2010 Tahoe with Mercruiser 5.0 MPI 260HP, Low top speed and low RPM

[alldodge]

I think going to a 19 pitch will get you better hole shot and more speed. Don't see you getting 51 mph so long as you have smart tabs installed. The dumb tabs are always putting drag on the boat

 

[burtonrider11]

Thanks for the update Jon, I was just thinking about this situation. For whatever reason, I am intrigued by this scenario and am curious to see a resolution. That said, I am going to agree with Alldodge above, the tabs are going to scrub some speed if they aren't able to be retracted all the way out of the water. Even with that, they could potentially be counter acting any positive trim from the outdrive, thus keeping the bow down. Bow stays down, more drag is created, less speed and potentially lower RPM's to boot. To cruise at 30mph in a smaller boat like yours and mine, bow angle isn't quite so critical. To start hitting speeds like 45, 50, 55 mph and up, you have to really get the hull out of the water and be running on the last few feet (or inches if you are going REALLY fast) of running surface. Of course, I am simplifying hydrodynamics, but this is the basis to speed and in my opinion, may be one of the causes.

The other factor which I referenced in my first post would be moving to stainless prop. In fact, in my humble opinion, I would have swapped the aluminum for stainless long before I would have added tabs.....I think a good 19" stainless will wake that boat up and get you closer to the rev range. I can't believe a boat with that power to weight ratio would need to move to anything less than a 19".

Please keep us posted!

 

[Rick Stephens]

I think unless you've tried Smart Tabs and have real life results that you are speculating. Often speculation can give a good answer. Just as often it doesn't.

 

[jonjon74]

Before I took it out, I did move the tabs to their least resistance position. They were still in play but couldn't have been holding much pressure.

I'm leaning towards a prop as well. What would you guys recommend in a stainless prop? 3/4 Blade? What series? so many to choose from? LOL

 

[Bonus Check]

I have Smart Tabs and run 47 MPH in my Tahoe Q6 with a 4.3L on a 3 blade 21 pitch Stainless prop.

 

[Fun Times]

I'm leaning towards a prop as well. What would you guys recommend in a stainless prop? 3/4 Blade? What series? so many to choose from? LOL

The SS vengeance from Merc typically works pretty well on boats like yours that run an Alpha drive and they make it in a 19 pitch. Merc part number, 48-16316A46

https://www.mercurymarine.com/en/us/...ies/vengeance/

Or see iboats In-Stock Equivalents
http://boatpropellers.iboats.com/Mer...ce-Propellers/

 

[jonjon74]

No Title

I believe I will commit to upgrading the prop. On the Merc site the diameter for a 14" is 19P but for a 14.5" its 17P.

So what's the right choice? Smaller or larger diameter? There are so many variables.

Thanks again!

 

[alldodge]

I will suggest the 19 pitch, but there are other manufactures which make a 14.5 diameter prop such as Stiletto
http://boatpropellers.iboats.com/Me..._10319_Propeller/767/17585/?cart_id=015895223

I have one of these that I used when I use to pull several skiers up at one time.

 

[QBhoy]

Hi
For what it's worth to you, in my opinion, it is Defo a prop issue.
I have the same engine in a lighter campion chase. Had a 14.5x17p on it when purchased. Set up for wake boarding by previous owner. Ran as follows;
Hit limiter very quickly at around 5100rpm around 55/55 mph
Quickly changed prop to recommended 21p laser 2.
Then ran at 62/63 mph at 4350rpm
Just purchased a 19p laser 2. Details to follow.
As mentioned, peak power and ideal rpm should be 4600-5000 rpm, according to power curve.
Having had a fair few older boats with 4.3 and v6 outboard, I presumed automatically that such an engine would require are least a 21"p, if not 23"p or greater.
Little did I know that the v8 mercruiser have a much higher ratio of 1.47-1 in comparison to 4.3 and v6 outboards. Hence, the pitch has to drop. Suggest 17-19p decent stainless to try out. Each " of pitch should raise the rpm by approx 200rpm.

