Horsepower/Sail Area equivalence

Ok, I accept that point of view. But while it applies perfectly to the ordinary language (we all know that asking "what's up?" doesn't require than one looks up to the sky in search of an answer), it clearly cannot be applied to scientific discussion. The former can be left to descriptivists, but the latter is necessarily a reign of prescriptivists (using your terminology), because it requires rigorous thinking and hence a rigorous definition of terms.
Confounding the terms "power" and "force" (for example) can lead to misunderstanding (at best) or even to catastrophic failures, when engineering (or more generally, a science) is concerned.

 

What is the purpose of this comparison?
To compare necessary sail area with installed horsepower of engine?


If so, it is unpleasant to say it, but such direct comparison is meaningless.
It is meaningless from both sides to compared:
1) Sail power to move the boat,
2) Engine power to move the boat.

First about sails.

The real measure of sailing boat power is the righting moment she is able to develop while sailing. All the rest is limited or measured by it. 10m2 or 50m2 of sail will produce the same max power -until your boat is heeled to her optimum heeling angle. And, if vertical CoE of 10m2 and 50m2 sails is the same, force (in kN, or pounds, or whatever) will be exactly the same. If 10m2 sail will have vertical CoE 40% (and this is likely) lower, it will be able to develop 40% more force. Of course, to develop same power as 50m2, 10m2 sail will need more apparent wind speed. But this is already another matter. And it rather prove the point, that it is waste of time and effort to search for horspower/sail area equivalence, if the real limiting factors are not set first:
* righting moment of spoken boat at optimum heel angle
* heeling arm of sail in question
We did not yet mentioned direction of sailing: close hauled, reaching or dead running. The problem is, unless a boat is dead running, not all the sail force is spent creating forward motion. In fact, when close hauled, only about 1/3 to 1/4 of total sail force is directed forward and directly create forward motion, the rest act to heel the boat and make leeway, thus increasing resistance to forward motion. It is possible to calculate horsepower equivalent from the calculated total resistance of boat, but it has to be understood, that boat doing 6 knots dead running (0 heel, 0 leeway 0 rudder) could experience only 1/2 the resistance of same boat sailing close-hauled to windward (~25 degrees of heel , ~3 degrees of leeway, 5 degrees of rudder). correspondingly, horsepower equivalent for same speed to windward will be 2 times that for dead running.
To avoid any misunderstandings, here I talk about DELIVERED power, immediately used to propel the boat. In powerboat world it would be only about 30% to 60% (at theoretically very best) of power, actually exerted by engine, depending on propeller efficiency and losses in gearbox with shaft line.

What lead us to engine power to move the boat.

There is also lots of cobblers and unknown quantities.
First, in normal everyday life only RATED power of engine is known, while actually delivered power is up to anyone's guess or to be calculated from measured fuel consumption. And then again, fuel consumption is different for different types of engines (diesel more efficient as petroleum motor, slow rewwing diesel is more efficient as high speed diesel, and all of this depend to some extent on how much of rated power is actually in use, and so on...).
Second big unknown is actual efficiency of propeller. This again depend on boat speed, if there is some additional resistance to motion of ship, if the flow to propeller is restricted (a boat is moving in very shallow water, perhaps).

To conclude:
When all the above mentioned is considered correctly and precisely (that's unlikely), it will be possible to solve sail area/power equivalence, but only for particular boat, carrying particular sails and sailing at particular angle to wind. If boat stability (5 instead of 2 guys on the rail, for example) is different -solution will be different. If sails carried are different -solution will be different. And so on with any of the variables.
It would be a really exciting game in boat physics and mathematics, but not much use for anything else.


Best regards

 

Absolutely. Answering the question correctly requires that you explain the difference between force and power. I think people only objected to the apparent condescending attitude.

Back when I was a cantankerous and unpleasant person (this was weeks ago) I used to sometimes refer to chimps as monkeys, bats as birds or whales as "big fish" just to annoy people. I refuse to be bound by the biological taxonomy when I'm not doing biology.

Sometimes I even used to pronounce nuclear as nucular because I also reject prescriptivism in pronunciation. That really pisses off American Democrats for some reason.

 

holding up weights

In the example of a weight lifter holding up a heavy weight, it is deceptive to say that he is doing no work while he just holds the weight at a constant height. Although no work is being done on the weight itself, the weight lifter is expending energy to hold the weight in place.

The case is similar when a boat is tied to a dock so that it can't move and the engine is running to put a force on the dock line. No work is being done on the dock or the boat, but there is still energy being expended by the engine.

 

The analogy is not correct. For the boat to exert a force by virtue of thrust from the prop it has to do work on the water. It is moving a large volume of water backwards.

The weightlifter can lock himself in position once the weight is lifted. There is no doubt that he is expending energy but most of it is do with keping the weight and himself stable.

