Calculating Pontoon Size?

Discussion in 'Boat Design' started by Davicus, Aug 16, 2009.

1. Joined: Aug 2009
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DavicusNew Member

Hi all, I hope this is the correct place for this question. I am building a boat / motorcycle, and I need to figure out how large my dual pontoons need to be to keep about 650 lbs afloat. Any suggestions? Thanks!

2. Joined: Jan 2006
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Location: Lakeland Fl USA

Davicus: Your question should go in the Boat Design section. Stability is a whole other subject. The moderator may move this thread to the other place. <mod note: moved to boat design main forum>

We need a lot more information in order to answer your question to best advantage. How long can the pontoons be? Do you have a particular section shape in mind ( like round tubing, or square sections, or...). Do you want to be able to go fast on this thing, or are you happy with slower pace? Is the Pontoon/motorcycle a deal where a motorcycle is used to turn a roller on the boat, and the roller turns a prop or paddlewheel?

Here is the general idea for calculating the size of pontoons or other things that float. Fresh water weighs 62.4 pounds per square foot. Suppose you build a box by using six pieces of material that are cut exactly 12 inches by 12 inches. The box will be in the form of a cube and will occupy a space of one cubic foot or 1728 cubic inches. Say that the box is airtight and it weighs 6.4 pounds Throw it into Lake Michigan and it will float. How much will it support. The box weight is subtracted from the weight of water like.........62.4 pounds - 6.4 pounds= 56 pounds. The box would support 56 pounds. You want to support 680 pounds. How many cubic feet of water is equivalent to 680 pounds? 680/62.4= 10.89 That is how much space the pair of pontoons must occupy in order to support that weight. Of course you have to account for the weight of the pontoons when you calculate the total weight you wish to support.

At the absolute least each pontoon should be able to support the entire load including its own weight and that of its twin. Much better to have a single pontoon capable of supporting twice the anticipated weight. So in our case the space occupied by one pontoon should be at least 2 x 10.89 cubic feet = 21,78 cubic feet. (or 37,659 cubic inches) Suppose the pontoon has a square section that is 15 feet long. divide 21.78 by 15 to get 1.452. That is how many square feet a cross section needs to be. It is easier to visualize this if we convert square feet to square inches. Just multiply by 144. So we need about 209 square inches. Any combination of width and depth whose product will be 209 will do. For example 14.456 by 14.456 will do. 18 inches wide by 11.61 inches will do, and so on.

There is some other important stuff to consider, so tell us more about the project and we can narrow the choices down a ways.

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DavicusNew Member

Build status

thanks for the input. here is a glimpse of what i'm working on.

the motorcycle will have an aluminum shroud around the back end - behind the fork and in front of the rear wheel - it wont be totally sealed, but it's function is to keep as much of the water as possible out of the engine compartment. we will build each pontoon which will have a 550 jet ski motor inside it, with the impeller sticking out the back. right now the pontoons measure about 60" long, 18" tall and 14" wide, which equals 15120 cubic inches per side, or 8.75 cubic feet per side - that means they should support 1092 lbs... right?

I can't really afford to go any larger than that, so I am hoping it works.

4. Joined: Jan 2006
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Location: Lakeland Fl USA

The combined displacement of the pontoons will be 1092 if the boxes are square ended. But if you have some curvature in them, as shown in your drawing, you will have lost some of the displacement and thus flotation. Your aritmetic is good. When you are dealing in inch dimensions you multiply three numbers and you have xxxx cubic inches. You can save a step or two simply by multiplying the total number of cubic inches by 0.03611. That will yield the number of pounds that the vessel will support.

The drawing is quite good but the concept is questionable to say the least. What is it that you wish to do here???? If I understand your description, the "thing" will have two 550 engines plus a bike engine of indeterminate size. You are flirting with a machine with a total of 200 HP and probably more. If you intend to use all that power on a 60 inch boat you will be way beyond disappointed. Worst case scenario; you will be residing in a casket ( that is if your survivors can find all of your water logged body parts) This rig needs to be at least double the length that you mentioned. It'll still be dangerous. Make it 15 feet long and you will have better results. Also the general performance will be better if you let the 18 inch dimension be the bottom and the 14 inch dimension can be the sides. In doing so there are structural consideration that you must address.

The bike is reminiscent of old time dirt track racers. Rigid rear, dirt tracker tires, no brakes, etc. I like the bike. Please tell us what function the bike has in this scheme. Do we have an amphibious vehicle here.......or what?

Do abandon the the notion of using short pontoons, and do so immediately.

5. Joined: Aug 2009
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DavicusNew Member

I appreciate your concerns, but this project is not intended to be used mulitple times. It's essentially going to be used once in a competiton - I need to win a race on the street, and then on the water. After that it'll be retired.

When you say I will be "way beyond dissapointed" - do you mean the boat won't have enough power?

6. Joined: Jan 2006
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Location: Lakeland Fl USA

Pardon my feeble attempt at humor. WHen I sad "way disappointed" it had nothing to do with power. In fact the boat will be much overpowered. Your power to weight ratio will be impressive. If you can get into the water at 600 pounds and you have two 75 HP jetski units, then the power to weight is 600/150 = 4 pounds per HP. Consider 60 to 70 MPH bass boats....At 1200 pounds with a 200 HP motor their ratio is 6. 4 is better for pure speed as far as power is concerned. The disappointing part is that the boat may not get more than 50 feet from the launch ramp before it dumps you. That is disappointing. The boat is very unstable as you have drawn it. At any speed above walking speed it is in grave danger of pitch poling ( Flipping end over end) Having enough flotation is only one of the factors that make a satisfactory boat. You must get some more length on the boat. I realize that a 10 or 12 foot boat attached to a bike will make street running problematic. This is a dilemma that you will need to resolve by sensible compromise.

7. Submarine TomPrevious Member

How about air pontoons that you inflate upon reaching the water?

8. Submarine TomPrevious Member

Oh 'ya, and an inflatable paddle wheel too...

Just kidding. A Rick W style flex drive prop to drop in the water would be good propulsion from the rear wheel via a friction

drive.

9. Joined: Nov 2003
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PARYacht Designer/Builder

How about a set of "scissor" pontoons, one on each side of course. They're "parked" along side each other on the street, then flipped open (rotated forward) to offer double the length, with the drive pushing the whole lot.

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BoilermakerNew Member

It Worked

Came across this post and had to giggle. Saw the show Dave and congrats on smokin the Bro. Look forward to upcoming episodes Steve

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