What is the reason that different sources report totally different speed-wave drag associations for seemingly similar hulls shapes?
Compare the curves you are presenting with the curves for the kayaks http://www.oneoceankayaks.com/kayakpro/kayakgrid.htm - totally different relationships. Does the sharply cut vertical stern of beach cats make the difference?
Actually, they are not different. The kayak data have a range from 0 to 7kts, which is 0 to 3.5m/s. While the scale in my diagrams ranges from 0 to 15m/s (0 to 30kts). Until 7kts the curves a quite comparable. Sorry, I don't have a zoomed view of my diagram. After 8 kts the wave drag stays more or less constant, and I would expect the same for the kayaks. There is always a risk of taking wrong conclusions if data of limited range are extrapolated, in other words: If we step beyond our knowledge.
As long as the flow is not attached at the vertical stern, a boat behaves as a slightly longer one, but with slightly reduced friction drag. If the flow 'attaches' around the stern, you get a different source of drag, called form drag. If the sterns of your cat are 'sucking', you should sit more forward to avoid this additional form drag.
