[thatch]

A vacuum gauge is a great tool for doing several things. First, it will help you find the most economic cruising speed. This is normally where you have come close to hull speed with the rpms in the 1800 to 2000 range, with the gauge still showing a relatively high vacuum reading. Obviously this can be a great help in finding the right prop for your boat. It can also serve as a "tell-tale" about the condition of your boat's bottom and when its time to do some cleaning. Don't be surprised if the needle tends to bounce at idle, it's just the gauge trying to read each cylinder individually. There are ways around this bounce but it's not really necessary to try to fix it.
Tom