Imagine that you’re stranded on a deserted island, and you’re lucky enough to find enough materials to construct a boat with two pontoons to attempt to make it back home safely. In that situation, how can you determine the amount of weight that you will be able to carry with you on your boat, and how big do you need to build your pontoons to ensure the boat will not capsize? With a little bit of knowledge of physics and the forces exerted on your boat, you can figure all of that out in no time! First, let’s evaluate the forces in play on a floating boat.
The Forces Involved
- Buoyancy – The upward force exerted on an object (in our case two pontoons) by the water, or whatever fluid an object is in.
- Displacement – The downward force of the object or the force that’s pushing away the water or our fluid when our pontoons are immersed. If the boat is stable, the buoyancy and displacement forces are equal.
- Volume – The physical space of our pontoons.
Let’s back things up just a bit and analyze the forces of water displacement a bit more. Archimedes Principle states that the buoyant force (upward force) of an object submerged in a fluid is the same as the weight of the fluid that is being displaced by the object. Also, if we know that the weight of water is approximately 62 lbs per cubic foot, that means that a square box that’s 1 cubic foot will support it’s own weight and can be filled with other objects up to 62 pounds before sinking.
Armed with this information, we can start doing a bit of math. To keep things on the simple side, we’ll use rectangular boxes as our pontoons for an example, even though that shape doesn’t provide the greatest amount of surface area to displace the most amount of water. (Think cylinder!)
Let’s say we construct two pontoons that are each 1 ft x 1 ft x 3 ft. The volume for each box is 3 cu ft, and multiplying the volume by the cubic weight of water (62 lbs/qu ft.), we can say that each pontoon can support 186 pounds of weight, and since we have two, we would think the entire boat can support 372 lbs. Here’s the math:
Length x Height x Width = Volume
- 1 ft x 1 ft x 3 ft = 3 cubic feet
Volume x 62 lbs/qu. ft = Displacement Force
- 3 qu. ft x 62 lbs/qu. ft = 186 pounds
- 2 pontoons x 186 pounds = 372 pounds
Understanding the Pontoon Effect
As anyone who’s been out on the open water in stormy weather knows, boats can be anything but stable. The pontoon effect describes a situation in which the majority of the weight of a boat is transferred over to one individual pontoon. The pontoon cannot fully support the weight and becomes submerged under water, resulting in the entire boat capsizing. Because boats can be top heavy and are subject to the forces of nature, it’s important to design a pontoon boat that meets a necessary rule to avoid the pontoon effect: Each pontoon should be able to support the entire weight of the boat and everything on it to avoid capsizing. In our little example, our pontoon boat could therefore carry the weight of itself, the boat and anything else up to 186 pounds.
The most difficult part of calculating our displacement force is calculating the volume of our pontoons or whatever we’re using to keep our boat afloat, which is why we’ve listed a few volume calculators for various shapes:
For learning more about buoyancy and the pontoon effect, visit this page provided by the University of Maryland.
This post was put together by Mike Hall, a boating enthusiast.