Her er hva Rod Stephens skriver (sakset fra boken
Rod on Sailing, Lessons from the Sea):
I’m often asked why we don’t step the mast on deck. After all, it makes for a
shorter mast, there’s no hole in the deck, and if the rigging fails it all goes harmlessly
over the side. Well, the big problem with stepping a mast on deck is that you need a
much stiffer section for the same load. This is because when the mast’s column ends at
the deck, it can buckle out wherever it wants to, maybe at the spinnaker pole fitting or
near the lower spreader, and it has to bend only at that one point. Whereas if the mast is
stepped on the keel, when it wants to buckle out at the spinnaker attachment or lower
spreader it must at the same time bend at the partners to compensate for the bend at the
pressure point, but it can’t because it’s secured at the partners. So there’s a strength
factor of about two to one involved. This doesn’t mean the mast has to be twice as big
when stepped on deck, but it does have to have twice the capacity to withstand a
compressive load, so it’s going to be a bigger, heavier spar. Which is why we discourage
people from stepping the mast on deck.
That also explains, incidentally, why it’s important to constrain a keel-stepped
mast tightly at the partners. Otherwise, the mast will buckle in whatever space is clear
around the mast at the deck, and you lose the stiffening effect of those six, seven or eight
feet of mast below deck.
The matter of what size and thickness a mast should be are highly technical
questions that depends on all sorts of complex details: boat size and use, type of rig, kind
of sails, number of spreaders, and so on.
In general, though, we can say that a boat designed for racing will have a thinner
mast with as small a section as possible. This has two chief advantages. First, there’s
less windage, which is important for speed. Second, the smaller section makes for a more
flexible spar that can be shaped to maximize performance. This means, though that the
mast must have a thicker wall so it can stand up to the very high compression loads
created by the need for extreme tension on the headstay for good upwind performance.
Needless to say, such a mast also demands the constant attention of an alert, competent
crew.
Now, a spar with a fatter section will be stiffer, and requires less sophisticated
rigging, runners and such, and so requires less concern by the crew. This mast, say for a
boat meant primarily for cruising, doesn’t need such a thick wall, and so, interestingly,
will weigh about the same as the thinner mast with its thicker wall.
The two masts will of course perform differently, but as in all aspects of boat
design the idea is to arrive at an artful compromise that best suits the need.
