How to obtain the maximum righting arm of a righting arm curves？

It is unlikely, quite improbable that GZ curve has a flat area values. Unless you understand as "stability curve" anything else.
A double humped curve stability can exist but is not "very common". It occurs mainly in ships with a forecastle deck that encloses much volume.

 

Oh，I forget the “ double humped stability curve” . Thanks

 

I meant to say "not very common"

 

Actually, when you accurately plot a stability curve, it can have a number of "bumps" in it. Typically when the side deck submerge, a flattening on the curve occurs, then when the cabin or deck structures start to submerge, the curve spikes up again. Depending on how you calculate the curve (many don't include deck structures) and the configuration of the deck, you can have a number of bumps.

In small craft design, the curve is typically hull only, through 180 degrees. This is how I do small craft, knowing the actual AVS may be a bit higher, than the calculations show, but an error toward the higher side isn't a bad thing. On larger craft ,the deck structures have to be included, but not so much on a 22' sloop (as an example) with a small cabin on it's foredeck.

 

ha ha

 

Thanks！
It is useful for me