Most sailors want to keep sailing in a breeze and are reluctant to turn the engine on

However, there is often some minimum acceptable speed required to ensure they reach their destination on time.

I have sailed with skippers who use 3.5-4 knots as a criteria on longish passages.

Light air performance is largely down to the design of a yacht and its rig, but will also depend on how it is sailed.

Knowing the design characteristics of your boat can help you understand why she sails the way she does, and how to keep her moving under canvas when the breeze starts to fade.

You might be surprised at how long you can sail without the engine, and there may be ideas of how you could improve your boat’s performance in a breeze.

Understanding wind speed and direction to help you sail in a breeze

Boat speed depends on the relative wind direction.

If you are just using white sails then the fastest point is with the wind on the beam.

The direction the wind appears to come from when you are on a moving boat is the apparent direction.

Someone on a boat anchored close to you will be feeling the true wind direction and speed.

On the boat you feel the apparent wind speed, which is greater if you are going upwind but less if you are going downwind, as illustrated in the diagram below.

Here I’ve used a true wind speed of 6 knots. If in this wind you can achieve 4 knots upwind, at 45º to the true wind, the apparent wind which you feel on the boat is 9.28 knots.

The apparent wind direction is 27º off the bow so it may seem that you are going very close to the wind.

I can’t be the only one who has checked the masthead wind indicator when on a beat and, thinking I’m close to the wind, put a tack in and got the boat going really close again only to check the compass and find the heading has changed by over 90º.

The reverse happens when you are sailing downwind.

The apparent wind speed drops to 3.27 knots, and the true wind may only be 17º off the stern but it appears to be 33º off.

So, when you gybe and sail at the same apparent wind angle on the other tack, the boat’s direction has only changed by 34º if you look at the compass, although it feels as if you have gybed through over 65º.

In such light conditions it is more comfortable to sail with the apparent wind on the quarter, 45º off the stern, because the sails are more effective than when going dead downwind and the apparent wind speed will be higher.

A quick way to find the best angle when sailing downwind is to put a target waypoint into your chartplotter and switch on the Velocity Made Good (VMG) display.

If you then alter the course and the sail trim until this is maximised you can find your best downwind sailing angle.

Sail force and Daniel Bernoulli

If you saw any of the coverage of the 2021 Americas Cup racing (still available on YouTube) you will have heard the crews continually talking about pressure.

It had nothing to do with anxiety levels, but the variation in wind pressure across the race course.

The brilliant 18th-century Swiss mathematician Daniel Bernoulli was the man who established a formula linking speed and pressure in fluids.

He showed that the pressure in a fluid is directly related to the velocity squared.

This is known as Bernoulli’s Principle and gives rise to the fluid force formula.

Fluid force =1/2 x coefficient x density x area x speed²

Air density, a sail force coefficient, the sail area and the apparent wind speed squared are used for estimating the sail force

Sail force =1/2 x sail coefficient x air density x sail area x apparent wind speed speed².

Using the velocity diagrams, we see the sail force will be proportional to 3.272, i.e. 10.6, going downwind and 9.282, i.e. 86.1, when going upwind.

So there is more than 8 times as much sail force when going upwind.

And although upwind much of it is trying to push the boat sideways and heel her, this still leaves about twice as much force as when going downwind to drive her forward.

Hence, other factors remaining the same, we go significantly faster upwind than downwind in light airs.

Polar diagrams

If there is a polar diagram for your boat you can find the best angle for downwind sailing in any wind strength.

The one shown only considers wind speeds of 6 and 16 knots.

Looking at the curve for 6 knots, we see that a horizontal line first touches the curve at about 150º true wind direction and the speed is 3.8 knots.

So, in this case, if you were to note the compass course when going dead downwind and then come onto a heading 30º from this, you should be going at the maximum speed for downwind progress and, every time you gybe, the compass heading should change by about 60º.

In principle one could do a similar thing upwind, but in reality it is easier to set the sails for the conditions and then to sail to the telltales, making the best course to windward without the boat slowing down.

As with downwind sailing, set a waypoint dead upwind and then look at your VMG while adjusting how high you are sailing to get the best compromise of boat speed and pointing.

At higher wind speeds the differences between real and apparent wind directions are reduced.

At 16 knots of true wind speed, the top of Force 4 and the point at which reefing is usually being considered, the difference between true and apparent wind angles is about half that at 6 knots.

As seen in the diagram below, a typical yacht can go a little closer upwind and needs to gybe through a smaller angle downwind in these winds.

I should sound a note of caution here about polar diagrams.

Most are based on generic velocity prediction programs (VPPs), which rely on assumptions and struggle to predict the effect of waves on performance.

However, there is one region that can be predicted more accurately than any other: running downwind in breeze.

It is also the easiest to estimate and, significantly, leads to a very useful ratio for assessing any yacht’s performance in a breeze.

Going dead downwind, the keel and rudder are not providing a lift force.

So the lift forces and associated induced drags don’t need to be estimated nor does that of the yawed hull.

The sails will only be providing a drag force and not the lift force they give when closer to the wind – so another factor drops out.

Light winds produce negligibly small waves – so their effects can be ignored.

This leaves a relatively simple calculation.

I recently designed and built a 6m trailer sailer and wanted to ensure it would sail at close to 3 knots at the top end of Force 2 (6 knots of wind).

