I don't have data on the Optimax 90 engine you have. On my 2 stroke carbed 2002 90 the gearbox is 2.33:1. On 2 stroke 115s the gearbox is 2:1. If you are comparing the running of the 115 to the 90 your problem lies in keeping the same prop on the same boat. Keeping the same top speed with the rpms you posted says you need a new prop with a couple more inches of pitch which would load the engine more, reducing the rpms but mph should increase.....but that's debatable and only a test will tell.
I went to the BAM prop slip calculator and ran some numbers. These numbers are without consideration for slip; just max possible speed if you theoretically had your prop (screw) turning in a solid medium:
Current 19P 6500 rpm 50 mph
Current 19P 6000 rpm 46
New 21 5500 47 (if it loads your engine down that much)
or
New 21 6000 51
Prop slip varies from like 10% to as high as 30 or so. Light fast hulls with lots of power and a good gripping prop like mine can run around 10. Boats with more weight and wetted area, more of a standard designed prop, and otherwise adequately powered and all may run 15 to 20.
HTH,
Mark
