Originally Posted by Trimix
Measure the capacity of the inside of the RIB hull - that will give you the weight of water it will hold. (Volume in liters)
Measure the capacity of air in the tubes - thats what will offset the water in the hull if it floods.
If the volume of air is larger, then that will offset the volume of water and you will float.
Not exactly, as the water in the rib would be not part of the mass, as the deck will be open, and at the transom the water will come and go.
As a rule of thumb, find out how much your boat weighs when out of the water, and for every tone you will need approx 1cubic meter of tube volume.
A circular tube of 500mm dia has as near as damn it 1 cubic meter of air in it, so will support 1 tonne of boat, irrespective of the boat being holed (or bilged to use it's technical term) as long as the tubes are intact they are reserve buoyancy.
To look at it in stability terms, your hull is not watertight, as it is open at the top, and the shape keeps it afloat when it is empty. Fill it with water so it overflows, and the tubes only need to hold up the weight of the hull material, engine and gear.
To get technical, you also need to see how much your fibreglass displaces, so if you know what the manufacturer says is the weight of the hull and tubes, you can work out how much it will need to keep it afloat.
For example, my OP 6.5 M according to Humber weighs 600 kilo, just hull and tubes.
Now fibreglass has a density of 2 tones per meter cubed so that hull will take up 300 liters of physical space, which will give 300 kilos of lift when submerged. Therefore to stop the whole lot going to the bottom you would need the tubes to hold only 300 liters of air, and completely flood the hull, and it would still float.
Add all the extras such as engines, tanks, batteries, gear etc etc. and you need extra buoyancy.
I know my boat weighs in at around 1500 kilos, and with 300 kilos of hull boyancy, I would need only 1200 liters of air in my tubes to stop it going under.
The tubes are 500mm, so I need a total of about 6 meters of tube to keep it afloat, even if I took a big drill and cut holes all through the hull deck and transom, it would still float.