Pumping a Foil Board Explained . . . with Skateboard

[Beasho]

Working with Numbers:

Take Kai Lenny ~ 150 lbs.  If he introduces an ARC with a Radius of Curvature of 10 ft and is traveling at 15 mph or 22 ft per second he would establish an Angular Momentum = 150 lbs * 10 ft * 22 ft/sec = 33,000 lbs * ft^2 / sec.

Reducing the Radius of Curvature by 6" or raising 100% his average center of mass by standing up or PUMPING, assuming no friction would increase his tangential speed by 5.3% from 22 ft/sec to 23.16 ft/sec.  This is the result from just 1 Pump.  In a friction-less world Kai could continue to accelerate 5.3% with every pump.

6" might not seem like a lot but going from a seated to standing position raises your body ~ 1 foot.  However your feet don't move at all, and your knees very little.  So I am using something less than a foot for an "Average" pump.  If you watch the GIF below it looks like he is also kicking the tail, making a smaller arc thereby reducing the radius while raising his center of gravity - a double whammy. 

I built a sensitivity test based upon the Initial Radius of Curvature and the Average decrease in curvature from Standing up or pumping.  An initial arc of 15 ft would only produce and increased speed of 3.4% per pump.  Conversely a shorter starting radius of 7 feet is much more influenced resulting in a 7.7% increase per 6" pump.

I am pretty sure this is college level physics, not too many 9th graders studying this stuff.   BUT any kindergartner knows that kicking your legs UP on a short swing, to bring their center of mass closer to the center point and reducing the radius, gets up and going faster than on a long swing. 

These numbers seem to be in the ballpark.  Would be nice to get some real data with GPS to show the direct result from each pump.

Shorten the Radius and Pump.

PS: I am pathetic because didn't surf or fish this morning but instead ran these numbers.  Hopefully someone else will benefit from these insights and make some cool videos or invent some Clayesque contraption like a bungee foil skateboard attached to the ass end of the space shuttle type of thing.

[Beasho]

The benefit of the Foil is that it takes the Huntington Hop and makes it work in more than CRAPTASTIC surf.

I ask: What is the radius of curvature that Austin Kalama is starting with???

For those of you who have foiled - What you will notice is that when you push down the board doesn't stick it accelerates like an airplane.  The key is to use that increased speed, establish your arc and then kick while simultaneously pushing up.   

[Beasho]

The skateboard ramp description translates exactly to riding a bike on a pump track. I think it also explains part of how pumping a shortboard on a wave works too. . . . .  I wonder when we'll start seeing people try to pump side to side rather than vertically...

I didn't know what a pump track was, but probably experienced them riding BMX's as a kid. 

Unlike the Foil example you can actually see the curvature and measure it visually ~ 8 feet.  This would increase the effects of a 6" pump.  The lower rolling resistance of a bicycle helps significantly.

The pump track video and Austin Kalama prove that vertical motion can produce forward thrust and perpendicular motion given the right geometries and gravity.  The pump track video alone might serve as motivation for how effective pumping can be when friction is reduced to almost nothing.

[Beasho]

[container]

thanks for those numbers beasho, its nice having some actual information to look at for once

not the best example cause the foils shit and the back foil is providing lift not downforce so pumping is super hard but you can see the amount of water being moved behind the foil during each pump. I have held on to the foil off the back of a small inflatable dinghy while mimicking the pumping motion and its surprising how much drive you get, almost faster than rowing!

[jondrums]

Beasho, while I do think there is an effect of angular momentum, from my viewing up videos of foiling pumping I don't think that's the primary effect at play.  It really looks to me more like the foil is moving through the equivalent motion of a dolphin's tail to propell forward.  So the pumping is more about introducing negative angle of attack, then applying downward force to "squirt forward", then using positive angle of attack and momentum to "reset" the foil back up high ready to do another squirt.  I'm sure angular momentum can be utilized as well, but for my money, I think its more about flapping the tail.

[surfcowboy]

Getting some popcorn...

[PonoBill]

I REALLY don't want to get involved in this discussion. It's the kind of notion that leads people to build a contraption that slides weights along the spokes of a wheel to "create" perpetual motion, because angular momentum is conserved, but velocity doesn't have to be. That's easy to confuse with magically generating force. Other than that, the last thing I'm going to say on this subject is simply to repeat that if you want to know what's going on in a system you need to determine all the forces and their vectors. No one is violating the laws of thermodynamics by pumping, whether it's a bike on a pump track, or a skateboard, or a surfboard. And trying to adapt what happens to a skateboard rolling with low friction on a static surface to what happens to a surfboard on a wave surface moving 20 mph and shoving tons of water around takes some big, unsupported leaps. But good luck with it.

I think you should have gone surfing, Beasho.

[southwesterly]

This summed it up nicely... at least for me.

[PonoBill]

Actually, what you want to look at for understanding a problem of this kind is Lagrangian Mechanics. Ahhh, damn, I just can't STFU.

[Beasho]

OK, what is this guy doing 

https://youtu.be/TkjNSQ3DKqY?t=102

[Beasho]

Here is some more sage advice:

The Huntington Hop seems tiring. Is there some kind of training you can do to prepare for it?
Get on a trampoline. You just gotta grind it out. Actually, the dance floor is great training for the Huntington Hop. Get all low. Twerking helps, too.

How many more points do U.S. Open contestants get when they successfully execute the Hop?
If you hop successfully into a nice section on the inside you could get three or four more points [in a 10-point scale]. The judges really like when you finish strong on the inside.

Is there any way to make the Huntington Hop a little more pleasing to the eye?
I don't think so. It's pretty grind-y. It is what is, you know? You can't try to hide it; it's more function and less fashion. Some guys dream weave [weaving through rather than hopping] it though. I don't know if you get more points for that—but it definitely looks better.

From here:
http://www.grindtv.com/surf/mechanics-behind-huntington-hop/

[Beasho]

This is just TOO GOOD! 

I made this clip

• Because it is hilarious
• Because PonoBill is going to love it

[surfcowboy]

HAHa, so aggressive.

[Bean]

Too funny!