9.9 Mercury 4 stroke~disappointed in performance at altitude.

[keninaz]

I bought a new 9.9 Mercury with a Lowe V1467 WT aluminum boat for fishing. The boat weighs in at 262# and is rated for a 20 HP motor max. I picked the 9.9 HP as it allows me to fish lakes in AZ that limit motor size. I live at some 5100' in elevation.
The stock prop had a pitch of 8.5" and with a 600# total load in the boat (including the motor and such) the boat could only get up to 6 MPH measured with a GPS. I found a chart which said that for every 1000' starting at 3000 to reduce the prop pitch by 1". Also, Mercury informed me that I needed to change the pitch on my prop to help the motor rev up for more speed at my altitude and they advised I change my prop out to a 4 blade bigfoot series prop, an 8.75"x 5.5". So I ordered the new prop and took the boat out yesterday to try it again. I did add another 100# to the load with a trolling motor and battery however.
I was amazed that the motor seemed to rev up more than it did but I could still only make some 6 MPH top speed as measured by a GPS.
Everything I have read and been told says that what Mercury told me should correct the problem but it's just not working.
Any ideas?? I would hate to have to change jets every time I change elevations but I may have to?

 

[Frank Acampora]

Correct what problem? Altitude reduces the amount of air an engine can pump and therefore reduces horsepower. Old piston powered airplanes were supercharged to help overcome this problem and even supercharged, they had a "Ceiling". This was the maximum altitude they could achieve where the engine was producing just enough horsepower to keep them flying.

Your boat will never get on plane let alone say, 20 MPH with that engine. You are asking too much from that engine.

Simply put, you don't have enough horsepower. It is what it is!

 

[keninaz]

I am well aware that I will lose some 15% of the power at my altitude. But when printed matter and the manufacturer tell you to reduce pitch to gain more power and RPM and still nothing happens I am still disappointed.
And they had full details on the boat, model and load. So knowing full details what are they selling me besides a bill of goods for no improvement whatsoever?
I think at higher altitude that I will just use the electric trolling motor.
But I certainly don't know what to expect at lower altitudes with the gas motor and the standard prop either.

 

[anzomcik]

What speed did your boat do at a lower elevation? Will it plane out?

I understand your disapointed, but I do not think its the motors fault. If merc said lower the pitch to increase rpm and you did and in your first post you said it sounded like rpm increased, then they didnt lie. Do you understand what you were doing by decreasing the pitch? It sounds to me that you didnt understand what you were changing, and what the changes would do to setup.

At 15% decrease in power your motor is about 8.5hp, what are you expecting out of it? I had a 9.9 on my old 14' boat it did 8mph with the wind, changing prop did nothing to my top speed. Then I hung a 25hp on it and would do 22-24mph.

 

[keninaz]

I have not used this boat at lower elevation as of yet so I have no idea of it's performance. What I noted in engine break-in was that I seemed to gain no RPM from 1/2 throttle to full throttle and the measured speed remained 5.5-6 MPH
Yes I understand what the pitch is doing but they told me it would not only increase RPM but speed. I am certainly not an expert on motors at altitude as this is my first experience as most of my boating was sea level in salt water or lakes close to that.
But if I gained nothing in speed going down 4 pitch sizes and going to a 4 blade prop from a 3 blade it was a waste of $110 in my opinion.
Again, I bought the boat and motor combo for fishing a wide variety of lakes here in AZ from 1700'-7000' and most of the Rim lakes at elevation do limit you to a 10 HP or less motor so for gas motors you are very limited. Even Lake Powell is at 3700' here.
Again I think that the trolling motor I have (50# thrust) will do me for the upper elevations and I will just keep the 9.9 stored for the lower elevation lakes or larger lakes where I don't want to wear my battery out getting someplace to fish.

 

[Sea Rider]

If you will constantly be fishing at different elevations you have a problem. If it wera at a fixed elevation you'll need to install a jet carb kit for that elevation + lower pitch change to compensate rarified air at that elevation, don't expect top perrformance as at sea level, at least engine will benefit achieving better performance as opposed doing nothing. Check with Merc for those elevation carb jet kits.

With a 9.9 you're way under powered. Is it possible to upgrade a 9.9 into a 15 HP. If both happen to be 350 CC probably a carb swap will do the trick. If not, you could maximize prop to get the most out of that engine. Will need to install an induction tach go for a wot spin you alone and lightly loaded boat on flat calm, no wind waters and check max wot rpm, compare them to factory stated wot numbers for that 9.9 and play with prop pitches sizes to be around minimum middle to max wot numbers.

Happy Boating

 

[steelespike]

You need to go back to the original prop, empty the boat and get us the wot rpm and gps speed.
You will need an induction tach to get the rpm Something like the tiny tach.Follow instructions carefully.
I figure your motor is a little over 8 hp we need to see if it acts like an 8 hp. with a empty boat.
6 mph is like fast trolling speed.
Has the motor had any work done for the elevation operation?
At 8 hp; empty boat speed should about 12 to 15 mph.

 

[keninaz]

Today, with a full load in the boat, all but the trolling motor itself, and the new 8.9x6.5 prop that Mercury Marine said to use at 5000' I took the boat out and it hit 5400 RPM max and over 12 MPH max. Steady speeds against the wind and all was 10-11 MPH easily.
Problem finally solved after a few experiments.
The only other thing that I did was at idle it was loading up a bit so I drilled the plug they had installed over the fuel screw for idle and adjusted it. It now starts better and idles better and does not load up at extending idling.