There is a very slight depression between the front and rear foot straps. It's so slight, it would difficult to measure, but light can be seen through it. It is between the back of the front foot straps and the middle of the rear foot straps. The first picture is a SB W49 and the second is an SB W44 (this picture came out upside down). Both boards have quite V from the front foot strap all the way the rear. They both also have a lot tail kick on the last 6 inches or so.

is this slowing me down, or is it just a tiny imperfection?

It maybe affecting your trim, it's putting more lift under the back foot, that can force the nose down and increase wetted area, so yes it could be slowing you down. Fangy had something similar with an old board, and asked What I thought about filling the gap. I said the increase in weight would probably cancel out any improvement. But typical Fangy, doesn't listen to me, and goes ahead and does it. He reckons there was a big improvement.
So all I can say, is mix some q-cells and epoxy, and fair it out, give that a coat of spray putty, to fill the micro holes in the sanded q-cells and wet and dry to 400 or 600 grit.
Then report here on the results.

The hollow at speed at speed gives a low pressure area that sucks the board to the water. Had a board for years which developed an even bigger hollow between the straps than yours. Slowed it quite a bit. Once I filled it I got the speed back.
Just a note though, it didn't last too long in my case as the hollow was cause by a breakdown in the foam underneath from the years of pounding over chop and it continued to deteriate.

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decrepit said..
It maybe affecting your trim, it's putting more lift under the back foot, that can force the nose down and increase wetted area, so yes it could be slowing you down. Fangy had something similar with an old board, and asked What I thought about filling the gap. I said the increase in weight would probably cancel out any improvement. But typical Fangy, doesn't listen to me, and goes ahead and does it. He reckons there was a big improvement.
So all I can say, is mix some q-cells and epoxy, and fair it out, give that a coat of spray putty, to fill the micro holes in the sanded q-cells and wet and dry to 400 or 600 grit.
Then report here on the results.

Defo fill it as per Decrepit's instruction ( I did listen to that bit :-) Made a big difference to my board, but to be fair, the hollow on mine was much bigger.

I have just checked my W49 and found a similar "hollow" on the centre line between the front and back straps too, however as it is a v shape at the rear straps I don't actually know for certain that it's a concave area because of the V. I also observe that this hollow is possibly more likely due the raised fin box then a depressed area. I do wonder if this was actually a manufacturing imperfection rather than high usage. The carbon Art series on youtube videos on carbon sandwiching briefly touch on how to correct this issue (Episode 6, part 1). Anyone keen on posting a step by step guide on how to fix?

Yes when checking it's best not to put the straight edge right on the centre line on top of the fin box. a check over the whole area will tell the real story.

It's not a hard procedure. Just bog up the depression sand flat and paint.

The surface will need sanding first so the bog sticks.
The bog can be anything, buying resin and q-cells just to do this may not be economic.
I think westsystems have "microbeads" in small quantities. I mix to a fairly thick consistency, the more q-cells the lighter it is and the easier to sand, but if you go too far it doesn't stick as well and is much harder to smooth out. It will "pick up" on the squeegee, leaving rough hollow areas.
If so panel beating car bog should be OK as long as no core foam is exposed, as it's probably polyester based and will dissolve styrofoam, it's probably also heavier.
I apply the bog with a squeegee, the final passes with a lot of downward pressure to minimise "pick up"

After this has cured enough to sand without clogging the paper, use a longish sanding pad and hand sand flat, to the surrounding surface. If there any misses, re-bog. Once sanding is finished spray from a can of spray putty, to fill the micro balloon holes, wet and dry, and paint as required. Most of the time I leave it with just a wet and dry spray putty finish.

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decrepit said..
Yes when checking it's best not to put the straight edge right on the centre line on top of the fin box. a check over the whole area will tell the real story.

It's not a hard procedure. Just bog up the depression sand flat and paint.

The surface will need sanding first so the bog sticks.
The bog can be anything, buying resin and q-cells just to do this may not be economic.
I think westsystems have "microbeads" in small quantities. I mix to a fairly thick consistency, the more q-cells the lighter it is and the easier to sand, but if you go too far it doesn't stick as well and is much harder to smooth out. It will "pick up" on the squeegee, leaving rough hollow areas.
If so panel beating car bog should be OK as long as no core foam is exposed, as it's probably polyester based and will dissolve styrofoam, it's probably also heavier.
I apply the bog with a squeegee, the final passes with a lot of downward pressure to minimise "pick up"

After this has cured enough to sand without clogging the paper, use a longish sanding pad and hand sand flat, to the surrounding surface. If there any misses, re-bog. Once sanding is finished spray from a can of spray putty, to fill the micro balloon holes, wet and dry, and paint as required. Most of the time I leave it with just a wet and dry spray putty finish.

