Is my propeller the problem?

[pydpiper]

I tried to Google this issue, everything brought me to this site.
I have a 25HP Merc outboard, tiller style. I have made no changes to the motor with the exception of putting on a new (garage sale) propeller at the end of last season.
The boat runs fine, but just as it is about to break plane the motor sounds as though it is thrown in to neutral (revs high and stops accelerating), yet looking in the water it seems to be turning the water just fine. As this happens the boat slows almost to a stop until I back off the throttle and accellerate slowly. When it gets up to speed it does it again. My only solution was to cruise at a speed just below this happens.
Any thoughts would be of great value!
Thanks.
David

 

[Leardriver]

Re: Is my propeller the problem?

Could the hub be spun? Put a magic marker mark on the inner and outer hub and see if it moves while you're motoring around.

 

[pydpiper]

Re: Is my propeller the problem?

Could the hub be spun? Put a magic marker mark on the inner and outer hub and see if it moves while you're motoring around.

I should have mentioned that I am fairly boat term illiterate, I will find out what the hub is and give it a try, thanks.

 

[steelespike]

Re: Is my propeller the problem?

The hub is the part inside the prop that engages the prop shaft.It is pressed into the prop on a rubber bushing and is designed to cushion the impact with an object.Over time these hubs will slip.Thus if you draw a water proof line across the underside of the hub and prop you can tell if it slips. A prop shop can test it for you and rehub if necessary.
If your old prop was damaged it could be repaired as well.
It woul;d help to know the year of the motor and the type/size and approximate weight of the boat.Would be good to know the size of the present prop and the old prop.
Numbers should appear near the base of the blades or under the prop nut.

 

[Bob_VT]

Re: Is my propeller the problem?

The prop has a rubber hub to protect it and protect the prop shaft from running into things........ that hub is actually just pressed in and they do go bad.

The magic marker test is easy......get a sharpie and mark the prop in relation to the prop nut. a straight line across the nut and marks on the outside edge of the hub.......... get the boat into the water and make it do it's thing. After the slip examine the marks and they will no longer be lined up then you have a bad hub.......... time to replace.



View attachment 108418

The other scenario is this......... you might be cavitating (ventilating) due to an improper set up. Try the marker test first

 

[pydpiper]

Re: Is my propeller the problem?

The engine is a Mercury 25HP outboard, tiller style from the mid 90's. The boat is a 15' Sea Nymph and I am unsure of the weight. The boat has a carpeted wooden floor and sidewalls, both packed with foam, it is a pretty heavy machine.

On the side of my original propeller it is stamped as follows:
48 19640 A40 13P

the new (used) one is stamped:
OP26023
OP47552

They look identical with the exception of a big nick in my original one (on one blade), and of course the numbers do not match.
I do see now where the "hub" is and what may be wrong with the original, I will bring both props down to the water and do the marker test, then swap the propellers back and see what happens. Until now I thought that was all one solid chunk of aluminum, didn't realize there was rubber in there.
Sounds like I am on track to getting this thing up and running properly, thanks!

 

[Texasmark]

Re: Is my propeller the problem?

I don't think you have a prop problem because you just come off the most stressful part of the operational curve (in the hole) to the least stressful part (planing out). I think your prop is sucking air and is ventilating. When you start to plane out, does your engine rev up? If so, lower your engine, get a cupped prop, or tuck it in a notch or two.

Mark

 

[SparkieBoat]

Re: Is my propeller the problem?

tuck it in 2 notches...

 

[pydpiper]

Re: Is my propeller the problem?

I am assuming that "tuck in" means bring the bottom of the motor coler to the boat?

 

[Texasmark]

Re: Is my propeller the problem?

I am assuming that "tuck in" means bring the bottom of the motor coler to the boat?

Yepper

 

[pydpiper]

Re: Is my propeller the problem?

I took off my new (used) prop yesterday to set it down beside my old one to try to find any variations in pitch, then to mark the hub as suggested. When I got the old prop off I noticed the hub was wore, to the point it is very shiny. I guess there is no need to use a marker now, it is pretty obvious what was happening.
So, thanks for all the help. I would never have suspected that as a problem, wouldn't even have seen it as a potential problem. I assumed the propeller was one solid piece.
Has anyone tried to have those two pieces welded together opposed to replacing the rubber? Just curious.
Here is my old prop that worked fine, on the left. The new one that appears to have been spinning hard enough to polish the hub on the right:

 

[Texasmark]

Re: Is my propeller the problem?

The rubber hub is there to absorb shock, the shock of shifting a stationary prop into motion by a rotating shaft and crankshaft. Second purpose of it is to slip if you strike an obstruction and automatically reengaging when you shift to N.

Merc did this years ago, back when OMC had a shear pin (with a rubber hub also, but not as elaborate) and when you hit something the pin sheared and you were dead meat.........now feature this....you are on the windward side of the rocks, it's rough, waves are pounding you, wind blowing you into the rocks, you are trying to get out of there and BAM you hit a rock and there goes the pin and your propulsion. BTDT

Mark