Slack means loose, not under tension, and sailors use it in several closely related ways. 1. A slack line is one with no load on it, and to take up the slack is to pull in the loose part until the line just goes tight. 2. Slack water is the short period around the turn of the tide when the stream stops running before it reverses. 3. In rig tuning, the leeward shrouds go slack when the boat heels, and how slack they go is a measure of how the rig is set up. 4. A sail or part of a sail is slack when it is not tensioned: a slack luff sags and scallops, a slack leech flutters. A beginner needs the first meaning to handle lines safely; the experienced sailor uses all four every day on the water.
Slack in Lines
Most rope-handling instructions turn on the difference between slack and load. Take up the slack on a halyard before you put it on the winch, so you are not grinding metres of loose line. Slack away, or slack off, is the order to let a line out under control; ease means the same thing done slowly and deliberately. A line that goes slack unexpectedly has usually parted, come off its cleat or lost whatever it was holding, and a sheet that goes slack while the sail is drawing means the sail has collapsed or is about to gybe.
Slack is also where accidents start: a slack sheet flogs, a slack mooring line finds the propeller, slack turns on a winch drum become a riding turn. Keep every line loaded, coiled or flaked.
Slack in the Rig
When a boat heels, the windward shrouds take up the load and the leeward ones lose tension. A little slack to leeward is normal; the wire stretches and the mast compresses. Rig tuners use the amount of leeward slack as a gauge. In the wind strength you expect to sail in most, the leeward cap shroud should be just firm or barely slack. If it is swinging like a skipping rope the rig is too loose, the mast is pumping in the seaway, and the fittings are being shock-loaded. If it is still bar-taut at full heel, the rig is probably too tight.
A slack forestay, or headstay sag, is the sailmaker’s concern. As the forestay bows to leeward under load it adds depth to the genoa and moves the draft aft, exactly what you do not want when the breeze is up. Backstay tension on a masthead rig, or cap shroud tension on a fractional rig with swept spreaders, takes the slack out of the forestay and flattens the headsail. UK Sailmakers cuts a hollow into the genoa luff to allow for the sag the rig will actually have, so keeping the forestay to the expected tension matters for the sail to set as designed.
Slack Water
Tidal streams do not simply stop at high and low water. In many places the stream keeps running for an hour or more after the height has turned, and slack water, when the flow is negligible, may be well separated from high or low water. Tidal stream atlases and almanacs give the times of slack for each location. Slack water is the moment to enter a harbour with a difficult entrance, to cross a race or overfalls, or to anchor and let the boat settle. Because slack is short, often twenty minutes or less in a strong stream, planning to arrive during it is a real navigational skill.
Slack in Sails
A slack luff on a headsail shows as horizontal wrinkles or scallops between the hanks or along the furler foil; tension the halyard until they just disappear, then a touch more in a breeze to pull the draft forward. A slack leech flutters, or motors, and a leech line taken up just enough to stop the flutter cures it without hooking the leech. A slack outhaul gives a deep foot, right for reaching in light air and wrong for beating in a blow. In each case slack is a setting, not a fault.
Frequently Asked Questions about Slack
What does take up the slack mean?
To pull in the loose part of a line until it is just taut, without yet putting real load on it. It is the step before trimming or winching.
Is slack water the same as high tide?
No. Slack water is when the tidal stream stops running, which can be an hour or more before or after high or low water depending on location. Check the tidal stream atlas rather than the tide table.
How slack should my leeward shrouds be?
Just firm to barely slack in the wind you normally sail in. If the leeward cap shroud swings loose, the rig needs more tension; if it is still rigid at full heel, it is probably too tight.
Slack is a word about tension, and sailing is largely the management of tension: in the lines you handle, the rig you tune, the sails you trim and the tide you plan around. Know when slack is a hazard, when it is a setting and when it is a window, and you will handle the boat like someone who has been doing it for years. See also ease, headstay sag, tidal stream and leech line.