Pre-Alpha Prop options

dusted_ya

Seaman Apprentice
Joined
Mar 22, 2008
Messages
39
I have a 1970 pre-alpha mercruiser 120hp I/O that I'd like to upgrade to a 19 pitch stainless prop but not sure what props are compatible? Any help would be greatly appreciated.
 

Watermann

Starmada Splash of the Year 2014
Joined
Jan 12, 2013
Messages
13,822
19"p in a SS prop is not a good option especially for a MC120 if the motor is weak with age. I think your STARCRAFT Holiday is pretty awesome but I don't know if you did any motor work during the restore?

What prop are you running currently, what RPMs are you getting right now with it and the speed GPS?

fetch
 

dusted_ya

Seaman Apprentice
Joined
Mar 22, 2008
Messages
39
Motor is in really good condition. I did nothing to the short block but updated everything else including pulling the heads and having hard seats and new stainless steel valves installed for unleaded fuel . Springs were tested for load and deemed worthy. Original owner turned the engine over a quarter turn every so often. I rebuilt the carb and installed a new fuel pump, Moeller spin on fuel filter, new fuel lines, Pertronix electronic ignition. Outdrive has new bellows, gimble bearing and complete water pump assembly. She had 110 to 115 psi compression test on first firing. Carb and timing was tuned. After initial break in with a 19 pitch aluminum prop she will run WOT at 4500 RPM with about 700# of crew and gear. I'm pretty sure a new 19 pitch stainless will do nothing but improve performance and fuel economy.

Last run was a long one. I took her approximately 20 miles including an 8 mile run upstream at WOT. She cruises at about 30 mph at 3500 rpm's on return route. Spent seven hours on the water, most of which was trolling for walleye's on the Columbia River and burned only about 6 gallons of fuel.
 

Watermann

Starmada Splash of the Year 2014
Joined
Jan 12, 2013
Messages
13,822
Yeah that is a great combo alright, enough power and good economy. The SS prop will eat up some of the power though from what I read, I really only have experience with AL props on my 3 boats.

One other thing is the 19p prop your using now new or one that came with the boat? Reason I ask is you never know the vintage of a well taken care of prop and without really knowing what your looking at, going to a newer prop with the same pitch can have some consequences as I've learned. Older props just don't have the cupping and aggressive rake like newer designs that grab and hold the water better which in turn eats RPMs. You really don't want to over prop a 120.

Sounds like you're dead set on a new SS prop and it's one heck of an investment so you don't want a prop decorating the garage wall.
 
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