Newbie...what prop do i need?

[tallguy63]

I just been given a '89 Bayliner Capri 19ft with a 3.0 Cobra motor. 140hp. the prop is chewed to hell and the boat is in good shape mechanically. the motor is spotless and runs well. i've never put it in the water yet. but it needs a new prop and i know absolutely nothing about props. it has a 13.25 X 15 on it now. i know nothing about pitch. can someone make some suggestions? give me the prop and pitch basics? i can't find a general explanation of the entire thing. the lake it will run in is relatively small so i think acceleration is a little more important than top speed. i think it might tach out at 5500? maybe? i really don't know much about boats. thanks

 

[Silvertip]

Re: Newbie...what prop do i need?

If you can't find "prop pitch theory" you didn't look very hard but pitch is essentially the amount of bite the prop takes on the water. Theoretically, your 15P prop moves the boat forward 15 inches with each revolution. But since the prop operates in a liquid, there is slippage in the neighborhood of 10 - 20% depending on prop design, hull efficiency, etc. To determine what prop you need you need to make some wide open throttle runs with an average load and note the maximum engine rpm and speed. Then and only then can you determine if you have the right prop. The correct prop allows the engine to operate at its maximum recommended wide open throttle rpm. For your boat that is very likely 4600 - 4800 rpm and definitely not 5500 RPM. If the engine runs faster than recommended you are under propped and need to increase pitch. If the engine does not reach the recommended rpm band the boat is over propped and you need to decrease pitch. Since you know what the boat came with, replace it with the same prop. Since you haven't had it in the water yet, you have no idea what current performance is and therefore should not even think about altering prop pitch until you see what you have. Maximum speed will be whatever the pitch of the selected prop gives you.

 

[Mark42]

Re: Newbie...what prop do i need?

Get the same prop that is on there now, it is probably the best all around performer for that boat. If you don't care for the performance, then go up or down a pitch or two.

The only way to know how it will perform is to try it out.

If you end up buying an different pitch, keep the first for a backup should you break one out on the water. It is also a good idea to get one of those floating prop nut wrenches in the right size for your drive.

 

[MikDee]

Re: Newbie...what prop do i need?

Chances are with that combo a 15" pitch sounds about right, see if you can get a used, reconditioned, or composite (cheap) prop, in that pitch, and if it's not enough pitch, once you get your act together, get a better prop, the 15" pitch will be perfect for tubing, skiing, or as a spare.

 

[steelespike]

Re: Newbie...what prop do i need?

There is a chance a prop shop can repair your present prop possibly for less than a used or reconditioned and much less than new.

 

[tallguy63]

Re: Newbie...what prop do i need?

thanks for all the advice, i think getting the exact same prop would be best, so.....an even dumber question: how do i know the pitch of the prop i have? pitch isn't mentioned on the prop itself. should i run a parts number? the prop has multiple chunks out of it, so i don't repairing it is an option, but thanks for the idea

 

[Silvertip]

Re: Newbie...what prop do i need?

You gave the pitch in your opening post. 13.25 x 15 means the prop is 13.25 inches in diameter and 15 is the pitch in inches. You can measure the diameter but it takes a pitch block to measure the pitch. I also think this is the wrong prop. If you go to the iBoats prop web site your outdrive calls for a 14-1/2 to 15-1/2 diameter prop. 13.25 diameter props are about the diameter a mid range outboard would use.


http://www.boat-props.com/propeller...cruiser&year=1989&rotation=Standard&x=38&y=10

 

[tallguy63]

Re: Newbie...what prop do i need?

thanks silvertip....yur saving me headaches...i really didn't know the pitch was included in that number...very new to boating...thanks

 

[tallguy63]

Re: Newbie...what prop do i need?

You gave the pitch in your opening post. 13.25 x 15 means the prop is 13.25 inches in diameter and 15 is the pitch in inches. You can measure the diameter but it takes a pitch block to measure the pitch. I also think this is the wrong prop. If you go to the iBoats prop web site your outdrive calls for a 14-1/2 to 15-1/2 diameter prop. 13.25 diameter props are about the diameter a mid range outboard would use.


http://www.boat-props.com/propeller...cruiser&year=1989&rotation=Standard&x=38&y=10

As much as i appreciate your help silvertip, i looked under iboats prop search under 15inch pitch and got nothing larger than 13.75 in diameter....where exactly is iboats saying i need a 14.5-15.5 prop?? not that i doubt u, i just don't see it myself. thanks

 

[Silvertip]

Re: Newbie...what prop do i need?

Go here first.

http://www.boat-props.com/propeller...cruiser&year=1989&rotation=Standard&x=38&y=10

You need to first select your outdrive which I will assume it's a 3.0 Alpha 1 (1986 or newer). It then opens a page with a listing of props for that outdrive. Select 15P and you are then given a list of props and they indeed show props from 14-3/4 to 15-1/2 inches in diameter. Don't worry about diameter within the prop range for your engine. Diameter changes a little with pitch.

 

[tallguy63]

Re: Newbie...what prop do i need?

my outdrive does not say mercruiser anywhere...only cobra. i'm pretty sure its the cobra 3.0 model 400 drive...

 

[steelespike]

Re: Newbie...what prop do i need?

I think silvertip missed you are running the Cobra 400.If you go to your application I think you will find a 15 inch prop.Go with that 13.75.
Perhaps the Cobra runs a different ratio than a Merc alpha 1.

 

[Silvertip]

Re: Newbie...what prop do i need?

Yup -- Sorry I sent you in the wrong direction. Blew right past the Cobra. If you have the 13 spline the unit takes small diameter props. The later 15 spline takes the larger diameter props.