Prop choice and RPM on 9.9 Mariner

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ChrisParish
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Prop choice and RPM on 9.9 Mariner

Post by ChrisParish » 21 Jul 2018, 19:11

Hey there folks,

I suspected that I wasn’t reaching full RPM on my engine so I bought a tacho. At WOT I am reaching 5100rpm and the manual indicates a WOT throttle range of 5000-6000.
Now obviously I am within that range but it only reaches that when absolutely flat out - the majority of the time on the plane I am at about 4600rpm and around 12 knots. Flat out I am reaching 13.5knots.

Is this ok or should I consider changing the prop?

On the subject of props - the markings on my prop simply state “F 8.5” I’m guessing this means 8.5” pitch. Any idea about the “F”?

Cheers!
Not a complete idiot - some parts are missing.

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Paul
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Re: Prop choice and RPM on 9.9 Mariner

Post by Paul » 21 Jul 2018, 22:17

You are within the rev range of operation so it's doing its job.
From the info supplied and a quick search gives this
Screen Shot 2018-07-21 at 23.01.08.png
To make sure this info is correct you'll need to remove the prop and look at the hub
IMG_1516.JPG

In basic terms, each change of pitch gives a change of 250-300rpm
There are prop calculators out there like https://www.mercurymarine.com/en-gb/eur ... !/step-one for your OB but beware they can be out by 2 pitches

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Andy B
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Re: Prop choice and RPM on 9.9 Mariner

Post by Andy B » 23 Jul 2018, 10:08

Sorry Chris, but I don't understand what you're trying to achieve here?
The engine is running at the lower end of its recommended rev range.
This is what you want, if you decrease the pitch on the prop to increase the revs you won't go any faster, in fact you are just making the engine work harder to achieve the same speed. And if you increase the pitch you will drop out of the optimum rev range.

If it's more speed you want then it's more power you need.
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ChrisParish
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Re: Prop choice and RPM on 9.9 Mariner

Post by ChrisParish » 23 Jul 2018, 12:36

Cool, thanks for that, Andy.
I know these questions may seem daft but it is only because I don’t know the answers that I am having to ask!
Looks like I want more power!
Not a complete idiot - some parts are missing.

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Re: Prop choice and RPM on 9.9 Mariner

Post by mm289 » 23 Jul 2018, 20:58

No substitute for cubes Chris :D :D :D

Thats why I have a V8 (car :roll: )

Paul.

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Martin
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Re: Prop choice and RPM on 9.9 Mariner

Post by Martin » 23 Jul 2018, 21:52

Chris, there’s no such thing as a daft question on here.
Everyone has to learn and we pride ourselves on helping everyone out regardless..
You ask away!!!

Ps... a stainless prop can sometimes help a little..
Less flex and a sharper cut in the water may improve the situation.
In your case with a 9.9hp and wanting more speed, I’d save the stainless prop money and follow the higher hp outboard advice.
The T38 runs superbly with a 15hp, stick a 25hp on it and hold on !!!!

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F-RibsAndSibs.Com.... :whistle
Because it's a big old WWworld out there :mrgreen:

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Andy B
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Re: Prop choice and RPM on 9.9 Mariner

Post by Andy B » 24 Jul 2018, 21:39

You can't buy a stainless prop for that motor Martin, you need the Bigfoot gearbox for a stainless.
I looked........
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Martin (30 Jul 2018, 14:31)
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