Chris,
The 391200 number is a stock number and you would need a cross reference chart to determine pitch. Aluminum props work very well, are cheaper, and are less likely to cause damage to internal parts in the outdrive during a prop strike (or during "dredging operations"
).
Yes, the Fast Facts chart does show the 14 1/2 x 19, and I would tend to follow that recommendation, for what it is worth. The 15 x 17 is listed on the 258. The larger diameter shouldn't change rpm that much, but larger diameters are typically used on larger heavier boats. You may gain speed and fuel economy with the 14 1/2 x19. Your 17 inch pitch may cause as much as 300 to 400 rpm more than the 19 inch prop listed in the table. I guess I would start with what your actual max rpm reading is with the outdrive trimmed up for best speed and with the boat topped out. My '95 190 Horizon with the 5.0EFI Ford had a max rpm range of 4200 to 4600 rpm, with a redline of 4600. I believe the 5.8 is the same rpm range. With the factory 14 1/4 x 21 prop my boat hit the 4600 rpm mark. Keep in mind that prop pitch and outdrive gear ratios can affect rpm, and may make comparing one boat and prop to another boat with the same prop very difficult. Most people will recommend choosing a prop that puts rpm in the top half of that rpm range. You may have specific reasons for changing to a lower pitch prop for towing skiers or tubes, or maybe changing to a higher pitch prop for cruising while lightly loaded. With these changes, it is hard to predict exact results.
Ideally we would all have a prop dealer next door that had demo props for trial use. In reality some dealers may still do this. I'm guessing that the 15x17 could be replaced with a 14 1/2 x 19 and you should see some improvement in cruise speed, top end, and fuel economy, with little change in initial acceleration. The factory usually has a reason for the prop they select.
In general,
1. A 2 inch increase in pitch (17" to 19") is likely to cause 300 to 400 rpm less at top end.
2. An increase in diameter will usually cause a minor reduction in rpm, but larger props are usually for larger heavier boats.
3. Different brands and models of props of the same diameter an pitch may perform differently.
4. A prop with a cupped trailing edge on each blade will have the same effect as an un-cupped prop with maybe 1 to inches more pitch.
5. Tachometers can be inaccurate!!! So, it is good to find some way of comparing your tach to another gage or tester.
6. OMC SX and Volvo SX props are 19 spline hubs. Mercs are 15 spline hubs. I found this out the hard way
Hope this helps.
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