Need prop diameter help.

[Commander 147]

I'm trying to work through all of the challenges to converting a commander to an inboard electric drive. I'm concerned that the motor I'm leaning towards does not turn at a fast enough speed (at 1200 RPM at full throttle) to allow for a prop small enough to fit in the keel/rudder cutout area. Can someone here tell me what is the largest size prop you can fit in a commander or ariel? Part of the problem is that with an electric motor the faster you travel the shorter the distance you can travel. I want to try and get to 3 knots at 40% of full throttle or at 480 RPM to help maximize the distance I can travel on a fully charged battery bank.

[mbd]

I've got a 13 x 11, 3-bladed prop on my Ariel an am probably at about the maximum size you could fit without modifying the cut-out in the rudder. I can't offer you any hard numbers on my RPMs because I still need to fix the tachometer. :o But I'm pretty sure I run well below the optimum RPMs for my power plant.

Here's a thread with all the gory details....

http://pearsonariel.org/discussion/showthread.php?t=1233

[Commander 147]

That thread you gave me the link to helped a lot. I really appreciate your reply. :)

[bill@ariel231]

i"m running a 2 blade 12x12 with my westerbeke. the original was a 13x8 for the atomic 4.

[Commander227]

Heres a link to a pretty good propeller calculator you can download;
http://www.castlemarine.co.uk/pitch.htm
One I really like is an Excel spreadsheet.
Google "Surfbaud Freeware Propeller Calculator"

Good rule of thumb #1. prop diameter should fill 80% (or less) of the hull aperture.

Good rule of thumb #2. for displacement hulls, assume 55% slip when calculating pitch.

I also recommend David Gerr's Propeller Handbook "the complete guide to choosing, understanding and installing boat propellers."

[Commander 147]

First Bill

Can you tell me what RPM the two motors ran at when at idle and full throttle? Also what is the HP rating for the two engines?

And Commander 227

What prop are you planning to use with the Mars brushless motor?

[bill@ariel231]

A-231’s engine is the Westerbeke 20B2 (18hp) driving the 12x12 prop thru a Hurth HBW50 gear box (2.05:1). I hit cruising speed at 2000-2200 rpm depending on the growth on the bottom or approx 1000rpm to 1200rpm at the prop. in hind sight, a 12x8 may have been a sufficient for my boat but my setup mimics a Cape Dory 25D that I used a model for my repower. In any event, i'm not planning to make any changes to my prop.

I have no direct experience with the Atomic 4. Looking at a couple sites like (http://www.hessmarine.ca/a4specs.htm) , it appears the Atomic 4 came in a couple variants; I assume the base model was used in the early 60’s Pearsons. If so, expect 30hp at 3500rpm (or 18hp at 2500rpm) with half that at the prop with a 2:1 transmission.

Idle for both should engines should be 600-700rpm.

good luck with your project. your motors 1200rpm available to the prop matches both of these inboards. If you are running the electric motor near it's limit, you might want to think about the finer pitch props 12x8 or 13x8 in order to reduce the torque load on the motor.

Is there a published torque curve for the motor?

[Commander 147]

Thanks Bill for the additional information. The electric motor produces significantly more torque than an internal combustion engine. And the torque jumps up high and stays up there through most of it's operating range. Take a look at this chart to see what I'm talking about. If we set this motor up with a 2.5:1 reduction it will produce at the prop 200 ft lbs. of torque at only 5.1 HP

[bill@ariel231]

It looks like torque is no issue; do you feel for the amount of heat you will need to dissipate at the top end of the rpm curve?? I'm assuming it's is some fraction of the wattage for the speed you will run (say 30%??)

[Commander227]

And Commander 227

What prop are you planning to use with the Mars brushless motor?

I'm starting with a 12X8 two blade.

[Commander 147]

Bill

I had not yet considered the heat portion of the equation so I am glad you asked that question. However while I need to have a good idea of what to expect as far as heat I will probably not spend much time at all at the upper end of the RPM spectrum. The reason is that the faster you push the boat the shorter the distance you can travel before the batteries are drained.

Today I spoke to Gregg at Thunder Struck Motors about the conversion. He sent me via e-mail an Excel spredsheet that allows me to put in data about the boat and the prop diameter and the anticipated prop shaft RPM. The software then calculates the prop pitch required based on 55% slippage. I plan to step down the high end RPM to 960 RPM and when I put in the 12" diameter prop it calulated the pitch at 13. That seems extream to me. And I'm guessing it does to Commander 227 also which is why he says he is going to start with a 12 x 8 two blade prop.

BTW Commander 227 I ordered the book David Gerr's Propeller Handbook "The complete guide to choosing, understanding and installing boat propellers." today. It should be here on Monday. Thanks for the info on the book and your reply on the prop. :)