I need some help with a calculation

[Jeff Monson]

I'm building a swim raft for the kids at the lake. It will be 8'x8' made with a few 2x8"s and 3/4" plywood for the top, I have 4 x 30 gallon plastic drums that I will use for floatation.

Question, is there a way to calculate how much weight a 30 gallon drum will support in the water? I'd like to add the same amount of water to each barrel for some ballast, but I'm
unsure how much I can safely add.

Any comments, appreciated.

[Rick Christopherson]

Each gallon of water weighs 8.3 pounds, so that is the amount you are displacing. Even though the air that is displacing the water does have weight itself, it is not important enough for you to consider in this problem. So the estimated displacement will be 4 x 30 x 8.3 = 996 pounds.

The wood will also displace some water if it is below the water, but you can partially ignore this added buoyancy. However, any wood that is above the water should be taken into account as part of the weight supported by the drums.

So take the total weight of the whole raft and subtract the displacement weight of the water, and this is the remaining weight that the raft can support.

[David Epperson]

I'm building a swim raft for the kids at the lake. It will be 8'x8' made with a few 2x8"s and 3/4" plywood for the top, I have 4 x 30 gallon plastic drums that I will use for floatation.

Question, is there a way to calculate how much weight a 30 gallon drum will support in the water? I'd like to add the same amount of water to each barrel for some ballast, but I'm
unsure how much I can safely add.

Any comments, appreciated.
Bouyancy is equal to the volume of water displaced. A gallon of water weighs about 8.35pounds so 4x30x8.35 = 1002 pounds. This would include any of the framework you wanted to hold above a waterline.

Water is not going to make for a very good ballast, some, but since it's the same density as the water outside the barrel, it's going to remove flotation as fast as you add it. Sand, rock, or something more dense (heavier) might provide better ballast without sacrificing water displacement quite as bad. But then again, you might want more barrels to make it float higher. Placing the flotation points as far apart as possible will provide some stability without ballast.

[Jeff Monson]

Each gallon of water weighs 8.3 pounds, so that is the amount you are displacing. Even though the air that is displacing the water does have weight itself, it is not important enough for you to consider in this problem. So the estimated displacement will be 4 x 30 x 8.3 = 996 pounds.

The wood will also displace some water if it is below the water, but you can partially ignore this added buoyancy. However, any wood that is above the water should be taken into account as part of the weight supported by the drums.

So take the total weight of the whole raft and subtract the displacement weight of the water, and this is the remaining weight that the raft can support.


Bouyancy is equal to the volume of water displaced. A gallon of water weighs about 8.35pounds so 4x30x8.35 = 1002 pounds. This would include any of the framework you wanted to hold above a waterline.

Water is not going to make for a very good ballast, some, but since it's the same density as the water outside the barrel, it's going to remove flotation as fast as you add it. Sand, rock, or something more dense (heavier) might provide better ballast without sacrificing water displacement quite as bad. But then again, you might want more barrels to make it float higher. Placing the flotation points as far apart as possible will provide some stability without ballast.


Great info guys, thanks!! The diameter of the barrels is about 20", so I dont want the raft that far out of the water, with a 2x8" frame, I would rather have the raft at about 12" out of the water. The barrels will sit inside and underneath the framework. So are my calculations right here, lets say there are 4 people with and average weight of 150lbs = 600lbs, the material and carpet will be right around 200lbs, so I could have 800lbs of total weight at one time. This would leave me with 200lbs of ballast, or 25 gallons of water, so I could add roughly 7 gallons per barrel??

[Robert Delhommer Sr]

I'd say 1,000 lbs. :)

[David Epperson]

Great info guys, thanks!! The diameter of the barrels is about 20", so I dont want the raft that far out of the water, with a 2x8" frame, I would rather have the raft at about 12" out of the water. The barrels will sit inside and underneath the framework. So are my calculations right here, lets say there are 4 people with and average weight of 150lbs = 600lbs, the material and carpet will be right around 200lbs, so I could have 800lbs of total weight at one time. This would leave me with 200lbs of ballast, or 25 gallons of water, so I could add roughly 7 gallons per barrel??
Jeff. You could do it that way - but your deck will be under water. Still floating, but right at water level or more likely just below at what ever corner someone is standing on. If it were me, I'd try and get some more barrels (jugs) - at least 8, if not 12, of them.
The raft will end up being only 10" or so out of the water rather than almost submerged.

[Brian Ashton]

Look up Archimedes principal and you'll find the answer

[Jim Stewart]

I would design for a high float and then you can add sand as a ballast. Your ballast probably should not be liquid since your kids will be running and jumping on the raft which would make the thing porpoise.

[Jerome Stanek]

Remember that the barrels won't be floating that high to start with. you may not need any ballast to get what you want

[Gregory King]

Jeff, if I were you I would hesitate to add the water to it. We did that the very first year when we built our 8' x8' -2" x6" joist - [4barrell ] deck. We only wanted to keep it lower in the water, which it did. The real problem started when we had to take it out of the lake before freezeup. There was no possible way to manually lift it up, so we had to cut the ties that held the barrels in the joist spacing below and force the barrels from under. Needless to say, that was the last time for adding water. Greg

[Art Mann]

I agree with the above buoyancy calculations. The barrels will displace just about 1000 pounds. I would question whether you can build a sturdy framework and platform out of 200 pounds of lumber and plywood. The plywood will absorb water and contribute significantly to the weight. The weight of the empty barrels must also be figured in. The truth is, your platform will probably not be 10 inches out of the water with no ballast and nobody standing on the platform. With four 150 pound people on board, the platform will be several inches under water. I agree with David Epperson. You need more barrels. Here is one way to look at it. If you want the barrels to be half submerged with a full load of people, then calculate the total weight of people and the true weight of the structure itself - say 1000 pounds. Each empty barrel will support (8.3 * 30 =) 250 pounds of weight but as David pointed out, that will be with the platform at water level. You need twice the buoyancy for the barrels to be half way out of the water. You need 8 or more barrels to achieve your flotation level.