Eagle Boat Docks - How To Calculate Floatation Requirements for Your Dock

Fresh water weighs in at almost 65 pounds per cu ft. So if we displace a cu ft of water with our floatation tank, it will create an uplift of the same amount. Minus the weight of the tnak itlself. Conservatively, we suggest, people use the figure of 55 pounds per cubic foot of "Floatation" or "Uplift".

- Now, wood docks, built to generally accepted standards, weigh in at between NINE pounds per square foot, and FIFTEEN pounds per square foot -- excluding any roofs or accessories constructed, such as ramps or storage boxes.

- The average "person" weighs 175 pounds, and making an educated guess about how many of these "people" will be on your dock -- you can generally use a "live load" figure of NINE pounds per square foot (each person occupying about a 4.5 x 4.5 foot area).

- Therefore: for a lighter weight dock you've now got NINE pounds per square foot for the dock, and NINE pounds per square foot for the people = 18 pounds per square foot total. And for a VERY heavy dock,
you would have FIFTEEN psf plus the NINE psf for the people, or a total of 24 pounds per square foot.

- To get the TOTAL uplift (floatation) you need, simply multiply the square feet of your dock by the "load." For example, a 12 ft. by 20 ft. dock at an estimated load of 18 pounds per square foot would require (12' x 20' x 18 psf) 4,320 pounds of floatation.

- Now rate floatation tanks for uplift at 100% submersion. But, good building practice and many codes specify that no more than 1/2 of the float should be submerged. In fact, typically loaded docks may be only 25% to 33% submerged. This gives excellent "reserve buoyancy" and provides for safety when the dock is temporarily overloaded.

So, to get a ROUGH estimate of the number of floats you will need:

1. Figure the AREA of your dock in square feet.

2. Multiply this figure by 18 psf (or the total load per square foot you need for your type of construction). This figure will be the buoyancy requirement in POUNDS to carry the dock itself and the live load.

3. Divide the TOTAL buoyancy pounds you need by ONE-HALF of the rated buoyancy of the float tank you pick from the chart. (remember to use 1//2 -- because we only want to sink the tank half-way and no more!)

4. This result will give you approximately the number of that particular size of float tank you will need.
Play around with the different sizes and depths of tanks and you will see the differences (bigger size = fewer number of tanks).

EXAMPLE:

How many 16 inch deep floats are needed for a floating pier which is 6 ft. wide and 40 ft. long?

6 ft. x 40 ft. = 240 square feet surface area of dock.

240 sq. ft. x 18 psf of load = 4,320 pounds of people and structure to support.

From the chart on the Floats Page we see that a Formex® Divide the needed 4,320 pounds by the 290 pounds of uplift provided by each float and you get just shy of FIFTEEN 16" x 2 ft. x 4 ft. floats necessary to properly carry the dock. Add one more float (sixteen) and you will have two runs of 8 floats each under
each side of the dock! Spaced about one foot apart on the long length, this works out nicely, and gives a very good looking and very stable dock.

Additional floats, or larger floats may be necessary under heavier loaded areas of the dock such as where a jet-ski lift might be, or where a gangway attaches.

Generally, docks with roof structures and overhead decks require almost 100% of the area of the dock to be floats, and those floats need to be 24 inches deep.

"Free Board" or the height of the dock above the water can be altered by the height of the floats, or simply by using a higher frame on top of the floats. Pure swim platforms can be quite "low" to the water, but docks for pontoon boats, for instance, work much better if they are pretty level with the deck of the
boat!

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