Prop size

[rmoore]

I have a 1993 crownline with a 4.3 lx engine. I have noticed that when i go to full throttle my engine will go up to 5400 rpm. The book said it should be around 4400 to 4800 rpm. The speed of the boat at 5400 rpms is 50. I have check to make sure the tach rpm's is correct with my shop meter.<br />The prop size is a 21 pitch with a dia. of 13 3/4 stainless steel. What prop should I be running with this boat.<br />

 

[ctak]

Re: Prop size

I have a 94 bayliner w/ a 4.3lx and have the same problem. I bought the boat with a stock aluminum 23 pitch prop. At full throttle I could run it up to 5300 rpms. Asked the local shops and they said to check the tach. So I got a fluke engine meter and everything checked out. I then bought a 5 blade stainless prop with a 25 pitch. When I put her on I could still run the rpms up to about 4900. I'm not sure if the readings are correct or not. My speedometer says I'm doing about 56. Anyway I just run it below full throttle keep it between 3300 - 4000 rpms.

 

[Frank Havens]

Re: Prop size

I don't have my shop manual handy so I'll try to answer you from memory. You say your manual shows that your RPM's should read 4400-4800 but at full throttle they are 5400. In order to bring the RPM's down where they should be, your prop needs more"bite." I believe you need to to increase the pitch. Also, your prop is only 13 3/4"? Maybe you could use a larger diameter in addition to increased pitch.

 

[Grant Ingle]

Re: Prop size

Depending on the length of your boat you definately need to increase the prop diameter. When you do this it will require you to drop the pitch size. Try a 14.25 x 21 from Mercury or a ss 4 blade like the 14 x 19 or 21 Offshore they offer.