# Shaft-generator on Tall ship

Discussion in 'Props' started by Nescio, Jan 28, 2020.

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### NescioNew Member

Hello,

Aboard a tall ship we want to use our generators less and charge our batteries at night by means of the wind (thus extracting power from the shaft).

We have a three blade propeller with a diameter of 1.5 [m], pitch of 0.92 [m] a J of 36.5 and a Fa/f of 56%. I assume its a Wageningen B-series since the prop is from 1978. The maximum RPM during sailing is 450 and when steaming 375.

Does anyone have data about how much power we can extract during sailing and at what RPM. This data is crucial for the selection of a shaft generator, we're thinking of a 3kW permanent magnet since these generators work at low RPM.

Regards,

Marco

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### gonzoSenior Member

Is the propeller fixed pitch or adjustable? You also need to consider the wear and tear on the transmission if it is turning continuously.

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### bajansailorMarine Surveyor

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### RooieDirkNew Member

The max power you can get is not depending on rpm (as long your generator is making enough, but that is only gearing) You can calculate the power available with
P = 0.5 x 1025 x 16/27 x V x V x V x pi x r x r
Where P = power in Watts, V = speed in m/s, pi = 3.14 and r is radius of prop.
A quick calc gives for your 1.5m prop at 6 knots a power of 15kW (in theorie;-) )
With a lot of losses in gear etc 5kW at 6knots should be possible.

The formula is the same as calculating the power for windturbines, only the specific weight of water is much higher as that of air.

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### DCockeySenior Member

That equation looks like it is based on actuator disk theory or similar. Such a formula will provide an upper limit of the power which can be extracted based on diameter and free stream velocity. How much of that limit which can be achieved will depend on the propeller characteristics.

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