Tomos new bottom

+1 on the channel bottom. Have a Byrning Spear in my quiver and whoa buddy. In the steepness the hold is incredible. Mine has a double wing swallow. At 18 3/4 and 2 and 1/4 It is really for a younger fitter PJ but for short surfs it's a blast.

As for the Tomo bottom, the fins at the edge of the concave isn't new. the Campbell brothers have been doing it for years with bonzers. The ventral fins are at the edge to extend the concave increasing the venturi effect. The Vid i saw showed how with the Tomo bottom the board planes like a pair of water ski's. Very interesting. I remember Stu Kennedy at snapper ripping on that thing.

Cool stuff Bass

 

Good find, BassMon... Just a couple quick thoughts to keep in mind as you consider the discussion. There's more, but for starters...

Looking at how water interacts with any form when it's moving in one direction... straight across the bottom of the form, from end to end... tells you a lot. But it doesn't tell you how water interacts when water flows at different angles across the form. I'm not saying "simple planing" (the term the scientist uses in the video) doesn't give you valid information. I'm saying it's only a fraction of what needs to be studied.

I'm glad they pointed out the changes in drag at different velocities. That information is REALLY important, practical information for surfers... not simply in terms of whether the waves you ride are fast or slow (translation: big or small, hollow or mushy, groundswell or windswell...) but how your board will react when riding on different parts of a wave (translation: high line vs. flats, coming into a turn vs. coming out of a turn, etc...).

Fins are not present in the analysis. How fins interact with bottom contours changes water flow significantly and dramatically (think about how side runners interact with channels on a Bonzer).

 

Whoa... we're on the same wave!

Although I'll say the Venturi effect is debatable. Many argue it does not apply because the bottom of the board is an open system, saying the Venturi effect applies to closed systems.

 

friends share.

I agree it's debatable. Let me posit this. if the reason to have concaves in the first place is to get some air under there to mix with the water then wouldn't the surface of the water put a lid on the system?

 

Bernoulli's principle and the venturi effect are very similar. You have the venturi effect working in jet engines where you're able to compress air into a smaller area. Beroulli's Principle is the same idea but allows the fluid to move over a surface rather than through a closed system.

"The idea is that as the parcel moves along, following a streamline, as it moves into an area of higher pressure there will be higher pressure ahead (higher than the pressure behind) and this will exert a force on the parcel, slowing it down. Conversely if the parcel is moving into a region of lower pressure, there will be an higher pressure behind it (higher than the pressure ahead), speeding it up. As always, any unbalanced force will cause a change in momentum (and velocity), as required by Newton’s laws of motion." See How It Flies John S. Denker

The concaves or channels create an area of unbalanced force as the water tries to fill the void where the channels are.

I've experienced this with that bonzer i've been playing with. It flies down the line. It scooted out from under me a couple times the first time I surfed it. Very directional and I need to play with the fin placement to crank out some better turns. These boards look like a more user friendly version of a bonzer.

 

I believe the ventilation effect and Venturi and Bernoulli Effects are secondary. The primary effect is to capture and direct water flow through the fins.

In a single concave, the water flowing up the face of the wave is captured by the inside rail and directed back toward the tail and fins, creating lift. This is why concaves are generally shaped in to reach maximum depth at the leading edge of the front fins. If it's a single-to-double concave array, the double concaves start to fade in half way between the leading edge of the single concave and the leading edge of the fins, and also reach max depth at the leading edge of the front fins. The whole idea of concaves is to allow less water to spill off the rail, to augment the affect of the fins (which create drive/speed, as well as directional stability and hold), and to force water out the tail behind the fins, where water should release as cleanly as possible.

At least that's my theory...

 

Doesn't having four or so channels help break up the suction that a single concave can create?

 

That's interesting. could be. I have both and the single concave thruster sticks on the face more for sure. I thought it was the wings on the channel bottom that helped break the suction.

 

Any kind of hard edge or chine will break up the surface tension of a smooth surface. Just a major B to glass.

 

My buddy who shapes, he will only make them for himself. I asked him to help me make one, he just laughed.

 

Do you run different fins on the two board? I find smaller fins work better on channel bottoms, but not so for concave bottoms.

 

Hard edges are release features. You can see that in the video above. Shapers have known this for a long time... soft, round rails without edges create more suction and hold as water wraps around the rail. That's why the best noseriders have soft rails the whole way. Hard edges and pinched rails release water, plane higher, and create less hold. So the compromise is to fade the hard edge in, and slowly reduce the tuck, as you get closer to the pod area of the tail. So the edge is soft and tucked ahead of the fins, then gets harder and closer to the outside of the rail as you go back, until it's basically square at the pod, with no tuck.

Bottom features like channels break up laminar flow, but if you go too far, you create turbulent drag. Now you're talking about Reynolds number and all that gobbly-gook.

 

yeah. The channel is FCS. it's the only one I have with FCS and I have one set of fins for it. medium honeycombs. the board rides great so I haven't needed to experiment. The single is futures. I ride blackstix and A6's on the single concave depending on what the waves are doing. the suction on the single just seems more noticable. Besides being HP thrusters they are not all that similar. tails shapes and rockers and such. pretty much two different ways to do the same thing.

 

I’m enjoying this thread, it’s getting long.

 

Yes... Bonzers have the added side runners, either a set of small fins on both sides, or on long runner on either side, that does something similar. And those fins are aligned with the channels, and set at crazy cant angles, too. Only with Bonzers, the side runners actually capture energy of water releasing from under the board... at least that's the Campbell Brothers' theory. When you put the board on a rail, the the water that would be spilling off the rail is deflected down and back by the runners, again, adding thrust and lift by harnessing what would be wasted energy. If you've ever noticed the really drivey way Bonzers ride... how they hold speed and power through turns... you could attribute that feeling to the "extended concaves" created by the runners, on top of the added lift and drive created by the channels and flutes.

 

It took me glassing a lot of boards to realize you can leave the blank with a pretty soft bottom rail edge (easier to laminate and get the cloth to wrap cleanly and not tend to want to lift) and then create the hard edges where you want them in the hot coat step. Its a lot easier to get resin to form a hard edge through taping and sanding, than to get cloth to wrap around a hard edge.

As far as chines go, i see their purpose as being more to keep the mid-forward rails from overly interacting with the water while going down the line on a relatively straight direction. I feel like they are hard to shape, easy to glass, hard to sand out.