90hp turning 6500 on 24' toon

[powerwagon360]

I dont have much info to give on my boats specs but...I i have a 24' suncruiser pontoon boat with a 1990 90hp johnson. My curent prop is a ss 11"pitch and i can turn up about 6500 to 6700 rpms a wide open. So i started looking around for info and figerd out that if im terning 6500 now with a 11p then if i go to a 15p i will have about 5700 right? i swaped props of my other 115s (one is 17p the other 19p) I tryed out the 19 the other day and it was doggy on bottom end take of and turned up4800 wot (beffor the prop spun) Should i try the 17? or will try it make it spin to?

 

[hwsiii]

Re: 90hp turning 6500 on 24' toon

No, if you go to a 13" pitch you will be turning about 5,600 to 5,700 RPM,these are small pitches they change RPM much more drastically than props in the 20" pitch range.


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[powerwagon360]

Re: 90hp turning 6500 on 24' toon

ok so the rpm dont drop 400 for every 2" of pitch?i figered that every 1" of pich wold drop 200 and a 13p would give me 6000 6100.(im learning please forgive)im not questioning your knowledge.im just trying to learn

 

[junior1113]

Re: 90hp turning 6500 on 24' toon

first thing i'd do is make sure tach is correct becuase those 90's ussually have a rev limiter of 6200 but it could be 6700 depending on power pack.

simple equation for the answer=old rpm(old pitch/new pitch)=new rpm

example=6500(11/13)=5500
6700(11/13)=5670
6500(11/12)=5960
this formula only works if both props are of the same material and same geometry.

yea 13 will work id lean toward a 12 if wot rpms are 6500 and not 6700. that motor will work best and last longer if wot rpms are as close to 6000 without going over. if it were mine id just get a 12

 

[Dhadley]

Re: 90hp turning 6500 on 24' toon

The tach is reading wrong. A V4 crossflow in stock configuration won't turn 6700 even if it had no limiter. Make sure it's an Evin / Johnson tach and is set on position 6 not 4.