Yes, generating heat from work has a very high Second Law efficiency as well - nearly 100%?
but props themselves are only about 50% eff., so that’s 5500 to actually moving the boat.
Actually, I think vehicles that carried/carry scientific probes to other worlds (Moon, Venus, Mars, etc.) get the best milage. Sure, they use a lot of fuel at first. But once they get coasting, that average milage starts to go up.
How about a paperweight? Zero energy goes in, and they hold stuff down, which counts as exerting some kind of force. I realize the amount of force is really small, but when the energy input is zero, the ratio works out pretty big.
But surely, most electric heaters also emit light, so their ultimate efficiency must be something less than 100%.
Hmmm… and your office is on which floor?
TheLoadedDog,
that’s what the SDMB is about! Classic. 
Err… my question meant, more specifically: are there any organisms having bodies that incorporate a wheel?
Heck, if I need an organism that just manipulates wheels, I don’t have to look any further than a mirror.
Quoth Ethelrist:
Force is not the same thing as energy. Energy is equal (roughly) to force times the distance over which that force acts (I’m leaving calculus and vector arithmetic out of this). A paperweight can exert a considerable force, but the distance moved is zero, unless it’s putting a dent in your table, so the energy is zero. Of course, if it does put a dent in the table, then that’s part of the energy that you put into the paperweight when you lifted it onto the desk.
Nope, sorry. When you calculate efficiency, you must look at the whole system, not just a small part of it. To make an electric heater produce heat, a power plant must produce electrical power by burning coal or gas (or whatever), making steam, turning a generator, etc. The power must then be delivered to your house via (resistive) conductors, transformers, etc. Anyway, when you look at the whole system, the “100% efficient” electric heater doesn’t look so hot after all. (Pun intended.)
Crafter_Man, I disagree with you. A machine or device takes one form of energy and outputs another and it does not care nor should it where the energy calme from or where it’s going. Your definition makes efficiency ratios meaningless.
The standard definition of efficiency is useful energy output divided by energy consumed. Note that in the case of a spcae heater heat is the desired output and therefore counts. In a motor, mechanical energy is the desired output and heat is counted as losses.
Along the same reasoning lines I have to disagree with k2dave on the propeller issue. Who cares where the energy is consumed? The motor outputs (say) 35% of the energy it gets from the fuel in the form of mechanical energy and 65% in the form of heat. That motor has an efficiency at the shaft of 35%. Now you can use the energy any way you like and each different use will have different efficiency of its own.
To say you must account for everything that happens before and after makes it impossible to give any figure. Or rather, as has been said several times, the figure would always be exactly 100% as you are just moving energy around.
Damn, CheapBastid, you beat me to it. I’m readin’ the thread, thinkin’, “Am I really gonna be the first one who remembered that Cecil talked about this?” And I kept readin’, and people kept talkin’ ‘bout it, and I kept thinkin’, nobody’s rememberin’!
And then you beat me by like 20 minutes. Bastid indeed. 
I understand what you’re saying, but I guess it all depends on how you define “efficiency.” I tend to think of it in more economic terms, even when it comes to technical applications. For example, if an electric whole-house furnace is 100% efficient, and a natural gas furnace is 60% efficient, then why is electric heat so much more expensive than gas heat to operate (all else being equal)? See what I mean?
I guess my point is that, most of the time, when we say “efficiency,” we really mean “economic efficiency,” i.e. we are looking for the least expensive process for a given result or outcome. This is why I would include the power plant, transmission cables, transformers - the whole nine yards - if I were to calculate the real “efficiency” of an electric heater.
>> most of the time, when we say “efficiency,” we really mean "economic efficiency
Well, you can say and mean whatever you like but that is not what the OP says and I doubt it is what it means.
In any case, if you are concerned with economic efficiency, (cost per unit of output) then you are still not concerned with where the energy came from etc. Only with cost of input per unit of output.
The OP clearly does not refer to that but, as I already mentioned in an earlier post, a person must be the most economically inefficient machine, especially if you feed him at an expensive restaurants.
An average male in good shape could probably generate 150 or 200 watts for a long period of time. But you still have to count the full diet. In other words, the practical mechanical energy a person could generate working several hours a day could be bought from the electric company for 2 or 3 cents.
If we could produce anything meaningful and useful, all those gym machines would be hooked up to the grid 
I remember from a attempt at human powered flight. They said that a male in very good physical shape could maintain 1/4 hp for a fairly long period.
Ahem. A few of the links I posted above cover the aspect of human power output on a short-term and daily basis. If you are interested.
Electric heaters can look to be very efficient isolated systems, providing you are looking at an enclosed, water (or even better oil) heaters. Fan based systems on the other hand have nowhere near this efficacy. Even coil-based systems are not as good.
Unfortunately as a useful machine a heater by itself is practically useless. It’s not the input I would question as much as the desired output. In most cases the output of a heater is not just heat but heat contained within a given area, which is to a large extent dependent on the environment, which would have to be considered part of the ‘machine’ generating the output.
A heat sink on the other hand makes an interesting case study.
Britt, I have no clue what the heck you are talking about. An electric heater converts 100% of the electric power consumed into electricity. I don’t care if it has oil or shammpoo, you are getting exactly the same heat from it.
This reminds me of a meeting where we were discussing how to heat a very big space (hold of a ship). A lady, who obviously didn’t have a clue, had seen some TV ads advertising some small electric heaters which were just awesome because they were ceramic. The heaters were tiny, like 600w. Anyway, she just kept going on about getting one of those while the rest of us rolled our eyes and contained ourselves. She just could not understand the concept of numbers. Lady, we are talking 10 or 20 Kw and that is 600w! She would just say "but it’s ceramic! They said it on TV! <sigh>
I don’t care if it’s ceramic or not, a 600w electric heater puts out 600 w of heat. No more and no less.
The only way to get more heat from an electric heater is to use a heat pump. And the only way to get less is… to open a window.