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What Is The Best Thickness For An Inflatable Paddle Board?

Board Stability Ratio Explained


The thickness of an inflatable paddle board affects its performance in many important ways. For this reason, thickness should be one of the main criteria in selecting a board, as it is at least as important as length, width, and outline shape.

The inflatable SUP industry has shifted hard in the direction of producing 6-inch thick boards, but not always for the best reasons.  The reason is simple: Making a board thicker will make it feel more rigid, even if the materials it is made from are not the strongest.  While rigidity is an attribute we look for in a paddle board, it can be better achieved with proper use of materials and construction methods, without having to resort to the shortcut of making the board thicker than it should optimally be.

An excessively thick board has some notable disadvantages.  It raises your center of gravity while riding the board, resulting in a wobbly feeling which can make it challenging for beginners to gain confidence. Extra board thickness makes it harder to get back on the board when you fall in the water.  A thicker board takes longer to inflate, is more difficult to carry under your arm, and has a bouncier ride than a thinner board.

Boards designed for all-around paddling by people in average weight ranges will have the best combination of ride feel, stability, and rigidity when designed with a thickness of 5 inches, provided the board is well constructed using the best materials, and is properly inflated to the manufacturer’s recommended pressure, usually around 15 psi.

6-inch board thickness has its place in longer touring boards, which benefit from the extra rigidity due to their length, in some whitewater boards where extra volume helps keeps the rider afloat in frothy water, in boards designed for multiple riders, and in certain sport-specific models.

 

Board Stability Ratio (BSR) And How It Affects Inflatable SUP Performance


When prospective inflatable paddle board buyers think about board size, the focus tends to be on length, which is a key determinant of speed, and width, which contributes greatly to stability.  What most buyers fail to take into account is board thickness, which has a huge impact on every aspect of inflatable SUP performance.

Here, we will go into some depth on how board thickness interacts with other design parameters to influence the stability of the board.

One of the guiding principles we use when designing a board is what we call Board Stability Ratio, or BSR for short.  It’s part of our time-tested process that goes into shaping an inflatable SUP to achieve the specific performance characteristics we’re targeting. The concept was conceived by the Earth River SUP Technical Design team and has been vetted by engineers in industry.  

Before we get into detail, we should state that we have several formulas we use in the construction of our boards which relate to aspects beyond stability, and our shaping procedure requires a lot of on-water testing of our prototypes.  But we felt discussion of the BSR was an interesting insight to illustrate how a relative stability parameter can be quantified with a basic mathematical model. 

BSR goes beyond the simplified concept of stability being a pure function of board width and encompasses additional parameters including board thickness and outline shape. The formula we have developed to relate board width, thickness, and outline shape to stability is this:

Board Stability Ratio = (Width x (Tail Width Factor + 0.5(Nose Width Factor))) / Thickness

Let’s define the terms used in the formula:

Board Stability Ratio  is a relative measure of the stability of the board, used for comparing one board to another.

Width is the measurement across the board from rail to rail, taken at the extreme end of the rounded edges

Tail Width Factor is a measure of the width of the board taken 12 inches from the tail, as a percentage of the total width of the board.

Nose Width Factor is a measure of the width of the board taken 12 inches from the nose, as a percentage of the total width of the board.

Thickness is the total thickness of the board

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EXAMPLES OF BSR IN USE


Applying the BSR formula introduced above to three boards with the same length and width, but with different thicknesses and outline shapes provides a sense of how this all works.

Board A – A 34” width board measuring 20 inches wide 12 inches from the tail, 15 inches wide 12 inches from the nose, 5” thickness:  Applying the formula, the result is:  BSR = (34 * (20/34 + 0.5(15/34)) / 5 = 5.2

To see how the BSR formula helps predict stability, let’s change some variables one at a time:

Board B – Same measurements as Board A, but with thickness increased from 5″ to 6″.  Applying the formula, we get:  BSR = (34 * (20/34 + 0.5(15/34)) / 6 = 4.58

Board C – Same measurements as Board B, except we narrow the tail from 20 inches to 16 inches. For this board, we get:  BSR = (34 * (16/34 + 0.5(15/34)) / 6 = 3.92

In the above examples, the BSR calculation predicts that Board B will be less stable than Board A, with the only difference in design being the board thickness of 6 inches vs. 5 inches.  Board C will be even less stable than Board B due to the effect of a narrower tail width.

Of course, a number derived from applying a formula is a simplification of a complex issue, and other design factors will influence how the board feels on the water.  However, the formula does help illustrate how changes in board thickness and shape affect stability, by putting a relative number to the reduction in stability from increasing thickness and reducing tail width.

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What is the takeaway from all this?


For the majority of paddlers, 5 inches of thickness is preferred over 6 inches thickness because it results in more stability for a given board width and outline shape.  For riders weighing over 220 lb, extra volume may be helpful for keeping comfortably afloat, so a 6-inch thick board may be called for, which is the kind of trade-off that is sometimes necessary.  But for riders who don’t need the extra volume, the extra inch of thickness of a 6-inch board is not always welcome, as it adds bulk, raises the riders center of gravity, takes longer to inflate, makes it harder to get back on the board after falling off, and is more cumbersome to carry.

There are some exceptions to the 5 inch thickness rule of thumb.  Longer touring or racing boards, generally 12’6” to 14’ in length, benefit from extra thickness to prevent bowing in the middle of the board.  Boards used primarily for whitewater are often 6 inches thick, as the volume helps the board skim over frothy water.  Some crossover boards used for a combination of running rapids, paddling upstream, surfing, and padding on flat water are able to meet some specialized performance objectives by having some extra thickness. But for all around paddling, we have found 5 inches of thickness to be the sweet spot for most paddlers, resulting in a board that feels properly balanced on the water. 

In general, quality inflatable SUP brands that focus on all-around paddling and use inherently rigid materials to construct their boards will have 5 inch thick all-around boards in their product line.  Whitewater-oriented or low cost big box store brands skew toward 6 inch boards, but for very different different reasons.

So why are 6 inch boards so prevalent in low cost brands?  It may seem surprising, but it is actually less expensive to produce a 6 inch board than a 5 inch board.  Why? Because adding thickness is a shortcut to achieving rigidity in a board.  Thus, cheaper base materials can be used to produce a board that has adequate rigidity.  In making a 5 inch board, the base material must be stronger with less flexion to attain the required rigidity.  This is why almost all of the bargain priced boards sold at Costco, Amazon, and high volume online dealers, are 6 inches thick, while higher-end all-around boards tend to be 5 inches thick.