Friday, June 26, 2009

Offwind Options

(This is an article we wrote for Multihulls Magazine two months ago. It's a good magazine, with some useful info.)



Watch out for the cliff! Every boat has it, every sailor feels it. Sailing upwind in 15 knots, the boat feels great, dancing across the waves. As you bear off onto a reach, the thrill increases as the boatspeed picks up and the waves feel softer. Heading down a bit more, you approach a beam reach, still reveling in the responsive feeling of power at the helm. Usually somewhere just below a beam reach, though, most boats, especially cruising multihulls, reach a performance cliff. Bearing off another 10 degrees causes the boatspeed to drop, swinging the apparent wind aft, slowing the boat further. While you were racing dolphins 15 or 20 degrees ago, now you’re watching jellyfish as the boat bobs along under main and jib, the collapsing wind over the deck has turned the heat up, and the evil engine key starts to beckon.

Modern multihulls led a design trend across the industry, moving sailplan power aft with larger mainsails and smaller jibs. Upwind, the jib completes the airfoil, shaping the flow over the leeward side of the mainsail and increasing its efficiency. But in terms of sheer driving force, the little jib, often less than half the area of the main, brings little to the aerodynamic party. As the boat bears away and the two sails begin to work independently, the jib sheet eases, the head twists out inefficiently to leeward, and little is left pulling the boat. Combined with (and a cause of) the apparent wind speed drop, this means the water gets stickier and the boat gets slower, quickly. At deeper angles when the main begins to blanket the jib, it becomes completely useless. What’s a speed hungry cat to do?

Fortunately, a range of sails plug the holes in the offwind inventory and allow you to leave the engine silent. Unfortunately, the broad diversity of shapes, materials, and deployment methods can be confusing to a guy just looking to come back from the Bahamas a little quicker. Choosing the right offwind sail depends on a variety of factors you must weigh do determine what’s best for the way you sail your boat.

CONVENIENCE
Most sailors choose a sail based on the reality of how they will deploy and douse it underway, because if it’s too hard to use, it doesn’t matter how powerful it is. Three primary options exist across the range of sails: a temporary furler, a permanent furler, and a spinnaker dousing sleeve.

Temporary furlers consist of a drum with a continuous furling line at one end that connects to the tack of a sail, and an independent swivel at the head. Designed for use with a straight-luffed sail supported by a strong, low-stretch internal luff rope pulled tight, the system allows for a sail to be hoisted or left in place like a furling jib, then rolled out for use. When done, the sail rolls up, and can stay in place for a short time, or more often is lowered and removed, furler and all, into a sailbag. Recently, some companies have made temporary furlers designed for use with large, loose-luffed asymmetrical spinnakers, and while promising, these systems are less tested than those for rope-supported sails.

Usually only larger (50+ feet) with permanent bowsprits use permanent furlers, extending the attachment point forward of the permanent jib furler. These work just like your jib, rolling the sail around the extrusion and leaving it permanently in place with a UV suncover on the leech and foot. They’re the simplest, but most expensive to convert to.

Spinnaker dousing sleeves are the least expensive, but limited to lighter nylon sails. With these, the sail is stored in the sleeve in a sailbag below and brought on deck for use. The sock is hoisted and hangs with the spinnaker trapped inside. Standing on ahead of the mast, you pull the control line, the bottom of the sleeve rises, and deploys the sail.

Again, for many, the dousing system determines the sail. Some people want to leave a sail up and furled when not in use, and others don’t really care what they have to do as long as they get the most power possible. The convenience vs. performance tradeoff often is no compromise, but sometimes, the need for speed takes over.

PERFORMANCE
Aerodynamically, headsails optimized for sailing close to the wind have straight luffs and relatively flat shapes – just look at your jib as the epitome of the upwind sail. Those designed to sail at the broadest angles look like parachutes, large, billowing spinnakers designed as large as possible to catch the wind. If you were to computer generate the animation from one to the other you would see the jib grow wider as the edges bulged out, and curvier as the middle got deeper. Sailmakers can design sails anywhere along this continuum, and advanced race programs might carry as many as 15 headsails, from the smallest jib to the largest spinnaker, with a wide variety of shapes in between designed for specific wind speeds and angles. Cruising sailors don’t have the need, crew resources, or budget for a highly specialized sail inventory, and as a result, four basic shapes dominate offwind sail design. Each of the shapes sails well in their intended usage range, but alas, no shape does everything well. Please note, terminology varies for many of these sails, and it’s more important to understand the shape and function than remember a specific word that different sailors might apply to different sails.

