Propeller for electric conversion of 25ft (Albin 25)

Hi

I'm trying to see if it's feasible to convert an Albin 25 (specifications) to electric propulsion with batteries and solar.

The goals are to be able to do 6 knots for 8 hours one one charge.

From the linked pdf above I've deducted that the original propeller need 3,73kW to propel the boat at 6 knots, 1900rpm. The pdf also say the propeller is 58% efficient.

In order to get the most range out of the batteries I figured that the boat need a new propeller with higher efficiency and that's better suited for the higher torque of an electric motor.

What propeller would you suggest?

 

You can play around with JavaProp to see how you can improve on the prop there.

I have attached two screen dumps from JavaProp. One shows the current situation. The result shown will be slightly higher efficiency than what you will get with a conventional marine prop because it is a higher lift section. The important parameter is the derived thrust.

You will see that for the design conditions the thrust is 804N. Looking at the chart I estimate the engine rpm is more like 1600rpm at 6kts rather than 1900rpm. Hence I am using 800rpm on the prop. This gives pitch of 300mm.

Now taking the 804N thrust you can play around with diameter, speed, number of blades and foil sections to see what is the most efficient. The diameter is going to have the biggest influence. I have shown what power would be required for the same thrust if the diameter can go as large as 500mm.

You need to determine how big the prop can be. At these low speeds and power you do not need to be too concerned about prop clearance to the hull.

For eight hours you are going to need about 35kWh of batteries. That is a large battery. Somewhere between 400 to 800kg of batteries depending on the type chosen. You can remove the heavy motor and fuel but the battery will be heavier for 8 hours so more drag.

Rick W

 

Electric Albin 25

I have done a prototype electric conversion on an Albin 25 and have gathered some preliminary data. If you are interested in comparing notes, please contact me at TheWireGuy@aol.com.
Jim McMillan