It's more complicated than that. The boat is going to use x amount of horsepower to go y fast. It doesn't care what rpm that is. The engines will use x amount of fuel to produce y horsepower. What you want is to produce the amount of horsepower required to go y fast as efficiently as possible. The term for this is BSFC. Best BSFC (brake specific fuel consumption) numbers are usually at or around the peak torque RMP. Where that is depends upon your camshaft, but with a stock marine cam it is probably near your current cruising RPM. You want your boat propped so that the best BSFC number is the rpm where you cruise at, assuming your hull and transmission ratio allow it. I had a camshaft custom made so that my peak torque comes in @ 2500 and the torque curve is very flat, particularly between 2500 and 3500 rpm. I'm propping it to cruise at around 2800-3000 rpm or so, even though the way I built the engine it achieves peak horsepower @ 5000 rpm. I think the torque peak is more important than the horsepower peak and the torque curve is what I focused on when I designed the engine. I expect you're propped correctly, assuming your engines are stock Chrysler Marine Engines. With those fuel flows you're running twin 440's?
John --- On Sat, 11/22/08, Rocco <[EMAIL PROTECTED]> wrote: From: Rocco <[EMAIL PROTECTED]> Subject: [UnifliteWorld] Prop sizing To: "UnifliteWorld" <[email protected]> Date: Saturday, November 22, 2008, 2:25 PM Here it is again, the ever so often discussion about prop sizing. I've read lots of info on this subject but I can't find the one piece I need. My boat is propped properly - so I'm told - and achieves 4600 RPM @ WOT. However, except for the occasional whoppie moment when I run it up to full speed - 26KTS and 60GPM - I consistantly cruise between 2800-3200 RPM. At these RPMs the boat cruises 12-13 KTS @ appoximately .5/gpm. I think my hull is very efficient considering its top speed. Speed goes up quickly above 3200 RPM but then secondaries open and the fuel consumption rises even faster. So, what's the problem with over-propping the boat so WOT is say 4000 RPM or so? If I continue to run between 2800-3200 RPM won't I get and increase in speed which will result in better mileage? I know the engine will be working harder at the same RPMs but there should be some gain in gpm. no? --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed to the Google Groups "UnifliteWorld" group. To post to this group, send email to [email protected] To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/UnifliteWorld?hl=en -~----------~----~----~----~------~----~------~--~---
