I was thinking how important are tail cut outs and do we really need them in all slalom boards ...is it excessive tail width where they are applied ,is the top speed dependent on them or are they only needed due to board shape .?
I have offen thought should I add cut outs to my 2014 100 litre Exocet ..or just leave it as is ,one thing I have noticed with it ,it seems to able to carry bigger sails than any other 100 litre board I have had and the narrow of the board starts way up to half way ..Are cut outs for top speed only ,I have noticed with starboard s their cutout started big and now they have gone shallower ..could this be for early planing .and less spin out .?..any thoughts on this subject .

All I know is that it is very hard to get a proper science based explanation of how and why they work from any of the manufacturers or board designers. Everything I have seen is simplistic marketing fluff.

That said, it is clear that cut outs work well on some designs and boards. But why.......

Edit: BTW, it appears that you have the means to test at least one board, with and without the cutouts. Let us know what you find.

true ..thank s sailquik ,.some points are good some not ,but so far i have found the 100 exocet so good in chop ,but it is a unfair test ,as it could be just the extra 15 litres on the middle board ..115 litre board in 25-28 knots ..kinda not fun ..lol.
with that said i couldn't imagine my bigger boards without cutouts .so maybe its a big board thing .

Can you try the middle patrik with and without plates to see what the difference is? With the exocets, think of the cutout as going the whole thickness of the board - ie the corners are cut off compared to a more rounded tail. My RS4 (no cutouts) is really nice in chop, but my wider RS6 does have cutouts to free it up a bit in the lighter winds its designed for.

the theory as i understand it as reported by american windsurfer

1. boards got wider because it improves low wind performance. But, control issues at speed happened due to too much lift that lightweights couldn't control... :-)

so to alleviate the excess lift at speed designers started moving the straps, fins and sailor towards the rear of the board. the idea being the further back the less surface area and more control at speed. this was good up to a point but too close to an edge of the board and the fin starts to spinout.

so 2 design paths occurred.

path 1. - AVS. this was a flap out the tail of the board which provide an area around the fin to help prevent spinout but allow reduced surface area and low drag at speed. - downside was reduced gybing performance due to reduced volume

path 2. - Pro-tech introduced the CAT. cut away tail. - by introducing cutaways the the lift at the rear of the board was reduced when sailing at speed. the reduced area also reduced drag. the shape of the cutaway left enough surface around the fin to prevent spinout. - the upside was the rail remained to help gybing performance.

path 2. won the race and we are still in a world of trying to push the sailor back as far as possible with varying cutout designs because it provides performance over a larger wind range.

Yeah I'm also curious and sus.
I have some boards with and without , I can't feel a pattern , but they are different boards . I think a lot looks good on brochures and that matters to sales . If you want a narrower rear end , make it narrower . I hear the trim argument about the smaller back not pushing the front down so much in chop and still having a wide early planing rear . I have an older formula board that has multi angles and levels that could only be good for a wankoff in brochures compared to the opposition that has only twelve angles.The main thing I can see with cutouts is it allows a rider to have a more outboard stance to push on a big fin on an effectively narrower board .
Not that long ago I saw a pic of a Goya freeride board and the tiny cutouts was less than a square inch area. That is purely only for looks. I can understand having cutouts with spacers and noticing a difference , but your effectively changing the board .
If you had two identical boards , one with and one without cutouts I'm sure they will feel different because they are different . I would think you could almost reproduce the feel of the cutout board by pulling in the edges of the non cutout board . As long , as above you still have standing area.
To the original question , I'm not sure if they are necessary .

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Gestalt said..
the theory as i understand it as reported by american windsurfer

Thanks Gestalt. That is one of the better theory's for sure.

Still pretty hard to find any supporting evidence that is not anecdotal though.

I guess the best evidence we have is that most, or all of the winning boards in top level slalom racing have cutouts. In that arena, speed and gybing counts big time so if it isn't faster, it should quickly disappear.

It's interesting to note that, as far as I can see, none of the fastest (narrow) speed boards have cutouts. Probably because they dont have to have a broad operating range as they are very specialised.

yeah only really applicable for wide boards.

or duck tails.

