By the efficiency, it's quite simple with machines. Most cases you can measure the output and input power and there you'll have efficiency.

The thumb rule in boating is that propeller's max efficiency is limited by slip. So a prop with 10% slip can have max 90% efficiency and the one with 50% slip can have maximum 50% efficiency. Usually the best props can achieve efficiency up to 80-85%. That means you'll get 40hp of pushing power from 50hp at prop shaft.

I've heard that some competition paddler could have reached about 80% efficiency. That's just something I've read on net, not sure if it's right. But however if the same works for paddler it means the paddle blade could not slip much, very little indeed. In that perspective GP could have slightly less efficiency on technical view. Or maybe the thumb rule does not work in this case?

One thing is true anyways - the efficiency in real paddling can be very different thing. In faster cadence the muscles might have higher efficiency so that might change the picture - less efficient paddle might turn to be more efficient in the whole big picture.

So quite much speculation, but in generally the GP can not really be poor thing - years of development in all kinds of cultures have lead to mostly quite similar paddles. Harvey notices this in his book where one paddle is from Africa.

: Energy conversion efficiency - desired energy output per energy input

: If you go here: http://en.wikipedia.org/wiki/Efficiency

: You see how many versions of efficiency there is.

: When you go up a hill your car goes into low gear because it strains the
: motor left in the higher gear.
: It becomes less efficient.

: In paddling, the human body has to be taken in to consideration when one is
: to assume that efficiency is the least motion to move something. If I was
: challenged to move my boat the furthest forward that I could with one
: stroke, I would pick the most efficient paddle I could - probably one with
: a blade twice the size of any made.

: But if I had to use that paddle for an hour, I would probably die pretty
: quick. It would not be very efficient for me if my goal was to paddle for
: several hours. Bicycle racers always pick the lower gears to keep from
: taxing their muscles from strain. Any bicyclist who uses too high of a
: gear will tire out quickly.

: So Nick's right.
: "As a general principle it is more efficient to generate a propulsive
: force by pushing a large mass slowly than it is to push a small mass
: quickly".

: The most efficient paddle is one that you can generate a strong propulsive
: force for the period of time that you would like to do that.