Board bottom rocker's effect creating negative pressure?

[SUPflorida]

Not sure if that's the best thread title for this but here goes....after listening to the FCS video and the effect of the pressure difference between the inside (flat to slight concave) surface and the outside (convex) surface of rail mounted fins...it occurred to me that the vast majority of SUP bottom rockers are in essence recreating this sinario as a board moves through the water.

 Common knowledge more rocker = more drag. On a wave, because of using the waves energy to plane, this is acceptable. On flat water this causes the convex rocker (nose to tail) to "suck" deeper into the water and increase drag as increasingly power is applied in an attempt to propell the board forward...and the shape accelerates the water over the bottom and creates low pressure.

I notice that on several race boards the latest trend is to have the first 6" to 12" slightly upsweep followed by almost dead flat to approximately 60%-70% aft of the nose...and then the remainder of the board bottom sweeps up 4-5" (in relation to the flat nose section) to facilitate clean tail release.

Has anyone here experimented with the transitioning between the flat forward section and the aft section?  I'm referring to how fast the transition is made... Think of one extreme being a constant curve with no discernible break and the second extreme of having two flat sections ( say forward section at 0 degree angle and aft section 5 degrees) with a crisp edge at the transition.

Which is faster? Maintains laminar flow and having this low pressure effect sucking you in a hole? Or having turbulent flow introduced at a more severe transition, but not suffer so much from the low pressure effect? PonoBill have any thought on this?
Seems like something very elementary that may have gone unnoticed or at least not widely talked about.

If a convex longitudinal rocker line is a major source of drag by sucking the board further down as power is applied, the question becomes how do you achieve clean water release at the tail without creating drag from nose & tail rocker?

For flat water...are we left with a perfectly straight longitudinal bottom rocker (or at least minimal curve) with the only way to deal with clean water release at the tail by way of pin tail?

Your thoughts?

[yugi]

Here’s the thing: water isn’t flowing over the top of your board so, no, your board is not acting like a foil. It’s not a Bernoulli “low pressure effect sucking you in a hole” that you are thinking about.

The kick in the tail is actually assisting a cleaner release off the transom since your board is going to be pressed into the water by gravity and overall weight of board and rider. A fully straight board would drag the tail (unless zero weight) causing turbulence behind the board and slow one down. More pronounced  kicks in tail are of course there for when the board is turning.

You’re going to need a very heavy transition and higher speeds than we just paddle at to get turbulent flow in the aft section. I can’t see how turbulent flow there is ever going to be faster.

Flatter aft sections help in earlier planing.

Lot’s of other stuff going on. The formulas blow my mind. Tunnel testing is more fun.

[PonoBill]

Actual testing is a lot more revealing than hydrodnamic formulae. It's not that formulae don't work, they do, but they're mostly marginally explicable, at least that's how it seems to me. For example, it's well understood that turbulence leaving a surface causes drag. But what's the mechanism? How does a working fluid that is no longer attached to a surface impart any force to the surface? It's an odd thing that both aerodynamics and hydrodynamics are macro-sciences, by which I mean they generally work with things that don't exhibit the strange nature of particles in the Planck scale, but still some of the phenomenon and formulas describing them are more heuristic than fundamental. Even something as fundamental as the theory of lift from a wing is not completely explicable. At least not in completely convincing terms.

[SUPflorida]

Here’s the thing: water isn’t flowing over the top of your board so, no, your board is not acting like a foil. It’s not a Bernoulli “low pressure effect sucking you in a hole” that you are thinking about.

The kick in the tail is actually assisting a cleaner release off the transom since your board is going to be pressed into the water by gravity and overall weight of board and rider. A fully straight board would drag the tail (unless zero weight) causing turbulence behind the board and slow one down. More pronounced  kicks in tail are of course there for when the board is turning.


You’re going to need a very heavy transition and higher speeds than we just paddle at to get turbulent flow in the aft section. I can’t see how turbulent flow there is ever going to be faster.

Flatter aft sections help in earlier planing.

Lot’s of other stuff going on. The formulas blow my mind. Tunnel testing is more fun.

All good points...any other thoughts on the "break" , where the forward/mid planing surfaces transitions to the tail section or kick. What effect the abruptness of the transition have on performance?

I see some see some SUP race boards (and successful catamaran racinging hulls) where the bottom is two very gentle curves with a fairly abrupt change (between the forward section and aft section) just aft of center 55%-60%...other designs where the transition is closer to 70% aft of center. Epic surf ski's it look like they are some where between 70%-75%.

