Surfboard design myths: the 'pushing water' nose rocker scare, is it real?

"Insufficient (nose)kick will cause the board to pearl while excessive kick will push water and slow the board down." www.essentialsurfing.com

 Lately we've all heard a lot about how low nose rocker gives early wave entry and greater speed, while higher nose rocker prevents early wave catching by 'pushing water' and slowing the board down. This is an oversimplification which has led to one of the most pervasive of surfing myths: The 'pushing water' myth.. Some of the fastest boards have pronounced nose rocker and can catch waves early with excellent acceleration... how can this be the case if the theory above is correct?

Pronounced surfboard nose rocker lifting at low speed. Note that nearly all the rail is submerged and only the last couple of inches are lifted ou. Spray Root  here 'pushing water' according to the aged pundits.

 The short answer is that the theory is not correct.

 'Pushing water' is crude way of referring to the Spray Root which exists ahead of the stagnation line on all planing hulls. Certainly this spray root will increase ( all else being equal) with an increase in rocker, but it exists to varying degrees at all times when surfing. What is almost never mentioned is lift: increasing rocker in the nose increases drag but it also increases lift. This increase in lift can allow a board with more nose rocker to catch waves earlier by lifting it on to the plane earlier ( contrary to the popular belief that it delays wave catching and/or makes it harder). So what does your ideal surfboard rocker look like?

Savitsky described the spray root in 1963

 What is also ignored is the fact that the nose rocker lifts the board more as speed increases, thus releasing the more highly rockered part of the board from the water. The nose thus becomes 'self tending': it is immersed at low speeds for lift during the transition to planing speed and released once speed increases.

Pronounced surfboard nose rocker lifting free as speed increases see also the root spray under the feet

 The higher nose rocker also simultaneously allows a steeper glide path into the wave ( thus giving more motive power to the board and rider on takeoff) and allows the hull to utilise that power via increased lift. When we add nose rocker what we are actually doing is increasing the angle of attack of that part of the hull. As it is with fins and wings so it is with the nose of the surfboard hull (tweet this): higher angles of attack produce higher lift and higher drag... but we have to be careful because 'more rocker' and 'less rocker' are terms which are used in inconsistent ways in the surf industry.

Usually 'more rocker' means only that the nose has a higher measurement above the base line, but this tells us nothing about rocker curvature, angle of attack, lift, or drag, since it does not take the length of the nose into account.

 Another definition of nose rocker is the rate of curvature or radius of the rocker at any given point, but this doesn't tell us what the angle of attack to the water flow at any given point is either, even though it can be assumed that a smaller rocker radius will tend to result in higher angles of attack. It's more useful than the measurement above the baseline method, but only tells us part of the story.

 A further complication is the overall trim angle of the board. Raising the nose also raises the trim angle, this increases the angle of attack of the nose... or does it? When the nose is raised part of it will rise out of the water and the wetted surface area will move aft. If the rocker curve is flatter aft the net result of raising the trim angle might be a lower angle of attack.

 We also have to take into account the curvature of the wave and 'effective rocker'. If for example the curvature of the wave matches the curvature of the board we have zero effective rocker throughout the board, regardless of how much curve the board has. It's often stated that more nose rocker is helpful to prevent nosediving aka 'pearling' on hollower steeper waves, but that this results in more 'pushing' of water and will slow the board down. This oversimplifies the situation and doesn't take wave curvature into account.

 Then there's water pressure: if the water pressure is lowered at a given angle of attack then the spray root or 'pushed water' will decrease. What lowers water pressure? One variable which affects water pressure is the amount of downwards force applied by the rider. In other words if there is less weight on the nose the water pressure and spray root  will be less. Thus the way in which the board is ridden has an effect, it's not solely a matter of the surfboard shape.

 What we are doing with the nose of the planing hull is creating lift when it is needed. If more lift is needed this can be achieved with a) a smaller radius to the rocker (i.e. increasing the angle of attack) or b) more hull surface area, since lift is proportional to wetted surface area, all else being equal.

 More hull surface are can be added by making the nose wider, or longer, or both The trend recently has been for shorter boards with wider noses and less nose rocker. The added nose area compensates for the lower rocker, so that the nose retains sufficient lift. This works, but it isn't the only way or necessarily the 'best' way.

 So, as is usually the case with surfboard design folklore maxims, it isn't as simple as is supposed.