Mainsail and jib. How to trim them to the best (with the help of science)


Give or treat yourself to a subscription to the print + digital Journal of Sailing and for only 69 euros a year you get the magazine at home plus read it on your PC, smartphone and tablet. With a sea of advantages.

Mainsail and jib trimming
Mainsail and jib. In this article we explain how they work together and how one wing affects the other.

Mainsail and jib. How do they work together? How does one wing affect the other, and vice versa? How can knowing their “scientific” behavior help us, on a practical level, in their perfect adjustment? We asked Professor of Naval Architecture at the University of Genoa, Paolo Gemelli*, former author for us of the article “The Carrying Capacity of Sails” which was a resounding success.

Mainsail and jib & airflows

Having seen how airflow behaves around a single sail let’s try to analyze what happens when there are two sails. Mainsail and jib (or genoa). Before proceeding, however, it is worth giving some definitions that will be useful in the lines that follow.

The stagnation line represents that ideal line of flow that separates fluid threads passing over the extrados from those passing over the intrados of the profile.

The term stagnation point, on the other hand, refers to that ideal point on the airfoil that separates the flow passing over the extrados from the flow passing over the intrados.

The term upwash finally denotes the air that passes from the upwind to the downwind part of an aerodynamic surface when it is presented to the wind at any angle of attack.

Consider now Figure 1 where flow lines (the ‘trace’ left by air particles) around a mainsail are depicted.

Mainsail and Jib - 1
Fig.1 – Flow lines around the mainsail alone (image from A review of modern sail theory; A.Gentry; 1981)

The backwater line (seen in the previous article) divides the flow passing through each side of the mainsail and is marked with the letter Sm (the m stands for main in reference to the mainsail itself).

The flow line passing through the forestay is marked with an H. The leading edge of the headsail will later be placed in the same position. The term slot air shown in the figure refers to the portion of air that will pass through the space between mainsail and headsail and which I will continue to refer to by the English term slot.

It is important to note that the stagnation point is on the windward side of the mainsail. On the downwind side, high velocities and relatively low pressure values will be measured on the forward portion while, on the remaining portion, there will be an increase in pressure associated with a decrease in velocity.


Mainsail and Jib - 2
Fig.2: Flow lines in the case of mainsail and headsail (solid line) and those for the case with mainsail only (dashed line) are shown. The line marked Si indicates the stagnation line relative to the headsail. (image from A review of modern sail theory; A.Gentry; 1981)

How they work together

If we now introduce the headsail, the situation is as shown in Figure 2. It is observed that the air that previously passed through the forestay now passes downwind of the jib (or genoa); moreover, the amount of air now passing through the slot is significantly less than that passing downwind to the mainsail alone in Figure 1; it is possible to vary this amount through sail adjustments as shown in figure 3.

Mainsail and Jib - 3
Fig.3 – Variation in the amount of air flowing through the slot as a function of sail adjustment. (image from A review of modern sail theory; A.Gentry; 1981)

The first image at the top of Figure 3 represents the basic configuration, capping the sheer sail results in a 60 percent reduction in air between the jib and mainsail, the modest amount left over must slow down to fill the available space, and, again due to the Bernoulli equation, there will be relatively high pressure in this region (second image from above).

Cocking both sails still shows a reduction in the amount of air, but to a lesser extent than in the previous case. The stagnation line of the flake affects the windward side of the flake. Rather high speeds should be observed in this area as the air tries to abruptly turn around leading edge causing, almost immediately, the stall. In order for this not to happen, it is secessary to slightly let go of the jib or hemming so that the doldrums line is returned to its correct position. The situation is depicted in the penultimate image from the top in Figure 3.

In the last picture we are in the situation where the jib is kept in its basic position while the mainsail is capped. The flow in the slot increases and the mainsail doldrums line has shifted slightly to the leeward side. The speed on the leeward side at the front of the mainsail tends to increase, and the pressure decreases. Under these conditions, the flow on the leeward side may separate near the mast, and to avoid this we should caulk the jib or heel slightly.

Mainsail and jib, the effects on each other

If we take up the concept of air circulation around the sail profile introduced in the previous article, we can make similar considerations referring to the situation with two sails. In this case, the individual circulations associated with mainsail and jib add up and tend to counteract each other in the space between the mainsail and jib as shown in Figure 4

Fig.4 Air circulation in the presence of two sails. It is observed that in the slot the velocities are opposite associating with an overall reduction in flow velocity in this zone. (image from A review of modern sail theory; A.Gentry; 1981)

Effects of the jib or genoa on the mainsail:

  1. Moving the mainsail stagnation point closer to the mast, resulting in reduced downwind flow velocities and reduced risk of stall even for very small angles of attack.
  2. By progressively cocking the jib or genoa the progressive decrease in speed in the slot tends to reduce the pressure differential between the windward and leeward sides of the mainsail. When the downwind and upwind pressures are equal, the mainsail begins to spin.

Effects on the jib

Regarding the effects of the mainsail on the headsail, it can be summarized by saying that the presence of the mainsail improves the efficiency of the headsail. The latter, in particular, produces much more thrust when acting within the flow influenced by the presence of the mainsail.

In particular:

  1. The flow of theupwash in front of the mainsail causes the stagnation point on the headsail to shift to its windward side improving the ability to tighten the wind.
  2. The jib leech or genoa is located in a region of high-speed flow created by the mainsail. Therefore, the air speed when leaving the leech is higher than when using only the headsail.
  3. The high airflow velocity allows the headsail to be used at rather small angles of attack with low probability of stall.

Who is our “prof”

*Paolo Andrea Gemelli is a lecturer in Naval Architecture in the Nautical Product Design degree program at the University of Genoa. From 1999 to the present, he has been involved in maritime security with a focus on weather routing and naval intelligence. He is a member of the expert panel of the European Maritime Safety Agency (EMSA) and the Italian Association of Intelligence and Geopolitical Analysts.




Leave a Comment

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.

Check out the latest issue

Are you already a subscriber?

Ultimi annunci
Our social

Sign up for our Newsletter

We give you a gift

Sailing, its stories, all boats, accessories. Sign up now for our free newsletter and receive the best news selected by the Sailing Newspaper editorial staff each week. Plus we give you one month of GdV digitally on PC, Tablet, Smartphone. Enter your email below, agree to the Privacy Policy and click the “sign me up” button. You will receive a code to activate your month of GdV for free!

Once you click on the button below check your mailbox



You may also be interested in.


Sign in