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ReadyHeli . Power Helis . Experience RC

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Off Topics Jokes-Puzzles-Riddles > ABT #3 Revisited
 
 
Mark C
Key Veteran
Location: Houston, TX - USA

My Posts This: Topic  Forum		This is a revisit of ABT #3 (http://runryder.com/helicopter/t243488p1/) which certainly sparked some debate.

To restate the ABT:

You're in a sailboat on a day with NO wind. However, there's a 10 knot current taking you directly toward your intended destination. What's the fastest way to get to your destination? Should you drop your sails to reduce drag, or use your sails to get there more quickly? You can only use the energy from the wind and/or water - no paddles, motors, etc...

Spork's answer was to raise sails.

I consider it sailing downstream faster than the stream which I believe is impossible; Spork calls it tacking into the relative wind which he believes to be possible.

Spork and I have touched on this in PMs agreed to revisit the issue. I would rather start a new thread to address it for a nice fresh start. It's easier than keeping track of PMs and you can post little diagrams a pictures here and format things little easier and others can ring in.

So with that said I will leave the thread open for Spork to post what he sent me in his last PM on the subject and we can start from there.

Mark C.
04-18-2006 03:35 AM
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GimbalFan
rrProfessor
Location: Copter County, Nv

My Posts This: Topic  Forum		This is definitely a horse worth riding twice -- sure to be at least as enlightening on the second go-'round as it was on the first.

I'm still amazed at how many exceptionally bright minds have been so far unable to see the utter lack of dissimilarity between a headwind over still water and a water current under still air, as viewed from the sailboat's (or the destination's) perspective.

Here we (happily) go again!

op-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-t
04-18-2006 03:41 AM
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AGRAV8
rrProfessor
Location: Mosquito Coast......Houston Texas

My Posts This: Topic  Forum		NO SAIL

Thats MY answer, duly researched. To "tack in relation to the relative wind" involved moving at an angle TO that relative wind, increasing the distance to the final destination.

That being said, IF the question was can the speed of that boat be increased by tacking, then yes.....in a direction OTHER than a straight line. This will NOT, however..make that boat get there faster than one with sail furled.

THERE, I've given THE answer. 'Nuff said.

Don't ROCK THE BOAT with replies.....unless you've brought oars to the party.

GOOD guy list-39, BAD guy list-0
04-18-2006 03:42 AM
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GimbalFan
rrProfessor
Location: Copter County, Nv

My Posts This: Topic  Forum
Quote 
NO SAIL


All due respect, wrongo.

A sailboat moving through still air in a water current whose direction is directly towards a destination can make better overall speed towards that destination if it tacks rather than drifts.

I'm in need of more helis and/or nitro/LiPo dough. Care to place a bet, or does the thought of a wager seem 'tacky'?

op-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-t
04-18-2006 03:45 AM
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AGRAV8
rrProfessor
Location: Mosquito Coast......Houston Texas

My Posts This: Topic  Forum		tell ya what.....

I have an ex-NASA engineer on staff at work. I'll put HIM to work on it and see what HIS answer is.

BTW, I got PLENTY heli's already, not interested in another one at the moment....yours or most anyone elses, no harm intended.

Ah, and its about time to bring in "The Prefessor".

Mark???

GOOD guy list-39, BAD guy list-0
04-18-2006 04:00 AM
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valerko
Veteran
Location: ny,dutches county

My Posts This: Topic  Forum		OK,I ain't no genius.Just a carpenter , but wouldn't this be same as driving my truck down the road when there's NO wind and then have a buddy open a sail on the roof
I bet it'll slow me down and on top of it I'd have to look for my buddy in dust behind me
04-18-2006 04:32 AM
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spork
Veteran
Location: Mountain View, CA

My Posts This: Topic  Forum		OK, I'm back. There's all sorts of arguments desciptions, glossy photos, circles and arrows back in the original thread. However, this is my latest stab at it. Of course I'd be happy to have your ex-Nasa engineer join the fray. Be sure and let him read the posts rather than just tell him you know some RC heli flying geeks that think you can outrun the current.

