Mainsail and jib interaction. Boundary layer and standoff, what are they

After telling you how mainsail and jib. work together and influence each other, today we deal with another important issue for the optimal adjustment of the two sails. In this article titled “Mainsail and jib interaction, boundary layer and standoff” Professor of Naval Architecture at the University of Genoa, Paolo Gemelli*, former author for us of the articles “The Carrying Capacity of Sails” and “Mainsail and jib. How to regulate them with the help of science” which were a resounding success.

Mainsail and jib interaction: boundary layer and standoff

We saw in the previous article on mainsail and jib interaction what the main effects of the genoa’s action on the mainsail are, which could be summarized as follows: the genoa causes a slowing of the airflow in the slot (space between the sails) on the leeward front of the mainsail. If the air does not slow down excessively, the mainsail can carry even in very tight gaits without stalling.

This happens, in a nutshell, because the genoa prevents airflow separation downwind of the mainsail.

It is worth trying to understand the mechanism of airflow separation from the profile because its implications are extremely important for the optimal adjustment of mainsail and genoa.

If we analyze the air flowing over a generic profile, we can identify two areas where the behavior is different: an outer area where the fluid keeps its characteristics unchanged, and one very close to the profile and of limited thickness, where the effects of viscosity become more pronounced, and the presence of the profile drastically influences the fluid’s characteristics. The same phenomenon can be observed with wind flowing close to the earth’s surface and taking on entirely different characteristics (in terms of intensity and direction) than flowing at high altitude. This limited portion of fluid is called the boundary layer or boundary layer.

Within it, three additional zones can be identified along the surface of the profile: the first, near the leading edge, of the forestay in the case of the genoa, where the behavior is nearly laminar (air particles move along nearly parallel trajectories); a transition zone; and a zone of turbulent flow extending to the trailing edge.

Several experimental tests combined with daily practice have identified an additional situation in which the flow detaches from the surface of the profile.

We realize that something has happened because the threads leave their correct position (parallel to each other and to the trajectory of the air particles) and begin a chaotic movement that indicates that flow separation has occurred. In this situation, the sail begins to lose its effectiveness and boat speed is affected.

It is important to understand under what conditions flux detachment occurs because the proper adjustment of sails is aimed at limiting this phenomenon as much as possible.

The condition underlying fluid release and subsequent profile stall is the excessively rapid increase in pressure along the profile surface corresponding, as is now known, to a decrease in fluid velocity.

A particularly interesting phenomenon is the formation of a boundary state separation zone near the leading edge of the profile. This phenomenon becomes evident by trying to gradually increase the angle of attack of a profile with respect to the fluid. In the initial stage, as soon as the angle of attack starts to increase a bubble-shaped separation zone will form near the leading edge; by further increasing the angle the separation zone will expand further eventually causing the profile to completely stall.

The three “microthreads”

Knowledge of this phenomenon can help prevent complete sail stall if the crew is able to catch, by appropriately placing windward marker threads near the leading edge, the early signs of boundary layer separation zone formation. This particular aspect deserves further study, however in the very first instance the idea of placing between the forestay and the first fillet, two or three smaller fillets, may be a good way to capture the formation and development of the flux detachment area.

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.

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