I believe it is the angle of the bow and stern. An extreme would be a plumb bow (perfectly vertical), such as what many surf skis have. This gives a lot of water line length, so good speed. But doesn't help you get over kelp and such. If you put an angle, or rake, in the bow or stern, it makes it so the boat can more easily get over things, but also reduces the water line.
Stern matters for the same reason. My old Looksha kayaks, with a vertical stern so they could put a rudder on it, wouldn't back up well in kelp beds and such, as the hull would catch on the plants. The Brit style boats I currently use have no problem with this, but need longer overall length to get same waterline length.
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