Johnson 9.9 Vs Force 15 prop pitch

[pecheux]

Good day.

Re: Johnson 9.9 hp with 10 pitch prop versus a Force 15 hp with 8 pitch prop. They both run around 5800-6000 rpm at Wot. Gear ratio on the OMc is 2.41/1 and the Force is 1.57/1. The Force is obviously stronger ... but not faster (maybe a bit slower) than the Johnson running on a 14 footer fiber that weights 260 pounds.

Question is: what prop to use on the Force OB so it runs at least the same wot speed, or a bit faster ?.

Thank you.

 

[Frank Acampora]

Re: Johnson 9.9 Vs Force 15 prop pitch

GEARING: There in a nutshell is the reason why Force engines of any size do not give the same performance as other brands with the same horsepower.

For recreational use it is always more efficient to drive a larger prop through the water slower. There is less slip thus more horsepower is used driving the boat forward rather than spinning the prop through the water.

On all my large Chrysler and Force engines, I try to use the 2 to 1 ratio lower unit with a prop 2 pitches higher. It usually gives between 3-5 MPH more than the 1.7 (1.72 or 1.78--I forget which exactly.) lower unit that most came with.

With your smaller Force, you really don't have much choice other than to run it like it is. There is not a big selection of props and no magic bullet.

 

[steelespike]

Re: Johnson 9.9 Vs Force 15 prop pitch

Like Frank says not to much you can do.
You could try fine tuning the setup.Raise the motor 1/4" at a time with
1/4" sticks on top of the transom.Raise it to a point just before ventilation becomes a problem.careful weight placement and trim adjustment.Be sure it has a good grip and pumps water.Might gain a little mph.Not likely to find a prop with a little more pitch but you could see if a prop shop could add a 1/2".
Be sure motor is tuned up, throttle opens all the way, prop in excellent shape.

 

[pecheux]

Re: Johnson 9.9 Vs Force 15 prop pitch

Thank you guys, much appreciated. On that Force OB the present prop is a 9 X 8 . Other props available would be 9 X 9 and 9 X 10.5 I am trying to figure out which one would be my best buy vs overhaul performance.

Obviously in my neiborhood Merc dealers tell me I should not spend a penny on that Force OB. Considering the cost of a new OB (any brand) would be in the $ 3000. range I dont agree with them and I will gamble going one pitch higher for my own satisfaction considering this OB is still in "mint" condition and cost me 25% of the price of a new one. If it does not pay off .. I'll have a spare prop ... LOL

Being a winter project I have time to try figuring it out.

Cheers

PS: when written 2.41/1 does this mean the prop turns 2.41 time for one turn of the motor ?? or the other way around ?

 

[steelespike]

Re: Johnson 9.9 Vs Force 15 prop pitch

That would be my thought ;go up an inch.hp is rated at 5,250.So you have some room to play. Just keep in mind you will lose some hole shot and loads may present a problem.As I understand it the motor turns 2.41 times.You can check by simply turning the flywheel by hand (easier with plugs out).If you compare the prop sizes its logical that the motor turns 2.41 the Johnson has a ratio advantage thus the larger pitch on a less powerful motor.