sailing puzzle

[DLT]

Scott, amd, Catigale, and others,

I composed a long winded dissertation on that whole idea of tacking back and forth inducing sideslip against the current.

I even went into discussing how all that sail that you would have up to catch the 5mph wind would be nothing but drag during a tack...

But, then I deleted it and left the "take the current out of the equation" post...

I'm still convinced that you'd have some sideslip up river, thereby somewhat reducing that 5 mph current.

But, if you do take the current out of the equation, then you are really left with one guy having no wind and the other guy having 5 mph headwind. Imagine all this on a still lake, with no current. If the side slip is really all that bad, how could that guy in 5mph wind ever make any progress? Even my meager (at best) sailing skills can manage some progess...

I just couldn't get away from the fact that if you can make progess in a 5mph headwind, given a still lake, then you have to be moving faster than the current, which in that case would be zero. So, if the water is moving, then you'd have to be able to travel faster than the water...


Yes, if you are sitting on a still lake and down't have any sails up, the 5mph wind will push you downwind. But, if you raise sail and head upwind (tacking of course) you can actually make progress toward the wind, even with the drag of the freeboat, all that canvas, and associated sideslip. If you can make progess on still water, why can't you on moving water? Yes, the there are slight differences in the hydro-dynamics, but they are probably negligable...

[Don T]

Hello:
You cannot take the current out of the equation without doing it for both. remember if there were no current, one boat looses his headwind and the other gains wind astern. Now who gets there first. Bottom line is, there is more energy going down stream (5knt current & 5knt wind) for one boat than for the other (5knt current & 0 wind). You cannot extract energy from still wind, the only energy available is the current. Think of it as the sails and foils have switched jobs. The foils are the force pushing the boat and the sails are the resistance.

[DLT]

Ok, I'm sorry, I wasn't clear.

Yes, I am talking about taking the current out of the equation for both boats. But, only partially...

By that I mean that the guy sailing on a calm day actually sees an apparent headwind of 5mph. This guy, I'm suggesting should be equated to the guy sitting on a still lake, with 5 mph wind blowing from the direction he wants to go.

The other guy, the one with both a 5mph current and a 5mph wind, I'm suggesting should be equated to the guy sitting on a still lake in no wind at all. Yes, this is hard to get to. But, in the original problem, he is drifting with the current and sees absolutely no apparent wind. So, as you are standing on a shore looking at him, in the original problem, you see the 5mph current and feel the 5mph wind. But, imagine your viewpoint being in a car travelling at 5mph along the river. In this case, the river bank goes by at 5mph, but he doesn't move with respect to the water (or you) and neither you or he feels any apparent wind. So, in my modification, I suggest he ought to be considered sitting in a still body of water with no wind...

If you can get through those two points, I think you'll see of the guy with the "apparent" head wind has to win, since we can make some progess against a 5 mph "apparent" head wind...

[Richard O'Brien]

My last comment: I hope some of you pilots step in here sometime? The main points to be made are 1. that any wind is better than no wind, even a headwind. 2. that a sailboat can excede actual wind speed because it is "sucked" forward by the airfoil shape. 3. that I can start a sentence with a participle, and everyone knows what I meant? 4. that the real thrill of sailing is when that puff comes along and the boat heels over and the gravity along with the wind allows the sails to fill, the vertical "wing" takes shape, and off you go?! Right?

[Scott]

not to change the riddle, but to sidestep it a little.

If you answer while standing in the woods and your wife doesnt hear you..... are you still wrong??

And is it still your fault

[RandyMoon]

OK Captains:

Downstream speed = boat speed + water current speed
Upstream speed = boat speed − water current speed

Problem. The speed of a boat in still water is 30 mph. It takes the same time for the boat to travel 5 miles upstream as it does to travel 10 miles downstream. What the speed of the current?

Maybe Heath can start a special winter cabin fever forum dedicated to boating puzzles to keep our minds off sailing until the tundra thaws out next year. Of course then I would post a puzzle like - how many California sea lions would it take to crawl up on the deck of a Mac and sink it.

[DLT]

10mph

[RandyMoon]

Damn it DLT.

I guess that was too easy.

[Catigale]

You guys are still wrapped up in trying to understand the Profs puzzle by deleting the current, which is a different

All boats sail in the apparent wind, which is the wind in the frame of reference with respect to the boat.

Boat one sails on a calm day.

Boat two sails with a 5 mph wind astern.

Both are in a river running downstream at 5mph

Boat one has an apparent wind of 5 mph on the bow.

Boat two has apparent wind of 0.

The Professor asserts Boat one can tack and use the apparent wind to win the race.

I assert that Boat one can tack, but will have to travel farther and at 45 degree pointing to the wind, cannot catch Boat two unless it sails the tack course at almost the same speed as the wind....I also neglected the fact that the apparent wind of Boat one on tack worsens when he starts on the tack of course.....so even at the 96 percent efficiency I calc'd earlier, he would still lose...

Now...suppose Boat One can assume a 45 degree position to the wind and drift down river holding this position....now the sails can generate some force in the downstream direction and he could win the race...but my gut feel is the friction force of a sideways presenting hull would be much larger than this force and so he would still only do 5mph down river....that would depend on hull shapes, etc....a whole new can of worms...

Stick with bugs Professor...

[yuri]

Catigate
All that you need is your two statemens:

Boat one has an apparent wind of 5 mph on the bow.

Boat two has apparent wind of 0.

This means that boat two can do nothing (it will in reality float with water speed). Boat one can use apparent headwind for progress forward by tacking. He WILL make progress against surrounding water and this will be on the top of the actual water speed, which is 5 kt.


I was away for two days and very surprised with the activity of this thread. The whole discussion actually demonstrate that theory is one thing and reality is another.

