One of the things that becomes very obvious after you've
been sailing for a while is you become a pessimist.
The addage "plan for the worst, hope for the best" is the
operative mind set for sailing.
You keep backups and spares for nearly all the systems on
your boat and you have to decide whether you need more food
and water or more supplies or fuel. It's always a balance
that you strike depending how far from the land of plenty
you are. If you're 100 miles off shore and something goes
awry there is nothing you can do except solve the problem
with what is on board because you are hours and hours if
not days from the nearest help.
When you plan for weather and distance you always have to be
ready for the worst. Weather forcasts are nortoriously bad
and nothing is ever what is says. So as an example:
We can make 7-9 knots under sail or motor but we don't plan
our trips for 7 knots we plan for 5. If we go faster we arrive
early and we're happy if we go slower we arrive late.
We plan our trips to arrive in the morning so that if we're
late we have an entire day to still reach the next anchorage.
We do things a certain way not because we're sure we can make it
without more water but because what might happen if we're late.
So we always fill our tanks even if we have days of water left.
Why? Because what might happen if we didn't make it on time
or if something goes terribly wrong. We always want to have the
maximum flexibility possible to change course mid stream in
the event of the unforseen.
