Following up on Capt Tom, who gave you the formula for hull speed
. Above that speed your transom is no longer supported by your first wave past your bow wave, so you are climbing the bow wave but not planing. That's horribly expensive and produces a monster wake. We're diesel
, but at 42' waterline and cruising at 7.5 knots, we get 2 NM/G, or just under 4 gallons per hour while pushing 60,000 pounds. Stay under your hull speed
, and fuel costs whatever your engine will not be a problem. I've met a sport fisherman skipper
who plowed the Tennessee-Tombigbee, Mobile Bay, to Carrabelle (our base) doing 1,800 miles and spending $25,000 in fuel. He still had to wait for the locks....