Just to add to what CJW was saying about BOR, a more extreme example is DN class ice boats. When they round thier top mark and head "downwind" they only ease about a foot of sheet. They do travel at over 100km hour though.

A) A river flows south at 10 knots. The wind is blowing from the North at 10 knots.
B) A river flows south at 10 knots. There is no wind.

You want to sail South down the river. Which of the above will get you there faster?

IDK

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evlPanda said...

A) A river flows south at 10 knots. The wind is blowing from the North at 10 knots.
B) A river flows south at 10 knots. There is no wind.

You want to sail South down the river. Which of the above will get you there faster?

Lock in option B thanks Eddy.

Haven't we had this question before?

Neither - same speed

For A there is no apparent wind (you are just drifting with current in both examples)

Interesting to listen in..you guys certainly have more analytical minds than me..I suppose I do err on the creative side though..

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evlPanda said...

A) A river flows south at 10 knots. The wind is blowing from the North at 10 knots.
B) A river flows south at 10 knots. There is no wind.

You want to sail South down the river. Which of the above will get you there faster?

A will get you there faster. There is air resistance in B meaning that you won't quite travel at 10 knots...

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ginger pom said...

evlPanda said...

A) A river flows south at 10 knots. The wind is blowing from the North at 10 knots.
B) A river flows south at 10 knots. There is no wind.

You want to sail South down the river. Which of the above will get you there faster?

A will get you there faster. There is air resistance in B meaning that you won't quite travel at 10 knots...

Ooops - of course!

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evlPanda said...

A) A river flows south at 10 knots. The wind is blowing from the North at 10 knots.
B) A river flows south at 10 knots. There is no wind.

You want to sail South down the river. Which of the above will get you there faster?

B) all you gotta do to beat A is to sail on a tight reach.

Yeah Bs correct.

You have 10 knots of apparent head wind to deal with so if you can sail upwind in 10 knots you're ahead of the game.

Maybe B is correct? You are sailing directly downwind and down current.

With A, the current will make you travel downwind with no air resistance so you will do 10kn... exactly 10kn.

With B, the current will try to make you sail downwind at 10kn but due to the apparent headwind, a correct sheeting angle will make power and you'll go faster.

So depends on the resistance of the hull and efficiency of the rig

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Mark _australia said...

racerX said...

choco said...


Or if you landing in an aircraft into a 40knot headwind at 120knots, you touch down at 80,

Oh my god!
What happens if there is a 120kn wind??? It would never come down

Actually, you just land like a helicopter or VTOL aircraft. I have done just that in a Cessna 152 in a about a 30 knots headwind. Had to keep quite a bit of power on to reach the landing strip threshold, then throttled back and as the stall warning went off I settled gently onto the ground and stopped within the length of the plane. Had to keep the throttle on again to stop being blown backwards!

^^^ err there was winks there dude

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sailquik said...

Mark _australia said...

racerX said...

choco said...


Or if you landing in an aircraft into a 40knot headwind at 120knots, you touch down at 80,

Oh my god!
What happens if there is a 120kn wind??? It would never come down

Actually, you just land like a helicopter or VTOL aircraft. I have done just that in a Cessna 152 in a about a 30 knots headwind. Had to keep quite a bit of power on to reach the landing strip threshold, then throttled back and as the stall warning went off I settled gently onto the ground and stopped within the length of the plane. Had to keep the throttle on again to stop being blown backwards!

yeah ive got the same prob landing my double loops when i do them no hands. tricky stuff.

Dude, get off ya high horse, I can land double loops no hands, and often no feet, all day long

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ginger pom said...

evlPanda said...

A) A river flows south at 10 knots. The wind is blowing from the North at 10 knots.
B) A river flows south at 10 knots. There is no wind.

You want to sail South down the river. Which of the above will get you there faster?

A will get you there faster. There is air resistance in B meaning that you won't quite travel at 10 knots...

