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The three magic numbers for optimum sailing comfort and efficiency

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What is the issue?
Newcomers to sailing can easily get overwhelmed by the complexity of the boat, the points of sail, the complex sailing environment and all the strings that need to be pulled aboard to make it happen. Worse, in this environment, it is all too easy to over canvas a boat and get into a right mess and even break gear.

Why address this?
No one goes out with the express desire to sail inefficiently and have a stressful time. Even cruisers want to get to their destination as quickly and as efficiently as possible.

How to address this?
All boats are different and conditions, of course, vary widely at sea. But there are three simple numbers that newcomers can keep in mind to keep them sailing comfortably and efficiently.

  • • Your Boat's Hull Speed

  • • 15° of Heel

  • • 2 to 4° of Weather Helm

There is an old sailing expression that goes any damn fool can put up sail, it takes a sailor to know when to reef'. It is this saying that all these numbers have in common.

1/ Your Boat's Hull Speed: Find out your boat's hull speed and don't try to push beyond it.

Every displacement boat has a mathematically defined maximum 'hull speed'. This is largely dependent on the waterline length of the boat and generally, the larger the boat, the faster it can go. However, it is not the boat’s length overall (LOA) we are talking about here but rather the load waterline length (LWL). This refers to the horizontal length of a hull at the water’s surface when a boat is carrying a normal load and it is this length, in conjunction with the laws of natural physics, that governs the boat's hull speed.

Hull speed can be expressed as a simple mathematical formula 1.34 X the square root of the length of the waterline (HS = 1.34 x √LWL). For instance, if a cruising sailboat has a waterline length of 36 feet, she should be able to sail 1.34 x 6, or approximately eight knots.

The physics behind this is driven by wave cycles. When displacement hulls proceed they push aside significant amounts of water creating a wave train at the bow and another at the stern. At lower speeds, the wavelengths between the waves are shorter so there is room for multiple cycles of waves to pass down the length of the boat before meeting the stern wave. As the boat speed increases the corresponding waves rise and troughs also extend and the number of cycles between the bow and stern reduce. Then there comes a point where the speed pushes the wavelength to stretch out to be that of the boat’s waterline length and the bow and stern wave merge creating one long trough that runs the length of the hull. Hull speed has been attained at this point.

This is also the maximum speed of the sailing vessel. After this point, the laws of natural physics starts to build a valley for the boat to climb out of which forms a barrier to further acceleration. More speed necessitates a deepening and lengthening of the wave cycle which itself acts as a natural break point. Once hull speed has been attained and further acceleration will push the wave cycle out aft of the stern so that the backend of the boat starts to fall into the through and the reciprocal heightening of the bow wave creates a mountain for the boat to scale at the front.

The boat thereby becomes effectively trapped in a hole from which any further speed requires disproportionately larger increases in power to achieve diminishing increases in performance. That type of power that is not available to a displacement sailing craft and the oldest saying in the book comes to mind when you are in a hole, stop digging.

Small yacht achieving hull speed
Photo: CC0

Knowing the waterline length of your vessel is therefore essential. Once you know that number you know the maximum speed physics will allow you to go. After that, unless you have lifted the vessel out of the water and are planning, you have no place to go. Adding more sail, or more power is just trying to push the boat through the laws of physics which it will not do. People arriving from planning hull dinghies have a tendency to think more canvas is always better' when in displacement hull sailing boats ‘less is more’. Trying to push on past hull speed is just loading up the equipment to be broken, stressing the boat out and crew.

So knowing that top speed, hull speed is important. Find out the vessels (LWL) and input it into easy calculations calculator to get your maximum speed. Here are some convenient formula for max speed calculations:

  • • In Knots =1.34 x (the square root of the waterline length of boat) in feet

  • • In Knots = 2.43 x (the square root of the waterline length of boat) in metres

  • • In km/h = 4.5 x (the square root of the waterline length of boat) in metres

2/ 15° of Heel. Traditional sailing advice has always been to 'sail the boat flat' and this has never been as true as it is today. Boats built close to the turn of the century are designed to want at least 5° of heel in light airs and with about 15° of heel being optimal rising to no more than 20°. Beyond this you are not really going faster, you just feel like it.

Increased heeling creates an asymmetrical underbody profile that results in:

  • • Sails start to slip wind.

  • • It becoming difficult or impossible to tack.

  • • Slipping to lee (leeway) so that velocity made good (VMG) deteriorates.

  • • The boat fighting to 'round up' after 25° requiring the helm to be put right over to counter it.

  • • The excessive rudder angle causing drag.

The net effect is the boat will be going slower and will be increasingly difficult to control. Most boats will round up when approaching 35°.

So although it may not feel like it, 'flat is fast'. Bottom line is if you are heeling past 15°, you are over canvassed so you need to reef or remove the jib or main. In the event that it is because of a gust, you can try to fight your way through it for few minutes at 20-25° degrees or turn slightly out of the wind to a beam reach before the gust subsides.

A low-cost investment that can help with this is a simple gravity-operated inclinometer, pictured right, that can be simply stuck to a bulkhead. Some digital instrument sets also include a clinometer function.

3/ 2 to 4° of Weather Helm. Keeping weather helm at an angle of 2 to 4° and you will balance the vessel and have her sailing to perfection. It will also be your guide to getting the sail balance correct.

Weather helm is the natural tendency of sailing vessels to turn towards the source of wind. It requires the tiller to be pulled to the windward side of the boat, hence 'to weather', in order to counteract the effect. It is a vital safety factor that a yacht turns to windward, head up into the wind, and simply luff if the helm is released and spill out its wind. Lee helm, the opposite risks the yacht creating a dangerous accidental jibe or even knockdown.

3 to 4° of weather helm improves performance providing lift like the flaps on the aft end of an aeroplane's wing. It makes the boat sail faster through the water and point closer to the wind. Too much weather helm, as mentioned above, adds resistance to slow the yacht, and constant adjustment also detracts from the smoothness of the ride and speed.

Achieving this optimum does not come easy, however, but if you can master this you will be sailing like a professional. Knowing the optimum 3 to 4° helps and sailing flat, but after that, it is all about optimising the sail plan to control the centre of effort upon the vessel. The videos below will help understand the theory and Skipper Tips article 'Cure Weather Helm with These Secrets!' will help with sail adjustment.

After adhering to this simple framework of three numbers for some time, you will get a feel for the boat and it will by then be telling you when it is happy or when it is not moving as well as it could/should. But you may be surprised to find very little variance from these three key numbers from boat to boat when it comes to smooth and optimum sailing.

With thanks to:
Michael Harpur & Burke Corbett

Hull Speed

Hull Speed tank run

Weather Helm Demystified

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