When a sail is involved, the phenomenon of apparent wind (the wind felt due to movement generated by the sail’s own power) increases, feeding the sail with more wind. As the effects of more wind generate more power from the sail, it goes faster, and generates more of its own power source.
The idea that we can gain extra power for nothing is the perfect illustration of a logical synergy. Boat designers are tackling this potential in a far more scientific manner than they have done for the last few thousand years, and so we see the exciting increases in speed that we have seen over the last 25 years.
The big problem for the boat designers is turbulence, which causes drag. As sailors we can’t worry about the design, although we can try to understand it. The big problem for us as sailors is to optimise the design that we are sailing, whether it be old school 470, 2019 18 Foot Skiff or foiling Moth, and to minimise the drag that the designers have left in.
Where does drag occur?
Drag, as can be seen from the picture, always happens behind the point of obstruction. Obviously the drag created by a tapered needle would be considerably less than that created by a flat surface perpendicular to the flow. Designers realised this early on, even if they were unaware of the theory, hence the reason that ancient boats usually had pointed bows.
The bubble of turbulence, be it air or water, is “dragged” behind the hull or sail, creating a barrier to going faster. This is something that may be inherent in the design of the boat, but we may be able to minimise it.
One aspect of drag that must be appreciated is this: Water is several times heavier than air. It follows that drag through the water (drag created by the hull) is much more punitive to performance than drag created by the sail. However, both these aspects are worth looking at, but always prioritise minimising the turbulence you see behind the hull before you worry about turbulence from the sail!
A good example of this minimised drag is where we sit on the boat in ultra light wind. A boat will rise up in the water with a little bit of speed. Displacement boats such as Laser or 470 need a lot of power to plane (when the bow overtakes the bowwave, and the boat feels as though it is skimming on the water rather than going through it). However, whether its a 470, Laser or 49er, when there isn’t enough wind to propel the hull fast enough to allow it to rise up in the water, we need to focus on different points of drag than when it does rise up. We can see the drag created by a hull, simply by looking past the transom at the wake. The less ruffled the wake is, the faster we are going. This applies at any speed. In ultra light winds on almost any boat the major source of wake is from the transom. No matter how poorly the hull is designed, the transom will almost certainly cause more turbulence (drag) than any other part of the boat. Moving forward until the wake minimises gives you another gear.
This study of the wake is important in any wind speed. A “rooster tail” where the wake stands up behind the transom in any strong wind is a disaster in terms of speed. Moving forward will minimise that. Moving too far forward will create bow drag. Finding the “sweet spot” is something that we at Toplevel Sailing tell you how to do – not where to sit or trapeze from!
Other forms of turbulent wake come from heel. As soon as a hull heels it becomes asymmetric. One side of the hull will have a different shape of water flow than the other, creating turbulence. The additional factor of needed corrective steering to stop the boat heading upwind (assuming the heel is to leeward) causes rudder drag, which can be felt through the tiller, and seen over the transom. That’s a good reason to keep the boat flat. FLAT IS FAST.
Whilst foils (rudder and centreboard) can and do cause turbulence, usually this is trivial compared to wrong tuning or flawed technique.
Sail and RIg Drag
The biggest concern for us is to tune the rig to our leverage (body weight ad what righting moment we can apply to the hull), so that the boat when sailed flat, will sail in as near as possible in a straight line. At Toplevel sailing, we encourage brief spells of letting go of the tiller during training to see which way the boat wants to go. This simple action will tell the sailor (and coach) if the boat is in the “go fast” zone. If it isn’t we simply keep working on different combinations of mast bend, rig tension and kicker/cunningham/outhaul until it achieves this ability to sail straight. If the boat wants to do its own thing and the helm has to combat that movement with the tiller, the rudder again creates drag and slows the boat down. Please note that sometimes it is faster to power up the boat and have a little corrective steering rather than be perfectly in tune. This is because the additional power more than compensates for drag created. This isn’t a usual occurrence, so don’t go there too often!
Drag across the sail can be very harmful to speed. The most obvious time this happens is on a reach, but though this is important it is easy to see through flying tell tales and varying twist. Same principles apply – flat is fast and the boat should be tuned to go in a straight line. largely we are concerned with upwind drag.
As the boat goes faster upwind the requirements of sail shape begin to differ. To get from 0 to 7 or 8 knots requires power. This means a very powerful (full) sail shape. To get from 7 to 12-15 knts requires some power but the top of the sail needs to begin to blade off. Above 15 knts for any boat, speed can only be optimised with an increasingly flat sail. The wing sails enabled this skill to become controlled by science. Most of us have to pull strings and compromise! When you get good enough, you do get a return on a flat setup in flat water and high wind.
We are at a point of evolution where foils are to be considered and explored, and knowledge built. Although a lot of people don’t wish to explore this topic yet, this heading is for those that do. Moths, Wazsps and derivatives are becoming increasingly popular – they usually have control of foil angle of attack, which provides lift. As soon as they are foiling (hull above water), the foil is the source of performance and the root cause of drag. That’s why we are covering this topic. Whilst drag from foils is still substantial, it has proven to be less than that of hulls. However as the speeds increase, foil technology becomes more suspect and the drag increases. This is a complex challenge – it has exactly the same basis as the challenge for sails needing power to get going and flatness for speed, but the foils are solid and underwater, therefore not visible…….
If foils are held at an angle to the water like an aeroplane wing on takeoff, there is a lot of turbulence. As they flatten out there is less turbulence. The challenge of making a foil that creates lift but has very pure flow is relatively new. What is for sure is that they have gone down the “glider” route. Very high aspect, long and thin foils….. Aircraft designers have found over many years that delta wings are effective – it is, I’m sure, only a matter of time before the foiling world goes delta and high speed increases are made.
Drag is our enemy! At Toplevel Sailing we consider this to be the case from development sailors upwards, and place great effort with minimising drag. Its a fascinating subject for the geeks among us, and an essential topic to understand for the non-geeks! Its most certainly a sailing topic, and whether you understand the theory or just key into practical feel, you can’t be fast if you are dragged backwards compared to the opposition.