Not much change in rpm between 14 and 17 pitch

I have a heavy aluminum 20' Polar Kraft mod V with a 1995 johnson 130hp.The 130 has a different lower on it with a different gear ratio.It is supposed to be a 12:27 and it is a 14:26.The 130 had a 17 pitch SS prop on it but I could only get 4800 rpms out of it so I went to a 15 pitch prop not much difference so I got a 14 pitch SS prop and I still can't get past 5000 rpm's.I went out today and got 38 mph at 4800 rpm with the 17.I got 36 mph at 5000 rpm with the 14. I think there should be a bigger change in rpm between the 2 props.I think I should be able to run a higher pitch prop.I read somewhere that a 14 pitch prop would be for a large cabin cruiser.I have a used gear set to make it stock but I was told there was only 21% between the 2 gear ratios.Not sure if it would be worth the money to have the gear set changed.Maybe it is my powerhead.I am going to take it in to get a link and sync.I would like to get the rpms in the high 5k range.Any ideas?
Thanks in advance,Jeff

Re: Not much change in rpm between 14 and 17 pitch

If my info is right the standard ratio is 2.25./If my math is right 14/26 is about 1.85.
about 20%.Motor is rated at 6000 max.Your speed seems in the ball park.
I'm thinking maybe check the tach.It just doesn't seem you would be running those speeds
at 1000 to 1200 rpm below the max rating.

Re: Not much change in rpm between 14 and 17 pitch

I am pretty sure the tach works because I can snap the throttle in neutral it goes past 5K.I have a friend with a boat that is close to the same in weight and hull design.He has a 150 he is running a 21 pitch prop I believe and he can run 47 mph.Would 20 horses make that much difference?

Re: Not much change in rpm between 14 and 17 pitch

Its interesting that the 150 uses a 1.85 ratio so you're running a 130 at a ratio
intended for 150 hp. On boats roughly similar to yours a 150 turns speeds ranging from 41 to 50 mph.The few tests I'ver found boats roughly like yours but glass,glass typically runs faster than aluminum, in similar sizes. w a 130- runs 38-42 mph. with a 16" & 17" prop 2.25 ratio 5700 5900 rpm. The key in my mind ius your short 1200 rp running a 14 and 17" prop yet making closae to speeds typical I supose it could be the ratio but think a tach check would cost less than a link and sync.

Re: Not much change in rpm between 14 and 17 pitch

Running both sets of numbers through a prop calculator shows negative slip so either the tach is wrong, speedo is wrong, lower unit ratio is wrong, or props have been reworked to pitch different than stamped. Negative slip is impossible. Just because the tach will zing up to 5K does not mean it is accurate. And by the way, 5K with no load on an engine is a good way to screw it up.

Re: Not much change in rpm between 14 and 17 pitch

Thanks for the info guys.I am using a lowrance fish finder with GPS for my speedo.I checked the ratio by counting the rotaions of the prop compared to the flywheel.That is how I came up with the 14:26.The 14 pitch prop is a SS omc with the black coating it looked brand new but I could be wrong.I will see if I can get a hold of another tach.I was wondering though if it is just my tach wouldn't my motor be bouncing of the rev limiter with a 14 pitch prop.My boat is a modified V so it is like a SeaArk.It is pretty flat at the back.That hull design probably is not very efficient.So I am not expecting a speed boat but thought it would go a little faster.
Thanks again for the info

Re: Not much change in rpm between 14 and 17 pitch

Tachs can be wrong either way (fast or slow) so just because it is not reving high does not mean it is correct. Fact is, something is wrong with your numbers. Period. You cannot have negative slip -- that is theoretically impossible.