What forces cause a surfboard to turn?

[fpjeepy]

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.

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.

 

[rcgldr]

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.

 

[fpjeepy]

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.

 

[berkeman]

I did a quick Google search and got some hits that look helpful.

https://adventure.howstuffworks.com/outdoor-activities/water-sports/surfing4.htm
https://www.exploratorium.edu/theworld/surfing/physics/index.html

 

[fpjeepy]

I did a quick Google search and got some hits that look helpful.

https://adventure.howstuffworks.com/outdoor-activities/water-sports/surfing4.htm
https://www.exploratorium.edu/theworld/surfing/physics/index.html

They do?

 

[njaisinghani]

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.

 

[hmmm27]

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.