Prop Pitch/Daimeter At elevation for Mercury 9.9 HP non Big Foot

[keninaz]

I have a Lowe V1467 WT that is 14' V hull and weighs in at 262#. Today I took it out for a break-in and found that it will not go very fast and does not seem to rev up all the way. I am at 5100' as is my local lake I like to fish. I had my wife with me and the total weight on board was about 560# counting motor gas and all. During break-in at 1/2 throttle or less for the first hour I noted that the GPS only showed me some 5-5.4 MPH, not even enough to plane the boat of course.
After that initial period it recommended 3/4 or less throttle for another hour and bursts of full throttle for 1 minute every 10 minutes for the next hour.
This is where I had a problem as the engine does not seem to rev up much further from 1/2 to full throttle and the speed only got to 6-6.2 MPH as measured on my GPS
I have a website showing props for my motor from a 9"x10.5 to 9.75"x 5.5 pitch. I am thinking that a 9"x8 or 9.25"x7 pitch may work but then again I might not be thinking straight. Or should I be looking at the 9"x10.5 direction???
Anyone have any experience with props at elevation? Someone told me that changing the prop at elevation will allow the motor to rev higher.

 

[Star]

I think that motor comes with a 9 x 9. you will do at best 11 to 15 mph in a 14' aluminum with the load you describe. With my 15 4stroke I may get to 17 mph! With one person in cool weather I get to 23MPH. 4 strokes do not wind up like 2 strokes.

 

[keninaz]

At altitude the motor is not getting enough air and I was told to change the prop pitch or diameter to improve the RPM and breathing. I am also aware that the motor will perform more like an 8.-8.5 equivalent but I think 6 MPH is ridiculous. and to see no RPM increase from half to full throttle has got to be telling me something too.

 

[keninaz]

Well I got a reply from Mercury but they were not much help. Apparently they don't even know what diameter prop they shipped the motor with by serial number. This is what they stated.

Listed below are the propellers for a 9.9 outboard. It shows diameter and pitch of propeller. You will need to figure out pitch of current propeller. From there you will likely need to go down in pitch to account altitude. Please consult dealer for assistance with getting outboard properly pitched.
In the chart that I cannot copy here it shows 5 props. it shows either a 8.5"x7.5 or a 8.9"x 8.8 prop as standard. And it shows the 8.5"x6.5, 8.9"x8.3 and 8.9"x 10 as optional props.
I pulled the prop looking for numbers and the only thing I see is "F8.5" So maybe it came with the 8""x7.5 prop that Mercury shows as standard?

Not sure which way is down in pitch is either so they were not a whole lot of help.
I just heard from the dealer by email and they say that they think I need to smallest pitch that Mercury has for the motor and that's an 8.9x6.5. I noted that they are decreasing the pitch and increasing the diameter.

 

[keninaz]

I just heard from the dealer and they are telling me that the F8.5 stamped on the prop is the pitch, not the diameter. They are telling me that a 6.5 pitch should correct the problems for me.

 

[keninaz]

OK, well I just talked to a representative at Mercury and he actually told me to go to the smallest pitch that they make and that part number for reference and anyone else researching this is 812954a10 and is an 8.75x5 4 blade prop which is the smallest pitch they make. He said at 5100' this is the best solution.
It's been a long time since I owned an outboard and they were two strokes of course. Times have changed.