 

[jonjon74]

Hi
For what it's worth to you, in my opinion, it is Defo a prop issue.
I have the same engine in a lighter campion chase. Had a 14.5x17p on it when purchased. Set up for wake boarding by previous owner. Ran as follows;
Hit limiter very quickly at around 5100rpm around 55/55 mph
Quickly changed prop to recommended 21p laser 2.
Then ran at 62/63 mph at 4350rpm
Just purchased a 19p laser 2. Details to follow.
As mentioned, peak power and ideal rpm should be 4600-5000 rpm, according to power curve.
Having had a fair few older boats with 4.3 and v6 outboard, I presumed automatically that such an engine would require are least a 21"p, if not 23"p or greater.
Little did I know that the v8 mercruiser have a much higher ratio of 1.47-1 in comparison to 4.3 and v6 outboards. Hence, the pitch has to drop. Suggest 17-19p decent stainless to try out. Each " of pitch should raise the rpm by approx 200rpm.

Thanks for the input. I have a topic in the prop sections but wanted you to see this info. Below is a list of props I've been testing with results. With this new info do you still feel like its a prop issue?

Thanks


Prop 1 (Original prop) Diameter 14 - 1/4", Pitch 21 3 Blade (Model832832A45 21P) - Aluminum
41 MPH (GPS) at WOT with 3900 RPM


Prop 2 (Test prop) Diameter 14 - 3/4", Pitch 18 3 Blade -SS
47 MPH (GPS) at WOT with 4500 RPM
- Better Hole Shot


Prop 3 (Test prop) Diameter 14 - 1/4", Pitch 16 4 Blade -SS
46 MPH (GPS) at WOT with 4600 RPM
- Great Hole Shot

Prop 4 (Test Prop) Diameter 14 - 1/4", Pitch 15 4 Blade -SS
44 MPH (GPS) at WOT with 4900 RPM
- Best Hole Shot yet. Really jumps out of the water.

 

[QBhoy]

I would say most definitely and/or the well known distributor problem on these engines (collects moisture very easily and needs cleaned or replaced every year or two).
All this info points towards a prop issue, especially the data ref the 14 3/4 x 18p.
Having thrown my own boat in the water last Monday to test the new 19p laser 2, I can tell you it ran better than ever. Great acceleration and ran as far as the speedo goes at 65mph. I didn't have the GPS with me on that occasion, but previous tests show that it's pretty accurate.
Given the old theory that every 2" of pitch dropped should raise rpm by 400rpm, I was hoping the revs would rise only just into the max HP range at around 4700rpm (dropping from the 21p). Things went even better. She pulled 4900 rpm trimmed up. I can only presume that she Defo wasn't in the full curve before (at 4350rpm) and bring the revs up produced the full whack and let the revs lift even further !
Couldn't have asked for better. Totally recommend a 19p laser 2. They have a smaller diameter than your tested 18p, which should allow more revs and greater speed.
Good luck !

 

[jonjon74]

Thanks for the input. I have a topic in the prop sections but wanted you to see this info. Below is a list of props I've been testing with results. With this new info do you still feel like its a prop issue?

Thanks


Prop 1 (Original prop) Diameter 14 - 1/4", Pitch 21 3 Blade (Model832832A45 21P) - Aluminum
41 MPH (GPS) at WOT with 3900 RPM


Prop 2 (Test prop) Diameter 14 - 3/4", Pitch 18 3 Blade -SS
47 MPH (GPS) at WOT with 4500 RPM
- Better Hole Shot


Prop 3 (Test prop) Diameter 14 - 1/4", Pitch 16 4 Blade -SS
46 MPH (GPS) at WOT with 4600 RPM
- Great Hole Shot

Prop 4 (Test Prop) Diameter 14 - 1/4", Pitch 15 4 Blade -SS
44 MPH (GPS) at WOT with 4900 RPM
- Best Hole Shot yet. Really jumps out of the water.

