13" X 19P Compared to a 13 1/4" X 19P

[stickflipper]

What kind of difference can I expect between the two? I have a 13" X 19P and have way to much RPM, tried a 13 X 21P. Not enough RPM. Will the extra 1/4" be in between? 2003 115 HP Merc 2 stroke 5250 RPM rated. I can get 5800 RPM with the 19P and barely 5100 RPM with the 21P. I've already raised the motor 2 holes. Does the altitude affect my HP much, 4500 ft.

 

[WillyBWright]

Re: 13" X 19P Compared to a 13 1/4" X 19P

Altitude makes a big difference. If you had a V-6 I'd advise a high altitude gearset. But I don't think those are available for inline-4s per-se (unless you use one from a 75 or 90Hp). You might want to have the carb jetting looked at. Any dealer should be able to look at the chart and give you the best choices. Dealers at higher elevations tend to specialize in that. But no matter what you do, you'll never get 115Hp out of it. The air is thinner and draws less fuel. So you'll never get as many molecules of gasoline and oxygen per burn as if you were at lower elevations. You'll be a bit lower than 100Hp. The only way around thin air is with a supercharger. Merc is coming out with a 135 Verado. Would that fit your boat?<br /><br />As for the diameter, there are so many variables of prop performance that it's hard to judge. But small differences in diameter seem to have the least effect. Thickness of the blade and total surface area of the blades are two far more major factors.

 

[Silvertip]

Re: 13" X 19P Compared to a 13 1/4" X 19P

If the two props were from the same manufacturer and the same design (style) you should not have lost that much RPM. As Willy pointed out, two props from different manufacturers with the same specs can produce very different results. That difference is can be even more with just one jump in pitch. Did you have the same load and were you trimmed the same for both tests?

 

[stickflipper]

Re: 13" X 19P Compared to a 13 1/4" X 19P

The 19P was the original Merc from factory the 21P was Michigan Match. Will a Stainless Steel 19P do any better?

 

[imported_Voyager]

Re: 13" X 19P Compared to a 13 1/4" X 19P

Stainless will perform better, but what it will do to rpms is guesswork. You can always have your prop shop do some final adjusting to pitch/cupping to get the performance and rpms that you want.