VERY inefficient, burning 16-18 GPH!!! Her most efficient speed is full out (mostly) 24 kt at about 4000-4200 RPM burning 20 gph (all by GPS and Flowscan), which is usually impossible to do offshore. Also, when seas are rough, I run about 14-16 kt, turning 3200 rpm, with the boat dragging her tail. What about 4 blade, would that be more efficient? Looking for range, mine's pretty poor, about 1 mpg. Lightly loaded in calm water she'll do 25 knots at 4400 RPM, and she has more throttle left (I've had them up to 5500 RPM) No room for any more diameter, prop shop suggested 20P, maybe with cup. She has twin Crusader 270s turning 17D x 16P wheels through 1.91:1 Velvet Drives. MMD, if you don't mind, what does your program call for for a Chris Craft 26' flybridge, 10' beam, 9'4" on the waterline, 8500# dry, 10,500# loaded. You may want to look at a higher-ratio gearbox to allow a slower rpm, bigger diameter prop, if there is room under the hull to fit it. Talk to your engine/prop supplier before you commit to the engine/gearbox combo. This gives a prop tip speed of 262 feet per second (178 mph), which is a bit high. To max out the new engines, you will need a prop of around 12D x 10P. This is optimized for cruising speed, so the engines may be overloaded if you try to max the throttles. I don't have any performance graphs of the Mercruiser 320 engine handy, but assuming that 250 hp x 2 is required for 27 knots, and assuming that the Merc 320 develops 250 hp at 80% throttle (4000 rpm) and the gear ratio is 1.01:1, then a 3-blade prop dimension of 14D x 12P would be in order and a 4-blade prop would be sized at 13D x 11P. I'd do some searching about (try contacting SeaRay) to determine if this hull runs well at that speed. ![]() ![]() Swapping the 225's for 320's should put the max speed at around 31 knots, which may be on the upper limits of controlability of the hull. Extrapolating this out to twin 255's at full throttle, the boat ought to max out at around 26-27 kts. ![]() It would also help if you could post the waterline beam and hull draft.Ī quick check using my proprietory software indicates that the boat at 22 kts is running with about 2 x 150 hp. You haven't told us what is the maximum prop diameter you can put on the boat (I assume that when you refer to the engines as "straight inboard Mercruisers" that you mean engines in the belly of the boat connected to the props via shafts, and not inboard/outboards with the engines mounted at the transom under the aft deck and directly connected to the exterior drive leg), which is a primary selection factor. Generally, the higher the prop rpm the smaller the prop diameter, and the less efficient the prop.
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