Iv'e read alot of posts here... trying to determine if i have the right prop on my boat or not...i have a 16 ft cajun demon pro with a rude 70hp prop is a 13 1/4 X 17 aluminum....when i plane it out it strarts to porpose until i trim it down..however i see four to five MPH increase with it porposeing...with all the posts i see I should start off with a tach reading...however i don't have a tach... what should i buy... how difficult is it and where to start....<br /> 