After finding nothing with a preliminary search of this topic I figured on asking the high IQs of the windsurf community this;

If the foiling Catamarans as seen in the GP series and previous Americas Cups can reach speeds both up and downwind that are far greater than anything seen before by similar non foiling craft, why can't windsurfing foilers do the same?

As an example I can get close to but have never exceeded 30knots in Lake Macquarie conditions (choppy/1/2 metre "waves") but on the new Naish set-up have peaked at 23 knots but realistically peak average at about 20knots. This is on free ride gear on both set-ups. Looking at other team members speeds it's the same scenario, just with different speeds.

I'm thinking it is because one or more of the following;

a) Foil/Mast drag
b) Reduced board stability and thus control
c) Sail "rake" and angle to windward
d) ????

But looking at any of these on their own or in combination it still doesn't explain the GP and AC foiling crafts ability to exceed speeds of non-foiling craft.

Any thoughts??

Big sailboats are not like planing windsurfers.
Your foil is too big for top speeds. Try the 600 Naish kitewing.
Your speed is very good for 1150. Excellent for the 1220 wing.
My old Cal 20 peaked at around 11 knots...then shuddered and bucked.

A foiling Cat versus non-foiling Cat is not a good comparison to windfoil versus fin as the non-foiling cat is is displacement mode.

I guess a fair comparison would be a foiling 18ft skiff (which hopefully does not exist) to a regular 18ft skiff. My guess is the difference in speed would be marginal.

Makes sense but the question that I can't resolve is why can't Windsurf foilers achieve speeds even close to Windsurf finners, same sailer same conditions?

www.seabreeze.com.au/forums/Windsurfing/Gps/Speed--the-final-frontier


have a read of this thread, same question, with answers.

Hey Belly25,
My take on this is power to weight, $, control and time on water/experience are the differences.
The America's cup boats would have had huge $ spent on them. This would give them a huge power to weight ratio. Their rigs are massive and really efficient so power would be sky high. Carbon hulls and componentry mean weight is really low. And the foils would be super efficient. Not that ours aren't. Theirs I suspect would be a little/lot better.
Am Cup cats have a couple of foil points of contact with the water which are quite wide apart, so I think (and it's opinion, not fact), that the control over the flight is better to some degree than for a windsurfing foil. You have a crew of 6+? running around each trimming sails, foils, rudders etc. So they can be REALLY precise with what they're doing to get max efficiency out of the craft. Plus they spend hours on the water to get tuning absolutely perfect.
Compared to one of us who has good gear, but suspect the power to weight would be down. 1 pilot doing everything. And probably not spending as much time on water as the professional sailors. The Windsurfing windfoiling pro sailors get some pretty fast speeds in light winds so that may be some comparison, but I think to compare one of us with lower spec'ed gear with Am Cup racing foils is a bit like comparing an F1 racing car with your good quality off the showroom floor BM or Merc.

Foil has way more wet surface area than a fin, the average speedfin is 20 something deep, speedboard hull only touches the water behin the fin and on one side next to it. A foil has to be 30cm deep with at least equally "huge" wings and a fuselage thus way more drag. For an americas cup boat this is definately not the case.

As to why a windfoiler doesnt go as fast as an AC boat, they have size on their side. If your boat is 75foot, a small imperfection is less important.

Tom would be the guy to ask, with 32 knots top on the foil and a 28 knot nauti (www.gpsteamchallenge.com.au/sailor/view/9954). Seems he got his top speed on a 550 front wing, which is quite small. But then, he's 10 knot faster with a fin.

Big difference to large boats is that they have a lot of stabilizing things built in, and that they have had a lot of races where top speed mattered. For windfoiling races so far, upwind/downwind speed has been more important than top speed. That was supposed to change this year, with foils in slalom races, but seem we'll have to wait another year. Once it happens, I'd expect windfoil top speeds to go up.

An important thing to understand is that for a planing board, drag increases linearly. For a foiling board, it increases geometrically. While the foil starts at a lower total drag, in the low 20s, things even up and from there on up, you are trying to push mast, wing, stab, fuse all through the water. There's a practical limit to how much power the foiler can generate so speeds top out in the mid to upper 30s for the best guys. Still pretty fast in my book.

There are few foilers and foilmakers chasing 40. It'll be interesting to see who does it and if there is an incremental increase in foil performance or something comes along which is a bigger step forward.

A foil will beat a fin on a reach in planing but underpowered conditions.

Foils produce much more power or lift than fins when you hit lulls so are able to keep going much longer.

They also get going better and accelerate faster out of gybes.

Upwind and downwind there is no comparison.

The downside - you lose the connection with the water and they are seriously slow across the wind in powered up conditions.

Just my observations.

