On a calm day, everything on a yacht happens slowly and gently. But if you head out in a gusty day it’s easy to find yourself on a boat that’s leaning (or heeling) over at worrying angles. New sailors find this quite disturbing – so what’s the best way to avoid this?

If you’ve spent time sailing small dinghies, you’ll be used to these boats capsizing once the angle of heel gets beyond what you can counterbalance with your body weight. If you don’t depower the mainsail by letting out the mainsheet, the sideways force on the sail from the wind simply rotates the boat until the sail is in the water. In contrast, on a larger keel boat we have a big heavy keel underneath the boat to counterbalance the wind force. The more the boat heels, the bigger the righting effect of the keel becomes. Consequently, keel boats can heel over quite far and not be in danger of capsizing. It’s even possible to do a stability test on your boat to see how far it will tilt safely.

Boats naturally heel whilst sailing upwind, since the majority of the lift force generated by the sheeted-in mainsail is pointing sideways rather than forward – it’s the job of the keels to act like fins to cancel out as much of this sideways component as possible. Crews quickly used to a constant heel angle. But what people find worrying is when a gust causes the boat to heel excessively. A sudden change to a heel angle of 30 degrees can easily be perceived by new crew as “the boat is about to capsize” even if that’s well within the stable range for the boat. (You can use your phone sensors to measure the heel angle for free, or spend money on a clinometer).

When the wind gusts – ie. wind is still coming from the same direction only stronger – it’s natural to assume that the consequent increase in heel is just due to the wind pushing on the sail more. This isn’t the case. The increase in wind speed changes the direction of the apparent wind that the boat is experiencing. The apparent wind is the combination of the true wind (that you’d feel standing still) and the ‘wind’ you create because you are moving forward. Let’s imagine we were on a beam reach, pointing at right angles to the wind. If the true wind speed was zero, and you were motoring at 5 knots then you’d feel a headwind of 5 knots. If the true wind speed was 5 knots, and you were moving on your beam reach at 5 knots, you’d feel a combination of those resulting in a 7 knot wind at a 45 degree angle. If the true wind speed doubled to 10 knots and you continued moving at 5 knots, the apparent wind would be 11 knots but now at a 60 degree angle.

The important lesson here is: as the true wind SPEED increases, the apparent wind ANGLE shifts backwards. If you were sailing on a close reach, then an increase in true wind speed could easily have you on a beam reach. Your sheeted-in mainsail that was perfectly set up for the close reach is now sitting broadside to the wind and the sideways force on it increases resulting in heel. The answer is to let the mainsheet out, since you are now effectively on a beam reach so long as the true wind speed stays high.

If the wind is consistently strong then the heeling force on a full mainsail will simply be too high. You can always let the mainsheet out to depower it, but it’s better to reduce sail area by reefing the sail. With less sail area, the sideways force on the boat is decreased. This is something you need to practise so you are confident that you can do it quickly when necessary, particularly if you are singlehanding. You should mark your main halyard at the position where the cringles reach the reefing horns, and it’s useful to have an elastic loop around the mast to hold the sail in place whilst you retension the halyard.

The other cause of worrying heel is during tacking and gybing. In an idealised tack, you go from a close-reach on one tack with main sheeted in, then turn the bow through the wind until you’re on the other tack – leaving the main sheeted in the whole time. In reality, it’s not always this easy. Some boats are harder to tack than others, and waves can push on the bow making it hard to get the bow turned. If you don’t tack quickly, the boat can start to lose forward speed – either resulting in being stuck “in irons” or having the boat “overshoot” the new tack. Overshooting is where your forward speed has dropped to the point that your rudder is no longer effective, and the wind pushes the boat far beyond the desired new heading. Rather than being on a close-reach, you are now on a beam-reach or beyond. Your sheeted in mainsail is now side-on to the wind and consequently the wind pushes the boat over. The answer is the same as earlier: you need to let the mainsheet out quickly to respond to the fact you’re now side-on to the wind. You should aspire to a textbook tack, but anticipate the need to let out the mainsheet.

When you’re gybing, a similar thing can happen. To gybe, you face downwind and pull in the mainsheet to centralise the boom. Once the boom has shifted over to the other side (a brief touch of opposite tiller can balance out the weight shift) you should let the mainsheet out quickly so you are sailing downwind again. If you wait too long with the boom centralised, two things can happen. Firstly, if you continue turning with the tiller you will end up side-on to the wind with a sheeted-in main, resulting in heel – the answer is to let out the mainsheet quickly. Secondly, even if you don’t overturn with the tiller, as the boat rolls on waves the sheeted-in main is causing a sideways weather-cocking force at the back of the boat that will tend to rotate the boat around toward the wind. This ends up with you being side-on to the wind with a sheeted-in main, resulting in heel. The answer? Let the mainsheet out!

In all of these scenarios (gusts or overshooting your tack or your gybe) the thing that will cause your boat to heel worryingly is having the mainsail sheeted in tight whilst you’ve ended up side-on to the wind. The solution is always the same – let the main out.

This is almost always a safe thing to do. If you are on a close reach or beam reach, letting the mainsheet out depowers the sail, which will sit downwind and flap. It’s only when you are heading downwind and the sheeted-out mainsail is pressed against the stays that it remains powered up. But if you’ve just gybed, then your choice is between accidentally rounding up (with main sheeted in) versus barrelling downwind (with mainsheet out). This does make it important to reef the mainsail appropriately for the wind speed, particularly if sailing downwind.

In conclusion, boats do not suddenly heel excessively for no reason. It’s a consequence of either gusts or less-than-perfect tacks/gybes resulting in the boat being side-on to the wind with a sheeted-in mainsail. The answer: let out the mainsheet.