Flotation Calculation - Dow Billets

1. Determining Your Live and Dead Loads

Your first step is to determine the live and dead loads of your floating structure.

The dead load is the weight of framing, decking, connections, flotation units, and all permanently-attached equipment, such as pipes, pumps, utilities, gangways, etc. The weight of lumber and hardware can be estimated to between 10 and 15 lbs/ft² of structure.

The live load is essentially the weight of the people who will be on the floating structure adding their weight to the dead load. It is recommended that the structure be designed for approximately 40% submergence, so that the remaining 60% can be used to support the design live load, which may add between 15 and 30 lbs/ft² more weight to the structure.

2. Determining the Quantity & Size of Billets Needed

Your next step is to figure out how many billets you will need to float the live and dead loads you've just calculated. Start by consulting Table 1. This table shows how much weight each size billet will support at four different depths of submergence.

Decide which billet size you wish to use and how deeply you want to submerge it. Then select from Table 1 the accompanying buoyant force for that billet at that submergence and divide your calculated dead load by that buoyant force.

For example, assume a swim raft with a calculated dead load of 1200 pounds. Table 1 shows that a 10" x 24" billet will support 290 pounds when 40% of it is submerged. So four of these billets will support the raft (1200 / 290 equals 4), with 60% of each billet floating above the waterline, a freeboard of six inches.

The available live load which can now be supported on this swim raft is 1720 pounds. Here's the supporting calculation: Table 1 shows that it takes 2920 pounds to submerge these four billets 100% (4 x 730 equals 2920). So then 2920 pounds of buoyant force minus the 1200 pound dead load of the raft leaves 1720 pounds of available live load.

3. Determining Bearing Area Needed

Your final step is to determine how many square inches of the structure's cross-members you should place in contact with the billets to transfer the structure's weight to the billets.

To determine the size of this billet contact area in square inches, multiply the dead load of the structure by the appropriate Design Factor in Table 2 (based on the expected wave action). Since this is the contact area for the entire structure, and since you want to determine the contact area for each cross-member which will be bearing on the billets, divide your answer by twice the number of billets you'll be using (assuming two cross-members per billet).

For example, if the swim raft is located in sheltered waters, multiply its dead weight (1200) by the Design Factor for sheltered waters, found in Table 2 (0.24). Thus, 1200 x 0.24 equals 288 square inches of billet surface. This comes to 72 square inches of contact area for each of the four supporting billets (288 / 4 equals 72).

And since each billet must have contact with at least two cross-members of the structure, each cross-member should have at least 36 square inches in contact with the billet (72 / 2 equals 36).

¹ Freeboard is the vertical distance between the water level and the top of the billet.
² The buoyant force is the load that can be carried by one billet at the stated percent submergence.