The basis of my argument is that the steady current is an inertial frame like any other and that wind is only measured relative to the water anyway. I can't see how the wind relative to the bottom of the ocean or land 1000's of miles away could have anything to do with it. That being said, here's my latest approach...


For the moment let's imagine a boat out in the middle of the ocean tacking into the wind in the normal manner (i.e. no current, 10 knot head wind). I think we agree that's fine.

So let's put the WHOLE thing in a big box. I mean a REALLY big box. Wind, water, boat, and all. In fact, let's make the box so big it contains the entire earth and then some. Still OK - right.

Now if that box is moving along at say 10 knots that should still be fine. After all the earth is moving along at a real good clip.

Now let's change nothing except we start making the box smaller. For now we'll make the box say 10-miles on a side. Everything inside it is just as it was. Still no current in the box. There still is wind in the box. Boat is still tacking into that wind.

Now we lower that box into a river that has a 10-knot current - but no wind. No problem because we're still safely in the box.

But let's let the box float along with the steady 10-knot current. If you buy what I have above, we're still OK (still in the big-ass box).

Now that I'm floating along with the 10-knot current my 10-knot head-wind inside the box is actually not moving relative to the still air outside the box. And my still water in the box is not moving relative to the water around the box. If I remove the walls of the box nothing changes.

I'll be curious to hear your thoughts on this.

I really wish I had the appropriate vessel and current to just do the experiment. It would be absolutely cool to get it on video in any case. Unfortunately, the tide through the GG bridge only gets up to about 6 knots. That's enough for a really respectable boat to tack into, but not an average day sailer (I'm told). It's about an hour to motor out and an hour back. We'd have to do it in the early morning to make sure of no wind. My old boss has the right boat, and a twisted enough mind to do it. But he's currently navigating for the ABN team on the around the world race. When he returns I'll try to talk him into it.

RC
04-18-2006 04:34 AM
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spork
Veteran
Location: Mountain View, CA

My Posts This: Topic  Forum
Quote 
wouldn't this be same as driving my truck down the road when there's NO wind and then have a buddy open a sail on the roof


No, it would be like sitting still in your truck on the road on a day when there's no wind but the road just happens to be moving in the direction you want to go. And then, yes, you could put a sail up and hurry yourself along.

RC
04-18-2006 04:36 AM
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GimbalFan
rrProfessor
Location: Copter County, Nv

My Posts This: Topic  Forum		And because everyone loves Smiling Island Girls, a reprint:

You're on the open ocean. The wind is steady out of the north at 10 knots. You have a destination that is due north of you -- doesn't matter how far, as long as the ice chest is full and the BBQ is lit when you get there. Wind from the north, destination due north -- CAN YOU GET THERE IN YOUR SAILBOAT?

Of course you can. First you tack diagonally left (alright, port), heading NW, then you tack diagonally right (alright, starboard), heading NE, and back and forth and back and forth and eventually, hungry and parched, you arrive at your destination EVEN THOUGH THE WIND WAS WORKING AGAINST YOU the whole way. You have traveled UPWIND successfully without any direct help from the ocean other than to gently caress your keel.

To celebrate, you pour a frosty cold beverage and toss some shrimp on the barby and party into the night (burrpprp).

Next day, meal digested, fresh pot of morning coffee, and now you want to get back due south to where you started from. But today there's not a hint of wind at the dock. Miraculously, though, there's a steady ocean current moving due south at 10 knots.

You cast off your lines. You wave goodbye to the sweet, smiling island girl who 'cooked the shrimp' for you last night. Since you were moored at the south side of your island paradise, you immediately begin drifting DUE SOUTH towards your starting point from yesterday.

