Even the best sailboat cannot sail much closer than 45 degrees to the
wind so this means at least 1/4 of the time you cannot sail directly
toward your destination (2*45 =90 which is 1/4 of 360). On a loaded
cruising boat, you will be lucky to get to within 55 degrees of the
wind so this eliminates 110 out of 360 degrees or 30%.
Where I live, NO wind at least 1/2 the time leaving only 35% of the
time you can sail toward your destination. About half the time while
trying to cruise, you get short of time so you end up motoring
directly toward your destination getting you down to 17.5% of the time
you can sail directly toward your destination. Around here, roughly
30% of the time the widn is blowing, it is a thunderstorm or tropical
storm getting you down to somewhere between 10 -13% of the time you
can sail toward your destination. This is why cruising sailboats need
adequate engines.