Guidance

[jonjon74]

Hey guys,

Hoping you can help me with this, I have a 2010 Tahoe Q7i SSI with a Mercruiser 5.0 MPI 260HP with an Alpha Drive. I picked up 2 test props on the shops recommendations - see below; performance increased. The shop suggested I go down 1 more inch of pitch. Wanted to get some options if this is the correct course of action, also should this boat need a 16P prop?

I did have the boat serviced and the engine checked out fine. Compression, fuel pressure, etc...

According to other sources this boat should run around 52mpg.




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 14 4 Blade -SS

46 MPH (GPS) at WOT with 4600 RPM

Great hole shot.








1) Performance issue you are trying to correct.
All around bad performance. Slow to plane and low top end.

2) Current prop manufacturer, model, aluminum or stainless as a minimum.
Mercury Marine, 832832A45 21P, Aluminum

3) Current prop diameter and pitch (required).
Diameter 14 - 1/4", Pitch 21 (Model832832A45 21P)

4) Wide open throttle RPM and speed with an average load (very helpful)
3900 RPM, 41MPH

5) Engine/drive make, model, year, and HP
Mercruiser 5.0 MPI, Mercruiser Alpha 1, 2010, 260HP, Drive Ratio 1:47

6) Boat make model, year, length and weight
2010 Q7I SSI, Length 20'6" Beam 96" 45Gal Fuel Tank, Dry Weight 3031 lbs.

 

[alldodge]

Using 1.47 as your gear ratio and ;looking for WOT RPM between 4600-5000 RPM

Prop 1 - 21P, 3900 RPM, 41MPH = 22.29 slip
Prop 2 - 18P, 4500 RPM, 47MPH = 21.43 slip
Prop 3 - 14P, 4600 RPM, 46MPH = -10.88 slip (negative)

Going from 14-3 blade to 14-4 blade tells me you boat is coming further out of the water and your prop is ventilating.

Actually I thing there is something wrong with your motor and you have the wrong prop or both. Test specs show the 5.0 MPI 260 HP should turn the motor 5000 RPM at WOT and achieve 54 MPH which comes out to a 11.76 slip

First I would suggest using a stainless steel prop, and maybe a vengeance line

Next I would suggest find a mechanic which can actually evaluate the issues. We can help but I think there is something wrong with your motor, or your dragging an anchor and don't know it

 

[jonjon74]

Using 1.47 as your gear ratio and ;looking for WOT RPM between 4600-5000 RPM

Prop 1 - 21P, 3900 RPM, 41MPH = 22.29 slip
Prop 2 - 18P, 4500 RPM, 47MPH = 21.43 slip
Prop 3 - 14P, 4600 RPM, 46MPH = -10.88 slip (negative)

Going from 14-3 blade to 14-4 blade tells me you boat is coming further out of the water and your prop is ventilating.

Actually I thing there is something wrong with your motor and you have the wrong prop or both. Test specs show the 5.0 MPI 260 HP should turn the motor 5000 RPM at WOT and achieve 54 MPH which comes out to a 11.76 slip

First I would suggest using a stainless steel prop, and maybe a vengeance line

Next I would suggest find a mechanic which can actually evaluate the issues. We can help but I think there is something wrong with your motor, or your dragging an anchor and don't know it

I brought it to the Tracker dealer in Jan (it was doing the same thing) and they performed a completed evaluation and said it was 100%. They checked compression, ECU, fuel pressure,etc.

They ended up changing the plugs since they had to come out for the compression check. changed Oil and filter, fuel filter and had them change the water pump impeller.

BTW the 2 test props where Stainless.

I don't think I have an anchor.

Any other suggestions?

Thanks

 

[alldodge]

Find out what drive ratio you have, look for the serial number

Just guessing, but you could be heavy (to much gear, or hull is water logged), not trimming correctly or maybe something is wrong with the hull (slim, paint, has a hook)

 

[jonjon74]

No Title

Find out what drive ratio you have, look for the serial number

Just guessing, but you could be heavy (to much gear, or hull is water logged), not trimming correctly or maybe something is wrong with the hull (slim, paint, has a hook)

Drive Ratio 1:47

Too much gear. Really don't have much in the boat other then life jackets and people. I'm trimming up to the point of cavitation and the hull is pretty clean.

6) Boat make model, year, length and weight
2010 Q7I SSI, Length 20'6" Beam 96" 45Gal Fuel Tank, Dry Weight 3031 lbs.

As far as waterlogged my understanding is the only wood in the boat is located in the transom which is fiberglass coated.

Attached is a pic.

 

[alldodge]

The issue from my view is you are not reaching WOT RPM (4600-5000) and keeping slip under 15%. One time you reached 4600 your slip was in the negative numbers, something is wrong here.

Even if you have a weak motor, you should be able to get your prop slip down, so either the GPS or Tachometer is reading incorrectly

 

[jonjon74]

In pretty confident that the GPS is accurate. Do you have any suggestion on testing the Tach accuracy? especially under way?

Thanks again.

 

[alldodge]

Search for handheld digital tachometer
I have one of these
http://www.amazon.com/Electronic-Specialties-325-Automotive-Tachometer/dp/B000UZO0L0

 

[jonjon74]

Good morning guys,

I was able to get a test tach and the in dash gauge is accurate. In addition I was able to try a 4th prop this weekend and below are the results. Prop 4 did get the WOT RPM's in spec but didn't get me anything on the top end. I did mistype the pitch on prop 3 which is why the slip number was negative, the correct pitch is below

Based on these results, what would you recommend?