Sorry but you have got to read some yacht racing in tide books here. and also understand the conceptual difference between apparent wind and true wind. Situation B will allow you to travel South at greater than 10 knots. Consider the frame of reference to be the body of water and not the shore or bottom ( the moving carpet effect). The GPS will show a speed of 10 knots South but your frame of reference velocity is zero. In situation A you have a true wind of zero according to our frame of reference and therefore cannot sail. You will travel South at a velocity of 10 knots according to the GPS. For situation B you will have a true wind of 10 knots South. Sail close hauled at whatever VMG your craft will make and your GPS will show your South bound velocity as VMG plus 10 knots.
The trick is to remember that true wind is relative to the body of water and that apparent wind is relative to the speed over water. not speed over ground.

If you dont believe this poke your head outside, you are actually hurtling through space at a speed greater than 900 knots but because of the moving carpet effect you can only feel a few knots of breeze (if you are lucky).

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frant said...
[


The trick is to remember that true wind is relative to the body of water and that apparent wind is relative to the speed over water. not speed over ground.

But what about the "wind against tide" effect. Never spent enough time in a tidal zone to make conclusions but apparently the effect is real. I can only assume the bottom or the edges of the estuary have an influence on the chop because if they don't, as you say Frant - all sailing is relative.

I'll go with B too.

Dunno frant "sailing close hauled" is fine but the question requires to you to travel due south not close to it

So what's the correct answer?

B, as long as you are allowed to sail i.e. take what ever course you need, in order to arrive due south (which I believe the question allows). You would need to make an even number of close reaches (upwind), ideally two.

You all have convinced me. Is there any theoretical explanation of this effect on the net ?. I researched this topic last year, but found very little and none of this was from researchers.

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Mark _australia said...

Dunno frant "sailing close hauled" is fine but the question requires to you to travel due south not close to it

Come on Mark. the question requires us to sail South down the river. Surely tacking into a Southerly fits that definition with the VMG being the Southerly component.

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Ian K said...

frant said...
[


The trick is to remember that true wind is relative to the body of water and that apparent wind is relative to the speed over water. not speed over ground.

But what about the "wind against tide" effect. Never spent enough time in a tidal zone to make conclusions but apparently the effect is real. I can only assume the bottom or the edges of the estuary have an influence on the chop because if they don't, as you say Frant - all sailing is relative.

I'll go with B too.

The "wind against tide" effect is exactly what I have referred to. The true wind seen by the body of water (our frame of reference) is the vector sum of the tidal stream wind plus the wind as seen from a shore or fixed frame of reference. Therefore wind against tide results in a greater true wind hence larger chop. ie situation B is wnd against tide c/f situation A and results in a 10 knot wave making wind from the South.

I understood there was more to it than just the relative wind on the water. ie. a 20 knot wind into a 3 knot tide produces far more chop than a 26 knot wind going with a 3 knot tide. Bit I'm not sure, maybe it is just observations that don't allow for the relative wind.

It IS possible to sail with a conventional type sail at speeds in excess of the wind speed. It is also possible to sail with a conventional type sail at a VMG downwind at greater than the wind speed. A windsurfer is capable of approx 2 x windspeed at 90 deg to the true wind and pretty close to VMG downwind of windspeed at 135 deg to to the true wind. The latest AC craft easily exceeded these numbers. My definition of a conventional sail includes wing sails but excudes turbines that drive a water propellor or those little wheeled craft that use their wheels to drive an air propellor. A turbine driven water propelled craft can "sail" directly upwind. The little wheeled craft can extract energy from the wind whilst travelling directly down wind at greater than true wind speed and thus "sail" directly downwind at greater than true wind speed.

With a conventional sail apparent wind provides no drive to the craft and only limits the speeds that can be achieved, it is only the true wind which provides thrust. Apparent wind is made up of the vector sum of true wind plus speed wind.
Speed wind is from dead ahead and therefore does not provide any forward thrust (only slows or restricts speed, drag). True wind provides the only forward thrust. The sails must be trimmed for apparent wind, as the speed of the craft increases the apparent wind moves forward and the sails must be trimmed in. With a windsurfer after the initial acceleration where the sails are trimmed it is possible to vary your course from approx 45 deg to wind to 135 deg to wind without trimming the sail from a close sheeted position. Your speed will adjust automatically so that the apparent wind is always from the same direction.

The illustrution of how it is possible to make a VMG downwind of greater than the true wind speed are most easily explained with vector diagrams. Might do some sketches and scan them in later.