Added power Prop size

[Autowizzard]

I have searched and not found a definate answer so I'll try here. I have a 1992 four winns Horizon with a 302omc I/O Stock prop is a 14.25x21. I have insalled a new motor with Id safely say 75 more hp. I know stock Max rpm is 46-4800. WIth the stock prop I can not get any higher top speed then before (50) but my rpms are 52-5400. I would figure spinning higher rpms would gain more speed? Acceleration is much better. I also have a 4 blade prop which keeps rpms down to 5000 ish but still 50mph. Yes speed is done VIA gps. My question is when adding power am I correct to go next size up for speed? I already have great acceleration and power but Like to go fast on occasion and not overrev my lower unit or motor. And Yes I keep rpms low 90% of the time as I know OMC stuff is hard to replace.

 

[steelespike]

Re: Added power Prop size

Do you know your geare ratio? guessing 1.62 and 12% slip 5400 should make about 58 mph.
Using 1.50 12% slip would be about 63 mph.
Using a gps of 50 same numbers 1.62 = 4600 rpm, 1.50= 4300.
I would say your tach is whacky.Unless you have very very high slip.
Perhaps the ignition is bad producing only 50 yet erroneous 5400 rpm.

 

[Autowizzard]

Re: Added power Prop size

I will definately bring out another tach to verify but The feeling and sounds of the motor, I would venture to say the tach is right. Besides the lower than expected top speed the boat performs very well. With the old stock motor I would also hit 48 to 50 at 4700 with the same tach. I am a bit surprised that there is no change from the higher hp and rpms.

 

[steelespike]

Re: Added power Prop size

The Gps should be accurate.50 mph at 5400 rpm indicates about 24 to 30% slip depending on the ratio.
That kind of slip is more like for a hard to push boat like a pontoon in fact some are lower.
What size and type is the 4 winns? If the tach is accurate It would seem you need a 25" prop.
But what of the speed If the tach and gps are accurate what's with the speed at 5400 rpm.
It would appear you have a really poor prop or some sort of hull problem.
Does the boat respond to up trim? Is there a foil on the outdrive?