What forces cause a surfboard to turn?

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Discussion Overview

The discussion revolves around the forces that cause a surfboard to turn, exploring the mechanics of turning while surfing. Participants examine the role of various physical principles, including lift, torque, and the effects of the board's shape and design.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that leaning on the edge of a surfboard causes it to "carve" by shifting the center of gravity, which leads to a combination of roll and pitch or yaw.
  • Another participant proposes that the surfboard acts like a plane, diverting water downwards to create an upward force, and that leaning the board diverts water outwardly, contributing to centripetal force.
  • A different viewpoint indicates that if the center of lift is forward of the center of mass, the outward component of lift will create a torque that rotates the board.
  • One participant notes that even a flat board has an upward-curving front tip, which can create an off-center force when turning, similar to a rudder on a boat.
  • Another participant draws a parallel between turning a surfboard and turning a bicycle, highlighting the role of the rider's weight in influencing the board's movement.

Areas of Agreement / Disagreement

Participants express various hypotheses about the mechanics of surfboard turning, but no consensus is reached. Multiple competing views remain regarding the specific forces and mechanics involved.

Contextual Notes

Some assumptions about the effects of board shape, lift, and torque are not fully explored or defined, leaving room for further investigation into the mechanics of surfboard turning.

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TL;DR
Leaning on an edge of a surfboard will cause it to "carve" in that direction. But why?
Summary: Leaning on an edge of a surfboard will cause it to "carve" in that direction. But why?

I was a little bummed there is not a Marine Engineering section, but there is an Aeronautical Engineering section. :frown:

The term carving describes turning on a surfboard. It is a roll + pitch or yaw or a combination of the two.

My question is what forces cause it? It is different from a boat because there is no rudder. There are fins, but these resist turning don't create it. A finless surfboard (Alaia Paipo) turns easier than one with fins. Searching for an answer on google was somewhere between comical and frustrating.

My intuition tells me that when CG shifts off centerline the board heels and the rocker of the hull bottom causes the board to turn with it, but a flat board will act the same way.

I assume the chine generates more drag on the heel side and maybe this plays a part. I know from experience that a board with soft chines doesn't turn well.

Thanks for anyone's input.
 
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My guess is that the board acts like a plane, diverting water downwards, in order to provide an upwards force. If the board is leaned, it also diverts water "outwardly", which coexists with a centripetal force exerted by the water onto the board. The board would also need to yaw in order to maintain a "forward" orientation with respect to the path it is traveling. I assume water skis operate on the same principle.
 
I like it. So if the center of the lift force is forward of the center of mass the "outwardly" component will create a torque rotating it.

The harder the rail/chine the move lift at the edge of the board... the further the board can lean... the larger the turning torque.
 
Even if the board is fairly flat, the front tip curves upward. In your mind, imagine turning the board all the way on its side and imagine water rushing at the front of the board. The tip of the board will be either a bit left or a bit right of the rest of the board and water hitting it is creating an off-center force which will create torque, turning the board. It would be like having a front rudder on a boat.
 
Probably the same as turning a bicycle.

Difference would be the bike has a front wheel, whereas the 'board has a rider, pushing the back of the board outwards, centrifugally.
 

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