It is not impossible that you could be correct and the aft exposure could be an issue.
However, again, the amazing thing about the JSD is the way that it minimizes such impact by keeping the boat going forward and allowing the wave to lift up the boat.
In other words, if you are going to be hit by a 100 mph baseball, would you rather be traveling 30mph away from it, or 30 mph towards it?
I remember seeing some great test video in a tank or something, or maybe it was just a graphical representation that demonstrated this concept.
Do you remember that catamaran that was abandoned on its maiden voyage? They took a wave from the front that stopped it in its tracks and even pushed it backwards. If I am recalling correctly, this bent the rudders and loosened the front window.
I would much rather take the punch from behind while I am moving forward away from it than be travelling into the punch or not travelling at all.
Also, keep in mind that the bigger the wave, the faster I am going to be going away from it. And, the faster I go, the more force the JSD will be applying to slow me. Seems like the optimum balance.
I also remember reading that if one is going to have a problem with a JSD, it will be from it being too strong and not letting the boat have suitable forward speed.
Anyway, does anyone know of stern issues actually caused by breaking waves with a JSD deployed?
I find this discussion very interesting and would like to point out the following.
In monohulls typically with a knock down the port lights on the up wind side (not the one immersed) blow out. It's the deformation of the house and internal forces that cause this. One would expect the large open houses would be exposed to similar forces by large boarding seas.
The forces generated by boarding seas can be extreme. I know of empty davits being severely bent by pooping and have had moderate boarding seas destroy well constructed dodgers. I have a JSD and hope to never use it in anger. I do not expect it to prevent damage to the aft sections of my and any other boat mono or multi if those regions are not constructed to allow for such a occurrence. I think with all due respect jzk is being over optimistic. Yes current port light glass is much stronger then in the past. Yes cored houses are much stiffer. Still, such large houses as seen on multi's will have much greater forces applied. They do not have extensive internal bulk heads spanning the large interior open spaces. Without thoughtful design I believe they are potentially subject to sufficient damage as to compromise watertight integrity allowing risk of down flooding. Returning to original question "What current multi's are safe offshore boats?"