Speed and DRAG
Boats are designed to go through the water, and over the past 70 years have been designed to ride ON the water. Over the past 10 years plus they’ve also been designed to fly over the water. These hull designs are termed as displacement (through the water), planing (riding on the water) and foiling (flying over the water).
What is the need for so much change? Over the past 40 years we have seen greater increases in boat speeds than in the previous thousand years. The round the world record has been better than halved for the big boats, and the course racing speeds have gone off scale. The reason for this dramatic improvement is down to drag. Drag is turbulence created by the passage of a body through a liquid or gas. It can be seen in scientific and design laboratories in wind tunnels or in flow tanks for liquids in the form of swirls occurring behind the body being passed through the gas or liquid.
One of the fascinations of sailing is that in an ideal world we would have a sailboat where drag would not exist. Because a boat can sail on the wind it generates by its own speed, a boat with no drag would have an infinite terminal speed, as theoretically it would keep accelerating by its own creation of wind. Few other vehicles of any sort can claim this sort of theoretical synergy potential.
We suggest that sailing has this massive appeal across all sorts of characters, from the relaxed cruising sailor to the fiercely competitive and athletic sailor, and of course ranges technically from those that really don’t care how the boat goes, as long as it goes, to the ultimate geek who needs to know the science behind the movement, regardless of speed.. We also suggest that the wider implications of development are often triggered by the friendly neighborhood geek who loves to bimble with boats, adding the occasional gadget that somehow has the effect of reducing drag, the enemy of speed.
Let’s begin to look at specifics. Drag occurs in boats both in the hulls and the sails. The drag felt from the hull can be minimized by weight placement and keeping the boat at a certain pitch and angle of the water. In waves the need to constantly alter that pitch by movement leads to the need for dynamic sailing, where the sailor is constantly shifting weight forward and aft, either by the movement of their butts or by trapeze technique. This will keep the water flowing around or under the boat at an optimum drag angle, especially in waves. Big waves send a further challenge – at the peak of a wave the wind is often stronger than in the trough, so the dynamic changes become even greater.
Now we look at the past and the future. Catamarans have always confounded the designer. They move swiftly once a hull is flying simply because they have less hull traveling through the water, with a longer sleeker profile on the hull once the windward hull has lifted, the speed they are already achieving forces the hull a little higher out of the water as water is forced under the moving hull.
They produce great speeds in a variety of conditions. Probably the most legendary cat design was the Tornado, designed by the Brit Reg White. As a forty year old design it was proven to be 25 to 30% faster round a course than the 49er when the 49er went Olympic for 2000. Aside from freak boats like the Tornado, the old designs occupy a place in many people’s hearts, and they are happy to push a lot of water in front of them, because sailing is great! Laser, 470,Finn and all the old school classes fall into this bracket. The real challenge to the designer is to introduce new speed with old school “raceability”. Producing a boat with the best of both worlds would make for great racing!
To a large extent the rig produces more drag the faster the boat is moving. At low speeds the rig drag is really minor compared to at high speeds, so sails are still often designed with great power (depth) and therefore big drag – power overcomes drag to a certain extent at lower speeds. We are however entering an era where hull drag has been reduced far enough that sail designers are having to become increasingly concerned with clean “exhaust” and reduced turbulence behind the sails. Large research is already underway, and progress is being made – led, again by windsurfing rigs! Its amazing that the square headed windsurfing sails which were really taken as a trendy addition back in the 1990s are now found on all sizes of sailing boats. Unstayed masts never behave the same as stayed rigs, yet the beautiful self sheeting principles of the block head are providing turbo boosts to all classes of boats, with a lot more to come from them as the Einsteins of the rig world learn more about them.
We at Toplevel Sailing are not too worried about the challenges of designers. We feel our support to sailors and coaches is better served by keeping abreast of these advances, but the most critical part of this knowledge is to be able to coach at an individual level, relating the sailor’s boat position and style to minimizing drag, and then going faster. We sail to win!