Bow how?
I was reading a travel brochure that told me that my cruise ship had a bow bulb on the prow of the ship. The guide says: 鈥淭he bow bulb is just below the waterline and displaces the same amount of water that would be pushed out of the way by the ship鈥檚 bow. This virtually eliminates a bow wave, resulting in some fuel saving as less energy is needed to push the ship forward.鈥� Intuitively, one would think that a sharp prow would be best. How does a rounded bow bulb eliminate the bow wave, and how does it improve efficiency?
鈥� Even with the evolution of complex fluid-modelling techniques, a good rule of thumb is still that water resistance on a ship varies in proportion to the square of its forward speed.
In general, the resistance on the ship鈥檚 hull can be broken down into three components. First, frictional drag is ever-present due to the viscosity of the water. This dissipates energy through shear forces in the fluid. Second, 鈥渇orm drag鈥� is caused by the change in fluid velocity as it moves around the ship. Finally, the waves generated by the moving ship dissipate energy through the creation of pressure fields. This is called wave-making resistance.
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To understand the concept of a bulbous bow, imagine a ball travelling at fixed velocity just below the surface of a fluid. The gap between the top of the ball and the surface constricts the flow, so to keep up with the surrounding fluid the flow must travel faster and, according to Bernoulli鈥檚 theorem, must reduce in pressure. This pressure reduction manifests itself as a dip in the free surface above the ball.
Generally speaking, there is a pressure field at the front of the ship and a suction field at the back, creating the bow wave and wake respectively. The speed of the ship and the distance between the fields determines if they will interact favourably or unfavourably. Careful positioning of a bulbous bow, keeping the resulting pressure reduction in phase with the forward pressure field, can reduce the magnitude of both the pressure and suction fields. With good design, this reduction in wave-making resistance can more than compensate for the increase in frictional resistance caused by the bulb itself. This means the same speed can be achieved with less power and therefore less fuel.
Michael Woodward
School of Marine Science and Technology
University of Newcastle upon Tyne, UK
Delayed reaction
Having just completed a half-marathon in the UK鈥檚 Great North Run on Tyneside, I was surprised to find that my legs felt stiffer two days after the event than they were the following day. Why was this?
鈥� Running is a form of eccentric exercise, meaning that the muscle is forced to lengthen while trying to contract. Prolonged or unaccustomed eccentric exercise often leads to pain, tenderness and stiffness in the muscles hours or even days later. This is known as delayed onset muscle soreness and is very common.
The sensation of discomfort usually develops approximately 24 hours after exercise, peaks at about two days and then gradually subsides. During the 24 to 48 hours post-exercise period, muscle swelling and stiffness usually result in a reduced range of motion and also muscle weakness.
Because the onset of muscle soreness is delayed, it cannot be attributed to the metabolic end products of exercise. In fact it is caused by localised damage such as microscopic tears to the membranes and protein filaments of muscle fibres. One hypothesis is that the damaged muscle cells die because they are subject to excessive calcium inflow. Another is that exercise-generated free radicals attack the cell membranes, leading to their death.
In addition, there is increased blood flow to the muscles, which causes the tissue to swell. Such swelling increases pressure on the neighbouring structures. The nerves in the muscle sense this and send pain messages to your brain as you move the morning after you have exercised.
Melanie Trickett
Sheffield, South Yorkshire
鈥� Delayed onset muscle soreness (DOMS) is a result of an excessive amount of tearing in the muscle. In order to improve performance when we exercise we need to progressively challenge our muscles with the amount of work we expect them to perform. This progressive overloading (usually achieved by increasing the resistance they experience such as lifting heavier weights or by running extra distance on successive days) causes tears in the muscle鈥檚 microfibres. And, in a gradual overload/repair cycle, we experience moderate soreness up to a day later.
DOMS is caused when the expected load dramatically increases, causing a greater number of tears (rather than an increase in the magnitude of each tear). In this situation it takes longer for scar tissue to form because it grows in perpendicular fashion across the repair sites. Once the new tissue is in place, we experience the soreness that comes with DOMS as we reactivate and stretch this new, less pliable muscle, until its strength and flexibility are restored.
Paul Carey
Personal trainer
David Lloyd Leisure Health and Fitness Club
Brooklands, Surrey, UK
This week鈥檚 questions
Seeing into the past
I was leafing through a book of photographs showing people 鈥� mainly Anglo-Saxons 鈥� celebrating the end of the second world war in 1945. I noticed that very few were wearing glasses, unlike in photographs of modern-day crowds where about a third are wearing them, to say nothing of contact-lens wearers. Why is there a difference?
Laurie Stott
Mudjimba, Queensland, Australia
Ordering a meal
When we are really hungry the quickest way for our body to gain energy is surely found in the kinds of simple sugars found in desserts. So why is it that we usually eat savoury foods followed by sugary puddings and not the other way around? I have noticed that on aircraft, where all the food arrives together on a tray, people often eat the courses in a quite different order from usual.
Toni Lluddmecy
London, UK