CHECK MY LOGIC AND SEE IF I HAVE THIS RIGHT

[DSwatloski]

My Boat is a 2015 Harris 220 Cruiser (22') with a 2015 90 HP Mercury Command Thrust

Top GPS Speed is 22 MPH at 4900 RPM. Using the calculator in the first post, the Theoretical Speed is 29 MPH. The slip at 22 MPH is 24%.

The current prop is a Power Tech stainless steel 15" diameter X 15" pitch. What these numbers tell me is to lower the pitch maybe to 13" pitch to reduce the slip and increase RPM.

That might increase speed and might unload the power head at higher RPM.

What am I missing?

Dave

[kryptonite]

You are thinking correct. Not sure if you will gain much speed, maybe a MPH?, but going to a 13P would bring your WOT RPMs up where they should be and not lug the motor.

[crookedlaker]

just for comparison i have a 2014 20' cruiser with a 90 four stroke not the command thrust and i run about 26 mph at 5,800
not sure about prop comparison as you can turn a larger prop with the command thrust but i run a 13.4 x 15
another thing to check is motor height, have you dealer raise the motor a hole and see if that helps. you definitely have a set up problem. make your dealer fix it. Good luck Mike

[DSwatloski]

So you are running a 13.4" diameter, 15 pitch prop? Do you know the brand or part number?

I am pushing a bunch of water onto the back of the boat. It is rising up the lower unit and pushing up between the transom and the lower unit of the engine. It might be too low as you suggest.

Thanks again.

Dave

[crookedlaker]

I am running a 13.4 x 15 mercury Spitfire prop part number 48-8M4026580
You can run a larger prop with the command thrust however unless the motor
height is correct you will be running in circles. Raise the motor a hole or two, it cost nothing to try
When i got my boat i was running 21 mph at 6,400 rpm they had to lower the motor 1 hole and i got up to
26 at 5,800, felt like a different boat.
hope you like your new Harris, I love mine
mike

[Soonertoon]

Regarding the motor height. I run a 115 Yamaha on a 22 ft twin tube Suncatcher . I had the same issues with water gushing all up the front of the motor and into the transom pan. Was hard as hell to steer as well. Had my Marina raise it up two holes. Was Much better but I found it was limited on available trim before prop blew out. Settled on a compromise of lowering it back down one spot. Picked up couple miles an hour and easier to steer while eliminating the water issue.
Contacted the manufacturer and come to find out they had set the motor up like it was for a tri toon . More buoyancy and higher position in water. Dumbasses.

[BobG]

Top GPS Speed is 22 MPH at 4900 RPM.

This IS the key. That's way too low, and to raise it, you LOWER the pitch of your prop.
I would guess that you're supposed to be seeing 5800 to 6200 RPM at wide open throttle (like my Merc 115 4 stroke).
If you're not getting up into that RPM range, you're never developing maximum horsepower (HP increases with RPM).

Now some people say "200 RPM for every X inches of pitch change". In my experience, you should throw that so-called "rule" out, and just try lowering it one step at a time (props come in 2" increments of pitch usually).

I run an 11" pitch prop, knowing that it is too little for the lake I boat on (elevation 7,500 feet above sea level), but it would be just right at the highest lakes I run on (elevation 10,000 feet above sea level). Yes, when you change elevation significantly, you may have to re-prop for optimum performance. I really should be running a 13 at 7,500, and on the rare occasions when I drop down to 5,000 feet elevation, I run best with a 15. Why is this? Because you lose 3% of your horsepower for every 1,000 feet of elevation change. So:

Sea Level: 115 HP
5,000 feet: 98.75 HP
7,500 feet: 91.5 HP
10,000 feet: 84.8 HP (Turquoise Lake - Leadville, CO)

[DSwatloski]

Crookedlaker, thanks for the info. That spitfire prop is a four blade right? BTW I do love the boat, I'm just trying to get the max performance out of it and unload the power head so I don't have problems with it later in it's life.

Kryptonite, BrokenArrow, Soonertoon and BobG thanks as well.

I put the boat away for the winter in a boatel... so I will try all this stuff next spring and let you all know how it works out.

From what I see, I need to raise the motor a hole or two first and then I will work the prop issues. I will post up and let you all and the community know how it works out.