I have been wondering about this a lot in Mini Simmons surfboard hull 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. The surfboard does not rise out of the water as speed increases.
Displacement hulls move through the water by pushing the water aside and are designed to cut through the water with very little propulsion.
Planing hulls are designed to run on top of the water at high speeds. Planing hulls are designed to rise up and glide on top of the water when enough power is supplied.
To achieve this, planning hulls are very flat at the tail or, as in most Mini Simmons designs, the flatness is through the entire shape with some slight rocker for more performance oriented Minis.
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.
You still with me?
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.
If 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 hear people talk about hull this and hull that, without any understanding of what a hull is.”
The best way to get to understand hull design is to start shaping a few. Then go ride them. after about five shapes, we can have a real conversation.
Some more links about hull design: > Swaylocks Surfers Forum > Surf Science > Shaping Surfboards
Thanks bro, appreciate the comment!
I agree, great post! I finally understood how it works! _and i searched a lot:)
I (even after researching and writing it) have to reread it sometimes to understand it too.
Thanks for checking in.
~ Derek, MSS
Hi, i agree with mostly what you say. But i still don’t understand the general accepted theory that as soon as you put an edge on the tail it turns into a planing hull. A displacement hull works more by pushing the water around it’s hull and through the rails and tail, where as a planing hull works more by channeling the water directly through the hull. A small amount of edge in the tail of a displacement hull will not convert the entire board, it will still act as a DH. If you were going for a hard back foot turn with little else board in the water than the tail only then you could call that planing, otherwise it would still feel and act like a DH majority of the time.
I’m no board designer but i’ve heard that “as soon as you put an edge on it” statement way to many times, and none has backed it up with an explanation!
I am totally with you on this, my experience is that a board reacts differently in a turn vs. going down the line and the hull dynamics will also depend on how much of the rail you bury on a turn.
Putting an edge on a tail does not automatically turn the board into a PH—totally agree.
Thanks again for the comments.
Digging this page dudes – thanks for taking the time to put this site together – awesome work. Mini Simmons hulls are more complex than people realize. Cheers! see you in the H20
Thanks bro, we dig you too.
Confusing, misleading, and highly inaccurate.
All surfboards (regardless of the intentions, theories, or fantasies of their designers) plane on the wave face.
Lord, Lindsay : Naval Architecture of Planing Hulls, New York (1946)
In particular note Preface, page vii, and pages 12 to 31.
Most simmons hulls I see are single concave, but I spoke to a guy in the water who was riding one with a v hull.
His explanation was simple; put a spoon on a table. When the spoon face down (like a concave hull), it is more difficult to pivot. Put the spoon face up (like a v-hull), and the curvature of the spoon allows it to wobble effortlessly.
The main gripe I hear with mini Simmons is that they are somewhat difficult to turn.
Would the v hull truly eliminate this?
Before I sink $500 into a board, if be nice to hear some feedback on the topic.
Great comment Jason, best way to test a board style is to buy it and try it. You can read about shit all day long but nothing beats the actual experience.
Go get one and come back and let us know what you think.