BOB SIMMONS AND HULLS, PART TWO

Rusty and friends on the physics of aspect ratio in surfboard design

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Last blog, John Elwell, a friend of Bob Simmons during the last five or six years of his life, shared stories from this great innovator. Here is Elwell's discussion on the planing hull:

Aspect ratio is a proper width and length ratio. Naval architect Lindsay Lord said the most common factor in a good planing hull was the width in the stern. If you divide the width into the length you'll get the Aspect Ratio. It will be a decimal number. Good numbers are .3 to .5. Wing design uses this depending on how much it is designed to lift with a power plant. With extraordinary amounts of power, a lower aspect number will work. (At Windansea, Simmons went to the shortboard because of the more powerful wave. He already did this in powerful shorebreak at Hermosa.)

In summary, there is a good ratio between length and width. Things too narrow don't plane well and shapes that are too wide are handicapped also. Examples are the U2 spy planes that are like gliders and can fly high and sustain themselves, as well as longboards that will pick up on big, fat waves and have increased resurgence at low speeds. These features also work against these shapes at high speeds. The tow-in boards are adapted because they can get a low aspect ratio up to planing speed where a paddler can't.

Archimedes displacement steps in for hydrostatics for the "plate" as it is called to support the load it is to carry. In other words, the optimum plate must float enough to not hinder lift...or too much, like paddleboards. Simmons was able to reduce a lot of extra weight by reducing flotation to a minimum. The boards in static position just barely floated with the tails squatting in the water, which is the attack angle. Moving a planing hull slowly, such as with a surfboard gets the kinetic energy (water flowing) to get initial lift to get it over the "hump" to plane while paddling in.

Tow-ins break the rules by having a power plant take you up to planing speeds to stay in the wave. Then as Lord says, "Everything changes." Pointed sterns start to drag. On surfboards when the pressure is on the inside rail to the wave the dynamic trans-pan flow comes across the bottom of the lower part of the hull. It is directed there by the monohydrean shape we call the "rounded rail" that is making the top of the rail low pressure and the bottom dynamic high pressure. The result is lift by the kinetic energy (moving water) that is being deflected. The surfer is controlling the pressure by the amount of weight he exerts on the rail from his feet in the right position. Planing is described as skimming on water. Lord says planing hulls adjust themselves with speed, so much that they can fly dangerously out of control at higher speeds in conditions that can be too rough causing cross waves and chop. The camber nose or turn up really helped surfboards from pearling -- an early problem with the old boards that no longer was a problem after Simmons.

Aspect ratio, put into use. Photo: Slavin

The old planks did not plane or turn well because of too much flotation, too much weight, and the wrong shape of rails. People were told to drag a foot to turn to angle the board. That ended with the Simmons' boards. All boards thereafter copied the foiled rounded rail, but changed nose and tail shapes to their own liking. No one knew what they were copying or doing. Simmons did not talk to very many surfers, nor did some of those surfers listen to him.

The modern board has some type of rounded foiled rails. There is still a lot of confusion of what the outline should be. But there shouldn't be if the surfboard makers knew how the electric strain gage works to identify shapes that have resistance. But aesthetics still win out in marketing. Today, so many people copy what works in surfing but are not quite sure how it works because there are a number of complex things working. One of the most important things that Simmons said and simply observed was that a surfboard really is going almost as fast sideways to the beach as it is going forward. He identified a surfboard's trajectory and designed a board to do that. What he did was not the last word in planing hulls as he was constantly changing and improving on what he did. What he did was on solid ground. As Curren once told me, "It was too bad he died, he may have come up with something better."

The summation of Lindsey Lord -- the MIT naval architect who wrote the book on the Naval Architecture of Planing Hulls, who used the Bernoulli equation, and tested his study with the Simmons strain gage for exact data -- said, " There is nothing revolutionary in this because one thing leads back to another. In other words, there are links after links to other known principles." And further more, "These things we did are from solid information and only the beginning."

I must smile on the definition of "flex". It is wide-open to some degree but there is a solid base and flex would fit into refining and improving the basic findings. There are restrictions but keep in mind that, "The sea is a hostile and ever-changing environment."

The parameters as Simmons saw it as a scientist was, "We are really not going that fast; you just think your are." He was right because waves only go so fast. He worked on reducing drag, with suitable size and flotation for the load of each rider, then attacked the enemy of planing hulls -- resistance, mainly eddy flow drag, and excessive fin(s) -- through Lord, hydrodynamic understanding, and his own observations and data with strain gages.