I was able to test today with a photo tach, looks like the RPM's are off around 450 RPM's. The Tach on the dash showed 4700 the photo Tach showed 4250. So with that new info a Diameter 14 - 1/4", Pitch 15 4 Blade -SS run 43MPH (GPS) at 4250 RPM WOT.

The motor runs really well on the low end, but never getting the WOT RPM's.

Any guidance would be much appreciated.

 

[alldodge]

Drive ratio 1.47
Prop 1 = 21x3 pitch, 3450 rpm, 41 mph = 12.15 slip

Prop 2 = 18x3 pitch, 4050 rpm, 47 mph = -0.08 slip

Prop 3 = 16x4 pitch, 4150 rpm, 46 mph = -7.54 slip

Prop 4= 15x4 pitch, 4450 rpm, 44 mph = -2.33 slip

New 15x4 pitch, 4250 rpm, 43 mph = -4.71 (negative slip umbers)

Something still isn't right, slip is never negative. Didn't go back and reread all post but if your using your phone GPS,this could be the issue, or the new tach is the issue. Your new prop with no more the 10 percent slip should put you at 36.2 MPH

 

[jonjon74]

This afternoon I tried pulling one plug at a time to verify each cylinder was firing and I couldn't tell any difference in the engine. Is that normal?

It also brings into question if the plug wires are correct. I looked through the service manual and it didn't make since of the order so I found this.

Does this look correct?

 

[alldodge]

There should be a change with each wire removed, just how much is noticed is unknown. If this is your distributer

Then just the wires, this kind of cap makes it easy.

The issue of prop slip would have no effect on how well the motor runs.

 

[jonjon74]

There should be a change with each wire removed, just how much is noticed is unknown. If this is your distributer

Then just the wires, this kind of cap makes it easy.

The issue of prop slip would have no effect on how well the motor runs.

I'm pretty sure I screwed up the photo tach testing. I was testing from the water pump pulley not the fly wheel.

Retested today and the on board tach does seem to be accurate. The prop was a power tech 15x4 pitch 4 Blade, 4750 rpm @ 42 mph. The mercury slip calculator showed a 8% slip.

I have the opportunity to pickup a new Mercury VENSURA 14.5, 17P, 4 blade SS. How would this prop run based on the numbers? Better or worse?

Thanks

 

[alldodge]

VENSURA 14.5, 17P, 4 blade SS. How would this prop run based on the numbers?

15x4 turns 4750, going to a 17x4 will reduce your rpm about 200 to 500 rpm. The rule of thumb for a 3 blade is 100 rpm per increase or decrease of pitch. The 4 blade has more drag.

Your going the wrong way.

 

[jonjon74]

Got a call back from the mechanic and he said all is good. Firing on all 8 cylinders, compress test were above spec. He is leaning to a prop issues.

​Thanks

 

[alldodge]

Boat test for the 2014 shows 53.7 WOT using a Black max 14.5 x19 (3 blade) at 5250 rpm. You doing a 17x4 blade and turning 4750 rpm is leading me more to the hull has a problem, or incorrect trim.

http://www.boattest.com/review/tahoe/3107_q7i

Just to check, you start with trim down and as speed increases your trimming up until it just starts to porpoise then come back down a tad, correct?

 

[jonjon74]

Boat test for the 2014 shows 53.7 WOT using a Black max 14.5 x19 (3 blade) at 5250 rpm. You doing a 17x4 blade and turning 4750 rpm is leading me more to the hull has a problem, or incorrect trim.

http://www.boattest.com/review/tahoe/3107_q7i

Just to check, you start with trim down and as speed increases your trimming up until it just starts to porpoise then come back down a tad, correct?

Yes sir, All the way down and trip up to porpoising. I did have Nauticus trim tabs installed but removed them for this testing.