How To Calculate Buoyancy Of A Pontoon Boat

How To Calculate Buoyancy Of A Pontoon Boat | Life of Sailing

Last Updated by

Jacob Collier

November 2, 2022

Key Takeaways

  • Every boat owner should know the maximum weight capacity
  • Every owner should be aware of the risks of overloading
  • Every owner should err on the side of caution than take risks

The most common cause of pontoon boats sinking is caused by overloading. How do you calculate the buoyancy of a pontoon boat?

The buoyancy of any object depends on its size and weight. The larger the boat, the higher the buoyancy force needed. Buoyancy is based on the formula B = ρ × V × g, - B (buoyant force) = p (density of fluid) x Volume (volume of the boat displacing the water) x gravity.)

One of the essential things for pontoon boat owners to know is the amount of weight that the boat can withstand. To figure this out, it is good to know the buoyancy of the watercraft. Whether planning a party aboard the boat, or purchasing size, knowing how buoyant a craft is can keep everyone safe during the summer. As a lover of boats, one of the questions most boat owners what to know is how much weight a ship can handle without going down.


Table of contents

What is Buoyancy?

There are two distinct forces at work whenever a boat hits the water.

First, gravity and the object's weight push down on the water. This is the old Issac Newton idea of gravity constantly pulling down on objects, keeping them from flying off into space. Our planet’s gravity is always present and always acting on objects.

Second, there is the force of a liquid, in this case, water, pushing against gravity and weight, fighting to keep objects afloat. This force is known as buoyancy. It is the determination of what it takes to keep your nose above water.

Buoyancy depends on the size and weight of the boat. A small craft displaces less water and requires less buoyancy force to counter gravity. Add a person’s weight to the boat, increasing buoyancy force with every pound you add. The less weight a boat has, the less water is displaced by it being in the water and the more buoyant.

Similarly, a larger boat will weigh more (even without any crew or equipment) and thus require more buoyancy force to remain afloat. The greater the buoyant number, the less buoyant the watercraft is.

How Do You Calculate Buoyancy?

The standard formula for calculating buoyancy is based on the formula of B = ρ × V × g, - B (buoyant force) = p (density of fluid) x Volume (volume of the boat displacing the water) x force of gravity. The buoyancy force is usually calculated in Newtons.

How Much Weight Can My Pontoon Boat Handle?

While buoyancy is an important piece of information, most boat owners just want to make sure that they do not overload their boat and cause it to sink. Since a pontoon boat tends to be a party boat, it is crucial not to allow it to be over the limit.

The easiest way to calculate how much weight your boat can handle is to take the dimensions of the pontoon tubes (otherwise known as volume) and multiply it by 62.43 lbs (the water weight per 1 cubic foot). The result is the amount of weight that the pontoons can handle. Simply subtract the deck's weight, and the result is the number of pounds the boat can comfortably handle.

For example, let’s assume that the pontoon boat in question has two - 20 ft tubes running underneath the deck and that each tube is 2 feet in width and height. (Most pontoon tubes are about 25 inches in diameter and about 20 feet long). The mathematical equation would be 20 feet (length) x 2 (width) x 2 (height) x 2 (two tubes, if tritoon, multiply by three).

Here is a rundown of the formula - 20 x 2 x 2 = 160 cubic feet displaced

Since scientists have already determined that the water weight is 62.43 lbs, we simply multiply 160 x 62.43 = 9,888.80 lbs. Subtract the weight of the deck (basically everything above the tubes). For our illustration, let’s assume that number is 7500 lbs. This calculation would mean that the pontoon boat could carry about 2.488 lbs in occupants and gear. This weight roughly translates to about nine passengers (averaging 200 lbs each) and 688 lbs of gear.

Where Can I Find the Maximum Weight for a Pontoon Boat?

For those boat owners who don’t feel like taxing their brains mathematically, the information on weight loads can be found in the owner’s manual or on the capacity plate, which is required to be visible from the helm for every boat that is less than 20 feet in length and motorized. This law has been in place by the US Coast Guard since 1971.

The capacity plate will detail important information such as -

  • The make and model of the boat
  • The manufacturer of the boat
  • The number of people allowed on the boat
  • The maximum weight of people, gear, and motor
  • Serial number of the boat
  • Adherence to all Coast Guard regulations

Why is Weight Limit Important?

The number one reason boats capsize is that they are overloaded, not just with weight but with people. The reality is that people tend to move around on a boat, which creates weight distribution issues. For example, assume that everyone on the boat gravitates toward one side, then the boat has to handle more weight on one side, and the chances of rollover increase. If alcohol is involved, people cannot tell what escapades they will get into. Every boat owner has the responsibility and can be held legally liable for violations of any accidents that might occur as a result of violating weight capacities.

Who Enforces Weight Limits for Boats?

While it is not a federal offense to exceed weight limits on a boat, many states have regulations and are not hesitant to enforce them. Just as an officer has the right to issue a ticket for operating a boat under the influence, many states give law enforcement permission to board a boat that seems to have too many party animals on it. A best practice is to be familiar with all boating regulations and keep the number of guests cruising the lake to a minimum. The motto should be to have fun, but most of all, be safe.

What are Some Considerations Concerning Weight Limits?

There are several important considerations regarding keeping everyone safe on the water.

The Size (Weight) of the Group.

Knowing the number of people who will be cruising with you is essential. If the Coast Guard states that the maximum number of people is 12 (1800 divided by 150 = 12 - based on the example above). I prefer to divide that number by 200 lbs a piece, which accounts for extra weight - making it nine people. If part of the passengers are children, then adjust. If it is an all-adult gathering, then err on the side of caution and adjust the number of people downward.

The Weather

Choppy winds and waves make for a choppy boat experience and increase the potential for an accident. A pontoon boat does not do well in choppy seas, so avoid taking the boat out on days when there is a wind advisory. A bobbing boat can roll over, mainly if the weight gets shifted to one side or another or the boat lifts high enough.

Other Boats on the Water

Accidents happen with boats just as they do with motor vehicles. By being diligent and conscious about the number of boats in the area, an owner can avoid any potential problems. Always be aware of the boat traffic around you and steer clear of any gathering places with a high potential for an accident.

How To Calculate Buoyancy Of A Pontoon Boat
Jacob Collier

Jacob Collier

Born into a family of sailing enthusiasts, words like “ballast” and “jibing” were often a part of dinner conversations. These days Jacob sails a Hallberg-Rassy 44, having covered almost 6000 NM. While he’s made several voyages, his favorite one is the trip from California to Hawaii as it was his first fully independent voyage.

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