So I did the calculation for running dead downwind, as I knew just under 3 knots downwind would equate to at least 4 knots upwind.

The question was how much sail area would I need to achieve this.

From a practical point of view, most sailors are unlikely to change their rig significantly, but it is possible to add more sail area with a main with more roach and a larger headsail, or off the wind, a larger spinnaker or a large furling sail like a Code Zero to improve performance in a breeze.

Wetted surface area

The excellent book Principles of Yacht Design by Larssen et al (Adlard Coles, £39.99) suggests that a Sail area/WSA ratio of less than two results in quite poor breeze performance and anything above 2.5 is quite good.

Of course you can always hoist a spinnaker (if you have one) to get more speed downwind, but upwind it is the ratio of the basic sail plan area to the wetted surface area that will be the key factor, all other things being equal.

Bilge keel boats tend to have low SA/WSA ratios: the Macwester27, for example, has a value of 1.97.

Long-keel offshore cruising boats also tend to have low values and for example the Dudley Dix Pratique 35 has a value of 2.01.

The same designer’s lifting keel Cape Cod 19 on the other hand has a value of 2.56, and some of his racing designs have values of over 3.5.

All these figures come from his very useful website.

Fin-keel cruising boats such as the Bavaria 31 and 34 have values of 1.97 and 1.99 respectively, at least in their early guises – the latest versions may be different.

The Dufour 32 Classic has a value of 2.44. I had one of these.

The first owner wanted to race her and increased the mast height by 0.75m and put bigger sails on her, raising her ratio to 2.8.

It gave her better performance in a breeze but did mean she had to be reefed earlier as the wind speed got up although reefing didn’t seem to affect her speed and just gave a more comfortable ride.

The more race-orientated Hanse 35 R has a ratio of 3.33 and a Swan 65 ketch 3.0.

The Beneteau First 18 has a ratio of 3.65 but you wouldn’t want to go offshore in one.

The graph below shows how the downwind boat speed for my 6m trailer sailer increases in 6 knots of true wind as the sail area/ wetted surface area ratio increases.

It also shows that doubling the sail area only increases the speed by about half a knot.

I went for a sail area of 20m2 which gave me a ratio of 2.52, and after five years of sailing I’m still happy with my decision.

The dashed line on the graph is for a geometrically similar 12m boat.

There are two reasons why this is different: first, the frictional resistance coefficient reduces as boat size increases, and secondly, the wind speed is greater the higher you go above the water surface.

This is one reason that the ‘fat-head’ mainsails found on many racing boats are effective.

Sail area displacement ratio

The sail area and displacement are normally available for any yacht so this ratio is readily calculated.

To see what it measures, it is useful to look at it in the context of Newton’s Law: force = mass x acceleration.

The force is determined by the sail area and the mass is the same as the displacement. So the ratio provides a measure of acceleration.

The ratio is usually quoted with displacement to the power 2/3, to make it independent of boat size.

In this form it is a less direct measure of acceleration but is still useful for comparing the ability of yachts to pick up speed after a tack or in a gust of wind.

The sail area displacement ratio for heavy deep sea cruising boats have values from 14 to 16.

Modern cruisers have values of about 16 to 19 and cruiser racers from around 19 to 21.\

Continues below…

Pure racing boats have values of anything up to 30 and above.

The actual value will depend on the rating rule they are designed to race under.

Large sail areas will be penalised and, if boats have to sail reefed most of the time, they will be disadvantaged.

Calculating your Wetted Surface Area

Unfortunately, the WSA is not generally quoted among the particulars for many yachts, and very few Owner’s Manuals include it.

For any yacht designed in the last 25 years or so the WSA should be available at the touch of a few computer keys to the designer or builder.

For older boats it is an hour’s calculation (for a naval architect) if the lines plan is available.

With a yacht ashore, a laser scanner can be used to get the WSA and all the other hydrostatic particulars very quickly given the software most naval architects now have.

The Offshore Racing Congress have scanned over 12,500 boats including most standard production boats.

If you can’t get the WSA any other way you can always measure it yourself when your boat is on the hard.

You will need an assistant, some string, some chalk, two marker pens, a tape measure, a notebook and a pocket calculator.

You start by marking off the waterline at 1m intervals, on both sides of the hull, from bow to stern.

Rub some chalk onto the string and, with your assistant on the other side of the boat, carefully bring it up vertically so that it touches a pair of corresponding points on each side of the boat.

Tighten the string a bit and then ping it so it leaves a chalk line on the boat, marking a girth.

Now, using the string, measure the length around the girth. Repeat for all the pairs of marks so the hull is divided up into a series of parallel strips, as illustrated in the drawing.

Now comes the difficult bit where some judgement is needed – you need to estimate the average width of each strip.

A reasonable estimate can usually be had by marking a girth at the midpoint between the waterline and the hull centreline and measuring the shortest distance from here to the next girth line.

The approximate area of each strip is then the average of the two girth lengths by the average width.

Totting up the areas for all the strips gives the wetted surface area for the canoe body of the hull.

In most cases the keel will get in the way of this exercise, unless it is very short.

So it will be necessary to draw a chalk line along the hull keel joint and measure from here to the waterline.

This will give half girths and a pair of strips each side of the keel.

The keel and rudder can be treated in a similar manner and results added up to give a reasonable estimate of the total wetted surface area.

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