another prop ?

[xtrema2]

I am new to pleasure boating and dont know anything about it. I enjoy skiing and tubing and was wondering if i have the right prop.
BOAT: 1994 maxum 1800 sr2
Top Speed: 4 people 52mph
Engine: 4.3lx 180hp
Outdrive: alpha one generation 2 i think
Prop on it know is 14.5x17

As i said i am more interested in hole shot than top speed but a happy medium is ok too. Thanks for your help
I almost forgot it is a michigan wheel aluminum prop

 

[Silvertip]

Re: another prop ?

You missed the most important bit of data. What is your wide open throttle rpm (observed) with an average load. RPM is what determines whether or not you have the right prop.

 

[xtrema2]

Re: another prop ?

I will find out tomorow and post the findings thanks

 

[xtrema2]

Re: another prop ?

Wot Rpm's Are 4400

 

[180shabah]

Re: another prop ?

Something ain't adding up here.

Small boat, 4.3, 17" prop, 52MPH and only 4400RPM????

What is the gear ratio on your drive?
How did you determine Speed?
Have you verified tach accuracy?

 

[xtrema2]

Re: another prop ?

dont know
speedo on boat
no


What should they be?

 

[180shabah]

Re: another prop ?

Gear ratio could be either 1.62 or 1.81, it is stamped into the housing right next to the S/N.

Dash speedometers are notoriously innaccurate, but speed is irrelevent most of the time anyway.

Other than generic recomendations for lower pitch and changing to a four blade, you will not get any good advice without an accurate WOT reading.

 

[Silvertip]

Re: another prop ?

Using a 1.6 gear ratio, the numbers you give present an impossible situation according to the prop calculator. Either speed is wrong, tach is wrong, both are wrong, or perhaps the gearing is wrong. I used both 1.5 and 1.6 gears. Both sets of numbers provide negative slip numbers which is not possible with a prop running in a liquid.

 

[180shabah]

Re: another prop ?

ST - What prop calculator do you use?

 

[xtrema2]

Re: another prop ?

I will try again this coming weekend hopefully. I have a gps I can check speed with. I appreciate yall's help

 

[180shabah]

Re: another prop ?

GPS speed will be nice, but accurate WOT RPM and gear ratio will still be needed. Also, double check the prop pitch

FYI - I get 50MPH turning a 21" prop at 5000RPM through a 1.81 ratio drive.

 

[wire2]

Re: another prop ?

Out of curiosity, I worked it out the long way,

Xtrema would go 44.27 mph at 4400/1.6 assuming zero slip.

Shabah, yours is 54.93 at zero.

This one is easy;
http://www.go-fast.com/Prop_Slip_Calculator.htm

 

[xtrema2]

Re: another prop ?

As I said I am stupid. So another stupid question.


What is prop slip?

 

[180shabah]

Re: another prop ?

it is the difference between the theoretical distance yor prop travels in one revolution vs the actual distance it travels in one revolution.

 

[xtrema2]

Re: another prop ?

Thanks guys, i really apreciate yalls help. I love this forum

 

[180shabah]

Re: another prop ?

Dialect Lesson:

the plural of y'all is all y'all

 

[xtrema2]

Re: another prop ?

Not where Im from feller