If you’re up for some fun… lets try a crude analysis.
Lets start with an ideal model, nothing complex here, basic stuff. It’s all contained in figure 1 – the Ideal Case.
[img_assist|nid=1042615|title=.|desc=|link=none|align=center|width=585|height=821]
By the way, hopefully you have some appreciation for what a torque is – it’s that which tends to rotate a body about some axis. Here, I’ve chosen that axis to be at the nose, but any other point would have just fine.
Some immediate questions come to mind: Do these forces from planing exist, have they sufficient magnitude, and does this treatment really make sense?
Do these forces of planing exist?
Below is a diagram taken from the work of man called Savistsky. The pressure profile drawn above the plank was Savistsky best guess, so some caution should be exercised in its interpretation.
[img_assist|nid=1042453|title=.|desc=|link=none|align=center|width=562|height=549]
Nevertheless, placement of the peak pressure above the what is called the stagnation line is likely accurate, as is the development of the spray root -i.e. all that water that is literally turned around and tumbles around in front of the plank. Notice Savistsky has the pressure profile tailing off as you move towards the tail. Does this mean that the forces of planing exist because Savistsky says they do? No, its because Savistsky did the research and backed it all up with (reproducible) data.
Do such forces have sufficient magnitude?
Savistsky provides us with a formula, lets use it just to see what kind of numbers we are talking about for the kinds of flow velocities that one might expect on a wave face. Lets take start with a guess of 1 m/s (that’s one meter per sec, or about 2 mi/h). The density of sea water, that’s the rho in the formula, is about 1020 kg per meters cubed. So the pressure as suggest by Savistsky’s formula, using these values would be about 510 N per meters squared.
To get an actual force we’ve got to multiple this pressure by area. Lets assume that the plank is 17 in wide or 43 cm wide. To get an area we’ll need a length of this, call it s, so the total area is s times 43 cm. Better convert to meters, so this becomes s times 0.43 meters squared, where s is now given in meters.
So now we have, for a length of plank equal to s, a force of roughly s(.43) 510 N (that’s newtons), or doing the multiplications, s219 N. You’re 160 lb, which is 712 N (-i.e. your mass is 73 kg). Which would mean that we would need a length of 3.25 meters to support your weight. Wow? Thats a lot of 17 inch wide board?
Lets tweak the numbers a bit, in fact let’s just tweak the flow velocity. It’s squared so we’ll stand to get the biggest effect with the smallest change.
Lets raise it from 1 m/s to 1.5 m/s that’s about 3 mi/h, so 510 N per meters squared becomes 1450 N per meter squared. Our final s-formula becomes s1450 N, where s is in meters. So again, to support your 712 N you would need 0.5 m of plank (a section around 17 inches wide and 1.5 foot long.) It would seem that velocity of the flow is likely to be critical. This is not a precise exercise, even so these numbers are not all that unrealistic either. (You can noseride in mushy slow conditions, but you’re likely to involve a whole lot more of your board to do it. Remember velocity is squared, so you get a whole lot more bang for the buck if you can put you and your nose in a more critical position on the wave.)
Lets say we’ve got your weight covered. Now we’ve got that other weight to worry about, the weight of the board. Lets assume the board’s weight (with fin) is about 20 lbs or 90 N. Using our s-formula and solving for s we get 0.06 meters, that is we’ll need an extra 6 cm or 2 inches of 17 inch wide board to handle that.
Thinks about this, crude as it is, if it even approaches what’s going on, we got your weight and the boards weight covered in .7 or meters of plank? Wow, you’ve just got to wonder what trouble the rest of the board is getting itself into.
Does this treatment make sense?
Well, okay it really is crude, but even so, its not ‘wild and crazy’ crude. The actual analysis would involve a lot more. Admittedly, I have taken some liberties here, but given the circumstances, I don’t think I’ve been all that unreasonable. The real point was to see if the forces and torques involved require introducing ‘extraordinary’ effects etc. like ‘suction’ etc. to balance your weight on the nose.
There is a lot going on under a surfboard, but until you can rule out the straightforward kind of reactions, like planing, evoking other effects seems, at least to me unnecessary – unless your trying to sell some gizmo.
…it this really a reasonable treatment?
Anyway, I’m sure people will find problems with this simplistic treatment. e.g. “Yeah, well what if the guys is like right on the nose” Fine, but then how fast is the flow, and exactly how is the bottom surface interacting with the flow, and for that matter, how long was he up there on the nose. Fact is if you move to an accelerated situation things change rapidly, and its amazing what you can get away with for a moment or two (especially if you’re in a sort of free fall), before it all collapses (or you do a nose dive.) The above treatment assumed something approaching what you can call a constant velocity case, or static analysis.
…torques?
Also, you may be wondering why the center of gravity of the board isn’t at the center of the board, or perhaps you understand why it’s not, which would be my hope. Fact is, if you add a fin and all that goes with it, a box, more matrix, etc. you push the center of gravity towards the tail. In fact, perhaps you’ve use a performer fin (a.k.a. hatchet fin). Sure hatchet fins, and its brethren (ask Probox-Larry for history) provide some real stability, but they also tend to weigh a lot. If there ever was a way to shift the center of mass of your board towards the tail, it was with one of those fat boys. But if you’ve been following, you might be wondering why shifting the center of gravity closer to the tail matters – heck it really didn’t seem to matter in this ideal analysis.
It matters because, in this model I’ve taken the liberty of modeling the forces developed during planing as localized directly under the position of the surfer and the center of mass of the surfboard, which is not likely to be the case. They may be close by, but they in general move they around. And when they aren’t directly under the surfer or center of gravity of the board, you’ve got to take into account where they are to resolve the torques. And when you do, pushing the center of gravity of the board towards the tail can really make a big difference.
… more?
So the next more actuate model might be to move them a bit, hither and dither. Then once we do that, we can get into how the angle of the board impacts the torques, and once we do that,… well how about before we do anything somebody goes out and gets some funding for all this research… perhaps we can get some from all those people making wads of cash selling those noserider and other enhanced performance gizmos… this is terrible…my envy shows doesn’t it… deep down I’m probably as much of a con-man as anyone… they were just first to market.
kc
