Force Prop

grouper9544

Recruit
Joined
Mar 22, 2010
Messages
2
Just bought a boat with an 84 125 Force Engine. Not really clear on how to decide which prop would be the best for this engine. The engine did not come with a prop on it so i can not base my decision on the old prop. the size of the boat is a 20 foot angular. If any one could help i would really appreciate it.
 

hwsiii

Commander
Joined
Jan 25, 2009
Messages
2,639
Re: Force Prop

Grouper, You might try reading this for a better understanding of changing props for better performance, and for the major differences between aluminum props and stainless steel.

http://forums.iboats.com/showthread.php?t=369057

Welcome to the forum, I will try to help you as much as I can but without more information no one can help you, and I will tell you I need more information than anyone else. But if you would like my help fill this form out and I will help you as much as humanly possible, if you would prefer to not go to this much trouble there are other people who will be here and gladly help you. My expectations are that you are willing to expend as much time and energy as I will to find a better prop for your boat.

Just copy this form to your computer and paste it into a word processing program and fill it out, then paste it into your post or just use the numbers for reference in your reply and type in your information.

Iboats Boat and Motor Info

1. Year, make and model of boat

2. Length, width and base weight of boat, look for boat decal on back of boat

2a.What is the maximum recommended HP for your boat

3. Number of people and gallons of gas normally on boat

4. What do you use the boat for

5. Is it a Deep Vee and if so how many degrees of Deadrise

6. Year, manufacturer and model of motor

7. HP and gear ratio of outdrive IMPORTANT
If you don?t know the gear ratio, you need to pull the plugs out of the motor and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing then do the same thing on the flywheel and turn the motor until the tape lines up with each other on the prop, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor

8. Manufacturer?s recommended Wide Open Throttle (WOT) range

9. Anti-ventilation Plate height above the bottom of the transom of boat if it is an outboard in inches, use a straight edge or a board under the keel and sticking out to the anti ventilation plate for a reference, and take about 4 pictures for us to see.

AntiventilationPlateStraightEdge-1.jpg


10. Is it a bass boat or does it have a pad bottom and is it on a bracket, if so how far does the bracket extend from the stern.

11. Does it have a hydrafoil, dolefin, whale tail or trim tabs and is the motor on a bracket, and if so how far back from the stern is the bracket

12. Manufacturer, model, diameter, pitch, number of blades and whether SS or aluminum props.
IMPORTANT
1.
2.
3.

13. WOT RPM and speed from your current prop and how much gas and how many people were in the boat for the test data and is the speed by GPS. Make sure you trim the prop up until it starts ventilating and then just trim in until it quits ventilating. If you do not have a tach you can buy a Tiny Tach for less than $ 50
RPM ___________ Speed (GPS)___________ No. of people ____________ Gal. Gas ________

14. Are you at sea level or a higher elevation, give us the elevation in feet __________

15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way by only using the control on the console, checked compression, looked at the plugs and checked spark, is the bottom of the boat clean and barnacle free, and have you checked the Tachometer against a mechanics tach, all of the foregoing could be the reason your prop is not attaining full RPM.

16. How long has this prop been on the boat and why, at this time, do you think it is the wrong prop.

17. Does the prop show any damage that you can see

18. What problems are you trying to cure or what are you looking for the boat to do that it is not doing the way you think it should or to your expectations

19. If you are trying to attain a better cruising speed and fuel savings or trying to attain a faster speed I will want you to take your boat and run it with 1 or 2 people and give me the RPM and speed readings starting at 3,000 RPM in 500 RPM increments all the way to WOT.
1. 3000 RPM
2. 3500 RPM
3. 4000 RPM
4. 4500 RPM
5. 5000 RPM
6. 5500 RPM
7. 6000 RPM

REMEMBER, The prop I recommend will be NO better than the information you give me.​

The only thing I ask of you is to come back and give me a report of WOT RPM and speed for my database.


H
 

Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
Re: Force Prop

13 diam X 17 pitch should be about the right size--more or less. The 84 Force 125 was not a particularly strong engine so most likely a 17 or even a 15 pitch will put it in the correct 4500-5500 full throttle range on your 20 footer. I would start with the 17 pitch unless you are carrying a heavy load or pulling skiers.

The correct prop is a closed hub design but if you can't find one of those, an open thru-hub exhaust prop will work. If your engine has the long prop shaft tail past the threads, then it should also have a "Flare Washer" and tailcone to complete it. Looks better but really doesn't do much so you can run without them.You can find used ones on ebay.

the photo with two props shows a closed hub stainless prop on the bottom and a thru-hub exhaust stainless on top. Both are Michigan made, the bottom is Chrysler stock. Stainless closed hub props have lightening slots in the hubs while aluminum are solid. That's why the closed hub stainless actually looks like thru-hub, but note the size difference in the opening. The red is the flare washer and tailcone on an open hub prop and the last is the standard black flare washer on an aluminum closed hub prop.

Grouper: Where do you live?
 

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Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
Re: Force Prop

If you can only afford one prop, start with the 17 pitch since it will be better for all around use and will most likely put the engine somewhere in the 4500-5500 range, unless you are carrying 7-10 people on the boat. Then see where the full throttle rpms are and if you have the money, buy a used spare or ski prop on ebay. Again. pitch will depend upon where the RPMs are with the 17 pitch. Dropping down two pitches gives a better hole shot and yanks out the skier faster, making it easier on the boat and easier for the skier to rise up on the skis.

Because of the greater area, wakeboards are not so critical--seems like they pop out no matter what prop you are running.
 
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