Stability calculations on RIBS

Dear friends, has some of you ever tried to make stability calculations on a RIB ?
I've currently done quiet a few test and calculations of stability for certifications according to iso rules, never had a problem on boats but have always the same problem on RIBS,after the inclining test conducted on the RIB when I make calculations to get the VCG of the RIB the result is always the same, a very very low VCG,probably due to the inflatables.
Any clue ?

 

I would expect VCG to be lower on a RIB than a comparable metal or glass hull, with the rigid hull being a large part of the weight compared to the tubes (you did include the air in the tubes?). Or is it that you are getting VCG's from the inclining lower than those calculated by the weight study? In that case, I'd start looking at deflection/distortion of the tubes/waterplane which will cause changes to VCB and waterplane inertia.

 

fede
We have performed several RIBs stability tests and calculations according to ISO rules, with lengths from 2,60 to 8,50 m. The effect of the tubes on the waterplane is significative and not all computer programs allow a good definition of forms. Tomorrow when at the office I'll have a look at what we have done and try to give you more detailed info.
In the mean time it would be good if you post the main characteristics of one of those you have done, with its VCG, just to compare with what we have done.
Cheers.

 

Thanks Guillermo I'll post the most obvious one, a 9 mt rib, that as from the test seems to have the CG underwater !!
It' not even a metter of software used, just basic math,done the inclining test,I was not there to attend the test in person, sombody else do the test and then send me the report on wich I make calculations.
Attached a basic RINA form that I use to find the basic datas of the RIB.
As you can see if you simply input the basic parameters the computed CG is ...underwater.
The form is based on the usual few steps taken to figure the r-a, pendulum movement->Tang H -> and then from that Msb/(mlps X tan H ) = r-a
Than from upright hydrostatics (KMt) KG is computed.
I guess there is no point that can be mistaken here.
Only thing that could be worng is the inclining test result itself, but then why this happen just with ribs ?
May be the tubes in the 3D model are not exactly in the same position of the real thing ?
No clue

 

3.391 deg inclination is kind of large for this small boat? Did the waterplane increase during the test (tube touching the water @ 3 deg)? If they gave you 3 points P/S you can check for linearity of deflections.

 

I concur Smartbight, maybe 3.391 is not a large angle, but something has thrown the wallsided requirement out of wack. Are you using the BM for the correct heel angle, or for zero? Is the trim the same as what is calculated for the stability curves? When inclining submarines, which are definately not wall sided, a large correction may have to be made for trim if it is different from the curves of stability.

 

Using BM for the correct angle,trim is the same reported to me by people that goes out to test,and then again...it just happens with RIBS.

 

sorry, fede. Had no time to check things at the office. I promise to do it tomorrow.
Anyway, we've never got a negative KG for the ribs we tested (around 15). Alaways reasonable results.
Although we experienced problems in the definition of forms with our NA programs at the beginning, because the hull has many nooks and crannies. One of them is the lower part of the tubes where they join the hull. Being the slope negative there (goes upwards) it may make some programs to perform crazily. At least this happened to us and we solved it by making those areas horizontal. Not highly precise, I know, but it worked and was good enough.
Another problem was related with the long prolongation of the tubes backwards of the stern, also difficult to interprete for the NA programs.
Cheers.

 

What are you using for weights, how are they attached/moved, and are you sure you included the inclining weights in the as inclined displacement (i.e. you show as inclined displacement = lightship displacement yet have 417 kg of inclining weights).

 

Thanks Guillermo, that's a good input,I will do that next time,I might get a wrong
KB/BM from the software and this could be an explanation.

jehardiman :I?m not doing the test myself but I can tell you that weights = persons
And yes I'm shure i'm including the inclining weight.