propeller loses thrust

[sdiet]

I have a 14 v bottom starcraft boat with a 25 hp johnson motor. I raised the motor so that the antiventilation plate was even with the bottom of the boat. But I continue to have the same problem of losing forward motion at top speeds. I have to back off on the throttle until the propeller "catches" again, then I can start slowly accelerating again. Anyone know what's going on? Another thing I noticed after raising the motor was that the over the prop exhaust was getting very loud like it was not exhausting into the water, is this bad?

 

[JB]

Re: propeller loses thrust

Hi, SDiet.<br /><br />You almost certainly have a prop hub failure.<br /><br />Mark a line across the hub and prop casting and try her out. If the line is misplaced when you come in it is proof that there was slippage between the prop and the hub.<br /><br />You didn't indicate the year of your engine. If it is the old, shear pin style prop, put in a new shear pin.<br /><br />Good luck.

 

[sdiet]

Re: propeller loses thrust

It is a 1971 motor, it does have a shear pin which engages the propeller, do these type propellers also have a hub that can slip? Would the over the prop exhaust, exhausting in the air mean that the propeller isn't enough in the water?<br />Thanks for your response.

 

[lastout]

Re: propeller loses thrust

sdiet, sheer pin props don't have the hub. sometimes the pin will shear partially and the particulat matter left will allow the prop to engage at lower speeds, but will not hold the prop at load or higher rpm's. make sure you put in a new shear pin and keep extra's in the boat. i have a 69 25 hp that shears often because the lobes on the prop are wearing.<br />as for the exhaust, not certain. seems that on this engine it shouldn't have much effect, but again i'm not quite sure.

 

[wikelam]

Re: propeller loses thrust

i had an older evinrude 40hp. it had the shear pin type prop. the prop does have the hub. i messed up a few of these hubs on stumps.

 

[sdiet]

Re: propeller loses thrust

I just pulled the prop, the shear pin looks to be in good shape. I did notice the prop had a bushing in the center and so I marked the prop to see if maybe the bushing had broken lose inside. It does seem logical that there is some type of prop problem since it feels like the engine isn't engaging the prop when it messes up at high speed.

 

[ob]

Re: propeller loses thrust

Sounds to me like you may have a case of worn down gear lobes or shift linkage problem causing the driveshaft pinion and the forward gear to dislodge when subjected to increased torque, then subsequently re-engaging when the engine speed is reduced.Do the hub check first and tell us what you find.