David Lock

I was listening to some commentary on the America's Cup which is currently in progress in San Francisco Bay (where NZ are stuffing the US boat).

The new generation of yachts are incredibly fast and apparently reach speeds of 3 or 4 times the wind speed.

Can anyone explain, briefly and in simple terms, how they can travel faster than the wind which is pushing them along as I can't get my head around this? I am guessing it's all to do with angle of sailing relative to wind direction?

Sorry if it's blindingly obvious

David

JGlanzaV

iTrader: (3)

I can never get my head round in travelling a different direction to the wind blowing. I understand the concept but it just is something that shouldnt make sense haha!

scoobyster

Don't think of the wind pushing them along as if the sails were parachutes. The wind passes over the sails to generate lift (as in an aircraft wing) which you can think of as a force rather than a speed. That force is actually greatest when the wind is at right angles to the boat, rather than behind it (which is actually a tricky and relatively slow point of sailing). The boat speed that force can generate is not equal to the wind speed, especially when the boat is essentially flying out of the water so there's next to no drag! Fantastic machines.

David Lock

Thank you Scoobyster. I sort of understand

Pretty hairy activity on board these super machines. Must cost a few bob!

David

David Lock

dpb

On the beat ( going into the wind ) the boats are actually being sucked fwd

Diffence between high and low pressure over the back of the mainsail

Imagine the sail as a vertical plane wing, the thing gets airborne because of the lift generated. The boat sail is the same, but obviously it's being lifted in a different plane

David Lock

Thank you skipper

D

hodgy0_2

correct, the term is "apparent wind"

all explained here



the boat will be sailing 12 mph (it is only a simple boat)

dpb

Formula 1 of sailing, ridiculous amounts of cash.

These things can essentially 'make thier own wind' ,by altering the apparent wind angle fastest point on broad reach with gennaker up.

But not 3 times wind speed! . And anyway America cup boats don't ever sail in more than 15 knts afaik

300gnspitzer

I watched some on the TV yesterday, when they're really going they hardly touch the water. There were captions on the yachts giving their speeds, the fastest that I saw was 27 knots, amazing.

David Lock

Well I promise you that the guy commenting quoted 3 or 4 times wind speed. This was on R5 Live with a guy who seemed to know his stuff commenting from San Francisco. Must admit I was a bit surprised.

David

dpb

Only sand and ice yachts can achieve those figures

dpb

This is the fastest thing on water off Namibia , not even 3x,

Leslie

Its most important that the captain can shout a lot and use very bad language!

Les

David Lock

I must admit to a satisfying smug feeling that the yanks asked for our help and Sir Ben Ainslie popped over to give them a hand. At the time USA were trailing 6-1 in the series.

Well USA, under Ainslie, have now caught up and it's level with one race to go.

Well done Ben

dl

speedking

If the wind suddenly dropped then the boat could be travelling 100x the wind speed but not continuously.

Also they're faster on the downhill sections. Normal boats get a boost on the downface of waves, doubt the America's Cup yachts which are suspended above the water really notice the waves though.

Tides can help. You can drift on the tide with zero wind.

It's a complicated game.

Fat Boy

"on the downhill sections" ?

Fantastic, I can finally get to use my Waterskis without a boat

The wind limit is 24.4 knots.

in pretty much of the races they have recorded boat speeds higher than the wind speed. The "lift" generated by the foils is incredible.

I was struggling to understand it as well - not the lift bit, but the seeming paradox of the boats travelling faster than the wind ( i.e. getting more out than goes in), but a big sailing freak friend of mine explained it by saying you have to think not just of the wind speed but also the energy behind that wind - so a kind of a momentum versus simple velocity type argument- and then see how efficient these foils are at converting that energy.

No idea if he's talking crap or not but it sort of made sense to me

a little bit of techy background:

The AC72 has the following maximum specifications:
overall length: 26.2 metres (86 ft)
waterline length: 22.0 metres (72.2 ft)
beam: 14.0 metres (45.9 ft)
weight: 5,900 kilograms (13,000 lb)
maximum draught: 4.4 metres (14 ft)
crew: 11
It was expected to sail faster than the wind: upwind at 1.2 times the speed of the true wind, and downwind at 1.6 times the speed of the true wind. But in fact it proved faster, averaging about 1.8 times the speed of the wind with peaks slightly over 2.3.Emirates Team New Zealand sailed at 44.15 knots (81 km/h, 50 mph) in 15.8 knots of wind (2.79 times the wind speed) on July 18, 2013.

Typical racing speeds are over 30 knots (55 km/h, 34 mph) with the boats capable of sailing well over 40 knots (74 km/h, 46 mph) in the right conditions.

The fastest race speed recorded was on Emirates Team New Zealand which was 47.57 knots (88 km/h, 55 mph) in 21.8 knots of wind (2.2 times the wind speed) on July 18, 2013.