Let’s Get Technical about Freeboard and Live Load Capacity

If you work in the marina industry it is important to understand the difference between freeboard and live load capacity and how the two affect the stability and use of a floating dock.

A floating dock’s live load capacity is basically the amount of weight the dock can hold without sinking.

Freeboard, in simple terms, is the height of a dock’s deck above the water.

Live loads are variable and include all unfixed items on the dock such as people and personal items. Basically live loads are how much weight you want a dock to support without sinking.

The two are related. The more live load placed on the dock the greater affect that extra weight has on how high the dock floats in the water.

This is where the discussion gets more technical. All things float based on the concept that they displace the water they are floating on. Salt water weighs approximately 64 pounds per cubic foot. So if we divide 64 by 12 we get 5.33 pounds, which means, if we’re using round numbers, if 5 pounds of pressure is applied to the top of an object, measuring one square foot, floating in salt water, the object will sink 1 inch.

With that simple concept in mind, we can show how live load capacity affects the freeboard of the two most common types of floating dock systems.

Let’s start with a concrete floating dock system, which offers continuous flotation meaning the entire pontoon up to the top of the deck is the float. In comparison, frame systems typically have 8” to 12” of structure that sits above the flotation tubs (see figure 1.0). This is an important distinction between the two when it comes to freeboard and live load capacity. We’ll see why in a minute…

Figure 1.0

Now, back to the concrete system… let’s say we have a 20’ x 4’ finger pier with 18” of freeboard. Using the formulas below we can determine how many pounds of live load that finger pier can hold before it goes “decks a wash”.

*Float Area x Flotation above the Water in Feet x Weight of Salt Water = Pounds of Support

Pounds of Support / Deck Area = PSF Live Load

It’s important to note that Float Area is calculated differently for the two types of dock systems. With the concrete system, we multiply the deck area by 90%. This allows us to account for the small difference between the surface area of the deck and the area below the deck which is providing floatation by being submerged in the water (see figure 1.0). When we calculate the Float Area of a tub supported system will calculate the area of the tubs using the same assumption of area being submerged in the water.

So let’s do the math for our concrete floating dock example…

((4’x20’) x .9) x 1.5’ x 64’lbs = 6912 pounds of support

6912 lbs / (4’x20’) = 86.4 PSF Live Load

Now, let’s look at this same scenario with a frame system. We’ll assume the same dock size, 20’ x 4’ with 18” freeboard. We’ll also assume the 20’ x 4’ dock is supported by two 4’x4’ tubs. As mentioned before, we assume that with a frame system approximately 12” of the dock’s freeboard is actually frame / not flotation. So, on a frame dock with 18” of freeboard we have only 6” of flotation above the water (or 0.5’).

(4’x4’x2) x .5’ x 64 lbs = 1024 pounds of support

1024 lbs / (4’x20’) = 12.8 PSF Live Load

When we compare the two examples, it’s easy to see that although the two docks are identical in their size and freeboard specifications their Live Load capacity is very different. This is important to understand because if we specify a certain freeboard, assuming it will meet our desired live load we could be unpleasantly surprised when the deck starts to go awash during a boat show or other public event!

Freeboard specifications should only be used to communicate the height of the dock for user comfort in mooring and accessibility. It should also be considered when taking into account the desired longevity of the dock. Docks with lower freeboards, can put vulnerable decking /frame materials in the splash zone, thus increasing the impact of the saltwater on the dock’s components.

On the other hand, Live Load capacity specifications should be used to achieve a desired function of the dock. For example, if you want the dock to accommodate larger crowds, equipment or cart traffic, specifying a higher freeboard will not give you what you’re after. Specifying a higher live load will.

Don’t be misled by a dock that provides a “false” freeboard. It’s that float area and the amount of flotation above the water that really matter when it comes to the stability and live load capacity of a floating dock. If you’d like more information about this topic please feel free to contact us at bmi@bellingham-marine.com.