confusing tandem pedal drive

This is a drive from the now closed Dads Boats. I have been looking at this for a while now, it confuses me because it looks like one person has to pedal backwards while the other pedals forward.

The rotation of the sprockets is opposite, and thus surely the cranks too. it looks like a normal gearbox not a differential. how does it work? am i missing something?

in the videos i have watched both people pedal forwards like normal.

 

Bevel gears I suspect.

 

ah that makes sense. im my head was imagining there was only two bevel gears, with one of the gears shared by one shaft.

 

I worked it out! Then saw Tiny Turnips drawing which is just as I imagined. "Dad" used a hand drill in his first incarnation for brvel gears for a single user. I read their website when they were still in business. Pity they closed but the boats were very expensive. There are few pedal powered options available but building my own was always important to me.

 

I can reach hull speed and cruise at a reasonable pace.

 

The Hobie Cat pedal kayak is quite unique

 

BTW, pedaling backward is good. My gym USED to have a stationary bike that would go either way and it seemed really good to change up the direction.
I like any exercise machine that allows you to 'go backwards' and seems like going backwards on these gizmos just feels better.https://www.amazon.com/Schwinn-411-...f5e6c9d158dc5e7c9885770eba595c&language=en_US

 

A clever guess, TT, but I doubt it's exactly like that. In that arrangement each person provides the reaction force for the other person. If they aren't closely matched in strength, one person will be driven backwards. Without breaking it open and looking, I'd guess that each crank has it's own bevel assembly, and like on a bicycle, each crank can freewheel if drivel slower than the driven shaft. As far as direction of pedalling vs direction of travel, you can change the direction the drive shaft is driven by changing the driven bevel gear's orientation on the driven shaft. In this case, the driven bevel gears would face the same way. Either both forward or both aft.

I think a lot about this sort of thing. I'm very interested in tracked vehicle drive trains. Engine, transmission, steering arrangement, final drives, sprockets, road wheels, suspension, tracks.

 

Squidly-Diddly- im a cyclist and im not sure my legs would like pedalling backwards, you become finely tuned after a time. one of the requirements of my pedal boat is that it has to be strong enough for my cycling friends to not be able to break it, they are all going to want to pedal it hard.

DogCavalry- i am sort of with you, it makes sense to me that in Tiny turnips diagram you just push back against each other. but looking at images of dads boat it looks like a straight forward "reverser" gear. it also doesn't look like there are free wheels on each shaft, it looks like shaft couplings. im not sure though still not totally getting it. i ride a tandem and both cranks are joined by a timing chain so you do have to ride the same cadence ,otherwise one person is being dragged around by the faster cadence of the other.

 

Hi DC
If one person is stronger, they will do more of the work; maybe most of the work, if they are very much stronger, but that's okay.

But they won't ever drive the weaker person backwards without pedalling backwards themselves.

Its like bit like two people turning a ships wheel, one on each side, One pulling up, one pushing down.

I guess you could build a freewheel in if you wanted to allow one of the pedallers to have a rest while the other pedalled, otherwise their feet will be whizzing around while the other person pedals, whether they like it or not!

And yes you could rearrange the bevels, or add additional gears in if you wanted a different arrangement, say both pedallers facing forwards.

 

Tiny Turnip is right. The rotational speed of the two input shafts is in a fixed relation, probably 1 : 1, (assumed a gearbox like his sketch). It is not very important if both people pedalling send the same power in the gearbox.
DC's idea of a reaction force will be right in case of a differential gearbox.

 

TT's diagram suggest that the boat will ride like a tandem. Both riders pedal at the same speed but can apply as much or little torque as they wish. The video shows one set of cranks are vertical whilst the others are horizontal. This will be to even out the speed of prop. rotation since one users is at maximum potential torque whilst the other is in more of a dead area and vice versa. As a single user you could try to even things out with an egg shaped chain ring. Some cyclists swear by them but they've never achieved big popularity, particularly when Shimano accidentally mounted their version 90 degrees out which caused a pulsating feeling. I had them on a 1992 vintage MTB.

 

Ah, you're right. I was thinking of a differential arrangement.