Expert: Jeff Grigg - 2/20/2008

Question
Jeff, i have a 2006 Mercury 25hp 4 stroke engine on my tracker boat.  It seems to be struggling to push myself and my son (we are bigger guys) around when we try to fish.  Our maximum speed is well under 20mph.  Rather than upgrading to a 40hp and missing out of my favorite local lake (which has a 25hp limit) I was wondering if there is any way to boost or tweek its performance so i can run 20-23 mph.  I believe the pitch on my current prop is 9.  However, to get a greater pitch I need to gain some RPM's.  Any ideas???  thanks

Answer
Tom to really get your boat running right you will need a tachometer. You need to prop the boat so the engine at wide open throttle, WOT, runs in the recommended RPM range at WOT.
To go faster generally you need more pitch, if you are in the bottom range of the correct RPM range you can simply add pitch. If you need to add pitch for more speed usually you will need to reduce the prop's diameter. The diameter and pitch always correlate.
Theoretically the pitch is how far forward the prop moves upon one revolution. So a pitch of 9" moves 9" forward. 23" pitch = 23" forward. However for any number of factors this number will be slightly lower. But you do want to hold the difference to about 15% or less.
So for all this you need a tach to see where in the RPM range you are so as to get all the HP you need, and to not over rev the engine.
Jeff