I got asked the other day why EPS feels so... different.
Its a great question and part of the answer lies in the foam’s density and its ability to rise and sink in the water at speed.
Its a widely misunderstood concept in surfboard design. That is buoyancy vs lift.
When it comes to a surfboards relationship with water, there is two different states your board can be in, that is resting and moving.
At rest, a board floats via its static buoyancy. As the board starts moving and increasing speed, this creates dynamic lift and the boards floatation transitions away from being supported by buoyancy to lifting.
Buoyant force:
When a surfboard is at rest it is using buoyant force to keep it a float. This means, the volume and the density of the board determine its ability to rise or sink in relation to the water. So a surfboard that is more dense (heavier) with the same volume will be easier to sink than a less dense (lighter) surfboard with the same volume.
Think PU vs EPS blank with the same dimensions.. PU is more dense, hence it is less buoyant at rest than EPS.
Dynamic lift:
As a board begins to move and speed increases, there is a transition phase where a surfboard shifts from being supported by its buoyancy to being supported by lift.
The Transition:
This is the missing piece to the pie.This transition from static to dynamic is where a lot of our perception on how construction changes riding feel comes from.
A denser board like a PU is going to take more speed to transition to lifting before a surfboard can liven up and be maneuverable but it will maintain this momentum easier and have more carry through. This is great when we need more control and predictability at higher speeds.
A less dense, EPS blank takes less speed to transition to lifting.It also loses momentum easier. Thats why EPS can feel particularly sensitive to good waves or texture, its lifting too much, and also why it feels more “alive” in smaller surf.
I hope that was informative.
There are other factors that contribute to why it feels different, including glassing layup and resin choice. I just thought it would be a cool to touch on this topic for those surf nerds who are interested.
Cheers!
Jake
