Elastic band powered boats

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

Fun item to build

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

Hello,

this started on the thread "Cassie, by Amy Jane" in that I asked what self cut propeller worked best for her boat. I explained I was interested as I was building elastic powered craft etc.

Discussion got going and included Ashley, so rather than hide it away and also take over Amy J's thread, I have started this one.

Our Commodore's challenge (one of them) this year is to build a simple boat (floating device) not exceeding 500mm to be powered by elastic strip supplied by him (1m of aeroelastic in a single strip, about 5 x 2mm). Any form of conversion of band power to the craft is permitted, water or air propeller, paddle wheels or oars. Object is to get distance and run time and some sort of control. Lots to see on the web, but try and build one. Distance and duration are hard to achieve. Theory and practice make for clever designs.

My attempts will pop up here as they are created, have a go too – if you're brave enough, as Ashley pointed out, it's not quite as simple as you first think.  Ashley has shown one of his on that thread, maybe he can add it here too?

Aye, Kim

Edited By Kimosubby Shipyards on 01/03/2015 09:52:23

Edited By Kimosubby Shipyards on 01/03/2015 09:53:38

[ashley needhamParticipant @ashleyneedham69188]

 

Why not, eh?

The issue of control. when the boat starts off, there is laods of torque, and the boat steers on this torque like a good un, until the motor wears down a bit, and then the rudder takes more control. This is a bit unavoidable unless a) you have govenor, like a Hornby clockwork locomotive, or b) twin props.

The govenor I thought could be of the simple pivoted weights on sprung restrained arms running inside a drum variety…the faster the shaft spins, the more the weighted arm drags on the drum and slows it down. NOTE; you are not wasting energy as would be the case for an electric boat, as the energy you have in the `band is a set quantity and is excpended, all of it, despite anything like a govenor. Twin props if made contra rotating, would solve this issue. Nylon gears would probably be adequate pushed on to shafts and fairly efficient.

The rubber needs to be lubricated with Glycerene to stop in sticking together when wound into knots. This makes a MASSIVE difference to the use of latent energy as stored etc.

The motor needs gearing UP to drive the prop to get the distance. The bigger the prop diameter, the worse the torque effect. My big stripy boat (4 foot long) was TURNED OVER by the twisting of a 70mm prop on its first test, not that the prop is in the middle(ish) of the boat for better leverage…. I had made a nice well pitched sheetr brass prop in good vintage style, but after this fitted a commercial 50mm 5-bladed prop I happened to have. Some commercial toy kits use a big flight rubber band type prop with no gearing…not sure how efficient this is. It would need to be placed deep in the water, and is too easy.

I have a feeling that of the types of propulsion suggested, a water screw is the most efficient and so would be the method of choice.

A long thin band or skein twists up more than a short one, I would go for max length of boat. On mine the (meccano) gear is as far back as poss so I could get the most rubber in. Two `motors` of 10m rubber each in this case. It would take a lot more, I never dared to try.

The PULL of the rubber is considerable to thought should be given to ensuring the rear drive/cog hook arrangement is adequately free…a little bearing or lots of grease….nylon washers..that sort of thing, and also push fit gears bay be a bit wanting especially the larger one that is connected to the motor although I suppose a grub-screwed collar would be ok to take this force .

I found a small keel useful in keeping the craft upright (to counter initiial torque), but mine was immensley powerful and yours is not likely to need this. My first one was a sort of Bowman Aeroboat copy and didnt have a keel, and was only 2 foot or so long. and 3m or so of rubber.

Obviously you want as light a boat as poss, but it needs to resist the rubber contraction.

A sub of course would be best as is it more efficient at getting through the water.

Notice below how much hull length is wasted by having a conventional shaft drive arrangement as opposed to the second one. One could I suppose use a cat hull for stability and have the motor as a stick in the mddle? Hull-on-water would be much less, whilst keeping with the 50mm limit.

