Patrick has it right, for a 2 blade broadhead. I always work in ‘broadhead mechanical advantage’ rather than ‘ratio’. The ‘ratio’ has been misused at times.
One particulat 3 blade BH was (and often still is) widely touted as having a 3:1 ratio, because each individual blade was 3 times as long as it was tall … when the ‘cut width’ was measured from the ferrule’s surface, and not from the center line of the ferrule, which gives the true cutting height of each blade. Very, very misleading, at best.
MA considers the cut width of each blade, from center line of the ferrule, and multiplies this by the number of blades. This becomes the “total cut width”. The “length of the blade” is a straight forward measurement of the length OF THE CUTTING EDGE (not the length down the center of the blade). The formula is: the distance the ‘load’ is moved divided by the distance the ‘load’ is raised. That’s the correct way to measure the MA of a wedge or inclined plane. Therefore, to obtain the MA we divide the length of the blade’s cutting edge (just one edge) by the TOTAL cut width of ALL THE BLADES.
The MA formula becomes more complex when the BH has blades of differing heights/widths, such as when bleeder blades are present.
In the case of a single blade BH 1″ wide by 3″ long, the MA of the blade will be a true 3:1 MA, and that’s the 3:1 ratio to which Howard Hill was referring. Be cautious of misuse of the term 3:1 ratio.
Ed
