Re: [electricboats] Elco vs ePropulsion

The only time and situation I've seen people use model airplane props for hydro propulsion was for pedal powered kayaks on Vancouver Island back in 1996.  The homemade propulsion units were based on the Eide SeaCycle pedal units but made much narrower, light weight and with a spring-loaded mounting arrangement that allowed the prop and water leg to pop up thru a dagger slot when the unit hit something as it was going along.  The person who made these called the resulting pedal kayaks "Kawaks".  They were super fast---we pedaled our Seacycle catamaran at top speed while pacing with the person who invented the Kawak drive.  Knowing the Seacycle was supposed to hit about 10mph at top speed, Bob called to me and said "is that as fast as you can go?"---then he pushed his pedal drive faster and took off ahead of us.  He told me that large model airplane propellers worked well for his drives.

 

-mt

 

From: electricboats@groups.io [mailto:electricboats@groups.io] On Behalf Of cpcanoesailor via groups.io
Sent: Wednesday, November 2, 2022 7:30 AM
To: electricboats@groups.io
Subject: Re: [electricboats] Elco vs ePropulsion

 

Has anybody tried or know how to pick a model airplane prop for electric outboard?

I first did some boat speed and electrical power measurements using the stock prop that came with the motor, to get a performance baseline.

I read an article that said prop efficiency increases with diameter, so I determined how large a prop could I reasonably use with my motor, based on submerged depth, length of the skeg, etc.

I also estimated the pitch of the stock prop. Then I translated that pitch value to the larger prop size to get the pitch number for a similar speed performance (but now more efficient).

That gave me an idea of which pitch numbers to look for in the larger sized airplane props. In my case, I thought I wanted about twice the pitch of the stock prop, since the motor was hitting its RPM limit but drawing only half the rated current.

That actually worked out well for me - the motor with the larger, higher pitched prop now draws rated current at max RPM. And power measurements indicate that for a given speed, the new prop takes noticeably less electrical power than the stock prop did.

One downside to slim airplane props is they slip a lot more than standard marine props. This is only a problem if you're pushing your boat into a strong wind, or trying to tow a heavy load with lots of drag. For me, on a sailboat, it's not a serious issue. But I do keep the stock prop and a wrench in the bilge...