A Quick Sailing Question.

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Alistairr

Alistairr

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A river runs straight from West to East at 10 knots. A 10 mile race is held: the boats sail downstream, from West to East. The first heat is held in the morning, when there is no wind. The second heat is held in the afternoon, when there is a 10 knot wind from the West. In which heat are the faster times recorded?

Please feel free to explain the answer!

Cheers

Al.
 
It's pretty simple:

If the wind is doing the same speed and direction as the water, all a sailing boat can do is sit and drift with the current as there will be no apparent wind. If the air is stationary then there will be 10 knots of apparent wind to a boat drifting with the current. The boat can use that wind to work to windward, thereby exceeding the speed of the current.
 
No wind (over the ground) is better.
In the morning with no wind over the ground the boats moving with the current will feel a 10 knot headwind. In the afternoon the true wind will cancel out the current-induced headwind and the boats will feel becalmed.
 
I took it to be a river in the sense that it flows continuously in one direction. What really intrigues me however is how the boats made their way back to the start against a 10kn current?
 
In the morning your boat speed is 10kts with no wind,
in the afternoon you still make 10kts boat speed, the 10kt wind is actually 0kt as you are making 10kts if you see what I mean.......therefore the boat finishes at the same time morning and afternoon???

Paul.
 
[ QUOTE ]
In the morning your boat speed is 10kts with no wind,
in the afternoon you still make 10kts boat speed, the 10kt wind is actually 0kt as you are making 10kts if you see what I mean.......therefore the boat finishes at the same time morning and afternoon???


[/ QUOTE ]
It's speed over the ground and apparent wind that are key here as has already been explained. In the morning you have a speed over the ground of 10kn. You also have an apparent headwind that allows you to make way through the water with a VMG to windward of, say 12kn, (10 kn due to current, 2kn due to sailing) so you will pass the 10 mile mark after after 50 minutes. In the afternoon the following wind is cancelled out by the current so you only make 10kn and it's an hour before you pass the 10M mark.
 
In the morning the boat drifts downstream ( or tacks and has to cover a long distance ) against an apparent 10kn headwind. In the afternoon ( after motoring back to the start ) the boat drifts downstream with a 0kn apparent wind.

I'll go for the afternoon.
 
If the boat drifts downstream in a straight line in the afternoon it needs to cover 10nm = 1 hour of drifting. in the morningiIf it can point up to 45 degrees then it has to cover 14.14 nm. Unless the boat can make 4.14kn through the water including the slowing down of any tacking then it is going to take longer than the straight line time!

So it depends on the polar of the boat in use.
 
It might be a quick question, the answer is not so quick /forums/images/graemlins/grin.gif

Although there is apperent wind in the morning it's also straight from the direction you want to go to. So you need to sail at an angle, making the distance to cover greater as you go in a zig-zag course towards the finish.
My bet is, for the most boats, it would be faster to drift with the flow in the afternoon.

Arno
 
Ok, I have no idea, and would have thought a 10knt wind with a 10knt current would give you more speed.

I am busy trying to read up on sailing as much as I can at the moment, (and for quite a bit longer until I can buy a boat!!), and came across this site this morning.
http://www.physclips.unsw.edu.au/jw/sailing.html

This is where i saw the question and couldn't quite believe it when I read it, after reading it 3 or 4 times, I think its making sense now.

Here is there answer: The faster heat is the one with no wind. When the wind and the water both move W to E at 10 kt, the boats drift down the river at 10 kt, with their sails hanging limp. In the heat with no wind (as measured on the land), a drifting boat has a headwind of 10 kt. You can tack into that.

Of course, you don't get something for nothing. In the heat with wind, the river does very little work on the boat. In the heat without wind, it exerts much greater force on the boat, in particular on the keel or centreboard. Much of that work goes into disturbing the air downwind of the boat's sails.

Right or wrong, I'll leave you guys to argue it out, I don't have a clue.

Cheers

Al.
 
Depends on the width of the river! And to some extent on the boat. In the end it comes down to whether it's quicker to beat down the river in an apparent headwind of 10 knots, or to 'tack downwind', broad reaching in a zero apparent wind. Probably the former, IMO. But maybe not on a decent catamaran.
 
Sorry but your math is wrong. You are mixing distance moved through the water and distance travelled over the ground.

In scenario 2 the sailboat has no apparent wind, so is unable to make any way through the water. It is simply drifting, and will float downstream at the same speed as a piece of floating paper. It will take 1 hour to travel the 10 miles.

In scenario 1 it will take no more than 1 hour, because of the current, unless it actually makes way through the water AWAY from the destination (upstream). Any way it makes through the water in the direction of the destination will reduce the time to finish - to something less than one hour.

It is just a question of vectors. All you have to consider is the VMG to windward. If the VMG to windward through the water is 2 knots, the speed towards the finish is 12 knots. So it should take 50 minutes.

Edit - I'm going to edit my reply with an important qualification. It occurred to me that, if the boat did not set any sails, it would take LONGER to get to the finish in the heat with no wind (10 knot apparent headwind) because the boat would be "blown" backwards (upstream). Provided the boat sets its sails and is able to make way through the water downstream (which it should), the heat with no wind (10 knot apparent) will be faster.
 
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