I`ll stop now.

Ashley

 

Edited By ashley needham on 01/03/2015 14:29:22

[ashley needhamParticipant @ashleyneedham69188]

Just re-read the other post. You really really need a reliable method to hold the wound motor, as if the boat slips a bit and the prop starts, there is little you can do about it. I used a simple long pin (nail or coathanger wire) that was inserted through a suitable meccano gear hole into a hole drilled for this in the supporting wood.

Pull out to go, easy.

Ashley

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

Well Ashley here's my first trial, construction a bit simple, but it does show up the major problems you highlighted, the main one being a decent thus washer. Body is hollow beach rod, champagne cork in end and bent aluminium to hold the gears. Yes the screw is far to small, but still on the bottom of that curve. It seems that the best thrust washer set up is two beads, and I'll have to drastically reduce the rod diameter of the gears shaft too. The copper squares are new props ready to cut and twist.

The wooden device above the CD is my band winder. The CD just happens to be an ideal size for the intended paddle wheels, two, either side the main body, which will be a triangular triple pontoon shape with stout central section to resist the band energy.

This is a first attempt at the flapper paddle wheel as I suggested to Amy J, I've seen them before and they stroke the water bending in use and take far less energy to turn than the usual paddle dipping into the water. With the boat as light as possible, all energy hopefully will turn the wheels and give distance.

All good fun, aye, Kim

[Amy jane SeptemberParticipant @amyjaneseptember49770]

Hi all

I shall be watching all this with great interest!

all good fun, for sure

[Bob Abell 2Participant @bobabell2]

Hello Kimmo and Ashley

Just a nostalgic chat and not really relevant here…………This article appeared in Model Cars Mag 1966

I was heavily involved with elastic power, some time back, as Ashley may remember

I just thought it may be helpful somehow in your current thinking?………And to do some trumpet blowing!

My elastic powered craft was a Dragster with 18 yards of lacky in it!

We had a Drawing Office competition and used to race down the aisle at lunch time

It was fascinating to see wide elastic bands, which were tight on the wheels, become airbourne when the wheels ran at top speed!

As can be seen, we geared four elastic harness together for fiercesome power!….And as you mention, thrust races were essential

A few pictures to ponder….

Bob

[ashley needhamParticipant @ashleyneedham69188]

Ah ha, I see that you have been doing work already!!

Much of what i said of course does nott apply to something really light and with only 1m of rubber, however any info is always useful.

Ashley

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

Thanks for that Bob,

truly amazing stuff, and all with some twisted rubber. I knew you'd have used the stuff along your career in everything.

Amy J and Ashley, I got the rubber size wrong in the other thread, its 1m and cross section is 3.5 x 1mm. On a 500mm long max model thats the whole length doubled. Not much power but what there is I've got to use.

Lets just be clear here, I'm going to prototype three designs, the underwater screw as in the picture, with a better boat to float it; a paddle wheel boat because it can be lighter; and an air screw boat using lessons learnt from the first two for power transfer and thrust washer design etc. I have ideas regarding a rowing mechanism, but the power loss would be too much when compared to the others, the oars only pull for 1/3 of a rotation. That is why my elastic band can be removed and fixed by the end loops, the same band can be used across the variety of boats.

The paddle wheel looks good, need another and then ponder how to transpose the lengthwise power twist in the elastic band to a crosswise rotation for the paddles. First ideas were a pair of bevel gears, but friction might be too much for such a small band. The gearing on Bob's car would be possible I suppose.

Two ideas come to mind, the first is to simply have a cord between the horizontal gear shaft and the cross shaft of the paddles. Wind loads of cord onto the paddle's shaft and tie to the band shaft. Set band shaft to rotate, and the cord is pulled off the paddle shaft, rotating it, and spools onto the band shaft. What could go wrong with that? Indeed, by varying the cord drum diameter on either shaft I can alter the speed. Too simple?

