bigshrimpin
Seaman
- Joined
- Jan 22, 2003
- Messages
- 69
http://www.iboats.com/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=28;t=032243 <br /><br />I just bought a used boat with a 150 evinrude ficht. I can't seem to get more than 4900rpm's out of the engine. Prop => 15 pitch stainless. The boat is a 20ft seacraft (weighs 1800lbs).<br />The cavitation plate is slightly higher than the bottom of the vee.<br /><br />Engine has a 2000 powerhead with the pinned heads. The compression is 105 - 110 PSI in all cylinders. When I did the compression test I noticed terrible soot like carbon buildup on the plugs. <br /><br />I worked out the numbers with a prop calculator.<br /><br />Gear ratio =>1.86<br />Pitch =>15<br />rpm =>5500<br />Theoretical speed =>42<br /><br /> http://www.go-fast.com/Prop_Slip_Calculator.htm <br /><br />42 - prop slip 10% = 38mph<br />------------------------------------------<br />I had 3 people in the boat and hit 38/39 mph via GPS. The tach only reads 4900.<br />------------------------------------------<br /><br />Could my tach be wrong?