A guide to asymmetrical spinnakers

Sail Trim

David Flynn from Quantum Atlantic answers frequently asked questions about asymmetrical spinnakers.

What makes an asymmetrical Spinnaker different from a standard spinnaker?

First, geometry. The Asymmetrical has a distinct luff and leech. The tack is always the tack, and both sheets are tied onto the same corner (the clew), just like a genoa. Second, the cross-sectional shaping is usually not uniform and semi-circular, as it would be with a spinnaker, but asymmetric. The position of maximum camber (draft), is likely to be forward of 50%. The sails typically have a rounder entry (luff) and flatter exit (leech).

How are they different from the cruising spinnaker we have all known and loved? (Flasher's, Thrashers, MPS, Gennakers, etc.)?

The geometry is basically the same. The difference is in the shaping. Modern asymmetricals use cross-sectional shapes, which are asymmetric to varying degrees, in order to enhance performance at specific wind speeds and angles. Also, as shapes have been refined, the distribution of extra area outside the straight line luff and leech has changed. This allows the sails to be larger, and project more useful area at broad reaching angles.

How big are they?

Asymmetricals can be made in a wide variety of sizes, depending on the wind angles and speeds the sail is intended to perform best in. Standard spinnaker sizing parameters of maximum girth = 1.8 x JC (where JC is the length of the spinnaker pole), and SL (spinnaker luff) is roughly equal to the “I” dimension, are not really applicable. Asymmetricals are often much larger if the boat is equipped with a bowsprit, and the sail is optimised for broad reaching and running. The best way to calculate the area of an asymmetrical spinnaker is to use a formula like the Whitbread Area Formula. One way for PHRF to address the question would be to have the sailmaker calculate the sail's area using the Whitbread formula and stamp it on the sail. The sail would receive no penalty as long as the area was not larger than that of a conventional symmetrical spinnaker.

How are they rigged?

An asymmetrical sail is set on a spinnaker halyard, with sheets attached to the clew and led aft to blocks on the leeward quarter of the boat. The sheets are led in front of the headstay, and outside all running and standing rigging. For boats with a bowsprit, sheets can be led either inside or outside the tack downhaul, depending on the preferred method of gybing. The tack is typically controlled by a short (3-6')pennant, or via an adjustable downhaul. The downhaul is usually led through a block at the base of the headstay or outboard end of the bowsprit, and then aft. Use of an adjustable downhaul, as opposed to a fixed pennant, allows for better control of the sail's shape.

Are two sheets necessary?

If you want to gybe easily, yes. When asymmetricals are marketed as “cruising spinnakers” they are often touted as needing only one sheet. This necessitates running the single sheet around the boat during a gybe; not a particularly elegant way to approach the problem. Sheets need to be twice the length of the boat.

Is a bowsprit necessary? What does it do?

A bowsprit will significantly enhance the performance characteristics of an asymmetrical spinnaker. It allows the sail to get further away from the blanketing effect of the mainsail. It also allows the sail designer to dramatically increase the girth of the sail, making a much larger spinnaker possible. Finally, a bowsprit makes handling, particularly gybing, easier. During a gybe the sail is less likely to run afoul of the headstay. If the bowsprit is long enough, the sail can be gybed inside the tack downhaul, between the downhaul and the headstay.

Can a normal spinnaker pole be used?

Yes. In fact many high performance mono-hulls like VOLVO 70's and Velux 5 Oceans boats set their asymmetricals in a conventional fashion.

An afterguy, running through the outboard end of the spinnaker pole takes the place of the tack downhaul. The use of a conventional pole allows the tack to be pulled to weather as the breeze moves aft relative to the boat, getting the sail out from behind the blanket of the mainsail. It makes an asymmetrical much more effective when broad reaching and running.

Not all rulemakers allow an asymmetrical to be used in this fashion; insisting that the tack be fixed on the centerline.