For close-to-the-wind sailing in light air, a large genoa look-alike often called a Screecher is preferred. A triangular sail about twice to three times the size of a cruising cat jib, the sail, positioned on a bowsprit in front of the jib, provides close and beam-reaching ability, while still serving adequately as a broad reaching sail in medium to heavier breezes. Set with a temporary or permanent furler, the target wind speeds and angles intended for the sail shape the choice of the sailcloth used, usually Dacron on a tropical cruising cat, but light laminates and heavy nylons are also possible.


Growing a bit curvier, a Code Zero-style sail sacrifices a little pointing angle to provide more power in the wide angle range around a beam reach. Still on a centerline bowsprit, with a larger roach and deeper profile, the sail drives a heavy cat more effectively than a screecher, but can’t get quite as close to the breeze, and still suffers from getting blanketed by the main at broad angles. Because it’s not used as close to the wind, sailcloth can be lighter, usually spinnaker Nylon, making it more efficient in lighter winds.


Even larger and rounder, an asymmetrical cruising spinnaker abandons the tight luff rope of the previous two sails for a large, positively-curved luff designed. Flown most often off the weather hull (or on the centerline when closer to the wind), the sail’s large, balloon-like three-dimensional shape is designed to float it out to windward, rotating out from behind the wind shadow of the mainsail. This makes this light nylon sail very effective from beam reaching angles in light air all the way through a broad reach.

Finally, a traditional symmetrical spinnaker offers great downwind power and ease of use when broad reaching and running. Usually trimmed with dual sheets and guys to facilitate gybing on long, wind-aft passages, it’s the most powerful and autopilot-friendly way to go dead downwind.

HOW TO CHOOSE
Sailors committed to longer-range cruising, looking to cover the entire wind range from close reaching through running, find a benefit in two of these sails, choosing one of the reachers (Screecher / Code Zero) based on their desire to leave it furler or remove it, and one of the spinnakers (Asymmetrical / Symmetrical) based on how much actual dead-down running they think they’ll be doing. Most sailors, though, are looking for just one to serve the majority of their offwind needs. The decision should be based on any predictable sailing plans for the boat, any preference for setting and dousing options, and desire for power.

South Florida sailors who make frequent summer trips back and forth to the Bahamas will use their main and jibs upwind, and then need one of the spinnakers, likely an asymmetrical, to speed the downwind passage home. Blue water sailors who spend considerable time reaching often choose the “set it and forget it” ease of a reaching sail, and choose between them by deciding whether they want to remove them after use or leave them hoisted furled for short periods of time. Those seeking the most convenience choose a Dacron screecher with a suncover that can be left furled in place for periods of time (though still removed when the boat is unattended). Or, if you want a bit more power – and color – in your sail and don’t mind putting it away when done, the Code Zero may be your choice. When you understand the tradeoffs in convenience and power, and the usage ranges associated with each sail, viewed in the context of how you sail your boat, the right choice becomes clear, and the temptation to fire up the engine on those light-air reaches drifts away with the jellyfish behind you.

1 comment:

  1. Thank your for the article which is perfectly in line with our own experience.

    This is the reason I am installing a spi-pole on our cat. This will allow me to fly the new dacron jib I ordered with wind directions up to approx. 25° wind from the beam + the gennaker to the lee side.

    My next problem is that the gennaker we have is perfect for light winds up to approx. 10 Knots apparent but it becomes a problem when you only have a limited deck watch because the only reasonable thing to do with this sail is to lower it entirely when the wind suddenly comes up (which happened every night out of 2 when crossing the Atlantic).

    As we have time , can you have a look at our gennaker in the front Port locker.

    Can you imagine a solution with a slightly smaller dacron gennaker (a new one our the present one modified) with rolling up characteristics which allows us to roll up entirely (temporarily) while leaving the sail on on bowsprit until the squall blows over.

    Rgds,


    Henri

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