Efficient electric boat

Jeremy,

What was the outcome of the flexible drive cable bending in a gentle curve over 90 degrees?

 

Jeremy
Just a note on your description of fabricating the prop. You state the sequence incorrectly. The sequence is important. It becomes obvious if you make one. But you may stuff up the first if you do it in the wrong sequence.

A friend actually had one blade facing forward and the other facing backwards. Thought they needed to be different on each side.

The other thing to note is that once you twist the blades you have set the handing of the prop so needs to be thought about before you start to twist. Although with a PMSM it is easy to change direction of rotation.

Rick

 

Thanks for that Rick, I thought it may have been your prop manufacturing method I'd seen a while ago.

That's a good point about a belt being more compact, I'll give it some thought. I have no need to twist it though, because I can just arrange the top shaft to be in line with the bottom one easily enough (I don't have the constraint of a fixed pedal shaft alignment). The only snag I can see with using belt over chain drive is keeping the lower seal lubricated, but I may have a way around that, if the low friction face seal idea I'm working on works as intended.

I like the curved shaft idea, but I'd then need to fit some form of rudder system, probably permanently immersed, as I'm not sure that your neat dipping rudder would be that practical when cruising marrow and winding canals and rivers. The need for good manoeuvrability is one of the things that keeps pulling me back to the swivelling drive leg option.

I haven't dismissed the right angle gearbox, it's just that I'm exploring other options in parallel, trying to decide on the best trade off between efficiency, cost and complexity of manufacture. As usual, this is complicated by a having surfeit of ideas to explore!

Jeremy

 

Re: swiveling drive system. That is the arrangement on my one man portable e-boat referenced in HPB. Like Rick, I made a smaller, but one man, scale model of a boat to test the concept for my specific uses. Some health issues, and never found a reason to scale up any further for what I like to do, anyway. There are many advantages to a swivel drive as listed in HPB. As regards racing or large boats, been there- just wasn't my thing- showboating, fighting crowds at the launch on the same rushed weekends or 4 holidays a year, expensive wasteful upkeep, environment destruction, etc. (IMHO-no offense meant to anyone- to each his own).

Anyway, you can run a shaft directly from your motor reduction shaft out to the prop. You only have to balance the weight of the prop lever arm against the motor/gearbox weight on the swivel. Study the "dragontail" or "mud" motors used in the Asian countries on their longish boats and longish motor drives. These "mud" motors use straight or sometimes curved, bulky shafts which are sheathed to protect from the considerable hazards of a powerful motor turning a "live" shaft at high rpm. My applications involve portability and very low power, so I did away with the sheath many years ago and went to a live flexible fiberglass prop shaft.

Porta

 

Jeremy
It shows how my mind can get stuck in a rut. Ian posted a photo of a twisted chain drive in the pedal power thread last night and I had that on my mind.

You are right. You do not need to twist it. I think you will get a belt more compact than a chain. It would be quieter as well

To my mind the little gearbox is the best solution for a little outboard leg but the others can be engineered to work. Gives you the opportunity to exercise your lathe.

Mitrpak have made a special for Greg K that includes a "T" box with crank inputs and an extended input shaft "R" box that mates to the "T" box. Something like this allows a very narrow fairing over the shaft so low drag. It will still serve as your rudder.

I expect you could make an extended shaft for a box if you want but it gets into dismantling the box. Mitrpak do have some optional extended shafts but they are not as long as they made for Greg. See photo - have I posted this before?

I have had success using simple lip seals on prop shafts.

With the pedal drive in the photo there will be a tube joining the boxes and the whole thing will be oil filled to ensure positive head on the underwater box. For intermittent use a lip seal on a stainless shaft should last a lifetime with just water lubrication or a little grease behind it. Not as if the boat sinks if the seal fails. The belt will throw a bit of water about.

I fiddled with many things before I purchased a precision sealed box and I have bought 8 of them now because I think they are real value and can be used for different things. A few have picked them up on Ebay for around half price. Some smaller ones very cheap in quantity.

Rick W

 

Thanks for those tips, very useful. I like the Mitrpak boxes, they do seem near-ideal.

