Prop Question

[mjc3276]

2 weeks ago, i was in the Potomac post sandy. Conditions werent condusive to open it up, but I definatly had to go through a lot of driftwood. It dinged my prop (found out yesterday)

Fast forward to yesterday... I went out on the lower Potomac and at WOT on a 9.9 Mariner, My RPM skyroketed and prop lost bite. I slowed to idle and turned it up again... 2 min later same issue. I shut her down and pulled the motor up and noticed the prop was dinged.

Questions- Would the dings possibly result in lost prop bite?
Can I file the edges to fix the ding and not effect balance?
Do I buy a new prop? If so... Im looking at a 10 pitch merc for more speed. (16ft starcraft carrying 500lbs of weight plus motor)

Any advice would help tremendously. I am trying to save for a 25 hp Evinrude 2 smoke OR a 20 hp 4cyc Merc. (boat is rated for 40)

 

[Texasmark]

Re: Prop Question

Don't go nuts over this man. You probably have spun your prop hub...aka the rubber interface between the spline geared to the prop shaft and the inner hub tube which is cast as part of the blades has worn to the point where it can't stand any kind of torque.

Get access to the rear of the prop and with anything scribe a line from the prop shaft splined bronze inner part of the prop to the outer diameters of the prop's hub....just before you get to the blades.

Go and run it. Get back to your marks and see if they are still aligned. If not get a new prop or that one re-hubed.

Mark

 

[jestor68]

Re: Prop Question

Since a new Solas prop for your motor is just $55, and most likely better performing than the original; I'd just replace it.

The Mercury prop selector recommends a 7" or 8" pitch prop in that application.

 

[steelespike]

Re: Prop Question

X2 stick with the size you have unless you have a tach and a gps for speed.
Arbitrarily increasing pitch to increase speed isn't likely to work.
Increasing pitch makes it harder for the motor to reach its rated rpm.
Harder to push the boat. A 2" increase in pitch could lower rpm 800 to 1000 on a little motor.

 

[jestor68]

Re: Prop Question

X2 stick with the size you have unless you have a tach and a gps for speed.
Arbitrarily increasing pitch to increase speed isn't likely to work.
Increasing pitch makes it harder for the motor to reach its rated rpm.
Harder to push the boat. A 2" increase in pitch could lower rpm 800 to 1000 on a little motor.

That would be 300-400 rpm change. The size of motor matters not.

 

[steelespike]

Re: Prop Question

I don't understand if a 50 hp has 12 sizes to deal with the required rpm problems how does a 9.9 manage with
only 5 sizes if the changes only net about 200 rpm per inch?

 

[jestor68]

Re: Prop Question

I don't understand if a 50 hp has 12 sizes to deal with the required rpm problems how does a 9.9 manage with
only 5 sizes if the changes only net about 200 rpm per inch?

Maybe because a 9.9 hp motor has a limited application? Hence the fewer number of pitch choices.

The 150-200 rpm thing is designed into all "over the counter" props for a reason; to make prop selection predictable instead of a nightmare.

 

[Texasmark]

Re: Prop Question

One variable comes to mind and that could be prop slip on the smaller engine. Seldom if ever do these props have any extra performance additions other than maybe some are weedless and provide less thrust than a standard bladed prop. But without extras and in pushing a large boat with a small engine, I think slip would go a long way in minimizing any dazzling performance variances you might obtain with one pitch or the other.

Other thing is small engines are usually not monitored, so you probably couldn't tell that much difference in performance between adjacent pitch variations in 2 props.

Mark

 

[steelespike]

Re: Prop Question

If you think in the scale of a 9.9 rpm and its typical boat size that the applications are about the same as say a 50 on
a boat typical for its size.I think you can see that a 50 is likely to lose less rpm than a 9.9 as you add people
If you had a 9.9 with a mid range 9" prop but it is running about 1000 rpm low do to its' typical load,
at about 5000 should be planing so should respond well to a prop change.
At 200 rpm per inch you can't make up the 1000 rpm because 7" is the smallest prop.

