Traditional wisdom has it that the beam reach is the fastest point of sail. But is this correct, and if so, why?
On a beam reach the hull travels at 90 degrees (perpendicular) to the wind angle. Sails are eased half way.
But is it the fastest point of sail? Well, that depends…
The best way to visualise boat speed at different sailing angles is to use a polar plot. This is one for a Hobie 20:
The bold black line shows the speed the boat achieves at different sailing angles. The furthest away from the centre of the circle this line gets shows us the fastest point of sail in that wind strength (in this case 10mph)
Sailing close-hauled the Hobie 20 achieves a speed of 5 mph. On a beam reach she reaches 14 mph and on a dead run, 4 mph.
This appears to support the view that a beam reach is the fastest point of sail. But what if we look at another Polar Plot…
This Polar Plot is for a laser dinghy. Unlike the last Polar Plot it has several bold lines (each for different wind strengths). The green line shows the speeds when the wind is 8.5 knots (Force 3), and the orange line shows the speeds in a Force 7 (30 knots).
As you can see, the fastest point of sail depends on the class of boat and the wind strength.
In a 19 knot breeze the laser dinghy travels at 10 knots on a beam reach (90 degrees to the wind). But it can reach 12 knots on a broad reach. In a Force 3 wind this disparity is reduced and it travels just as fast on a beam reach as a broad reach.
The polar plot above is for an RS 400. As you can see there is a clear speed increase on a broad reach in all conditions. The stronger the wind the lower the fastest angle moves.
This all goes to show that generally a beam or broad reach is the fastest point of sailing. Though this depends on your class and the wind strength. The sea state also has an effect.
Why is a reach usually the fastest point of sail?
Lift and drag determine what point of sail is fastest. The more lift you have the faster you go. But the higher the drag, the slower you go.
Lift is created by air flowing over both sides of a curved foil (the sail in our case). Therefore optimal lift is achieved by sailing close to the wind (close-hauled). Think of an aeroplane. Aeroplanes like to take off into the wind as that creates maximum lift. Boats are just like aeroplanes turned on their side (with the sail like one wing and the board as the other wing).
However, there is more drag the closer you sail to the wind. Also, on a close-hauled course the component of lift in the direction of travel is less than on a reach. Imagine a juicy lemon pip. If you squeeze the tapered point of the pip between finger and thumb it will shoot out. But the pip won’t go anywhere if you squeeze the flat sides of the pip.
On a close-hauled course there is less lift force propelling you in your direction of travel. This, combined with increased drag, is why we slow down as we approach a close-hauled course.
Running is also never the fastest point of sail (as you see on the polar plots above). As you bear away onto a run the sail stops working like an aeroplane wing (creating lift) and starts to work like a plastic bag being pushed in the direction of the wind. The later being less efficient.
Somewhere in between a run and close-hauled you’ll find the fastest point of sail.
All sailors know that they go fastest on the plane. If there’s less hull dragging in the water you’ll sail faster. So the fastest point of sailing should be a point of sailing where you are planning. In marginal planing conditions it will be the first point of sail where you are able to get on the plane.
Optimum planing happens on beam and broad reaches as the centreboard must be lifted to achieve planing.
If you stick your hand out of the car on a windless day you’ll feel a breeze. This is called a head wind. It’s the wind created by an object traveling through air. The faster you go the more of a head wind there will be.
With high-speed craft the head wind can be stronger than the true wind (if the boat is travelling faster than the wind). The AC72s used at the last America’s Cup are a good example of this. They travel so fast that they are effectively close-hauled even on a broad reach.
To sum up
A beam reach is not necessarily the fastest point of sail. Generally, you’ll go fastest on a beam or broad reach. The exact angle of maximum speed will depend on the class you sail, the wind speed and the sea state.
Why not try creating a polar plot for your class? All you need is a GPS, a pen and some paper (preferably waterproof).
As always if you have any questions pop them below and I’ll endeavour to get back to you.
If you want to read more on how sails work then the following will be of interest: How do sails work against the wind?