Boat Forum / Building / October 2005

Willliam,
Consider the basic principle that thrust = mass x velocity, but power
consumption is proportional to !/2MV2.
This means that while a small jet at twice the speed has the same thrust as
the larger jet at twice the area, the power consumption is four times
greater!
Big Slow Pumps!
For anyway near the same effectiveness the jet area needs to be around the
blade area of an oar.

Regards barry lawson

Have you considered the idea of using foot-operated paddlewheels
instead? It would seem to me that they would give you better propulsion
for the same amount of effort. Also, I would think that they would
actually be easier to build and maintain. You could have 2 wheels (1
each side) and have them set up to mount on top of the gunwales. They'd
be nice and quiet too, for those nature pix that you might want to
capture with your camera.

Hope this helps,
Paul

That makes sense.

I'm afraid you're baseing your design on a false premise. The small
muscle groups of the upper body are not capable of driving the heart to
as high a rate as the large lower body muscles. I'm also an avid cyclist
and can tell you from experience that paddling is far easier on the
cardiovascular system than cycling is. With the exception of high
intensity paddling while surfing or playing in rocks, I never even get
winded while paddling. By comparison, I can drive my heartrate into the
high 180's on a bike.

It's not even remotely possible to generate the same amount of power
with the upper body muscles as with the lower body muscles, which is why
they cannot drive the heart rate as high. It's also why kayak racers use
boats that allow them to rotate at the hips using the legs to help drive
the boat.

That's because straining to lift something increases blood pressure,
especially if you bend over to do it.

Thanks for all the suggestions.

You can get more power from the lower body (the muscles are larger) but
for the *same* amount of power the upper body puts more strain on the
heart. That's what I read while researching heart disease.

I have worked before with hull resistance figures for canoes and kayaks.
You can get a pretty good estimate of the horpower required to move a hull
at a certain speed. Non-athlete paddlers can sustain about 1/20 of a
horsepower. It's also easy to calculate the amount of water pumped from
the size of the cylinder and the rate of pumping. It's possible to convert
the amount of foot pressure on the pump into the speed of the water
leaving the pump. I should find out about the efficiency of water pump
propulsion. In one book I was lookign at last night it says the shape of
the outlet nozzel is important. I once estimated the energy comsumption
efficiency of paddling a canoe at about 13% (it's on my website under
"Boats").

I agree with one poster that innovation is the fun part about designing
and bulding boats. But I'm not crazy about the building part,
time-consuming messy physical work costing money, so I try to do as many
calculations as possible before starting to build, even though trying to
figure out the formulae sometimes drives me crazy.  :)

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Hobie makes a foot powered boat, but I wouldn't call it a kayak.