 

[jestor68]

Re: Prop Question

If you think in the scale of a 9.9 rpm and its typical boat size that the applications are about the same as say a 50 on
a boat typical for its size.I think you can see that a 50 is likely to lose less rpm than a 9.9 as you add people
If you had a 9.9 with a mid range 9" prop but it is running about 1000 rpm low do to its' typical load,
at about 5000 should be planing so should respond well to a prop change.
At 200 rpm per inch you can't make up the 1000 rpm because 7" is the smallest prop.

It will respond well to a pitch change; just not as much as you think. Sometimes, you need a smaller boat.

 

[steelespike]

Re: Prop Question

It will respond well to a pitch change; just not as much as you think. Sometimes, you need a smaller boat.

I'm speaking theoretically of a boat of reasonable size for a 9.9. I think that with a 9" prop that a a 9.9 at 5000 isn't over loaded
Again a theoretical setting.I wonder if a 9.9 could make 5000 with a 9" and a load.But I think it could plane but need a prop change to get into its rpm range.
A boat that is too small will be harder to push with a load than a larger light boat ie.: a big bare bones 14 footer.

 

[jestor68]

Re: Prop Question

And he should still get 150-200(actual) rpm change per inch of pitch.

Example: In the Merc prop selector; working in his weight category with a 9.9 hp, when I change the weight by 200 lbs, it changes the pitch by one inch.

The weights I'm using for that example are: Boat=400lbs. Motor=100lbs. People/gear=500lbs. For a total calculated weight of 1039 lbs with 6 gal of gas. Recommended pitch=9"

 

[steelespike]

Re: Prop Question

I guess its hard to argue with the numbers.
Do they give a theoretical rpm and speed with that weight and prop?
What is the weight category?
Is that setup like say 900-1200 lbs,, 1200 to 1800,1800-2000 ?

 

[jestor68]

Re: Prop Question

I guess its hard to argue with the numbers.
Do they give a theoretical rpm and speed with that weight and prop?
What is the weight category?
Is that setup like say 900-1200 lbs,, 1200 to 1800,1800-2000 ?

The weight category (in that selector) requires you to know an accurate weight. You can "play" with the weight to see how much of a change causes a change in the pitch recommendation. Of course, that change varies with the amount of HP you're dealing with. As you might expect, the smaller motors are more sensitive to a moderate weight increase/decrease, triggering a pitch change well before a larger motor does.

While it only takes a 200 lb increase with the 9.9; using the same weight with a 25 HP, it takes a 400-500 lb increase to trigger a change to the next lower pitch.

No speed/rpm is given. You need to go to the slip wizard for that.

Of course, the best method is to test with a tach. But I have found that particular selector to be pretty accurate IF you can put in accurate weights. That's especially helpful for picking a prop initially.

 

[steelespike]

Re: Prop Question

Doesn't seem to be logical.The 9.9 requires a prop change for only 200 lb change, makes sense but there are only 5 sizes to pick from.
The 25 only requires a change at 400 to 500 lbs yet probably has twice as many choices.??
What am I missing?
I think I've figured it out. We don't know how many rpm a 9.9 loses with a 200 lb increase.
I'm betting it's more than 200.
Did a little research Found some boats of similar design but different weights.
25hp Yam. PolarCraft V1670 996 lbs 11 1/4" 27.8 mph @5800
25ho Yam. PolarCraft 1578 1344 lbs. 101/2" 6000 @ 26.2mph. While not exact matches the is only 3/4" to make up for 348 lbs.
20hp Yam. G3 PanFish 145, 833 lbs 12" 5700 @ 26.7
20hp Yam. G3 PanFish 155 1085 lbs. 10 @ 6000 @ 23.mph a 2" to make up 252 lbs. rpm increased 300
with more weight so probably an 11 would have been close .
Evinrude 25 AlumaCraft 1648,NCS 978lbs, 12" 5700 @ 26.2
Evinrude 25 Sea Ark Stealth 1887 lbs. 11" 19.2 @ 5385. about 900 lbs more
Only a 1" pitch change Probably an 10" would have had more pep but I have a feeling the 11 was little faster. I think the Stealth has a similar hull shape to the Alumacraft 1648,Jon boat.