Using an SUP unlimited board as a reference,which would not be executing bouy turns, how far (%) aft of the nose can the main straight/minimal curve section be to be optimized an sup from strictly a "form drag" perspective? What has proven itself in the real world?

[yugi]

I actually have a board, Rogue Rage (14’ DW board), with a very abrupt transition from a very flat mid section planing surface to a pronounced kick. So I can comment on that and also compare to boards with gentle rocker there (JL M14, Naish Nalu 11’4) and boards with a very straight aft section (SIC Bullet).



On flats the Rogue has a very good glide. It leaves very little wake behind it. For me an excellent indicator of clean tail release of water. Of course what I really feel is how it makes the board turn. I find that with that abrupt transition I feel a corresponding abrupt character difference when I step back. Stand back onto the tail and it becomes a very quick turner. Standing up in the big, very fast mis-section it takes off on planes very well. Exactly what one would expect looking at it. I like it! Dr Jekyll and Mr Hyde.

Round continuous rockered boards like JL M14 or Naish 11’4 Nalu are obviously nice turners off the tail and are correspondingly more progressive in character as one moves back. Both, I find, have a very nice and clean release of water from the tail (v little wake) and are really very good in flat water. I’ve done a lot of flat water with both and remain, over the years and comparing to other similar boards, impressed with their flat water performance.

Basically, in non-planing applications, a watercraft that disturbs the water the least is going to be pretty slick.

Flatter aft sections way to the tail definitely help you get up on a plane earlier. EG SIC Bullet. Also on the Bullet one can stand about a foot aft of the handle and just stay there. On Boards with tail rockers one definitely needs to return to the front to get the board to plane. Just as one would expect from windsurfer hulls and getting to know the ones that work.

What has proven itself in the real world?
Well, as you mention it’s always good to look around what works.

I would like to see some tunnel testing done, which is really what is needed to answer your questions. Kind of surprised it isn’t being done. Make up some different shapes and test then against each other.

Not hard to do. You just need a good tunnel test facility on hand. I actually did that once, for keel shapes. The test, and the results were used to for adjusting the Offshore Racing Council (ORC) and the IRC  handicap rules. They needed an overhaul following the 1979 Fastnet race disaster with more stability in mind. Part of a class I did in University (on the side of my major - just out of interest as I was a competitive sailor).

Pono, you need any more projects? I’m happy to come help, especially if it’s in the Gorge or on Maui. Or could facilitate some contacts to spark a project at the tunnel testing facilities in Boston.

[PonoBill]

I used to pass the MIT testing tank frequently on my way to the library. I always thought it would be great fun to get to play around in there. At the time I was building a robotic telemetry submarine and I had a lot of questions about waterproofing, but couldn't find any answers. Never did, the sub was never completed and would have gone straight to the bottom if it hit water. I always wonder what happened to it. Probably Stoneaxe knows. He probably made a sled out of it.

I think the current span of my projects puts me safely past 120 years old, but that's never stopped me from adding more.

Most of the data to design low speed hulls is already available though. You just need to find books old enough--and understand the math. If I were going to try to find such data the MIT library would be the obvious choice. I wonder if they'd honor my aincient pass, it didn't have an expiration date, but it's probably paper mache' in the pocket of some jeans in a landfill.

I do have a test tank here--two actually. One is a conveniently straight stretch of the Hood River with bank access. Windsurfing mast and a load cell from a $29.95 scale: Presto.  The other is a set of three cascading tanks made from big storage containers with a glass window installed in the middle one. Keep the flow speed low enough and you can see a few things. I made a manifold with pinholes for ink mixed with jello to see the flow over surfaces. Unfortunately it needs a flow straightener section. Too much mixing going on. I guess that's why water tunnels are so big. Mine is satisfyingly messy though. I used it for comparing slip flow on KeNalu paddles vs other manufacturers. Didn't learn much.

[willi]

This is a very critical subject cause if your gonna build your own boards to race -- or have fun goin fast --it is one of the most important areas -- besides shape > outline
So how about the concave used today -- I am finishing a 12.6 board with some concave and a recessed cockpit ---some of the races we do in south Florida get a little nasty (Key west)
so looking for some extra speed and more stability + (love my indo cushion) if ya don't have one its a great tool ;]
thoughts are eagerly awaited.
aloha
willi

[PonoBill]

I just reread my posts on this thread. Obviously written during some kind of experiment with scotch, or perhaps after a head injury. Nothing to see here, move along.

