Using the Boatdiesel.com Prop. Shaft Calculator I entered the following:
Perkins 4-236 85SHP - 2,800RPM
Reduction Gear - 3:1
Shaft Safety Factor - 5 (recommended)
Here are the results:
Stainless 304 - 1.94"
Stainless 316 - 1.94"
Monel 400 - 1.54"
Monel K500 - 1.30"
Aquamet 17 - 1.28"
Aquamet 18 - 1.35"
Aquamet 19 - 1.28"
Aquamet 22 - 1.19"
So, if your 1.25" shafts are made of Aquamet XX alloy or Monel K500, you should be safe. If stainless 304 or 316, they are considerably undersized.
There is a very old "Rule of One Fourteenth" where the prop diameter is divided by 14 to roughly evaluate the prop shaft size. In your case, 23" divided by 14 = 1.64" which is sort of in the middle of the calculated results above.
Finally, using Skene's shaft size calculations:
Shaft Horsepower - 85
Shaft RPM - 933
Bronze & Mild Steel - 2-1/8"
Monel - 1-3/8"
You can draw your own conclusions from the above. They all seem to indicate that 1.25" shafting will be too small for your application. The exception would be Aquamet 22 shafts. If there is any doubt, it would be better to err on the side of safety.
Just my $.02 . . . .
Larry
M/V Boomarang