150hp running 40mph

[Wessmith]

I have an 18ft. cobra fish/ski. Boat only runs 40 mph at 4900 rpms. This is WOT. Any prop pitch suggestions? How can I find out what pitch prop I currently have?

 

[roscoe]

Re: 150hp running 40mph

First check for spark on each cylinder.<br />Then check compression.<br />Examine plugs for consistent appearance.<br /><br />Remove prop and look for any and all numbers stamped into it, inside outside and on the blades. poist those numbers here or in general outboard forum to get help in id'ing the prop.<br /><br />Finally, don't expect miracles from that motor, and don't change the prop pitch too much or you will be way out of the rpm range (5000-5500).

 

[Wessmith]

Re: 150hp running 40mph

Mechanic just replaced the switch box and said there is good compression on all 5 cylinders. Plugs all look the same. On the inside of the prop most of the numbers are pretty much rubbed off but I do see a 21P. I am guessing this means 21 pitch? Should I move down to a 19 pitch?

 

[LubeDude]

Re: 150hp running 40mph

There is a lot more to correct setup than just prop pitch. Poorly set up boat with 150, 40 MPH, correctly set up boat with 150, 55-60.<br /><br />We need to know how long you have owned the boat, if it has always ran this way, how far is the ventilation plate from the bottom of the boat, stainless steel or aluminum, are the numbers you are giving us at WOT with full trim. Many things may be problematic.