# How much weight can my pontoons hold at half emersion?

Discussion in 'Stability' started by christoph, Jan 31, 2010.

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### christophNew Member

I have a calculation that I need answered by you guys.

I have 3 pontoons that are 33 feet long with a 24 inch diameter. How much weight will these pontoons hold at half emersion in fresh water? The pontoons are aluminium.

If some one could post the actual equation that would be awsome to! I have found no place on the net that has this equation.

Thanks for the help

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### sharpii2Senior Member

What you are really asking for is the displacement at the half diameter.

I'll assume the pontoons are not just straight tubes with a flat bulkhead at the ends.

You need a separate calculation for the ends which is based on their shapes.

For the straight tube portion, the formula is:

(Diameter in ft squared)*0.785*Length*Number of pontoons*62.2 pounds/2

This will give you the displacement of just the straight sections at half diameter immersion. The weight of the pontoons must be subtracted from this to give you the weight they will actually support.

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### christophNew Member

is this correct?

Thanks for the quick reply .I do have some questions though. I was looking on a web site called u-fab pontoons and according to there spread sheet it says that a pontoon of 24 inch diameter at 30 foot long will hold 5500 lbs . why is this so different than the calculation that you gave me?

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### sharpii2Senior Member

It looks like they are either loading the pontoon down to almost all its diameter or they are assuming you are actually using two, as most pontoon boats do, and using the full volume of one to calculate the half volume of two.

3.14*30*62.4=5878

which is pretty close to what you got on their spread sheet. They may have allowed for the lesser displacement per foot of the ends.

5. ### Submarine TomPrevious Member

Each cubic foot equals 62.4 pounds of water.

You can figure out the rest.

-Tom

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### pistnbrokeI try

feet inches pounds ...god help america

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7. ### Submarine TomPrevious Member

I get ~8500 pounds of buoyancy half submerged. That's assuming tapered or rounded ends and circular cross-section.

Your total vessel weight should not exceed ~6000 pounds as this is the total buoyancy available from one pontoon.

-Tom

8. ### Submarine TomPrevious Member

Did you mean "feet, inches, pounds..."?

I agree with you on the second part!

-Tom

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### froudedudeJunior Member

Really, unless we get grammar and punctuation correct, America is done for. Oh, did I end a sustenance with at preposition?

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### JFdezNew Member

weights pontoon

Weight that the pontoon will support=Weight of the pontoon submerged volume at the desired draft- total weight of the pontoon

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### JFdezNew Member

......-total weight of the pontoon (of course, without considering the added weight)

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### netjawsJunior Member

cristoph, be careful with using 1/2 pontoon depth as your design draft. beyond the pontoons being 1/2 submerged, your TPI decreases exponentially with each slight weight addition, leading to a very precarious stability situation

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### conceptiaNaval Architect

netjaws has made a good point.

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### hoytedowWood Butcher

Never use a preposition to end a sentence with.

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### ddrdanJunior Member

I received conflicting info on this same topic. How will the pontoon stability fair in the info below?

(used 62#/ft3)
Hull Shape: Square
Pontoon Size (ft): 2.0625'(bottom)* 2.02083' * 14.7' * (2)
Area (ft2): 61.25
Displacement (ft3): 126.3
Max Displacement (#): 7892