[yugi]

Check the finish of the long distance race at the isaworlds. Titouan's board is upside down for a while at the finish. Dispelling any myth that a race board needs to be less rockered.

In fact DW boards may have less tail rocker to help them jump up and plane quicker.

[SUPflorida]

That was good for a laugh ponobill...took me right back to the scene in Star Wars ..

[SUPflorida]

Yugi...it could argue that wasn't the flattest of water...heats looked like a chop fest with the photo boat and jet ski chugging around making already choppy conditions even more confused. Watching it there were times where the boat's presences had a profound effect on paddlers in the wrong place at the wrong time...usually those just behind the leaders got the worst of it.

Near the end of the women's finals it looked like Candice got caught in slop from the bozo-boat that made her board veer off her intended line (while setting up for the last buoy turn). Would it have changed her position? Probably not from that one incounter...but it sure didn't help...she propably lost two to three board lengths because of it. The two leaders we're still to far ahead at that point for her to catch them.

[yugi]

Yugi...it could argue that wasn't the flattest of water...heats looked like a chop fest with the photo boat and jet ski chugging around making already choppy conditions even more confused. Watching it there were times where the boat's presences had a profound effect on paddlers in the wrong place at the wrong time...usually those just behind the leaders got the worst of it.

Near the end of the women's finals it looked like Candice got caught in slop from the bozo-boat that made her board veer off her intended line (while setting up for the last buoy turn). Would it have changed her position? Probably not from that one incounter...but it sure didn't help...she propably lost two to three board lengths because of it. The two leaders we're still to far ahead at that point for her to catch them.

florida, I was talking about the distance race (yesterday). A bit over 10 Mi of very flat water. Booth won, Titou 2nd. Kai Lenny [7th]'s take (see SupRacer's vid on facetube) was "that's the flattest race I've done in a long time". It was flat.

You are mentioning the tech race. Yes, chop fest. On purpose just BTW. Interesting you mention Candice's board getting veered. She was one of the few with a dispacement'ish nose board. Olivia also fell but earlier on (Allstar). The guy who took the most dunks was Georges [cannverspellhislastname] , two at least flying.

Women's long distance in about 6 hours

[Area 10]

Yugi...it could argue that wasn't the flattest of water...heats looked like a chop fest with the photo boat and jet ski chugging around making already choppy conditions even more confused. Watching it there were times where the boat's presences had a profound effect on paddlers in the wrong place at the wrong time...usually those just behind the leaders got the worst of it.

Near the end of the women's finals it looked like Candice got caught in slop from the bozo-boat that made her board veer off her intended line (while setting up for the last buoy turn). Would it have changed her position? Probably not from that one incounter...but it sure didn't help...she propably lost two to three board lengths because of it. The two leaders we're still to far ahead at that point for her to catch them.

florida, I was talking about the distance race (yesterday). A bit over 10 Mi of very flat water. Booth won, Titou 2nd. Kai Lenny [7th]'s take (see SupRacer's vid on facetube) was "that's the flattest race I've done in a long time". It was flat.

You are mentioning the tech race. Yes, chop fest. On purpose just BTW. Interesting you mention Candice's board getting veered. She was one of the few with a dispacement'ish nose board. Olivia also fell but earlier on (Allstar). The guy who took the most dunks was Georges [cannverspellhislastname] , two at least flying.

Women's long distance in about 6 hours

It's a bit off-topic I know, but can I just point out that so far the US seems to be getting its ass handed to it by the rest of the world this year at the ISA World Champs? Do you think that the racing scenes outside of the US might be growing relatively faster than within the US? Or have the US team just been unlucky with their performances and who could attend? But difficult to see e.g. anyone matching Boothy right now in a flat water ocean slog (even Danny Ching, who competed last year) and even Candice looked unusually vulnerable throughout the tech race.

[PonoBill]

They're probably all a bit burned out at the end of a long season. I don't know how these people do it. Some of the US racers are specialists, but most of the people I know do just about everything you can do in the water except America's Cup racing, and Kai races against the AC on occasion.

[yugi]

At the end of Bill Clinton's presidency a reporter asked John Cleese (who just moved to LA) the main differences he noticed having moved from england.

He answered :
- In England we speak english
- we only need to get down on one knee for our sovereign and
- when we host a world championship we invite other nations

Last year was the first year any of the US racers actually even showed up to the ISA worlds. Already seems like they forgot about it. That's all. Kind of like Hockey World championships. US players don't go, they have the Stanley Cup on.