Prop Advice

alekseyss

Cadet
Joined
Jun 19, 2017
Messages
15
Hey guys, new to the thread and new to boating, I have a 1990 Four Winns Freedom 190 with a 4.3 OMC Cobra Inboard. I'm not sure what numbers count as what on my prop so I'll list all I have and hopefully you guys can help me out. My current prop is chipped and slightly bent on tips from trim being too low when loading boat. (Learned my mistake the hard way). Before my max speed was about 40MPH at about 4400RPM. Rubex Solas, 14.25x19, 5464A. Figured if I'm going to be getting another prop I might as well look for something that can give me more speed.. any advice? I literally know almost nothing about the boat world so any advice is much appreciated. Thanks Guys!
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
Welcome to Iboats,

What's engine min- min.max wot rpm range factory stated ? If with current prop obtained 40 MPH at 4400 wot rpm ( if happen to be near or at max wot rpm range) as loaded. Going + 1-2 pitch sizes will pull top end speed slight more while OB revs around middle wot rpm range, but will lose slight top hole shot.

Don't expect to achieve a wow top speed increment though. Can't have best of both worlds at same time. How loaded was combo when achieved 40 MPH at 4400 wot revs ?

Happy Boating
 

Maclin

Admiral
Joined
May 27, 2007
Messages
6,761
Your engine max recommend RPM is 4600 from the manufacturer. You want to select a prop that will let you get close to or achieve that rpm when running at WOT with your normal load. You are pretty close to ideal RPM right now in my estimation.
 

Maclin

Admiral
Joined
May 27, 2007
Messages
6,761
I should have included this too:

To "raise" RPM go to a smaller pitch. This will let the engine rev more freely, top speed may go down some but holeshot will improve. Go too low and top speed suffers and the engine can overrev too easily and sustain damage.

To lower RPMs go to a larger pitch. If the engine can turn the larger pitch at the top RPM's the top speed may go up some at the expense of holeshot. Go too high and the engine lugs, performance suffers on all counts, and can overheat internal components and sustain damage.
 

alekseyss

Cadet
Joined
Jun 19, 2017
Messages
15
I appreciate all the reply guys! Even though I must admit slightly confused, what I'm understanding is that what I have currently best for my boat? Just find another prop with exact same numbers?
 

Maclin

Admiral
Joined
May 27, 2007
Messages
6,761
When you wrote "before" I took that as when the prop you have was in good shape before the damage. If so then yes repair or replace that one with the same specs. The WOT numbers you gave show pretty much optimal for that setup with that prop in good condition.

Much depends on how accurate this set of numbers is...:
  • Drive ratio (I used 1.8)
  • Pitch
  • RPM at WOT (assumed your tach reading is accurate)
  • MPH achieved (best to use GPS over Speedometer)
That number set will give the slip factor. The calculated slip from your numbers is 10% or below, this is usually considered optimal. The top RPM of 4400 is in the range of the mfg recommended 4400-4600.

Does it have a good holeshot now? Or does it struggle at first to get on plane in just a few seconds? Is holeshot important, like for towing and skiing? If that is the case, then since you are 200 rpm from the top you could try a 17 pitch prop, but expect only the same top speed or a little lower.

You only mentioned top speed so that is why I said stay with a prop with same specs as yours. Without more help from the engine (to get that last 200 rpm now) you are close to what it can achieve speed wise with the 19 pitch.

When was the engine last fully tuned?
 

Sea Rider

Supreme Mariner
Joined
Sep 20, 2008
Messages
12,345
To dial the perfect pitch prop for your combo, It's all down to how loaded you currently go boathing with, how many passengers were on board when OB was running at 4400 wot revs ? Near fixed load is perfect to dial a spot on prop if like running at middle or max wot rpm range. variable loads really screws the equation.

Happy Boating
 

alekseyss

Cadet
Joined
Jun 19, 2017
Messages
15
Ok. Then I'll be on the lookout for an exact replacement. My holeshot I am very pleased with, it gets up and takes off without hesitation! I don't know if the engine been tuned, I bought it from a friend who is a boat dealer, but I've had two mechanic friends look at it and they both agree the engine is running very clean and strong, said it purrs lol. So once I get the new prop I should be all good to go! Thanks for all the help!
 
Top