A Tale of 2 Props... 14x11 vs 13.25x17.

[GranvilleDave]

I bought this boat from a 'friend' ('74 Avenger 16.5' with '74 Johnson 50 HP (re-homed to this boat)) and it included 2 props. I've tried both with totally different results and think I may need something in the middle. Here's what happened.

Prop 1. 14x11. Ran it 2 times (both without the Tach configured, I know that I need to retest with the Tach but wanted to float this out to the forum for your thoughts and input). Test 1: My wife and I, calm day, very light tail wind, boat was slow to get up on plane and would run 22 MPH WOT (again, not sure of RPM) here. Test 2: My 'friend', his friend, and me, slightly choppy lake and the boat would not get up on plane. I didn't even measure speed. It seemed that my 6.5 kicker pushed the boat as fast as my 50 HP!

Prop 2. 13.25x17. Test 1: My daughter and I on a calm lake, the boat would plane very easily, but the top speed was only 17 MPH and the RPMs would exceed 8,000 at WOT. Of course, we would not leave it at that level for longer than the test.

It looks like I need something in the middle. Would you recommend that I retest Prop 1 and look at the RPMs, or just choose a 14 pitch prop and be done with it?

Thanks for your thoughts. Assume that all 3 adult men, weight about 250 lbs and my daughter is about 150 if that helps in your feedback.

 

[bruceb58]

Prop #1 is lower pitch than #2. If #2 was hitting 8000RPM why would you go to a pitch between the 2?

 

[Chinewalker]

Something is very not right here. If the 17 is in good shape, (.ie hub not spun, not set to high and ventilating) it should NOT be revving that high. Period.

And the 11 pitch prop, if anything, should pop the boat on plane instantly, but should be the one that is exceeding the redline at top end. Prop pitch is like shifting gears on a car. Lower pitch is like lower gear - lots of oomph off the line, but if you keep it in first, you'll rev too high as you speed up. Higher pitch is like a higher gear, it keeps revs down as you speed up the boat. Ideally, your motor should be peaking in the 5000-5500 range at wide open throttle.

Also noted, 50hp on a 16.5-footer will likely not perform very well no matter what prop you try.

 

[Maclin]

The results do not make sense, almost like they are reversed, or the props are mislabeled or labeling misread. There is no way a 17p would rev higher than an 11p on the same boat and conditions. The lower pitch prop should have the lower speed and higher RPM and better take off using conventional wisdom.

 

[Maclin]

And I am with Bruce, if the 17p is really over-revving then you need a higher pitch like a 19, certainly not anything lower than a 17p.

 

[Maclin]

Chinewalker's post was not there yet when I started mine, good points made. I had a 55hp on a 15.5' Trihull and could not run anything higher than an 11p if I wanted to get up on plane with anyone else in the boat.

 

[ttankmoran]

Sounds to me like both props you have are a total mess, worn out, spun hubs, re-pitched, change in diameter, etc, because neither of those props yield any predictable results what-so-ever.

I'm going to go on the limb here and assume you must have the large gear case otherwise that 14 inch prop wouldn't fit, so you must have a 2.66 gear ratio.

Im also going to go on the limb here and guess that your 50 in good running condition will push that boat about 25-30 mph.

Going off that I recommend you get yourself a new aluminum prop 13.5"x15 Pitch and go from there.

Also are you saying that you had 3 adults that weigh atleast 250 lbs a piece in the boat when it wouldn't plane? Thats asking a lot of a 50 hp engine

 

[GranvilleDave]

No Title

Thanks for the feedback! The motor runs solid. Great compression and I had a mechanic go over the whole thing (replaced stator, power pack, plugs, rebuilt carbs, set timing, etc). It has 120+ PSI in both cylinders.

I wonder if the RPM gauge needs adjusted since the motor is not the original motor on the boat and I am somehow getting bad results. I read something about this in an earlier forum.

Also, I do not have trim on the boat, so I set the boat to the second to the last pin (closest to the hull at the bottom). Maybe, I'm setting that too deep too.

I've attached a side pic of the two props. It's not the best, but maybe it would help.

 

[bruceb58]

Maybe it's the lighting but does that prop in the foreground have a chunk of its blade missing?

 

[GranvilleDave]

Yes, a small chunk of the front prop is missing.

 

[bruceb58]

Then any test with that prop doesn't mean much. Also, running with that prop is going to damage your lower unit.

So, what is the condition of the rest of the blades of the two props?

 

[GranvilleDave]

The other prop blades are nearly perfect. The two other blades on the prop with the chunk out are not bad, but not perfect either. I'm leaning towards trying a new 15 pitch prop and see if that is a reasonable compromise.

I didn't realize that a banged up prop could damage the lower unit!

 

[bruceb58]

I didn't realize that a banged up prop could damage the lower unit!

When that piece is missing, the prop is then out of balance.

 

[GranvilleDave]

I just discovered that my Tach Settings were wrong because of repowered boat. I will do another test this week with both props and hopefully get good RPM readings to go with them.