Help me re-prop

[NOSLEEP]

I have a 19ft 2000 Starcraft Islander - 3.0L 4cyl Alph 1 Gen II 2:1 ratio - 2600lbs Using Smart Tabs (They work great)
Prop I have now - 15x17 3 blade aluminum.
The engine is in excellent running order. Tach and GPS Confirmed.
At 4800 ft above sea level. Wide open throttle - trimmed appropriately 4200 rpm = 29 mph
At 0 ft sea level. Wide open throttle - trimmed appropriately 4500 rpm = 32 mph

I am considering a 4 blade replacement but 3 blades may be more appropriate.?
The boat is used for fishing. I have never towed a skier or boarder, tubber.

I would like to get greater top end speed as well as good performance if possible. I don't believe I would care for separate props for the difference in elevation but would consider it if I had too..

I used the general formula for determining mph and was spot on I believe.

RPM divided by gear ratio X pitch divided by 1056 - 12% slip = speed (29.7 mph) pretty much bang on!
I am trying to get the prop right on the first try because I have to order the prop and shipping and duty apply.

Thanks Steve

 

[jestor68]

Re: Help me re-prop

The Solas Amita 15-4/5 X 15 should satisfy both altitude situations. About 4800 down low and about 4500 up high.

The speed isn't as important as keeping your motor within it's recommended (4400-4800) operating rpm range, which it presently is not at altitude.

 

[NOSLEEP]

Re: Help me re-prop

The Solas Amita 15-4/5 X 15 should satisfy both altitude situations. About 4800 down low and about 4500 up high.

The speed isn't as important as keeping your motor within it's recommended (4400-4800) operating rpm range, which it presently is not at altitude.

Thanks for the responce jestor68

That should keep me to a max of about 28 mph at 4600 ft and 30 mph at sea level according to the formula.
Would that be more efficient than a 4 blade?

 

[steelespike]

Re: Help me re-prop

Using 10% slip the prop calculator says about 28 at 4600 ft. and 30 @ sea level.
I do wonder if you release the tabs so they are just effective enough it may help your top end.
I don't know if the Starcraft is hard to push but it seems to me that a 19 footer with 135 hp could use a 19" prop and run
in the high 30s.In your split altitude the 17 seems more normal but at sea level should do better 4800 rpm or more.
Are you sure the compression is good?Does the throttle open all the way?

 

[NOSLEEP]

Re: Help me re-prop

Hi steelespike

The 3.0l has very low hours. I have done a compression test. My 9.8 kicker motor gets around 500 hrs a year. I have rebuilt the carb and replaced all gaskets including the head gasket 2 years ago. I am a licensed mechanic for 35 years. I have adjusted and tinkered with the trim tabs as well. The throttle does open all the way. The 15x17 prop has been on the boat since I bought it in 2005. I did feel the boat could be over propped with the 15 diameter and may benefit from a 14 diameter and as you mention with a 19 pitch but I don't know enough about that to just go ahead and buy one to test it...

 

[steelespike]

Re: Help me re-prop

The reduction in diameter is normal with a pitch increase.If we can't find a reason for the lower rpm the pitch increase will
make matters worse. The 15" Suggested by jester may be the only choice.
I'm just a little puzzled a 15" is close to what you might find on a pontoon.
I checked around a little; found a 26 ft 3.0 pontoon with a 14" prop
and and a 19 ft islander 3.0 with a 19"
Didn't spend a lot of time looking and 2 examples don't mean much
but I do wonder...

