Re: Pitch and RPMs... While it is a "rule of thumb" that 1" pitch = 150 RPMs ( some say 200) it is no way "linear", i.e. 4 " pitch not necessarily = 600 RPMs. The RPMs that an engine gets is related to the torque curve and to the load. The torque curve is no way linear, and the load varies with prop pitch, diameter and the hull characteristics...all very non linear. I've changed prop pitch by 2 " thinking I'd get 300 to 400 RPM change and had the change be more like 600 RPM.
The real issue is not if you reach the specified MAX RPM, but if you are within the recommended operating range. For example, on my I/O ( a 5.7L, 260 HP MERC MIE (2bbl) in front of a Volvo 280 drive), the MERC factory manual says... operating range 4200 to 4600. Just so long as I am within the range, the engine will be happy. However, within that range, there are differences in operating characteristics. If for example, a 19 pitch prop would get me 4550 RPM and a 21 would get me 4250. Then I would be able to get a bit more top speed and a bit better fuel consumption.
Why is this... Assuming a 1.66:1 reduction and 14% slip at 4250 and 15% slip at 4550, the 19 pitch at 4550 would get me about 42.4 MPH, while the 21 at 4250 would get me about 42.3 MPH.
So where does the improvement in fuel comsumption come from?
On 4 stroke engines there is a parameter called (MEP) Mean Effective Pressure ( internal cylinder). As the MEP increases, the engine is working more efficiently and fuel economy improves somewhat. Increasing the prop pitch to 21 causes the engine to "work" harder ( again within design limits) and the MEP increases.
Some folks think that the prop that gets them the highest RPMS within limits is the best...not always so. Sometimes, top speed actually drops at the higher pitch with the engine still within operating range, then you get to choose betwen a bit more speed and a bit better efficiency.
The non linearity of the amount of "push" needed to move a hull under various conditions, and the non linearity of the engine's torque curve (how much "twist force" an engine generates at a given RPM) is what makes prop selection a bit of a "trial by error" :-) procedure.
BTW... From what one prop shop told me, changing a prop's pitch by one inch or so is about all you can do.
interesting...
this is why i explained to joe,a little is alot.if the engine is allowed to have more turns,more of the hull is gonna be out of the water,meaning the rpms will climb.this is the reason that 2" of pitch you changed gave you 600rpms-it's all about load...
it's like this there's a thing called "load",if that engine is allowed to run up to it's full rpm,the load on that engine is minimal...take the same engine,load it,so it will not reach it's full operating rpm,the load is much greater,right ? the engine is working harder,common sense,right ?i believe that's what you mean...i prop boats to run at the top of the reccomended range,i do that for a reason,it's referred to as "propping lite",by doing this the boat will plane at a lower rpm,the load on the engine is greatly reduced and you're gonna burn less fuel...as i explained to joe earlier,most of the boats i see,they're overwheeled.
there's other variables that come into play with propping a boat,and it is trial and error,there's no exact science to this.
stainless outboard and sterndrive props,that i 1 inch thing is correct.inboard props can usually be moved ahead or backwards 2" in pitch,meaning,if a prop starts out at 17" pitch it can be changed to a 15" pitch and it can be changed to a 19" pitch...
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