The "mud motor" idea is certainly simple and effective, I've seen these in use on TV programmes, but am sure that our inland waterways authorities would have a "Health and Safety" fit if I tried such a system here!

I've just done some quick sums over breakfast and it looks like a belt drive to the prop will struggle to take the torque. The maximum for a 15mm wide 3M HTD belt running on a 20 tooth lower pulley looks to be about 1.6 N-m or thereabouts. Going up to a 16mm wide 5M HTD, or switching to a Gates GT2, increases this a bit, but still nowhere near the 5 N-m or so that I need.

It looks like a small chain might be the best bet. A 1/4" pitch chain is still about as compact as the belt, as I can get away with a 10 or 12 tooth sprocket. The great advantage is that the allowable working tension for the chain is much greater than that for the belts. Ordinary chain in this size, on a 12 tooth sprocket, is good for nearly 50 N-m, much more than needed.

As always, the devil is in the detail with this stuff!

Jeremy

 

I'm risking talking to myself here, but have been experimenting with a tiny chain drive today. The chain in question is really tiny, 6mm pitch, with sprockets just 2.8mm wide. The spec is 04B, a British Standard metric chain. What's impressive is the torque rating when used on small diameter pulleys (around 10 times that of a 15mm wide, 3M HTD belt).

The best feature is the low frictional loss compared to a belt. I haven't properly tested it yet, but just turning the shaft by hand is noticeably much easier than turning the belt drive. I've been able to get this slim chain drive inside a 1.5" OD, 1.25" ID tube, as a drive leg, with a 10 tooth pulley at either end (I'll fair the tube in due course, like the gearbox drive one I made previously).

As this works so well, I'm going to change the main reduction drive over from belt to chain as well, in the hope that it will reduce the losses further. At the low power levels I'm working at, these small savings seem worthwhile.

The only remaining concern is the noise that a chain drive might make. Hopefully I'll get a chance to test it properly tomorrow.

Jeremy

 

Sounds interesting to me. Let us know how it goes. We have a saying here, "there's more than one way to skin a cat".

Porta

 

The chain drive seems to work OK, but tends to be a bit noisy at certain speeds. The good news is that the power loss is very small indeed, about 3 watts at maximum speed. I'm currently testing it using a leg made from 1 1/4" x 18g stainless tube, with a pair of 10t sprockets. I think that, to make this reasonably quiet, I need to look at adding some form of chain tensioner.

I tried pushing a strip of 1/16" PTFE down the slack side of the chain and this works well to stop the noise and doesn't seem to make any difference to the power consumption. It needs a bit more engineering to get this to the point where it is a reliable solution though, I think.

The current test setup has a 4:1 belt reduction drive from the motor to the top sprocket, using a 15mm wide, 3M section, HTD belt. The chain drive uses 10t sprockets at both ends.

The no-load power breakdown is this:

Motor only, full speed, no load = 22 watts

Motor only, minimum speed, no load = 3 watts

Motor plus belt reduction, full speed, no load = 26 watts

Motor plus belt reduction, minimum speed, no load = 4 watts

Motor, plus belt and chain drive, full speed, no load = 29 watts

Motor, plus belt and chain drive, minimum speed, no load = 5 watts

Full speed = propshaft RPM of 780

Minimum speed = propshaft RPM of 40

The major part of the losses are now in the motor, so any further improvement would be best aimed at making the motor no-load current lower.

To compare the chain drive with a gearbox, I'm going to order one of those Mitrpak units and see what it's like. I've tried to order one online, but their online shop is flagged as insecure, for some reason. Hopefully they'll get back to me tomorrow to sort out a way to order securely.

Jeremy

 

Jeremy
The Mitrpak seals and gear mesh are tight when you get the box. It needs a few hours of operation to loosen up. I also run some light oil on the shafts to the outside of the lip seals.

If you have trouble getting a reply from Mitrpak I have another email that could work better.

Also if you do decide to use the box you will need to cut/grind off the prop shaft flange and fair the housing. I expect a little box will have similar losses to the chain. We measured 3W at 300rpm on the box with the 1/2" shaft under load.

You can measure the load losses if you have another motor you can set up as a brake.

