When I started this project I planned to design and build a trailer-sailer and go sailing it along the seashores of the Whitsunday Islands in Queensland on the East Coast of Australia. But I got side-tracked into investigating the physics of sailing to windward. I found much of this theory to be questionable and not supported by practice. The main outcome of this investigation is that the windward performance of my catamaran WRC6800 was greatly improved by locating its centreboard well forward of its sail centroid and installing it at an angle of 4-5 degrees to its fore-and-aft hull centreline to create a positive angle of attack. A description below covers my experimental results supporting this conclusion.
I class my beamchange systems as the best outcome of this project. It simply increases the beam of my catamarans from 2500mm to 3800mm and provides a tremendous improvement in sailing, comfort and safety. Details of it are included below and photos of my l beach-launching trailer.
The Quest for Speed16.1 Forces on a sailboat when sailing to windward
If a perfect rig is mounted on a perfect hull, but one without a centreboard or keel, and it is sailed upright, it will not sail to windward. The 'wing in the air' (the rig) produces a force 'OS' (Fig 16.1), which is roughly at right angles to the sails.
The bottom of any planing hull is necessarily reasonably flat, and so in unable to produce much cross-boat force (unless it is heeled to put the chine deep in the water). As a result, in Fig 16.1, 'upright no keel nor centreboard' situation, the boat will accelerate in the direction 'OS' until the hull's drag increases to the point where it becomes equal and opposite to the sail force 'OS'. This situation, with the hull's centreline headed in the direction 'heading', but the boat tracking across the water in the very different direction 'OS', will neither win races, nor be much fun.
The best way to think about any sailing boat is to regard it as an assembly of three basic components:
The keel or centreboard is a 'wing' which 'flies' through the water. Like the rig, it produces a force roughly at right angles to its surface 'OC', Fig 16.2 (b).
When a sailboat sails to windward or on a close reach, the forces acting on it when viewed from ahead, i.e. those forces which act across the boat, take the arrangement shown in Fig 16.2 (a). Appreciation of these forces is helpful in understanding which boats can and which cannot sail fast. For Fig 16.2 we will use figures typical of the many popular classes of non-trapeze conventional dinghies about 14 to 15 feet long which are of approximately similar performance and are well understood, such as the Gp14, Enterprise, Albacore, etc. These boats weigh about 350 pounds (300 for the hull and 50 for the rig and foils) and are designed to be crewed by two adults who will typically weigh about 310 pounds themselves and 330 pounds dressed for sailing and wet, so the total weight at which the boats are designed to sail is about 680 to 700 pounds. We will assume that the water is flat, that the True Wind is 12 knots, that the boat's heading when sailing to windward is 45 degrees from the direction of the True Wind, and that the boat speed through the water is 4.5 knots. The triangle of velocities in Fig 16.2(c) shows that the Apparent Wind becomes 15.5 knots, and blows at 12 degrees from the direction of the True Wind, and at an angle 'across the deck' of 33 degrees from the bow. These speeds and angles are reasonable, in that they are suggested by theory and confirmed by observation and measurement.
The leeway angle is different. Other authors have reported that they have found it hard to measure. We took one of our own designs - the marque shown in Fig 20.12 - which is designed to sail to windward in the design wind, with its centreboard running at an angle of attack of 3 degrees (i.e. with a leeway angle of 3 degrees), and towed a thread with a small weight at its end. While two of us sailed the boat, a third read the angle of the thread across the aft deck from the bow of a following motor-boat. It never showed anything like 3 degrees. It did not read even half a degree on any of a number of runs. We tried the direct observation approach. An observer stood on one shore of a narrow bay without tide and the crew sailed from near the shore on a fixed heading, which was about close-hauled, directly towards a conspicuous object on the other shore, The observer moved if necessary, until he was sighting along the boat's centreline at the aiming point on the far shore, and watched to see how much the boat drifted to leeward of the line of sight. It didn't drift to leeward either.
PHOTO 2 - Steerable yellow-centreboard and the self-tacking aluminium centreplate which was successfully trialled on WRC6800 during year 2003 - published in the Dec 2004 Instalment.
PHOTO 3 - Setup for measuring the lateral force on starboard leeboard of my planing trimaran
Joseph Norward,Jr "HIGH SPEED SAILING" page 58:
"The best steering, leeway resisting, and yaw stabilizing system for a proa is probably twin steerable leeboards near either end of the outrigger (leeward hull). The system would be very like that shown in Fig 5-10 except that the cases will be mounted with zero leeway angle and the bottom of the leeboards will consist of a spade rudder. The two rudders will work together driven by a wheel amidships as shown in Fig 5-11. As the major job of the shunting operation the rudders are turned around 180 degrees to put them in a balanced position and the bow board is raised somewhat with respect to the aft board. This can be done with tackle or, on a large proa, with hydraulics."
Photo 4 - Norwood's illustration.
This scheme is applicable to all multihulls.
PHOTO 5 - Beamchange system for WRC 5600
Photo 6 - Trailer for WRC 6800 catamaran, trailering beam 2500mm, sailing beam 3800mm.
Photo 7 - Launching WRC 6800..
Photo 8 - Reloading WRC 6800.
Photo 9 - Wimbie Beach I km from my home - note the gap in the fence.
Photo 10 - Loading my planing trimaran onto the beach-trolley (the wheel/axle/Ubar) from its road trailer.
PHOTO 11 - Trimaran loaded onto the beach trolley.
My plan is to carry the trimaran to Wimbie Beach on its road trailer and off-load it onto the beach-trolley and manually wheel it through the gap in the fence to the sea.
PHOTO 12 - My latest creation - Atlantic proa using fore-and-aft rudders for steering and lateral resistance.