I have been wondering about this a lot in Mini Simmons surfboard design (surfboard design in general)—what is a hull and how does it influence the board and the ride?
Two factors are at work when a planing surfboard is in motion:
- the propulsion power, or force, that creates forward motion (the wave energy)
- the resistance that opposes it (your fat ass standing on the board)
When at rest, a surfers weight is borne entirely by the buoyant force (also depending on how much beer you drank the night before).
At low speeds every hull acts as a displacement hull, meaning that the buoyant force is mainly responsible for supporting the surfer.
As speed increases, hydrodynamic lift increases as well. Ok, you got to reach back to some physics for this one.
But check out this diagram which will help:
A fluid flowing (water) past the surface of a body (surfer + surfboard) exerts surface force on it. Lift is the component of this force that is perpendicular to the oncoming flow direction. It contrasts with the drag force, which is the component of the surface force parallel to the flow direction.
If the fluid is air, the force is called an aerodynamic force—if it’s water it’s called hydrodynamic force.
In contrast, the buoyant force decreases as the hull lifts out of the water decreasing the displaced volume.
This is the reason that surfboards with lots of rocker need more speed and their no rocker cousins scream across the water.
The major difference between a planing surfboard hull and a displacement hull is the way in which the surfboard travels through the water.
A displacement hull has a belly, or convex, bottom contour and planing surface. This design does not ride high on the water like a planing hull, instead plowing through and parting the water. Think of an ocean liner moving through the deep ocean versus a speed boat racing around a lake.
At high speeds a displacement hull’s tail will sink down lower and lower as a result of the “hole” created in the water as the surfboard moves forward.
A planing hull, on the other hand, will have a flat or concave bottom contour and plane up on top of the water.
The board will almost skim across the water’s surface.
Displacement hulls push through the water as they have no hydrodynamic lift, or the surfboard does not rise out of the water as speed increases.
Planing hulls are designed to run on top of the water at high speeds.
To achieve this they typically are very flat at the tail.
The hull design (shape) does not limit the maximum attainable speed but does affect the power required for it to get on plane (on top of the water).
Semi-displacement / Semi-planing Hulls
Semi-displacement or semi-planing hulls have features of both planing and displacement hulls.
They have a maximum hull design speed. Exceeding this speed can result in erratic handling and unstable operation.
There is not one hull design characteristic that differentiates semi-displacement from semi-planing hull.
The greater the hydrodynamic lift and higher the hull design speed the more likely it will be referred to as a semi-planing hull.
I came across this blog post by Steven Mast of Mast Surfboards and it is an excellent discussion of the topic.
“Although the recent fascination with hulls has centered around the Greg Liddle “modified transitional displacement hulls”, any surfboard can be considered a hull. There are displacement hulls, planing hulls and as usually is the case, some variant of the two.
As soon as you put an edge at the tail of your board you have created a planing hull. The very nature of the release provided by that edge, by definition puts that board into planing mode. It the edge were left soft and round, you still have a displacement hull. Now whether it is a good one of either type is another matter.
Have you ever seen the old footage of guys towing behind motor boats on their logs? As soon as they get going, the tail end of the board starts submarining and they can literally walk to the nose and go. This demonstrates a the principle of displacement hull theory. A displacement hull has a theoretical hull speed, above which the water actually sucks the hull deeper into the water (I’m simplifying here). Take a sailboat or any other displacement hull and tow it. At anything above the theoretical hull speed, the boat begins to submarine, actually being pulled deeper into the water.
Old, soft edged boards are the same, as are any true displacement hulls being produced today. As soon as you put an edge at the tail, you release the water and the board begins to plane. The modern surfboard, most “hulls” included, balance these principles to achieve the desired effect or feel.
Now I’m sure I’ll get some flak for this, but displacement hulls, by their very nature, are not as fast as planing hulls (editors note: . They may “feel” faster in a section, but without the release, they are constantly dragging more than a sharp edged board would. Now this is not a bad thing. The feeling of a well balance hull is one of the great pleasures of surfing that most people fail to credit. Surfing one well takes a different approach, especially if you are stepping off a thruster. Single fin riders tend to have an easier time.
Another thing I’ll take flak for, and I’m saying it anyway, Greg Liddle did not invent the displacement hull surfboard. He refined it to an amazing degree, made it work in a short package, championing it when it was completely against the trends of the time, but have you ever seen a Weber Foil? Have you ever really looked at almost any board before the mid sixties?
All displacement hulls, although arguably not “modified transitional displacement hulls”, whatever that means. Please don’t take this as me dissing Greg Liddle. On the contrary, I think his boards are brilliant and have been a huge design influence for me. It’s just I get a little frustrated when I here people talk about hull this and hull that, without any understanding of what a hull is.”