Prop Stupid

Matt Tyson

Matt Tyson

Member
I know virtually nothing about propellers (sp?), other than I have a banged up aluminum one on my Nissan 20hp and, importantly, doesn't seem to push my 14' welded jon as fast as I think it should. (I struggle to get on plane with an empty boat and a hydrofoil that helps with the current prop.) Where do I begin?
 
Matt Tyson said:
I know virtually nothing about propellers (sp?), other than I have a banged up aluminum one on my Nissan 20hp and, importantly, doesn't seem to push my 14' welded jon as fast as I think it should. (I struggle to get on plane with an empty boat and a hydrofoil that helps with the current prop.) Where do I begin?
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Any banged up prop is not going to work very well or push much of a load. Really need more information. Did the prop work well before getting all banged up? If so replace it with a new one of the same size and pitch.
 
Huntindave McCann said:
Matt Tyson said:
I know virtually nothing about propellers (sp?), other than I have a banged up aluminum one on my Nissan 20hp and, importantly, doesn't seem to push my 14' welded jon as fast as I think it should. (I struggle to get on plane with an empty boat and a hydrofoil that helps with the current prop.) Where do I begin?
Click to expand...

Any banged up prop is not going to work very well or push much of a load. Really need more information. Did the prop work well before getting all banged up? If so replace it with a new one of the same size and pitch.
Click to expand...

X2
 
Geo Kaiser said:
Sounds like either the props to big (diameter)

Or it could be a spun hub also
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Spun hub was my first thought too.
Even a banged up prop on a 20hp should easily plane an14’ boat out with no load.
 
It pushed my previous, and much lighter, boat just fine. I'll try to post a pic soon, but I would call it at least "moderate" wear on the prop. Edges are scuffed quite a bit but no major tears, etc.

I'll do some research, but I've seen folks refer to a "spun prop" before on here and, frankly, never knew what it meant...could I have spun it when I transferred the motor to the new boat?
 
Wow. Thanks, all...even a quick search on "spun hub" has me thinking that might be the issue (especially since it doesn't seem like the boat picks up speed as I increase RPMs after a certain point).

I'll run the test soonest (by marking and running it) and circle back on this thread...
 
Hard to say why a prop hub lets loose when it does, outside of hitting something of course.

We were 20 miles offshore one day when another boat flagged us down. Spun prop on a 150hp, and they had a relatively new boat. THey sure Didnt hit anything, we were in 200' of water. It just spun as it got to 3/4 throttle.

I have a 25 with an aluminum prop on my 15' semi-v. I'd bet I've worn off a 1/4" of the prop, edges look horrible. Still jumps me up on plane and runs 24 mph in calm water at 3/4 throttle.
 
In my experience a prop with a slipping hub will perform perfectly at any speed below the point at which the shaft torque exceeds the holding ability of the defective rubber hub. When that point is exceeded, the hub seems to slip completely, and the motor races as if there is no load on it whatsoever. Very much like the way an older outboard acted when a shear pin was sheared. Each time it happened, I just kept my speed reduced to a point where the hub could maintain its grip on the prop and never had any trouble getting to my destination. I took the prop to the dealer and had a new rubber hub pressed in. I don't recall it costing too much, at least compared to the cost of a new prop. Luckily, it has been decades since it last happened.
Regarding your immediate problem, the first question to answer when trying to solve propeller problems is "What are your maximum RPM's at full throttle". If your boat doesn't have a tachometer (most don't), you can google numerous ways to determine engine speed. Each motor has a maximum full-throttle RPM range. It is listed in your owner's manual or can usually be found by searching online. If your motor is over-speeding, you might need a bigger prop or, more likely, a prop with more pitch. If your motor can't get the RPM's up into the recommended range, your propeller has too much pitch and the motor is being overworked, preventing it from getting into its maximum power band. Of course, it could also be that the motor is worn out or needs a tuneup.
 
I opted to try a new prop, as the inside (and outside) of mine was sufficiently worn for my taste. I went with the same pitch as my factory prop but will change if the spun prop was not the primary problem (there was a lot of wear on the inside of the prop, so I'm guessing that was a big part of the problem).

Here's a pic of the old prop for the sake of history:



View attachment KIMG0394.JPG
 
Carl said:
That prop looks brand new compared to mine!
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Yup, by my shallow-rocky-water and ancient old motor standards, that prop is pristine.

After 15 years of use and abuse by me, 10 years of similar use by the former owner, and who knows how much of what kind of use by the owner before him, I did finally end up with a spun prop on my 15 HP Yamaha 2 stroke last summer.

It was unmistakable--I could make slow headway (5-6 knots) at idle speeds, and none whatsoever at higher rpms. It was a long run back to the launch site, and we wouldn't have made it if the 2-4 knot tide was not with us.
 
Matt Tyson said:
I opted to try a new prop, as the inside (and outside) of mine was sufficiently worn for my taste. I went with the same pitch as my factory prop but will change if the spun prop was not the primary problem (there was a lot of wear on the inside of the prop, so I'm guessing that was a big part of the problem).

Here's a pic of the old prop for the sake of history:
Click to expand...

That prop is fine, you will hardly notice that in PRMs or top end. I wonder more about all the stuff you have above the prop on the anti-ven/cav plate. I can't tell exactly what is going on there, but there appears to be a lot of drag inducing stuff. As a side note and it is almost impossible to tell in photos, but the motor looks high in relation to the keel.

T
 
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Don't know much about props. Had a free prop that fit my 9.9, but would spin some days. Took it to a prop shop and they pressed in a new piece and ran like a top. Wasn't expensive. Look in the yellow pages...
 
[/quote]

That prop is fine, you will hardly notice that in PRMs or top end. I wonder more about all the stuff you have above the prop on the anti-ven/cav plate. I can't tell exactly what is going on there, but there appears to be a lot of drag inducing stuff. As a side note and it is almost impossible to tell in photos, but the motor looks high in relation to the keel.

T[/quote]

I was thinking the same thing, what is all that stuff??? A 20hp motor on a 14' boat shouldnt need all that "stuff" to get it on plane and run 25 mph.

And yes, the motor appears to be sitting too high, cavitation plate should be at keel level.
 

That prop is fine, you will hardly notice that in PRMs or top end. I wonder more about all the stuff you have above the prop on the anti-ven/cav plate. I can't tell exactly what is going on there, but there appears to be a lot of drag inducing stuff. As a side note and it is almost impossible to tell in photos, but the motor looks high in relation to the keel.

T[/quote]

I was thinking the same thing, what is all that stuff??? A 20hp motor on a 14' boat shouldnt need all that "stuff" to get it on plane and run 25 mph.

And yes, the motor appears to be sitting too high, cavitation plate should be at keel level.[/quote]


I agree there is a lot of stuff going on down there!
 
All the stuff is an SE Sport hydrofoil with the no-drill fitting, which definitely improved my motor performance (although I still don't get up on plane as quickly as I suspect I should).

I went with a 9 pitch prop and may try a 12 pitch, if for no other reason than to experience the difference (hopefully I can borrow one rather than sink another $100).

As for the prop height, not sure what to say...I'll look again, but I recall checking that I was spot on when buying the boat (and outfitting it with my current motor).
 
The cav plat is less than an inch above the keel. Possibly still too high? I've read recommendations about checking the cav plate height on plane rather than while sitting on the trailer. I think I'm a few holes up on the motor bracket, so I can futz with it next time on the water and check results.
 
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