rhino/righting arm data

[3dig]

hello!

I having trouble understanding the the righting arm curve graph output by Rhinomarine 4. Am I tasked with determining how far a particular hull can heel without rolling completely over. Would this not be the righting arm curve peak? In my case @ 30 deg.(max stability?). Any help would be apreciated. Cheers!

[Crag Cay]

No, the value you are looking for is the 'Angle of Vanishing Stability'. That is the angle at with the 'righting moment' of a heeled boat becomes zero. The 30deg value you quote is likely to be the angle at which this 'righting moment' is at a maximum.

I'm not familiar with the output from RhinoMarine, but you are either looking at the righting moment curve or the righting lever curve. (It doesn't really matter as the moment is only the lever multiplied by the displacement.) If you continue to heel passed your maximum you will see the value of the graph then starts to decrease. The tendency of the boat is still to right itself (ie the heeling moment is still positive) but its willingness to self right will be less and less until some angle at which its self righting tendency has become zero (ie, the graph has returned to the zero x axis).

Beyond this point, the heeling tendency is negative which means the relationship between the centre of gravity of the boat and its immersed centre of buoyancy will combine to continue to invert (capsize) the vessel. The heeling angle at which the graph becomes zero is the AVS.

If you are talking about an open boat or one with openings in the deck which will down flood, you need to add this information to the analysis as it will radically affect the behaviour of your vessel.

[Raggi_Thor]

Have you added all the weights, or calculated a cg?
Then you are looking for the angle where the rightening arm or moment is zero, as Crag wrote.

[3dig]

Much thanks on the speedie and detailed response!
Hull has been modeled, CG located.

I will post some results asap.

[3dig]

Does the curve look as what might be expected? Hull is a 30'x 10' "Cat".
What determinations can be made from the output?
All comments welcome.

Thanks!

[Raggi_Thor]

The shape of the curve is quite normal, the values is hard to comment on.
50 degrees sounds reasonable, but that depends on your loading, of course.

[tananaBrian]

You need to re-do your RhinoMarine analysis and this time, enter some critical points along the inner edge of your sheer deck (or anywhere else that water will enter the boat as it heels). If you do that, then you'll get a curve more like the one that I show below (assuming my image posting works ...first try here.) Note how the righting arm (GZ v. Heel) is truncated at about 60 degrees for the boat illustrated. To be honest, I'm surprised that your 10' wide cat's curve is only going up to 50 degrees. The beam is appropriately wide for a 30' boat though, so I suspect your freeboard is a bit low. What's your transverse metacentric height (GMt)?

Brian D

Originally Posted by 3dig

Does the curve look as what might be expected? Hull is a 30'x 10' "Cat".
What determinations can be made from the output?
All comments welcome.

Thanks!

[tananaBrian]

This is a very important step as well. You can use RhinoMarine to keep track of your component CGs (and whole-vessel CG in (x,y,z)) or you can use a spreadsheet. I find a spreadsheet to be more valuable since I produce separate tabs for each subsection of the boat (according to my slightly-modified US Navy format spreadsheet.) If you have not accomplished a very accurate and detailed determination of the boat's CG, then the curves and analysis mean nothing. Design is an iterative process and can be slow, e.g. assume a CG and design the hull form (meet desired parameter values, form, etc), then accurately find the CG, then adjust the hull form, repeat. When you think you are done, then do a hydrodynamics and dynamic stability analysis and go back to square one if you need to. You should repeat your analysis at a) light ship loading, b) design load (average displacement), and c) to 115% of full ship (maximum design) loading as well. Take your time and have fun.

Brian D

Originally Posted by Raggi_Thor

Have you added all the weights, or calculated a cg?
Then you are looking for the angle where the rightening arm or moment is zero, as Crag wrote.