hi nebbian,

a flat bottom means a surface which is smooth without any dimples etc. microscopic stuff.

all speed boards have concaves and some have cutouts.

concaves are used to change rocker line and also feed air under the board. typically the v at the back of the board is a result of concaves. normally speed boards need V to stay in control.

cutouts reduce wetted surface area (not required on pin tail speed boards.) cutouts also reduce the forward moving waves created by the tail of the board. therefore reducing pressure and allowing more speed.

so in short. speed boards have both concaves and cutouts. the big difference with speed boards is rocker and the length of the flat section in the planing area.

"cutouts reduce wetted surface area (not required on pin tail speed boards.) cutouts also reduce the forward moving waves created by the tail of the board. therefore reducing pressure and allowing more speed."

Rubbish..

You've been reading to many marketing blurbs by the board manufacturers. What keeps a planing surface afloat with "X" weight on it is a counter balancing force related to the surface area , the angle of attack of that surface and it's velocity. If you reduce the surface with cutouts and the weight and speed are the same then an equivalent amount of area further up the board must be engaged to keep afloat.

As Slowboat said.. it is a way to change the trim. The reduced wetted area is just marketing spin.

no concaves on carbon art boards... just V all the way through.

The rocker on the CA boards is quite a bit flatter than most "go-fast" boards.

The V and flatter rocker work together to produce what I considered a desirable balance of control and trim stability (even in chop). I wrote some modelling software to do this and the result was used to make the mould.

quote:

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Originally posted by yoyo

"cutouts reduce wetted surface area (not required on pin tail speed boards.) cutouts also reduce the forward moving waves created by the tail of the board. therefore reducing pressure and allowing more speed."

Rubbish..

You've been reading to many marketing blurbs by the board manufacturers. What keeps a planing surface afloat with "X" weight on it is a counter balancing force related to the surface area , the angle of attack of that surface and it's velocity. If you reduce the surface with cutouts and the weight and speed are the same then an equivalent amount of area further up the board must be engaged to keep afloat.

As Slowboat said.. it is a way to change the trim. The reduced wetted area is just marketing spin.

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interesting yoyo.

you seem to be in dissagreement with Peter Thommen and Sebastian Wenzel though.

i don't think it is marketing hype. once a board is planing wetted surface area is it's biggest source of drag.

Most boards can easily sustain less wetted area than we are giving them. Putting cutouts on the board makes that possible.

If it was rubbish we wouldn't be able to pull up on the front foot at speed and reduce the wetted area of the board since it's supposedly planing already on it's minimum wetted area countering the riders weight.

Second point is 2 boards, 1 with 70cm tail, other with 30cm tail in 15knots both plane with a 80kg rider travelling at a boardspeed of 20knots. Yoyo's relationship dosen't work then, 1 has either excessive wetted area, one sinks or the 30cm wide board has enough hull in the water to equal the area of the 70cm hull.

Yoyo's relationship is fine for non-planing.

Alot of boards now inherently push a little more hull into the water than needed to keep control, riding styles have also changed to accentuate mast foot pressure forcing the board into the water. Removing some of that area from where it is doing little aside from altering trim slightly is a logical choice.

Ok, it is a bit more complicated but Gestalts comments show why the marketing guys do what they do. Thommen and Wenzel said it is so, so it must be so , stuff the laws of physics.

Nebbian got what I ment, but now to confuse you all, cutouts DO reduce the wetted area. But not in the way the marketing departments present it and many windsurfers think.. which is related when you think about it. The upward force on a planning surface is directly proportinal to the AoA. By cutting away the surface area at the rear the board , it has a greater angle of attack, therefore less surface area is required BUT the drag is also related to the AoA so even though you have less surface the lift requirement is still the same as essentially is the L/D ratio and hence the drag. There are limits . A powerboat starting with a huge AoA has heaps of extra drag and a flat plate skimming dead flat also is not efficient. The US Navy found in tank tests that 7 degree was the best (least drag) for their flat bottom planning patrol boats but there wasn't much difference between 3 and 10 degrees.

I was hoping to avoid the long answer but it is all about trim.

Which laws of physics? Boards are not boats and I still believe we are presenting more hull into the water than necessary for anything over 60 wide where cutouts are used.

that's a big call slowboat.

so you're saying that most of the worlds respected designers have fluked where they are currently at. more luck than anything because of a lack of understanding of physics.

this is an extract from *board- "But more boards than ever were made simply to experiment, to investigate and to understand new design theories: aerodynamic lift and its effects on the nose of a board -- a board's optimum angle of attack on incoming chop -- reducing hydrodynamic drag in the tail -- the effects of a planing surface's aspect ratio -- optimizing the leverage between fin/planing surface's width and footstrap position -- the relationship between a board's centerline rocker and its outside-edge's rocker."

in 2006 starboard made 240 dfferent prototype shapes.

clearly there is more going on than just AoA. fin rake, tail drag, foot pressure, board roll, mast pressure all have their own laws of physics. i don't think you can just isolate one law and say that it is the reason for everything. particularly when sailor position and sea state is constantly changing. drag is a very big player as well including drag aspect ratios. what about board stability as well.
just applying the laws of physics doesn't cut it. the theories need to be tried in real world conditions.

there is some good stuff here. http://www.deepfried.tv/news/default.cfm?Pg=640&ID=197

"so you're saying that most of the worlds respected designers have fluked where they are currently at. more luck than anything because of a lack of understanding of physics."

