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

[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. It's a little slow to plane but my main concern is the top speed. I'm only getting 41 MPH (GPS) at WOT with 3900 RPM (The engine is rated 4600-5000 RPM). I have 2 YouTube videos showing the top speed should be around 52 (I also found other references in forums).

I did the Mercury prop selector and it's showing a 16 to 17 pitch prop. So I'm wondering is it a prop issue or possibly an engine issue.

Any guidance would be appreciated.



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.



Comparison

https://www.youtube.com/watch?v=tLLwZgRmGmw

https://www.youtube.com/watch?v=0CV-_HuOL0s

 

[alldodge]

Your WOT rpm should be in the 4400 to 4800 range. Being at 3900 your way down in the area. So my guess is either your engine is not preforming or the wrong prop. Is this the original prop and how long have you had the boat?

 

[jonjon74]

I purchased the boat this fall. The previous owner didn't say.

The sticker on the engine shows RPM 4600-5000.

 

[alldodge]

I purchased the boat this fall. The previous owner didn't say.

The sticker on the engine shows RPM 4600-5000.

Then I would say check compression and if it's good, then you need to prop down to about 19 pitch, maybe 17 but would start at 19

Currently your showing 22 percent slip, so the hull may be an issue along with trim setting

 

[SkiDad]

you most likely have engine trouble - maybe some of the cylinders not firing. you should be at least high 40's

 

[Fun Times]

The 2nd video at the 3 minute make said that boat was running a 19 pitch Black Max aluminum prop from Mercruiser and if the performance numbers were correct, the RPM seemed a bit to high at 5250... The prop you have now according the part number you posted above is a Black Max 21 pitch which is almost close enough to the same as the 19 they were running...So the 21 really shouldn't make that much of a difference to keep you in the 3900 RPM range under normal circumstances.
https://www.mercurymarine.com/en/us/performance-tests/bhb/2408/?units=imperial

Before we move your topic down into the Mercruiser I/O & Inboard Engines & Outdrives for possibly more help on diagnosing engine problems, Are you trimming/raising the Alpha stern drive up a few degrees once you are up on plane? And what is the "altitude" of the body of water you're boating in?

 

[jonjon74]

Yes, I am trimming up just to the point where before cavitation.


Thank You!

 

[burtonrider11]

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. It's a little slow to plane but my main concern is the top speed. I'm only getting 41 MPH (GPS) at WOT with 3900 RPM (The engine is rated 4600-5000 RPM). I have 2 YouTube videos showing the top speed should be around 52 (I also found other references in forums).

I did the Mercury prop selector and it's showing a 16 to 17 pitch prop. So I'm wondering is it a prop issue or possibly an engine issue.

Any guidance would be appreciated.



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.



Comparison

https://www.youtube.com/watch?v=tLLwZgRmGmw

https://www.youtube.com/watch?v=0CV-_HuOL0s

My two cents, I would imagine it to be a prop issue. It sounds as if you boat is over propped. Myself personally only use aluminums for spares, although I boat in deep clear water and don't have a high chance of hitting something (though the risk always exists). So I would move to a stainless prop to start, you should see gains in hole shot, mid range cruise and top end. I would be willing to bit any of those sites referencing 52mph top end are using stainless. Lots of good options in the $300-400 range for stainless. Unfortunately, there is no magic bullet and you will need to experiment a bit to find the best one for your boat and/or needs.

Every drop in pitch should result in very roughly 100 rpm's increase per inch pitch. I think that motor/boat combo should still be a 21" pitch prop and at least be at or near the bottom of WOT range.

It may be worthwhile to check the hull bottom to see how clean it is. A dirty bottom can result in more drag and lower RPMs. Another issue could be the boat being water logged, if you leave it in all the time and had a leak of some sort, your stringers and transom could be wet, thought I find this scenario very unlikely.

I'd be curious if a lower pitched prop would bring the revs up. If it doesn't then you most likely have an engine issue.....Sorry this is somewhat rambling, hopefully this will help.

 

[jonjon74]

No Title

I checked the hull and it looks pretty clean to me See attached.

It's only been in the water for a few hours and the previous owner keep it on the trailer. I couldn't image it;s water logged. but you never know.

I also attached a pic of the prop, It has a few nicks and dings, How does it look to you guys.

I'm planning to bring it to an authorized Mercruiser shop, the closet one is an hour away. They are doing to check compression and put it on the Dyno.

Hopefully it's something simple.

Thanks again for all the suggestions.

 

[Rick Stephens]

I would also use a shop tachometer, like one built into a decent timing light, to make sure of your tach readings. More often than can be counted boat tachs aren't near close enough. Well worth the effort - don't even have to put it in the water.

Nice looking boat, BTW.

Rick

 

[burtonrider11]

Interesting that the boat has trim tabs. In my experience, a boat that size with that kind of power should not need trim tabs. Of course they can't hurt if used correctly, but I am wondering if those could be affecting trim and running angle. I once had a boat that if you didn't trim her up, it would only run up to about 3600-3700 rpms. But, give her some trim and you could really feel it and the RPM's would come up like they should.....

