As a data point, my Islander 30 MkII will move at 6.4 kts (multi direction pass with GPS average) at about 10 KW (~14HP). Transom drops at least a foot with large (by sailboat standards) trailing waves. Roughly speaking power should be proportional to square of speed up to hull speed, which is close to 6.4 kts on my boat. By that formula, 4 kts should cost me about 3.9 KW.
Islander 30 MKII "Sojourn"
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