Learn · 8 min read
When to reef: heel, drag and speed
More wind does not always mean more speed. Past a certain point, extra power turns into heel and drag instead, and reducing sail makes the boat both calmer and faster.
Reefing means reducing sail area: lowering the mainsail partway and tying down the spare cloth, or rolling part of the sail away on a furler. Every sailor learns the saying “the time to reef is the first time you think about it”. This guide explains the physics behind it, so you can see why an over-canvassed boat (one carrying too much sail) is slower, not just less comfortable.
Why more wind stops making more speed
Sail force grows with the square of the apparent wind speed. Double the wind and the sail pulls four times as hard. If the boat could use all of that, it would get much faster in a breeze. It cannot, for three reasons.
1. The hull-speed wall
A displacement boat (one that moves through the water rather than skimming on top of it) makes waves as it moves. The faster it goes, the longer its own bow wave becomes. Near a certain speed, that wave is as long as the boat, and the boat is effectively trying to climb its own bow wave.
That speed is called hull speed. A common rule of thumb is:
hull speed (knots) ≈ 1.34 × √(waterline length in feet)
For the simulator’s 40 ft monohull, with an 11.7 m waterline (about 38 ft), hull speed is about 8.3 knots. Near it, the energy spent making waves (wave-making resistance) climbs very steeply. Extra power adds almost no speed; it only makes bigger waves.
2. Heel wastes power
The sail’s sideways heeling force tips the boat over until the boat’s righting moment balances it. The righting moment is the boat’s resistance to heeling: keel weight and hull shape on a monohull. More heel costs speed in several ways:
- The sail tilts away from the wind and presents less area to it, so it loses power.
- The hull sits lopsided in the water and drags more (heel resistance).
- The boat wants to turn into the wind (weather helm), so the rudder has to be held at an angle, which adds drag.
- The keel tilts, so it resists sideways push less well, and the extra drag it makes doing that job (induced resistance) grows.
3. The deck goes under, and the rudder lets go
Two thresholds make things much worse:
- Deck-edge immersion (“burying the rail”): past about 30° of heel on a typical cruiser, the side deck goes under water and drags through it. In the simulator, this is where more wind starts making the boat slower.
- Round-up (also called a broach): past about 35°, the rudder loses its grip. The boat spins up toward the wind, the sails flap, and speed collapses until the boat straightens up again. Then it often happens again.
These thresholds are rule-of-thumb estimates for a 40 ft cruiser in flat water. Waves, gusts and hull shape move them.
What the numbers show
The simulator’s 40 ft monohull, sailing 50° off the true wind with the sail at its best trim. Full sail is 70 m²; 55 m² and 45 m² are like a first and second reef.
| True wind | Full sail (70 m²) | 55 m² | 45 m² |
|---|---|---|---|
| 12 kn | 6.8 kn, heel 17° | 6.4 kn, 13° | 6.1 kn, 11° |
| 16 kn | 7.5 kn, heel 23° | 7.2 kn, 19° | 6.9 kn, 16° |
| 20 kn | 7.6 kn, heel 28° | 7.7 kn, 25° | 7.5 kn, 22° |
| 24 kn | 7.1 kn, heel 32° | 7.6 kn, 29° | 7.8 kn, 26° |
| 28 kn | rounding up repeatedly | 7.1 kn, 33° | 7.6 kn, 30° |
Read the table from top to bottom:
- Up to about 16 knots, full sail is fastest. The boat is not yet near hull speed and heel is moderate.
- Around 20 knots, the first reef is as fast as full sail with 3° less heel. Reefing costs nothing.
- At 24 knots, full sail is clearly slower: the deck edge is in the water. The second reef is 0.7 knots faster with 6° less heel.
- At 28 knots, full sail cannot hold a course. The boat keeps rounding up, and its average speed collapses.
In the simulator, the chart What is slowing the boat splits hull resistance into skin friction, wave-making, heel and rudder, deck-edge immersion and keel side-force drag. Compare the first two links above: with full sail, the heel and deck bands grow large; reefed, they shrink, and the boat speeds up.
The heel gauge in the top-left corner shows the same story. Its amber zone starts where the deck goes under, and its red zone where round-ups begin.
Catamarans: no heel, different warning signs
A catamaran resists heeling with the distance between its two hulls, not with keel weight. It stays almost flat, so it does not warn you by leaning over. Instead, the heeling force tries to lift the windward hull out of the water. If it succeeds (flying a hull), a cruising catamaran is close to capsizing, and unlike a monohull it will not come back upright on its own.
The simulator shows this as hull lift: the heeling moment as a percentage of what would lift the windward hull. For the 44 ft catamaran in 25 knots of true wind, 60° off the wind:
| Sail area | Boat speed | Heel | Hull lift |
|---|---|---|---|
| 80 m² (full) | 11.5 kn | about 1° | 44% |
| 60 m² (reefed) | 10.3 kn | about 1° | 32% |
Notice the difference from the monohull: the catamaran is still faster with full sail here, because it has no heel penalty and its slim hulls make small waves. That is exactly why catamaran crews reef by wind speed (following the boat’s reefing table) rather than by feel. The boat stays comfortable right up to the moment it does not.
What reefing looks like in practice
Every boat has its own reefing system, so follow its manual. The usual sequence for a mainsail reef is:
- Take the load off the sail: ease the mainsheet, or turn the boat closer to the wind.
- Lower the main halyard (the rope that holds the sail up) to the reef mark.
- Secure the new front corner of the sail (the reef tack) at the boom.
- Pull in the reefing line that holds down the new back corner (the reef clew).
- Re-tension the halyard, trim the sheet and settle back on course.
On many cruisers with in-mast or roller furling, reefing simply means rolling part of the sail away. The physics are the same: less sail area means less heeling force, and in strong wind, more usable speed.
Practical rules of thumb
- Reef early. Shaking out a reef when the wind drops is easy; putting one in during a squall is hard.
- On a monohull, watch the heel. Many cruisers sail fastest below about 20–25° of heel. If the rail is going under or the helm feels heavy, reef.
- On a catamaran, follow the reefing table. It lists the wind speed for each reef. Do not wait for the boat to feel overpowered.
- Reef for the gusts, not the average. In gusty conditions, set the sail for the strong puffs.
- Easing the sheet is a short-term fix. It spills power in a gust, but if you are easing all the time, it is time to reef.
Key takeaways
- Sail force grows with wind speed², but boat speed is capped by hull speed.
- Past that point, extra power becomes heel and drag, not speed.
- On a monohull, burying the deck (about 30°) and round-ups (about 35°) make an over-canvassed boat slower than a reefed one.
- A catamaran stays flat, so watch wind speed and hull lift, and reef by the table.
Real reefing decisions also depend on waves, crew, daylight and your boat’s own manual. Use the simulator to understand the trends, and the manual for the numbers.
The simulator uses a simplified physics model: airflow around the sail, stall and luffing, hull resistance, heel and round-ups. It shows the right trends for learning; it does not predict exact numbers for a real boat. Numbers quoted on this page come from the simulator's 40 ft monohull unless stated otherwise.