Prop size help please??

[tygray6]

Hello all,

I previously had a 120 hp merc in a 19' boat that weighed around 3k pounds. The motor couldn't keep up with the demand I wanted with kids and water sports.

I recently changed the motor out to an mcm 898/ 305 V8 and changed the top end of the outdrive to a 1.62/1 ratio. After all the install work the boat runs great but I'm in an uncharted area with sizing this prop needed.

I tried the prop that bogged down my 120 which was 14.5/19P and it's hard to give details because I pegged the RPM's. My guage goes to 6k and at 6k I was a little over 3/4 throttle so I could of probably been closer to 6.5k when full open. I was at around 30-35mph with that rpm and I was hoping I could get some help with what prop to try next to get me within range, or what couple props to try. Granted this was with minimal weight in the boat and no water sports as I did not run long with the rpm's that high as to not damage anything. The ideal rpm's for that motor is 3800 WOT.

Any help would be great as I'm taking it to the lake next week.

 

[Poormansboating]

I know someone with a similar setup and he uses a 21 pitch and still he's at 4200 and his calls for 3800 also his boat does great at top end maybe 55mph or so. Find a local marine shop and see if they will let you try out different props to find the right one alot of them will.

 

[Scott06]

Hello all,

I previously had a 120 hp merc in a 19' boat that weighed around 3k pounds. The motor couldn't keep up with the demand I wanted with kids and water sports.

I recently changed the motor out to an mcm 898/ 305 V8 and changed the top end of the outdrive to a 1.62/1 ratio. After all the install work the boat runs great but I'm in an uncharted area with sizing this prop needed.

I tried the prop that bogged down my 120 which was 14.5/19P and it's hard to give details because I pegged the RPM's. My guage goes to 6k and at 6k I was a little over 3/4 throttle so I could of probably been closer to 6.5k when full open. I was at around 30-35mph with that rpm and I was hoping I could get some help with what prop to try next to get me within range, or what couple props to try. Granted this was with minimal weight in the boat and no water sports as I did not run long with the rpm's that high as to not damage anything. The ideal rpm's for that motor is 3800 WOT.

Any help would be great as I'm taking it to the lake next week.

Would check your tach as 6 k plus likely inaccurate

 

[tygray6]

Would check your tach as 6 k plus likely inaccurate

The tach worked fine with the 120 and the idle rpms read around 1k. I can manually check the engine to make sure but by those accounts the tach should be okay.

 

[Scott Danforth]

You went from 4 cylinders to 8 and didnt change the tach settings?

My guess youre at 3000 RPM

 

[Scott06]

The tach worked fine with the 120 and the idle rpms read around 1k. I can manually check the engine to make sure but by those accounts the tach should be okay.

There is a knob or dial on the back with 4 cyl, 6, cyl, and 8 cyl settings and idle of 1000 would be very fast