help with picking a prop

destroyer_85_golf

Seaman
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Joined
Dec 3, 2011
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70
I have a 1998 johnson 40 hp on a a lowe 1645 MT johnboat. the boat weighs about 350 pounds im about 230 and usually have one other person in it with me i dont haul any loads with it, just wondering if i can step up to a 17inch prop and still be alright?? Any help would be greatly appreciated!!!
 

steelespike

Supreme Mariner
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Apr 26, 2002
Messages
19,069
Re: help with picking a prop

That would depend on the present prop and your present performance.Its best to have a tach and a gps to get a wot rpm and speed with the present setup.A tach is important to help tell how the setup is working and without spread a the rpm doesn't mean much.
A Tinytach is about $47 and will work on any motor.now or in the future.
Some cells have a gps app.
Your 40 has the same gear ratio 2.42 and uses the same series of props my 88 50 does.
I run a 19" pitch on my 18ft 375lb Crestliner voyager,
I have no Tach or gps so I can't say its perfect but it has a strong hole shot and runs
in the high 30s.Has been setup this way since 88.My son has the boat now perhaps some day He will get a tach and use his gps
to evaluate the setup.
Long story short if your running a 15" with a strong hole shot and top end a 17 may work fine.
It will lower rpm about 300 to 400.
Might be a good idea to check your motor height and trim.Does the throttle open all the away?
Is the bottom clean and smooth,motor in excellent tune?
 

destroyer_85_golf

Seaman
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Dec 3, 2011
Messages
70
Re: help with picking a prop

im getting ready to retune the carbs this weekend i have a rebuild kit for the trim unit and a new set of plugs so it will be tuned up, i have been thinking about getting a tach just havent pulled the trigger yet. im pretty sure the prop that is on it is a 16 it has a great holeshot and decent top end just wondering if it would be ok to step up to a 19 .... thanks for the input and sorry it took so long to write back im working out of town.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: help with picking a prop

No problem on your getting back I sometimes have the same problem and I'm retired.
Anyway the best bet for performance and motor health is a tach and gps and some test runs to get a based line and
be sure the motor is up to snuff.
 
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