# Bouyancy of air

1. Aug 19, 2013

### rkeene007

i'm building my own floating dock.
how do I calculate the volume of air I need to float a 1000 lbs boat completely out of the water?
thank you

2. Aug 19, 2013

### SteamKing

Staff Emeritus
Boats don't flote on air, they flote on water.

[STRIKE]And why are you shouting 'Buoyancy!!!'[/STRIKE] << OP's shouting removed by Mentor >>

Last edited by a moderator: Aug 19, 2013
3. Aug 19, 2013

### rkeene007

didn't mean to shout, just a cut and past thing

obviously the air would be contained in a tank or container of some sort.. just need to know how big a tank or container I need to have

4. Aug 19, 2013

### davenn

5. Aug 19, 2013

### BruceW

well... depends what he means. I'm guessing he means to use a big tank of air partially submerged in the water, with some kind of dock on top of this tank. In this case, the weight of the water displaced by the air would need to be at least equal to the weight of the dock (and the tank itself). Another big question is whether you can get this thing to be stable in the water.

Really, it is the same principle a boat works by. The boat has a wood or metal structure, but inside it has lots of air. And the upward force on the boat is equal to the weight of the water which is displaced by having that air submerged under the water line.

6. Aug 19, 2013

### Staff: Mentor

... and I guess you'd have to have a big air pump associated with the dock, so that the thing can be mostly submerged when the tanks/pontoons are filled with water (so you can slip it under the boat), and then pump in air to force the water out of the tanks to lift the dock and boat up out of the water...

7. Aug 19, 2013

### B4ssHunter

the weight of your dock is is almost 435 kg .
so you need to displace an amount of water that weighs 435 kg .
if a cubic meter of water = 1,000 kg
then you need to displace around half of a cubic meter of water * ofcourse you have to fill more than this incase of any temperature change in water , which could increase the density of water meaning tha t1/2 cubic meter of water will hold less weight than 435 , also you have to account for extra mass that might add up to the dock ), thats 1/2 a cubic meter of Air or any less dense material

8. Aug 19, 2013

### Staff: Mentor

The temperature of the water alters its density? Not by much, at least until the water freezes!

9. Aug 19, 2013

### B4ssHunter

if there is an increase in temperature of the water , wouldn't that cause an increase in temperature of water which would cause an increase in density ?

10. Aug 19, 2013

### Staff: Mentor

It looks like it varies by a few percent over the 4C to 100C temperature range. Probably not enough to matter in this floating dock application, IMO.

http://www.engineeringtoolbox.com/water-thermal-properties-d_162.html

.

11. Aug 19, 2013

### Redbelly98

Staff Emeritus
Technically yes, but the change would be less than 1%. So the submerged volume would increase less than 1%, and not cause the dock to completely sink -- unless it was designed stupidly and was over 99% submerged to begin with.
I'm thinking that lake water doesn't get hotter than 100 F, or about 40 C. So less than 1% change, from 40 to 100 F (4 to 40 C)

12. Aug 19, 2013

### B4ssHunter

thats what i meant to say
he should make such that not all of is submerged , i couldn't put it in your words though

13. Aug 19, 2013

### SteamKing

Staff Emeritus
The OP has not provided any details about his proposed drydock, so any speculation about how much air is required must remain just that, speculation.

There are several important issues to consider when designing a floating drydock or whatever the OP has in mind.
The first two are these:

1. Will the dock remain stable at all times during the lifting and lowering of the boat.
2. Will the boat remain stable at all times while it is being lifted or lowered.

Point #2 is not as simple to guarantee as one might think.

14. Aug 19, 2013

### rkeene007

I'll do my best to explain the goal.
Where I live has restrictions as to how big my dock can be, how high, etc.
There are no restrictions though on how many boats we can have.
Since here in Florida it's best to keep boats out of water when stored because of veg and little critters like to attach and grow.
Basically, I can remove the axle and wheels from a typical boat trailer then mount air tanks or containers under the frame in place of the axles.
My idea was to have a hole on the bottom of the container towards the stern of the boat.
The front of the trailer would be hard mounted like it would be on one's car.
Towards the front, top of the containers would be where I would attach the air line.
My thought is that as air is pumped into the container, they would fill with air, forcing out the water thereby raising the boat on the trailer bunks until out of the water.
When I wanted to launch the boat I would simply release the air, water would flow back into the tanks slowly lowering back into the water to the point the boat floats on it's own.
Trailer structure is aluminum and securing to "guides" is not a problem to keep it from drifting away.
So, I'm at the point where I need to find out "how much air" I need to contain to go any further.
Looking at salvage bags I was surprised how effective air bags are... I just can't figure out how much I need.
Thanks to all so far for attempting to help me figure this out... I think it will work, but I'm not sure

15. Aug 19, 2013

### rkeene007

the trailer and tank rig will remain in the water

16. Aug 19, 2013

### B4ssHunter

1 cubic meter of air would be enough i guess

17. Aug 19, 2013

### rkeene007

in reading about buoyancy in wiki what I really need to do is figure out the volume of 1000 lbs of salt water and replace that volume with air?

18. Aug 19, 2013

### B4ssHunter

yes , but beware if you only do this , your dock's air chamber will be totally submerged in water , so you get this quantity and you multiply by 1.5 or 2

19. Aug 19, 2013

### B4ssHunter

also beware if you over fill it , not only it will displace an amount of water to carry the weight of the boat , but it will also displace an amount of water to displace a great part of the air sac , which could lead to the ship flipping over , i will do the calculations for you and i will post them later today

20. Aug 19, 2013

### rkeene007

good point, I hadn't thought of that.
i'd probably do best if I used two containers mounted port and starboard... like outriggers I suppose to reduce chance of tipping.... thanks