How fast will she go?
Light slender hulls have much less resistance
A DN ice yacht can sail 4 times the speed of the wind and has a top speed of 60 mph and it’s only 12 feet long. Water has considerably more resistance than ice, making sailing at merely a speed equal to the wind speed an impressive achievement for a boat. It is easier to analyse the resistance or drag of boats (and how to reduce it) when it is split into three parts: Skin Friction, Form and Wave.
Skin friction drag is caused by the tendency of water to cling to the hull and to itself. Think of dragging a spoon through honey. Skin friction depends on the size of the wetted surface and the surface finish. Polish is much better than barnacles. For a certain weight of boat, a hemisphere would have a very low wetted area. So stubbier and rounder is better. The long thin hulls of a multihull have a higher wetted area than a beamy monohull. In this matter, multihulls have some inherent disadvantage.
Skin friction drag is the biggest part at low speeds, when wave drag is small.
Form Drag (sometimes not separated
from Wave Drag) is the effect of turbulence and can be
reduced by smooth curves and gentle hull lines. A wide
flat transom below the waterline would be bad. The idea
is to minimise the disturbance to the flow. There is no
major difference between mono or multihull.
Wave drag is harder to grasp intuitively.
Wave drag is the reason that a submarine has less resistance in a dive than is does when cruising on the surface, even though the wetted area will be less. Perhaps the extra effort needed at speed near the surface is the reason that dolphins jump from the water.
Waves are caused by the deflection of water as the hull drives through it. As the bow approaches, water near the bow gets pushed aside. The pressure increases and the water level rises. As the boat passes the water level drops under gravity and a wave is created. Making the wave takes energy, which is the Wave Drag. Water is not only deflected by the bow, but all along the boat where there is curvature. Amidships, water movrd away by the bow is deflected back towards the boat causing low pressure, this time, and a drop in level. At the stern, inwards moving flows meet, making another high pressure area and height increase again. As the boat increases in speed, the length of the waves generated increases. When the length of the wave is equal to the length of the boat the influences from bow, midsections and stern combine to generate a large wave with a peak at the bow and stern. At this point the wave drag is increasing very rapidly. For heavy boats is so high that any further increase in speed is impossible.
Beating Hull Speed
This is known as the Hull Speed and depends only on hull length.
i.e, if:
- LWL = 30’ Hull Speed = 7.34 kt
- LWL = 40’ Hull Speed = 8.48 kt
- LWL = 50’ Hull Speed = 9.48 kt
The penalty of Wave Drag at Hull Speed depends on the shape of the hull – or how much the water is deflected. Heavy boats will have much more drag as there is more water to be deflected. Wide beamed boats will have more wave drag as water will be deflected more severely.
LWL 45 ft
Sail Area 1354 sq ft
Displacement 20944 lb
Ballast 9460 lb
SA/D 28.5
Hull speed 9.0 kt
Actual performance - in 20 kt wind 12 kt – Hull Speed x 1.3
LWL 35 ft
Sail Area 758 sq ft
Displacement 5500 lbs
Ballast 0 lbs
SA/D 34.7
Hull speed 6.9 kts
Est. performance - in 20 kt wind 18 kt – Hull Speed*2.2
Unfortunately, two of the things we might do to increase performance in a monohull – increase ballast and beam to increase stability, and have rounded forms to reduce Skin Friction, are bad news for Wave Drag and probably mean that Hull Speed is as fast as we can ever go.
Light displacement monohulls, however, can crack the hull speed barrier and a high performance racer can expect to get as high as 1.3 times hull speed before the Wave Drag gets insurmountable.
Light, slender hulls have much less resistance
Multihulls are less limited by Wave Drag than monohulls as long slender hulls don’t make big waves. On top of that they have the advantage of lower displacement as they do not need ballast for stability. Consequently multihulls typically have a larger speed range and higher top speed than monos.
Slender, light displacement hulls are very easily driven at speed. Racing kayak illustrate this very well. A 17 ft olympic kayak can be paddled in a sprint at 9.98 kts, which is 1.8 times hull speed with human paddle power alone. Even at 7 kt (1.27 times Hull Speed) wave drag is roughly only the same level as skin friction drag.
Typically a fast cruising catamaran or trimaran could be expected to do 1.6 times Hull Speed. Racing designs are capable of well over twice Hull Speed and will go faster than the wind. Load it up with tons of cruising ‘extras’ and the higher displacement will clip its wings, though.
How slender is slender?
The more slender the better – better than 8 to one – 12 to one good. (paragraph to be added)
