40 hp mercury high rev but no power to the prop

carlyai

Recruit
Joined
Apr 28, 2012
Messages
2
I have a 2006 40 HP Mercury Lightnihg XR. Under load it revs high and there is no prop power to get to the plane. Plenty of water is being churned up, but no power.

I thought it was the propeller slipping so I replaced the propeller. Same thing.

It seems to be cavitating all the time when it gets under load and adjusting the trim so the prop is more towards the surface does not seem to have any effect on the problem.

On a good no waves, no wind day, with my wife up the front it seems OK an now and then overrevs like I've hit something.

It seems that the boat is full of lead, but I drained all the wave water our before my test run.

On land it runs fine. When stopped and in gear, there is good drive when I turn the prop and no slackness.

No unusual noises from the engine, just churning up lots of water but no push power.
 

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: 40 hp mercury high rev but no power to the prop

Hi, carlyai.

Welcome to iboats. :)

Where is the anti-ventilation plate located compared to the bottom of the boat? What you are describing sure sounds like prop ventilation.

The AV plate should be about level with the bottom of the boat and the PT&T should be fully down (in) when accellerating.
 

79Merc80

Senior Chief Petty Officer
Joined
Nov 22, 2007
Messages
673
Re: 40 hp mercury high rev but no power to the prop

There is another less common cause. If the diameter of the prop is too small, the exhaust gasses not only go thru the prop, but also around the prop causing the cavitation. I had this happen once when I let my brother change the prop and he forgot to re-install the thrust washer. Pulled him on a ski for about 30 minutes, changed back to the normal prop and noticed that the prop actually chewed it's way forward, eating the rear of the "Bullit" as well as the leading edge of the blades of the prop. After that, there was a big gap between the lower unit and the prop hub. If I accelerated slowly, I was able to get up to about 3500 RPM before it would start to cavitate, or if I stomped on it it would cavitate from the start. I ended ujp having yo get another lower unit and have the gears transfered.

HTH, Craig
 

Wingedwheel

Lieutenant Junior Grade
Joined
Jun 16, 2009
Messages
1,071
Re: 40 hp mercury high rev but no power to the prop

The question I have is this a problem that just started happening and have you had this motor for a while? If so was anything done to the motor prior to this starting to happen?
 

JWH0420

Seaman Apprentice
Joined
Jun 6, 2011
Messages
49
Re: 40 hp mercury high rev but no power to the prop

is prop size appropriate for the boat and motor? if u have the wrong prop it would behave that way as well
 

carlyai

Recruit
Joined
Apr 28, 2012
Messages
2
Re: 40 hp mercury high rev but no power to the prop

Hi, carlyai.

Welcome to iboats. :)

Where is the anti-ventilation plate located compared to the bottom of the boat? What you are describing sure sounds like prop ventilation.

The AV plate should be about level with the bottom of the boat and the PT&T should be fully down (in) when accellerating.

I have have not changed anything on the boat or engine, only the prop. Sorry, can you tell me what the PT&T is?
 

JB

Honorary Moderator Emeritus
Joined
Mar 25, 2001
Messages
45,907
Re: 40 hp mercury high rev but no power to the prop

I have have not changed anything on the boat or engine, only the prop. Sorry, can you tell me what the PT&T is?

The vertical position of the prop is critical, carlyai. We use the anti-ventilation plate, the horizontal plate above the prop, to position the motor. It should be about level with the bottom of the boat.

PT&T is your power tilt and trim unit. For takeoff, trim it all the way in. For planing trim it out until it is just short of ventilating the prop.

You say you have changed nothing. Are you saying it has always done bad or that it worked fine before this ??
 
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