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. . . Haven't fixed the props yet, but I think I know what I want. Same engine F5s(stainless) was 4600 rpm in the Seabird. These are D2s (aluminum) and turn 5050 loaded. The volvo book says 200-250 rpm for each size larger in duoprops and go one size larger for equivalent in stainless. That gives me a F4 which should give about 4800 loaded. Max rpm range 4600-5000 on this engine. I'll have several conversations with prop guys first, but thats what I think for now. Does being in the middle of the max rpm range with the boat loaded close to the max its likely to carry make the most sense?
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Connor,
I'd get those aluminum props fixed and run them again before making a final decision on SS props. As for rpm at Max load, being at the middle of the range is probably pretty close. High end of the range is probably easier on the engine, low end may give a little better mpg, so splitting the difference is a reasonable compromise. I know Fr. Frank says to prop for Max rpm at Max load. I'm sure that's true for 2S outboards that run needle bearings, have no valve train to worry about, and love to rev.
Lugging an engine (high load at low speed) increases cylinder pressures and temperatures, which increase loads on pistons, rings, bearings and valves while the low speed REDUCES oil/coolant flow and pressure, so it's generally better to let 'em rev! It's probably true for 4 strokes as well, but they can have valve train issues (resonant vibration that can wear or even break valve springs, dampers, studs and rocker arms if you spend much time at a critical speed). 200 rpm isn't a big deal, but I'd try to find a Volvo engineer, and ask him if you should prop it for 4600 or 5000 at max load, and why. If there is a critical speed, say 5100-5200 you'd like to know that too!
[/QUOTE]One thing I don't understand, maybe someone can shed a light. The volvo book says most efficient cruise speed is 15-25 percent below max speed at max rpm. That results in real high rpm that the vacuum gauge says is too high and looks too high to me as well. Any comments?
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That 15-25% recommendation is a very general statement and not necessarily right for a specific boat. It should probably say MAXIMUM cruise rpm, not the most efficient speed! You're right, the vacuum gage is a much better indicator of best efficiency, and I'd guess 3750-4250 rpm is pretty close to 5-7" vacuum or max continuous power for that engine, if it had such a rating. If you're gonna take that boat to the Bahamas, you'll be burning enough gas that I'd spend the money up front to get either a Floscan system or the Volvo equivalent of the I-Command/Smartcraft system if there is one. It'll eventually pay for itself by helping you to always run the boat at optimum trim, plus it's good to know exactly how much fuel you've got left and not have to depend on a gas gage that may not be very accurate!