How Will She Perform?
Resource
The basic speed formula
A first principle of design is: a moderate or heavy-weight cruiser will have a maximum speed in knots of about 1.4 x the square root of her ‘sailing length’ in feet.
From this it follows that the bigger the yacht (whether sail or power driven} the faster she will go – other things being equal, which they seldom are.
Rule of thumb
Maximum normal hull speed (in knots)
When not planing = 1.4 x √Sailing Length (in feet)
This formula tells us that if a yacht has a waterline length of 36 feet, then we take the square root of that number, which is 6 and multiply it by 1.4, giving 8.4 knots. This is known as the ‘hull speed’. To attain this speed the vessel must have ample power, either from an engine or from a large enough sail area exposed to sufficient wind strength. In addition, the hull must be the correct shape, and in this connection buttock lines which run steeply upwards aft, at 20 degrees or more, make it more difficult to achieve the speed.
There is another complication: what matters is ‘sailing length’ and not the static waterline length. So our length figure from which the square root is taken includes some or even all of the length of the bottom of the counter along which the stern wave will curl, and maybe part of the bow overhang. That is partly why our ocean cruiser design has an overhanging counter – to give extra speed in moderate and high winds.
Speed in light winds
When sailing slowly, the whole overhang of the counter is above the water. In light airs there is no friction between the water and the hull forward or aft of the Designed Water Line.
If we want a yacht to do exceptionally well in light winds we keep the total area of the immersed part of the hull, keel and rudder as small as possible.
This is known as ‘minimising the wetted surface’ and the aim is to have the least resistance against forward motion. At low speeds there is no wave-making resistance, just the drag of the water on the wetted part of the hull, keel, rudder and skeg.
A low wetted surface is illustrated on P88 and P89 where we see that the racing yacht hull sections are as near to a semi-circle as possible, while both the keel and the rudder are made as small in area as the designer dares. Because the ‘engine’ of a sailing yacht is the rig, the sail area is made large if a sparkling light wind performance is the aim.
Wind performance
If a yacht is required to be a wizard to windward, the designer gives her a very deep keel and a tall mast. The former makes it impossible for her to go into lots of harbours but, as in most matters of design, a gain in one area calls for a sacrifice in another.
A high rig needs skillful handling and is expensive, which explains why moderation usually prevails. Lightness aloft is also a bonus, but this puts up the cost. In passing it has to be said that speed costs money which again explains why our ocean cruiser is not excessive in any direction.
Other things being equal, the lighter a boat is, the faster she will be. This explains why our ocean cruiser has a ply deck and cabin top instead of a steel one – weight-saving high up is doubly beneficial because it improves the stability and so gives a double bonus.
Going to windward, two identical yachts will perform equally, unless one is kept more upright than the other. This explains the importance of devices like water-ballast tanks, tilting keels and broad mainsheet horses to give forward drive with the minimum heeling moment. Other requirements are the minimum windage and weight from the deck upwards, as well as the maximum weight as low as possible in the keel.
An ocean cruiser can seldom have a deep keel, so we enhance its efficiency by cramming everything heavy we can down in the fin. She has lead instead of iron ballast because its centre of gravity will be lower and also the space saved can be used for other heavy items like tanks and anchors. Even the engine may be partly down in the fin.
When it comes to the driving forces, the bigger the sail area the more wind is ‘captured’ so the faster the yacht goes. Under power, other things being equal, the yacht with the most powerful engine will be the fastest until ‘hull speed’ is achieved. Once this is reached, extra horsepower is a waste of money.
Using standard tables and formulae it is easy to get a reasonably accurate forecast of speed likely to be achieved under power for a given engine – provided the engine is giving the output promised by its manufacturers, the propeller is right, the engine is exactly aligned and so on. It is a good idea to predict the speed with more than one technique and build up data based on actual performance under power.
A heavy boat will not plane, that is ‘sledge’ along the surface of the water. This means she will never go faster than 1.4 x the square root of the waterline length. Even a vast spinnaker, set in a blast of wind, will not increase the speed as the drag increases as the bow wave moves aft and the hull ‘squats’.
Planing – At least partly
Up to now, it has been assumed that the speed is limited by the sailing length because this is true for medium weight and heavy craft. However, if the weight can be reduced and the shape of the hull made broad and flattish with a gentle slope at the bow and straight, almost horizontal, buttock lines aft, then, when there is sufficient wind pressure on the sails, the yacht will plane.
She is now no longer just making a trench in the water for herself, but partly slithering over the face of the water.
The concept is easy to explore by taking a flat plank and cutting one end to a sharp bow shape in plan view. Tow it along the edge of a pond and the faster it is pulled the fewer and bigger the waves are which run along the plank sides.
Now fair away the bow by rounding the forward end so that it is sledge-like in elevation. Repeat the towing process; as the speed builds up there comes a time when the bow rises and the plank skeeters along on the surface of the water. This is planing. At this point the rule which states that the top speed achievable is about 1.4 x the square root of the sailing length melts away.
Now the only limitation to the top speed is the driving force which can be applied, the strength of the hull and rig (and the nerves of the crew). The lighter the boat, and the better the planing shape, the faster she will go. However, a planing shape is not ideal for going to windward, so the designer either compromises or fashions a yacht which is a downwind demon, but may be a bit weak to windward.
From a designer's point of view, planing is a mixed blessing because, although it gives extra speed, it means structures have to be stronger to stand up to the increased loads. Strains are proportional to the square of the speed, so the bow area, the hull bottom, the skeg and rudder all need to be beefed up. This increases the weight, so weight has to be skimmed off elsewhere. A vicious circle like this is likely to add to costs.
How is she going to perform?
Anyone who has designed a yacht and wants to know how fast she is has to build her first, then sail her well, to find out. However there are short cuts. If a closely similar yacht has less sail area, or less sailing length, or more weight, or a shorter mast, or more wetted surface then the new design should beat her most times.
Once a design is complete, but before the yacht is built, an approach can be made to a test tank like the one run by Southampton University. Here a model of the yacht can be made then towed and tested. The experts at the tank can give predictions as to how the yacht will perform. It’s just a pity this process is not cheap, but then a lot of expertise is involved.
Anyone who designs on a computer and is interested in performance will need a piece of software which gives a ‘Polar Diagram’. This predicts speeds on different headings and includes curves for different wind strengths. When comparing different yachts, one of the first things any new designer must realise is that few boats are brilliant all-rounders. This means that the probable weather and wind direction for a given race or series or cruise has to be taken into consideration before, during and after doing a design.