# stock prop for Mercury 9.8, 2 stroke motor



## cwphoto (Aug 31, 2014)

Hi folks:

One more question about a motor I just purchased. I have a Mercury 9.8, two-stroke I just picked up, and would like to get an extra prop for it. This is a 1985 model, just before they came out with the 9.9. While it's 8 inches in diameter, I cannot determine the pitch. It's quite possible I may need to change it as well, but would like to have some basis or starting point. The prop is stamped with 8P A1. Easy enough to measure the diameter, but does the A1 indicated a 10 inch pitch or how does one read these things? does anyone know the stock prop the motor came with? This engine has a whopping four hours on it, so it has not been changed. The paint has not even rubbed off the prop at this point. See attached photo.

Thanks Troy


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## Boat2fast (Sep 3, 2014)

8P designates 8in of pitch. Mercury make this easy. 9P would be 9in of pitch.


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## cwphoto (Sep 4, 2014)

[url=https://www.tinboats.net/forum/viewtopic.php?p=365269#p365269 said:


> Boat2fast » 02 Sep 2014, 20:39[/url]"]8P designates 8in of pitch. Mercury make this easy. 9P would be 9in of pitch.



Thank you. That seems all too obvious! I was thinking that the diameter would come first. So, is there any logic to the numerical part of this? A1 indicates what? I Do know through further research that this is an 8 3/8" dia. but how that relates to A1, no clue on the logic there. 

When I talked to a marine shop about this, they came back with part number 032020. This is a 9 inch diameter by 7 inch pitch. An online manual indicated the stock prop was 8 3/8" (Now obsolete) so not sure how a 9 x 7 compares to an 8 x 8. 

I'm sure there must be a formula for figuring this out, but I'm assuming one would need to know the gear ratios. I may just throw the 9 x 7 on and see were my RPMs are too. I can't imagine there would be a huge difference between and 8 3/8" to 9". 

Motor also has a 17 1/2 inch shaft, which I've never come across before. I thought there was just a standard short shaft ( 15 inches) and a long shaft ( 20 inches), and a few extra long for special applications. 

Thanks again!

Troy


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## JMichael (Sep 7, 2014)

With only 4 hours on the motor, the first question that comes to mind is "Why exactly are you looking at changing the prop"? :roll:


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## Boat2fast (Sep 8, 2014)

The A1 is not a size designation. It has to do with Mercury part number series. Personally, I would get some good tach readings on the current prop before even guessing which other prop might be better. This unless the current prop is damaged. 

As for shaft lengths...Motors are either '15in shortshaft' _models_ or a '20in longshaft' _models_. These are NOMINAL sizes to fit the engine to the right size transom. Transoms are designed to 15in or 20in heights. Some are a little taller, 16in or 21in, in accordance with the manufacturers hull design. 

Example: A 16ft 1988 Checkmate has a 'longshaft' transom. It actually has a 22.5in transom height, the elevated height is for high speed efficiency. It is still a 20in _nominal_ size or 'longshaft'. The engine was further lifted 3bolt hole positions up to an ACTUAL height of 24.75in. This gave the best top speed but holeshot ventilated a bit.

Outboard engines are built to put the lower unit anti-ventilation plate a certain distance below the bottom of the boat. This is designed by the motor manufacturer for the motor to perform best under most circumstances.

Again 'shortshaft' and 'longshaft' are NOMINAL designations. 15in and 20in are Nominal sizes. Your motor appears to be 17.5in but is designed to work properly on a shortshaft (or 15in) transom. The same motor designed for a longshaft transom is exactly 5in longer shaft.

Michigan Wheel has a good resource for prop information:https://www.miwheel.com/uploads/resources/Propeller_Basics.pdf

Also, check out Mercury's Guide to High performance Boat Operation manual part number: 90-849250

It is not likely all the information will apply to a 10hp aluminum boat.


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## cwphoto (Sep 11, 2014)

[url=https://forum.tinboats.net/viewtopic.php?p=365665#p365665 said:


> JMichael » 07 Sep 2014, 04:06[/url]"]With only 4 hours on the motor, the first question that comes to mind is "Why exactly are you looking at changing the prop"? :roll:



I run a lot in shallow water, so want a spare. Always good to have a spare! Agree some good tach readings are necessary. Not even sure what max rpm numbers are though, as I can't find much about the motor. The original diameter is 8 3/8" x 8 pitch (from what I've learned here). The replacement is said to be a 9 x 7. Looking at the specs for the 9.9, produce one year later. I think that has an 8.5 inch prop, and need to see if this is an 8 spline or not. Seems like it should work. 9 inches would leaves very, very little room, and just about touches the cavitation plate. Certainly close enough for a branch or something to wedge in there. 

Troy


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## cwphoto (Sep 12, 2014)

[url=https://forum.tinboats.net/viewtopic.php?p=365748#p365748 said:


> Boat2fast » 08 Sep 2014, 04:44[/url]"]The A1 is not a size designation. It has to do with Mercury part number series. Personally, I would get some good tach readings on the current prop before even guessing which other prop might be better. This unless the current prop is damaged.
> 
> As for shaft lengths...Motors are either '15in shortshaft' _models_ or a '20in longshaft' _models_. These are NOMINAL sizes to fit the engine to the right size transom. Transoms are designed to 15in or 20in heights. Some are a little taller, 16in or 21in, in accordance with the manufacturers hull design.
> 
> ...



I will check out the resources mentioned. I decided to raise my transom 2.5 inches. This puts the cavitation plate about a half-inch above the bottom of the boat. Since I run a lot in shallow water, I want as little of the prop in the water as I can get away with! Whether or not I will get cavitation remains to be seen, but this is where I'm starting at least. I have not compared this motor to other short shaft motors, and perhaps they are all similar lengths. This motor is something to get started with for me, and I will more than likely upgrade to something early next season. 

I'm already not liking the 2 stroke. I did go with some royal purple synthetic, but still smokes like all get out. I think the engine needs a good tuneup as well, as the engine was missing a bit, and suspecting a dried out diaphragm in the fuel pump. Died a few times and extremely difficult to get started again, without re-priming, choke, etc. Tossed in a couple of new plugs today as well, although old ones look pretty darn good, just 20+ years old. I'm waiting on some parts to try again, and will probably give the carb a good cleaning in the interim. Very simple motor to work on, so that is a real plus. 

Thanks again.

Troy


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