Since you're eager to get back to your home on the mainland and check the posts in a very interesting thread on some obscure forum that has nothing to do with sailing, you HOIST YOUR SAILS AND TACK, first to the left (ok, port), heading SE, and then to the right (ok, starboard), heading SW, and back and forth and back and forth.

Amazingly, this slightly INCREASES your overall speed due southward relative to the due southerly speed of the ocean current, enabling you to arrive slightly sooner than expected to your mainland abode, permitting you to more quickly log on to peruse the threads of your favorite forum.

Once again you have traveled INTO the wind successfully, only THIS time your overall speed was due mostly to the ocean current. Your overall speed southward was increased slightly by tacking -- the same way your northward trip was MADE POSSIBLE by tacking the day before.

Does this short story make the concept any clearer, or would you rather tack back northward and talk tuna with the sweet island girl?

op-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-t
04-18-2006 04:37 AM
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valerko
Veteran
Location: ny,dutches county

My Posts This: Topic  Forum		Dam you guys.My head hurts again
04-18-2006 04:48 AM
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spork
Veteran
Location: Mountain View, CA

My Posts This: Topic  Forum
Quote 
Dam you guys.My head hurts again


Then my work's neary done here!
04-18-2006 04:54 AM
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GimbalFan
rrProfessor
Location: Copter County, Nv

My Posts This: Topic  Forum		I love Sweet Smiling Island Girls almost as much as nitro helis.

op-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-t
04-18-2006 04:57 AM
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Mark C
Key Veteran
Location: Houston, TX - USA

My Posts This: Topic  Forum
Quote 
I'll be curious to hear your thoughts on this.


I have pretty much the same thoughts on this analogy as I have on all the others. It overlooks the forces at work and their differences given the opposed situations. What I would like to do to show you what I am talking about is break it down step by step. By doing this, I will find the exact point that you and I disagree on. I'm not going to use numbers or equations or analogies. I'm going to put the boats on the water.

To clarify lets state situation 1a: There is a boat on a stream. Its sails are DOWN. There is NO current. There is a 10kt wind blowing upstream. There is a goal or "finish line" upwind from the boat.



Situation 2a: Same boat, same stream, sails still DOWN. Now there is NO wind and a 10kt current moving downstream toward the finish line.



Now in both situations, after everything has reached its steady state and you were standing on the bow of either the boat everything would seem equal. You have a wind blowing in your face. Water is flowing from the stern of the boat to the bow. The boat is moving backwards with respect to the water. If you were on the open ocean you could not tell the difference unless you had a GPS unit in your hand. But since you can see the shore and the goal, you know that in situation 1a you are drifting away from the goal. In situation 2a you are drifting toward the goal.

Now let’s move to situation 1b. You are in the boat in situation 1a. You turn the boat left (north) as the diagram shows in preparation to set sail but with your sails still down you pop open another beer and after everything has reached it's steady state again the boat is moving off to the left and back (moving northwest on the diagram)



Conditions now are very favorable to set sail. The apparent wind approaches from a nice angle. When ready, you can set sail with the sail almost parallel to the boat, trim it out, tack into the wind and make a couple of knots progress toward the goal.

I doubt seriously that we disagree on anything up to this point. We have been over it many times.

Now get ready. The next situation is where I am pretty sure you and I part ways.

Let’s go to situation 2b. You are in the boat in situation 2a. You turn the boat left as the diagram shows in preparation to set sail with your sails still down you pop open another beer and after everything has reached it's steady state again the boat is now drifting off to the RIGHT and forward (moving southeast on the diagram).



In this situation, conditions are now very poor for sailing. The apparent wind approaches from a poor angle. The boat is sliding backwards. Raising the sails in parallel with the boat would only make matters worse. You are going to have to reposition the craft to keep it from sliding backwards and the only way to do that is to point the nose of the craft away from the goal line. Not so good.

