The rigging. A bit tough?

[Elmar Bijlsma]

I used broadside in the manner of all guns on a side able to fire, not the firing mode of ETW.

[Gaiseric]

I've noticed the same thing. Multiple small ships attacking the same, larger target, seems to take out the rigging quicker.

[antisocialmunky]

Your guns all become synced if you use broadside once. There's no reason to do one manually after that.