Interesting facts about the AC72...

[flaming]

I have but I think you need to explain it to me

Ok, Imagine a sailing boat sat on the water with no wind and no tide. It just sits there.

But now imagine a 10 knot tide. This moves the sailing boat at 10 knots, giving a wind over the deck of 10 knots in the exact opposite direction to the tide.

The sailing boat can then use this 10 knots to move through the water.

If however the sailing boat was an AC72, which is capable of going downwind at over the speed of the wind (and thus in this case the tide) it could make progress up a fast flowing river on a windless day.

Messes with your head right!?

 

[Tempus]

No. 1.8 x zero wind speed = zero boat speed and no headway. They would just drift with the tide.

But your comment was a joke, right ?

Sighmoon is quite right. He's assuming that the wind speed over the ground is zero, gives 10 knots of wind over the water. The boat goes 18 knots downwind, which is actually 8 knots up tide.

Bizarre.

 

[stevebrassett]

I understand about being able to sail faster than the wind, but how do you sail faster than the wind on a dead run? Say you had a 10-knot wind directly behind you. If you are at a stand-still, you have a 10-knot apparent wind. Once you have accelerated to 10 knots, surely the apparent wind is now zero? What makes you go faster? I can understand it if the wind is on a quarter, as you will always have some apparent wind.

 

[[23929]]

So, a 10 knot adverse tide will sweep the AC72 backwards at 10 knots. The air flow over the sail is leech to luff. How will this make it sail 18 knots against the tide ?

 

[Tempus]

It's the same question as novice sailors being unable to understand how a boat can sail up-wind. So long as you are not sailing dead downwind there is a sideways component to the wind which produces lift.

There's a brilliant explanation here:
http://www.nauticed.org/sailing-blog/americas-cup-apparent-wind/

 

[flaming]

I understand about being able to sail faster than the wind, but how do you sail faster than the wind on a dead run? Say you had a 10-knot wind directly behind you. If you are at a stand-still, you have a 10-knot apparent wind. Once you have accelerated to 10 knots, surely the apparent wind is now zero? What makes you go faster? I can understand it if the wind is on a quarter, as you will always have some apparent wind.

It's not on a run, it's VMG, so they're actually going even faster, but making 18 knots VMG downwind in 10 knots true. So gybing back and forth.

 

[Tempus]

So, a 10 knot adverse tide will sweep the AC72 backwards at 10 knots. The air flow over the sail is leech to luff. How will this make it sail 18 knots against the tide ?

If you watched the light wind race carefully there were some moments when they sailed away from the mark - this was to build speed so they could carry the apparent wind forwards. They'd never run as you suggest, start on a reach and go deeper, as you do sailing an assymetric, dragging the apparent wind fwd.

 

[DJE]

I understand about being able to sail faster than the wind, but how do you sail faster than the wind on a dead run?

You can't. That is why they tack downwind.

 

[flaming]

So, a 10 knot adverse tide will sweep the AC72 backwards at 10 knots. The air flow over the sail is leech to luff. How will this make it sail 18 knots against the tide ?

Forget the tide, the boat knows nothing about the tide, all it knows is that the difference in velocity between the water and the air is 10 knots. It can use those 10 knots in exactly the same way it would on a lake with a 10 knot wind blowing. And sail downwind by gybing back and forth.

What's unusual about the AC72 is that sailing their normal angles it can make progress downwind (VMG) at greater than the windspeed. Which would mean it can go against the tide in this example...

 

[stevebrassett]

It's not on a run, it's VMG, so they're actually going even faster, but making 18 knots VMG downwind in 10 knots true. So gybing back and forth.

I know about tacking downwind. The original statement was "* The boats go directly downwind 1.8 times faster than the wind." I took that to mean a dead run.

 

[DJE]

Type DDWFTTW into Google and we'll see you in a month or two!

 

[stevebrassett]

Type DDWFTTW into Google and we'll see you in a month or two!

Quote from the page:- "Conventional sail-driven boats, not going directly downwind, can achieve speeds - reaching the destination downwind, at a greater speed than the wind on line to the destination, in what is called velocity made good[n 2][n 3] but cannot move faster than the wind while sailing in the same direction as the blowing wind ("dead downwind").

By using a propeller instead of a conventional sail, and coupling the propeller to its wheels, a land yacht can proceed dead downwind faster than the wind.[18]"

So an AC72 won't sail faster than the wind on a dead run.

 

[snowleopard]

Forget the tide, the boat knows nothing about the tide, all it knows is that the difference in velocity between the water and the air is 10 knots.

That's the key fact. A boat doesn't get its propulsion from the wind but from the difference between wind and water velocity. A boat drifting with a 5 knot current with 5 knots of wind in the same direction will be becalmed. An adverse current means the difference between wind and water velocity is greater so there is more energy to harvest. In boats that can make a VMG > wind speed it will be advantageous to seek out the stronger current.

Here is a question: two boats are going down-tide in 5 knots of tide. One has a calm, the other has a 5-knot tailwind (both winds are true, not relative to boat or water). Which gets to the next mark first?

 

[flaming]

So an AC72 won't sail faster than the wind on a dead run.

No, as explained it's VMG is greater than the true wind speed. So it would win the race with the balloon released at the top mark, but would sail a longer course.

 

[[23929]]

That's the key fact. A boat doesn't get its propulsion from the wind but from the difference between wind and water velocity. A boat drifting with a 5 knot current with 5 knots of wind in the same direction will be becalmed. An adverse current means the difference between wind and water velocity is greater so there is more energy to harvest. In boats that can make a VMG > wind speed it will be advantageous to seek out the stronger current.

Here is a question: two boats are going down-tide in 5 knots of tide. One has a calm, the other has a 5-knot tailwind (both winds are true, not relative to boat or water). Which gets to the next mark first?

They both get there at the same time

 

[flaming]

They both get there at the same time

Nope....

The boat with the clam actually sees a 5 knot headwind, so can sail into that and add it's upwind VMG to the tide. The boat with the 5 knot tailwind actually sees no wind so will just drift at 5 knots.

 

[flaming]

Another question.

You have been challenged to a sailing race across a tidal estuary. The rules are that you can pick the time to make the crossing, and the fastest time across the river is the winner.

The wind will be constant all day, blowing straight down the river (same direction as the Ebb).

So should you go at slack water, on the Ebb, or on the Flood?

 

[prv]

So should you go at slack water, on the Ebb, or on the Flood?

Flood.

Pete

 

[stevebrassett]

Flood.

+1

 

[JumbleDuck]

No, he's right.... It's difficult to get your head around but he's 100% right.

Think about it some more.

If I may suggest a little thought experiment:

Imagine that the sea is fixed and you have an 10kt tailwind. The boats have a VMG of 28kt downwind.

Now change to an axis system moving with the wing. In that axis system you have zero wind, 10kt adverse tide ... and you've got a VMG of 18kt into the tide.

High speed drudging, basically.