Aluminum Prop!

[bluewaterposer]

Theoretical question?
My current hull weighs 2800 lbs ready to go, My 1.7 115 4 cylinder, 2 stroke Max's out at limited 5150 rpm at 31 mph,. 5150 is max allowed due to 4 cylinder design.

If I repower with a new 150 Mercury 4 stroke, 3.0!
The 150 is max 5800 rpm, oops, I haven't given you gear ratio, 2.08 on the 115!
BWP

[bluewaterposer]

Mercury 150 is 1.92, now, what prop pitch should I shoot for, the prior owner said max speed he saw with a 2.0 2.5 v6 was 38, only information I have.
With the 115 there is a 2k delta between rpm vs mph.

4000 rpm is 20 and 5000 rpm is 30, very even between these ranges.

Much advice is requested, thanks in advance!

BWP

[massbay]

I've done a bunch of repowers with that motor.
heavy 19, 20, light 21. I use a 17" 4 blade al merc prop.
Runs in the 40 to 42 mph range.
3.0 4 cyl is faster than the 2.5 v6

[bluewaterposer]

I've done a bunch of repowers with that motor.
heavy 19, 20, light 21. I use a 17" 4 blade al merc prop.
Runs in the 40 to 42 mph range.
3.0 4 cyl is faster than the 2.5 v6

That's interesting!
This is for future use to ensure proper break-in!
I think a 2.1 liter 4 stroke would equal my 1.7 liter 2 stroke in the torque area.
So 3.0 liters would be a blast!
BWP

[massbay]

A 2.1 would give more performance than your 2+2 motor.
2+2 was the slowest motor merc ever built.

[ken2]

if a 115 HP "propped correctly" and "running properly" topped out at 31 MPH.
a 150 HP would run 35 MPH on the same boat.

Massbay brings up a good point. The 115 was a fairly weak 115, and I'd argue the new 150 is a strong 150, so hopefully you'll see a little better speed increase.

Slip ratio's will be different either way cause your running different props.
your old motor if it was a regular Mercury 115, had a 4750-5250 RPM range, and a 2.07 gear ratio. It also had a 4.25 diameter gearcase/lower unit
The new motor, will have a 4.8 diameter gearcase, and larger diameter prop.

I'd say 15 pitch aluminum, but I don't run aluminum props: I'm basing this pitch on an expectation of 10% slip and 35-39 MPH.

[bluewaterposer]

Rock on Ken!
Now to get the Admiralty to agree with a new outboards!.

My report is complete, now to see what happens!
BWP

[massbay]

Not quite complete!
That motor needs a good steering system. Needs a full size hydraulic steering not a cheapie (baystar).
Had an old timer bounce his factory rigged 21 sailfish off a channel marker because he couldn't pull that motor around.
Had a repower cust wanted to return his motor cause it was dangerous with old mechanical steering.
That motor needs much more trim than you're used to.