If you look at this diagram it shows that the upward lifting force is greatest at the leading edge of a planing surface and decreases to very little at the trailing edge. Friction drag is relative to the surface area so at the rear of the board the lift to drag ratio is low...high drag, low lift. So in theory if you remove surface area at the trailing edge you reduce friction drag with only a small decrease in lift.
Cutouts are a way of doing this.

Here's another couple of thoughts .
All the slalom guys have to use the catalogue comparing pretty boards .
Why is it that the cutouts are so vividly showing off that they are there ? It seems like it's the most eye catching part of the board yet offers way less than the overall design.

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mr love said..
If you look at this diagram it shows that the upward lifting force is greatest at the leading edge of a planing surface and decreases to very little at the trailing edge. Friction drag is relative to the surface area so at the rear of the board the lift to drag ratio is low...high drag, low lift. So in theory if you remove surface area at the trailing edge you reduce friction drag with only a small decrease in lift.
Cutouts are a way of doing this.

Thanks Martin, Now that's more like what I was hoping for.

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mr love said..
If you look at this diagram it shows that the upward lifting force is greatest at the leading edge of a planing surface and decreases to very little at the trailing edge. Friction drag is relative to the surface area so at the rear of the board the lift to drag ratio is low...high drag, low lift. So in theory if you remove surface area at the trailing edge you reduce friction drag with only a small decrease in lift.
Cutouts are a way of doing this.

So why not just make a narrower rear ?y
Id bet that the top slalom guys would be just as fast on a purpose built custom board without cutouts .

If you make the board narrower then your stance will get closer to the center of the board and you will not have the leverage to hold the big sails or control the larger fins. Cutouts allow you to have an outboard stance while reducing the surface area in the tail of the board and lowering drag.

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jamesf said..
Can you try the middle patrik with and without plates to see what the difference is? With the exocets, think of the cutout as going the whole thickness of the board - ie the corners are cut off compared to a more rounded tail. My RS4 (no cutouts) is really nice in chop, but my wider RS6 does have cutouts to free it up a bit in the lighter winds its designed for.

yes i tried it with them in ..it drove the nose down and gave a slightly rougher ride ..so took them out and left them out .but it planed earlier ,


i like those theory's Gestalt .I remember watching a Patrik vid a few years ago ..where he took his router to the beach
and was cutting pieces out as they where testing them with their pro riders ...so the shape of cutouts seem to be in a rounded shape now rather tham the straight line shape ...so thats another story .

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mr love said..
If you make the board narrower then your stance will get closer to the center of the board and you will lot have the leverage to hold the big sails or control the larger fins. Cutouts allow you to have an outboard stance while reducing the surface area in the tail of the board and lowering drag.

I agree with that.

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Tardy said.. >>>
yes i tried it with them in ..it drove the nose down and gave a slightly rougher ride ..so took them out and left them out .but it planed earlier ,>>>

This is also a reason for cutouts, lift behind the rider will force the nose down, shifting the entry point forward and increasing wetted area. Ideally lift should be under the feet not behind it.
Lack of cutouts will improve early planning and help at low speed, when the entry is far enough forward to make the centre of lift under the feet, but as speed increases, there's a point where point of entry is inhibited from moving back, by the lift going behind the rider.

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Tardy said..

jamesf said..
Can you try the middle patrik with and without plates to see what the difference is? With the exocets, think of the cutout as going the whole thickness of the board - ie the corners are cut off compared to a more rounded tail. My RS4 (no cutouts) is really nice in chop, but my wider RS6 does have cutouts to free it up a bit in the lighter winds its designed for.

yes i tried it with them in ..it drove the nose down and gave a slightly rougher ride ..so took them out and left them out .but it planed earlier ,


i like those theory's Gestalt .I remember watching a Patrik vid a few years ago ..where he took his router to the beach
and was cutting pieces out as they where testing them with their pro riders ...so the shape of cutouts seem to be in a rounded shape now rather tham the straight line shape ...so thats another story .

A mate of mine (who owns Patriks) asked the man himself about cutouts, and what you experienced above was the explanation he got back.

