Sailing downwind: angles, pressure and puffs.

The downwind points of sail

ASA's points of sail run from a broad reach (wind over the back quarter) to a run (wind directly behind). The US Coast Guard Auxiliary says the centerboard is partly down on a reach and not needed dead downwind. The basics are on points of sail.

Best angles by wind strength

Dave Flynn of Quantum Sails (2023) gives these best true wind angles for a standard monohull, measured from the wind (180 degrees would be dead downwind):

The exception: boats that plane or surf in more than 14 knots should head back up into the low 140s, Flynn says.

Why you cannot steer by apparent wind angle

Instruments and telltales show apparent wind, the mix of true wind and the boat's own motion (SailTrain explains this well). Downwind that number swings with boat speed. Flynn says the best apparent wind angle on a displacement boat can range from about 90 to 130 degrees as boat speed rises from 4 to 10 knots, even though the best true wind angle barely changes. That makes apparent angle too volatile to steer by.

His alternative in 10 to 14 knots is to steer by feel: keep a constant pressure on the spinnaker sheet and on the helm, and avoid over-correcting. Pip Hare (Doyle Sails) makes a related point: if the boat keeps slowing down downwind, the usual cause is over-steering.

Waves

Flynn notes that bearing off slightly to stay in line with the waves gives a steadier ride and can add speed.

Mainsail and vang

• Ease the main fully on a run. Rupert Holmes (Sail) says sheeting in to keep the main off the shrouds only adds weather helm.
• Let the vang control twist. Once the sheet is eased, the boom wants to lift and the leech opens. Quantum says this is where the vang does its main work, and Holmes says on reaches and runs it becomes the main twist control on every boat.
• Do not overdo it. Gary Jobson (Cruising World) calls the vang most useful downwind but warns against over-trimming it in strong wind, and Quantum notes too much vang strains the mast, boom and gooseneck.

Finding puffs downwind

Downwind the new wind comes from behind you, so you have to look back for it. Moose McClintock (Sailing World) advises scanning astern for dark patches you can reach. He also notes you usually get headed as you enter a puff and lifted as you reach its center.

David Dellenbaugh (Sailing World) gives two downwind rules of thumb: gybe on the lifts, and sail toward the puffs. On reaches he suggests sailing lower in puffs and higher in lulls to keep the apparent wind angle roughly steady.

Guard against the accidental gybe

The US Coast Guard Auxiliary notes that light winds from astern are hard to detect, and that wind shifts and careless steering can cause an accidental gybe. North Sails recommends a preventer to guard against an accidental gybe when you are not planning to gybe. The risks and principles are on tacking and gybing.