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Author Topic:   MONTAUK 17 with 90-HP Johnson

motoboy posted 09-24-2010 12:57 PM ET (US)
A [1999] Johnson 90-HP [OceanPro] with 13-7/8 x 19 SST three-blade propeller [produces an engine speed of] 5,700- to 5,800-RPM at wide-open throttle [on a Boston Whaler MONTAUK 17]. The tachometer has been verified as accurate. The 13-7/8 x 19 SST three-blade propeller has some minor dings on edges, but no bends. I'm not hearing or seeing any ventilation.
I wanted to get the WOT engine speed closer to 5,000- to 5,100-RPM, so a 14 x 21 four-blade propeller was ordered. My rationale was 200-RPM per inch of pitch would mean a 400-RPM reduction. Extra blade would reduce another 200-RPM.
Are my calculations right? Do the dings on my curent prop cause excessive slip? My current prop looks like painted aluminum. It's the propeller that came with the motor in 1999. Propeller I ordered was aluminum.
Thanks!

Tohsgib posted 09-24-2010 02:31 PM ET (US)
Your first post says you have a SST prop, your last says aluminum--which is it? Aluminum propellers have round dull edges; stainless steel propellers are sharper and after 11 years should still be fairly sharp. SST's are painted black but the blades will have a bit of surface rust on them, alum will be alum colored. No SS prop in 19" would fetch you those RPMs. Correct prop for your engine would be a 13.25x17 SST or 15" Stiletto.
You are going to be way overpropped with a 21" 4 blade which in theory should perform like a 23" 3 blade. Your redline is 5500 and with a good $250 or so Stiletto advantage 15" 3 blade polished SS from Tom Clark would fit the bill and get you around 5300rpm WOT. Stilettos have aggressive cupping and hence why a 15" acts more like a 17" conventional SS prop. Every Montauk I owned with your combo I ran a 17" SST which is equiv to the 15" stiletto.
motoboy posted 09-24-2010 05:52 PM ET (US)
I know it seems weird that I'm seeing the engine speed that high. Maybe it has a lower gear ratio or something that lets it spin a higher pitch. It's a 1999 OceanPro 90.
The propeller says SST on the outside. It's painted black. I took it off just now and it feels pretty heavy so I guess it is stainless. I know for sure it's a 13-7/8 x 19. It's stamped right on the propeller. I know for sure [the engine speed is] 5,700-RPM at WOT. Maybe I have the turbo-charged 90. LOL. Seeing 43-MPH on GPS speed, again at 5,700-RPM. The propeller shaft has 13 splines.
Tohsgib posted 09-24-2010 09:35 PM ET (US)
If you truly think the tach is right--which I doubt--have it double cupped which will bring you down to the 5,500 redline, maybe 5,400. There is no way a 19-pitch is spinning that fast, if so you would be doing at least 50-MPH. Something is amiss.
Tohsgib posted 09-24-2010 09:39 PM ET (US)
Lastly with a 17" she should almost throw you overboard if you nail it from a standstill. With a 19 it would be more conservative. With a 21 it will take you a LONG time to plane out and RPM's would not break 5k. If you can pull a skier semi-easily, you do not have a 19" prop. Props can say whatever, they can be repitched. My 17" on my 13 has been repitched to a 15 and there is NO evidence of it. Sure you got it with the engine but it does not mean it was NEW, 90hp engines do NOT come with props so they might have thrown you one off the shelf that was used?
jimh posted 09-25-2010 11:44 AM ET (US)
Damage to the blades of a propeller will affect the performance. A propeller with dings, dents, or bends in the tips of the propeller blades will not perform the same as a similar propeller without such damage.
Running the outboard motor at high speeds with a damaged propeller is not recommended. The balance of the propeller will be off, and the vibrations that result could cause wear on the bearings of the propeller shaft.
motoboy posted 10-28-2010 01:57 PM ET (US)
Well, the 13x21 pitch 4 blade was to much. She still hit 42ish mph on gps but only at 4600 rpm's.
Strange that it lost over 1000 rpm's but I did notice the "ears" if you will, of the new stainless 4 blade are bigger (ie more surface area) than the 3 blade sst.
So they are going to swap me for a 4 blade 13x19 which hopefully will hit my 5000rpm target.
Really liked the 4 blade for smoothness and maintaining lower planing speed of the boat.
pcrussell50 posted 10-29-2010 02:30 AM ET (US)
What rpm does your motor's manual suggest as the maximum? At what rpm does max power occur? If you prop yourself up such that you do not reach that rpm, you will not be the getting full rated power of your motor. You might even be lugging it, which is hard on the motor.
-Peter
Tohsgib posted 10-29-2010 11:30 AM ET (US)
Redline is 5500, you are making another mistake with the 19", go for the 17.
knothead posted 11-04-2010 02:00 PM ET (US)

I have 2 props for my 1987 Montauk which is mounted with a 2001 90hp Johnson.
The 13x19 SST will top out at 5450-5500 RPMs with 2 people, 22 gallons of fuel, and a light load. The 13x17 Stiletto tops out around 5300-5350 RPMs with same load. I'm too cheap to buy a GPS so I don't know the speed. I also prefer to stay lost when at sea.
My motor needs to be raised, it's mounted in the blind hole and I'm too lazy to raise it. Just another project in a infinately expanding list.
Best regards---knothead