If controlled remotely, are you pulling the tabs all the way UP when at speed? If not, these would work to counteract your positive trim on the outdrive, keeping the bow down and potentially causing the low speed/RPM's.

So in regard to the trim tabs, were they installed because the PO was having hole shot issues? If so, I am wondering if that motor has not been running right for a long time, leading to a diagnosis of needing trim tabs. You note in the initial posting that poor hole shot is an issue. With that power AND trim tabs, time to plane should be minimal...... I don't know Tahoe's very well, it may have just been a factory option. I am really just wondering out loud.

I am starting to wonder if it is a prop issue. With a 21 aluminum, you should at least be in the ball park of 4400-4600 rpms. For comparisons sake, I have a 20' Starcraft which weighs almost exactly the same, with slightly less HP (235 VS 260) swinging a 22" 4 blade (which is too much and getting replaced this spring) and I can hit 4400 rpms at WOT. I would think with slightly less prop and slightly more power, you should at least be in that same neighborhood....

Keep us posted on what the dyno and Merc tech's say. Since it's an MPI, they can plug in to see if there are any error codes.

I second what Rick says, very nice looking boat!

 

[steelespike]

It all points very likely to a motor issue. With a 4600 - 5000 wot rpm range. You are 1100 rpm off with a corresponding loss of top speed.
​ 3900 rpm probably accounts for the relatively high slip
Props for that drive; range from 11" to 28". A 21 is roughly in the middle.The weight isn't particularly heavy for a 260 hp.
If we were to try to make up the 1100 rpm with a prop we would probably end up at about 16" pitch.
A 16 at 5000 rpm would only make about 45 mph. As suggested your speed should be close to 50.

 

[jonjon74]

No Title

I purchased the boat this fall and was able to take it out twice. The first time I took it out it did not have the trim tabs installed and that's then I noticed the RPM/Speed issue. After researching ways to improve performance a lot of folks reported Trim Tabs helping out. And they did, bow rise was down and it stopped wondering at low speed but the RPM/Speed issue was still there.

These are the Smart Tabs from Nauticus which has no real-time adjustment. They use a piston system to keep pressure one the tabs.


I'm planning on bringing out boat to the authorized Mercruiser shop this Saturday. Will keep you all posted.

Attached is a better pic.

Thanks Everyone and have a Merry Christmas.

 

[steelespike]

Smart tabs do a great job but applied to aggressively can cause odd handling and eat up some speed.
They should be applied only just enough to be effective.

 

[Faztbullet]

I have seen this same problem on 27ft Regal before, It had been to several shops and a dealer asked me to look at it in his shop. On starting engine the exhaust seemed a little dull so I pulled drive and found BOTH exhaust shutters blocking the Ypipe.Removed shutters with long needle nose pliers and reinstalled drive and lake test ..ran like a champ.Did a full service on drive and replaced shutters and customer stated it had never ran that good...also it was the first boat I had seen with brake lights in the hull!!!!!

 

[Four Winns Fanatic]

Suggest you pull spark plugs may not be running properly on 1 or 2 cylinders , check for wet spark plugs .

 

[Bonus Check]

I have an 2006 Q6 which is the equivalent of your Q7 in size. I only have the 4.3L but I run a 21 P 3 blade prop. I run about 47mph GPS at 4600 rpm WOT. My prop diameter is 15 -1/4 " if I remember correctly. Hole shot is great and I also use the Smart Tabs set in the middle hole. I use a SS Turbo prop.

 

[Four Winns Fanatic]

When I mentioned yesterday check your spark plugs I should have clarified that I had loss of RPM similar to your experience on my 5.0l Mercruiser carb engine turned out I had 2 bad spark plugs they were wet from unburnt fuel I had sprayed too much fogging oil in carb during winterizing and these 2 plugs were carboned up and not igniting .A fresh set of plugs cured problem RPM returned to 4800 .

 

[HT32BSX115]

Howdy,


It has been mentioned I think, but you NEED to use GPS to determine speed, a KNOWN TACH, and you need to VERIFY the prop pitch and drive ratio before you do any sort of trouble shooting.


Until you know the above information for sure, you're wasting your time doing anything else.


Regards,


Rick
By the way, Welcome aboard!!

And since you're new here you also might have a look at the following thread! Lot'sa helpful stuff in there!

Focus particularly on #15

http://forums.iboats.com/forum/engi...77-must-have-technical-information-for-diyers

 

[jonjon74]

No Title

Well, I received the boat yesterday and took it out. The engine along sounded much better, and the RPMs were up to around 4300 and speed is now at 45 at WOT.Still how for that boar.

The mechanics Checked the compression in each cylinder and averaged around 160-175. They did find one discolored plug and went and replaced the entire set. They also changed the impeller and foot oil as well as engine oil, oil filter, and gas filter.

While it's better I don't think it's where it should be at. We should be around 4800+ RPM and 51+ MPH at WOT.

I did buy this boat used last year and know it had been hardly used. The tech checked the computer and it had 24 hours on that engine. The boat is 5 years old. WOW. The Outside and seats need work some woo so i'm thinking I did well

So this brings be back a prop issue. The prop is a Mercury Marine, 832832A45 21P, Aluminum. I attached the mercury selector and it's recommending a lot lower pitches.