Can't get 'er up!

[maxxxair]

1972 12' Glassmaster w/1978 70-hp Johnson OB- I'm having a hard time getting the boat to plane. It just chugs along at full throttle but won't plane unless I move forward in the boat. I tried to move the tilt angle and this worked but at full speed the motor over-revs like the prop isn't all the way in the water. Can't tell you what kind of prop I got other than it has 3 blades. Motor runs strong. People look at me funny when I'm standing up in front of the steering console rocking the boat so it will go! Someone please save me some further embarrassment.

 

[WillyBWright]

Re: Can't get 'er up!

Unknown if it's grabbing air from too much trim, or maybe the rubber hub in the prop is slipping. The antiventilation plate above the prop should be parallel to the bottom of the boat, maybe tipped-up in back one notch. If you pull the prop and notice shiney metal where the prop meets the hardware, that could indicate a spun hub.

You also might have too steep of a pitch, or you may be running on two cylinders. See if any of the sparkplugs are burning differently than the other two. Look for dark brown deposits that look slightly damp. That would be the target. Clean plugs in a 2-stroke usually indicates a problem.

Are you sure about that 12' length? A 70 would sink most twelve footers.

 

[maxxxair]

Re: Can't get 'er up!

sorry -14'

 

[steelespike]

Re: Can't get 'er up!

If it appears to be developing full power and can't plane a 14 ft boat the hull may have absorbed water.This would put a lot of weight at the stern making it difficult to plane.

 

[WillyBWright]

Re: Can't get 'er up!

Boy, I don't think you can overprop a 70 on a 14. Either the motor isn't running on all three, or the motor is way too heavy for the boat. What's the max rating on the boat? Most 14 footers are rated under 60, some way under 60. Might you have a longshaft motor on a shortshaft boat? The plate above the prop should be about even with the bottom of the hull.

 

[tashasdaddy]

Re: Can't get 'er up!

any one thought of a spun prop?

 

[kenmyfam]

Re: Can't get 'er up!

Put more than one spark plug in it !!! LOL
Seriously though if a 70 wont plane that boat you have a serious problem. What are the RPM's at WOT ??

 

[rickdb1boat]

Re: Can't get 'er up!

Are you trimming the motor in or out when taking off? Should be in...

 

[maxxxair]

Re: Can't get 'er up!

Thanks for all the replys. Let me respond to a few questions and please excuse me for any dumb answers as this is my first boat...........RPMs at WOT?, no gauge but it has new plugs and I had to install a head gasket when I got it.Check the plugs and there all firing........Trimming the motor?,sure, I jump in the water and change the trim by hand(LOL)........Let me also add that i can get it up alot quicker if turn the boat and straighten it up when it starts coming up.Another question is what is the adjustable little fin at the bottom of the shaft by the prop?

 

[maxxxair]

Re: Can't get 'er up!

Also will someone please answer my other post "Transom Mount Steering Cable" THANKS........

 

[WillyBWright]

Re: Can't get 'er up!

The little fin is a trim tab to reduce steering torque. Looking from behind, it should be a tad right of center. It's to help keep pressure even on the steering wheel.

In order to achieve top Hp, you need to reach full RPMs, probabaly 5500. You don't have a tach, so not much we can do about that.

The spun hub tashasdaddy mentioned would make the boat go, but rev high and just not grab and go. Like a slipping clutch in a car.

It's possible one of the three carbs isn't delivering the right amount of fuel and may need cleaning. Deposits left on the sparkplugs could show which one (if any).

The head gasket is interesting. The head could be warped and you could have a bad gasket again. Water may be entering one of the cylinders and extinguishing combustion. If you get droplets of water on a sparkplug, that would be an indication.

 

[atticus3600s]

Re: Can't get 'er up!

Since you said that all of the plugs are firing, you need to do a compression test to make sure the engine is ok.
That motor should pop that boat up like nothing.
Like others said, check for a spun hub on the prop. Also check to see if the anti-ventilation plate is level with the hull, if not you need to adjust the engine position on the transom. Then trim the engine all the way in (towards the boat) when you try it.
Wot RPM's means the engine revolutions at full throttle which I think for your motor should be 5000-5250 RPM's, but somebody with better OMC knowledge than mine can verify that.
Good luck.8)

 

[maxxxair]

Re: Can't get 'er up!

Thanks for all the replys to both posts.Called the online parts dealer about the steering and he sent me everything to swap the cables.(they dont even make my type of cable any more) As for this post, I also agree it's not the prop as the motor bogs down and doesn't over-rev. I'm doing the compression test next weekend.I must say that weight is an issue as it's alot worse when I have a couple people onboard vs. just my self.I'm a novice mechanic so I guess the shop is the best answer as I really don't know what to fix and I've checked everyones suggestions.You guys have definitely narrowed down my search..........Excellent forum with quick responses! Very impressive!