[Bushwacker]

Offhand I'd say that's pretty good performance for a hull with that much deadrise with a set of nicked up props! Very impressive that you can plane at 12 mph, as many boats with 25 degrees of deadrise can't do that! I always thought that hull might need a big block engine for good performance, but that small block/duoprop setup is lighter and very efficient, so it sounds like it's a good match. By the way a friend of mine named Herman Johnson had a Seafari 25 and lived in Palm Beach Gardens - any chance you bought his boat?! We must live pretty close; we ought to get together some time!

I'd take those props to Bobby Soles and ask his advice; he might be able add some more cup to them to get the rpm down. Best to tweak them to determine right dia/pitch before you spend big $ on a stainless set! You might also want to keep that aluminum set, as they sound ideal for heavy loads. As Fr. Frank says, most folks end up over propped because they don't test the boats with the loads they finally end up running! 5100 doesn't sound like a lot of rpm for a small block Chevy, but maybe they're worried about the valve train. Those ball-stud rockers with the pressed in studs are the first thing most guys change when they try to soup up a Chevy.

Vacuum numbers with no load on engine aren't particularly meaningful. Main thing is that vacuum is steady at idle; 13" at idle sounds a little low, but that's a function of cam lift, duration and overlap. The fact that vacuum picks up so much at 1000-1500 is further indication of fairly aggressive cam timing. The 5.2L in my truck runs about 19" at about 600 rpm idle, but it's only 235 hp with a very mild stock smog motor cam.

The only comparison I can give you on speed/vacuum is from the "Unohu", a friends old SeaCraft 21 when it had a carbed Mercruiser 350 Chevy/Alpha 1 outdrive. I believe it was rated at about 260 hp with the gear ratio he was running. He used to cruise at 20 kts/~23 mph at about 9" vacuum (with very heavy Bahamas-cruising type load), and I believe top end was around 45-47 mph; used to average about 2.5 mpg. I think you're doing quite a bit better than that! He later had a carbed Volvo (non-duoprop)/350 Chev in it rated at 270 hp, and he claimed it would run 50 mph, but I believe it was jetted richer and he indicated he lost some mpg.

Regarding optimum cruise rpm, IC engines are generally most efficient, as indicated by minimum specific fuel consumption (fuel flow/HP), at the point of maximum torque. If you can find out the max torque/rpm rating that would be a starting point. Maximum mpg of the installed engine/outdrive is a little different and involves a lot more variables. Installing a flowmeter is the best way to nail down optimum cruise settings; the modern systems that combine speed with fuel flow with speed will calculate instantaneous mpg and can even help you optimize engine trim and trim tab settings, in addition to tracking total fuel burn. The next best way might be to look carefully at the vacuum/speed relationship. I'd suggest plotting vacuum vs. speed and then looking at inflection points on the curve, after you're up on plane. You're looking for the combination of highest speed/highest vacuum. Large speed increases with a small vacuum drop indicates you're in the optimum speed range. When vacuum starts dropping faster than speed is increasing, then you're running faster than optimum. (Caution: this approach is only valid above about 5" vacuum; on most engines the carb or FI system will dramatically increase fuel/air ratio or richen the mixture at or below 5", so you'll see big increases in power and speed for small decreases in vacuum once you get below 5". Part of this increased fuel flow is to help cool the exhaust valves, because lean mixtures burn very hot!)
Denny