 

[NOSLEEP]

Re: Help me re-prop

Interesting. Thank you for taking the time to check I very much appreciate that! The boats hull is very much like a 19 ft Lund. I thought if I wanted to bring up the rpm I could reduce the diameter and if I increased the pitch I could get a little more speed. I thought it should be able to reach 33 or 35 mph with the right prop and pitch. Its only a 19 ft aluminum with a deep V & 135 HP... Did the 19 ft Islander 3.0l have a 14x19 ? or something else... This is the number stamped on the hub of my prop. 48-82328228-C1-17

 

[steelespike]

Re: Help me re-prop

the 19"was a 14 X 19 and the question was something like what would work on the new 3.0 just installed replacing an old 3.0 120.
It was not stated but implied the 14 X 19 was on the 120 originally.
While diameter can affect rpm It is a secondary consideration.When looking at the 17" props about half were 15" the others basically 14.5.
Typically as pitch goes up diameter goes down. A 1/2" change in diameter means nothing when a 2" pitch change results in a 300 to 400 rpm change. A 1" diameter change might result in 50 to 100 rpm change But a 19" prop should behave like a 19" prep.
I wonder if someone may have messed with your 17. However your slip numbers seemed reasonable for a prop operating below the wot rpm rating"4800". 14% @ 4200 and 11% @ 4500.By my figures.
I wonder ,if your motor has mechanical timing, if so it may be set wrong or perhaps not advancing as it should.
Does it have a strong hole shot at sea level?Typically A 3.0 does real well for sking with a 17" prop,possibly over revving under a light load.Don't assume because the boat is relatively young that the throttle opens all the way or that the tune up is correct.
Have you tried wot with the tabs fully released?

 

[NOSLEEP]

Re: Help me re-prop

In your expert opinion. What could, or would likely be the result if I replace with 14 x17 prop. Stay the same, make such a small difference not worth the change, lose bottom end performance. lose top end performance ?

 

[steelespike]

Re: Help me re-prop

I'm not the guy to be guessing what that kind of prop change may result.
I think your better off with the 15 4/5 X 15 for the split elevation you are working.
While you might lose a little speed The motor will do a lot better at higher elevation.
and be really strong at sea level.
The 14 X 17 might gain a little rpm,50-100- Maybe a little more hole shot but slip might go up with the smaller blade size.
Not a good change in my opinion.
The higher rpm from the 15 4/5 X 15 might improve your slip numbers so there may not be any speed loss.

 

[NOSLEEP]

Re: Help me re-prop

Thanks for taking the time to answer my many questions. I was concerened the larger diameter would be harder on the motors crankshaft, wrist pins and over all longevity.

Cheers,

Steve.

 

[steelespike]

Re: Help me re-prop

Actually the lighter pitch will be easier on the motor.Instead of struggling most of the time it will be easier to get up to speed
and it should stay on plane easier at reduced throttle.
I still wonder why your boat Requires a prop you might find on a very difficult to push pontoon.Its not hard to figure a 15" pitch on say a 23 or 24 foot pontoon.

 

[NOSLEEP]

Re: Help me re-prop

I wish I knew the answer too. Just a shot in the dark. Is it possible for a anti fouling paint to cause enough drag to cause this type of situation.

 

[steelespike]

Re: Help me re-prop

Yes anti fouling paint can cause drag especially if its starting to flake off
and/or if it was applied coarsely like with a roller.
Whether it could reduce speed 6 or 8 mph I don't know.
Does the boat respond to up trim?
Do you run regular gas, 87 octane?

 

[NOSLEEP]

Re: Help me re-prop

The anti fouling has been sprayed on. Its not flaking. It looks good but I suspect it would not slow the boat down more than a mile or two at the most anyway. The boat does respond well to trim up but will soon porpoise if trimmed two high. I have gone thoroughly through the tunning and checked gauges and timing advance and compression. The motor is spot on.
Sooo.... I have ordered a prop from iboats tonight. Hopefully with the help of yourself and jestor68 I have narrowed it down to the best choice. I have decided to go with the 4 blade Solas equivalent of the 3 blade 15-4/5 x15 ; the 14-3/4 x 15 Amita salos 4 blade. With any luck the boat may perform better at lower rpm, docking and still have similar WOT speed. Fingers crossed... Thank you for the generous information and I will report back with the results as soon as the ice comes off the lake...

Cheers, Steve