?

Log in

No account? Create an account

Tue, May. 10th, 2005, 04:51 pm
Rowing

I had a thought about rowing the other day. Might it be more efficient to have a human-powered canoe where the human powers a propeller using an ergometer-style mechanism, or are oars already just as efficient as that would be but with less weight?

Wed, May. 11th, 2005 12:29 am (UTC)
mrcommonreader

Paddling provides the highest level of steering control. The single paddle acts as both the motive force and the rudder. With a single paddle you can stop a canoe and spin it in place. This is nearly impossible with two-oar systems.

Two-oar rowing is certainly a much more efficient use of muscle power since the legs do the overwhelming majority of the work (assuming a sliding seat). You can steer by using just a single oar, but you don't have as fine a level of control.

An ergometer-style drive would require some additional mechanism to steer.

Tue, Jun. 21st, 2005 05:30 pm (UTC)
(Anonymous): spinning in place

if you're even a little coordinated you can move the oars in opposite directions and spin the boat in place. it's tricky the first time, but after you get the hang of it it's easy. the main thing isn't the oars, it's that a shell is awkwardly long.

Wed, May. 11th, 2005 01:40 am (UTC)
misterajc

Rowing is pretty efficient. It makes efficient use of the strongest muscles in the body with very little energy wasted generating turbulence in the water. I doubt if a human driven propeller craft could do as well, though there have been attempts in that direction :
http://www.water-play.com/2003Site/PBC_future_bike1.shtml

Sun, Jun. 26th, 2005 08:00 pm (UTC)
bramcohen

According to a response to my question about propeller efficioncy the benefit of pedalling is that it allows you to output a lot more power quickly, but for distances long enough to require aerobic respiration the comparison is highly unclear.

Thu, May. 12th, 2005 02:40 am (UTC)
zx2c4

What about a boat with two propellers on the back each operated by a seperte set of feet. The legs are the strongest on the body and having the two propellers seperated would allow for turning.

Thu, May. 12th, 2005 06:05 am (UTC)
(Anonymous): Rowing

I saw a boat with this in Kirkland-
http://www.hobiecat.com/kayaking/miragedrive.html

It's quite interesting, though I don't quite see the benefits of the odd propeller design.

Legs are stronger than arms and rowing is not the most efficient motion, this makes sense.

Thu, May. 12th, 2005 06:31 am (UTC)
7leaguebootdisk

There was someone trying to make a hydroplane rowing shell, it was faster in simulation, I don't know if it got built. You would have the drag from the lift, but a much smaller wetted area, a net win.

Thu, May. 19th, 2005 10:21 am (UTC)
(Anonymous): Weight is critical

I used to be a competetive sculler, so I can say with some authority that weight is critical. Racing sculls are 14kg minimum weight. Anything that added to that would require a larger surface area being in contact with the water and therefore more drag.

The Physics of Rowing FAQ is fairly authoritative:
http://www-atm.physics.ox.ac.uk/rowing/physics/

Thu, May. 19th, 2005 05:10 pm (UTC)
7leaguebootdisk: Re: Weight is critical

Well, 14kg is insanely heavy, if you build it out of carbon fiber, so weight is not the issue. Plus the rowers weigh quite a bit more than that anyways. Sure the foils do add weight, but not much, I'd expect you have weight to spare.

Thu, May. 19th, 2005 05:29 pm (UTC)
7leaguebootdisk: Re: Weight is critical

The way it would work, it would start out slower (because of the drag of the foils), but then you would reach your planing speed, and rise out of the water, so you would have the area of the foil and supports, but no longer have the hull in the water at all. You would have the drag from the lift from the lift produced by the foils.

But it would seem that rowing is as ossified as bicycling, and no one has built such a hull.
Kayaking would seem to be different. Serious hobiest in a hydrofoil kayak beats the world record for 500m, and the math implies that a world class kayaker in a foil kayak could beat an 8 man rowing hull in the 2000m.

A pedaled propeller hydrofoil hull would be best, much higher sustained output that the kayak, you do not have the 20%+ loss from the oarlock, or the dramatic change in oar angle you may have with the foil rowing hull.

Fri, May. 20th, 2005 09:09 am (UTC)
icheyne: Re: Weight is critical

(I'm the anonymous guy who posted earlier.)

You may be right, but the cost would probably be be prohibitive in terms of racing. A cheap racing scull costs ~£3,000 ($5k), which already puts the sport well beyond the reach of most people. Racing boat design has been ossified for some time, mainly due to this constraint. Imagine saving up for a sculling boat and training like a madman, only to be beaten by a flabby wimp in a £10,000 boat...

There is still some innovation:
http://www.frontrower.com
http://www.rowvirusboats.com/virus/sliding_rigger.html

You can discuss this until you're blue in the face at news:rec.sport.rowing.

:)

Tue, Jun. 21st, 2005 02:33 pm (UTC)
(Anonymous): human-powered light craft

MIT had a man-powered hydrofoil based on pedals in the early 90s... the sculling motion may well generate more power (since you're using not just arms or legs but both, as well as your core mass) but the problem is converting that into the kind of continuous rotational energy necessary for a propeller -- the ideal mechanism is some type of flywheel, but the inherent weight of such a device is prohibitive in a human-powered boat.

An alternative might be a propeller with some kind of automatic pitch mechanism that folds when the person is in the negative part of the pull -- sailboats have a similar design that lets the prop fold away when not in use.. but it might be too inefficient.

Tom K

Tue, Jun. 21st, 2005 04:11 pm (UTC)
(Anonymous): conversion efficiency kills you


The process to get the energy thru any mechanical conversion is where you lose the efficiency. Any gearing or pulleys would cost you 20-30% loss of power that is transmitted, not even counting the weight as other have mentioned.

Sun, Jun. 26th, 2005 08:06 pm (UTC)
bramcohen: Re: conversion efficiency kills you

Yeah, apparently all that swinging of human arms loss is fairly comparable to mechanical lossage in anything which uses gears, surprisingly enough.

Tue, Jun. 28th, 2005 10:29 pm (UTC)
(Anonymous): human-powered hydrofoil

You might want to search for human (powered) hydrofoil on google. The original was in Scientific American many years ago. It bycicle style. Apparently there's a recent one based on rowing-style motion. But all such machines use propellers rather than oars.