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Author	Topic: MONTAUK 17 with 90-HP Johnson
motoboy	posted 09-24-2010 12:57 PM ET (US) A [1999] Johnson 90-HP [OceanPro] with 13-7/8 x 19 SST three-blade propeller [produces an engine speed of] 5,700- to 5,800-RPM at wide-open throttle [on a Boston Whaler MONTAUK 17]. The tachometer has been verified as accurate. The 13-7/8 x 19 SST three-blade propeller has some minor dings on edges, but no bends. I'm not hearing or seeing any ventilation. I wanted to get the WOT engine speed closer to 5,000- to 5,100-RPM, so a 14 x 21 four-blade propeller was ordered. My rationale was 200-RPM per inch of pitch would mean a 400-RPM reduction. Extra blade would reduce another 200-RPM. Are my calculations right? Do the dings on my curent prop cause excessive slip? My current prop looks like painted aluminum. It's the propeller that came with the motor in 1999. Propeller I ordered was aluminum. Thanks!
Tohsgib	posted 09-24-2010 02:31 PM ET (US) Your first post says you have a SST prop, your last says aluminum--which is it? Aluminum propellers have round dull edges; stainless steel propellers are sharper and after 11 years should still be fairly sharp. SST's are painted black but the blades will have a bit of surface rust on them, alum will be alum colored. No SS prop in 19" would fetch you those RPMs. Correct prop for your engine would be a 13.25x17 SST or 15" Stiletto. You are going to be way overpropped with a 21" 4 blade which in theory should perform like a 23" 3 blade. Your redline is 5500 and with a good $250 or so Stiletto advantage 15" 3 blade polished SS from Tom Clark would fit the bill and get you around 5300rpm WOT. Stilettos have aggressive cupping and hence why a 15" acts more like a 17" conventional SS prop. Every Montauk I owned with your combo I ran a 17" SST which is equiv to the 15" stiletto.
motoboy	posted 09-24-2010 05:52 PM ET (US) I know it seems weird that I'm seeing the engine speed that high. Maybe it has a lower gear ratio or something that lets it spin a higher pitch. It's a 1999 OceanPro 90. The propeller says SST on the outside. It's painted black. I took it off just now and it feels pretty heavy so I guess it is stainless. I know for sure it's a 13-7/8 x 19. It's stamped right on the propeller. I know for sure [the engine speed is] 5,700-RPM at WOT. Maybe I have the turbo-charged 90. LOL. Seeing 43-MPH on GPS speed, again at 5,700-RPM. The propeller shaft has 13 splines.
Tohsgib	posted 09-24-2010 09:35 PM ET (US) If you truly think the tach is right--which I doubt--have it double cupped which will bring you down to the 5,500 redline, maybe 5,400. There is no way a 19-pitch is spinning that fast, if so you would be doing at least 50-MPH. Something is amiss.
Tohsgib	posted 09-24-2010 09:39 PM ET (US) Lastly with a 17" she should almost throw you overboard if you nail it from a standstill. With a 19 it would be more conservative. With a 21 it will take you a LONG time to plane out and RPM's would not break 5k. If you can pull a skier semi-easily, you do not have a 19" prop. Props can say whatever, they can be repitched. My 17" on my 13 has been repitched to a 15 and there is NO evidence of it. Sure you got it with the engine but it does not mean it was NEW, 90hp engines do NOT come with props so they might have thrown you one off the shelf that was used?
jimh	posted 09-25-2010 11:44 AM ET (US) Damage to the blades of a propeller will affect the performance. A propeller with dings, dents, or bends in the tips of the propeller blades will not perform the same as a similar propeller without such damage. Running the outboard motor at high speeds with a damaged propeller is not recommended. The balance of the propeller will be off, and the vibrations that result could cause wear on the bearings of the propeller shaft.
motoboy	posted 10-28-2010 01:57 PM ET (US) Well, the 13x21 pitch 4 blade was to much. She still hit 42ish mph on gps but only at 4600 rpm's. Strange that it lost over 1000 rpm's but I did notice the "ears" if you will, of the new stainless 4 blade are bigger (ie more surface area) than the 3 blade sst. So they are going to swap me for a 4 blade 13x19 which hopefully will hit my 5000rpm target. Really liked the 4 blade for smoothness and maintaining lower planing speed of the boat.
pcrussell50	posted 10-29-2010 02:30 AM ET (US) What rpm does your motor's manual suggest as the maximum? At what rpm does max power occur? If you prop yourself up such that you do not reach that rpm, you will not be the getting full rated power of your motor. You might even be lugging it, which is hard on the motor. -Peter
Tohsgib	posted 10-29-2010 11:30 AM ET (US) Redline is 5500, you are making another mistake with the 19", go for the 17.
knothead	posted 11-04-2010 02:00 PM ET (US) I have 2 props for my 1987 Montauk which is mounted with a 2001 90hp Johnson. The 13x19 SST will top out at 5450-5500 RPMs with 2 people, 22 gallons of fuel, and a light load. The 13x17 Stiletto tops out around 5300-5350 RPMs with same load. I'm too cheap to buy a GPS so I don't know the speed. I also prefer to stay lost when at sea. My motor needs to be raised, it's mounted in the blind hole and I'm too lazy to raise it. Just another project in a infinately expanding list. Best regards---knothead