T-Pain Is On A Boat: Re: [Electric Boats] propellers

Friday, March 5, 2010

Re: [Electric Boats] propellers

Larry,

I didn't mean regenerate - that would be super but I believe from what I've read here - it's not practically attainable. I meant just going backwards, as reversing out of a slip. I'm under the assumption that folding props tend to fold up when generating reverse thrust. Maybe not??

My "I bet..." statement had more to do with using a much higher aspect ratio prop than normally found under water. Human powered boats, usually driven by a bike pedal crank turning a prop shaft, use props that look more like airplane props with long thin blades. My own experiments with a 12v trolling motor showed a 10 x 6 APC model airplane prop gave about 30% more range than the stock 9x4 "normal" prop, both pushing the canoe the same speed. Of course long thin blades are fragile but maybe strong enough for the very low power most E boats use.

More tests to follow when the water reverts to liquid form.

Denny

----- Original Message -----

From: gramplarry

To: electricboats@yahoogroups.com

Sent: Friday, March 05, 2010 11:20 AM

Subject: Re: [Electric Boats] propellers

Denny thank you. My copy of Gerr"s book is a signed first edition that is dog eared from use.
Hiding a two blade prop behind the skeg is a standard operating procedure in sailboat racing when the racer cannot afford the $1000+ folding or feathering prop. Usually the shaft is marked with a dab of paint to indicate when the prop is up. Then locked in place with the transmission. However some transmissions do not allow this and i have seen some pretty crude methods as a vise grip and a block of wood. But starting and stopping engine can become a fire drill.

Your comment "A folding prop that would work in reverse would be even better.' is interesting in that i have heard this before and actually plan on testing the concept when the test boat is launched. I have several theories why this might be an advantage and would like to understand your reasoning without influencing the discussion. after all, you are more experienced with electric power and how to use it and regenerate it than I am.

Larry

--- In electricboats@yahoogroups.com, "dennis wolfe" <dwolfe@...> wrote:
>
> Welcome, Larry.
>
> Your expertise will be welcome. You must have Dave Gerr's "Propeller Handbook" in your office. It has a wealth of info about props.
>
> It seems to me an electric drive is much easier to optimize than ICE as the power absorption of the prop is simple to measure (volts x amps = watts). Watts vs. speed for various throttle positions measured in controlled (no wind or current) gives a good picture of efficiency.
>
> Props are easy to change and motor rpm / prop rpm ratio with is easy to change with belt drive. Comparing watts vs. speed curves shows which prop / drive ratio is the best for a particular boat / motor setup.
>
> I bet a large diameter, high aspect ratio 2 blade prop would give excellent performance as a sail aux drive, especially if a method could be devised to lock the prop in the vertical position hiding in the wake of the skeg. A folding prop that would work in reverse would be even better.
>
> Denny Wolfe
> www.wolfEboats.com
>