Hi folks, Been following this thread- Agree it's a good one. But while efficiency is a useful tool for development, it's varies greatly in practice. Look at the Torqeedo efficiency curves here http://www.torqeedo.com/us/hn/background-knowledge/performance-indicators.html . What you see is that efficiency is anywhere between zero and a maximum value even with fixed hydrostatic resistance parameters. In reality, there is also quite a difference in efficiency from resistance parameters; different boats, different loadings, even different weather and sea conditions. So it's not surprising to see some customers happy and others unhappy with a single product based on their particular set of use-conditions. Inboard guys know this and it's why they will go through several props until the motor operates within its most efficient rpm range. This is called impedance matching.
For example, our Electric Paddle, in a tank that matches the hydrodynamics of a Necky 16 ft kayak, has the same efficiency as the Torqeedo 401s stated efficiency/ power curve when tested in their tank. If you have a dinghy that is shorter and fatter, than efficiency drops. If you point that same dinghy downwind, the efficiency goes up even if you account for the wind lift. This is a rule of physics and it's reality for all motors and brands. What is true is that both Torqeedo and Electric Paddle match impedance to higher speeds on more relevant boat hull shapes using advanced technology so they are very light for the available power and range. Trolling motors impedance match to a bassboat at very low positioning speeds and use lead acid technology and that's why trolling motors are much heavier and less efficient as main propulsion.
Guess what I'm saying is that the new wave of efficient motors are truly well-designed to provide performance but this performance will vary based on what kind of boat you use. If you try one and it's not keeping up with your expectations, talk to the makers. They should be able to either satisfy your needs with a prop change or take the motor back.
I've started a new forum just for electric outboards. The electric boating group is expanding and with that it must segment. I hope to make this new forum useful to customers and makers of of all brands of electric outboards as well as to folks converting gasoline outboards.
http://groups.yahoo.com/group/electricoutboards/
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