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Prop Help

I have a 2012 SD 187 with Yamaha 115. I bought it with an aluminum 13x17 prop and WOT is around 6400 rpms per the tach and around 30 mph top speed. I was worried about the rpms being too high and top speed being too low so I bought a 13x19 prop per the Yamaha prop selector tool. Did a test run with the new prop and WOT was only around 4800 and about 28 mph. Also with the new prop and adding 3 adults and 4 kids the boat would not get on plane. I also have smart tabs for reference. Why would going from a 17P to a 19P drop the rpms that much?

Answers

  • formerlyFlyingV5formerlyFlyingV5 Beaver Lake, Arkansas, USAMember Posts: 8
    RPMs dropped because you put on too much prop. The standard test is that the engine should just reach max rated rpm at WOT with a light boat (no extra passengers or weight and less than 1/2 tank of fuel) on smooth water. I think the Yamaha 115 is rated at 6000 rpm. So whatever speed the boat makes at 6000rpm w/ WOT is top speed because the engine is producing max hp. (assuming it is running right). You should be able to see 40mph with that motor on a 187. Was the 13x17 prop in good condition or was it maybe a little bent and missing some metal?
    Fastest speed will usually occur with a 3-blade prop. Going to a 4-blade will get you up on plane sooner and better hole shot but will cost you a couple mph on top end. Every boat / motor combination is different and you may need to try a few props to get it perfect. Lots of good info and test data in this forum if you search. And the folks at PowerTech were very helpful to my prop selection. Good luck!
  • sbalexander1980sbalexander1980 USAMember Posts: 2
    I knew the rpms would drop going from a 17P to a 19P but 1600 rpms??? I was under the impression that each change in pitch was equal to around 200-400 rpms. Both the 17P and 19P prop appear new so I don't think the prop is bad. Engine runs and sounds great so I don't suspect that is an issue. I tested both props the same day with just me in the boat so conditions should have been equal for both. I guess I will have to see if a dealer will let me try out some props until I find the right one? Or take it and let them figure it out.
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