Re: Only 4500 -4800 RPMS- 90 HP Johnson on 20'SF
The gear ratio makes a huge difference. I used to run a 15"P on my current Seafari when I had the 120 Chrysler. HP at the prop was about 95-100, and I turned 5200 rpms. The gear ratio on my Chrysler was 1.98, and I had a top speed of about 28-29 mph, and cruised at 23-24 mph.
Now that I have an '06 Mercury 90 hp Optimax, I have about 100-105 hp at the prop, my gear ratio is 2.33, I turn that same 5200 rpms, running a 12.75"x20"P SS 4-blade, with a top end of 32 mph, and cruise at 4500 rpms at 25-26 mph.
I was running a 3 blade Apollo SS, in 13"x21", with a top end of 34 mph at 5100 rpms, but it would fall off plane below 21 mph.
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Common Sense is learning from your mistakes. Wisdom is learning from the other guy's mistakes.
Fr. Frank says:
Jesus liked fishing, too. He even walked on water to get to the boat!
Currently without a SeaCraft 
(2) Pompano 12' fishing kayaks
'73 Cobia 18' prototype "Casting Skiff", 70hp Mercury
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