how big or small prop can i go

[lizardfish]

1986 mercuryxd 25hp stand prop 10 3/8dia-13pitch how much can i move these numbers before it is bad for my engine,,, speed is not a big deal with me,, jus want good gas performance and the best for my engine life.

 

[gcboat]

Re: how big or small prop can i go

Prop sizing is aimed at your WOT rpm's. It would be a great help to you to find what the WOT setting is on that particular motor and guide your selection to those figures. Hopefully a Merc shop would be able to guide you in your selection.

 

[steelespike]

Re: how big or small prop can i go

You should read the stickies at the top of the page.We need your present prop size,max rpm and speed.You may be able to borrow a shop tach,or buy a Tinytach.Reasonable easy to install,fits any similar motor.Wouldn't hurt to know what kind of boat,description not brand.Motor height is critical to performance as well.Anti vent plate(just above prop) even with the bottom
of transom is considered a starting point.

 

[lizardfish]

Re: how big or small prop can i go

16ft alum flat bottom 5000-6000rpm, long shaft, top speed not for sure but a figure 25-30 give or take a few, prop size 10 3/8dia 13 pitch

 

[Silvertip]

Re: how big or small prop can i go

Your info doesn't help much. First -- the 5000 - 6000 RPM you provide is what the manufacturer recommends proping for. Second, you show an "estimated" top speed with a 5 MPH spread. It's tough enough to predict what will happen when we have "exact" numbers much less figure what's best using "guess work". We need to know what the engine is "actually" running at wide open throttle with an average load. If it is in the range you specified (actually closer to the top end than the bottom) you already have the best prop. If it can't reach 6000 you need less pitch but don't expect an increase in speed because it will likely be less. If it exceeds 6000 RPM then an increase in pitch is called for and a slight increase in speed may be seen. How much of a change is needed depends on how "wrong" the existing prop is. Propping for 6000 RPM will give you whatever speed that produces. There simply is no free lunch here. Besides -- you only have 25 HP. It ain't a rocket to start with. As for diameter, there is no choice here. Diameter is function of prop design. In other words you cannot slap a 15 inch diameter prop on an engine designed for a 10 - 11 inch.

 

[lizardfish]

Re: how big or small prop can i go

Your info doesn't help much. First -- the 5000 - 6000 RPM you provide is what the manufacturer recommends proping for. Second, you show an "estimated" top speed with a 5 MPH spread. It's tough enough to predict what will happen when we have "exact" numbers much less figure what's best using "guess work". We need to know what the engine is "actually" running at wide open throttle with an average load. If it is in the range you specified (actually closer to the top end than the bottom) you already have the best prop. If it can't reach 6000 you need less pitch but don't expect an increase in speed because it will likely be less. If it exceeds 6000 RPM then an increase in pitch is called for and a slight increase in speed may be seen. How much of a change is needed depends on how "wrong" the existing prop is. Propping for 6000 RPM will give you whatever speed that produces. There simply is no free lunch here. Besides -- you only have 25 HP. It ain't a rocket to start with. As for diameter, there is no choice here. Diameter is function of prop design. In other words you cannot slap a 15 inch diameter prop on an engine designed for a 10 - 11 inch.

If it is in the range you specified (actually closer to the top end than the bottom) you already have the best prop.

above sentence is all i wanted to know thank u very much