marine propeller

1. Marine Propellers
By,
Anand P Gaikwad
Sujith Patil

2. Propeller (Thruster)
• A propeller is a type of fan that transmits power by
converting rotational motion into thrust.
• A pressure difference is produced between the forward and
rear surfaces of the hydrofoil-shaped blade, and a fluid is
accelerated behind the blade.

3. Basic Nomenclature:
• Hub
• Blade
• Blade Root and Blade Tip
• Blade Face and Back
• Leading and Trailing Edges
• Diameter
• Pitch Propeller Geometry

4. Number of blades
The choice of the number of blades is one of the first decisions to be made in the
design of a propeller.
• Two blades: It requires very large diameter to get the blade area required for
effective thrust.
• Three blades: it has been proven to be the best compromise between balance,
blade area and efficiency.
• Four or five-bladed propellers and even more blades are useful for two
reasons. First, their extra blades create more total blade area with the same or
less diameter. 4 blades propellers, however, would seldom be as efficient as the
three-bladed because the closer blades create additional turbulence, literally
scrambling up each other's water flow.

5. Figure represents propeller efficiency w.r.t no. of blades

6. CAD model of 4 bladed propeller

7. Propeller calculations ( 4 – bladed )
Known values,
• Break Power, PB = 0.38 Hp = 288 watts
• Ship speed, VS = 2 m/s = 3.88 knots
• Rotation per minute, n = 1500 rpm
i) Speed of advance,
VA = VS ( 1 - w ) [ w wake friction = 0.15 ]
= 3.88(1 - 0.15)
= 3.298 knots = 1.696 m/s
ii) Shaft power,
PS = PB nS [Where, nS is the shaft efficiency and have value of
= 0.38 * 0.97 0.98 for ships with engine located aft and 0.97 for
= 0.368 Hp ships with engine located amidships. For the
purpose of this work 0.97 was used.]

8. iii) Power delivered,
PD = PS * nS = 0.368 * 0.97
= 0.357 Hp
iv) Power coefficient,
BP = PD
0.5 n = 0.35 0.5 * 1500
VA 3.29 2.5
= 42.85
From the chart of type B series of 4-bladed,
The value of BP = 42.85 is read. The point of intersection between the BP
line and optimum line (in red line) was traced to get ,
( P /D ) = 0.66, no = 0.55, δopt = 256, AE/ Ao =0.55, to/ D = 0.05

9. v) Propeller thrust,
T = PD no = 0.357 * 0.54
VA 3.29
= 0.0585 N
vi) Optimum diameter,
D = δopt VA = 256 * 3.298
n 1500
= 0.56 ft = 6.72 ins
vii) Pitch,
P = 0.66
D
P = 0.66 D = 0.66 * 6.72
P = 4.435 ins

10. vii) Maximum blade thickness,
0.05 = to
D
to = 0.05 * 6.72
to = 0.336 ins
viii) Hub diameter,
d = 0.18 D
d = 0.18 * 6.78
d = 1.20 ins

11. Non-Dimensional Characterization of propeller performance:
i) Thrust coefficient,
CT = 2T = 2 * 0.0585
ρA VA
2 1000 * ( 3.142 /4 ) 0.170 2 * 1.696 2
= 0.0179
Ideal ( actuator disc ) efficiency = 2
1+ ( 1+ CT )1/2
= 2
1+ ( 1+ 0.0179) 1/2
= 0.995
= 99.5 %

12. vii) Advance coefficient,
J = VA = 1.696 [ n = 1500 rpm = 25 rps ]
n d 25 * 170
= 0.339
Absolute advance coefficient,
λ = VA = J = 0.339
π D n π 3.142
= 0.127
From Kramer’s diagram,
Ideal propeller efficiency = 0.99
= 99 %

13. Conclusion
The actual propeller is much less efficient than either the ideal propeller
or the actuator disc result.
< <
55% < 99% < 99.5%
Actual
propeller
efficiency
Ideal
propeller
efficiency
Ideal
actuator
efficiency

16. THANK YOU