There was an article in the German Surf Magazin on this topic not long ago. It boiled down to you can speed and low lift or you can good lift but low speed. Its all based on fluid dynamics. In essence every wing design has an inherent top speed that is quickly reached. At that point drag and lift become to big to go any further.

the article explained it a lot better... I'll try and repost it here.

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petermac33 said..
A foil will beat a fin on a reach in planing but underpowered conditions.

Foils produce much more power or lift than fins when you hit lulls so are able to keep going much longer.

They also get going better and accelerate faster out of gybes.

Upwind and downwind there is no comparison.

The downside - you lose the connection with the water and they are seriously slow across the wind in powered up conditions.

Just my observations.

So you are always underpowered every time I see you on the water ????

When you boil it right down it's simply just a function of power (from the sail) vs drag from the fin/foil. Drag on a fin/foil increases with the square of speed, that's why you hit such a 'wall', it's an exponential relationship. As you get faster and faster the amount of 'power' from the sail you require to increase boardspeed speed goes up exponentially. In simple terms, because the foil has a much larger 'area' in the water it hit's that wall at a much lower speed than a fin; following on it's the same reason a small fin is faster than a large fin and a small foil is faster than a large foil.

AC cup boats go so fast for a few reasons:
- They have massive righting moment (IE can generate huge amounts of power for their weight) compared to a windsurfer.
- Have very efficient rigs (even the new twin skin soft rigs) and overall aero.
- Can actively trim their foils so can better optimise their foil lift/drag ratios.
- Windsurfers will never match them, particularly if they ever actually optimised one for pure speed rather than course VMG.

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CJW said..
When you boil it right down it's simply just a function of power (from the sail) vs drag from the fin/foil. Drag on a fin/foil increases with the square of speed, that's why you hit such a 'wall', it's an exponential relationship.

To clarify, over the useful range, the drag on a fin is much more linear. The crossover is a bit higher (still mid 20s) than I recalled but the point remains that the drag on a foil increases much faster than that of a planing board.

mauiultrafins.com/technology-2/hydrofoil-mechanics/

p 12 of the pdf, fig. 11

"While the drag of the windsurfing board in gliding is relatively constant over the speed, the drag of
the foil wing changes quadratically with the speed."

Yeah I agree, that's a better way to put it. I think that it's important to distinguish that the actual drag on the fin still follows the exact same laws as a foil and the same rate of change. It is just that due to its small area and induced drag its percentage of the overall drag compared to the board at the speeds you talk about is low.

As a kitefoiler and sometimes windfoiler the foil drag is much more noticeable on a kite foil. We have no physical join between the board and kite except for your legs. As your speed increases you reach a point where your legs get so loaded up you can't go any faster due to foil drag. A smaller foil allows for faster speed but eventually you get to the same point between 30-40 knots.
A windfoil needs a larger foil due to the extra weight of the board, mast etc so top speed will be limited. Yes you could run a smaller foil on a windfoil but then you would have no chance of gybing it.
The foiling cats have no human holding the power, instead using carbon masts, stays and sheets so will have much more power available to drive the foils much faster through the water.

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Paducah said..

CJW said..
When you boil it right down it's simply just a function of power (from the sail) vs drag from the fin/foil. Drag on a fin/foil increases with the square of speed, that's why you hit such a 'wall', it's an exponential relationship.

To clarify, over the useful range, the drag on a fin is much more linear. The crossover is a bit higher (still mid 20s) than I recalled but the point remains that the drag on a foil increases much faster than that of a planing board.

mauiultrafins.com/technology-2/hydrofoil-mechanics/

p 12 of the pdf, fig. 11

"While the drag of the windsurfing board in gliding is relatively constant over the speed, the drag of
the foil wing changes quadratically with the speed."

This makes the most sense to me. The Naish has a heap of lift and thus peak speeds are lower.

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airsail said..
Yes you could run a smaller foil on a windfoil but then you would have no chance of gybing it.

Exactly what another kitefoiler has told me. If you really got into speed sailing you wouldn't have to worry about the gybe. It's just a matter of scaling. An appropriately sized foil will have less drag than a planing hull at the target speed. It's just the start up from zero that is the problem. (Not so much for kites) . Maybe a bi plane with an upper wing that can retract up the mast into the hull?


The fastest planing hull in non-nuclear wind was Macquarie Innovation. It had all the leverage, fully designed for speed, only went on starboard tack, but is now matched by foiling cats designed to go around triangles.

Is windfoiling faster than kite foiling? I will have to race my buddy on his kite foil to see if I can pass em

Maybe a bristle-coated launch ramp? Anything goes in speed sailing.

Tow-in speed sailing?

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MagicRide said..
Is windfoiling faster than kite foiling? I will have to race my buddy on his kite foil to see if I can pass em

Kite foilers are much faster until they end up swimming ;)