Second, now being an old farm hand from way back we used to run equipment off the tractor side drum, and by twisting the belt get right-angled rotation to that of the drum (and crossing the belt gave reverse) – but you need a long belt. My idea would be to use two rubber belts, each angled through 45', in line and in that way I could get an UP gear rotation on each one of say 1:5, 1:10 total but probably too much for the paddles, but their design should also act as a governor slowing the rotation due to interaction with the water surface. Well that's what I think.

Any offers out there, aye, Kim

 

Edited By Kimosubby Shipyards on 01/03/2015 18:44:07

[ashley needhamParticipant @ashleyneedham69188]

Kim. I dont think that a pair of push-fit Nylon bevel gears will create that much friction. The twisty band thing will eat up valuable boat length which i dont think you can afford. Perhaps as large a bevel as you could get as this would give the cogs a better mechanical advantage. You may not need to gear down paddles due to the increased water reistance compared to a prop?

Similarly the drum thing which I have read of before (in boys own magazine) has merit but would require endless experimenting with several variables to get the right amount of cord and so on.

I like the stick setup…perhaps having that sort of thing between hulls and just dipping the gear end in the water with your large prop would be an idea. How about even putting the whole thing in a free flooding tube with a pointy front end and suspending it under one hull like a torpedo?

I am sure i tried 1m of rubber on the first boat, and it ran for so long in the bath that I gave up waiting for it to stop. was abysmally slow though.

Ashley

[Keith LongParticipant @keithlong89920]

Kim

For the paddle wheel boat – what about a catamaran hull with the elastic doubled up to 4 strands and across the hull. Gearing would be by ordinary spur gears then rather than having a right angle drive.

Keith

[Kimosubby ShipyardsParticipant @kimosubbyshipyards]

Hi Keith,

thought of that, but means a wide boat, not long and thin for directional stability – though could easily give it a try. I assume the band folded to four will still give the same power twisted as the length folded into two? Need some of those flybys to put us straight. Now with a catamaran hull why do I require two wheels, surely one in the middle would be enough? Oh joy, another design to build and test, thanks Keith.

Ashley, the elastic is twisted the full length oolong the boat near 500mm, the pulley at the end then has a twist belt down onto a smaller pulley the shaft running back up the boat. This then drives the sec on twisted belt onto a shaft with pulley underneath the across boat paddle shaft, with belt up onto that shaft. A bit like your own design with the drives running back along the boat a bit. This puts the paddle wheels about (rough calc here) 1/4 way up boat length at most possibly less.

I have the belts and pulleys, could use other elastic bands as they do not supply any power, just a transfer capability.

My original twisted band stick, first picture, does in fact float almost like a surface torpedo, or surfaced sub. That might be easier to stick with because all the weight is under the floatation volume. I'd have to add ballast and keel the other way up. Or as you say, stick it under a hull as the drive unit, but flood the chamber, the water would act as a dampener too.

Oh dear, will sleep tonight or will I creep up to the attic workshop at midnight and start scratchahdoodling about?

Aye, Kim

[TelstarParticipant @telstar]

Hi As an ex. (old) rubber powered flyer, I would recommend:-
A.lubricate the rubber band. Use a soap based lubricant, oil or grease damages the rubber, I used to make lubricant from soft soap and glycerin but I should think there is a commercial lubricant now.
B. Prewind/pretension the rubber band, this allows you to get more turns with less strain, It can give 20-30% more than just winding the band straight.
We used 'huge' coarse pitched props on direct drive, but on semi scale a step up gear ratio sometimes was used to put 'load' on the rubber and stop a short burst of high speed whizzing in favor of a longer run of moderate speed.
There was examples of rubber powered experimental test models on display with Tubinia, at Newcastle, that Parsons used for high speed prop. trials. Some had about 10 to 1, or 15 to 1 step up on the prop.

Cheers Tom

[Author]

Posts