Dynamics - Surfing the Force

Surfing the Force

When you’re surfing, ever notice there’s this kind of area, usually around your forward foot that seems to be pushing back? Well, it is, and in the theory of simple planing surfaces, it roughly correlates with the peak of the pressure distribution that develops from the flow impacting the underside of your board.

Ever notice that it moves? More to the point, ever notice what you instinctually do when it does? It actually does move around, and when it does, my bet is you do what most do, try and find it again, or recapture it, without given it much thought. Usually a shift in body stance (shift in weight without a change in position), possibly a bit of shuffling (shift in weight through a change in position), or a small adjustment in the angle of the board (shift in bottom presentation) does the trick, though occasionally recapturing the ‘force’ requires a lot more.

It moves around because its position is a function of weight distribution, bottom presentation and the characteristics of the impacting flow on the bottom of the board. In general, the greater the flow, the farther back in moves, but not always. It also need not occur along the centerline. The reason it does appear where is usually appears, has a lot to do with the surfer making the right adjustments to place and keep it there, hence the title of this post “Surfing the Force”.

In longboards the area is generally more diffuse, but its still palpable. And this is what still makes longboards neat, if it moves you’re not restricted to changing your stance or shuffling your feet a little, you can start walking. (Sort of sneak up on it with wee little steps.)

I guess the point I wish to make is that it’s there, and you surf it, but that most aren’t aware that that’s what they’re actually surfing. But don’t take my word for it, close your eyes the next time your catch a wave, you’ll find you can still surf just by tracking the ‘force’. (You may want to try this on less than crowded days.)

Once again I may have exaggerated to make the point, obviously vision is a big plus when surfing.



It was hard to avoid the Star Wars puns.


I’m still trying to find the variable flex blah blah’s of my boards. Now I have to find the “force?” Mike


…SORRY! just had ta…


sweet spot.

fall line.


it’s all semantics, but it’s all good.

at the end of the day it’s just surfing.

When you’re surfing, ever notice there’s this kind of area, usually around your forward foot that seems to be pushing back? Well, it is, and in the theory of simple planing surfaces, it roughly correlates with the peak of the pressure distribution that develops from the flow impacting the underside of your board.

I love it!

Hey Kevin,

have you considered how well designed bottom contour affects this pressure distribution?

Can we optimize this distribution with proper bottom design?



Some Preliminaries:

Savitsky_Orig.jpg has Savitsky’s origin planing diagram and, hopefully it’s self-explanatory. I have never been able to get good numbers to pop into Savitsky’s treatment. However, this isn’t something I consider critical, for I’ve never considered Savitsky’s original treatment as more than a departure point. Nevertheless, at this point no numbers.

Savitsky_45.jpg is Savitsky’s original diagram rotated 45 degrees. The exact rotation was arbitrary, the point was to bring the water line in Savitsky’s original diagram more in line with the water line on the face of a wave.

Savitsky_Extended.jpg here, no longer using Savitsky’s original diagram, I extend Savitsky’s ideas. I’ve drawn the plank so that you can see the deck, and placed a number of flow lines (which terminate with Savitsky’s root spray) in the diagram (which are actually impacting the bottom and shouldn’t be visible.) The plank is at an angle to the face of the wave. (See below.)

I’ve also drawn in the Resultant Force from the impact of the flow, its a guess of course, but I think is probably a reason one for illustration purposes. I’ve also resolved the Resultant Force in three perpendicular directions, Up the face, Down the line, and Towards the beach (for lack of better terms.)

Even though this is still a plank (possible something I shaped, but chose to forget) you can begin to see where propulsion is coming from. Balance the Up the face component with some Weight (surfer and surfboard) and you’re left with a Down the line component and a Toward the beach component. As for the Toward the beach component, you might not think its there, but trust me, if somebody somehow stop the wave you’d fly forward toward the beach (as a result of your beachward velocity.)

