SaltInMyVeins
Cadet
- Joined
- May 1, 2013
- Messages
- 24
Hey this is my first post but I have been lurking for years and have found lots of useful information on this great site that has helped me work on my boat. I have over 35 years of shadetree boat and engine work under my belt. Glad to finally be participating.
I recently had my 1983 Evinrude 25 hp powerhead rebuilt (blueprinted, balanced, polished, intakes matched, exhaust ported, pistons ported, 35 hp rebuilt carb installed and rejetted, etc.), new water pump, new seals in foot, etc. Rebuilder estimates new power at 40hp. It runs like a pure dream. Idles great and cranks every time and doesn't cut off until I shut it off. The boat is an aluminum flatbottom jon boat that is used for saltwater fishing and carries relatively higher loads.
I purchased a brand new (old stock) 12" (actually measures 11.5") pitch OMC SST cupped stainless steel propeller for my new ride. This prop turns 4500 rpm at WOT with a top speed of 31.5 mph. As a comparison, I tried my old, beat up, nicked up, Michigan Wheel aluminum propeller which is a 10.5" (measures 10.5") pitch. It spins up to 5280 rpms with a top speed of 28.5 mph. The sound of the motor at wot with the aluminum prop is harsh and sounds like it will come apart if its run for any length of time. The stainless prop sounds smooth and nice at wot. Speed measured with depthfinder and paddlewheel type sensor. RPM measured with temporary, simple digital tach with sensor wire that wraps around spark plug wire. Both tests performed same day with same load and no other changes. This is my second set of measurements that confirms earlier tests. The name plate on the engine states an operating range of 4500 to 5500 rpm. On a seperate day i carried a normal but heavier load with the 11.5" stainless prop and the rpms only got up to 4400rpm.
The rule of thumb according to Evinrude, is that a change of 1 pitch size, or 1" of pitch for this motor, equals 200-300 rpm change. Cupping can reduce rpm another 200-300 rpm. In that case, the rpm spread between the two props should be 400-600 rpm vs 780 rpm that I recorded. In addition, a stainless prop should turn up a little higher than an aluminum prop of the same pitch.
Question: Can I run with the 12" (11.5") stainless at around 4500 without damaging (lugging) the motor? I like the way it sounds and accelerates. Also, would the reduced rpms extend the engine life? Would the extra hp acheived during the rebuild prevent lugging? Or is the lower limit of the operating range sacred?
If not, how much should I change the pitch? My prop man tells me that these smaller props (9.25" diameter, non-thru-hub exhaust) are difficult to alter without cracking.
I recently had my 1983 Evinrude 25 hp powerhead rebuilt (blueprinted, balanced, polished, intakes matched, exhaust ported, pistons ported, 35 hp rebuilt carb installed and rejetted, etc.), new water pump, new seals in foot, etc. Rebuilder estimates new power at 40hp. It runs like a pure dream. Idles great and cranks every time and doesn't cut off until I shut it off. The boat is an aluminum flatbottom jon boat that is used for saltwater fishing and carries relatively higher loads.
I purchased a brand new (old stock) 12" (actually measures 11.5") pitch OMC SST cupped stainless steel propeller for my new ride. This prop turns 4500 rpm at WOT with a top speed of 31.5 mph. As a comparison, I tried my old, beat up, nicked up, Michigan Wheel aluminum propeller which is a 10.5" (measures 10.5") pitch. It spins up to 5280 rpms with a top speed of 28.5 mph. The sound of the motor at wot with the aluminum prop is harsh and sounds like it will come apart if its run for any length of time. The stainless prop sounds smooth and nice at wot. Speed measured with depthfinder and paddlewheel type sensor. RPM measured with temporary, simple digital tach with sensor wire that wraps around spark plug wire. Both tests performed same day with same load and no other changes. This is my second set of measurements that confirms earlier tests. The name plate on the engine states an operating range of 4500 to 5500 rpm. On a seperate day i carried a normal but heavier load with the 11.5" stainless prop and the rpms only got up to 4400rpm.
The rule of thumb according to Evinrude, is that a change of 1 pitch size, or 1" of pitch for this motor, equals 200-300 rpm change. Cupping can reduce rpm another 200-300 rpm. In that case, the rpm spread between the two props should be 400-600 rpm vs 780 rpm that I recorded. In addition, a stainless prop should turn up a little higher than an aluminum prop of the same pitch.
Question: Can I run with the 12" (11.5") stainless at around 4500 without damaging (lugging) the motor? I like the way it sounds and accelerates. Also, would the reduced rpms extend the engine life? Would the extra hp acheived during the rebuild prevent lugging? Or is the lower limit of the operating range sacred?
If not, how much should I change the pitch? My prop man tells me that these smaller props (9.25" diameter, non-thru-hub exhaust) are difficult to alter without cracking.
