For the sake of clarity, let's assume you're operating a boat with inboard power, either straight or V-drives. Shafts, struts and props come out of the hull at an angle, although some more than others. Therefore the props will push water down, pushing the boat up at that same angle, either at the engine or V-drive location. Thus trim tabs are necessary on planing and some semi-displacement hulls. A good hull design will incorporate lift characteristics, which will determine how much tab is needed.