Mercury 1984 125 HP Force Outboard. Need prop suggestions

Mythran

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Joined
Jan 17, 2012
Messages
3
I just received a boat from my uncle that came with the Mercury 1984 125 HP Force Outboard engine. I've been trying to find information on the engine (cylinders, cc's, etc) and have found many forum posts stating that the Force engines are crap. While those that seem to have owned them say the opposite, that they are very "decent" engines, but not top of the line. While all that is fine and dandy, the one I have runs great (out of water). The boat that came attached to it is not pretty at all and needs quite a bit of time out of the water to get it in-water shape.

I'm a new boater (this is my first boat) and I would like to know the questions I need to ask to purchase a new propeller for this boat. I know now (after speaking to Mercury Marine over the phone) that it's a 1,626 cc engine with a 99.2 ci/displacement. I definitely need a new propeller but don't have a clue where I need to start...other than posting here. Any helpful tips for a new boater are also welcome.

The boat is a bayliner...(so I've been told) with the names "Sierra" written on several parts of it. Am I even asking the right questions? <shrug> Would help if I knew what to ask I guess.
 
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Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
Re: Mercury 1984 125 HP Force Outboard. Need prop suggestions

Starting point would be if you already have a prop and your tachometer is working. The engine should be propped to run about 5000 to 5500 RPM at full throttle with the regular load in the boat.

Now, yeah, that doesn't tell you much but we don't have a lot of information to go by.

If there are no numbers on your boat like 190, 180, 165, etc. take a tape measure and find the length from the transom to the bow.Round it out to 16 17, 18, 19 feet etc. Now, if you do have a working tachometer, let us know the RPM at full throttle. If you do have a prop, remove ot and look for numbers on it.

Now, that said, props for these engines are pretty much standard: All are approximately 13 inches in diameter and have 15 splines. Pitch is what varies and this is like gearing on a car.

Typically, if your engine is on an 18 foot Bayliner it will use around a 17-19 inch pitch prop. On a larger boat it would use around a 15 inch pitch prop and on a smaller one, perhaps as high as 21 pitch.

You can find used props on the auctions and you can get a good deal on a new prop here at iboats stores.
 

Mythran

Recruit
Joined
Jan 17, 2012
Messages
3
Re: Mercury 1984 125 HP Force Outboard. Need prop suggestions

Starting point would be if you already have a prop and your tachometer is working. The engine should be propped to run about 5000 to 5500 RPM at full throttle with the regular load in the boat.

Now, yeah, that doesn't tell you much but we don't have a lot of information to go by.

If there are no numbers on your boat like 190, 180, 165, etc. take a tape measure and find the length from the transom to the bow.Round it out to 16 17, 18, 19 feet etc. Now, if you do have a working tachometer, let us know the RPM at full throttle. If you do have a prop, remove ot and look for numbers on it.
When I spoke with Mercury Marine, the guy told me that my engine is propped to run at 5300-5400 max at wide open throttle.

Now, that said, props for these engines are pretty much standard: All are approximately 13 inches in diameter and have 15 splines. Pitch is what varies and this is like gearing on a car.
I heard about splines. I was told to make sure of the # of splines on my current prop to make sure they match any new splines I get, since if they don't, the prop won't fit, correct?

Typically, if your engine is on an 18 foot Bayliner it will use around a 17-19 inch pitch prop. On a larger boat it would use around a 15 inch pitch prop and on a smaller one, perhaps as high as 21 pitch.

You can find used props on the auctions and you can get a good deal on a new prop here at iboats stores.

I'll probably buy new since I'm going to put in up to $1k on the boat repairs anyways. I was told the benefits of steel vs aluminum props and that if HE (the guy from Mercury Marine) was going to purchase one based on what I told him I was going to use the boat for (ski'ing but mostly fishing) he would purchase an aluminum one because if I hit rocks, it would more likely mess up the prop more than the engine and gears, which are more expensive. Would you have the same recommendation?

Thanks for your help.
 

Frank Acampora

Supreme Mariner
Joined
Jan 19, 2007
Messages
12,004
Re: Mercury 1984 125 HP Force Outboard. Need prop suggestions

Well, Props and lower units were not designed to run through rocks! If you hit hard enough, it doesn't matter what the prop is made of, you are going to do damage. Stainless generally runs a bit faster in the exact same prop and, of course, is more durable. It does cost more than aluminum. Aluminum is cheaper but less durable and will run slower than the same stainless prop. NOTE that I said the same. Different brands of props run differently and can not be directly compared. Personally, I run nothing but stainless props but I don't make it a habit to cut rocks with them.

Again, If you buy a composite prop. the blades will break before major damage is done to the lower unit---but they break easily and don't run as well as metal. You will probably be buying a lot of blades (if the replacable blade type) if you buy a composite prop.

1984 is a 15 spline prop.

Understand that you have an old Bayliner. You will most likely NOT recover the 1000 bucks you will put into it for repairs or refurbishing. That is a personal choice you need to make.

I forget the exact numbers but the usual RPM range for the four cylinder engines is 5000 to 5500 with rated power developed at 5250RPM. The 125 may be a little different but 100 RPM will not mean anything. These engines can rev well over 6000 because there is no valve train.
 
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