Thanks Mike. I'll recheck and try the fix if required.

Im gonna go Check Steves boards, it probably whats slowing him down! LOL

Thank's for all the replies. One last question, If I do fill in the depression, should I sand it flat (this would be altering the shape) or should I sand it to follow the original V?

I'm guessing the V shape is there for a reason?

Follow the existing bottom shape!
Depending on your skill level, fairing a negative tail rocker might mean later planing and less glide, balanced by higher top speeds and better slipperyness.
Some old Naish Supercross's had slight negative, and when faired out, hardly any gain to be had because it already had a very narrow tail for it's expected wind range.

The simple answer is yes the negative rocker is slowing you down and if you can fix it accurately then I would. Don't mess with the V just fair out the negative. This is something I have observed on Cobra boards for years and has to be something in their production process. I have seen brand new boards in the rack with it.
It is worth checking your boards as they can also develop this negative in front of the fin box over time especially if you sail in rough conditions.

If you fair it in nicely, the V shouldn't change, it would take a lot of work to flatten the V and still have it fair.

And for anybody building a board...... if you want to avoid this negative tail rocker, put a Tee stringer in when you build the board.
Dead easy to do, holds the rocker while you build and they never go negative in use.

Brilliant, i'll Defo sort out the negative rocker on my W44. I'm waiting for the wind to fill in, so checked all my boards I checked all my boards. 7 out of seven have negative rocker, all very mild, but it's there. The newest is an iS107 2015, bought it new last year on a close out deal, used maybe 7 or 8 times. So I think mr love I spot on about cobra.

I'll get it sorted and let you know if I notice any improvement in speeds.

many thanks all

Lots of information about this too on the dutch forum.windsurfing.nl

First of all, this is REALLY important for performance. One of the first things (apart from structural integrety) to check on a board.
Easy to correct on flat bottoms, a bit more difficult to repair when it's a V bottom, difficult when double concave.

When V, you first correct a strip 1-2 cm wide in the centreline.
Then fill the rest of the hollow in the bottom between the strip and the outline. Just use a straight edge to make sure it is straight (duhhh...) from centreline to outline. Sanding with sandpaper on a plank.

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ned321 said..
Lots of information about this too on the dutch forum.windsurfing.nl

First of all, this is REALLY important for performance. One of the first things (apart from structural integrety) to check on a board.
Easy to correct on flat bottoms, a bit more difficult to repair when it's a V bottom, difficult when double concave.

When V, you first correct a strip 1-2 cm wide in the centreline.
Then fill the rest of the hollow in the bottom between the strip and the outline. Just use a straight edge to make sure it is straight (duhhh...) from centreline to outline. Sanding with sandpaper on a plank.

Those Dutch guys know how to go fast so I guess that makes it unanimous. No negative if you want the best out of your board.

Mark that's shocking dude all 7 of them! I know u ain't 70kg but u sail west Kirby so can't really blame chop! Well not chop that I know!

Hmmm hope patrik don't use cobra? S@@@ where's my straight edge!!!!!

Ok so update checked my favourite slalom board patrik v4 115 and it negative so I'm pi@@ed that's 2017 board and yeah I ride it hard but it's a board supposedly designed to be ridden hard and it's not old so not impressed with that...that said it's fast and recently broke my pb for my local spot on it 10 year standing so I'm a bit bemused at that????
checked 2008 carbon art sp44 er well used...not a hint
both checked with an expensive box level, patrik probably about 1/2mm maybe 3/4mm doesn't sound a lot but the daylight that poured through I though Jesus that's 5mm...carbonart no daylight whatsoever?

It's that old trade off, soft bottom gives soft ride, but fatigues the core. stiff bottom gives harsh ride but is more durable.
And I think how much negative rocker affects speed, is down to trimming ability. If you can adjust other stuff, (like foot strap/mast track positions) to compensate for the extra lift behind you, there shouldn't be much effect. I don't think the slight concave in itself will slow you down, it's just it's effect of pushing the nose down, moving the entry forward and increasing wetted area, that slows you.

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decrepit said..

And I think how much negative rocker affects speed, is down to trimming ability. If you can adjust other stuff, (like foot strap/mast track positions) to compensate for the extra lift behind you, there shouldn't be much effect. I don't think the slight concave in itself will slow you down, it's just it's effect of pushing the nose down, moving the entry forward and increasing wetted area, that slows you.

Good point. I mean of all the trillions of subtle tail rockers out there? What physical reason is there that the one in a trillion dead flat is best? Possibly because that's the curve that's easiest to check and correct with a straight edge and we get used to trimming flat boards?

