SkySails • Hacker News Board

Re: SkySails

#41

Post by murgindrag » Fri, Jul 05, 2019, 2:02 PM UTC

kirrent wrote: post #14

ian0 wrote: post #8

Kind of a dumb question but I'll ask anyway. If the wind is blowing in the exact same direction as the ship is travelling and you begin to retract the sail - do you recoup some of the energy you use during this retraction process through increased pulling power of the sail? Is the ship being pulled a little stronger during the retraction process or the exact same amount as if the line wasn't being retracted? Im imagi…

Newton's 3rd law. If you're applying a force to reel in the kite, that force is equally propelling you forward. Plus, the faster apparent air flow over the kite will produce more power, which is why sailboarders pump their sails while they race. In the real world, I've only ever pumped a kite like this to momentarily get some more energy into it to help with ram air inflation.

I don't think that's quite how the physics works out.

Speed of airflow is approximately constant. Extreme case is reeling it in at 1mph in a 200mph wind. Force is approximately fixed.

Consider that I have an object with a fixed force on it (a good approximation in that case). If I reel it in, I do work equal to force times distance. That work goes into heating up the air. The force on my boat is the same.

I'm not sure it's a problem we care about, but you know how we solve the problem? Collapse (or partially collapse) the kite before reeling it in.

Re: SkySails

#42

Post by alecmg » Fri, Jul 05, 2019, 2:32 PM UTC

sunkenvicar wrote: post #40

sunkenvicar wrote: post #28

Nuclear could really clean up the shipping industry and provide numerous other benefits.

Thorium (U-233) is one way to solve the proliferation risk.

You even put U-233 in there

How proliferation of fissionable U-233 is different from proliferation of fissionable Pu-239? Both share ability to be separated from fuel without enrichment factories, both can be used to build a bomb.

I like thorium cycle, but proliferation should not be used as argument for it.

Re: SkySails

#43

Post by kirrent » Fri, Jul 05, 2019, 10:31 PM UTC

murgindrag wrote: post #41

kirrent wrote: post #14

Earlier quoted context omitted.

Newton's 3rd law. If you're applying a force to reel in the kite, that force is equally propelling you forward. Plus, the faster apparent air flow over the kite will produce more power, which is why sailboarders pump their sails while they race. In the real world, I've only ever pumped a kite like this to momentarily get some more energy into it to help with ram air inflation.

I don't think that's quite how the physics works out. Speed of airflow is approximately constant. Extreme case is reeling it in at 1mph in a 200mph wind. Force is approximately fixed. Consider that I have an object with a fixed force on it (a good approximation in that case). If I reel it in, I do work equal to force times distance. That work goes into heating up the air. The force on my boat is the same. I'm not sur…

No the force is not the same. To actually reel the kite in you will need to pull on the lines with more force than during normal constant distance flight. It is this delta that I was implicitly referring to. The work done as you reel the kite in will largely go into kinetic energy for the boat. You're literally pulling on something in front of you.

Your wind figure is crazy large by a factor of 10. Nevertheless, my point wasn't on the magnitude of extra energy produced through apparent wind, it was referring to the fact that apparent will in general produce more apparent.

Re: SkySails

#44

Post by Gustomaximus » Mon, Jul 08, 2019, 5:07 AM UTC

dmurray wrote: post #35

skribbj wrote: post #30

Earlier quoted context omitted.

Exactly. If the charterer has to pay less fuel for your ship because you put a kite on it, surely they will choose your ship over others? Is there no competition in this market or why does fundamental market principles seemingly not apply?

It's one extra level of indirection. In the housing market, landlords do sometimes pay for upgrades to their rental properties, but they are less likely to do so than owner-occupiers. Additionally if the ship owner was also the operator, they could benefit from some vertical integration, training their crew on the new technology. If the effect of the kites on prices becomes large enough or the technology becomes stan…

> In the housing market, landlords do sometimes pay for upgrades to their rental properties, but they are less likely to do so than owner-occupiers.

Is that a fair companrison? An owner occupier will often over capitalise because they desire something or have a lifestyle benifit beyond money. A rental owner wil look at dollars in/out. With the ships, its all coming down to a spreadsheet of cost vs benifit for both owner and hirer.

Re: SkySails

#45

Post by murgindrag » Sat, Jul 13, 2019, 10:31 PM UTC

kirrent wrote: post #43

murgindrag wrote: post #41

Earlier quoted context omitted.

I don't think that's quite how the physics works out. Speed of airflow is approximately constant. Extreme case is reeling it in at 1mph in a 200mph wind. Force is approximately fixed. Consider that I have an object with a fixed force on it (a good approximation in that case). If I reel it in, I do work equal to force times distance. That work goes into heating up the air. The force on my boat is the same. I'm not sur…

No the force is not the same. To actually reel the kite in you will need to pull on the lines with more force than during normal constant distance flight. It is this delta that I was implicitly referring to. The work done as you reel the kite in will largely go into kinetic energy for the boat. You're literally pulling on something in front of you. Your wind figure is crazy large by a factor of 10. Nevertheless, my p…

My point was that Newton's 3rd law isn't the right way to think about it. Newton's 3rd law means that if I throw a rock with 1 N-s of impulse, I will also gain 1 N-s in the opposite direction. It's a conservation law: "Equal and opposite reaction."

Yes, you do get pulled forward a little bit more, but you can reel in the kite with arbitrarily little "pulled forward a little bit more" (by pulling in slowly) or arbitrarily much (by jerking hard).

The force on me and on the kite is equal-and-opposite, but it was equal before I started reeling it in too.

Re: SkySails

#46

Post by kirrent » Mon, Jul 15, 2019, 2:07 AM UTC

murgindrag wrote: post #45

kirrent wrote: post #43

Earlier quoted context omitted.

No the force is not the same. To actually reel the kite in you will need to pull on the lines with more force than during normal constant distance flight. It is this delta that I was implicitly referring to. The work done as you reel the kite in will largely go into kinetic energy for the boat. You're literally pulling on something in front of you. Your wind figure is crazy large by a factor of 10. Nevertheless, my p…

My point was that Newton's 3rd law isn't the right way to think about it. Newton's 3rd law means that if I throw a rock with 1 N-s of impulse, I will also gain 1 N-s in the opposite direction. It's a conservation law: "Equal and opposite reaction." Yes, you do get pulled forward a little bit more, but you can reel in the kite with arbitrarily little "pulled forward a little bit more" (by pulling in slowly) or arbitra…

>The force on me and on the kite is equal-and-opposite, but it was equal before I started reeling it in too.

Yeah, of course. The force is always equal when objects aren't accelerating, so when you reel in the kite at a constant speed with an increased force, the force on your ship is increased as well. When you reel in with more force you reel in faster, which means that you can make your increase in force arbitrarily small, but in doing so you'll make you're increase in speed arbitrarily small and make your reeling time arbitrarily large. In doing so, you'll end up accelerating the ship by the same amount.

The work done in reeling in the kite is obviously larger than the kinetic energy imparted on the ship and is minimised by reeling in the kite arbitrarily slowly. That energy doesn't just go into fluid interactions but also the potential energy of having your foil back at the ship as well as the ship's kinetic energy.

All of that is a bit of window dressing. To reel in your kite you need to increase the pull or force on its lines. That increase will equivalently act on the ship. The ship is being pulled a little bit stronger in the retraction process just like the original commenter asked.