View Full Version : Cutouts - fact or fashion

5th January 2007, 12:56 PM
Well i last posted this sometime ago as i was sceptical about the deep cutouts.
Seems that in general i was right and that it was a fashion.
Fanatic and Exocet removed deep cutouts from their new slalom boards. Exocet removed them from their formula and the cutouts are more conservative like Starboard.
Cant say that the Falcons or Warp Slaloms are missing the cutouts as the 07 boards are so fast, plane early, and gybe so well.

F2 seem to be stuck with them on their boards, but supply adjustable plates which add Kg's and most people use to reduce the depth.
Question is, will they keep them to justify the air-pipe concept?

6th January 2007, 09:27 PM
Guest...Exocet removed them from their formula and the cutouts are more conservative like Starboard...


Yes, really interesting Exocet approach for its 2007 Warp Formula:


Rather than "more conservative". it seems that Exocet combines both deep and thin cut-outs: at first stage water flow under board meets thin cut-outs, then it undergoes an expansion in a wider area in a way comparable to older (and deeper) cut-outs...

It would be interesting someone REALLY QUALIFIED in hydrodynamic issues could explain for once the working of cut-outs (and air pipes) and their REAL influence on windsurfing performances and efficiency.

- Expander.

7th January 2007, 10:27 AM
I have and sail the Exocet WF 07, i cant really say that the tail shape is in effect a deep cutout.
However it has a very clean water release and i think will be right up there with F161 this year.

8th January 2007, 02:02 AM
It seems to me that the main purpose of cutouts is that of combining a very wide tail and its huge leverage over the fin, with good longitudinal pitch stability. In fact reduction of planing surface at the tail should force the board use more forward surface to glide on, therefore obtaining a longer planing surface with more resistance to pitch variations. A no-cutouts "full width" planing surface would be shorter and therefore be more prone to pitch variations, somewhat like the board was hinged on the very tail.

On the other hand, probably a calmer water surface that one may encounter at lower wind speeds would make it possible to use the superior efficiency of a superwide supershort planing surface.

Thin cutouts IMHO are used to obtain gradual transition from a "full width" sub-planing surface to a narrower surface at speed.

Air pipes are to me a deep mistery, explanable only in terms of marketing bells and whistles. The side of cutouts has plenty of surface for the air to pass through and fill the void, if any, left under the board by the cutouts. I observe that in the case of F2 boards the cutouts are triangular, therefore air is shielded from getting directly under the board from the sides. I see that after 2 (?) seasons other board makers than F2 do not copy/use them.

8th January 2007, 10:51 PM
Ooops, I forgot. I am the author of the post here above. Maybe I am not completely "really qualified", but at least I am a graduated mechanical engineer who studied fluid dynamics.
I see that the forum software did not recognize me. I usually post with the ".g.e.o." signature.

9th January 2007, 05:38 PM

Thank you, GEO, for your analysis... let me investigate about some issues risen by your reply.

Some other comment about ?

- Exp.

16th January 2007, 05:47 PM

I want to give a full open contribution about a issue that, after some years from their appearance, is still unclear; this isssue regards cut-outs devices and will be treat with a close analysis of windsurfing board behaviour and using detailed schematic (in scale) drawings.

Following analysis is based on accurate surveys crossed with personal engineering / scientific background; windsurfing board shown is a generic free-formula board (270 cm lenght x 87cm width with 61 cm fin) with a 8 m. sail.

First of all we should distinguish between: a ) a sub-planing (full displacement) and b ) a high-speed planing condition where a complete windsurfig system (board, rig and rider) assumes different peculiarities.

As visible in Fig. 1, at low sub-planing speeds board bottom surface in contact with water (wetted area), is larger than same area in planing condition, of a size of approximatively three times.


To develop a more efficient windsurfing board in terms of hydrodynamic efficience, one of ways followed by manufacturers has been to reduce wetted area as shown in Fig. 1 (bottom).

This can be achieved using a so called "cut-out device" that has taken geometrical forms different from manufacturer to manufacturer: thin cut-outs are used by Starboard while deeper cut-outs are used by F2 or Fanatic; anyway, that is the simpler way to reduce drag resistance and increase speed performances, above all when board is running free from wind.

Just in this running (free from wind), I think F2 or Fanatic approach should be more efficient than Starboard but this involes some problem that makes Starboard solution preferable (above all in low wind condition), as we can see further on.

A closer view of board reveals interesting details about board dynamics in sub-planing (full displacement) condition (Fig. 2 and Fig. 3).



