Yet another Prop Question

[Littlehoe]

So the general consensus I have found is that rpm should change by about 200 rpm per degree of pitch????

Well I have a spitfire 13.4 X 15p and a spitfire 13.8 X 13p, the engine is a 2012 yamaha 115 and its running on a 23' xcursion with 2 full toons and the center 2/3 toon. Lifting strakes on the insides of the toons, fully covered underneath.

I run one person no extras on the boat at about 5200rpm WOT with the 15p and hit GPS 27mph, when I switch to the 13p I am banging off the rev limiter at 6300rpm and cant even come close to WOT and my top speed is about 26mph. These are with the motor in the lowest holes. This weekend I lifted the engine 2 holes and ran the 15p expecting to pull some extra rpms however to no avail. I ran the same rpm and speed there as I did all the way down.

What is the next step? They don't make a 4 blade spitfire in a 14p and the spitfire has been the best prop for holding and not spinning out so far. Can anyone point me in the right direction. Even with a load and pulling a 3 man tube I am banging the rev limiter with that 13p. with the 15p and a load and tube I am at like 4800 rpm and only 20 mph going straight, turn a corner and my revs pull down to near 4000 and I lose all momentum.

Help.......

[DWRAT]

Have your current 15 repitched to a 14 or have your 13 repitched to a 14.
I personally moved 350 rpm going from a 15 Enertia to a 14 Enertia..
I don't know where the 200 rpm change ever came from.

[zoraduntov]

I just installed a 14x14p Spitfire propeller on a Mercury 115hp Command Thrust. It replaced a 16x13p
Black Max 3 blade prop. The engine WOT rpm stayed the same at 5700 rpm. The top speed went from 27 mph to 26 mph. I haven't had a chance to pull any tubers yet. I was hoping the 4 blades would hold better while taking sharp corners with tubers. It does feel like it accelerates quicker.

Mine is a 14p, maybe they don't make a 14p in the diameter you are looking to use.

[crookedlaker]

can you raise the engine higher, go up another hole or two, it is less expensive than buying new props
if you did not see a change raising it the first time you may still be too low.

good luck

Mike

[rancherlee]

You need to find another prop or drop the motor DOWN for better traction, The 15p is slipping 27% and the 13p 39% A 13p its about what you should be running, and with typical 10-15% slip for a straked pontoon would be right around 30-31mph. Since you are getting 26-27 mph it should be up on the strakes at that speed and 30-31 should be achievable with a light load though I due expect the F115 struggle to get on the strakes with moderate/heavy load. Does the pontoon accelerate smoothly? Does it hit a wall at a certain speed where the motor just revs up higher? are you playing with the trim at all?