YES.

Imagine squeezing a greasy steel ball between 2 lengths of steel (say about 2 meter long) that are hinged at one end. The pieces of steel may only need to close 100mm but the ball needs to move 1000mm to escape the squezeeing force being applied near the hinge.

Now the sails on these boats are very efficient at converting wind into movement, so that the when on a reach, the sail may need to move twice as fast as the wind to release the pressure being applied by the wind. This is not normally possible on a displacement hull due to the drag losses but when a boat is on the plane or up on foils, it all becomes possible. Instead of healing over they are able to move at speed to release the force instead of healing over like a displacement boat.

Hope this simple way of looking at the concept helps you on the road to enlightenment.

And as always, I could be wrong again :)

Well said Crusoe. I'm sort of equating the concept to an electric motor charging a battery with an alternator that
produces more amps than the system uses, thereby creating perpetual motion. But then I am insane!!!!.

Yes, well, given the yacht doesn’t need to carry tonnes of lead in the keel to keep it upright then it is possible. Most foiling cats, racing cats and even some cruising cats do go faster than the wind – easily. Check this video between 0:30 and 0:40, 20knots speed in 18 knots wind. That will put a smile on your face.

samsturdy said..

Well said Crusoe. I'm sort of equating the concept to an electric motor charging a battery with an alternator that
produces more amps than the system uses, thereby creating perpetual motion. But then I am insane!!!!.

Sticking with the electrical analogy you could say it is like a transformer- lower voltage in, higher voltage out.

Hi Sam,
The way I understand it is, reverting back to physics, is a function of lift (power) vs drag, with the real focus being on decreasing drag, rather than increasing power. Drag is a much bigger value than lift in most traditional hull designs. Typical ways to reduce drag is by reducing weight and the wetted surface area.

The wide beamy concept enables you to not only harness more power (can handle a bigger sail area) but it spreads the weight of the boat over a larger surface area, and the by product of a flatter hull shape both combine to reduce drag by decreasing the wetted surface area. This is usually only when reaching or running, lots of poorly designed flat beamy shapes are terrible at going to windward.

It is worth noting that this comes at another cost, the flatter hull is a lot more uncomfortable in short sharp seas when beating to windward, ie: it lands on a flat surface where a displacement hull will ride more comfortably.

In our trials last week, we were reaching at 7.8 kn in 8 knots of breeze without planing, this I can only put down to a sufficient reduction in drag (wetted surface area)whilst maximising the available power (efficient/larger ?? sail plan) .

The Americas Cup 72' cats when they were foiling were easily doing 2x wind speed!! How? Mostly by decreasing drag (wetted surface are)
Hope this helps!
SB

HI Sam,
I'm with you.I think the answer is efficiency of design, if your feather was shaped like a wing , like the example earlier with Ice yachts, the wing aerodynamics can provide more than enough power, it is getting rid of the drag that is the key.
Your feather is a good example, you are not gaining any efficiency, so it cannot go faster than wind speed. But the way your sails works, by creating a low pressure and high pressure side hence creating lift, is your golden ticket, it is increasing the power of the wind. This is why when you are badly trimmed, you cant achieve wind speed or greater, there is no multiplier on your available energy,
So, does that mean the old chestnut of needing more input energy than output is not a rule of physics, it is a engineering challenge??

Sam,

As you say a feather is never going to blow faster than the wind.

But with regards to sailing another thing to consider is the apparent wind speed.
More apparent wind over the sails such as on a close reach, will create greater differences in pressure either side of the sails. High pressure on the windward side and lower pressure on the leeward side - this is converted into forward momentum.

It is this difference in pressure along with the aforementioned principles that can allow a fast, low drag, planing type yacht to sail faster than the wind.

My cruiser will probably never sail faster than the wind!

Right N&Z. The more I think about the theory the more it makes sense. two things seem to be of most importance,
.....very low drag (low weight and ability to plane)
.....optimum low pressure.
So all the talk about rig tension- luff, leach, stays. slot effect, boom angle and sail shape is all to do with creating
optimum low pressure. There's more to sailing than meets the eye......isn't there!!!!.

I found this book one of the best reference books I have ever read.
It might have something to do with being super keen on the subject!

I would recommend this book to anyone.


The Modern Cruising Sailboat: A Complete Guide to its Design, Construction, and Outfitting.
Author: Charles J Doane.

Sailing works by changing the direction of the wind towards the back of the boat. You need force to do that, and that force drives the boat forward. All the rest is detail, but that is the principle. As long as you can create that change in direction, it does not matter what speed the boat is travelling at.

Anyone mentioned waterline length yet ?

samsturdy said..
...if a force is PULLING the boat ( low pressure in front of the sail)
then the maximum speed is related to strength of the pressure rather than the strength
of the wind.

Bingo! :)

Good onya Chris. Frankly, I've really enjoyed this little mind excercise. Thanks for your input
everybody.