Also, I’ve marked in the Savitsky’s maximum pressure line (green) and once again its my best guess. The Resultant Force, in general, will not originate from the maximum pressure line. Its a function of the total wetted surface area. It will however be biased towards the maximum pressure line.

I’ve also marked in the Savitsky’s Root Spray line. This is actually important, it the origin of all that spray from under the board that visible from the beach. It can tell you a lot about what’s happening under the board. Also you can use this line to get a better sense of the angle of the plank with respect to the wave face.

I haven’t marked in the wake line, but it follows the waveside edge (rail) that is wet. This is also important, as most hear ‘wake’ and think ‘that’s gotta be something that comes after the tail’, but not so it would seem, at least if you’re buying into all this. Hence my opening comment in the Bottom Contours Thread regarding rail-to-rail flow.


“Hydrodynamic Design of Planing Hulls,”

Daniel Savitsky, Marine Technology, October 1964 Issue

Kevin if you ever get around to giving those forces some value then the ‘balancing force’ of gravity must be given a value which is proportional to the mass of board and rider. . . this makes a big difference to the result.

Also remember that the amount of work done by the wave when lifting the board is proportional to the mass of board and rider. . .

Assuming that the wave drives the surfboard directly can lead to an error, namely that the force which moves any two surfboards and riders of different mass on the same wave is constant in magnitude… . .in fact it is not, the force which moves board and rider is directly proportional to the mass of board and rider. .



First things first, that is one cool and interesting product (your tunnel fin.)

Yes and no regarding your post.

A surfer can adjust power generation by controlling bottom presentation. For example, he might change the angle of the bottom with respect to the flow. In Stavitsky’s original diagram he has a tau angle, which (more or less) is the angle between the oncoming flow and the bottom of the plank. If you increase that angle while maintaining a constant flow, you will increase the pressure profile and peak.

In general, the impacting flow amounts to a pressure, and pressure times surface area gives force - control bottom presentation, control the force.

I admit its a subtle balancing act, but surfers learn how to do it very quickly without giving it much thought (if any.)

Thanks mate (re. tunnel)

I understand what you are saying about controlling bottom presentation and thus controlling pressure on the bottom, and that pressure equals force.

My point is that the maximum amount of force which can be achieved depends upon the mass of the board and rider. . . in an extreme case where the board and rider have no mass, then no bottom pressure is possible. No matter how much bottom area is presented to the wave, the maximum force which can be exerted on the bottom by the wave is limited by the mass of board and rider. . . . . which is why the amount of mass ( and thus the amount of gravitational potential energy) of board and rider must be taken into account. . .




Here’s an attempt at some analysis (see attachment.) Its a picture of Whitaker at the Pipe trimming. Perhaps someone has a different take on what’s happening, share it if you care to.



Here is an attempt to apply Savitsky to water skis, in case you were not already aware of it. Since water skis and surfboards would appear to have more in common with one another than either has with boats, it may lend some information useful to this discussion.

“The Waterskiers Paradox” http://tinyurl.com/npzwb


Some Notes On the Formal Application of Stavitsky’s Treatment

In Stavitsky’s original diagram he labels the relative velocity of a water particle moving towards the plank with V. The V and his tau angle are related in that they both assume the same horizontal –i.e. sea level, or the resting level of the body of the fluid… This, or at least understanding how the flow in a wave differs from his V is, I believe critical in understanding why his equations don’t seem to give realistic numbers (with any reasonable guess of V.)

The attached diagram is meant to be a cross-section through a wave headed for the beach, the trough is to the right. On it are approximate (-i.e. as in a rough guess to make a point) velocity vectors of two water particles. Both are resolved into their vertical and beachward components (the beachward component appearing horizontal.) Technically then the flow of water on the face of a wave is actually upward and toward the beach. (Try and explain that to someone on the beach, or anywhere.)

