Rev -limiter going off when triming up

[moffett cr.]

Have bass tracker 2005 170tx with 40 h.p. four stroke mercury. When i get on plane and start trimming up the motors shuts down to idle? Called a shop and gave them prop information and was told it was the right prop. Called another shop and was told to move up to a 15" pitch? Mine is a 13". #816704a40p13mercury. Not sure were to go from here? Does anyone have any idea's? Thanks

 

[alldodge]

If it's hitting the rev limiter then you need to increase pitch. The question that needs answering "is it hitting the rev limiter"? Do you have a tachometer to check?

 

[bruceb58]

Typically, a rev limiter won't force the engine down to idle. What RPM are you hitting?

 

[Frank Acampora]

If the engine is running well while tucked in and over-revs when you trim out, then there is the possibility that the anti-ventilation plate is too high. Check the engine for correct transom height mounting.

 

[moffett cr.]

I do have a tack but at the time i had no clue what was taking place or what to look for. I do remember the tack in the high five to six range. So what does the rev-limiter do when over tacked? Why would engine shut down to idle? The transom is all factory as far as i no,but will check it out. Once i go to neutral i can power up and get right back on plane,then when i trim it goes back to idle.

 

[bruceb58]

It all depends on the motor. On my 90Hp Mercury motor, it starts shutting down the injectors. Non fuel injected motors will start turning off some of the spark pulses. The fact that you are getting into the upper 5K region means maybe you hit the limit.

Your service manual will tell you what is supposed to happen with yours.

 

[Fun Times]

Why would engine shut down to idle?

Once i go to neutral i can power up and get right back on plane,then when i trim it goes back to idle.

While reading your symptoms I'm thinking maybe/possibly that the "boat" is coming down to idle speed (off plane) once you start to trim the engine and maybe the engine is still revving high hitting the rev limiter until you bring the throttle down to idle RPM...then all seems fine until you get going again and once you use the trim system then the boat slows back down...Not the engine..... If so than this would be called cavitation. [h=4]cavitation[/h]

• physics
  
  the formation of an empty space within a solid object or body.
  • the formation of bubbles in a liquid, typically by the movement of a propeller through it.
    
    
    Does this scenario seem right to you?

 

[fhhuber]

My guess is you need to lower the motor and/or add a "whale tail" to aid in preventing pulling air into the prop.

If the motor is too high, the whale tail will do no good...

 

[steelespike]

While reading your symptoms I'm thinking maybe/possibly that the "boat" is coming down to idle speed (off plane) once you start to trim the engine and maybe the engine is still revving high hitting the rev limiter until you bring the throttle down to idle RPM...then all seems fine until you get going again and once you use the trim system then the boat slows back down...Not the engine..... If so than this would be called cavitation. [h=4]cavitation[/h]

• physics
  
  the formation of an empty space within a solid object or body.
  • the formation of bubbles in a liquid, typically by the movement of a propeller through it.
    
    
    Does this scenario seem right to you?

It has been called cavitation for 100 years but now is called ventilation because the prop is fouled by air.
Cavitsation now refers to erosion to the under side of the blades from water boiling in the low pressure area.

 

[Texasmark]

While reading your symptoms I'm thinking maybe/possibly that the "boat" is coming down to idle speed (off plane) once you start to trim the engine and maybe the engine is still revving high hitting the rev limiter until you bring the throttle down to idle RPM...then all seems fine until you get going again and once you use the trim system then the boat slows back down...Not the engine..... If so than this would be called cavitation. [h=4]cavitation[/h]

• physics
  
  the formation of an empty space within a solid object or body.
  • the formation of bubbles in a liquid, typically by the movement of a propeller through it.
    
    
    Does this scenario seem right to you?

Right on the definition of cavitation sir but that is caused by irregularities in the prop, usually along the outer edge of the blades....small bubbles following the blade as it turns......same phenomenon occurs in fluid pumps due to irregularities in the system preceeding the pump causing turbulent flow rather than laminar flow across the pumps "impeller". I had a 1971 Chrysler 85 hp with the factory prop. Frank A can show you a picture of the shape of the prop, but there was a very distinct reversal of the perimeter of the blade just after the root. As a result, cavitation caused a hole to develop about the size of a dime coin, just inboard of that offset that ate through the paint and on into the interior of the casting. The rest of the blades were unscathed and the prop was new with the new engine.

Ventilation on the other hand is the venting of the solid medium due to the sucking action of the prop too close to the surface, or being subjected to turbulence. The prop sucks on the medium in front of it and blows it out the back. If there is a disturbance in front of the prop, like a keel on an alum. boat, it can generate bubbles, especially in turns where the keel is essentially sliding sideways, which decrease the density of the water entering the prop and reduce the effectiveness of the prop's thrust capability. Other form of ventilation is that the surface is of less resistance to the sucking action than the oncoming water so taking the path of least resistance, the prop sucks in air resulting in the same effect. This one is caused by the AV plate too close to the surface, too big of a boat for the pitch of the prop, or a prop that has not been designed for surface performance.... high rake, cupping or cleaver style to name a few.

On the rest of what you said sir, I agree.

 

[fhhuber]

Cavitation can occur to a submarine's prop 200 ft underwater. (where its impossible to draw air in from the surface)
It is from creating a low pressure area due to the speed of the prop. The water boils at the low pressure creating bubbles. Tiny bubbles are generally not much of an issue (except to the desired stealth of the sub) but larger ones can cause the surface of the prop to get eaten away.

I forgot how deep our subs have to go before they don't have enough power to cause cavitation... its deep.

Ventilation occurs only from drawing air into the prop from the surface. (for whatever reason)

All that white foam behind any boat running at high power is mostly from whipping in air due to ventilation.

 

[steelespike]

I had aluminum Mercury props for 65 Merc 500 All are Mercury made props same part number but for the pitch.Motor on the same 57 18 ft. Crestliner utility
Since 65. Original motor was a 57 Johnson 35 Electric start tiller.
Always operated in the 1000 islands.These props all have the same erosion from cavitation. You can look at the erosion and figure the props with
the least service.Present prop with little use has just a dot about 3/8".The oldest with lots of use from 65, has pretty severe erosion About 1/3 of the back side of the blades with lots of metal gone.The erosion originates in the same spot.
I guess they didn't make any changes to the basic design.

 

[moffett cr.]

Thanks all, now i have some things to look for and a place to start

 

[steelespike]

Some folks feel that cavitation scars indicate the prop is working really efficiently.

 

[Texasmark]

If you ever see an underwater picture of a prop turning you can see the bubbles that lead to cavitation burn. They follow the tips of the blades where, as spike said is the low pressure vacuum caused by the blade running through the water.