Fig. 3 shows a detailed view of board tail: it is clearly visible that in full displacement running, thin cut-outs are in contact with water flow and contribute to increase overall board wetted area while deeper cut-outs (Fig. 3a), as used in F2 and Fanatic Formula boards, do not allow a contact between board bottom surface and water flow, so reducing overall wetted area (and early planing qualities in low-wind condition).

On the contrary, in planing hi-speed condition, when board finally rises on tail, wetted area is naturally reduced of three times, as seen above.

Fig. 4 and Fig. 5 show what happened in this condition (hi-speed): with a wetted area strictly limited by board contact surface with water flow (Fig. 5), cut-outs (Starboard like) top surface are now finally freed from water allowing higher performances (higher top speed) when board is running free from wind (compared to board without cut-outs).


Horizontal plates placed on cut-outs, as introduced by F2 and Exocet in last season board (FX-100 iv and Warp 2006), in my opinion are the attemp to gain a better board control on longitudinal plane but, substantially, even a larger wetted surface on board with good qualities in mid to hi wind condition but not so good in terms of early planing.

So, returning to starting question: Cutouts - fact or fashion?, what I feel to answer is that this is a good solution for increase speed performances in particular conditions (i.e. when board is running free from wind), but that they have to be carefully designed if one has to do with "early planing" questions too.

In other words (a issue as old as the hills) I think it's really difficult to combine different qualities in a board: for this reason Exocet approach in tail design of new Warp 2007 (see image shown at the beginning of this thread) is really interesting because it seems to combine both deep and thin cut-outs positive characteristics.

- Expander.

16th January 2007, 08:01 PM
I think that in planing conditions cutouts determine the planform shape of wetted surface, rather than its extension. In fact extesion (surface) is the much that is needed to support total weight (body + board + rig etc.) at a given speed.

Otherways, it would be possible to reduce wetted surface even to zero, by designing cutouts corresponding... to the wetted surface itself.

Of course, the shape of wetted surface (together with rocker line) will affect angle of attack and therefore lift coefficient and, consequently, wetted surface extension. But to say that cutouts outrightly reduce wetted surface seems to me inappropriate at the least.

17th January 2007, 03:07 AM
A very interesting discussion. While I'm really not qualified to comment technically how cutouts actually work, it's clear to me that they have gained an important significance as boards have gotten wider with shorter high aspect planing surfaces. I have to think that they are here to stay, but what gets more difficult to say with any certainty is what configuration is the best. I often wonder how scientific the result is, or whether some of the development is more intuitive and experimental in nature. I think it's fair to say that trial and error is an ingredient in the mix.

One thing missing from the discussion above is how turning is affected by bottom and rail oriented cutout relief, and how things blend with rockerline's influence. Also, there is a bit of a broadening of concepts at work here with influences at work with light wind slalom designs, especially since high speed turning becomes crucial to performance. It would be interesting to hear more about this and how the configuration of cutout concepts vary between formula and slalom influence respective performance.

Lastly, I thought I would be of value to comment on a portion the bottom design of the high wind B&J/wave boards that I have been using over the last 11 years. Although these boards include a number of unique bottom characteristics, the tail of the boards feature a cutout that is shaped as an arc that starts from the rails at about the half way point of the US type finbox and intersects the finbox down about 7.6cm from the start. So, the entire tail aft of the arc is cutout to a depth of about 1.0-1.5 millimeter. Not a real deep cutout by todays standards, but it still must be considered a cutout or relieved area. I should point that the tails are quite narrow. I guess one of the noteworthy differences or departures from what is being done today is that it emcompasses the tail as a whole rather than implementation of balanced cutouts on either side of centerline. Even though I can't comment on the science of the design, the boards work very well, particularly in strong wind and rough water conditions. Very playful in nature, yet tracking is smooth and controlled.

17th January 2007, 06:45 PM
Hi Steve C,

I guess it's more experimental than scientific; otherwise, probably there would be some scientific explanations readily available. Nevertheless, I think there are no big mysteries in board designs and shapes usually speak for themselves.

As for the cutout configuration issue, I think that probably "thin" cutouts provide for a more gradual transition from "subplaning" to "full planing"; while deeper ones should generate a sharper acceleration when the central part of the hull gets free form the water flow, enabling for a quicker adaptation of wetted surface to the higher speeds. Probably, thin cutouts provide for more even performance spectrum while deeper ones provide for more "powerful/extreme" performances in a narrower speed range.
Last Spring tests from PlancheMag seem to confirm this idea, with regard to the F2 SX S (strong acceleration and good control in high winds, but less low end) and the JP Slalom 94 II (strong acceleration and high low end performances, less control at high end); both with deep cutouts. Other boards with no cutouts, such as the Sonic 85/95, were praised for their wide performance spectrum.
Of course, this can not be 100% valid as many other elements contribute to the performance spectrum, rocker line in first.