Rick W

 

Thanks very much for that, Rick. Even though the gearbox is more expensive, it's encouraging to know that it's probably no worse than the chain drive in terms of losses.

I also did some experiments with a 12mm stainless steel shaft and some lip-type oil seals today. A standard seal, running with a touch of grease on a standard ground finished bit of stainless thick wall tube absorbs about 6 watts at 700 rpm. Polishing the shaft for a few minutes in the lathe, using a bit of Scotchbright pad, reduced the seal losses to less than 2 watts.

This probably explains why the Mitrpak gearboxes need to be run in.

I shall most probably fit the gearbox inside a composite fairing to drag down. As you say, i shall most probably have to trim down the output flange as well.

Jeremy

 

AS an old motorcycle driver, here is how to take care of the chain, if its not run in an oil bath (best but messy).

Soak the chain in solvent , gas , diesel or ATF. and let it dry.

Get a set of old pans and put water in the larger and chain grease in the inner pan..Bring this double boiler to a rolling boil till the grease is thin as it gets , and dump in the chain.

Move the chain about a bit to aid grease flowing .

Tale out the grease pan and let it cool.

Then wipe off as much grease off the cool chain as you can, any left on the surface will hold grit and wear the sprockets faster.

The grease you need for chain lubrication is inside the chain, right where it should be.

Chain is a wonderful drive system,quiet and powerful, low loss.

FF

 

Jeremy,

While I don't disagree with FF's idea, here is another for your chain treatment. As a ex-cycling competitor, I waxed my chain.
Same process, just use paraffin instead and hang the chain to drip.
I'm sure Rick will have a comment to add...

Cheers, Tom

 

Thanks for the chain tips, very reminiscent of the antics I used to get up to when I was young and rode around on an old (well, 1959) Triumph Tiger. The chain on that used to get taken off and immersed in some black, wax like, grease once a month. The stuff came in a big, flat, round tin and was solid until heated over a gas ring. A bit of wire was tied to one end of the chain and the chain was then coiled over the solid wax-like grease. The stuff was heated up until liquid, the chain left to soak for a while, then lifted out, hung up and the excess was allowed to drip back into the tin for re-use. A quick wipe down when the chain was cool, then back on the bike for another month or so. Thankfully, O ring chains were invented towards the end of my motorcycling period, they made this monthly ritual a thing of the past!

Overall I'm pretty impressed with this tiny chain drive. Because it's running after the primary speed reduction, it's only turning fairly slowly, so grease doesn't seem to get thrown off at all. Being totally enclosed, the chain should stay clean, as long as I can keep water out of the lower bearing.

A new motor arrived to day, a Turnigy 63-74 170Kv motor, rated at 3250 watts (although I'll be running it at a much lower power). Here's a link to it: http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=7870

Overall I'm very impressed with this small motor, it's much better made than the slightly cheaper TowerPro 5330 that I have been experimenting with, plus it's a bit bigger. The good thing about this particular motor is that the Kv is low enough to be able to use a modest reduction ratio without messing about with rewiring from delta to star, although as all six winding ends are brought outside the case reconfiguring it for an even lower Kv would be very easy.

The hull design is firming up and is essentially very similar to the Winsome pedal launch, with a slightly smaller cockpit, greater deck area and no keel/skeg or other underwater projections. The motor drive leg will most probably be fitted into a swing up section of hull fitted into the sloping transom and hinged at the upper edge. This will allow access to the prop for weed clearance and also allow beaching. I'll arrange for the motor to swivel inside this "pod" for steering; steering will most probably be by Morse Teleflex cable. When the motor is down there should be no obvious external sign that the boat has an "outboard" type motor. The attached very crude sketched side view shows how I think this might work.

I'm torn between continuing with the chain drive or trying out a gearbox at the moment. Although the chain is efficient, it is fiddly to make and assemble. It will also need regular maintenance. Decisions, decisions..............

Jeremy

 

Jeremy
Are you going to fit the hall sensors into the motor or will you run it open loop?

I doubt that you will be disappointed with a little Mitrpak box. Have you had any success working out how to get a box?

Rick W