At this point, whether you think my diagrams are exaggerated is not really relevant. Their purpose is to show the differences in the situations. Move the wind over the still water and the boat moves one way. Move the water under the still air and the boat moves the other way. In order for your analogies and examples to work, the boat in situation 2b would have to move exactly as it does in 1b and unless you break the laws of physics that that just is not going to happen.

And these forces are here to stay. You can't outrun them or start the boat of in a steady state that ignores them. Any steady state example that has the boat sailing downstream has to account for these forces. Your examples need to be revised to show these forces.

To put it out there bluntly, I do not believe tacking into the relative wind is the same as tacking into the wind. The forces at play are certainly different.
And that is about as clear as I can possibly illustrate my point. Instead of numbers or equations or any technical jargon which tends confuse folks like myself I used cute little diagrams.

So this is where I believe this thread is short lived. Moving forward, I can only see a couple of different possibilities:

1. You say "Yep. I didn't account for those forces and they will likely hinder my ability to sail downstream faster than the stream." And we call it quits. But since you are a pretty headstrong guy I'm not thinkin that is gonna happen.

2. You say "I always knew those forces were there but I choose to ignore them since they are insignificant or they are cancelled out by some other forces." But since I believe these forces are relevant and I don’t believe opposing forces exist, I would rather avoid a pi$$ing match that would blemish our spotless relationship and therefore call it quits.

3. You say "What are you? Some kind of lunatic!?!? The boat in 2b moves exactly like it does in 1b!" And although I lack the sailing experience to counter I have certainly peed on my rubber ducky in the bathtub enough times to know which way the current pushes it so once again, I would rather bow out of that one even though I am a lunatic.

4. You are a bright guy and type something really cool and I have to go off and think about it some more.

Take it away......
04-18-2006 05:01 AM
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GimbalFan
rrProfessor
Location: Copter County, Nv

My Posts This: Topic  Forum
Quote 
I have certainly peed on my rubber ducky in the bathtub enough times to know which way the current pushes it


Eeeew! Is that rubber ducky YELLOW?

op-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-thwõp-t
04-18-2006 05:11 AM
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spork
Veteran
Location: Mountain View, CA

My Posts This: Topic  Forum
Quote 
4) You are a bright guy and type something really cool and I have to go off and think about it some more.


Yup, it's gonna be number 4 for sure. There is actually one place where we disagree ever so slightly prior to

Quote 
I doubt seriously that we disagree on anything up to this point. We have been over it many times.



And that is just that your first two situations do differ in one minor way. For them to be identical the goal line must drift with the current in the second situation.

I don't have time to post the really cool answer right now (I promise I will), but I'm curious why your boat moves toward its bow in the first situation, and toward its stern in the other. I'm guessing its because you feel the wind is the only thing pushing on the boat in the first case, and the water is the only thing pushing on the boat in the 2nd?

In any event, that's the crux of the problem I have with your explanation. I'm curious where you feel the hole is in my latest explanation (above).

RC

P.S. I'm not trying to baffle anyone with complex math or diagrams. I'm just trying to approach this every way I know. I'll be happy to operate either way.
04-18-2006 05:20 AM
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Mark C
Key Veteran
Location: Houston, TX - USA

My Posts This: Topic  Forum
Quote 
And that is just that your first two situations do differ in one minor way. For them to be identical the goal line must drift with the current in the second situation.


I'm pretty sure I covered that when I said "In situation 1a you are drifting away from the goal. In situation 2a you are drifting toward the goal"

Quote 
I'm curious why your boat moves toward its bow in the first situation, and toward its stern in the other. I'm guessing its because you feel the wind is the only thing pushing on the boat in the first case, and the water is the only thing pushing on the boat in the 2nd?


Yes. In situation 1 only the wind is moving (no current). In situation 2 only the current is moving (no wind).

Quote 
In any event, that's the crux of the problem I have with your explanation. I'm curious where you feel the hole is in my latest explanation (above).


