Efficient electric boat

Rick & Jeremy,

Thanks for the folder feedback. I've got my hands full right now finishing the boat but will try a folding prop is weeds prove to be a problem. The motor comes out of the well when sailing so prop drag is not an issue. I'll post a picture the nest time I remember to take the camera out to the shop.

Jay,

The 12" prop sweeps more than twice the area as the 8" - efficiency is probably a lot better. Good luck on gettting warm weather in Florida. I spent two weeks there in Feb and it got above 70 once.

 

Bliss, thanks for sharing your measurements on prop efficiency.

One thing I wonder about is the trade off of gearing on the motor vs. the APC prop pitch and diameter. The Torgeedo prop may be optimized for that particular motor, gear ratio and power draw. If you substitute the APC 12X10 on the specific Torqueedo motor that uses the 8X8, will you still have a lower power draw at any or all speeds and by how much? I guess you will have greater efficiency with APC IF the Torqeedo prop is built for weed shedding, since shedding props are known to be inefficient. MCDenny's test seem to show a big efficiency improvement with APC, at his target speed of 4 knots but less at lower speeds, post 233. Of course, MAYBE a DIFFERENT APC prop could have been found to outperform the standard weed prop at a lower target speed of say 2 knots. Am I thinking correctly here?

One thing I have found in tinkering with RC props over the years is that the brand and profile shape of the RC prop matters, at least at power draws below 50 watts (30 watts typical). Generally, the APC pylon styles are the worst and the thinner, wider electrics the best despite their tendancy to flex at peak. Some of the scimitar profile master airscrews do slightly better than the equivalent APCs at lower cost. The very stiff carbon bollys were not significantly better than the APCs. Maybe, at my low power draws, gearing, rpm, etc. stiffness is not a factor.

There appears to be an optimum gear ratio for any particular motor when using RC props. When I take a motor, geared so that it runs very efficiently with a particular APC prop and lower the gear ratio by 50% downward (lower rpm), I usually cannot find a larger APC prop that will restore the efficiency back to the previous.

Hope all this rambling info helps.

Porta

 

Rick, the 10X8 folding blades deflect about 1" using about 8# of pressure at about 75% of the length.

Vic

 

Vic
This one would then have no chance of handling 35lb.

I produce about 8lb of thrust at cruise speed on V14 boat. With 1" of deflection it would not be distorting excessively but not the best choice.

If you get any bigger ones for your own needs I will be interested in test results for them.

Rick

 

Yes, I thought it would be too weak also. I did the deflection by clamping ONE of the blades in a vise and using a scale with a hook located about 3/4" from the tip to pull until the deflection was 1". I guess with 2 blades that would 16#, which is still not enough. The folding blades are for electric sailplanes which are thinner and deflected more than regular non-folding pattern prop blades. The carbon makes them stiffer than corresponding nylon folding blades, but maybe still not enough for serious human power range.

Vic

 

Jeremy
How is the building going?

I am interested in any testing you do with the outboard.

Also would like to know if you have devised a clever way to mount the solar cells into a panel.

Rick

 

great thread. i got through about 6 pages before my head started to spin, i'll pick up later on.

 

electric tri

Rick, there seems to be a lot on u-tube, somewhat similar to your proposed type design:

http://www.youtube.com/watch?v=7F-ZQyX1H0k&feature=related

V.

 

I've been working on the outboard leg and virtually have the gearbox fairing completed, but have a problem. I test ran the thing today, with the folding prop fitted, and the gearbox noise is very loud. I had hoped that encasing it in the composite fairing might help to quieten it down, but it doesn't seem to have done.

I now have to decide whether to continue with the gearbox leg, or revert to the belt drive unit with a fixed propshaft. The belt drive unit has the advantages of being quieter and slightly more efficient than the outboard leg, but the disadvantage is that I'd then need a rudder (or rudders).

This leads me to ask about the effectiveness of dipping rudders. Because of the need to be able to swing the prop up to reduce draft and because of limited space at the relatively narrow stern, I wouldn't be able to fit dipping rudders right aft, behind the prop, but might be able to fit vertically sliding rudders in a pair of centreboard cases maybe 400mm or so from the stern. I'm reasonably sure I could get a good sliding fit using PTFE strips and I'm sure I could devise a workable cable linkage to alternately push them down as required.

The worry I have is that they may not be adequately effective if not in the propwash.

