Why use mainsheet systems described as 5:1, 7:1, or 9:1 rather than 4:1, 6:1 or 8:1?
These systems are quite common (see attached picture as example).
However, since it is only the blocks on the object which is moved (i.e. the boom) which contributes to increasing the mechanical advantage, these systems are actually 4:1, 6:1 and 8:1, respectively.
All systems which terminate the fixed end of the sheet line on the boom necessitates an extra length of rope and a block, without increasing the mechanical advantage. Hence, even if the sheet line must be pulled 5 inches for each inch movement of the boom (5:1), the mechanical advantage is 4:1.
Please correct if I am wrong! I assume that the boom must be considered the moved object and the block on the traveller the fixed point (I know the theory of relativity, however in the moderate speeds obtained in sailing, I assume Newtonian mechanics and empirical measurements may be trusted).