As far as the whole debate on the word power goes the original post specifically stated horsepower. This is a defined term in an engineering sense specific for the rate of doing work. It is analogous to watt. We do not talk about the weightlifter being hosrepowerful or wattful. Those terms do not have any recognised usage. So, by usage of horsepower in the very first post, bob had asked an engineering question that any reasonable person would expect to evoke an engineering response.

To relate horsepower to sail area without any idea of the intended speed of the boat does not make for very useful comparisons.

 

The situation is similar because in both cases there is a simple vector diagram that you can draw that shows no net forces and therefore no work being done on the objects in the diagram. However, that does not imply that no energy is used within the actual physical system.

My complaint is that by focusing on the diagram and not mentioning the other physical aspects, you will confuse people who are not familiar with physics. If someone doesn't know the difference between force and power, he probably also doesn't understand the subtlety of isolating different aspects of a physical system for analysis.

As I said before, I agree that a correct response required that you explain the terms being used. In fact I didn't say anything until someone implied that physicists get to define the "correct" meaning of words that predate physics, and that anyone who does not use the technically correct definition even in casual conversation is making an error.

I'm sorry, but I have a strong negative reaction to things that I view as pedantic. It is probably because I used to be an annoying pedantic twit who used to correct people for things like that. Now that I've converted to relativism, I feel the enthusiasm of the convert, and now I correct people for correcting people for things like that.

 

See inserted comment and attachment.

Dave
You have created an interesting dilemma here. You say you are not pedantic but in continuing to argue for the rights of individuals to use engineering terminology loosely are you not being pedantic about that right?

Anyhow I am over it. bob will do what he chooses and believe what he chooses. He was simply having a bit of sport with certain pedantic individuals. Few caught on that he had subtly changed the reference to power rather than horsepower.

 

If I used a winch and anchor then there would be no energy being expended and so it would not be similar in the relevant way. My whole point was that even though the major structural elements of the system were static, there was still energy being expended to keep it that way. Maybe part of our disagreement is in the amount of "locking" that we think is done by a weight lifter. I am a former weight lifter, and at least for me, holding a weight up over my head was almost as exhausting as getting it there in the first place --it was by no means a semi-relaxed braced condition; I had to work very hard to keep my muscles tensed and prevent the joints from folding (although this probably varies quite a bit from individual to individual). There is work being done in the micro contractions of muscle cells analogous to the work being done by a boat engine in moving water.

Also, let me say for the third time that I was not criticizing your answer for explaining the difference between force and horsepower. I don't know why you keep implying that I was.

Finally:

This is, of course, the irony of conversion. My beliefs changed, but my personality seems to have remained the same.

 

The original question was never about power - it was horsepower. bob made the subtle change and sucked a lot of people in. He is good at it and I enjoyed the banter.

Once the weight is in the elevated position the weightlifter could be replaced by a couple of pieces of timer and stays. These items cannot produce horsepower. The weightlifter is developing some horsepower to keep the weight and his body stable but it is a fraction of the horsepower required in the period to elevate the weight.

The boat has to use horsepower constantly to move the water to sustain the force - very poor analogy to a weight lifter holding a weight.

 

everybody, we have to re-vitalise rick. he's not as young as he used to be


rick and i both agree
and
KNOW that stoneage math is blunt and SILLY, but if the engineering straight jacket fits, then wear it.



it's obvious all power measurements and terminology should be based on the POWER required to hold 1Kg steady 1m of the floor.

so much engineering drivel would just disappear



"Relating sail area to horsepower has to be in the context of boat speed or some reference that can lead to an assumed boat speed."

no it doesn't. rick i'm surprised at this dodgy engineering, you could have assumed joules per second rather than distance.

also, quite frankly, i'd have a hard time paying an engineer that didn't use SI units and conventions. so i suggest it's your fault in the first place for listening to a question about non SI units.




"bob will do what he chooses and believe what he chooses."

i REFUSE point blank to believe what i choose, OR do what i choose.

society has spent so much money on filling my brain that i would be an ingrate of the worst sort unless i regurgitated parrotwise the norm, and only did what was expected of me.


no foul independence for me

the onerous neverending crushing responsibility to have and maintain your own personality?
never!
i'll believe whatever society decides i should believe.

let the brave and noble mob be my guide


i was my parents puppet till my teenage years,
but then, i rebelled and swapped them for my peers,
I had to keep this compulsion to myself,
This need to be like everybody else.

But now I'm out of the closet and I feel free





"bob made the subtle change and sucked a lot"

ok rick, the gloves are now officially off.

a show down.


right here, right now.

we'll redefine power, and see who can come up with the briefest maths AND satisfy the most number of people here.



game?




me first -
1 power unit = the weight of 1 liter of water

vectorless, calculusless, distanceless




the answer then, is also just a straight 75 Kilograms on the 1 meter flat square sail.

which equals 21.21 m/s wind speed



i'm fairly certain i should be in charge
(of everything)

can you all let me know?


thanx,
mal