Positively Buoyant
Posted By Lucie Wright on 3 February 2005
On my first Red Sea dive, the divemaster suggested that I needed 8-10 kg in weight, to which I snorted, implying that he was sorely mistaken, oh how wrong was I! In order for the divemaster to assess that I was as good at diving as I had implied, we began with a shallow shore dive. I ungraciously took extra weight as recommended - 6kg, two whole kilos more then normal. Embarrassingly the descent was very tricky and I began to wish I had listened and brought more weight! Needless to say, it was an uncomfortable dive and I learned my lesson.
So what is buoyancy?
Legend holds it that Archimedes suddenly realized the principle of buoyancy while watching his bath water overflow, delighted at his discovery, he ran home without his clothes, shouting "Eureka"! A diver is subject to Archimedes Principle, which states that 'an object floats if it weighs less that the amount of water it displaces'. For example if a diver weighs 70kg but displaces 71kg (71 litres) of salt water, they will float, to get neutrally buoyant they need to add 1kg of weight to their weight belt! A penny thrown in the water will sink because the density of metal is much greater than that of water. However, a steel ship will float because, the effective volume of the ship includes all the air inside it, so that its average density is then less than that of water.
In layman's terms, a diver needs to be able to achieve the three states of buoyancy throughout a dive, positive, neutral and negative. To descend at the beginning of a dive we need to be negative, to swim smoothly in mid water during the dive we need to be neutral and to be comfortable at the surface at the end of the dive we need to be positively buoyant!
Why is buoyancy important to divers?
Buoyancy is one of the most important skills to learn in diving; to become a better diver, to streamline your body, to conserve air thereby gaining longer bottom times, to move naturally amongst the local marine life avoiding injury and more importantly to avoid crashing thorough and damaging the reef. Photographers need it to take good photographs safely, there is nothing worse then watching a photographer crushing corals under their knees to take that perfect shot! Cave divers need buoyancy to be able to move along passageways without disturbing the bottom sediment, or the person following will see nothing but clouds of sand.
What affects our buoyancy?
There are so many factors to consider when diving in a new place; is it salt water or fresh, have I put on, lost weight since my last dive, is my wetsuit thicker, or thinner, is my tank steel or aluminium, am I calm or excited? All these things affect our buoyancy.
The reason for my faux pas in the Red Sea was that unknown to me it is one of the saltiest seas on the planet, due to its high rates of evaporation. Therefore, in general, a warm climate means saltier sea and the saltier sea means the more buoyant we are going to be. Think of how people are able to float on the top of the water in the Dead Sea, which has 5 times the normal content of salt. Fresh water means no salt and therefore less buoyant, so less weight needed.
Fat is positive! Each person has a different body makeup and as such will have different buoyancy issues. In general a person that finds it easy to float on the surface of the water, is naturally 'positively buoyant' and probably a little on the chubby side. A lean, mean athlete will sink like a stone because muscle is negative!
With lungs, size matters. When my little brother (all 6ft 4in of him) did his open water, his 8ltr lung volume seriously affected his diving. When he fully fills his lungs in shallow water, he rises very quickly which means he has to control his breathing to ensure stability. By breathing slow, long, and remaining calm, a diver will maintain far better buoyancy and use less air, making for a longer dive.
- Bookmark with :
- what are these?
