The most important thing is getting the sterns so that they are not dragging - it's like sailing with the brakes on, and the 18 seems to like dragging the sterns (at least mine does). Waterline length certainly is a consideration. But in this case, if the difference was between having the bows free of the water or not, I'd say the waterline length was the issue, but it's the difference between the bow being a third in the water or two-thirds in the water - same waterline, more of the hull in the water, butt not dragging...