Electric outboard on "Iona!"(a Storm 15)

[Michael Rogers]

Hello, all you electric lot

Not of particular interest to me, but following a phone conversation with Pete Greenfield (editor/publisher of Water Craft), the following might cause a spark or two -

1) You may be aware of the 'Cordless Challenge' at the Beale Park Show this year, being a series of races for boats powered by (the electric motors of) cordless tools. It was won by a young man from Germany, married and resident in UK: his boat was probably several knots faster than any of the other boats which competed, and he won by a huge distance. He used a model aircraft, rather than outboard, propeller. The competition was covered on the UK HBBR Forum website. Jeremy, who contributes knowledgeably to this forum, had his entry ruined by a spectator who switched the motor on while the boat was on terra firma, apparently to see whether it was any good for ploughing. Shame his carefully designed craft couldn't compete. Both Jeremy and the competition winner (whose name I do not have to hand) are scheduled to have articles on the Challenge in one of the next few issues of Water Craft.

2) Pete Greenfield tells me that there is great interest in electric outboards in Finland, where the first thing that knowledgeable users do, having bought their electric outboard, is to fit a model aircraft propeller from the nearest model shop, in place of that supplied with the outboard.

[Jeremy]

I'm afraid that my article in the next issue of Watercraft isn't very Swallow Boats related (although the next article from me will be).

It's very true that changing the prop on an ordinary, cheap, electric outboard will significantly improve efficiency, as I have mentioned here before.  An acquaintance from another forum has undertaken some very interesting experiments to show the improvements in performance that are possible by modifying a cheap electric outboard.  They are documented here on this Boat Design forum thread: http://www.boatdesign.net/forums/boat-design/efficient-electric-boat-27996-15.html#post302305 and the subsequent follow on post by MCDenny.

Jeremy

[Paul Cross]

I've had a look at this idea too...(never ignore the Finns)

The idea of switching for a model aircraft prop seems to be centered on(or should that be "revolve around" ?.....) very lightweight craft..ie canoes, kayaks etc. I suspect that, as most electric outboards are designed to replace small petrol models, their props have been optimised for the largest craft that they could reasonably be bolted to the back of. Consequently there may be large gains in efficiency to be had when using them on very low displacement boats.

IONA is probably at the top end of the "comfortable with a cheap electric outboard as a means of auxillary propulsion" range of boats.

Swapping out the prop may not yield much but at around £5-8 for a suitable model aircraft propeller...I feel an experiment coming on..........

Paul

[Jeremy]

Denny Wolfe's experiments (the link I gave above) were aimed at propelling a fairly high displacement sailing boat, of around 14 to 20ft LOA.

Having done a lot of experiments myself I can confirm that a correctly sized narrow blade prop, like a model aircraft one, is massively more efficient than a typical outboard prop.

The downsides are that the long thin blades are relatively fragile, plus they tend to collect weed.  Outboard props (and boat props in general) seem to be optimised for small diameter (for reduced draft) and good weed shedding properties, both things that very significantly reduce efficiency.

There is a way around the weed fouling problem, by making the blades fold, like this prop I made a while ago, for one of my experiments:

[Jeremy]

Further to the above, I've attached a graph that Denny Wolfe derived from some testing of a Minkota electric outboard modified with a fairing around the tubular leg (to reduce drag) a 10 x 6 APC model aircraft prop, a model aircraft spinner (again to reduce drag) and a model boat PWM speed controller instead of the rather lossy resistors that some of these motors normally use.

I've also borrowed a couple of photos that Denny posted on the Boat Design forum thread that illustrate the mods he made to his motor.

http://www.boatdesign.net/forums/attachments/boat-design/35904d1254966402-efficient-electric-boat-img_2187.jpg

http://www.boatdesign.net/forums/attachments/boat-design/35905d1254966402-efficient-electric-boat-img_2185.jpg

The performance improvement he obtained from these mods was pretty good.  Denny kindly detailed the performance change with each of his mods (the speed setting numbers in the first three tests refer to the Minkota speed settings with the switched resistor control):

Baseline with unmodified motor:
-Speed #5 (WOT) gave 4.3 mph at 330 watts; range 1.7 hrs, 7.3 miles.
-Speed #4 gave 2.9 mph at 178 watts; range 3.4 hrs, 9.9 miles.

Switching to an APC 10x6 prop:
-Speed #5 gave 5.1 mph at 410 watts; range 1.3 hrs, 6.6 miles.

Adding the PWM controller and adjusting 'throttle' to give 4.3 mph speed:
-Standard prop pulled 330 watts; range 1.7 hrs, 7.3 miles
-10x6 prop pulled 260 watts; range 2.2 hrs, 9.5 miles
-10x6 prop plus fairing and spinner pulled 190 watts; range 3.2 hrs, 13.8 miles.