Efficient solar powered (electric) kayak

Re: double UJ prototyping: http://www.alltronics.com/cgi-bin/category.cgi?item=24Z028

Re: Flex shaft/housings
Unsheathed flexible shafts always run at horizontal when they are properly tensioned to the thrust. The reason this works is that the pusher prop is self stabilizing. When it starts to produce thrust, it maintains a level horizontal attitude and pushes against the angled shaft which forces it to curve up to the gear box- even without a strut at the prop end. A minimal strut or even a cord tether can be used to keep the prop from hitting the hull during turns and for pulling the flex shaft up out of water or when beaching. You can prove concept by putting a low pitch prop on the extension end of this 3/16" (not 1/4") electricians shaft and using a VS cordless drill for power while holding in your hand- prop in water. http://www.grainger.com/product/GREENLEE-Flexible-Drill-Extension-4HEC8 (can be found at Lowes/HD).

A shaft housing introduces too much friction loss when they touch the shaft. Whether they be used with a solid or coiled spring shaft (no reverse) and whether the casing, be copper, flexible conduit, weedeater shaft or whatever.

Re: Belt gearing prototyping: http://www.mikessubworks.com/drive.html Recommend you would need a heftier version of this design, preferably with a self tensioning idler. Go to 540 model or "Drive reduction unit".

PC

 

RE: unsheathed and unsupported flex shafts

These are very intriguing and more to my liking than the rigid universal joint approach. Thanks for suggesting them!

I can get this easily at my local store: http://www.homedepot.ca/product/1-4-x-54-inch-flex-bit-extension/918700. It is 1/4 inch by 54 inch flexible drill extension. Suitable or too stiff? Why did you say 3/16 (not 1/4)?

 

Haven't tried thicker, but I know thinner 54" shows a nice curve at around 30-60 watts- more pliable to that power input range. Thicker is heavier to handle, and prop may not stay horizontal at lower power. Maybe call around, my local HD stocks 3/16" X 54" and takes them back if not altered. I haven't pushed it to the hand hold limit.

PC

 

I bought the 3/16" by 54" flex extension. Installed the prop, mounted it on my drill and then went for a test in my sea kayak. Results were very good. No issues at any speed up to about 11kph. It definitely works!

However when I tried to apply power too aggressively or at power levels needed to exceed 11kph the shaft started to oscillate wildly and uncontrollably. At first I thought I was inducing the oscillation being handheld but even when pinned to the deck it would do this. I tried different depths and different angles thinking the prop may not be aligning properly but with the same results. I also tried putting the drill tip in the water thinking the portion of the shaft in the air was being excited and not damped well enough but no change. What causes this and can it be fixed?

I've included some pictures. Note...when I view the uploaded pictures the shaft looks like twisted wire but it's not. It is a smooth piece of spring steel. I assume this is an artifact of compressing the pictures too much.

 

This oscillation also sometimes happens when the speed is too low. Not a factor for my applications at high speed since I run at 3 knots maximum.

My best guess at the cause is that the shaft curve became too severe. If you want to experiment, you might try to make your shaft go to adjustable length/stiffness. Maybe find a way to mount the prop at any point of length along the shaft. This will also help if shaft length harmonics are a factor. The thicker 1/4" may take you to higher speeds, but sacrifice horizontal prop position/efficiency/oscillation at low speeds.

Some other possibilities also exist for the oscillation. Sometimes occurs with a very light wooden prop, so there may a gyro factor? Maybe cavitation when exceeding prop design speed adjusted for water?

BTW, if you didn't see these on the dual props post by DW, here are some other perks of using a flex shaft:

Vid post #53 & 54 below:

http://www.boatdesign.net/forums/inboards/prop-shaft-systems-24636-4.html

Much better/responsive steering and no rudder resistance/complications as well as shallow water operation....

PC

 

Is is possible you just hit the hull speed of the kayak?

 

Hull speed is a soft number but 11 kph is certainly getting up there in terms of power requirements. Euler's column buckling formula for 3/16" spring steel of that length would predict around 55N for critical load. This is a believable thrust for the kayak at that speed. So shaft buckling is probably the most likely cause since the shaft carries both the torque and thrust loads. Most people support the propeller with a rear strut to address this problem.

11kph is a reasonable speed but the allowable thrust given a bend with infinite fatigue life would be much lower than 55N. So unless one were working at very low thrust levels the shaft will eventually fail.

Also a given thrust can be reached at lower speeds if heading into wind or current.

