sailing puzzle

[Rich Smith]

The guy sailing on a calm day will win...

To the guy sailing on a 5 knot wind day with a current of 5 knots there will appear to be no wind at all... the boat is moving at 5 knots from the current and the wind is moving 5 knots in the same direction so he will feel no wind.

The guy sailing without wind will also get pushed along by the current. However, this time he will feel an apparent wind in front of him caused by his boat moving along through the still air (Just like you feel wind on a calm day when driving your car with the window open). If he sets his sails correctly, he can take advantage of this apparent wind and gain extra speed...Remember that sailboats aren't pushed by the wind (except on a run), they are sucked forward by the low pressure zone created just in front of the sail (Bernoulies principle)

So, even though it seems counterintuitive, the guy that races on the calm day will win the race!!!

Rich.

[Catigale]

Hmmmm,,,i wonder how you trim your sails for a 5 mph wind on the bow...ie....in irons.....something sounds Danish about this to me....

[Richard O'Brien]

Answer : The "dead calm" sailor. The water is carrying the vessel at 5 mph, creating a 5mph wind, filling his sails a bit, and allowing him to take advantage of the airfoil shape, thus increasing his hull speed just slightly more than the existing 5 mph. The other vessel has no opportunity to take advantage of sailshape as he is limited to 5 mph flow, matched by the 5 mph of the wind. He is limited to 5 mph hull speed, and actual speed. Tacking does not actually slow the minimum 5mph downstream progress.

Congratulations to Yuri, Frank C. and Rich Smith....and maybe Richard Lori?

Ken Phillips is an entomologist who learned to sail on a 40' ketch, who likes physics too.

[Billy]

OK, let's take this 1 step further. No current, no wind. You motor @ 5 mph on calm water. Will you go faster if you open your sails and "take advantage of the apparent wind" since you are moving forward? Or is this like mounting a fan on the stern to fill the sails?

Or will you just look silly to the other sailors who are motoring in?

(I didn't start this.)

[Catigale]

thinks....I get it....I didnt understand the puzzle correctly until now...I was trying to understand how two boats could 'run' differently downwind and got hung up with how to trim the sails.

The point I missed isyou are not constrained to run with the wind, one guy can beat down current in the apparent wind and the other cant....

[DLT]

Billy,

The problem with your scenario is that motoring straight down the river at 5 mph is different than motoring back and forth accross the river at 5 mph, while tacking.

The way to look at the orginial problem is to just remove the current from the equation, as they both have that same advantage. The guy with no apparent wind would be just stuck there... While the guy with a 5 mph headwind will be able to make some progress, however slow...

In practicle terms, I'd still be the guy doing nothing and letting the current take me down river as I drink a beer and listen to the tunes... Unless I need to be somewhere, then I'd fire up the motor...

[Billy]

Was just having some fun. But, DLT, if you were sailing in that scenario and drinking beer, wouldn't you be going slightly faster since you were consistently lightening the weight of your vessel?

[DLT]

Yeah, and if I pi$$ off the back I'd get a boost from good ole Newton...

[RobertKing]

The vessel would only get lighter only if you relieved yourself overboard. If you used the portapoddy the weight would remain onboard. However if you relieved yourself overboard you would have to hang on to something tight so the acceleration caused by the weight reduction didnt also topple you overboard.

[Catigale]

This is fun.

I assume a 30 foot boat for laughs

I dont agree with the statement above that Tacking does not slow the boat, as everyone knows it does. To minimise the effect I will let each boat 'sail' at least 10 boat lengths so that the tack loss is neglible to first order.

Boat one drifts 300 feet at 5 mph.

Boat 2 tacks in the apparent wind, and has to sail 220 feet, then tack and sail another 220 feet to arrive at the same point as the first boat.

It thus sails 440 feet vs the first boats 300 feet.

The Second Boat current vector is 2 1/2 mph, so the second boat must be able to sail at 440 feet/300 feet * 5 mph - 2 1/2 mph equals 4.8 mph component of the tack vector direction due to the sails only.

That is a sail efficiency of 96% to win the race. If I did all that right, I dont think you can win with this strategy in the real world.

On edit: Assumes ability to point 45 to wind - a boat that could get up to 30 degrees to the wind could sail less efficiently and pull this off, of course.

[Richard O'Brien]

This is fun.

I assume a 30 foot boat for laughs

I dont agree with the statement above that Tacking does not slow the boat, as everyone knows it does. To minimise the effect I will let each boat 'sail' at least 10 boat lengths so that the tack loss is neglible to first order.

Boat one drifts 300 feet at 5 mph.

Boat 2 tacks in the apparent wind, and has to sail 220 feet, then tack and sail another 220 feet to arrive at the same point as the first boat.

It thus sails 440 feet vs the first boats 300 feet.

As was mentioned previously "take the downriver current out of the question", and boat one didn't travel anywhere while boat two tacked upwind 220'? of course I don't know of any Mac that points well in 5 mph winds, but then there's the Nicole Kidman factor to the advantage of the "dead calm" sailor?

[Catigale]

take the downriver current out of the question

That approach is faulty. Because the other conditions are different (namely the wind) the effect of the current is different, so when you delete the current they are changing the problem.

Kind of like the famous three sailboat problem with the hidden gybing centerboard.

[Scott]

I agree with catigale and disagree with richard and the poser of the riddle.

You would need to be at an angle to the wind to gain some forward momentum for steerage. this would create apparent wind opposing the current and your sail would create initial lift opposite the direction of travel adding more time to cats equation. Is there such a thing as "apparent lee slip" if so (and this exersize seems to indicate that there should be or could be (where is stephan Hawkings when you need him)) that would also add distance thereby reinforcing cats argument.

Now my head hurts, I need a sick day.

[adm]

I do not want to sound like dumb ass and spoil the fun but the "correct answer" posted today by Richard O' Brien is wrong. It violates Newton physics principles.

Here is why:

Answer : The "dead calm" sailor. The water is carrying the vessel at 5 mph, creating a 5mph wind, filling his sails a bit, and allowing him to take advantage of the airfoil shape, thus increasing his hull speed just slightly more than the existing 5 mph.

The "dead calm" sailor boat is not encased in concrete but floats in water. The apparent wind will push him back "against" the current and thus slowing him down over ground. Is is impossible to say by how much. (It depends on size and shape of sails and rig and size and shape of part of the hull above waterline and size and shape of bottom of the hull submerged in water - about 800 times denser then air - and many, many other factors) If he set sails and try to tack he will recoup some of the lost speed but it is impossible regain 100 percent as some of available energy will inevitably be conveterted to heat due to friction. Also the boat will be making leeway from the finish line. According to puzzle his only source of energy is kinetic energy of current. This is may be tricky but what energy you take from wind to propel the boat you are taking it from current too. You move over the water but entire body of water is moving slower over ground from your perspective. His overall progrees to finish will be less. Some of the available energy will be converted in the form of heat which is not usefull for moving toward finish. The total amount of energy must be preserved. It is unquestionable law of physics. It can not be broken.
It is all a matter of available energy.
It does not matter how efficient sailing of the first guy will be.
Other sailor has additional source of energy in form of kinetic energy of the wind on top of energy of current. The wind flowing with the direction of current will nulify effects of air drag of sail rig and portion of hull above water to his advantage over "dead calm" sailor. He do not have to to anything to counter any drag as the additional wind will help him win this race.

[RandyMoon]