On your 1965 Mercury 50hp, the 11-spline prop shaft means you’re dealing with the older "small gearcase" setup Mercury used on a lot of their inline 4-cylinders from that era.
- Prop Size Range: Most commonly you’ll see props around 10–11" diameter with pitch options ranging from 10" up to 15", depending on whether you’re prioritizing hole shot or top speed.
- Interchangeability: The 11-spline hub props from the 1960s–early 1970s 4-cylinder 40–50hp Mercurys are generally interchangeable, so you don’t need a prop specifically labeled for 1965—anything for an 11-spline small-gearcase Mercury should fit.
- Where to Find: Michigan Wheel, Quicksilver (OEM Mercury), and Solas all made replacements. If buying used, just confirm 11 splines, small gearcase (not the 13-spline large gearcase used on bigger inlines).
- Tip: If you’re running a light boat, a 13–15" pitch will usually give you good speed. For heavier hulls or if you’re hauling loads, stick to 10–12" pitch to keep RPM in the right range (these motors like ~5000–5500 WOT RPM).
If you’re unsure of pitch, start with a
12" pitch prop—good middle ground—and check your wide-open throttle RPM on the water. Adjust pitch up or down from there.