To address the buckling the choices are shortening the shaft, increasing its diameter or adding a thrust bearing. I've found a local supply of 6mm x 3m spring steel for about $10 including tax so will experiment with that. Unfortunately it is a garden variety spring steel with a yield strength of only 380Mpa so the final bend radius will almost certainly not allow infinite life. I haven't found a good formula for calculating the bend required for infinite fatigue life for a flexible shaft such as this so if anyone knows that would be helpful.

EDIT: PC, I know you operate at low thrust levels but have you been using a 3/16 x 54" shaft long? Any failures?

 

I see that there are manufacturers claiming solar panel (cell) efficiency of up to 37%, and more claiming they could offer 60% and more in the near future...

If those become available then electric propulsion is going to be very desirable.

 

Hope this helps.

PC

 

PC: I didn't notice any wobble or vibration until trying to exceed 11kph or accelerating hard.

 

I have two 50watt flexible panels ( http://www.sungoldsolar.com/Product/Flexible-Solar-Panel-50W.html ). They claim cell efficiency of 22%. I've felt efficiency needed to about double before they were really viable. 60% would be very useful indeed!

 

Mine occurred while accelerating very slowly with a wood prop and continued if you maintained that low target speed. The prop was spinning slowly with very little thrust and the shaft was making a circular orbit at the same time on the prop end (while shaft straight- no curve at all noticable). It did not occur with rapid acceleration or once the prop had a good bite and the speed was reduced back down gradually that I could notice. Anyway, it is of no consequence in normal operation, just thought it might give some clues- prop weight or shaft length harmonics?

PC

 

I guess a lot of these threads grow stale as people achieve their goals or move on to other things but since it's been a few months here's an update on the project.

I ended up buying a used Minnkota Endura C30 for $25. I removed all the miscellaneous bits keeping just the motor and then added a $13 chinese "3000watt" PWM speed controller, a $20 3/16" flexible shaft from Home Depot and a $3 APC 10x6P prop. The whole package weighs in at 7.5 lbs (thanks in large part to the aluminum fin/housing of the MK).

This setup has proven very good and I've been using it heavily for the past several months in all kinds of conditions on both the sea kayaks and inflatable kayak. Efficiency is improved over the results I posted earlier for the optimized electric outboard version.

I haven't encountered any problems running the MK out of the water.

I'm liking the flexible shaft drive. It reduces drag to the minimum possible levels and is much more forgiving when hitting the bottom (or rocks and logs).

The flex shaft design can be easily raised in shallow water so the prop becomes surface piercing (with some loss in efficiency and increase in vibration but still quite usable) and allows operating in water depths not much more than the draft of my kayak.

I have found a couple downsides however:

Handling is more fiddly especially out of the water. It is easy to bend the shaft accidentally. Especially when using the Home Depot drill extension shaft as it is low grade spring steel. There are much higher grade spring steels available but I haven't pursued those yet. 17-7 (Condition C or RH950) stainless steel is probably the best material to use but hard to find locally. Actually it is easy to find locally, the hard part has been finding someone who can straighten it adequately as it comes coiled and is very hard.

I've bent the shaft while at sea in rough conditions. The prop tends to stay in a fixed position but the kayak pitches, rolls and yaws about from the waves which can put too great a stress on the shaft resulting in a permanent bend. A shaft with higher yield strength would probably solve that problem (as would avoiding excessively rough conditions!).

The second downside is a certain amount of vibration compared to the electric outboard approach. I believe the vibration is due to the shaft not being perfectly straight and stress relieved. It doesn't seem to take much so vibration free may not be possible using an unsupported flexible shaft drive.

All in all, it has performed very well. The efficiency boost from losing the submerged motor and downshaft is well worth it. The serendipitous gain in shallow water performance has been great and may ultimately be the most useful aspect of the system.

Having said all that, I recently made a version using the Turnigy Aerodrive SK3-6374 148kv R/C aircraft motor and have started using that. If there is still interest in this little project I can writeup that experience as it has a much higher learning curve for someone not used to the R/C environment.

 

You have to be careful with batteries in parallel as one battery can something drain and even damage the other. But I really like the idea of switching battery banks from 12v to 24 volt mode. With 6 x 6v batteries you could even switch from 6v to 12v to 18v to 36v. Batteries can be like having multiple wives though. It's hard enough treating one of them equal. ;-)

p.s. Thanks for the update.

 

I had wondered what your progress was, thanks for posting. I would be interested to see your Turnigy write up. Some suggestions/questions are inserted in your original message below, hope they help.

PC