What he had and did for his time was remarkable. Proper aspect ratio was of course a key factor. To make it simple, there are important parts to the whole. Although as Lord said, "You change around a little but everything else is a compromise." Which was apparent for better planing because it keeps the water under pressure and directing by the monohydrean rail for sudden and dynamic release, for lift making the hull lighter. Changing your weight deflects the board for turns and trim. The rail reduces pressure on top and increases pressure from the bottom, and it also tracks and holds the board in the wave unless it gets too steep. This application is unique to waves from standard planing.

We can follow the history of surfing. We look back and see small Hawaiian or Oceanic types of plates. They were used for different types of surfing. All these designs were done by "rule of thumb" -- simply, what works is copied. They had no mathematics or language, and only the materials available. The variable is, of course, the rider's skill. Some rider's can ride anything. Dempsey Holder use to say, "A good surfboard rider can ride a door." That is what it has been. Surfboards got screwed up from old Hawaiian shapes with "planks" and "paddle boards". Flat decks and U-rails have too much weight. Paddleboards are surface shapes and have serious drag problems planing. This all had to be sorted out.

We started out in '47 riding borrowed paddleboards and planks. They were dangerous and impossible to ride well except for the gifted few. We asked our mentors and every one advised, "Get a board that floats." Usually surfers picked a board with nice grains and a shiny finish. No one knew how a surfboard worked...until Simmons came along. Bob defined surfing as planing and surfboards were supposed to be planing hulls. Simmons snarled and gnashed his teeth, "Paddleboards are not." He was more specific and despised pointed tails and tails under 10 inches, but favored wider tails for quicker lift. Soon, his adversaries, which were a few of the ignorant and jealous, started to generate vicious hearsay about wide tails, spin out, nose pushing and so forth.

Simmons would snarl, "Go someplace with better waves! We are really surfing on our rails." He was right and it was too difficult to explain to the general population found at the beach. He was referring of course to Bernoulli and Lord's research and basic hard knowledge of aero and hydrodynamics.

Others over the years without really understanding all that he did and meant have said, "Simmons was way ahead of the pack by light years."

Stan Pleskunas is one of those quiet understated, highly-achieving, mad scientist surf dudes. His shaping machines were state of the art in the late '80s -- Channel Islands,
Linden, Nectar, and Rusty all used them. He designed a line of shaping hand tools, which are still used by many shapers today. Stan also worked with Lis, Greenough, and countless other visionaries on boards, sailboards, machines, and fins. His Fumunda Marine Products are globally distributed.

He goes further into Aspect Ratio for us:

The "whole" surfboard plan form is distinct from "wetted area" plan form. That said, it stands to reason that overall plan form relates in a general sense, to wetted plan form if the shapes considered are more or less conventional surfboard shapes. Another thing that seems to be overlooked is the rule (which is set in stone) that it takes a given amount of area to plane, a given weight at a given speed. Gravity and weight are the constants where speed and area are the variables in this rule. This is sort of the bedrock or foundation that all other factors are based on. To be more concise, the slower you go the more area you need to plane a given weight.

The next thing to consider is the power available. If you look at a modern jet, the wing plan form is decidedly inefficient or low aspect ratio. This is because of the fact that there is essentially unlimited power to push the jet forward.

Digging deep with a Simmons-inspired hull design. Photo: Slavin

On the other hand if you look at a sailplane the wings are long and narrow which have extremely high aspect ratios. The reason for this is there is nothing but gravity powering a sailplane so it has to be very efficient. To recap: low aspect ratios are less efficient. Higher aspect ratios are more efficient. In aeronautics, efficiency is measured in the ratio of lift to drag.

Throughout this rant we are assuming that the areas for both high and low aspect bodies are the same and the weight is the same. So low aspect ratio wings will have a much steeper natural glide path than a high aspect wing. A jet might have an unpowered glide path described as 2-to-1. That is, it goes forward two-feet for every foot it falls. A sailplane might have a glide path of 20-to-1 or it goes forward 20-feet for every foot it drops. The reason is the differences between the drag that low aspect and high aspect wings have. Remember, in this case gravity force/weight and area is the same for both bodies.

