Boat motor question?

[DareDevilDave]

Other question is my prop says LM78-11P 13 spline. On google it says my prop is 12 diameter x 11 pitch. So would a 11 5/8 x 11Pitch 13 spline on Amazon work?

 

[OL Deadeye]

Found a peice of tinfoil when I took the prop off should I be worried?

Only if your boating in "Kansas"

 

[Allen]

I wouldn't be terribly worried about finding some tinfoil in there as it's pretty soft stuff, weird though.

As far as the diameter of the prop, you will be fine with the 11 & 5/8 diameter prop. It's actually pretty common to use a prop that is slightly smaller in diameter than the max. In this case, it would appear a 12 inch diameter prop is your max before you start having clearance concerns with the lower unit.

Props are far more complex than many boat owners realize. Diameter, Pitch, Rake, Cupping, Ventilation, Number of Blades, and Material make this a real pain in the derriere if you are trying to balance speed, control and efficiency.

For example, I once ran a composite 4-blade prop on an old Smoker Craft with a 75 hp Mariner. I actually really liked that prop's performance, but it was fragile as hell. It had a lot of flex which allowed for faster revving of the motor on takeoff, but then the prop would flex back into its designed shape once on plane.

Aluminum is/was the standard and they could flex, but they were also relatively inexpensive and soft enough to help save your lower unit if you hit something. The bad though it that the prop would take damage on any impact, even in sand/gravel. You can get them reworked, but on one prop that I had to get fixed twice on that same boat, I noticed a 150 RPM loss each time it got repaired. I finally had to relegate that prop to being a spare, even though it looked brand new.

In general though, larger diameter and higher pitch = more speed and efficiency, but slower hole shot. Smaller diameter and pitch produces faster hole shot and lower top end speed. Rake is a bit of a mystery to me. Lastly, cupping reduces blowout/ventilation (catching air in the prop) when on plane and you have the motor trimmed up for maximum speed, especially when in a turn.

One other thing, I like hydrofoils on a boat. They don't usually increase one's top speed, may actually cost you 1-3 mph, but it will improve your boat control and help you get on plane at lower speeds. I've used them on a 16 ft Alumacraft with a 25 horse tiller, a 16.5 ft Smoker Craft with a 75 hp Mariner, and a 17.5 Lund Fisherman with a 115 Yamaha. All were greatly improved with a hydrofoil. The only one that had a higher top end speed though was the Alumacraft, maybe because it also did not have power trim/tilt. The others were a no change in speed, or 1-2 mph slower at WOT.