Hi John,
Dave's sheet is helpful but you'll find the speeds predicted out by half a knot or more. The voltage and current will be reasonably accurate but your loaded motor rpm will be lower than the usual quoted no load rpm/volt. Anyhow on a boat your size you could easily go direct drive but then your prop p/d effectively becomes your gearing.
As you know as big a diameter as possible with as low shaft speed as possible is best for efficiency but if you are restricted by diameter then you have to play with blades and pitch and gear ratio.
For a cruise speed it would be ideal with lead acid to be around .2 to .3 of battery capacity current draw or as close to the 20 HR rate as you can to reduce Peukert effect.
Diameter and pitch are related to efficiency and generally that increases a little from about .8 to 1.2 p/d ratio. The downside subject to gearing is you reduce voltage and go up on current as you increase pitch, ie amps/torque.
It's all about balance v bollard pull v static cruise speed in open water v top speed in calm waters without exceeding the continuous motor current rating at highest continuous in choppier waters.
On my 4 ton, 26ft boat with direct drive I've played with 12x10 2 blade, 12x14 3 blade and settled on 12x11 3 blade for my needs. The key is to get your motor running at peak efficiency at constant cruise with the lowest shaft speed possible. In your case a 12 by 14 should be a fine starting point with 2:1 reduction. My approach was to use a motor with the highest torque per amp which rpm per volt is related to, but doing that you have to be careful you don't run out of volts/shaft speed as it were. No real need for an electric specific prop but if you do talk to James at Electroprop. If I needed or wanted to experiment more I'd try an overdrive folding 2 or 3 blade prop which you can kick into effectively two different pitches/shaft speeds.
Hope that helps and see my spreadsheet on my Elektra Yachts Facebook page for real figures. I have my 12 X 14s too but way too many amps without gearing. A final note, I now run 2 X 200 Lithiums as for my 24v direct drive Peukert Ah reduction was too much with lead. The voltage stability under load of the LFPs is amazing as is negligible Peukert.
John R.
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