flotation

[daedong]

I am building an aluminium boat and I am trying to work out the volume of flotation material needed. I am looking for advice on formulas to calculate this.

The boat weighs about 800kg (hull) plus outboard motor of say 200kg plus say 4 people @ 80kg, fuel etc. another 300kg.

[jonsailor]

In a basic nutshell, 1 cubic meter of vloume (air or foam etc) will support 1000kg of weight. The imersed weights of materials varies accordingly in salt and fresh at different levels.

Someone else will have to help you with cubic feet to pounds of bouyancy as this is not my unit of measure

[lewisboats]

Browse thru here, the calcs according to the USCG. Common sense says use 2.75 cu ft per person plus 1 cu ft per 60 lbs of boat, motor and gear. Foam comes in various densities...so calculate that in too. Eg... factor in the weight of the foam in the overall wt of boat, motor, gear and persons...figure 170 lbs per person. USCG uses 145lbs, unrealistic nowadays! Oh...and 1 cu ft of fresh water weighs 62.4 lbs and 1 cu ft of salt water weighs 64 lbs...so the same weight will float slightly higher in salt water than in fresh. Everything weighs less in water than in air because of the bouyancy of water but if you calculate for air weight you will never go wrong.

Steve

PS: according to your numbers...65 cu ft or 1.81 cu meter of foam would be about right. This should be distributed with consideration to the densities of the various components of the boat...put sufficient around the motor to support it and the rest distributed where needed, paying attention to keeping it as high up in the boat as possible.

[alex fletcher]

As I see it. the real question in Boat Design Terms is of Bouency or Displacment but as all engineers Know Bouency cant be considered on its own in Boat Design A consecpt known as Vessel Stability is used. This not only looks at how deep a boat floats but looks at the way it will float and Its tendency to roll over.
You will need to Find the Displacement of the hull(how much water it displaceces), Centre of Gravity of the hull(the point the hull would spin around if spinning in the air). The Metacentric Hight of the Hull(a point that if you where to hang the boat upright on a sting would be the point where you would attach the string to the hull) The higher the Metacentric Hight in relation to the centre of gravity the more the vessel will want to stay upright and the more water the hull displaces the higher the boat will float
Hope it helps Alex

[Ike]

To calculate the amount of flotation required you need to know the weight of three things.

1. The weight of the hull and any stuff like fittings, attached to the hull,

2. The weight of the people

3. The weight of the engine, controls and battery.

Each of these weights has to be multiplied by a factor depending on the material to account for their density.

Then the amount of flotation is calculated by using 62.4 pounds per cubic foot for the weight of salt water and 2 lbs per cubic foot for the density of the foam. That is, one cubic foot of foam will support 64.4 pounds minus the 2 pounds the foam weighs = 62.4 pounds.


Also the Coast Guards Boatbuilders handbook has an explanation at http://www.uscgboating.org/safety/bo...on_landing.htm Click on flotation

Go to my web site at http://members.aol.com/spinners/ and click on flotation, or downlad the pamphlet at http://www.uscg.mil/d8/mso/louisvill...bp16761_3b.pdf

On my site I have fill in forms you can use. If you want a more detailed explanation send me a personal message.

[daedong]

Thanks for the help fellas, I am thinking of using coke bottles, calculation is already done on each bottle 1 litre =1 KG. Well not really but thats what a mate told me to use. That would be a lot of bottles.

[alex fletcher]

Be carefull where you put your Coke Bottles or you could end up upside down in a flood
Oh, and Coke Bottles are Designed to appear as large as thay possibly can make them (product Design).

[Ike]

Cokes bottles will work. Plastic bottles that is. This reminds me of a boatbuilder that used plastic milk bottles. It would have worked except they had metal caps and the caps rusted through. I've seen beer cans, ping pong balls, rubber bladders, and other clever methods. Have you seen those long circular foam toys that children use in pools. They have been used for flotation too.

Make sure you glue the caps on so they are sealed. You will have to figure out how many pounds of flotation each bottle provides.

[Raggi_Thor]

The gas stations are full of empty windshield wiper solution bottles, approx 4 liters.

[rayaldridge]

Originally Posted by Ike

Make sure you glue the caps on so they are sealed. You will have to figure out how many pounds of flotation each bottle provides.

Couldn't you just fill one with water and weigh it?

Ray

[Raggi_Thor]

I am glad I live an the metric world :-)
One litre of water is (very close to) one kg.