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Surfboard Hull Design

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:

Surfboard Hull Design

Surfboard Hull Design

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.

Surfboard Hull Design

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.

sink

A planing hull, on the other hand, will have a flat or concave bottom contour and plane up on top of the water.

lift

The board will almost skim across the water’s surface.

Displacement Hull

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 Hull

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

Mini Simmons Rail Design

First of all, we got to think about what happens at that edge where your surfboard rail and the wave meet.

That edge is where the magic happens in surfing.

There are two options at that edge in a turn, sink or unsink your rail. Did you get that? Sink or unsink, that is the question.

But we’re not always turning, so that rail also has to be designed for flying down the face and gathering speed—thus your design must accomplish two goals for down-the-line surfing: reduce drag and release energy.

You have to design for turns and speed.

The ultimate dichotomy of surfing.

Ok, now we have to think about the different rail types and the functions of each. Which surfboard rails meet our ultimate objective of, well, surfing our hearts out?

I like Dave Parmenter’s description of rails, so I’m just going to quote him here:

 “An over-simplified primer on rail shapes would probably contain these main premises: the lower and harder a rail is, the faster and stiffer it will be. The softer and rounder, the slower and more neutral-handling it’ll be.

Fuller, boxier rails are harder to sink, so have more potential for leverage. Thinner, tapered rails sink easily, but with less stored energy, are not as likely to leap out of turns.”

Leaping and going fast: these are the characteristics that I want for my Mini Simmons.

Let’s look in more detail at the types of rails:

  •  50/50 Rail: A description of the point where the bottom of the rail meets the top, in a 50/50 they meet in the middle.
  • 60/40 Rail: This is a 50/50 rail but slightly turned down, the meeting point is 10 degrees past center.
  • Round Rail: Rounded rail that looks like a complete semi-circle.
  • Down Rails: A rail coming to an edge at the bottom of the board.
  • Rolled Rails: Down Rails that are rolled under the board.
  • Egg Rails: Similar to Round Rails, but egg shaped and fatter.

Here is an image to help visualize what we are talking about.

Mini Simmons Rail Design

Round & Soft vs Hard & Low Rails

You got to consider your surfing style and pick the rail design that best matches how you want the board to perform.

The rounder and softer the rail, the slower and more easy handling it will be. Think of a long board, most of these shapes have rounder rails and are for slow turns.

A hard rail provides better traction but can catch an edge easier than a soft rail. Think Slater in the pit or Dane throwing a big turn.

The lower and harder a rail is, the faster it will be and the knifier it will cut into the face of the wave and assist in the slash.

Thinner, tapered rails are good for quick turns, they sink easily—but they are not able to carry speed coming out of a turn.

However, fuller rails are harder to sink, which can translate into more drive coming out of turns as they bounce back from being sunk underwater.

Dude, I know it’s confusing.

I have shaped quite a few Mini Simmons with different rails and what I have found is that the 60/40 rails to 80/20 rails that get harder towards the rear work the best.

In fact, these were the same types of rails that Bob Simmons was using on his boards towards the end of his shaping experience after years of testing and refining.

I am more than happy to follow Bob’s lead.

Whatever rail you pick, have some fun and please let us know about your shaping experience.

Finless surfboard

Finless Surfboard

Inspired by my involvement with Salsa (Jason Salisbury) and his desire to surf Desert Point in Indo, this is the refined version of that interlude.Friction free with a rail that holds and drives, this board will take your surfing to a level previously unknown. Are you experienced?

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