Tacking2 Challenge

No

Look at tacking, relative to the medium we're moving it, the water. The wind is blowing one direction and the boat is moving in the other. On a river the water is moving but using this as a reference point, a floating boat would be stationary and the wind blowing past it, backward (okay okay...maybe the boat will slow down a bit from aerodynamic drag bit this is negligible). Anyway, tacking uses the angular summation of forces across the sail and keel to propel the boat forward....so let me reword the question: forget the river...a boat on the open ocean can tack into the wind...this we all agree on. If there is a current behind the boat it will only help propell the boat forward! So in the case of a "large" river, yes it will move slightly faster by tacking.

seems too good to be true... ideally it works, but by the time you come about several times you might have enough losses to overcome the gains achieved by tacking. also, you'd spend a lot of travel distance traversing the river bank-to-bank instead of taking a straight shot down it. this might be a job for the Mythbusters.

It is the same as being on a stationary river on a windy day, you could tack into the wind, in this case, downstream

Seems like we have a No, a Yes, a No and a Yes! A nice engineering stalemate! Who is going to cast the deciding vote?

Well, I had (now expired) a U.S. Coast Guard Captain's license to carry passengers in vessels up to 60 tons, sail or power. I also had my own sailing yachts from 1939 until 1971, and I taught 2 daughters to sail in dinghys (including tacking!)Also, I was race committee chairman for the largest international yacht race in the world, the Newport to Ensenada Race, for 6 years, and I developed the PHRF sail boat handicapping system now used in California. What are the qualifications of the other judges?

Douglasbook is right! Scientific/engineering solutions should not be decided by popular vote!

And yes, the boat will gain speed downstream if correctly tacked - as Douglasbook previously pointed out: it's the same as a stationary river with a headwind.

B.S. The making of another urban legend. The current is doing its best to get you down stream. Any thing that you stick up into the wind is going to present drag. This includes a sail no matter what you think that your doing with the sail. Race me.

would the actual DOWNRIVER speed increase?? it seems that the overall speed of the boat would increase as a result of the cross river component, but the boat would be traveling a longer path zig-zaging across the river. (just restating the obvious) Also, would it matter how fast the wind is blowing in relation to the current? tacking across an open lake is one thing, but for example... if the current is taking the boad downriver at 5m/s and the wind is blowing upriver at 5m/s, then the sail is actually moving through the air at 10m/s. doesn't drag increase with the square of velocity? would someone smarter than me help me out here: )

Douglasbook (see posts #6915 and #6971 above) will be a good candidate to help you out. Your statement of the problem is somewhat confused – there is no wind blowing upriver at 5 m/s. The stream might be taking the boat downstream at 5 m/s, inducing an effective wind over the boat of 5 m/s. Once you tack (sig-sag) into this wind, your relative wind speed becomes more than 5m/s, but will not reach 10 m/s, I think. Douglasbook can perhaps help us out with max speed?

Hi Phil, did you read Douglasbook's post #6915 above? In #6962 you wrote: "...The current is doing its best to get you down stream. Any thing that you stick up into the wind is going to present drag. This includes a sail no matter what you think that your doing with the sail. Race me." What you say is effectively that sticking up a tacked sail presents drag, making it impossible to sail 'upwind'. I will race you!

Yet another great question!

With bare poles, your speed across the ground might be 4.8 knots in a 5 knot stream. If you were a non-sailor, you might be perplexed by the fact that the rudder seems to work in reverse.

If you hoisted the sail, and let it luff, you'd go even slower across the ground. It would feel as if a 4.6 knot apparent wind was blowing you backwards, relative to the water... but you would look ashore and see the trees (with leaves quiet) moving toward your stern.

Then you bear off onto a close-hauled course, trim the sail, and suddenly you are making good to windward, and sailing forward through the water. If the shore were out of site, you'd swear you were in the middle of the ocean with a 5 knot wind (with mysteriously flat water.) Depending upon your boat, your progress downstream might be at 7 knots or so, with a speed through the water of around 3 knots.

A sail acts more like a verticle wing than a parachute. The "wing" creates lift, in a forward direction, countered by the weight of the keel, keeping the wing verticle as opposed to allowing the force to push the sail over on to the water. Given that, and given that the question IMHO does not intend to take the boundries of the river into effect. The answer is yes, the boat speed will increase. If the boundries of the river are to be considered, and the angle of attack of the tack in relation to the direction of the river, then no, the general downriver overall speed will not increase, because the tack must take the boat back and forth across the river. However the boat will pick up speed in relation to its surroundings.