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Prop Selection Advice

mstahl1171

mstahl1171

Contributing Member
Boat: 1992 Angler 204 CC
Engine: 1992 Johnson 155 Commercial
Boat Weight with fuel, gear, persons: Approx 2600lbs
WOT: 5000rpm
GPS Speed: 30mph
Prop size 15x17

Previously I had on the same boat a 1988 Johnson 150hp with a 15x17 prop
WOT 5000rpm
GPS Speed 42mph
Prop size 15x17

I would like to get a little more speed if possible. Would a 14.5x19 help or should I go with a 15x15?

Thanks
 
I would go with less pitch and get the rpms up i am going to guess the difference in speed is the fact you have a commercial motor maybe double check the tach.
 
Thanks flyingscott. I also have a donor 175HP lower unit. I wonder if I could swap lower gear case out to get a little more speed? Or maybe live with what I got....lol

Cruise seemed to be about 28mph....4000rpm. Push to 5000rpm and it only gained 2-3mph. Seemed like I ran out of prop or maybe since the lower unit gearing is way different
 
So I sat down and compared my service manuals for both the old 150 and 155. same gear ratios for both..so I guess your right flyingscott...low compression heads are probably the main reason for loss in speed...I will try a 15 x 15 prop to see if I get a little bit more speed. Does anyone else have any other suggestions?
 
You have some kind of problem your numbers don't line up. Same gear ratio same rpm and same prop should equal same speed. Your 1988 150 would not have had high compression you would have been lucky to see 100psi on that motor new. The low compression heads aren't your problem you are missing something 12 mph difference is huge
 
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OK...clean hull...Factory Manual says WOT is between 4500-5500. Power Pack limit is 5800
All cylinders are 80-85psi cold. Timing is set at 7 degrees idle, 28 degrees WOT
Plugs are Champion QL78V Fixed Gaped. Good spark with a spark tester set at 7/16" gap
Carbs cleaned and rebuild kit installed last week and link and sync checked. All carbs open fully.
Engine decarb with OMC engine tuner

My buddy was running a 1976 20 foot Grady White CC with a 1992 140HP Johnny....he had to slow down to 4400 rpm for me to keep up with him....lol

What i do not know about this FREE motor is whether or not the lower unit is the original one. Factory Manuals says gear ratio should be 14:26. Maybe its not the right one? Guess I could drop the gear case and look at the P/N. If memory serves me right, the omc number is stamped on the inner casting. I still have the lower from my old 150hp. Cross reference says it will fit. Guess I could try that too. Maybe the gear case ratios are not right?

Don't get me wrong...the motor runs great...nice and smooth. Just seems lacking on the high end of the RPM range. I normally cruise at 25-30mph @ 4200 rpm...depending on water chop but was able to jump up to 35+mph if i wanted too with the old 150hp. Now its 30-32mph @ 5000rpm with the newer 155 commercial. All the wiring and instruments are the same on the boat. Just swapped the motor. Too bad my old 150 blew a cylinder and scored the wall beyond repair. Planning on having a powerhead rebuilt for it when I find a good head worth rebuilding.

I will go thru the motor again tomorrow afternoon and let you know if I find anything.....P.S. the pic shows how flat the water was the day we were out for a "fun run"
 

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So I checked everything again except removing the gear case. All checks out. I have a friend who is lending me his 15x15 and 14.25x17. I will try them both and see what happens. I'll keep you posted.
 
mstahl1171 said:
Boat: 1992 Angler 204 CC
Engine: 1992 Johnson 155 Commercial
Boat Weight with fuel, gear, persons: Approx 2600lbs
WOT: 5000rpm
GPS Speed: 30mph
Prop size 15x17

Previously I had on the same boat a 1988 Johnson 150hp with a 15x17 prop
WOT 5000rpm
GPS Speed 42mph
Prop size 15x17
Click to expand...

Assuming a gear ratio of 14/26 = 0.538, the first example has overly slip and the second is almost slipless (too good to be true).

You can calculate the speed for the slipless case with:

rpm * gear * pitch[in]
---------------------- = theoretical speed through water[mph]
1056

Then substract 5...15% for prop slip.
 
Thanks Werner....I'm not completely sure the GPS speed is total accurate but I did find a problem. I changed to a 15x15...rpms came up to 5400 WOT. Speed increased to about 34mph. My buddy who was giving chase noticed that my cavitation plate seemed to be too deep in the water. I measured my old motor mounting holes and my new motor holes and realized that i have the motor about 1" too deep. Duh me!....lol I mounted the motor too low and didn't notice. We were able to raise the motor up and tighten bolts. Did another run with the 15x17 prop and now Im at 5000 rpm and 37mph GPS Speed. Im going to leave that alone and call it done!!
 
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