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Issue No 64, 14 October 2024
By: Anthony O. Ives
Aerodynamics is as important in sailing as it is aviation more so than in any other application of aerodynamics. The sail functions as wing and parachute in a way with the exception that the aerodynamic forces are used to move the sailboat forward through the water instead as in as aircraft to fly. Probably not known to many people not familiar with sailing a sailboat can use the wind to move in any direction regardless of wind direction because of the sail's ability to function like an aircraft wing.
I am not anywhere near to describing myself as an experienced sailor. In reality I have very little practical experience of sailing. Most of my sailing experience was a short stint when I was PhD student where I got talked into running the university sailing club mainly because none of the real sailors wanted that painful task. I had wanted break from aviation to try something different but I really have not sailed since that adventure however, it was through learning sailing theory that I finally understood the mechanism of lift (see Ref [1]) I never really ever grasped it in those aerodynamics classes maybe all the long derivations of equations distracted me.
Building a small sailboat or sailing dingy is also something I would like to do at some point in future as I would like to use and improve my woodworking skills. Boatbuilding is great for developing and enhancing your woodworking skills as well as applying your mathematical skills in building something. I also recently inherited a RC (Radio Controlled) Sailing Yacht so I was thinking if I have the time I could revise my limited sailing skills and possibly build more RC sailboats working my way up to a full sized sailing dingy!
In the future sailboat content I will look at using mathematics to size sails and boat hulls as well as using CAD (see [2]) and possibly CFD (see [3]). Actually a lot of the theory and skills used in sailboat design and boatbuilding are very similar to that used in building aircraft, the fact is the aircraft industry and aviation developed lot of its knowledge, skills and terms from the shipbuilding and maritime industries. Common example are things such as port and starboard used to the reference the left and right side of an aircraft as well as a boat. Also the rules of the air navigation are very similar to the rules of sea navigation. The one advantage of boating especially pleasure related, is that it is no way as nearly regulated as aviation. For the rest of the article I will just talk about basic sailing theory as well as some basic sail aerodynamics. Most of this content was originally on the university sailing club website which I spoke about earlier. This content really focuses on full size sailing dingys which in reality I will, at least initially focus on RC sailing yachts possibly later expanding to include full size sailing dingys. However, in most cases the terms are very similar for sailing dingys and yachts with RC sailing yachts using the same terms a full size sailing yachts.
To start I just discuss in a bit more detail how a sail works just elaborating on what I discussed earlier in the article. Sails convert the energy in the wind into forward movement of the boat. They do this in the same way as wings of an airplane provide lift. The wind causes the sail to take an airfoil shape. The wind passes around the sail and is slower on the windward than on the leeward side of the sail where the wind travels faster (see Ref [1]) . This means the pressure on the leeward side is lower than the pressure on the windward side. This pressure difference results in a force acting perpendicular to the boom in the downwind direction. This force can be resolved as a force on the boat pushing it forward and a force on the boat pushing it sideways. The sideways force results in drift but is (for the most part) avoided by using a centerboard/daggerboard.
A sail has a particular angle to the wind at which it is most efficient. This is usually 45 degrees, which means that boats can travel at 45 degrees to the wind.
When moving in a downwind direction the sail no longer acts like a wing, but more like a parachute, catching the wind and moving the boat along with the wind. It is downwind that boats travel fastest.
Now I will give a brief description of the parts of a typical sailing dinghy it is important to understand the terms used for each part because they will be referred to when discussing the 'Points of Sail'. Most boats have parts unique to their make and class but only the more standard parts are described here.
Hull: The boat itself. Most fibreglass dinghies have double hulls
Cockpit: Where the crew sits. In dinghies everything (sheets, yards, etc) is accessible from the cockpit.
Transom: The (usually) flat vertical part at the stern of the boat. The rudder is attached to the tramsom on most dinghies.
Bow: The front end of the boat.
Stern: The back end of the boat.
Starboard: Looking forward, the right side of the boat.
Port: Looking forward, the left side of the boat.
Mast: The pole which boom and mainsail are attached to.
Forestay: For a mast standing alone to have adequate strength, it would need to be heavy. Much better to have a light mast with supports. Some bigger dinghies have wire ropes (usually) from the top of the mast to the bow. This is a forestay.
Shrouds: Same as a forestay, except to the port and starboard, and slightly aft of the mast
Boom: The horizontal spar to which the clew corner (and on some boats, the whole foot) of the mainsail is attached. The boom is used to maintain the sail shape. The boom is attached to the mast close to the base of the mast and pivots about the mast depending on point of sail.
Kicker: A tensile member attached at one end to the base of the mast and at the other end to the boom, usually about a quarter way along the boom's length. The kicker keeps the boom from rising and so maintains sail shape.
Parts of a Sail: Most sails on sport dinghies are triangular. The leading edge of the sail is called the Luff. The base edge (along the boom) is the Foot. The trailing edge is the Leech. The corner at the top (between the luff and the leech) is called the Head Corner. The lower leading corner (between the luff and the foot) is the Tack Corner. The lower trailing corner (between the leech and the foot) is the Clew Corner.
Jib: A forward sail on some dinghies, the jib is attached to the forestay.
Jib Halyard: The rope used to raise the jib; attached to the head corner and runs down the mast.
Jib Downhaul: Used to tie down the jib; attached to the tack corner and the bow.
