Confused on which prop to get...

spindoc

Recruit
Joined
Apr 7, 2015
Messages
5
I have a 1991 Force 120 Outboard on a Bayliner Cobra Bass/Ski boat. The Model is 1208C91H. The current prop on the outside says 21C, and on the inside says 12 5/8 DIA x 21P.

Here are my choices

14 X 13 Cup
13 1/2 X 15Cup
13 3/4 X 15 Cup
13 1/2 X 17Cup
13 1/4 X 17 Cup
13 X 19 Cup
12 3/4 X 21 Cup

I dont necessarily need more speed, I would like to tow a raft with some kids on it, so if I go with the 12 3/4 x 21, which I have I can pretty much expect the same speed and torque I get now with my existing prop right?
If I go with the smaller pitch, that gives me more speed and less power, or less speed and more power?

What would you do?
 

roscoe

Supreme Mariner
Joined
Oct 30, 2002
Messages
21,788
The decision needs to be based on your WOT (wide open throttle) rpm's with your current 21p prop.

Are you reaching 5000-5400 rpm's?

Smaller pitch will give you more power for pulling and better hole shot.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
I have a 1991 Force 120 Outboard on a Bayliner Cobra Bass/Ski boat. The Model is 1208C91H. The current prop on the outside says 21C, and on the inside says 12 5/8 DIA x 21P.

Here are my choices

14 X 13 Cup
13 1/2 X 15Cup
13 3/4 X 15 Cup
13 1/2 X 17Cup
13 1/4 X 17 Cup
13 X 19 Cup
12 3/4 X 21 Cup

I dont necessarily need more speed, I would like to tow a raft with some kids on it, so if I go with the 12 3/4 x 21, which I have I can pretty much expect the same speed and torque I get now with my existing prop right?
If I go with the smaller pitch, that gives me more speed and less power, or less speed and more power?

What would you do?
You have it reversed A smaller pitch gives more power and more rpm. possibly less speed.
Your 21" is the theoretical distance the prop moves forward in one revolution.
Your 21" is like high gear in the car not much power but good speed.
If your boat is light and reasonably fast the 21 may work for sports.
What is the present wot (wide open throttle) rpm and gps speed lightly loaded.
If you decrease pitch it increases rpm.
The motor needs to operate within its rated rpm range at wot.5000-5400
Once propped correctly you can operate at what ever rpm suits you.
Sometimes if you operate with a lot of weight you may have use a prop
that may over rev with a lighter load. You need to watch that you don't over rev.
 

SkiDad

Lieutenant Commander
Joined
Jul 18, 2010
Messages
1,518
my dad has a similar boat - it's a faster hull than mine - a 19p will pull tubes no problem - if you want to pull slalom skiers than 17 is the way to go. This is assuming your boat has the original 1.93:1 lower unit. But if you give us your top RPM at full throttle the guys on the forum can get your dialed in.
 

Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
While a 14 inch diameter will clear the anti-ventilation plate, it may have a hub that is too large for the gear case. Stay with props that are nominal 13 inch diameter--ie. 12 5/8 to 13 3/8. Stay away from 13 and 15 pitch props.they will almost certainly over-rev your engine.

After that it is mostly experimentation because different props will load the engine differently. If at all possible test drive a couple of different ones. If you buy a high rake prop like the Michigan Ballistic, the blades will hit the exhaust snout unless you modify the thrust washer system. In the past I have posted on this so a search of my posts should show this.

The engine is rated to run 4500-5500 at wide open throttle with rated horsepower at 5000 RPM Try to keep it in that range even though it is capable of running 5900-6000 RPM for extended periods.( I had a 125 on a flat bottom hull which I always ran at 5900 and 55 MPH) I used a number of test props but never less than 19 pitch.
 

Slip Away

Lieutenant
Joined
May 11, 2010
Messages
1,431
. If you buy a high rake prop like the Michigan Ballistic, the blades will hit the exhaust snout unless you modify the thrust washer system. In the past I have posted on this so a search of my posts should show this.

Not totally accurate. If the Force 120 is using the Mercury 4-1/4" gearcase, there is no exhaust snout, only a trim tab. The only two Ballistic props that require a special exhaust snout (part # F744116) are Ballistic XL 3 blades in 13-1/2 x 22p and 24p. these have an extra large blade area, thus special thrust washer required) Which would not be what the OP needs. Plus, for his boat, I would not recommend stainless props like the Ballistic. It's a performance prop, which is not his application.
 
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Frank Acampora

Supreme Mariner
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Jan 19, 2007
Messages
12,004
Yes, but we are talking about a 1991 engine. Both the dual exhaust and single exhaust versions of Force engines prior to Mercury lower units have a large exhaust snout. High rake props will hit the snout with ANY thrust washer. Since the hub is pressed in from the thrust side of the prop, with the closed ( bow or gear case)) face of the Ballistic prop A stainless split collar is required to provide enough clearance and still let the hub enter the gear case casting. The thrust collar is used on the open face under the prop nut washer. Although the Ballistic will load down his engine drastically, it might still be a viable choice for his hull. I have had a Cobra with a Force 120 over 45 MPH with two people in it with a 13 X 19 stainless. However, I only used ballistic as an example of high rake props.
 
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Slip Away

Lieutenant
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May 11, 2010
Messages
1,431
Got it, but there would be no reason to consider a prop for this outboard that would never fit. Thus why use Ballistic as an example ?
 
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