If you then go back to Stavitsky’s diagram, all of a sudden his tau (which is with respect to sea level) takes on a whole new value, now the angle of attack of the flow is actually almost perpendicular to the bottom of the board, or using Stavitsky’s tau, its damn close to 90 degrees, actually often better!?.

When this first occurred to me, my first though was to start new –i.e. assume Stavitsky didn’t apply and find some other treatment. I haven’t done a complete search of all the literature, but I’ve never found any other treatment that comes as close. Anyway, I wasn’t all that sure that Stavitsky’s treatment had no value. In fact, I think is does, if only for its visual clarity and standard language (names we can give to things.) The little formula and his analysis in general however, isn’t all that useful, if at all.

Surprisingly, the impact of all this really doesn’t change the visual analysis much all. It’s likely that something similar to a maximum pressure line exists, same for the spray root line, but the pressure profile and size of the profile per flow is likely to be significantly greater (flowing the reasoning, greater tau, greater pressure per given flow.)

My guess is that it pushes the maximum pressure line back towards the wake edge or the wake rail, and that there is a greater separation between spray root and maximum pressure line or just a lot more spray root production (which is actually important.)

There is also another consequence of all this, another flow to consider in design.


That’s an interesting analysis. I wasn’t aware of it.

Water skiing would appear to be a perfect candidate for applying Stavitsky, however water skiing has an independent (reliable and constant) source of propulsion. In the case of surfing , applying Stavitsky is sort of dangerous, the pressures generated are themselves the source of propulsion.

Please see my post above on formally applying Stavitsky’s treatment to surfing.

Nevertheless, the piece was worth the read. Thanks.


May da scwahtz be wit ya.

waht movie?


co-opting terminology

from pop cultural hooks to lead off into

the theoretical analytical void of

detatched quantifying of virtually

the last remaining phyisio spiritual


mankinds headlong rush to measure and

and control : with the primary goal of reproducing

marketable results has become the appedicitus

of our contemporary stage of ongoing evolution.

Boris Spatzky’s forays into the dominion of chess

was validated over and over again within the checquered grid.

Unlike these grid works

surfings unlimited access

to unlimited unmeasurables

and the concurent quest

for momentary coincidences

with perfect trim

are to brush palms with the creative "dharma head "

of all creation.


as posed by O.B. Juan Kanobi *

in the beloved …out of our miserable worldliness

trilogy…is a belief that the oppertunity to commune with the unfathomable spontaneous creativity of the cosmos indeed exists.

To graph on and Savitsky up

is a pitch that the surfing industrialists

have taken to heart.

and god bless their hearts,

may they too live long and prosper.

Has the lasting effect done more to further

communion with The Kanobi precept

or dampened the quest and hopes

of millinea of study

to access a communion with the stars ?

by the way once we master gravity

in the aqueous training ground

can we be the first navigators

to fold space,

with or without the spice

from arrakis?

AS Surfers

we are the next step in evolution.

tim leary said that

was he just a raving psychiatrist on Lsd ?

or is there some truth to his hope that we are evolving?

choose the path you might,

this is indeed the dawn of a new set of oppertunities

or just another day to buy gas.

it IS up to each and every one of us.

qantify your daily life

in graphs and numbers

removing yourself from natural happenstance

or surrender to the void and



dogs CAN trim

my son saw two

stuck together

on the side of the road

as he was

delivering food

to a banquet

in rickys truck

on saturday.

are there Omens in daily life?


(sp? sorry trekkies)

I admit its a subtle balancing act, but surfers learn how to do it very quickly without giving it much thought (if any.)

precisely. so why not call it trim & be done with it.

rather than measuring it, perhaps we would do better to enjoy the reality that it feels good to have saltwater moving beneath the bellys of our surfboards.

if your not interested in the science !

thats cool

i personaly am very interested

silly, it’s trim man. it’s not science, it’s metaphysics!

but you’re right, each to their own.


as posed by O.B. Juan Kanobi "…

…I think HE may be a close friend of a guy who posts at surfer mag ,

“juan kerr”