Pitch vs. Diameter + Determining correct RPMS

[Striper500]

I own a 1995 18 ft Hydra Sports, that when loaded with fuel, gear and people weighs about 2800lbs. It's powered by a 1995 130HP 2 stroke Johnson outboard. When I purchased the boat, it came with a 15x17 OMC Aluminum Prop. This past weekend, I tried the OMC SST Stainless Prop 14.75X19 and was successful in picking up about 3or so knots at 4750rpms.

1) The increased diameter seemed to have no effect on the take off. I thought I was supposed to lose 200 RPMs for every 2 inches of pitch. The boat can still tack 5500RPM (not that I keep it there)

2) Most of the bigger pitch props appear to be smaller in diameter, is that to maintain the "whole shot"

3) does a 14.5 X 21 make sense given the fact that I can still tack so high ?

Any feedback or insight would be appreciated. This is an extremely heavy boat for it's size, and I am trying to maximize the speed while being congnisent that with 85 galls of fuel and 3-4 people it may struggle.

Thanks
Chris

 

[Silvertip]

Re: Pitch vs. Diameter + Determining correct RPMS

Picking up 3 or 4 knots at 4750 RPM is meaningless. The key is to select a prop that allows the engine to operate at the manufacturers maximum recommended wide open throttle RPM. Since you said you can see 5500 on the tach, that's where it should be so the prop is correct (or nearly so). Whatever speed that provides at WOT is what you get. Increasing pitch in an attempt to get more speed would be a waste since the engine would not likely have enough power to twist the prop. (More pitch needs more power!). Diameter is function of prop design. As pitch goes up, diameter comes down a bit. As pitch is reduced, diameter increases slightly. Diameter has very little to do with performance and everything to do with design.

 

[Dhadley]

Re: Pitch vs. Diameter + Determining correct RPMS

5500 is about as low an rpm you'll want to see with that motor. 5500 is border line lugging it. Those motors like rpm and you should be looking to get as close to 6000 as possible with an average load.

You are probably still able to get 5500 with a larger pitch because the SST will have more lift than the aluminum prop you had. More lift equals less drag. The other design function is that the SST will run higher than the aluminum prop. Try raising the motor, that should gain a few more top rpm.

Diameter alone may or may not mean anything. It all depends on what the manufacturer does with it. Generally speaking, the larger the diameter the higher you can mount the motor. On some props even a 1/4" more diameter can mean a lot as far as performance goes. Again, depending on design.

If I were setting up your boat I'd stay with the 19 SST and raise the motor. That should gain some rpm and gain some efficiency too. Only drop pitch as a last resort to simply gain rpm.

 

[Striper500]

Re: Pitch vs. Diameter + Determining correct RPMS

That was very helpful. One last question. I felt as though there may have been some vibration when the boat was digging out of the whole onto plane. Is it common to feel a little vibration ? the prop was used, but I stripped all the paint to inspect before installing. It appeared to be in good shape, and doesn't vibrate on plane or an other speed for that matter. Thoughts ? could it be the way it was manufactured ?

 

[Dhadley]

Re: Pitch vs. Diameter + Determining correct RPMS

It's probably a little out of balance, most used ones are somewhat. It may be on the verge of venting too much at holeshot too.

 

[Striper500]

Re: Pitch vs. Diameter + Determining correct RPMS

Is there any damage that can be done if it's a little out of balance ? to the seals ? etc ? as mentioned it only happens probably between the 3700-4000 range as the boat is planing. Any other RPM its fine.

 

[Dhadley]

Re: Pitch vs. Diameter + Determining correct RPMS

I guess there could be eventually. It all depends on how bad itis.

 

[Randybeall]

Re: Pitch vs. Diameter + Determining correct RPMS

Yes, vibration and pulsing of load will take their toll on any mechanical system. You could have an out of balance condition or even one blade which does not have exactly the same pitch as the other two. That will also cause a vibration. Perhaps that inexpensive used prop is not quiet so good of a deal when it needs to go to the prop shop before it gives you other issues. Sorry 'bout that.