Prop selection

"I just bought a 2002 Mercury

"I just bought a 2002 Mercury 125hp 2 cycle motor for my 1986 Bayliner 19' cuddy
My old motor, a Force 125 ran well with a 13"x17p prop on the boat
The new motor does not come with a prop and the selector I used suggests a 13x21 or 13x23p prop for my usage (fish/ski)
Does this sound right?
Should I be looking for a 21p or 23p prop rather than a 17p prop?
The boat ran well with the old 17p when using the old Force motor
Just wondering if the new motor change would make that much difference in prop selection
Thanks"

"Joe, it looks like whatever y

"Joe, it looks like whatever you used as a prop selector is pretty much bang on.

Your (new) Merc has a gear ratio of 2.07:1 with a WOT range of 4750-5250 rpms

Your older Force would have had 1.7:1 gears with a similar range (4500-5500).

The lower gears on the Merc indicate that you can turn a larger prop (pitch/diameter or a combination thereof) on your particular rig.

I threw all your numbers into my software and it looks like the 21 pitch would be the optimum prop for your new motor.

That would be in a standard, non-cupped, 3 blade made of either aluminum or stainless..."

"Thank you Graham,
I was just

"Thank you Graham,
I was just concerned as I had no "reference point" with the new motor and the difference (17 to 21) seemed like a big jump
The difference in gear ratios explains it
Thanks again,
Joe"

"Joe, another question if you

"Joe, another question if you don't mind. What kind of top end speeds were you getting with the 125 on the back of that rig? And how did it generally handle?

Since you did not go with more horsepower, sounds like you were generally pleased.

I ask because I am working on a project for a guy - similar boat and we have been trying to decide exactly what to hang off the back in terms of horsepower..."

"Glad to help,
especially aft

"Glad to help,
especially after all the times you've helped me
I have a 19' Bayliner cuddy and the max hp for the boat is 125
I was running a 12 x 17p prop and was getting about 35 mph by the factory speedometer
I went to a stainless 12 1/2 x 19p prop and was topping out at 40
This was also a 20" motor on a 25" transom that added to the performance but created overheating headaches
Overall if not for the overheating, I would have kept the 12 1/2 x 19p prop and been very happy with the force motor
There was some breaking on cornering but that was probably because of the short shaft motor
Hope that helps,
Joe"

"Thanks Joe, that was more tha

"Thanks Joe, that was more than helpful and sounds like 125 horses is an exceptional match on that particular hull. I think you will be well pleased with the Merc, it's a very nice motor..."