Jibsheets: A rope from the clew corner of the jib. There are two jibsheets; one to port and one to starboard. The sheets maintain jib sail shape. The port jibsheet is used on a starboard tack with the starboard jibsheet slack. Vice versa on a port tack.
Mainsail: The main sail of a boat. Usually the biggest sail on a boat (spinnakers and genoas are bigger).
Halyard: The rope used to raise the mainsail; attached to the head corner and runs down the mast.
Downhaul: Used to tie down the mainsail; attached to the tack corner and the base of the mast.
Outhaul: The outhaul holds the clew corner of the mainsail at a set distance along the boom.
Clew Strap: Holds the clew corner down to the boom.
Mainsheet: A rope from the end of the boom. The mainsheet is used to control the sail position - it is taken in on a beat and let out on a run.
Traveller (Horse): The mainsheet runs from the end of the boom around a block and into the helmsman's hands. This block is attached to and moves along a rope running abeam. This rope is called a traveller.
Rudder: The rudder is used to steer the boat. Attached to the transom on dinghies. On most dinghies the rudder can be raised to allow easier launching.
Tiller: Used to control the rudder.
Tiller Extension: An extension to the tiller. The tiller extension usually pivots about the inboard end of the tiller on horizontal and vertical axes. It allows the helmsman to steer while hiking.
Centreboard: The "fin" which protrudes from the bottom of the hull. The centreboard prevents sideways drift. Centreboards are pivotted about a pin and can be raised or lowered as required. Used in larger dinghies and some small yachts.
Daggerboard: The daggerboard is a centreboard which, instead of pivotting about a pin, is raised or lowered vertically. Used mainly in smaller dinghies.
Toe straps: There are straps attached at both ends to the deck of the cockpit. Hook your feet under these when hiking out.
Self bailer: In some dinghies handy device in the deck of the cockpit which can be used to release water in the cockpit. Self bailers only work when the boat is moving at considerable speed.
Bung: Most boats are double-hulled. In most dinghies you'll find a bung (usually in the transom). A bung is just a hole in the outer hull with a sealing cap, which can be opened to empty out any water which somehow managed to get into the gap between the hulls.
Finally I will introduce the 'Points of Sail'. 'Points of Sail' basically means the direction of a boat with respect to the direction of the wind. Sailing vessels can not be taken directly into the wind. If you go too close to the wind the boat will stop, as the sails can not get energy from the wind. The sailing ships of the past (the tallships or square riggers) could get only 65 degrees to the wind. Modern vessels can get a lot closer, usually about 40-45 degrees. You can of course go upwind by taking a zig zag course.
In Irons: When a boat is in irons it is pointing into the wind, or is too close to the wind to make headway. In other words it is stopped dead in the water. In smaller dinghies (without a jib) you have to push, push and pull, pull. That is, push the boom and the tiller away from you, which reverses and turns you, and then pull in the mainsheet and pull the tiller towards you. With larger dinghies and yachts (i.e. boats with jibs) you just back up the jib. That is, sheet in the jib on the windward side (so long as you are not directly into the wind). The jib catches the wind and pushes the bow away from it. When you are far enough away, sheet the jib in on the leeward side.
Beat: A beat is as close to the wind as you can get. With most modern sailing boats it is at about 45 degrees to the wind. On a beat, mainsail should be close hauled and centerboard/daggerboard should be fully down. You can move upwind by beating on one tack and tacking to the other. This is known as tacking.
Close Reach: Anything between a reach and a beat. Mainsail and centreboard are at intermediate positions.
Reach: Moving perpendicularly across the wind. Mainsail should be at 45 degrees to the keel and centreboard/daggerboard should be half raised.
Broad Reach: Moving at a slightly downwind angle across the wind. The mainsail should be let out a more than 45 degrees.
Training Run: Moving downwind, but not directly downwind. The sail no longer acts like a wing but more like a parachute, catching wind. With some larger dinghies and most yachts a spinnaker is provided.
Run: Moving directly downwind. Mainsail should be fully out to one side, with the jib out on the other (the jibsheet should be let out so that the jib takes a more balloony shape). Spinnaker should also be out. Centreboard/daggerboard should be fully up. Boats move fastest when running, but if you are on a run you would feel the wind has dropped; it has not, you are just travelling with it.
I hope this article has given you a good introduction to sailing theory and explained why aerodynamics is important in sailing, more detailed information can be found in the books [4,5] given in the reference section at end of the article.
Future content will mainly focus on RC sailing yachts at least initially and boatbuilding. Building boats is good way of demonstrating how mathematics can be used to practically build something. Sailing in general is a great way to better understand aerodynamics because you are applying it directly in a very practical way when you are sailing.
Please leave a comment on my facebook page or via email and let me know if you found this blog article useful and if you would like to see more on this topic. Most of my blog articles are on:
Mathematics
Helicopters
VTOL UAVs (RC Helicopters)
Sailboat Design and Boatbuilding
If there is one or more of these topics that you are specifically interested in please also let me know in your comments this will help me to write blog articles that are more helpful.
References:
[1] http://www.eiteog.com/EiteogBLOG/No23EiteogBlogLift.html
[2] http://www.eiteog.com/EiteogBLOG/No60EiteogBlogFreeCAD.html
[3] http://www.eiteog.com/EiteogBLOG/No63EiteogBlogCFD.html
[4] The New Complete Sailing Manual, Steve Sleight, 2005, Dorling Kindersley
[5] Maximum Sail Power, Brian Hancock, 2003, Nomad Press
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