Pushing Water
When we go sailing, the boat may just push water out of the way (usually called displacement mode), ride on top of its own bow wave (planing) or foil. This week we look at how water resistance effects boatspeed, and what we can do to minimise this resistance.
Let’s first look at displacement sailing. Most people think that displacement sailing is slow, until we consider that catamarans sail displacement mode. Essentially cats can be far faster than planing boats, which is something that used to perplex the late, great Frank Bethwaite (the incredibly gifted designer who planned the 49er among other serious boats), who stated clearly it was impossible to push water out of the way and go as fast as a boat riding on top of the water whilst planing. Well, displacement IS slow, unless the hull cuts through the water like a knife. Obviously a wafer thin hull would cut through the water and give minimum drag, but as the hull has to support the weight of the boat and rig, plus crew, it’s not really practical to sail a wafer thin hull. We then go to the other extreme, which may be considered as a bathtub (aka an Optimist!!). When we push a blunt bow through the water we have to use a lot of the power developed by the rig to push the water aside – the faster the boat tries to go, the resistance increases almost exponentially.
Consider how difficult it is and how much power you need to displace water. If you have ever had a bucket of water thrown at you, you know it’s quite heavy – 1 litre of water weighs 1 kilogram (In contrast to 1 Liter of air which weighs close to nothing). So how many litres of water does a blunt bow have to displace so the hull can pass through it? The answer is several bucketloads. That’s all power generated by the rig that is used to push water aside rather than make the boat go faster. The less water we have to push aside, the more power will go into making the boat go faster and the less power is drained from pushing those mountains of water aside..
If we look at planing boats, they’re great – we float on turbulent water and skid across the surface on bubbles of water and air. Terrific news. So why do planing boats kick up so much spray when we sail? Very often we leave rooster tails of water when we go sailing. How many litres of water per minute are we spraying into the air? When we hit waves, and briefly stop planing (by definition), how many litres of water are slowing us down?
Essentially it doesn’t take much to realise that any splashing of water is energy that is taken from making the boat go forward to stopping progress. It doesn’t matter whether that water is coming from the bow in big splashes, or from a rooster tail from the rudder stock or pintles, or from a dragging rope, or from a wave at the tail – all those things slow the boat down. If you’re making waves you’re displacing water, which takes energy which prevents the power from the rig going into driving the boat forward. This applies to all forms of boats.
As sailors how can we improve our speed by eliminating most of the splashes, waves and tails? Steering the boat to avoid the worst of a wave is a start, using our body weight to stop the bow plunging or lifting, understanding what is the most effective path through a set of waves or chop, and coordinating the sheets (when you slow down you need more power!), angles and body positions is all work that has to be done. The top sailors spend many hundreds of hours on these skills, and beginning to understand the drag that water produces is the beginning of really going faster – and don’t forget it IS something we can do something about, not dependent on the age or condition of equipment, but just pure skill development. At Toplevel sailing we always work on gains, and this is a big one, so get working at reducing those splashes and waves!