This might seem a bit complicated but it is not. Lower aspect ratio wings are more swept back. Higher aspect ratio wings are more perpendicular to the flow. What that means is at any point along a low aspect ratio wing, the flow is in contact with the wing for a longer distance than on a high aspect ratio body. This offers more opportunity for the flow to develop turbulence and increase drag, so it has more drag for the lift it generates. On a high aspect body the flow is in contact with the surface for a shorter distance so it has less opportunity to develop turbulence hence less drag for the lift it generates.

Aspect ratio diagram

So if we have two wings of exactly the same area carrying exactly the same weight, the low aspect ratio body will require more power to go exactly the same speed as the higher aspect body. And if we try to relate all this to surfboards, let's start with the assumption that the wetted area will more or less reflect what the overall plan form aspect ratio is. Keep in mind that the rule of speed, weight and wetted area still applies.

Let's take a 6'2" x 19" board. Steve Coletta's outline (which is what I have to work with) has 1060 square inches. The span is measured as the width of the shape or perpendicular to the flow, parallel to the stringer. In this case the span is 19", the square of 19" is 361. The span squared 361, divided by the area of 1060 is .3405. .3405 is the aspect ratio of the outline shape.

That same board scaled to be 9'8" X 20.5" has an aspect ratio of .2349. This is a substantially lower aspect ratio than the 6'2". The gun has an aspect ratio, which is only 69% of the 6'2" board.

So the question begs to be asked: why does a gun go so much faster than a shortboard if higher aspect ratios are more efficient? First, it has to do with amount of power available to push the board forward. Just like the jet that might fly at mach 2 the gun has a heap more power available. The gun on a big wave has more power to tap than a shortboard on a small wave. It is certainly arguable that a shortboard will go faster than a gun on a smaller wave more suited to the shortboard. All things equal, especially the available power, efficiency wins over all. Otherwise, we would all be riding 10-foot guns on three-foot waves. A 9'6" longboard designed for smaller waves will be much wider which will push the aspect ratio up considerably perhaps even equal to the shortboard's aspect ratio. Or put it this way, the 9'6" board would have to be much wider and have much more area to have the same aspect ratio as the 6'2", which would end up looking like a longboard not a gun.

The next obvious question is why not ride a shorter, higher aspect ratio board in big waves where we have all that power available to go that much faster? Again, for the same reason a jet has low aspect wings -- it is all about control. The flow laying on the wing for a longer distance makes the jet more pitch stable than the sailplane. It has more drag but the advantage of lower aspect ratios is it does not pitch up and down or pearl and stall as easily as the high aspect wings of a sailplane. With all that power, the jet, just as the gun, must be controllable especially in the "pitch" plane. So you are dropping into 15-foot Sunset with three-foot bumps coming up the face, the lack of relative efficiency of the gun to the shortboard is of minor concern compared to plowing into bumps or getting launched off one. Besides there is so much power to tap who cares if the shortboard is a touch more efficient? Control is the key to riding bigger, bumpier waves.

This is really simplified but the basic premise of this argument is correct, in my opinion. This is more or less proven by the fact that smaller waves require more efficiency and they are generally ridden on higher aspect ratio boards. The same is true for big waves where longer lower aspect ratio boards are ridden in big waves. Today's shapers have it sorted out -- just look at what they prescribe for different wave conditions, it all pans out. One thing that is very hard to get a handle on is the power that a competent surfer can add to the equation. When a guy starts pumping and the board begins to flex a bit and he un-weights, the energy he is imparting is substantial. This profoundly affects the efficiency and ultimate speed of the board. This is where today's shapers have made most of the improvements in design. This is especially true for specific riders who can accurately communicate their feeling/desires in a design to the shaper.

This treatment does not take into account rocker, thickness, rail shape and a myriad of other design features that make a complete board. However, using aspect ratio as a common denominator for board measurement/design might be a very useful (if overlooked) tool to tune surfboard shapes with. Using aspect ratio as a measurement will be an empirical process but may really help those who have had a family of boards designed with CAD software. Area measurements, which are critical to calculating aspect ratio, are easily known from CAD drawings.

Where is the next frontier in board design? I think it will be in the quantification and control of flex. This has not been done and cannot now be easily designed for. Given the variability in shapes, materials and construction techniques it will be a long row to hoe to get flex sorted out. When a guy gets that "magic" board, that "magic" is flex, in my opinion. I could go on and on about flex but that is entirely another chapter.

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