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Propeller performance

pke94

New member
"Hello all. I'm a long-ti

"Hello all. I'm a long-time lurker and first time poster.
hopefully I can get some help as i am in quite a bind. I just spent a bunch of money on two new propellers and they seem to have slowed my boat down by about 10 knots. I am currently running twin v6 4.3 liter mercruisers to bravo 2's.
My local propeller shop told me my current propellers were 18.25" by 19 degree pitch and were unrepairable. He said I should achieve 250 more rpms for every deg. of pitch I go down....(I was full throttle max at about 4200 rpm) so, he set me up with a 18.75" 17 degree pitch propeller. these props are cupped as my last ones were (im not sure as to how much cup variance though ..make a difference?)They are also alluminum as my last ones. I should have achieved higher rpms but I am revving the same if not a bit less than before and my boat is sluggish as all hell!!

Please someone help!"
 
"Patrick,

Here is a link to


"Patrick,

Here is a link to a prop calculator: http://www.boatramp.com/prop_applet/PropAnalyzerAppletG.html

Each 1" change in diameter is roughly equal to 1 deg pitch change. So you've essentially gone down 1.5 degrees on the pitch. Also, a full cup is equa l to another 1 deg pitch. Depending on the cup variance, you could essentially be running roughly the same pitch with a higher diameter prop, which will slow you down.

Try plugging some numbers into the calculator. You should be in the 4-6% range for slip.

Also, additional information about the boat and outdrive (weight and gear ratio) will help too.

Mike"
 
"Mike,

Thanks for the help.


"Mike,

Thanks for the help. I will check the cupping as it did seem to be a bit more severe then my old props..is this something that can be altered? Cant I basically have the pitch changed on these also by my local prop shop? The weight of my boat is roughly 9500 lbs loaded and my gear ratio is 2.2 I think."
 
"I notice a slight growling no

"I notice a slight growling noise when I turn the propeller of my merc alpha one by hand. When I pull the prop back and turn it, the slight growl stops.

Under load, the boat seems fine. There is just about 1/8" endplay in the shaft and no side to side play.

Is this something I should be worried about?

Thanks, Larry"
 
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