Electric propulsion for Personal Watercraft

I'm developing a personal fishing craft for rivers & streams.
It's 4 feet wide, 8 feet long, and will be rotomolded.
Currently, I'm using 2 trolling motors in tunnels under the hull but
cavitation is still a problem.
I would like to explore some sort of 12v volt powered jet pumps.
Maximum speed is about 7 knots, so speed is not a requirement, but
'reverse' motion is!
Efficiency of power and longevity of battery life is a concern.
Any thoughts or suggestions?

John

 

Pacific Scientific makes industrial grade DC motors for what your looking to do. Anywhere from 12V to 180V, but they are expensive. You will also need to make or buy a DC controller for it which is also expensive, but would make for a great project. Dc controllers are fun to work with, you can build in a tachometer into them, control AMP output, Variable speed, Reverse speeds.
As far as a drive, what would be perfect IMO, would be an old Kawasaki jetski 550 or 650 jet drive. The 330 or 440 would be smaller and may work for you also. They are from late 80's and probably cheap on ebay. They also have a long drive shaft that could be adapted to fit any DC motor.

 

Electric Boat

I have a 14 ft electric boat, using a Pacific Scientific 24 VDC motor, with two 12 VDC deep cycle marine batteries. 14" x 17" 3 blade prop at 400 RPM.

The circuits do not use an expensive electronic controller, instead there are two simple DPDT relays, giving two forward speeds and two reverse speeds (basically full speed and half speed). The relays are about $23 each from WW Grainger.

Cruising range is quite good, because I have an efficient propulsion system, and in a slow speed displacement application that means a "Big Slow Speed Propeller". A jet system, typically made for higher speed planing boats, will show poor propulsion efficiency.

 

Thanks, Fred

It appears you have a better understanding of the basics than I do.
You already knew that jet propulsion isn't for my slow speed application.

My problem is trying to find the right propulsion for the application.
The boat was a catarmaran 4 feet wide and only 8 feet long.
Originally, I mounted a trolling motor on each side and used differential steering. That worked okay but looked cumbersome and homemade.
It also had a cavitation problem when the props got too close to the surface.
The props really needed to be mounted under the hull for a finished look.

I've studied a few kayaks and I know that displacement speed can be improved by a long narrow boat, but length becomes a storage and transport problem. So, the bottom became a tri-hull design with the center section resembling a kayak and the outer hulls only 4 inches wide for stability.
In between the hulls (tunnels) is where I need the props or propulsion system.

Trolling motors have 10" props requiring 11'' of tunnel height & width. That's a problem in a boat that is only 10" deep drafting only 5". The tunnnels are shallow (3.25") at the front and back. Then for about 24" in the center, the tunnels rise to accommodate the props and the deck is humped in that area. As feared, the props cavitated heavily. So, recently, I lowered the tunnels and props by 3". Now I have decent forward speed with only slight cavitation, but reverse makes a lot of noise and no movement. Also, the props are now exposed 3" below the bottom of the boat which is something I had hoped to avoid.

The initial objective was to have a whitewater craft that could be electric powered in the flats between rapids.

Is there any other way to address the propulsion system?

 

It works! It works!
I've exhausted all sorts of drives . . . ducted props, centrifugal pumps, paddle wheels, etc. Then I used an Archimedes Screw (predecessor to the propeller) and it boat now rips through 6" water, weeds, trash, mud, whatever. Three years of trial and error paid off! Check it out at www.streamdancer.com The site and the boat are a bit rough. Please give me your comments and suggestions, so I can build a final prototype.

Thanks,

Stream Dancer
www.streamdancer.com

 

John,

How does Archimedes feel about you fooling with his screws?

Well done for your persistence.

Pericles

 

Jet Boat - Low Speed

An efficient propulsion system in a slow speed displacement application implies a "Big Slow Speed Propeller". A jet system, typically made for higher speed planing boats, will show very poor propulsion efficiency. Water Jets are much worse than propeller type trolling motors in this application, and for decent efficiency you need an even larger propeller, probably in the range of 14 to 18 inch diameter.

 

interested in product

interested in your product, I am the owner of Enve marine, we manufacture a 10ft, 12ft and 20ft hovercraft , as well a 32ft, 52ft and 64ft custom designed yachts all electrically driven ,,,,

 

John, very cool!! I hope you sell thousands of 'em!! Let us all know when you get to market. Price range?
I'm sure I'm not the only one here who wants to see the underside, as soon as you get the patent of course.:]
Good Luck!

 

Very Cool indeed! Wish you the best of luck in selling tons of these.

 

Innovative Ideas!

John, without letting the fish out of the creel, can you show us more about the screw type propeller?? How many 'blades' or thread-starts does it have??

Are you using trolling motors??