# Calculate Emergency Flotation For Kayak

**Article By Shorty**

**Floatation**

There are 2 types of floatation that you are probably going need to calculate before you design and build your own plank rat. The first one is the waterline, and how much weight your boat can carry. The second is the airbox float capability, so you can calculate if you will be building enough reserve floatation so you can self rescue in deep water.

Box boats such at these are extremely easy to calculate, all you need to figure out is the area on the side panel, and the width of the boat. This will give you the volume, which you multiply by 62 lbs per cubic foot (the weight of water) and you will know how much it can float.

I drew the curve and end of the plank in Autocad, and measured the square area of each portion shown here.

Now, lets do some example calculations

**CALCULATING DISPLACEMENT**

(how much weight the hull can comfortably carry)

**EXAMPLE 1**- When coming up with the first one, I knew I wanted the boat to be 32" wide. So if I had an 8' board, I cut off the pieces for bulkheads and had 65.5" left. Each end has a curve on it, which takes up 17". So I am now left with 31.5" between the curves. I figured that a 6.5" waterline was the most part of the hull I wanted underwater, leaving 4.5" above the water. So lets see what all these numbers add up to in terms of volume:

53 sqin = curved area at the bow

204.75 sqin = space between the curved areas, 31.5" long by 6.5" high (waterline)

53 sqin = curved area at the stern

----------

310.75 total square inches

x 32 (the width of the boat)

----------

9944 cubic inches (amount of water displaced when boat has a 6.5" waterline)

/ 1728 (number of cubic inches in a cubic foot)

----------

5.75 cubic feet displaced

* 62 lbs per cuft (the weight of water)

----------

356 lbs

So we know that if the combined weight of the plankrat and the paddler was 356 lbs, then it would sit in the water with 6.5" underwater, and 4.5" above water.

**EXAMPLE 2**- Lets say that we want to build an 8' long plank rat, with the same 32" width. We would now have 62" between the curved ends.

53 sqin = curved area at the bow

403 sqin = space between curves 62" long by 6.5" high (waterline)

53 sqin = curved area at the stern

----------

509 total square inches

x 32 (the width of the boat)

----------

16288 cubic inches (amount of water displaced when boat has a 6.5" waterline)

/ 1728 (number of cubic inches in a cubic foot)

----------

9.4 cubic feet displaced

* 62 lbs per cuft (the weight of water)

----------

584 lbs

Wow !! That is a lot of weight!! Could put my whole family in a boat that can carry that much.

**CALCULATING EMERGENCY FLOATATION**

For these calculations, I'll assume that you will be putting in a full bulkhead across the entire width of the boat, and then putting a deck on top of it sealing in an airbox.

**NOTE:**It is VERY important to have adequate floatation, otherwise if you swamp your boat in deep water (anything over 5' deep), then it will be impossible to self recover your boat, and you can die quickly from hypothermia.

**EXAMPLE 3**-Lets say that we will put a bulkhead right at the edge of the curve in the stern, which is 17" in from the transom.

125 sqin = curved area at the stern

x 32 (the width of the boat)

----------

4000 cubic inches

/ 1728 (number of cubic inches in a cubic foot)

----------

2.31 cubic feet displaced

* 62 lbs per cuft (the weight of water)

----------

143 lbs

So if we fell out of the boat, and to reboard it we climbed over the stern, the stern airbox would have to support the weight of both the boat and your body, plus any leftover water sloshing around in the cockpit of the boat.

Now lets calculate how much 1 additional inch of airbox space would carry:

11 sqin = 1 additional inch x 11 high sides

x 32 (the width of the boat)

----------

352 cubic inches

/ 1728 (number of cubic inches in a cubic foot)

----------

0.2 cubic feet displaced

* 62 lbs per cuft (the weight of water)

----------

12.4 lbs

So for every inch we move the bulkhead, we get another 12.4 lbs of lifting power from the airbox.

Now for the last question, exactly how big of an airbox do you need? At this point I am not sure, because it depends on the technique used to reboard the boat. Some people aren't that athletic, and to reboard their swamped boat, they have to sit in the cockpit as the boat is rolled back upright. In this scenario, you will need a pair of airboxes that are big enough to support your weight while the boat is sideways, and when rolled back, not have too much water leftover. In other cases, people will be able to climb aboard one end, pulling themselves up over the airbox. In this scenario, the airbox you climb over needs to be big enough to support the boat and you. A third scenario involves the absolute minimum amount of airbox space, it is just enough to get the water out of the boat, and then the person floats their body till it is horizontal, then quickly pull themselves across the water and over the gunnels landing in the boat. I saw a guy do that into a D4 that had no floatation built into it, was amazed at the trick. (not sure if I could do it though).