Buoyancy Calculations

[zenrat]

I've been working out how big floats need to be to support an aircraft.  I think i've got it but I just need to check i'm thinking about it correctly.
Please tell me if i'm right.  I've tried to be as clear as I can.

My thinking.
For an object of a given mass to float it needs to displace a greater mass of water.
So for a 12,000 kg (max take off weight - assume that includes the weight of the floats) aircraft to float it will need to displace at least 12,000kg of water.
Taking the density of water to be 1,000 Kg/m³ that means the aircraft in question must displace at least 12 m³ of water.
So, if the 12 Tonne aircraft has two floats of 6 m³ volume each it would sit in the water with the top of the floats awash (the correct nautical term for level with the surface of the water I believe).
And if I increased the volume of each float by 1/3 then the top 25% (depending on the shape of the floats) of each float would be above water level.

Is that correct thinking?  It make sense to me.

[PR19_Kit]

I reckon you've git that dead on, it equates pretty much exactly with the sort of calculations I did when I was into serious model boat designing yonks ago. I doubt Archimedes Principle has changed in the intervening years. 

[kitnut617]

Your displacement should equal the volume of a float/hull to your proposed waterline, so whatever the max weight of the aircraft is, has to equal the displaced water

[zenrat]

Thanks folks.
Kitnut - you put it much clearer than I could.  I know what I wanted to say but couldn't find the words.

Now I know what i'm starting next - a torpedo armed Hind on floats in a navalised variation on this Peruvian camo scheme.

I assume on a helicopter the centre of the rotor will be above the centre of gravity allowing me to locate the centre of my floats?

[PR19_Kit]

Absolutely, single rotor choppers are very CG sensitive so it has to be like that or they turn over rather quickly.

One reason why Boeing sold so many Chinooks, it's nowhere near as big a problem with them.

[zenrat]

And there off - Floats started and gunners ordered from Elhiem...

[sandiego89]

Ahh, since you are making floats for your Hind, they might be more like the simple pipes on a pontoon boat, rather than the more complex shaped floats on an aircraft than needs to get up to planing speed.  To double check you calculations, here is a handy chart for pontoon boats showing the load capacity for different size pontoons when both pontoons are halfway submerged:

http://u-fabboats.com/brochure/weightcap.pdf

Bring on the Hind!

[PR19_Kit]

Think Bell 47 floats maybe?

This one's even in Oz.