That boat with that engine and dialed in right should run 62-65 on any given day. 47 MPH is way too low...

That is actually a good setup with that boat and engine and was one of the faster 18 footers out there...

I have a 00 Stratos 283 (91-92" beam) With 115 HP that will do 47 loaded (GPS)and will hit 50 unloaded. That is almost the same boat with 35 less HP. The 150 should run in the 60's. Industry standards for a new 18' boat with a 150 is lower to mid 60's (source nitro website). I know there is many factors that control speed, but it seems that boat should do at least 10 mph faster.

Boy, never thought I'd be getting so many different answers. I might try and get an actual radar calculation of the speed (I just happen to have access to a radar gun ha-ha). However, I thought that the GPS calculations were supposed to be pretty accurate. I thought that my boat's motor was just old, but it sounds and runs great. I don't know how to figure out how high or low, the motor should sit, the "proper or correct" pitch of the prop. What else do I need to consider? Thanks again for all the information.

check out this prop calculator .

I believe your gear ratio is 1.85:1, and normal prop slippage would be maybe 15%. If this is correct, you should be going 55mph with a 23p propeller.

Experiment with the jackplate and trim button.

My mistake, for some reason I was thinking the 285 was a bigger boat. The way Stratos numbers their boats, 201 is bigger than a 285. Something is wrong with with your boat or motor. I just worked on a 284 with a 150 Fast Strike had a 24 pitch renegade 4 blade, 2 people, full of fuel and tackle ran 59.4 GPS at this altitude and jumped on plane. Motor is on a 8in manual jack plate with prop shaft center 2in below bottom of hull. Would go thru carbs, timing, fuel pump and filters first (do`nt overlook the screen in the fuel tank). You could be dropping a cylinder at speed, have seen it. Keep us posted

NTF...what exactly can this fella do for 15MPH more top end when he is already maxed out on RPMs?

Prop change, or motor setup... We would just have to take a look at it. A three blade prop will also run faster than the 4 blade in most applications.