[Chip Hindes]

You're stating this as fact while providing no explanation. It's simply not so...

...I think your bug guy should have stuck with bugs.

I've thought about this a bunch more, and I've now managed to convince myself that the bug guy had it right after all.

Don't know how I got so confused. My apologies to the bug guy and everyone who got it right the first time.

[Zoran]

This puzzle is easy for anybody that sails in light wind and strong currents and there we are, guys from the West Coast. I have been in situations where I had no wind but I had GPS speed of 3 kt (which is close to 5 mph). I realized that the wind is blowing with the current and I had no apparent wind. I wanted wind to die and than I would catch some apparent wind and do some sailing (not the hull a lot but at least it will not be so hot in the boat).
The other thing that many are forgetting is that even the skipper with no wind and 5mph apparent wind does travel longer way he always travel 5mph down the stream. At the beginning of the puzzle we are asked to consider that river flow is equal at the shore and the center of the stream (even it is not like that in real life). Regardless of his tacking the current is carrying him always 5mph down the stream and his real speed is higher. The same way as if you were swimming right across the river. Your speed through the water is slow (at least in my case) but the current is carrying you down and you will come on the other shore downstream from your starting point. Skippeer with no apparent wind will have 0 speed through the water and 5 mph speed over the ground. Skipper with the apparent wind will have some speed through the water and definitely over 5mph over the ground.

Zoran

[adm]

During totally calm day you are sitting on the shore and you are watching the sailor preparing for the race. You can see 5 mph current and you see him drifting in this current at 4.92 mph without the sails. He is slower the current by 0.08 mpg due to windage of his rig . You see him setting sail in preparation to race in his apparent wind . Now he is sailing closehauled in outmost imaginable expertly manner, tacks in seconds without losing any speed and so on ... You envy him of his ability to maintain his speed and style. He is crossing start line. Oh boy, he is sailing like hull ALL THE TIME. What you now see. His apprarent wind is now 4.89 mph due to drag of sails. Additionally you see that he is making progreess on the water 2.57 mph but at he same time he is falling behind the current AND HIS FORWARD PROGRESS OVER THE GROUD TOWARD THE FINISH is 4.4 mph.

DISCLAIMER: Numerical values are for illustration purpose only and do not represent anything pacticular.

Later on You are sitting in the same spot on the shore. Now is windy (5 mpg in the direstion of current ). Current did not change at all and it is still the same 5 mph as before. You see other sailor sitting in the same boat you see before, drifting down in the current and preparing for the race. Preparing ?!?! . What a looser. he does nothing. He did not bother raise the sails. But wait, you see his point. He is sitting in still air. You are feeling the wind but this wind is cancelling his apparent wind. He is sitting there dead on the water with no wind, no windage from his rig. And you see him drifting trough start line as fast as 5mph current without any slow down. Ok This loser is going to win this race in comparison to the first SAILOR.

Now why such injustice?

The first guy only souce of energy is current. The second guy has current AND the wind as sources of energy.

They both (and entire Universe) must maintan energy equlibrum which means that energy can change its form - go from potential energy ( to kinetic to heat etc but TOTAL amount remains CONSTANT!!!. This is non negotiable principle of physics which can not be broken without cosmic scale consequences and anihilation of entire Universe as result.

Let me explain energy trasfers. In the moving body of water each moving molecule has kinetic energy (equal mass of each molecule times its velocity squared ). Wnen we put stationary boat in, some molecules of the water will start hitting boat and decelerate. Because each molecule has the mass this deceleration will generate force (which is equal mass times deceleration). This force will immediately be couteracted by other equal force in opposite direction. The energy is transfered at this point (do you remember from school: each action causes equal reaction - Newton). The The last force will push boat and it will start to axcelerate and move away from molecules of the water. Part of boat above water has rig and sails attached to it and they will also start moving. Resin, glass, aluminum, steel and dacron or spectra will start hitting stationary molecules of air and proces identical to the one below water line will take place but with opposite effect on boat momentum. In every case some of energy exctracted by boat will be tranfered into heat by friction and will cause the boat to slow down. All of those dramatic events will continue until energy transfer reaches equilibrum and then continue indefinitely until some change take place. But in our puzzle this will not happen. Note energy tranfer direction - from water to boat to air. Key point is that there will not be transfer from air to boat to water. Air has no energy to offer. It is stationary and do not have kinetic energy and whatever is transfered to air is converted to heat and do not contribute to boat propulsion - just the opposite. From boat perspetive it is drag. Also note one important this. The faster boat movement will cause the more drag. It is the cheef reason the first sailor will loose. The faster he moves the more goes to drag

Those one who will advocate that it is possible to gain speed by sailing are asked to explain how to get something from nothing - in this case from energyless air. Please try to convince me if that is possible.

Keen observer will ask: where is equilibrum. Here is is one spelled out. The whole process will cause water to cool down and air to heat up. Sounds strange - but it is how it is. How come? Get you answer from Intelligent Designer not me.

SO the first guy can take energy from current trough the apparent wind and use it to move his boat but what his boat gains in movement over water (one form of kinetic energy) he will also lose from the push of the current (in form of its kinetic energy) and because object are mowing some of energy will be converted to heat by friction. Total amout will stay the same in his case.

The other guy has the advantage of ADDITIONAL kinetic energy of moving airmass and can use it. As trange as it is the less he does the better off he is. Now You should use your brain again to figure out why. I am exosted and done.


Above text may contain gross misspellings and lack of consistent argumentation but I am fed up.

[RobertKing]

If the total amount stays the same then the correct answer is Its a Tie

[adm]

The amount of energy stays the same within each case separately. But each of the gays have diffrerent amount available to begin with. Therefore no tie.