Bulkhead increase your Structual Str.
One fact is that when your DP drops below a point at about 30~40%(it depends on
your engine), you'll be unable to maintain speed. With bulkhead the point will be
postponed.

IJN armor isn't good because the Defense/Weight ratio is the most inefficent.

For example, if there is a UK CL and a IJN one both with 1000 free displacement and both
are trying to fit the maximun armor (assuming money isn't an issue here), the UK CL will
probably be able to get twice more defense from armors than IJN(the ratio isn't corect,
but you get the idea). IJN has the worst ratio in the all of the nations, that's why no
matter how much displacement you get, you will mostly out-armored by other nations.

I don't know if bulkhead has HE defense or not. However, having more bulkhead can
certainly increase the crew protection in a noticeable way.

well, this is a new forum....all the old topics got killed :(

anyway, belt and deck basically substracts damage from HE damage, until each shell have an
probability of doing 0~50 damage. It does not deplete. To get good protection, you need
thickness of 1/3~2/3 of the caliber of gun you are facing, more if you are IJN. One thing
to note is that if you have armor, it can sometimes "bounce" shells from an specific angle
(usually ~10 degree deck hits, for example) resulting in 0 damage, and this can result in
an armor immunity zone where you are basically immue to enemy fire from an certain range.
IJN probably can't do that against similar classed ship at any point in the ship tree
though.....

bulk to mostly nothing, other than perhaps lowing sailor death....

that just the point, i havent been able to find any....

read the bazillions of posts on armor. It block some damage from hitting you! Bulk
saves ur sailors...