Hydraulic pedal drive system

[Okefenokee]

I was thinking about adding a pedal powered propeller to a kayak and I had an idea:Use a hydraulic pump/motor combo to simplify the design. That way I could have the pedals in front of me and easily route some hydraulic lines to a propeller in the back.

I came across this gear driven hydraulic pump at Northern tools. It weighs less than 7 lbs which is nice. I was wondering if it would be possible to use two of these pumps together with one driving the other. It seems like it should work. The pedals could drive one pump and the other will act as a motor.

I'm worried that the pump will not work as a motor. A gear driven pump should be able to act as motor, correct?

 

[Baluncore]

A gear driven pump should be able to act as motor, correct?

Yes, that is correct.

I see two big problems. Firstly; hydraulic oil does not go well in water, environmentally. Secondly; to move the pedals your knees will have to rise and fall which is not really possible in most kayaks.

An oil filled hydraulic system requires a reservoir tank to allow for thermal expansion and oil use. You would need to use a closed oil safe rubber bladder to avoid spilling oil or mixing water into the oil.

So, how to improve the design, reduce the weight and eliminate the oil? If you took water into your pedal pump you could eject it backwards through a nozzle and so get jet propulsion. That would eliminate the motor and the external propeller.

Two cylinders would balance the load on your two legs without a need for gearing. If the pump was composed of two plastic and or rubber cylinders, each with one-way plastic flap or ball valves then you would eliminate corrosion. The steel pump you link to would corrode rapidly if used with water.

Pressing pedals will push you against the seat back.
So that leaves the problem of your knees hitting the kayak deck.

 

[Okefenokee]

I have a sit-on-top kayak. No worries for my knees. Also, I have an old broken bike and an itch to do some welding.

I was thinking about using a brake reservoir with the system for expansion. The only problem is that I would like to be able to reverse the prop too. I would need a fancy check valve system to keep the reservoir on the low pressure side. I could live without reverse though.

I do like the idea of just using clean water as the hydraulic fluid. That would eliminate any oily messes.

 

[SteamKing]

Dubious assumption: adding hydraulics simplifies things.