I have a 2007 mercury 115 lower unit that took a crap. Will a 2001 mercury 115 fit it?
2:1 gear ratio on 115. 90 has 2.3. Depending on the boat, if you backed off your pitch with another prop having the same characteristics, the same percentage (15%) that may give you the same results. Faztbullet should know the answer...... is there more to it than that.
Never quite figured the reason for the lower ratio on the 90 unless it's used in heavy load applications and either props aren't available in shallow enough pitch, or there is some magic I missed. It's all about gear ratio. A given prop of a given dimension on an given boat turning at a given rpm (the prop) gives you a given result.
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You're talking about a gear train and propeller torque/thrust requirement. The gears in the system are drive shaft, prop shaft, and propeller pitch/slip. A x B x C = the desired result. If I change the value of A and B but the mathematical "product" remains the same, what difference does it make if I swap their values by 2.3/2 = 1.15 or 15%. I never could figure that one out.
It did matter to me as on my last boat, 2002 90 hp, I was running a 13 1/4x24 Ballistic XL 3 blade at 5600-5800 rpms. There were no higher pitched props available from them and to change prop mfgrs. or types would have thrown in a bunch of variables. Had I had the 2:1 gearbox like is on the higher 4 cyl engines of the series, I could have had my engine loaded a little more and used my 24P getting a few more mph.....which I would have done and put me over the 50 mph mark rather than kissing the underside.