need help in choosing right prop

[steelhead1]

i have a 90 evinrude 120hp pushing a 22ft starcraft islander aluminum boat.the motor is in excellent shape inside and out.i was using a michwheel aluminum prop 131/4x17 and it was getting 5500 to 5600 rpm's.then a couple years ago iput on a new stainless rapture 131/4x17.i am getting great performance but barely getting over 5000 rpm's at full throttle.i didn't realize i would lose that many rpm's converting over to a stainless prop in the same size.will i have to go to a 13 1/4x15 stainless to get my rpm's back or aluminum.i see michiganwheel came out with a high performance aluminum prop called the vortex that i could keep in a 13 1/4x17 and not lose a lot of performance.my rpm range at full throttle for this motor is 5000-6000 i should be in the top half right? i would greatly appreciate any help or advice.<br />thank you,<br />scott

 

[steelespike]

Re: need help in choosing right prop

Steelhead1,Sounds like you hit it it pretty good.<br /> Im pretty consertive and would probably stick close to the bottom of the rpm range as long<br />as the motor isnt lugging.<br /> Try the michigan wheel site at: www.michiganwheel.com .Does the motor still have the same hole shot and top speed?

 

[steelhead1]

Re: need help in choosing right prop

hi,it is ok if you stay closer to the bottom of the rpm range?i could really feel the difference in performance from the stainless compared to aluminum performance wise.no my motor doesn't lug.i take it in every other year and have it serviced last year i had the carbs rebuilt and they tell me the compression test in each cylinder is to its highest.would i lose top end speed going to a 15 pitch instead of a 17.<br />thank you for your reply<br />scott

 

[miloman]

Re: need help in choosing right prop

hey steelhead I just posted a thread for you that should answer any questions you have re prop

 

[ob]

Re: need help in choosing right prop

Steelhead,Most of the ss props are double cupped.You could also consider letting a good propeller shop remove some cupping in an attempt to gain a few more R's at WOT.

 

[steelespike]

Re: need help in choosing right prop

Steelhead1, I want to stress Im not an expert here.I did some basic math and i think you would be in the same ball park for speed and probably <br />turn something around 5,300 to 5,400 rpm.<br /> your hole shot would be amazing if it is allready good.Your present prop should resist cavitation better than your old one so::<br />before you change the prop you may want to consider raising the motor virtically<br /> till you get cavitation then down to where it is tollerable.It will change how much you can trim<br />but your prop should help that.If you can gain an inch(not likely)it could give you 5 mph.<br /> It depends on where your at now.

 

[Boatist]

Re: need help in choosing right prop

Steelhead1<br />1 question first, are you sure the decrease in RPM's is from the prop?? What happeneds if you put your old aluminum prop back on?? If your boat is like most it slowly gains weight which will slow it down. You add a kicker motor or a depth finder with a transducer that add drag. Add a second anchor more tools ect. also any dents in bottom or hooks can have a big affect.<br />In general you want to be near the top of your motor's RPM range for best overall performance. Having said that I run a 17 and a 19 pitch prop on my 21 foot Crestliner aluminum boat. Both are double cupped and same style. The 17 runs about 400 RPM's higher but is actually 2 miles per hour slower and uses more fuel. We use the 19 pitch in ocean where we make long trips and never run flat out anyway so can get the better fuel mileage. Kind of like putting your car in overdrive.<br /><br />Most Stainless steel props have more cupping than aluminum props. The cuppimg does alow you to run the motor higher for more speed.<br /><br />I am no expert either but as long as your motor is running in the recommended WOT RPM range I think is ok. For pulling heavy skiers and tubs with lots of starts and stops you might want a lower pitch.

 

[steelhead1]

Re: need help in choosing right prop

i traded in my aluminum on my new stainless so i dont have one to try without buying one.maybe i would be all right and in better shape if i try a 19 pitch aluminum since the double cupped stainless rapture prop is in the rpm range?