Surfing Explained: Ep9 Surfboard Bottom Contours

In this weeks episode of Surf Simply's animated surf science series, 'Surfing Explained', we explore the different bottom contours (Concave, Double Concave, Convex, Flat, Channeled, Vee) and their effect on our speed and maneuverability.

This episode was written and animated by Surf Simply Coach Will Forster.

Transcript:
In the episode we going to be talking about bottom contours.

If you’ve never heard the phrase bottom contours, then its what we call the particular shaping of the underside of the surfboard, the surface that the water flow will obligingly follow thanks to something known as the Coanda Effect - in which a fluid flow attaches itself to a surface and remains attached even when the surface curves away from the initial flow direction.

What this means is that we can control the water flow direction under the board, and with it encourage either more speed, or more maneuverability depending on what we are attempting to achieve from the design of the board.

We can use a combination of Concave, Convex and Flat surfaces to create an almost infinite number of variations depending on what the board is designed for, so lets take a look at some of the most common components.

Concave
We'll start with a concave bottom. A concave, or single concave as its sometimes refereed to, creates a channel that runs along the length of the board between the nose and tail, guiding the water into a clean and direct flow. If you remember some of the physics that have featured in previous episodes, you’ll know the the more water we have travelling towards the tail of the board, or the faster that flow of water, the greater potential for lift and thrust there is.

Surfers have been employing this technique since the 40’s, with Bob Simmons credited with its introduction, along with many of the fundamental hydrodynamic principles of surfboard design.

A concave design typically suits surfboards that have speed in mind, for example small wave boards that need to maximize the little energy available to them, and powerful or barreling waves where down the line speed and grip are essential, and where turning maneuvers are not the priority.

Once up to speed, boards with Concave bottoms have a tendency to continue in that direction, a feature known as “tracking”, which to some is a blessing as it means it will hold its line well, though if the rider wants to change direction, one has to overcome that tracking by transferring more weight into the back foot to turn the board.

Although it’s not specifically known why, a concave also assists in producing grip from the rail when in trim mode, and from it comes down the line speed. It is thought to be from a more favorable angle of attack where the water flow coming up the wave face is the redirected by the rail, and this combined with the lift generated over the length of the board, it is generally accepted as the fastest bottom contour.

Convex
The convex bottom was the first innovation in bottom contour design, moving away from flat bottoms in an effort to make the heavier solid wood surfboards more maneuverable.

A convex is almost entirely the opposite of a concave channel, and resembles somewhat of a belly underneath the board across it’s width. This encourages the board through the rail to rail rolling motion during turn.

Convex bottoms, though more maneuverable, create more form drag as a result of greater frontal area and so are typically slower than an otherwise identical concave board. A convex bottom can reduce the hold and tracking nature of the board down the line, and so is at risk of slipping out of trim and dropping down the wave face if the necessary rail pressure is not maintained.

Flat
A flat bottom isn’t so much a contour, but more so the absence of a contour, and was once the standard before much thought had gone into the underside design of the surfboard.

We can safely say that a flat contour lands somewhere between the concave and convex as some of the water will move out towards the rails and some will travel down the length and out the tail, and so no specific benefit or loss will occur assuming we have no expectations for the design.

Vee
In 1967, famed shaper Bob McTavish created the ‘Plastic Machine model’, a 9ft board with a deep vee-shaped keel through the back of the board and a wide tail. This particular board became one of the first examples of the coming "short board revolution", with designs eventually going from 10ft down to 7ft or less over the course of just a few years. It was during McTavish’s quest for a more radically maneuvering board that he employed the deep vee bottom contour.

Similar to the convex bottom, a vee will direct water out to the rails and so although compromising the lift for down the line speed, it does allow for a lightning fast response when transitioned rail to rail by the rider.

When a surfer is riding in trim, a vee bottom provides only half the board...character limit met.