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.

 

[steelespike]

Test: 2014 TahoeQ7i 5.0 260 MPI , 19" prop 1.50 ratio 2 adults makes 5250 rpm at 53 mph.
Wondering where your rpms are?
All your slip is about 0% (not possible) seems to indicate your tach may still be wrong.
Typical runabout slip is about 12% I checked 1.50 and 1.62 gear ratios Essentially 0%.
If I use 18" prop at 5200 rpm 10% slip I get 53 mph.The motor is rated for 4600 - 5000.

 

[jonjon74]

Test: 2014 TahoeQ7i 5.0 260 MPI , 19" prop 1.50 ratio 2 adults makes 5250 rpm at 53 mph.
Wondering where your rpms are?
All your slip is about 0% (not possible) seems to indicate your tach may still be wrong.
Typical runabout slip is about 12% I checked 1.50 and 1.62 gear ratios Essentially 0%.
If I use 18" prop at 5200 rpm 10% slip I get 53 mph.The motor is rated for 4600 - 5000.

Thanks for the info, Glad to see someone with the exact boat and engine. I attached a pic of the Tach from this weekends run.

So are you saying the Tach is reading low or high? Wouldn't the engine cut out if I went over the max RPM?

Just not sure what to do. I brought it to the tracker dealer and they said it was 100%.

At a loss.

 

[steelespike]

If we assume the rpm is accurate,which is, possible, Then the GPS speed is not accurate.Perhaps the GPS is set on knots?
47 knots would be about 54 mph. But it is impossible to go 54 at 4500 rpm. I'll be back.

 

[jonjon74]

I took a screenshot of the GPS and it's MPH.

Thanks

 

[steelespike]

Probably right. I really think there is a power problem The 18" prop should behave much like the 19 in the test.
Only more so. If there is a rev limiter it wouldn't kick in at 5200 .
Your fastest prop the 18" should be over 5000.Your 6 mph slower than the 19" tested.And at 4500 and not 5250.
The test boat made 5250 at 53 mph. Tahoe says the tach is right the GPS appears to be accurate.
I think we have a performance issue.
Is there a wale tail on the outdrive?
I know its electronic ignition but::
Is the timing set right and advancing as it should?
Are all cylinders firing?Plug wires on?
Throttle opening all the way?
Don't assume that all is right just because it is relatively new.
Is the bottom clean and smooth?

 

[jonjon74]

Is there a wale tail on the outdrive? No

I know its electronic ignition but::

Is the timing set right and advancing as it should? Tahoe dealer checked and said it was good.

Are all cylinders firing?Plug wires on? Tahoe said it all cylinders were firing. They also did a compression check and said it was good. All withing 10psi of each other.

Throttle opening all the way? Not sure on this one. I will check. Any advice on checking?

Is the bottom clean and smooth? Yes, It's not waxed but clean.

 

[alldodge]

Your speedometer is GPS?

 

[jonjon74]

No sir, that pic was just to show the RPM. I used a GPS app on my iPhone to track speed.

 

[steelespike]

I don't know fuel injection,I just know yours is Multi P Injection same basic idea get fuel to the cylinders ,right amount and time.
If its fuel injected have them confirm they all working. Did they confirm the condition of the plugs when they did the compression check?
They should all look the same.Throttle body should be at the end of the throttle cable.Just be sure it opens all the way.
Look on the motor for anything that seems odd,loose or bent.I had a friend that lost his brakes on his car seems a soda can got
stuck between the pedal and the fire wall.
Could be a problem at the motor or at the remote control.Exhaust fittings,rubber tubes crimped collapsed,etc.
Squirrel nest or corpse,mice etc.If all really checks out you have to look elsewhere.
I'm pretty sure a 220 4.3 could easily out perform your 5.0 260.
Good luck.

 

[alldodge]

No sir, that pic was just to show the RPM. I used a GPS app on my iPhone to track speed.

I think there is the issue, phone apps are nice but I have not found them to be that accurate. Some are off but 600 meters in distance and 6 to 9 mph. Not saying that yours is not accurate, this could add some clarity as to why the numbers are not adding up

 

[jonjon74]

I don't know fuel injection,I just know yours is Multi P Injection same basic idea get fuel to the cylinders ,right amount and time.
If its fuel injected have them confirm they all working. Did they confirm the condition of the plugs when they did the compression check?
They should all look the same.Throttle body should be at the end of the throttle cable.Just be sure it opens all the way.
Look on the motor for anything that seems odd,loose or bent.I had a friend that lost his brakes on his car seems a soda can got
stuck between the pedal and the fire wall.
Could be a problem at the motor or at the remote control.Exhaust fittings,rubber tubes crimped collapsed,etc.
Squirrel nest or corpse,mice etc.If all really checks out you have to look elsewhere.
I'm pretty sure a 220 4.3 could easily out perform your 5.0 260.
Good luck.

Yes, the dealer confirmed all cylinders were firing and getting fuel, also the computer had no errors and was reporting everything good.

I had them change the plugs during the compression checks.

I can check the throttle tomorrow.

I've gone throught the engine and don't see anyting crimped, rats nest....

Thanks