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Author Topic:   Most efficient prop?

wadams posted 09-07-2008 09:45 PM ET (US)
Maybe you can shed some light on my prop situation. I have a classic 17 Montauk with a 90 Etec. When I put a 13x19 prop on I turn up 4800 rpms and go 42 mph. At 3500 I'm doing 26 mph and at 3000 I'm going 23 mph.
With a 13x17 prop I can turn up 5600 rpms and go 41 mph and at 3500 I make 23 mph and at 3000 I go 19 mph.
The dealer told me I should turn up about 5000 but I know the manual says 4600 to 5500 for WOT so the 4800 should be within range without overworking the engine. I know I get a better power curve with the 17" prop but am wondering if the 19" prop would be more efficient(mpg) since I can go 23 mph at 3000 vs 19 mph at 3000 with the 17" prop? Any opions

Peter posted 09-08-2008 08:15 AM ET (US)
The 800 RPM difference between the 17 and 19 inch pitch doesn't make sense. Are the propellers from the same maker? The 17 inch propeller would be the better choice of the two because the 19 inch doesn't let the motor above 5000 RPM at WOT.
Tom W Clark posted 09-08-2008 11:36 AM ET (US)
Walt,
I share Peter's confusion. Exactly what "13x19" propeller and "13x17" propeller have you been using? There is far more to propeller design than just diameter and pitch.
While the E-TEC 90's WOT range is a very broad 4500-5500 RPM (not 4600-5500), the optimal WOT operating range is 5000-5200 RPM.
wadams posted 09-08-2008 01:34 PM ET (US)
Both props are OMC stainless props and were 13x19s but I had one re-pithced down to a 17, they came off my old 1985 90 HP Evinrude.
wadams posted 09-08-2008 01:37 PM ET (US)
I think maybe the only real way to know for sure is with a fuel flow meter but I hate the spend that much $$ just to find out, doubt I would use the flow meter much once I had the info on most efficient rpm and prop.
Tom W Clark posted 09-08-2008 01:49 PM ET (US)
Ah. This is a good example of how repitching a prop is not the same as using a prop designed and built to a pitch. While and inch up or down can be down by a good prop shop, a two variation of pitch is quite a bit of modification especially on a small prop like these.
The numbers are telling:
42 MPH @ 4800 RPM with a 19" pitch prop suggest a prop slip of less than 3 percent, which is an unusually low slip figure for the OMC SST props. In fact, if you hadn't said otherwise, I would have guessed you were turning a 21" pitch prop.
41 MPH @ 5600 RPM with a 17" suggests a prop slip of over 9 percent, a much more normal figure.
Having propped numerous classic Montauks with the E-TEC 90, I can recommend with some confidence the Stiletto Advantage 4.25 in the 13-1/4" x 15" size. This is a prop that runs like a the OMC/BRP 13" x 17" SST/Viper.
ratherwhalering posted 09-08-2008 05:09 PM ET (US)
I have run both props on my 1987 17-Montauk, and I feel that if the motor is low on the transom, perhaps only one hole up, then the OMC/BRP 13" x 17" SST/Viper might be a better selection, as it feels a bit 'softer' (perhaps due to less cupping Tom?) than the Stiletto. The Stiletto is less forgiving when mounted low. When the engine is mounter higher on the transom, the 13.25 x 15 Stiletto outperforms the OMC/BRP 13" x 17" SST/Viper. The SST/VIPER feels too soft, and the Stiletto comes into its own, feeling much like the SST/Viper might the lower engine mounting height.
cooper1958nc posted 09-08-2008 06:42 PM ET (US)
Huh?
You go faster at less RPM with one prop (the larger pitch), and we are debating which prop to use?????
Another data point confirming what I have been saying.
Keep engine speed down for best economy and longevity.
Here, the speed is better as well.

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Author	Topic: Most efficient prop?
wadams	posted 09-07-2008 09:45 PM ET (US) Maybe you can shed some light on my prop situation. I have a classic 17 Montauk with a 90 Etec. When I put a 13x19 prop on I turn up 4800 rpms and go 42 mph. At 3500 I'm doing 26 mph and at 3000 I'm going 23 mph. With a 13x17 prop I can turn up 5600 rpms and go 41 mph and at 3500 I make 23 mph and at 3000 I go 19 mph. The dealer told me I should turn up about 5000 but I know the manual says 4600 to 5500 for WOT so the 4800 should be within range without overworking the engine. I know I get a better power curve with the 17" prop but am wondering if the 19" prop would be more efficient(mpg) since I can go 23 mph at 3000 vs 19 mph at 3000 with the 17" prop? Any opions
Peter	posted 09-08-2008 08:15 AM ET (US) The 800 RPM difference between the 17 and 19 inch pitch doesn't make sense. Are the propellers from the same maker? The 17 inch propeller would be the better choice of the two because the 19 inch doesn't let the motor above 5000 RPM at WOT.
Tom W Clark	posted 09-08-2008 11:36 AM ET (US) Walt, I share Peter's confusion. Exactly what "13x19" propeller and "13x17" propeller have you been using? There is far more to propeller design than just diameter and pitch. While the E-TEC 90's WOT range is a very broad 4500-5500 RPM (not 4600-5500), the optimal WOT operating range is 5000-5200 RPM.
wadams	posted 09-08-2008 01:34 PM ET (US) Both props are OMC stainless props and were 13x19s but I had one re-pithced down to a 17, they came off my old 1985 90 HP Evinrude.
wadams	posted 09-08-2008 01:37 PM ET (US) I think maybe the only real way to know for sure is with a fuel flow meter but I hate the spend that much $$ just to find out, doubt I would use the flow meter much once I had the info on most efficient rpm and prop.
Tom W Clark	posted 09-08-2008 01:49 PM ET (US) Ah. This is a good example of how repitching a prop is not the same as using a prop designed and built to a pitch. While and inch up or down can be down by a good prop shop, a two variation of pitch is quite a bit of modification especially on a small prop like these. The numbers are telling: 42 MPH @ 4800 RPM with a 19" pitch prop suggest a prop slip of less than 3 percent, which is an unusually low slip figure for the OMC SST props. In fact, if you hadn't said otherwise, I would have guessed you were turning a 21" pitch prop. 41 MPH @ 5600 RPM with a 17" suggests a prop slip of over 9 percent, a much more normal figure. Having propped numerous classic Montauks with the E-TEC 90, I can recommend with some confidence the Stiletto Advantage 4.25 in the 13-1/4" x 15" size. This is a prop that runs like a the OMC/BRP 13" x 17" SST/Viper.
ratherwhalering	posted 09-08-2008 05:09 PM ET (US) I have run both props on my 1987 17-Montauk, and I feel that if the motor is low on the transom, perhaps only one hole up, then the OMC/BRP 13" x 17" SST/Viper might be a better selection, as it feels a bit 'softer' (perhaps due to less cupping Tom?) than the Stiletto. The Stiletto is less forgiving when mounted low. When the engine is mounter higher on the transom, the 13.25 x 15 Stiletto outperforms the OMC/BRP 13" x 17" SST/Viper. The SST/VIPER feels too soft, and the Stiletto comes into its own, feeling much like the SST/Viper might the lower engine mounting height.
cooper1958nc	posted 09-08-2008 06:42 PM ET (US) Huh? You go faster at less RPM with one prop (the larger pitch), and we are debating which prop to use????? Another data point confirming what I have been saying. Keep engine speed down for best economy and longevity. Here, the speed is better as well.