The Design and Analysis of Hydroprops

The methods behind the propeller design and analysis pages can be used to design and analyze propellers operating in air as well as fully submerged in water. The differences between air and water are reflected by different values for density, viscosity and the speed of sound. Generally, the density of water is approximately 1000 times the density of air. Additionally hydrofoils as well as propellers are plagued by the phenomenon of cavitation. As soon as the local pressure drops below the vapor pressure, the water starts to boil. This means, that the maximum allowable lift coefficient of hydrofoils (e.g. on a propeller blade) is usually much lower than for an airfoil.

Altitude H
[m]
density ρ
[kg/m3]
kin. Viscosity ν
[m2/s]
0 1.225 1.4645E-05
2500 0.957 1.7875E-05
5000 0.736 2.2088E-05
7500 0.557 2.7675E-05
10000 0.413 3.5231E-05
12500 0.287 4.9204E-05
15000 0.194 7.2981E-05

Table 1: Properties of air in the atmosphere.

Depth H
[m]
density ρ
[kg/m3]
kin. Viscosity ν
[m2/s]
speed of sound a
[m/s]
0 999.7 1.3053E-06 1447
100 999.8 1.3043E-06 1448
1000 1000.2 1.2959E-06 1463
2000 1000.7 1.2877E-06 1480
3000 1001.2 1.2806E-06 1497
4000 1001.6 1.2747E-06 1514
5000 1002.1 1.2700E-06 1532

Table 2a: Properties of water (salinity=0%).

Depth H
[m]
density ρ
[kg/m3]
kin. Viscosity ν
[m2/s]
speed of sound a
[m/s]
0 1078.7 1.5105E-06 1572
100 1078.8 1.5096E-06 1573
1000 1079.1 1.5018E-06 1587
2000 1079,6 1.4942E-06 1602
3000 1080,0 1.4876E-06 1617
4000 1080,4 1.4812E-06 1632
5000 1080,8 1.4778E-06 1647

Table 2b: Properties of water (salinity=10%).

Design

Power and thrust of a propeller depend directly on the density of the fluid into which the propeller is submerged. As the density of the fluid can be changed on the Options card, you can design a propeller without any special specification. Typically, seawater has a density of 1025 kg/m. This means, that a propeller, designed for air, will produce 1025/1.225 = 836.7 times the thrust when it is immersed into water. The same factor applies to the power consumption.

In order to avoid cavitation, the design lift coefficient should be fairly low, depending on the thickness of the airfoil and its velocity (pressure) distribution. Also, the vapor pressure depends strongly on the water temperature. As a rough rule of thumb, lift coefficients in the order of 0.2 (thick airfoil) to 0.4 (thin airfoil) should not be exceeded. This will lead to wide chord blades as they are common in marine applications.

Analysis

The analysis module can be used without changes. The resulting power and thrust coefficients can be used to calculate the power and thrust for any given fluid density.

See also

[Design a Prop]

[Considerations for Hydrofoils]

Last modification of this page: 21.05.18

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