Having said that I bogged and flattened the hollow bottom of my Naish Hybrid years ago (should never have sold that board) and I reckon it gybed better.

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Ian K said..
I mean of all the trillions of subtle tail rockers out there? What physical reason is there that the one in a trillion dead flat is best? Possibly because that's the curve that's easiest to check and correct with a straight edge and we get used to trimming flat bottom

NACA tank test data on planning hulls.

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yoyo said..

Ian K said..
I mean of all the trillions of subtle tail rockers out there? What physical reason is there that the one in a trillion dead flat is best? Possibly because that's the curve that's easiest to check and correct with a straight edge and we get used to trimming flat bottom

NACA tank test data on planning hulls.

How did dead flat compare with 1mm of hollow?

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Ian K said.. >>> I bogged and flattened the hollow bottom of my Naish Hybrid years ago (should never have sold that board) and I reckon it gybed better.

I'll go along with that, it's why we put heaps of rocker in wave boards, well heaps compared to speed/slalom boards. If you want to carve on a wave you need rocker, negative rocker would want to turn you in the opposite direction.

So that made me curious and I checked my boards:

Patrik 48 speed : about 1mm - this board has had very little use
JP69 (litres) 2008: nothing - this is an old board and I have sailed about 2000 kms with it in high winds
Exocet rs2 and rs3 2013: about 0.75 mm (rs3 has done 4000 kms)
Exocet rs5 2018: nothing
rrd lightwind 2014: nothing

Just wondering if it is induced by fatigue would it be better to add a layer of carbon in the area ?

That's what all my boards have, 6ox carbon at 45deg tapering to just underneath the mast base

i meant instead of using bog , to fix it

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decrepit said..

Ian K said.. >>> I bogged and flattened the hollow bottom of my Naish Hybrid years ago (should never have sold that board) and I reckon it gybed better.

I'll go along with that, it's why we put heaps of rocker in wave boards, well heaps compared to speed/slalom boards. If you want to carve on a wave you need rocker, negative rocker would want to turn you in the opposite direction.

Funny u should say that decrepit went to west Kirby yesterday with one aim to do some tweaking on settings with the 115 and 7.0 koncept, now at my local spot with let's say 1/2mm of negative the board is fantastic and fast this is over decent chop 1-2 foot very gusty lake conditions and flys off the fin beautifully with a Lockwood sl2 38 top speed so far 33.5kt by far the nicest easiest fastest board I have sailed and the first thing I noticed when I got it was how high it rode and seemed to fly off the fin by default something I had to work hard on with other boards
.on real flat water at Kirby it sat real low, different board completely, started fiddling, took it to wall, board stuck couldn't send it and trashed the wall, oh dear, test was inconclusive due to no board to test but feel it could of been tweaked,
perhaps the differing bottom shapes on boards will have different effects with the negative?
So with the patrik that has inverted v around the footstraps the contact is the rails with the water and creates a lot of lift with the air going straight through the middle coupled with some negative rocker this counters the lift? Inadvertently creating a board that flys off the fin really well..but in mega flat water too sticky and different setting required???
Just sharing findings

You definitely need different settings for chop and flat. Sitting flat in chop is good, because you're not crashing into the back of the chop, just sitting on the top of it. But sitting flat in smooth just results in more wetted area.
The common adjustment is mast foot, forward for chop, back for smooth. But negative rocker may mean you can't get the mast base back far enough to unstick the board, you may have to move foot straps back as well.

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powersloshin said..
i meant instead of using bog , to fix it

Well I was sort of agreeing with you, I think it's a good idea. As long as you don't create a hard edge, that can mean the board flexes over a very small area and fatigues the board just in front of your new carbon, it needs to be tapered so the change in stiffness is gradual.

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eastcoast said..
Ok so update checked my favourite slalom board patrik v4 115 and it negative so I'm pi@@ed that's 2017 board and yeah I ride it hard but it's a board supposedly designed to be ridden hard and it's not old so not impressed with that...that said it's fast and recently broke my pb for my local spot on it 10 year standing so I'm a bit bemused at that????
checked 2008 carbon art sp44 er well used...not a hint
both checked with an expensive box level, patrik probably about 1/2mm maybe 3/4mm doesn't sound a lot but the daylight that poured through I though Jesus that's 5mm...carbonart no daylight whatsoever?

Interesting story on that. was out in my mates shed where he'd just finished repairing his patrik 115 (not sure what version it is) just for sh!ts and giggles i put a straight edge on the tail end. I was slightly shocked when i saw negative. He's not had the board long, and he's not slow on it either.

It might be a design thing given your story, might not be too. I also know someone else who managed to put negative in a board in the space of a season.

Subsonic was it as negative as this?