Smaller surface area at the back when the tail hits the next bit of chop, keeps the nose up where it should be. I didn't understand them either till i heard that explanation.

yep i'd argue it is more to control lift then drag.

in Tardy's example he could put the plates in and move his sailing position back t control the AOA. but this then leads to spinout and is why the CAT design happened.

With martins diagram. i don't see how adding cutouts reduces drag on a plate. surely adding cutouts merely just moves the wetted length forward and does nothing to reduce drag. what drives the plate AOA is the load position. in this case the sailor.

we can reduce drag by increasing aspect ratio.

I spent a bit of time testing Starboard protos around 8-9 years back when cutouts started to go from thin little steps in to the much more complex stepped things we see now (ie, the Toro Tail which every brand has replicated from Finian Maynard / RRD ideas). At the time, there was big focus on trying to make the medium board (107) have an obscene windrange from something like 15-35 knots. Why? Because there's quite a lot of situations on the PWA where you sail in those conditions (Fuerteventura, Costa Brava, Sylt occassionally). The 107 was getting wider and wider at the OFO mark and around your back foot so you could get more leverage to pump out of a gybe, but this got out of control in bumpy seas so they needed to reduce the tail. Honestly, the slalom ranges are so focused on making sure the top rider(s) is fast against the other brands in a specific condition so I'd say the evolution of cutouts has really stemmed from trying to get the tails much, much wider for light winds whilst not giving away any control in highwinds. I would doubt there is any "top speed increase" whatsoever compared to no cutouts. But definitely the boards have got wider wind ranges out of this trend. And that is good for the consumer market.

Downside is they are complex/expensive to make, horrible to repair! But performance wise = YASSSS!

According to that pic above, flat-plates (and thus boards?) have anti-gravity capability and thus suck water uphill.

Even if the the cutouts never touching the water, - then there wont be any signification drag reduction, vs the drag at the board/water interface, and the large-area for about a 1/4 to 1/3 of the board that is touching the water. But... then consider formula boards which have massive cutouts... those may have some drag reduction, vs the slower speeds traveled while formula-sailing.

All planing boards make a hole in the water - it is how planing works, which is what that pic is meant to explain ** That hole eventually that gets filled in some finite time later once the board is out of its way. The implication is there is some upward force at the tail of the board.

Cutouts may have some effect on that upward force, so "control" is probably the real answer.

** this excludes "flying on the fin only" such as when speedsailing. But then cutouts aren't usually used on speed boards.

Matthew if you are reducing wetted area you must be reducing friction drag. And yes there is lift at the tail of the board but less than at the leading edge.
And yes boards make i hole in the water...I know, I have sailed behind Spotti

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SeanAUS120 said..
I spent a bit of time testing Starboard protos around 8-9 years back when cutouts started to go from thin little steps in to the much more complex stepped things we see now (ie, the Toro Tail which every brand has replicated from Finian Maynard / RRD ideas). At the time, there was big focus on trying to make the medium board (107) have an obscene windrange from something like 15-35 knots. Why? Because there's quite a lot of situations on the PWA where you sail in those conditions (Fuerteventura, Costa Brava, Sylt occassionally). The 107 was getting wider and wider at the OFO mark and around your back foot so you could get more leverage to pump out of a gybe, but this got out of control in bumpy seas so they needed to reduce the tail. Honestly, the slalom ranges are so focused on making sure the top rider(s) is fast against the other brands in a specific condition so I'd say the evolution of cutouts has really stemmed from trying to get the tails much, much wider for light winds whilst not giving away any control in highwinds. I would doubt there is any "top speed increase" whatsoever compared to no cutouts. But definitely the boards have got wider wind ranges out of this trend. And that is good for the consumer market.

Downside is they are complex/expensive to make, horrible to repair! But performance wise = YASSSS!

wow ...cheers ..

If they are so wonderful, why do they provide blanks to cover them up ?

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forceten said..
If they are so wonderful, why do they provide blanks to cover them up ?

Because not everywhere is choppy, and a shallower cut out aids early planing.