As for turning, any vertical surface in the water flow would tend to impair it, but cutouts surfaces are comparatively small with regards to fin and rails surfaces; so their influence should not be that much.

As for high wind / bump and jump boards, I guess that the drawn out outer outline provides good tracking for high speed jibes and the stepped bottom provides a short, wide planing surface with a sharp release for acceleration and speed. The step/cutout shape with small longitudinal vertical surface should not impair turning abilities.

I really like this discussion and I would really love to see some contributions from board designers.

17th January 2007, 07:05 PM
Although I do not know much about hydrodynamics IMHO, the main target for cut outs is to improve control at high speed and reduce drag since both are related. Reduced drag might also help in putting a hull on a plane earleir...maybe.
Obviously it will depend on a correct design for it to work properly. It might even improve or change slightly the way a given fin works.
The idea of setting different possible depths sounds good however in the real world I believe it might be not very practical.

This subject seems to me a good application for fluid dynamic softwares. I do not know if they're in use in the windsurfing industry but I know from expeirence that some time should be spent to validate it ( in that applicatin ) otherwise results would be meaningless.

My two cents.

17th January 2007, 10:26 PM
Yes of course, but the point is in how drag is reduced and/or control is enhanced.
And, of course, in how fluid dynamics software models are conceived, which requires to understand the physical facts as the first thing.

18th January 2007, 06:38 PM
Knowing why and how is always best however, sometimes not really necessary. Provided you validate your models and metrics you can come up with say version A x version B and choose the one with the best results.
The hardest part is to validate your models specially when there are lots of variables which are evalueted subjectively. One can say in which area a new board is better but cannot objectively measure it.....(yet). The task and job would be really huge but it has been done in other engineering areas.
Simulation programs are target to reduce the number of prototypes required to check a new design.
As far as I know the windsurfing industry has been improving its products through massive experimentation. Sometimes trying things that shouldn't work but that end up working in a different hull, for instance.
Working with simulation would in time reduce experimentation a lot and therefore the money spent in so many prototypes.
Example: in the auto industry a few years back sometimes one would have to actually crash 10,15 even more prototypes to get the desired results. Nowadays they crash only 1,2 to confirm simulation.
Hopefully all the money saved would mean cheaper products .....maybe :)

20th January 2007, 07:06 AM
I am not all convinced that cutouts, contribute directly to board efficiency. This is because the cutouts on most boards leave an irregularly shaped planing surface, with reduced aspect ratio.

A positive effect of cutouts may be that they remove lift from the back of the board, thereby bringing the center of board lift forward. This is important as the center of board lift normally move towards the back, as speed increases.

In other words, a benefit may be that we can keep a well balanced stance in the foot straps, from marginally planing to top speed.

20th January 2007, 09:08 AM
I'm not sure that simulation testing is less expensive than prototype testing, however, I can't solidly validate this opinion one way versus the another. Yet my thought is that prototype testing is probably the most versatile, and ultimately less costly, especially if a number of candidate designs or versions being experimented with is numerous. What it really comes down to is the number of focus concerns. In reality, I think a practiced tester is better positioned to discern the range of performance in a practical way to the designer, particularly if varied testers are used.

Regarding erling's comments, I have to think that cutouts aren't targeting an irregularly shaped planing surface, as most designs are clearly balanced off the centerline. Of course, the actual shape of the wetted area can be viewed as irregular shaped when considered solely off centerline, but the sum of both is balanced overall. No asymmetrical concepts in today's offerings. Nonetheless, I have to think that overall lift is affected by cutouts. Similarly, I would think that overall range of performance is crucial to an optimum result, so cutout concepts must be efficient and productive across a broad range from light to strong wind conditions.

21st January 2007, 08:47 PM

Knowing how and why is best, period. It may not be necessary, but it can also save lots of time and energies. I think that knowledge of physical facts that are behind board performance is even more useful than models, due to the high degree of subjectivity and the complexity of our sport.


that is what I was saying: IMHO a superwide tail with no cutouts is the most efficient, and the need for cutouts is more related to longitudinal stability than else. As for the irregular surface, it should not be a problem as the trailing edge of a cutout would behave just like a tail's trailing edge. Only, I guess the angulation between centerline and the side of the central portion of the hull (the one between the cutouts) could be critical as it could not have a sharp release and therefore generate turbulence and drag.


I agree about the idea that proto testing is probably the best way to go, both economically and practically. One can get new ideas even from a screwed up proto, plus I don't think there can be a sophisticated and complete enough model for numeric simulation of a sailboard (+ rig + sailor) hull's behaviour. This makes clear ideas about the physics even more important.