5m sit on top plywood kayak with electric propulsion

Since i won't be able to start working on my own boat until summer i thought i might practice on smaller project. My fishing buddy got infected with electric propulsion, but he's certainly not a DIY type. So i thought i might try to build it for him, materials and required equipment he will buy and i'll do the work. I can practice boat building skills on small scale and of course use completed kayak sometimes myself, if successful sounds like a ton of fun.

But before i even start, i'm not too sure on required power. I'm planning on using one of electric trolling motors (brushless) with some modification to control and steer from seat, lifepo4 battery. Not decided on design yet, but something like our current kayaks Hobie Revolution 13, narrow flat bottom hull, works quite well and looks simple to build. 5m long, 80-85cm wide. Total weight with one person, fully loaded with gear and ice should be below 150-180kg. What kind of speed can expect on continuous 500w/750w output? That's probably the practical limit where it still makes sense to us.

I want to use exterior grade 3mm/5mm plywood and try to choose sheets myself, seal everything with fiberglass and epoxy. It's complicated and expensive to get marine grade plywood in Japan and for this size i hope exterior grade will be sufficient.

 

You could look in the electric propulsion sub-forum for some ideas. A 5M narrow boat is big enough to use a lead-acid battery which would save some cash, and chargers are available anywhere. There are ways to increase the efficiency of a trolling motor - replace the propeller with one for a model aeroplane, and add a spinner from the same source. Put a fairing around the round vertical tube of the motor, and if it has a resistor switching speed control, replace that with a PWM type from model cars or 'planes.
I have a 14ft skiff powered by a brushless outboard and 54kg of lead batteries and it goes nicely on just 400w.

 

I gotta ask, Why don't the manufacturers do that so they can then say 'ours is better'?

Are stock props more rugged, weed free or designed to push bigger boats? I'm thinking in a kayak you'd want to be as weed free as possible, since it might be harder to pull the whole works up and de-weed in a kayak VS typical alum runabout. Plus, IMO, kayaking is all about the weeds and getting into spots other boats can't.

Why aren't manufacturers using PWM to one-up other brands? Cost? I'd think the extra $10 would be worth it for increased sales for something even non-tech folks can sort of understand.

 

A sleek pontoon is better for fishing applications, IMHO. Better for Comfort, stability, boarding/ egress, detangling weeds, etc.

PC

 

Portacruise, thanks for explaining. Although I suggested a model plane prop (uses 1/2 to 2/3 the power for the same speed compared to trolling prop) I can confirm that they are weed magnets. Xellz did mention brushless motors which are PWM anyway, plus the chopping and changing of phases.
Go ahead Xellz and build a small boat; it's a great feeling to finish it and launch for the first time and say "I made that!" I've recently finished my first but I'll admit that the launching was better than the building.

Sorry, don't know how to reduce picture size.

 

Really nice looking boat, Alan!

What kind of motor and prop are you using?

I've always wondered if this older technology could be adapted to make more efficient, weed friendly, troll motors: https://www.eetimes.com/document.asp?doc_id=1278828
It might solve or at least improve on a lot of issues that go with the present conventional troll motors.

PC

 

Thanks PC, there is a thread in electric propulsion sub-forum which shows my brushless/Evinrude conversion. Turnigy brushless and standard prop. Yes, you could have piezo propulsion, or pump, but if I understood the article, it would have only one speed.
The boat in the picture is floating with the 54kg of batteries on board.

 

Thanks for suggestions, how about shaft? Can in be noticeably more efficient than trolling motor with better prop?
Almost no weeds around here, only big waves and often strong winds. Quite deep too, only 200-300m from shore and it can get over 200m deep. I'm trying to find a way to pull out this canoe from water on concrete ramp and drag it about 50m from launch place to home for charging and storage. Thought about similar concept i'm doing with my Hobbie kayak, i.e. put wheels into scupper holes and drag it out without touching concrete. With more heavy and longer, but more stiff kayak 2 pairs of wheels that can be stored on kayak or just leave on rope somewhere i could pickup them from water. But i need to be able to install them on the water without flipping over.

 

Here's the most efficient system I know of, similar to the one I use- scroll down for pics of the finished setup: Efficient solar powered (electric) kayak https://www.boatdesign.net/threads/efficient-solar-powered-electric-kayak.52833/page-6

You can mount a single or double set of non-buoyant wheels on the same shaft for the heavy kayak at the center of gravity (COG), but it will require some tinkering to get it just right. Mount a square socket firmly on the sides at the COG, attach with a removable pin to roll your kayak, detach and place in the kayak while cruising. You don't want to lose the non buoyant wheels or pin overboard, so use a tether or small chunk of floatation.

PC

 

A brushless DC motor and PWM control costs much more than a brushed DC motor and resistor / transistor based controller because of the need for a programmable device to commutate the phases and the additional power transistors.