Cheers Scott,
As I said elsewhere:
Overall system efficiency of direct drive can be better than a geared drive. My Lynch direct drive system is great on my 3.5 ton sailing yacht because it means it is simpler, smaller, lighter, cheaper and more efficient overall for my cruise speed than an equivalent geared drive.
I am constrained by space and at the original install about 4 years ago, by money - so running a 24V direct drive meant less batteries than geared which meant less cost etc., which all fitted with my criteria originally of repowering for the same money or less, the same weight in the same place or less compared to a diesel repower. It all comes down to how much power one requires, continuous and peak and the variables of money, space, weight and system efficiency.
With direct drive using a Lynch or Motorenergy type motor - It's a case of choosing a motor with a high torque per amp and therefore low rpm per volt married to a low pitch, small-ish diameter prop, which keeps RPM (volts) up and amps down, balanced against prop efficiency for ones cruise speed and to ensure peak power is sufficient. Such a system is classed as an auxiliary drive for a sailing yacht, as in my case it is a 24V system. This will not suit the majority. It does me, especially now I have 2 x 200 Ah Lithium batteries. Good enough for nigh on 20 miles range at 4 knots in calm conditions.
Here are my measured results comparing bollard pull v open water electric and my old 10hp diesel.
http://john.rushworth.com/Elektra_Performance_June_2016.xlsx
John R
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