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Ideal Sail Shape – Get it right and leave the other boats behind!
Get the sail shape right and your sailboat will perform well. Get sail shapes wrong and performance will be poor. Good sail shapes and the controls required to achieve it are not ‘just for racers’. Sails are the means of achieving the best out of your boat and, therefore, the most pride and pleasure from the considerable investment that we spend on sailing.
It’s often said that increasing your speed from five to five-and-a-half knots will save you nearly an hour on a fifty mile passage, thereby offering a better choice of berth when you arrive, as well as uncrowded showers and the certainty of a table in your favourite restaurant. This is all true, but for me the most important reason for improving your sail trim is to provide comfort rather than boat speed. With the sails correctly trimmed most sailboats will more or less steer themselves, the ‘balance’ will be improved and the boat will ride more easily through the water. Overall, life on board becomes easier for both crew and gear. Having said that, I see nothing wrong with an extra hour in the Yacht Club nor an hour less spent dodging freighters in mid-passage. Fortunately, with the right knowledge and a few simple adjustments to the sails we can enjoy all of these benefits.
How sails work
Before talking about the mechanics of sail shapes it’s important that we review the terminology used to name the various parts of a sail, and also the way we define and describe sail shape.We spend much of our sailing time looking upwards at sails, a viewpoint that makes it easy to see their vertical shape. Yet, almost all of our descriptions refer to their shape in the horizontal plane – that’s to say, as the wind flows across the sail. Many sails have horizontal ‘camber stripes’ built in so that you can see their shape more clearly. But sometimes you will have to lay on your back on the coach-roof or foredeck and look aloft to fully appreciate the finer details.
Flow theory
Sails harness the wind’s energy by generating ‘lift’ in much the same way an aeroplane’s wings will lift it off the ground. There are similarities in their shapes and both sails and wings are known as ‘aerofoils’. So far as sails are concerned, the lift is generated by the flow of air across them, horizontally from luff to leach. They are not flat sheets of fabric but are cut and fabricated so they form a shallow dish shape. To trim a sail we point the leading edge – the luff – directly into the wind and then pull the trailing corner – the clew – slightly across the wind. This sets up a steady airflow around both the concave (inside) surface and the convex (outside) surface of the sail.
The Swiss physicist Daniel Bernoulli (1700–1782) discovered that a fluid’s pressure reduces when it accelerates and increases when it slows down. Air is a fluid and it follows this principle when it flows over a sail. The air flowing around the inside slows and its pressure builds. Whereas air flowing around the outside of the sail flows more freely and speeds up, reducing the pressure. Since there is now a pressure difference on each side of the sail, it is drawn towards the faster flow on the outside, creating the lift that powers our boats.
You can test Bernoulli’s Principle by dangling the back of a spoon against the flow of water from a tap. The spoon will be pulled into the flow.One might conclude that the deeper the dish shape – meaning the more the sail is curved – the bigger will be the pressure difference and the greater the power produced. One might also think this illogical, since the soon is being pulled in towards a fluid of much greater density.
But power isn’t everything. Look at the top diagram to the right. It shows a boat with a single mainsail having a nice even curved shape. The coloured arrows represent the forces produced by the pressure difference between the inside and outside of the sail. Because the sail has an even curve the forces are evenly spread. The green arrows are producing some drive and would like to pull the boat forward, but they are opposed by the red arrows wanting to ‘drag’ it backwards. Their combined force will certainly heel the boat over but, otherwise, it isn’t going anywhere. However the boats ‘entry angle’ to the wind is pretty good.
In the lower of the two diagrams the same boat has a sail cut with more curve forward near the mast, and having a flatter trailing section. Now you can see that there’s more drive in the leading part of the sail, with the relatively flat trailing section experiencing much less pressure difference and, therefore, producing less drag. This boat stands a good chance of moving forward but the large entry angle means that it won’t sail very close to the wind.So, sails must first be cut by the sailmaker so they can take up the right shape, then trimmed by the crew so they work optimally.This will involve trying to shape the sails to match the entry angle to the wind direction while producing as much forward drive as possible.
Some fundamentals
Figure (C) Shows a sail that’s over-sheeted. The entry angle is too large; the flow of air around the outside of the sail has become detached and turbulent. Air from the inside of the sail is being drawn around the leach to fill the void and the sail is stalled. It might look OK but, without an accelerated, attached flow around the outside surface, the sail isn’t providing any drive. The sheet should be eased so that the outside flow can reattach.
Putting theory into practice
The last figure in this series (D) shows a ‘full’ sail in a very light breeze. The airflow around the outside of the sail is too weak to remain attached to the surface. It has broken away, and become disturbed. The sail should be flattened and the sheet eased so that the flow can re-attach to the outside surface. “Don’t ask a small wind to do too much,” is a very wise saying. So, if we want to maximise the effectiveness of our sails, we must adjust our sheets to get the entry and the exit angles just right.
Mostly, we will be sailing with two sails – a jib or genoa (foresail) and a mainsail. Those two sails will combine, effectively to produce a single airfoil so we need to consider the entry angle at the leading edge of the combined surface (the jib luff) and the exit angle at the trailing edge (the mainsail leach).
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As a general rule, ease your sheet until the sail just starts to luff (shake on the leading edge) then pull it in slowly, just enough to stop the luffing. Start from the front of the boat in clear air and work your way back. This means you should trim the jib first before trimming the mainsail.
This article is adapted from content in Rob Gibson's RYA Sail Trim for Cruisers
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Seamanship & Boatcraft Ideal Sail Shape - sail trim secrets
