeller area (i.e. more thinner blades) will help, but then you are into a custom prop. If you are willing to go to a special design, then the polynomial coefficients for the B series can be found in the 1988 edition of PNA Chp VI, Sect 8, Tbl 17.

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### gonzoSenior Member

I think the first calculation is whether the AUV can attain 4 knots with 300W.

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### jehardimanSenior Member

Yes, the first lines are to see if it is possible to attain 201W effective HP with 300W delivered HP. Based on what I saw during the few minutes I spent on this, it seems like a class problem, the requirements kept falling near the max efficiency line for the standard Bp-sigma charts. Nothing really strange. But it really needs a custom 2x2 counter-rotating prop set to optimize 300W dHP.

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### gonzoSenior Member

I meant that the AUV may be the size of a torpedo and will need more than 300W to reach 4Kt.

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### jehardimanSenior Member

Oh, I see your point. But the OP started this thread with a required thrust of 10 kgf @ 4 knots so I assumed he had already made that calc. FWIW, 300W @ 4 knots for a 21" (~525mm) diameter torpedo sized AUV (based off a stated prop of 300mm)