I've not done any further work on the solar cells, as I want to get the propulsion unit finished first. I'm pretty sure that I'll try making them using thin (1mm or maybe 1.5mm) polycarbonate sheet, coated with clear silicone resin, with the cells laid face down on the resin, backed with more resin and a layer or two of glass cloth. To get rid of air bubbles I plan to gently vacuum bag the whole thing, using the lo vac process pioneered by the Cozy Girrrlz (http://www.cozygirrrl.com/menupage.htm).

I'm off to see the boat builder on Thursday, to sort out the final details on the hull.

Jeremy

 

Jeremy
It surprises me that the gearbox is noisy. Have you tried it underwater.

Having a rudder behind a prop is going to make very little difference. You have such low velocity ratio across the prop that it will be insignificant most of the time. You may be able to produce a very small side component at zero speed with forward thrust with a rudder in the wash.

I have been working on the idea of connecting the twin dipping rudders through gears so they statically ballance each other. When one goes down the other goes up. It means I could operate them with a turning shaft made from aluminium tube and I do not need spring returns. It would be a neater arrangement than what I have now.

Rick

 

Vic
The single long slender hull with stabilisers has the lowest drag so that is the common approach for low power boats.

Rick

 

To be honest, I'm quite disappointed with the gearbox, particularly after the many hours of work I've put in to it. I have tried it in water but it's still pretty noisy. I think that part of the problem may be torque ripple from the motor making the gears chatter. At the low input speed (cruise gearbox input rpm will be around 700 to 800 from the motor), coupled with the relatively low inertia in the motor, there will be distinct torque pulses for every magnet pole pair (the rotor has 7 pole pairs). Added to the trapezoidal drive waveform from the controller (the ideal would be sine drive) this means that the gearbox input rotation isn't that smooth and will have a number of torque pulses per rev that will be close to half the number of teeth on the input gear, probably the worst condition in terms of exacerbating gear noise. The 2:1 Mitrpak box also seems to have some backlash, which probably adds to the noise, by allowing some gear chatter at low speeds.

What makes me think that this is a significant contributor to the noise is the fact that the gearbox tends to get a bit smoother and quieter at full speed, when the motor torque ripple smooths out a bit due to the rotor flywheel effect. One fix might be to add a flywheel to the motor. This wouldn't fit within the mounting constraint I have though!

The belt drive smooths the motor torque pretty well, plus the big pulley I'm using on the output must help (it's a 72 tooth, 15mm wide, 3mm pitch HTD).

Thanks for the advice about dipping rudders. It hadn't occurred to me that prop effect would be so weak, but now you've highlighted the low velocity differential across the prop it seems obvious!

I like the idea of a single control operating both rudders differentially. I have been sketching up a few ideas, using rudders designed a little like pivoting centreboards, rather than like sliding daggerboards. A pivoting design seem easier to devise an actuating system for than a sliding system.

Jeremy

 

Jeremy
I believe there is an adjustment nut on the input shaft that you can alter to set the backlash in the gears. I only have drawings of the set up, not actual box. I think it can be adjusted without disturbing the seal. I expect tightening the mesh will add a little drag though.

Using a more compliant connection between the motor and gearbox might help reduce noise. Machinable aluminium is much more compliant than steel but has the strength of mild steel. It tends to dull noise.

The electromagnetic noise in these motors using the trapezoidal waveform is something to behold. I have not heard what they are like if fed with sinusoidal voltage.

I got some 40mm diameter large teeth nylon bevel gears to try on my rudder control. I also have some PTFE rod to make simple bearings.

I expect you will do some testing once you get the hull and then work on the little annoyances. When I pedal on the water my mind wanders into pleasant places for thinking up how to address little issues.

Rick W

 

Jeremy, have you considered to experiment with magnetic tooth wheels as per attached article. That must be pretty silent. What the article did not say is the actual power transferred.
Bert

 

Rick,

I've had the gearbox apart to remove the top seal (as I didn't need it and it was creating friction) and the gears are meshed about as well as they can be. For information, getting the box apart (should you feel the need) is a bit of a struggle, as the big threaded bearing retainers are Loctited in place. Being of fairly large diameter and with only a peg wrench to try and turn the retainer made this an interesting exercise!

I've already got an alloy drive shaft, machined up from a bit of 16mm diameter 6082-T6 bar. This engages via fairly tight pin drive to the gearbox shaft (I had to make it this way to that it could be assembled easily).

Bert,

Interesting idea about magnetic drive, but I need proven technology that I can buy now, as I want the boat in the water this Summer!

I think I may stick with belt drive, as all around its probably the best compromise. It seems that if you want quiet gears you need to pay a lot of money for them!

Jeremy