201 Liberator OMC cobra drive help

[201Thumper]

I bought a 1988 201 liberator with 270hp king cobra drive, seller had all the records of repairs and the drive was replaced couple of years ago. boat had a damaged 14.25x23 ss viper prop on it, I did some research over winter and four winns site said i should have 14.25x19 viper so I bought a 14.25x20 SS Raker, I just ran the boat and full throttle it topped out at 5200rpm ( WOT is 4200-4600 according to 4winns ) by the way speedo didn't work so I don't know top speed, I think it would have gone higher if I trimmed it more but didn't want to blow motor.

Ok now my question, it looks like the 23 degree prop would be the right for WOT 4200-4600, but i'm confused when drive was replaced it appears to me it is a different gear ratio then what it was built with (1:41:1 ) is that right? and if so what would it be since it looks like i have the highest or lowest ratio (however you figure it) available?

As I understand it the motor turns 1.41 turns to 1 turn of the prop, correct me if wrong

1988 liberator 4winns ratios 1:68:1 4.3 liter
1:50:1 5.0 liter
1:41:1 5.7 liter
1:43:1 460ci

 

[Howard Sterndrive]

Re: 201 Liberator OMC cobra drive help

your theory and testing is a bit flawed. First of all don't trust the dash tach - verify it with a second unit.

I did some research over winter and four winns site said i should have 14.25x19 viper

Not valuable info from them. Boats should be propped by WOT rpm, not by suggestions from the builder. unfortunately its a trial and error thing.

so I bought a 14.25x20 SS Raker,

the pitch on Rakers is a different concept it seems, they may run quite different on a heavy hull than they are designed. You might not be getting beyond the slip designed into the prop with the vent holes. Rakers are for lightweight bass boats. but they'll work well on an a small 16 or 17 foot bowrider with a big i/o V6 or V8
They are bow lifting props. so you need to trim in to get the bow to stay down.
Some weld in the vent holes, but I don't know what that will result in personally

it appears to me it is a different gear ratio then what it was built with

opening the cover and counting gear teeth, or counting shaft rotation the only ways ratios can "appear" to anyone. If you want to know for sure you'll have to do one or the other.

you probably have the correct unit on there.

 

[HT32BSX115]

Re: 201 Liberator OMC cobra drive help

Howdy,


That's a fairly heavy boat....


Mine had a 460 and a 20p 3 blade prop on it (19 cupped and repitched to 20) and would turn about 5100 at 60mph. (300 rpm higher than max rated)

The slip calculated out to about 12% or so.... The following calculator works well. Use 12-15% slip as a starting point. http://www.go-fast.com/Prop_Slip_Calculator.htm

Once you have accurate RPM and speed (GPS), you can get accurate slip numbers.


I do not think you have a 1.41:1 drive ......or That boat NEVER had a 23p prop on it. Or, if it did it was SEVERELY over-propped!


That boat (if it could even get to max RPM) with a 1.43:1 drive & 23p prop would go almost 60mph at 4600 RPM.

Before you buy props, you NEED to determine the drive ratio. either pull the top cap and count teeth, or pull the plugs and slowly turn the engine (in gear) and count revolutions. Have a helper count prop revolutions.

You MUST have have an accurate drive ratio, to use any prop slip calculator. You also MUST have accurate speed ad rpm measurements too....


Just because Four Winns (and your OMC owners or service manual) says you have a 1.41:1 drive etc, doesn't make it so!


Regards,


Rick


Oh, and by the way, for the "MODs", this thread might need to be moved to "Prop Questions and Topics"