90HP alluminum Boat prop

[hotrod76car]

I have a 18' alluminum boat with a 2005 mercury 90 with a stock 21 pitch prop my rpm is 5800 at wot and needs to be about 5500 so I was woundering if going to a ss like the raptor in the same pitch would solve the rpm since it is cuped. I read that a cuped prop you can go down 2 pitches with the same rpm so if I go with the same pitch in a cuped prop then that should lower my rpm. Is this corect or did I read rong.<br />Thanks

 

[hotrod76car]

Re: 90HP alluminum Boat prop

What about the apollo props from M W

 

[Major Woods]

Re: 90HP alluminum Boat prop

That sounds about right if your current prop is not cupped.<br /><br />You could also bring your current prop to a repair shop and ask for a pitch change or add a cup to bring the rpm's into spec.

 

[Ron G]

Re: 90HP alluminum Boat prop

Just curious if it really needs to be5500??or if 5800 is alright on that motor.

 

[Major Woods]

Re: 90HP alluminum Boat prop

You want to be in the upper end of the WOT spec not over.<br /><br />Lugging the motor (under the WOT rpm spec will harm the motor over time. A little over the spec should not. <br />I would still try to achieve the 5500rpm @ WOT with a prop mod/change for best fuel effiency and life of the motor.

 

[hotrod76car]

Re: 90HP alluminum Boat prop

Thanks guys the prop on it is a black max so I dont know if it cuped any or not max rpm is 1750 in the book and min is 5000

 

[walleyehed]

Re: 90HP alluminum Boat prop

You want to be in the upper end of the WOT spec not over. IF YOU ARE LOADED <br /><br />Lugging the motor (under the WOT rpm spec will harm the motor over time. A little over the spec should not. <br />I would still try to achieve the 5500rpm @ WOT with a prop mod/change for best fuel effiency and life of the motor.

WITH A FULL LOAD <br />If this rig runs 5800 with just you in it, I go up 2" in pitch and raise the engine mounting possition until you gain the 5800 back with just you in the boat...this is "maximizing" the set-up....you don't want 5500 with a light load.<br />The RPM range is 5000-5750, so if it ran 6000 with just you it would be OK...5800 is in NO WAYOVER-REVING THE ENGINE.

 

[hotrod76car]

Re: 90HP alluminum Boat prop

thanks Kenny

 

[Texasmark]

Re: 90HP alluminum Boat prop

I am running a 17' alum bass boat with an '02 merc 90 with a 21 pitch Mich Wheel Ballstic; 3/4 " jack on engine and boat is padded.<br /><br />When I bought the boat it was way under propped; like it had a 17" pitched MiW Ballistic and you don't want to know the WOT rpm's with just me on a winter day. Can't believe the guy ran this boat for 2 years with that setup.<br /><br />Did my figuring and replaced it with the same brand 21" and brought my rpm's down to 6250 and speed to 49....when I could see to run...eyes watering.<br /><br />Ran the numbers thru the "Prop Slip Calculator and it said I had a 10 percent slip and I believe that. For now I keep feeling that I want to go up at least another inch, but my trim tab is in the way and I haven't tested summer operation with the wife and all the "fishin goodies". So for the time being I'm just gonna wait.....<br /><br />So it sounds like you are not that far off, from me, with your rig, depending upon setup, weight, and hull type. You're only 400 rpm's down from me and my boat, Lake Sport by www.rivertrailinc.com, is real light and being padded, at 49mph, the wake...spray... leaving the boat is behind the corners of the transom; so the engine isn't working very hard.<br /><br />HTH<br /><br />Mark

 

[hotrod76car]

Re: 90HP alluminum Boat prop

OK went out today bass tracker 185 5 1/2 in jack plate 21 pitch black max cavatation plate level with botom of boat. Just me in the boat 5800 rpm loaded 5500 rpm. If I jack the motor up about 2 in I can not trim the boat all the way out it ventalates or plows the front end and max rpm with me is 5500. So going with ss prop will that let me rase the motor up and trim all the way with out ventalating and if so should I go with a 21 or 22 ss prop. I am thinking 23 so when I lift the motor I will gain the rpm back I loose with a 23 pitch. Does this sound write? My main concern is will I be able able to trim the motor if I rase it up because now I can not.I gess the black max does not run close to the top of the serface lick I read a ss prop will. Is this corect?

 

[hotrod76car]

Re: 90HP alluminum Boat prop

one more thing do I go with turbo or stiletto<br />Thanks