Prop sizing questions

[atthelastminute]

I want some opinions on my prop selection. I recently replaced my outdrive with a sterndrive unit and purchased a piranha prop at the same time. I got two sets of blades, 14x19 and 15x17. This weekend with my engine finally tuned to what I consider to be perfect, on reasonably smooth water I got 28mph at 4600 rpm with the 15x17, and 33mph at 3980 rpm with the 14x19. My engine WOT range is listed as 4200-4600 on the engine. Running Piranha's prop finder on the 14" prop specs it says to go to a 15x19 prop, but normal logic says that would make the engine run slower. Anyone have any input on what would be my best choice here?

By the way, I am using a speedo for the speed, and a digital tach off of the #1 plug wire for the rpms.

Thanks,

 

[Silvertip]

Re: Prop sizing questions

You proved your point. Increasing pitch lowers engine RPM. But because you lower engine rpm speed may actually increase. However, you pulled the engine off its recommended wide open throttle rpm band so it is now lugging badly and very likely burning way more fuel than necessary with the engine reving more freely. I take it this is a 3.0L engine? Depending on the boat, I would suspect that you should be seeing faster than 28 MPH. Your statement about "what I feel is perfect tune" may not be accurate. What you feel and what is indeed perfect tune are two different things. Is it tuned to spec? Since some work was apparently done, does the throttle open the butterflies in the carb all the way?

 

[atthelastminute]

Re: Prop sizing questions

The engine is a 3.0. As far as tuning goes, the compression tests between 145 and 155, within spec; I have all new ignition parts, the dwell is staying right in the middle of the range, and I am using a digital dwell/tach/timng light to set timing and check rpm and timing is set to spec.

With a lower pitch prop the engine goes into the WOT range without hesitation, so I don't feel like the carb is causing a problem. I am hitting 28mph at 4600rpm using the lower pitch, but 33mph at 4000 rpm with the higher pitch. So yes, the boat can go faster, and should go faster than 33. My question though was how a 15x19 would increase engine speed over a 14x19. Speedwise, I am not trying to reach something higher, the boat is a 16' runabout, and at 33-35mph it gets a little too bouncy if the water is choppy at all.

 

[MikDee]

Re: Prop sizing questions

I doubt 1" in more diameter, is going to make it better, it will just increase the load on your motor, you'd be grabbing more water, you are overpropped now with the 14X19? probably slow you down at tha same time.

I may be wrong, but I think those Mark Twains are kind of a heavy, and not speed designed.

 

[Silvertip]

Re: Prop sizing questions

Your original post said the two props were 14 x 19P and 15 x 17P. Now you are saying they are both 19P -- which way is it? You also need to realize that unless the two props you are comparing are from the same manufacturer and same family (prop design) you cannot draw positive conclusions on how they will perform. A 19P prop from Company "A" may not and usually does not perform the same as a 19P prop from Company "B". High Rake, Low Rake, cupped, uncupped, vented, non-vented, stainless, aluminum, progressive pitch are all factors that affect prop performance. To comlicate matters even more, what works well on one boat may not work worth a darn on another.