Swapped lower unit gear ratio now ned help finding correct prop

[rocketman87]

I had a 1997 force by mercury 120 hp and lost the lower unit .

So I put on a 1995 90 hp lower unit

the 1997 120HP lower unit is 2.07:1

1995 90HP lower unit is 2.3:1

I have a Laser SS 48-16992A45 22p


But the rpm is now about 6000 wot not sure because I really didnt want to take it that high for very long...


I would like a prop size reccommendation....


I want to drop about 800 rpm .....


any help would be greatly appreciated....

 

[Frank Acampora]

Re: Swapped lower unit gear ratio now ned help finding correct prop



Well, it really depends upon the hull and the prop. Are you in a go-fast or a fishing boat?
Have you experimented with other props?

This is a 14 foot flat bottom with an older Force 125 and a 2 to 1 ratio lower unit. With a 21 pitch fully cupped stainless cleaver prop it turns 5900 RPM and 60 MPH. With a stainless Ballistic 21 pitch RPM drops to 4500 or 4900-- I forget which exactly-- but top speed remains at 60. So, you can not directly compare props of different design and from different manufacturers.

6000 RPM is not a speed to worry the engine but you will be using more gas so if you can maintain top speed at a lower RPM then a prop change is in order. Generally 2 pitches drop RPM about 400 and cupping the prop drops it about 200. So, if the laser is a cupped prop, and you want to lose 800 RPM try for a stainless cupped prop about 4 pitches higher.

However, since props are expensive and money is usually tight, try to borrow a test prop before buying or negociate a test then trade deal if at all possible.

 

[rocketman87]

Re: Swapped lower unit gear ratio now ned help finding correct prop

Thanks.. The motor is on a 1997 nitro bass boat 18 ' dual console.

I was wondering if I wanted to get a bigger diameter prop or just play with the pitch ?

 

[rocketman87]

Re: Swapped lower unit gear ratio now ned help finding correct prop

Will this one fit my boat

Mercury Laser ll 2 Prop 13 3/4" 25P Quicksilver 48-16550 Propeller Alpha 3 blade

 

[Frank Acampora]

Re: Swapped lower unit gear ratio now ned help finding correct prop

You need to be careful about the outer hub diameter. Most Mercury engines have a larger diameter gearcase than the Force line However, since it is a Mercury designed lower unit they may have changed the gearcase diameter. I have not had much experience with the late model Force engines yet. SO: measure yours first. ALL larger Mercury and Force engines have 15 spline propshafts so the hub kit will fit.