Floatation / Displacement formula

[Andrew Leigh]

Hi,

rather than re-invent the wheel and make costly mistakes I was curious to know if there is a formula to aid with the design of pontoons.

There is an annual charity raft race that I wish to enter. I want to make my raft from two pontoons and would like to know what size these should be for a given mass.

Any takers?

Cheers
Andrew

 

[deejaycee_2000]

Re: Floatation / Displacement formula

I use a formula when constructing my pencil pontoon inflatables .... but it is a bit different from what you wanto construct here, what is your pontoons made out of and how many people should it be able to take?

 

[Andrew Leigh]

Re: Floatation / Displacement formula

Howzit DJC

It's for the Tugela River Raft Race in April next year.

10 Adults max per raft, that's the rules. No rudder allowed.

Assume that the average mass of the adults is 110kg, that will include all the things they would need to board with.

Pontoons would either be steel or PVC tube, I think.

Most the boy's use a flat piece of polystyrene and they race down for the win. Mine needs to cruise down, powered by beer swilling, overweight, middle aged men.

Cheers
Andrew

 

[Paul Moir]

Re: Floatation / Displacement formula

It might seem an odd way to do it, but I think the Canadian Small Boat Construction Regulations will provide the formulas you need.

Construction Regs

Now the idea here is a stable platform. If you want something that will flip (for added fun and excitement) you'll probably want to go outside those rules.

 

[JB]

Re: Floatation / Displacement formula

How much does water weigh? Around 64lb/cubic foot?

You calculate what that is in Kg per cubic meter. I am not sure of the weight of water and my gonkulator is out of reach.

What does your loaded, ready to race, raft weigh?

It will have to displace water of equal weight with about half of the volume of your pontoons.

Bon voyage. 8)

 

[steelespike]

Re: Floatation / Displacement formula

To keep it really simple I figure 8 lbs per gallon.
55 gallon drum half submerged is 216 lbs.30 gallon drum
half submerged 120 lbs.If your using some sort of pvc tube fill a section with water and see how many gallons.
Primative back yard science but effective.

 

[Andrew Leigh]

Re: Floatation / Displacement formula

Thanks guys,

Yup all the info is good and roughly correlates. About 10 44gal US drums will do between 1000 and 1200ks total mass. Will err on the safe side, would hate to sacrifice a crewman for a couple of crates of beer:8)

Cheers
Andrew

 

[deejaycee_2000]

Re: Floatation / Displacement formula

I see you are from Johannesburg, so it is easyer for me to explain in South African terms, I built one for a race once with a braai (BBQ) and builders toilet on (to be funny), obviously we didnt' go for the win cause we threw some steaks on the coles with loads of booze ... guess you also going for the same thing ... I used 9 plastic drums in 3 rows of 3 (those huge 200L blue or green ones you can get from industrial soap manufacturers) and about 10 wooden pallets and 8 support beams to keep everything together, we were about 12 people on and it was fine ... i found a pic of my floating jetty, and my raft was built on the same concept, I circled the plastic drums so you can see ....

 

[deejaycee_2000]

Re: Floatation / Displacement formula

oh yes, the drums are tied down with strap (safety belt strap)

 

[bjcsc]

Re: Floatation / Displacement formula

Easiest way to think about it is metric. 1 cubic centimeter of water = 1 milliliter of water = 1 gram of water. So for every kilogram of weight, you will displace 1000 cc's of water.