As I stated, your latest explanation does not account for these differences.


Mark C.
04-18-2006 05:24 AM
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spork
Veteran
Location: Mountain View, CA

My Posts This: Topic  Forum
Quote 
I'm pretty sure I covered that when I said "In situation 1a you are drifting away from the goal. In situation 2a you are drifting toward the goal"


Actually, that doesn't cover it. That points out that the two situations are different. But for the piece we're discussing at the moment, it's not an important difference.

Quote 
Yes. In situation 1 only the wind is moving (no current). In situation 2 only the current is moving (no wind).


By definition "moving" is measured relative to something. I'm not trying to be coy here - that's really the crux of the whole issue. In situation 1 the air is moving relative to the water. In situation 2 the air is moving relative to the water (or the water is moving relative to the air - same thing). The only thing the boat "knows" is that it's like the proverbial watermellon seed being squeezed between the two by its two wings (the keel and sail).

Quote 
As I stated, your latest explanation does not account for these differences.


I'm sorry, I don't understand how to apply that info. Again, not being coy. I don't think there's anything I didn't account for. If you could take the time, I'd like to know which step in my scenario contains the error, and how you think it's in error.

Incidentally, it seems the closest you and I ever got on this particular ABT is when we both agreed that a land-yacht on a tread-mill would in fact be able to tack into the relative wind created by the motion of the treadmill. Unless I'm mis-quoting you, doesn't that suffer the same problem you describe above - in one case the wind pushes the land yacht and in the other case the treadmill does?
04-18-2006 06:28 AM
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spork
Veteran
Location: Mountain View, CA

My Posts This: Topic  Forum
Quote 
tell ya what.....

I have an ex-NASA engineer on staff at work. I'll put HIM to work on it and see what HIS answer is.


Incidentally, I want to clarify one thing ahead of time so no one can accuse me of back paddling...

I *think* the ex-NASA engineer will agree with my arguments if he's given the chance to read them. But I can't guarantee it. There's one thing I can almost certainly guarantee. There are ex-NASA engineers that would agree and those that wouldn't. I've got a very close friend (that I was kitesurfing with today) who is also an ex-NASA engineer (and currently consults for NASA). He will most certainly attest to these statements.

More to the point perhaps, I'm an ex-Aerospace Corporation engineer. Not to cast any dispersions on NASA or its engineers, but it's a matter of record that the Aerospace Corporation (a non-profit company instituted by the U.S. government for the benefit of the Air-Force Space Division) has a more highly educated staff overall (approx 25% PhD's and somewhere near 75% with M.S.)

So I'm happy to have your friend join the discussion, and I'm even reasonably sure he and I will agree. But I don't guarantee that we will, and I won't admit defeat if he doesn't. I'll just try and bring him around. It's never too late to learn
04-18-2006 06:40 AM
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spork
Veteran
Location: Mountain View, CA

My Posts This: Topic  Forum		Wow, third post in a row without anyone to argue with.

In my post above I forgot that I had PM'd you that I hope to do this "experiment" under the GG Bridge when my old boss returns. This reminds me of one of the participants in the original debate - who called me an idiot (perhaps not in those words - but perhaps IN those words - I don't recall).

Anyway, this guy claimed he had a boat that was perfect for this experiment. He said he had 10 inlets within 10 miles (St. Pete, FL if I recall correctly) that had 10 knot currents on a regular basis. He said he could easily do this experiment and prove me wrong.

I wonder what ever happened to that fellow?
04-18-2006 06:47 AM
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spork
Veteran
Location: Mountain View, CA

My Posts This: Topic  Forum
Quote 
I said: Actually, that doesn't cover it. That points out that the two situations are different. But for the piece we're discussing at the moment, it's not an important difference.


Now I say...

Sorry, I was mistaken. I re-read your post. What you said w.r.t. this was accurate.


RC
04-18-2006 06:51 AM
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