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Tardy said..
I was thinking how important are tail cut outs and do we really need them in all slalom boards ...is it excessive tail width where they are applied ,is the top speed dependent on them or are they only needed due to board shape .?
I have offen thought should I add cut outs to my 2014 100 litre Exocet ..or just leave it as is ,one thing I have noticed with it ,it seems to able to carry bigger sails than any other 100 litre board I have had and the narrow of the board starts way up to half way ..Are cut outs for top speed only ,I have noticed with starboard s their cutout started big and now they have gone shallower ..could this be for early planing .and less spin out .?..any thoughts on this subject .

I have no idea why they work, although the theory exposed above, that they increase control because they reduce surface area in the tail, makes intuitive sense ...

But I owned a Patrik slalom 100 (64.5 wide) that had medium sized deep cutouts. The board was an absolute dog unless it was completely lit: the drag at take off or jibing, in just powered conditions, was something remarkable. Plates on or off seem not to make a difference. They did in overpowered conditions, where taking off the plates helped controlling the board. Compare to the venerable Exocet Slalom 90 I inherited last year. It is a 2008 board, 62 wide probably at least 95 liters, with no cut outs. It is probably slower. but much better in a jibe if you are just powered up. The power simply does not comes off, where with the Patrik if you had a second of hesitation you literally heard the turbulence in the the rear before you came on a stop! (This again in just powered situations, if you were lit the Patrik was very happy.)

An anecdote, maybe not even accurate ... but I suspect that unless you are racing on a medium/big slalom board cut outs are probably unnecessary ... talking of which: the reason why foil boards have cut outs completely escape my understanding ... and certainly does not fit the control theory!

Cutouts on wider boards and fully powered.
No cutouts on narrower boards or underpowered.
'08 Exocet Warp is as fast as anything out now 95% of the time.
I guarantee a good sailor on 2002 Exocet Speed Slider can pass 90% of good windsurfers in the water.

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LeeD said..
Cutouts on wider boards and fully powered.
No cutouts on narrower boards or underpowered.
'08 Exocet Warp is as fast as anything out now 95% of the time.
I guarantee a good sailor on 2002 Exocet Speed Slider can pass 90% of good windsurfers in the water.

Unfortunately I do not have any GPS on the Exocet Warp 90, and it is probably a moot point anyway because the omni-present chop makes me top out at 30-32 knots most of the times. But the Patrik did feel faster. Or at least easier to un-stick when overpowered to really ride the fin ... but feel ain't speed and again if you are not racing ...

I think Sean is correct that most of the development with cutouts now is around control and wind range, however cutouts these days are a given on wider slalom boards. Back in the day when designers started to explore cutouts I will guarantee it was about drag reduction as they explored high speed in light winds.


Does this diagram make you happier Matthew?

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mr love said..
Matthew if you are reducing wetted area you must be reducing friction drag. And yes there is lift at the tail of the board but less than at the leading edge.

Do the calculation yourself, on a 100L slalom board there is somewhere around 0.7m2 of wetted surface area, of which cutouts reduce it by about 0.02m2... or something of that order. That is about 2% surface-drag reduction.

The dominant drag, is due to the force required to move the water out of the way to make the hole - it is a volume, which it is a function of downforce and speed. Some downforce is mitigated by flying the board. The remaining hole-making effort, is the dominant component ... at least until that component becomes close to zero like when speedsailing.

I said "insignification" because apparently I dont know how to spell... but I'm sure everyone got the idea. Surface-drag is big and some of it can be mitigated via various types of surface prep, but my claim is cutouts much sweet-FA any difference to that value.

Well I will disagree with insignificant, If you look at this graph you will see that skin drag at high speed becomes the most significant form of drag, the faster you go the more significant it becomes. Also bear in mind that as the speed increases the hull lifts higher from the water and the stagnation point moves rearward, therefore as the wetted area decreases due to this effect the proportion of area you have removed with the cutouts increases and their impact on reducing wetted surface is more significant.

Tail cutouts are also a great way to stabilize trim over a bigger speed range. Think of it like a trainer wheels. You end up with two lifting / balancing points, one in front of the cutout and one behind.