Pedal Powered Boats

I have commercial drive units which use twisted chain on my pedal boat and I've been quite happy with them, but your idea seems pretty interesting - it would allow a shorter leg, apart from anything else. I'd say that at 175lbs (and its gonna be stronger in stainless) its got to be worth a go compared with the difficulty of achieving strength by adhering/clenching plastic balls onto a line. Because the propellor to water interface is viscous, you're only likely to have an issue if the prop strikes an obstruction, and a shear pin should protect the drive in such an incident.

 

My gut feeling is that 175 pounds while inadequate for driving wheels would be ok with a propeller due to slippage. Assuming a 200lb person with the leverage of the cranks could exert aprox 300lb on the chain if the output shaft was locked but the propeller slips a lot when the boat is at low speed and at high speed the higher shaft speed means less force can be applied.

If you consider that most lightweight efficient human powered boats reach "hull Speed" by 2olb of resistance I think it would be ok if the sprockets weren't tiny compared to the propeller diameter.

 

We measured it on SUBHUMAN II back in 1991. A semi-pro cyclist in a recumbent position with backboard and hand grips was able to generate ~ 450 ft-lbs on the crank axle. The 33" CR prop set could absorb that much from a standing start (i.e. maximum torque at maximum slip). It got much easier as speed built up so we had to have a gearing change to keep up delivered thrust with the typical high efficiency (i.e. high pitch, low area) props that we ran.

 

As I believe this thread shows, somewhere, torque is a huge issue with pedal drives.
It's that moment, not long after top-dead-centre, that huge torque is suddenly applied.
Rick W was twisting spring steel shafts and reporting "mushiness" in even thicker ones.

 

Given a good CAD model for your STL file and a suitable printer, it should be possible to print a complete, closed ball chain (links as opposed to beaded cable) in one pass, in a medium which uses a support material. I don't know if such a medium would be able to provide the tensile and shear strength needed at a reasonable size (and cost.) 3d printing/sintering in metal (stainless steel?) should be able to provide the strength, but I am not aware if it is possible using a support material.

 

If the beads were shaped like so, they would press against each other so slippage wouldn't be a problem.
The cable would just be to hold them together. Would still need a "master link" though.

 

The thrust would then be transmitted through a column of beads in compression. The beads are designed to rotate on each other, and it relies on the tension in the cable to keep the beads aligned and not buckle out sideways. I fear that is a very big ask.

 

Re. my post #2098, beaded chain is not the same as beaded cable, beaded chain is bath plug chain - very low tensile strength. Beaded cable or whatever Google now calls it runs on specially made pulleys for light industrial transmission of rotary power and easily twists between shafts with axes at 90 degrees. Duplex beaded cable is two cables moulded side by side, joined by cylindrical beads, which can work on ordinary chain toothed wheels/pulleys.

 

up to 77" total for around 30" of what I'd call "working distance" around gears, 1/4" or 3/8" wide and rubberized toothed belts running on metal or plastic gears should be OK in water, and should be OK to turn 90degs over any 10" span.McMaster-Carr https://www.mcmaster.com/toothed-belts/xl-series-timing-belts-8/

McMaster-Carr https://www.mcmaster.com/6495K733/ McMaster-Carr https://www.mcmaster.com/6495K711/

For an experimental craft, I'm thinking it would be fine, and simplest, to bolt the gear directly to the prop and have the toothed belt run around that small gear into the water. At the speed the belt will be going around the bike pedal gear I'm not anticipating any annoying spray issues.

 

True, a toothed belt is better than a chain for this application in just about every way.

Yeah, I tried shaping the contact point between beads so that individually they wouldn't get out of alignment but that won't fix the bowing.
I was overthinking it anyhow. An actual chain-chain would do ok if they had bulges in the middle.

 

Anybody ever try curving the tails of the outrigger floats outward so they could be used as dipping rudders just by leaning?
Upside is no moving parts, downside is rough water catching them and making you turn accidentally.

Second idea: Give a monohull a lot of flare only in the middle just above the waterline to guard against tipping without using outriggers so it looks like this from above -=o=-
Upside is no outriggers makes it narrower and easier to transport, downside is probably more drag in rough water due to waves catching the flare and the tipping protection isn't going to be as good as with outriggers.

 

That's a popular way of designing recreational kayak hulls, although the flares on those stay below the waterline. It gives extra initial stability at the expense of secondary stability and speed. Similar to a box keel, used on larger boats (Bolger did a lot of those). Take it a bit further and you have a "tri-hull" design, which then adds hull slap.

 

Yeah, this wouldn't give you any initial stability. It's just to prevent flipping. Dynamic balancing by leaning is required which would pair well with dipping rudders. Performance is situational, but that's a compromise some might want for the sake of a making the overall width less.

Next silly idea. This has been done a hundred times but surely this time it will be a winner!
Swooshy fin drive.


Driven with a stair-step motion. The whole thing can be turned around to paddle in reverse, though it's going to be a little annoying to do so.
Fin is rubber with stiffening rods in it so it flexes in the most efficient way.
The arm is a flat plate of aluminum or spring steel so it can flex up if it hits a log.
Length of the arm would be determined by the amount of leverage needed to swoosh the fin at an efficient cadence.
The strut is fixed and contains the axle connecting the arm to the pedal mounting.
It can be made narrow and low-drag since the axle is just an inflexible hunk of bar stock.

 

Actually, it has more to do with the stiffness distribution in the fin than anything else. There was a paper back in 1991/92 on this which lays out what you need to do. Many of the "swimming" AUVs make use of this method.

 

Chain already bulges enough. Could use pulleys liked those used in chain hoists if you can find a source.