Mainsail base: what it’s for and how to adjust it, but watch out for exceptions

Aboard a sailboat, every single line serves for a maneuver, and among the most well-known ones is undoubtedly the mainsail base, one of the first we learn to handle. However, what exactly is the mainsail base adjustment useful for and how/when should we use it?

Trivially, one might think that the line that adjusts the base of the mainsail serves to keep the sail sheet point integral with the boom, in fact this is not the function of this maneuver. In fact, the reason we have a line that adjusts the base is to change the shape of the sail, that is, its fat.

The rule is quite intuitive: the stronger the wind, the more the base should be tensioned, and vice versa. Be careful not to overdo it, however, especially with little wind, because there are exceptions to the theory. In fact, if in 4-5 knots of wind we keep the base of the mainsail excessively loose with the sail taking on a very deep profile, we risk having a mainsail with an un-aerodynamic shape, as we will explain in a moment.

Indeed, it is necessary to find the right size especially in light wind adjustment. For this purpose it can be useful to mark with tape, on the boom, the position of the clew in the various wind conditions, as soon as we are satisfied with the adjustment obtained, so that we have a reference for subsequent outings.

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Mainsail base – How to adjust it with very little wind

When the wind is very light, say less than 6-7 knots or even less, intuition would suggest keeping the mainsail base as loose as possible, actually in these cases an adjustment against intuitive should be chosen. In very light breeze an excessively deep sail, with a very leaved base, will tend not to discharge well the air it catches.

So let’s not be surprised, especially on racing boats, if we see experienced randers holding the base with a tension almost similar (just softer) to that of 8-10 knots even when there is half as much wind, the reason being the one just described, that of providing the wind with an aerodynamic and efficient profile, which will perform better.

In general, subject to this exception, the rule applies that in breezes the sail should have greater depth than it will have in strong winds where we are going to heavily slacken the profile, and we will do this not only with the base but also with the halyard and backstay, vang and high shrouds in the case of a quartered spreader rig.

Mauro Giuffrè