Hi Jim,
With your 7,400 lb displacement, you should look for a drive with a minimum of 3.7kW capacity. Displacement is a much better indicator of power requirements than boat length. The same recommendations on drives and batteries that I made to Jason are still applicable.
A two bladed prop will work as well as it did with the diesel, if you gear it to keep the same prop speeds. I don't think that many people here have custom props, just different ones from a regular catalog. You can peruse a number of options at Deep Blue Yacht Supply http://www.deepblueyachtsupply.com/ . I ordered my new prop shaft and flange from them, I already had my 13 x 15.5 four bladed prop from a local prop shop to replace the 13 x 10 two blade that was already on the boat. You can see both props side by side in "Eric's Serenity" photo folder in this group
http://groups.yahoo.com/group/electricboats/photos/album/1967401930/pic/list
Here's a quick excerpt from a previous post that I made back in August concerning prop slip of my old and new props...
"With the old prop and the diesel engine with 2:1 reduction, engine rpm was about 1200 (shaft speed of 600) at 3kts and 3200 engine rpm at 5.5 kts. With the new prop and electric motor with 2.55:1 reduction, the motor speed is 830rpm (shaft speed of 325rpm) at 3kts and 1900 motor rpm at 6kts. Doing the calcs, the old prop showed 39% slip at 3kts and 58% slip at 5.5kts. The new prop calcs to 27% slip at 3kts and 37% slip at 6kts."
So, of course you can use your old prop, but there is definately opportunity for efficiency improvements if you get a prop with more pitch, the electric motor can handle the slow speed load better than the diesel. My motor will pull smoothly from 200 rpm (prop speed 78 rpm).
Gear boxes like the Browning are generally considered to lose less power to friction, but there are some synchronous belt drives, like the Goodyear Eagle NRG systems, that are close (before you add the losses from the requisite bearings that you would need for a complete drive). Belts are quieter that gear boxes, but my Browning is pretty quiet.
There are lot's of options and it's your money, you get to chose what best for you.
Fair winds,
Eric
Marina del Rey, CA
--- In electricboats@yahoogroups.com, "Jim" <jmills500@...> wrote:
>
> Greetings!
>
> This is a GREAT thread with key questions to be answered for the Bristol 27.
> My questions are basically identical for a Tartan 27 (7400 lbs displacement
> with an Atomic 4 removed).
>
> So, can the existing 2-flute prop be used? The prop is "hidden" by lining up
> (vertically) with the keel in the aperture while using sails for power (as
> you already mentioned). If the prop RPM range is maintained as it originally
> was with the gas/diesel engine, is the prop "useable" or is it always a
> forgone conclusion that a custom prop is required? (and I don't have a clue
> how expensive one of these custom props are).
>
> Also as mentioned, the electric motor's reduction can be accomplished with a
> Browning gearbox attached to the "face" of the motor or with some type of
> belt-driven system. I don't know which would be most "efficient" as far as
> power losses are concerned, though.
>
> Finally, I always thought that the higher voltage configurations would
> always be better because the motor would draw less current though the power
> cabling (allowing smaller gauge electrical cables to be used and less
> current running through the motor windings). But, as you said, the higher
> voltage systems require more batteries.
>
> Great post!
>
> Jim Mills
>
> jmills500@...
>
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