Prop sizes and pitch

[sren1958]

New to owning a boat. Purchased a 1979 Sea Ray Sundancer 260, 260 hp to keep myself busy. Just replaced gimbal bearing, lower shift cable, all bellows. That being all put back together and cleaned up and painted upper and lower units, it's onto prop now. Very badly corroded/pitted, cannot read part number. Sea Ray Owners web site lists 15.75 x 17 Special, but no part number. I am a little confused as I have seen some specs on web sites listing 4.25 inch gear case. Talked to someone about a 4 blade, when I looked up it listed it as 4.75 gear case. Being that I am wet behind the ears, what does that mean?

 

[jimmbo]

4.25" gearcases were used on mid power(a long and torturous list to try and write out) OMC outboards and Stern Drives, and on Mercury Older Inline >60cubic inch 4 Cylinder and inline 6s, and some of the older smaller I/Os
The 4.75 Gear cases came out on the Larger Merc I/Os back in the 60s, and were later used on Merc and OMC V6s starting in the mid 70s. But, and there is Always a But. Over the years the Smaller case has been used on larger Engines and the Larger Case on smaller Engines.
Your 260 I/O would be speced to use the 4.75" gearcase, and the 4.75" Hub Props. That said, a 4.25" Prop would likely fit on the Propshaft. It will however create a situation where some of the Exhaust would pass over the Prop Blades instead of being contained through the Hub. Usually that results in the Prop not retaining a good 'Bite' on the Water upon Acceleration, and lots of Revs but not much go.

 

[sren1958]

4.25" gearcases were used on mid power(a long and torturous list to try and write out) OMC outboards and Stern Drives, and on Mercury Older Inline >60cubic inch 4 Cylinder and inline 6s, and some of the older smaller I/Os
The 4.75 Gear cases came out on the Larger Merc I/Os back in the 60s, and were later used on Merc and OMC V6s starting in the mid 70s. But, and there is Always a But. Over the years the Smaller case has been used on larger Engines and the Larger Case on smaller Engines.
Your 260 I/O would be speced to use the 4.75" gearcase, and the 4.75" Hub Props. That said, a 4.25" Prop would likely fit on the Propshaft. It will however create a situation where some of the Exhaust would pass over the Prop Blades instead of being contained through the Hub. Usually that results in the Prop not retaining a good 'Bite' on the Water upon Acceleration, and lots of Revs but not much go.

I thank you so much for your insight, Jimmbo, very much appreciated. Many times I read too much and not ask enough questions, thanks again!

 

[Stinnett21]

Nice looking old SR.