I didn't say that at all... I implied that some of them *might not* understand much of the physics going on. I've certainly met a few that certainly don't understand, but still make fast boards- which look very similar to others on the market where it counts.

There is indeed a lot going on. There are a lot of things to balance over a wide range of operating conditions. Some of these things are linked well enough to isolate other aspects from the design method. Rocker, V/concaves, fin design, strap/mast placement, and outline are strongly linked and fairly independent of other windsurfing variables. If you have to design it to work with specific rig types then theres something diabolically wrong with the rig.

By using some of the known relationships and understanding the physics, we are less likely to get distracted by holes in the board, funky looking exposed-carbon cutaways, and other "interesting" features... and we'll have more confidence that what we are doing is done for substantial reason.

Dont get me wrong- trail and error testing results in worthwhile knowledge of how shapes interact on the water, but it doesn't impart knowledge on why its doing what its doing. You never really know if the thing you change is making the difference or whether its something you didn't realise was important that got changed as well... I did it that way for 15 years... But my designs have improved *significantly* since I applied a bit of grey matter to the problem and did some study.

BTW none of the fastest recorded boards have cutaways or blowholes.

cheers

My 2 cents worth.

Coming from a sailing and windsurfing background:

The smoothness of any surface for maximum speed is determined by the the theoretical max speed and the medium it is travelling on. Anything going through air, with limited boundary layer effect, should be as smooth as possible, one of the reasons smooth mylar films are used on all sails irrespective of the load holding material (carbon, kevlar, dacron etc). In the case of water there is a huge difference to air - the molecules are larger and stay together (drops etc) and there is also the surface tension aspect. Having said this a sanded bottom using 600-800 has been proven faster in slow to medium speed (early planing) watercraft as it causes molecules of water to attach and form a boundary layer which other waters molecules slide over. In relation to fast watercraft, windsurfers etc this type of bottom helps in the release stage of going through the early planing speeds but a smoother bottom will be fractionally faster when fully powered and at full speed. In relation to the surface flow film that Dennis Connor used on the 12 metre, they are very slow in comparison (even compared to m ost large yachts of their time) and the surface film used created a larger boundary layer and supposedly assisted with the direction (tunnel effect) of the water.

In relation to the cutouts on modern boards, personally I think they have been somewhat overdone, but with the new iSonics reducing them. At any given speed and given the other characteristics of a boards shape (rocker etc) a certain amount of surface area is required in contact with the water (talking flat water, but in chop it averages out) and the wider tail sections enable early planing. If there were no cutouts as speed increases there would be less forward length in contact with the water and more width, probably resulting in more nose lift and less control. With the cutouts there would be more length in contact with the water enabling a better control of nose lift and producing a better angle of attack meeting the water. Ever wondered why speend boards are still so narrow. The width in modern boards is not for speed, purely for control and transitions.

Please anyone correct me if I am off the mark.

Regards

Tony

Yep, the aerodynamics of the board are mostly overlooked in the forums but with speed it is important. Slowboat has hit 90ks at Sandy Point with averages usually in the 80s. For those who are unlikely to get to those levels (most of us) just think about taking a board off the roof racks in a 30kn seabreeze (55km/h) and then realize that as the force is related to the square of the wind speed it would be 2.25 times that at 45knots. Certainly something to consider.

From Mal Wright's write up on that epic day when Slowboat averaged over 46kn last year at Sandy Point.

"But the star of the show today was Chris Lockwood. His first run he clocked 47 on the GPS and just got faster from there. Amazing speed. He was kitted up well with the new KA 4.4 which seems to work well. It certainly meant Chris could put the pedal to the metal and he didn't muck around doing that. There were many runs where Chris was literally tail hovering for up to 60-70 meters at a time with the amount of wind he was getting under the board, but it didn't phase him at all and he kept powering on. Probably went faster doing that... "

Hers my 2 Elmo's worth.

I think a point a number of people are missing is that there is considerable difference between a dedicated speed board and a slalom board.

A speed board is designed primarily for one purpose only that is scaring the crap out of Elmo's. Dealing with chop and turning ability are generally not a high priority in the design.

Whereas a slalom board, although designed to go quick is also designed to deal with a whole plethora of other factors as well such as chop, going around corners, going upwind, float etc so cut outs and all the other fruit is of benefit to its over all performance.

At the end of the day speed sailing is considerably different to slalom sailing.

Chris (and others) have put a lot of work into developing true speed boards.

A lot of companies market also their boards as speed boards but in reality they are just small versions of their slalom range, don't get me wrong they're quick in their own right due to their small size and narrow width but generally a dedicated speed board on the right course will be quicker than a slalom boards.

Now going fast in chop is probably where a slalom board will come into it's own for the average punter.


I once again may be wildly inaccurate with my theories but that wouldn't be the first (or last) time.

naaah. its pretty basic and definitely hard to drive. But it allows me to see and indirectly adjust the lift distribution on the bottom of the board. It was a one-off to get a good overall balance on the CA designs. And it did its job quite well One day I might spice it up but I'm happy with the boards as they are so no need for now. Concentrating on fins and sails at the moment... no need to mess with something thats not hitting its limit yet.

Nah, The bar at Pt Walter is mostly under water now with the winter tides so the chop comes through. Also the wind is mainly westerly, not a good angle. Additionally the part that isn't under has built up with grass. It ain't what is used to be. Hardie's run may be the best option this winter.

have any of you gurus tried Aquaspeed????