25HP Mercury 4 stroke efi

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jayace77

Cadet
Joined
Sep 16, 2008
Messages
8
OK got a 2006 25HP Mercury 4 stroke EFI just purchased on a 16ft tracker. Owner said it ran about 21mph, took it out today only runs about 7mph @ 4500 RPM. He did change the prop before he sold it to me not sure what was on before. Right now it's a 9.75x9.5P. Do I have a prop problem? I used a different gas tank with fresh gas, and a tiny tach to verify RPM, can't seem to find any other issue. Any help would be appreciated.
 
Silvertip

Silvertip

Supreme Mariner
Joined
Sep 22, 2003
Messages
28,771
Re: 25HP Mercury 4 stroke efi

25 hp on a 16 foot tracker of unknown "Model" may be a bit too little HP. What does the maximum HP tag say. That tag is usualy an aluminum plate on the right side of the boat or on the transom. If 25 HP is less than 70% of the maximum then the boat is extremely underpowered. At 4500 rpm using a 9x9 prop I doubt there is an engine issue. To get the rpm up to 6000 you would need a prop with about 7 inches less pitch which would mean you need a 9x2P. Hardly a reasonable conclusion since such a prop is not even available. I think the boat is extremely under powered. If this boat is a side console it is probably rated for a 50 - 75 HP which means you have roughly half the HP the boat should have.
 
S

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: 25HP Mercury 4 stroke efi

A 9.5 pitch isn't extreme but may not be right..
If you could get it to plane I think you would get about 19 mph.
What model is the tracker Dry weight?
Is it a bare bones jon or a Side comsole 16?
Calculator says an 8" prop would make about 21. at 10% slip and 6,000 rpm.
A lot of guesses in this estimate.
 
P

pootnic

Chief Petty Officer
Joined
Aug 11, 2007
Messages
447
Re: 25HP Mercury 4 stroke efi

I think you should aleast plane off with the 25hp.
If the previous owner was able to get 21mph(on the same boat),you should to,unless your "load"is alot more or you changed the setup.
A 9.5 pitch prop seems like it should be in the ballpark.
Could you give us alittle more info?
Just you in the boat,200lbs anchor added to the bow,trim pin setting,prop is in good shape...?
4500rpm and still plowing water,hard to beleive the bow didn't lift to get on plane...
Will it rev above 4500rpm out the water for a sec...?
 
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