New technology :
Loadmaster
It's not just a case of throwing it all in the hold and hoping the ship dont sink, you know! In your Academy each of your captains are instructed in the Dark Arts of the new science of Tetrisology : they learn how to squeeze the last crate into those odd-shaped empty corners : additional lessons in calculating such wonders as 'Centers of Gravity' and 'Load Distribution' ensure an even keel is maintained at all times. Thus with your officers educated and fully certified, each level of research will add 1% to your vessels cargo capacity.
Cost should be comparable with Galleons technology research.