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prop sizing 6.2L Merc DuoProp

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allenkuhn

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[FONT=&quot]I am looking at replacing/resizing my current prop. I purchased a 2013 Crownline 285 SS bowrider with 6.2L MPI Mercruiser 320 hp. This includes Bravo III drive with 2.2 gear ratio and Mercury 22.5 pitch duo props. The dry weight is 5600 lbs. and wide open throttle around 5000 rpms on light load. The current maximum MPG is 2.5 in the 25-35 mph range (10-14 GPH). How much could fuel performance improve increasing the prop pitch? What reasonable limit would prop pitch cause too much engine 'lugging'? Any other recommendations on prop sizing/fitting? Apologize if this is incorrect forum. Thanks, Allen[/FONT]
 
allenkuhn said:
I am looking at replacing/resizing my current prop. I purchased a 2013 Crownline 285 SS bowrider with 6.2L MPI Mercruiser 320 hp. This includes Bravo III drive with 2.2 gear ratio and Mercury 22.5 pitch duo props. The dry weight is 5600 lbs. and wide open throttle around 5000 rpms on light load. The current maximum MPG is 2.5 in the 25-35 mph range (10-14 GPH). How much could fuel performance improve increasing the prop pitch? What reasonable limit would prop pitch cause too much engine 'lugging'? Any other recommendations on prop sizing/fitting? Apologize if this is incorrect forum. Thanks, Allen[
what is the wide-open throttle rpm range for that engine? I can’t recall if that year is the small block Chevy based 383 or the newer MerCruiser design 6.2?

If it is the small block Chevy based your WOT is 4800 to 5200 RPM so you would be propped perfectly. If it is the MerCruiser designed engine the RPM range is 5000 to 5400 RPM and you would actually want to go down in pitch to avoid lugging. Bottom line I don’t think you would see much improvement in fuel economy with either scenario
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It looks as if the previous owner of your boat already went up in pitch in an attempt to get more top end. You are now apparently at the bottom end of the specified WOT RPM range with a "light load".... You should be within the specified RPM range ( see decal on top of flame arrestor) when normally loaded (normal number of passengers and gear) and a mostly filled (if not full) gas tank in calm seas.
 
Capt Bob, thanks makes perfect sense - the WOT is 4800-5200 - sounds like I am there.
I might be more "frugal" but trying to maximize fuel economy - thanks !!
 
allenkuhn said:
Capt Bob, thanks makes perfect sense - the WOT is 4800-5200 - sounds like I am there.
I might be more "frugal" but trying to maximize fuel economy - thanks !!
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Only if you still rev to within that range with normal load of passengers and gear and full load of fuel... Typically, only one specific prop size works for most boats. Best way to optimize fuel economy is with use of a fuel burn computer/display and a GPS and find most economical RPMs, i.e. best miles per gallon..... beware on some boats this gets to be gallons per mile. . Typically this is somewhere between 60 and 80% of actual observed MAX WOT RPMs.... Best control of fuel efficiency is with your throttle hand :) .
 
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Thanks again, that helps. I thought the best fuel economy was just above getting on plane, lowest rpm's... but sometimes it seems its as efficient from above being on plane to another 500 rpm's.. maybe from the boat riding a little higher with less drag ? This boat seems generally in the 20-35 mph range it gets around 2.3-2.5 mpg ...always wishing for better economy, but on the other side, this does allow me to adjust trim, speed to give it the smoothest ride for the conditions at the time. Thanks !
 
I guess that you are on protected waters that don't get big sea conditions. When you're slugging it out with 6-8 footers into a headwind, you don't want a prop with a ton of pitch, you want one with more "bite" the keep the nose into the wind so you don't fall off and broach. If you only run in fair weather on flat water then a pitchier prop may be OK. Honestly, the only way to get the right prop is with measured-mile testing and several props to try. A good propeller shop can help immensely. They can get you the right prop and even tune one specifically for your boat. That will gain you a couple extra knots at the top end.
Since the Bravo 3 comes in many sizes in about 1" pitch increments, it is possible that your perfect prop is on a shelf somewhere waiting.
 
Thanks, yes - mostly protected waters - 3-5's is where I start thinking to postpone it for another day. (not to mention the family). The Crownline 285 at 28 feet is also designed more an inland water type boat. For open water I could see a Deep V, Flared bow would be a big advantage. However point well taken, if I go up some pitch, if I am caught in bad weather, I will wish I didn't :) Thanks again!
 
Your boat is considerably larger than mine, and heavier too. But i need to confirm O2beatsea's post.

Find a decent prop shop and try some different prop sizes. (I used AMPRO myself) Each boat is different, as well as each skippers expectations of performance. Common sense and what works for one boat, may not work the same on yours. I ended up going from a 3 blade to a 4 blade with less pitch and it made an incredible difference in how the boat handles in rough water at slower rpm levels. What amazed me was that i did not loose all that much top end speed..... But that's ok because i rarely run at WOT. I never actually checked MPG or GPM, but i run my boat at lower rmp levels now and the fuel economy is noticeable. I think it has a lot to do with how easy it comes out of the hole and planes out now. I don't need WOT to get out of the hole any more, and i'm not running it as hard....... I HIGHLY recommend finding a decent prop shop to help you find the perfect combo. I wish i would have done that as soon as i bought my boat and saved a lot of frustration..... Kirk
 
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