RIB buoyancy calculation according ISO-6185/3

hello designers and "wanbe" designers!
I would like to ask a question in those who have used this standard.

The "buoyancy of the rib" witch is involved in some basic equations according to the standard is (paragraph 3-3.4) :the inflated buoyancy and the permanent inherent buoyancy or at least two compartements of permanent sealed buoyancy ,fixed to the rigid hull, not exceeding 20% of the total buoyancy.

can you please explain that to me.

for example:
if we say a=inflated volume
b=the perm.inherent buoyancy or
the perm.sealed buoyancy
then a+b=T(total volume)
but b<=0.2T
so a+0.2T=T
a=T*0.8
T=a/0.8 which means the volume is 10/8 *a=1.25a

Am i wright?

thank you very much and sorry for stealing you time!

 

I think you should read it this way:

buoyancy of a rib is: inflated + inherent OR perm. sealed + inherent

and the condition on the permanently sealed buoyancy is that is should not be more then 20% of the total