Originally posted 2011-07-13 13:32:52.
Scuba Buoyancy Masterclass #5
PRECISION TRIM & POSITION CONTROL
by Andy Davis
Whenever we use our fins for propulsion in the water, we create directional thrust. If our body is not horizontally trimmed (‘flat’) in the water, then we will be accidentally directing that thrust at an angle (upwards or downwards) that will interfere with our scuba buoyancy.
Why Trim Is Important For Scuba Buoyancy
Achieving a reliable flat/horizontal trim in the water makes our propulsion more effective (all our thrust goes into covering distance) and does not cause buoyancy problems. Being flat/horizontal also prevents any water movement (either current upon our body, or our body moving through the water) from pushing us vertically in the water column.
For an exaggerated demonstration of this factor, simply stick your hand out of a car window when driving. You can hold your hand still if it is flat. If you angle your hand upwards, the air-flow will force it upwards. For a diver who is in a bad, head-up/feet-down trim position, the effect of simply moving through the water will subtly force them upwards. When added to the directional (downwards) thrust from fining in that position, it will give the false impression of having more buoyancy than they actually have.
This is why many divers can maintain a constant depth whilst fining but then find themselves sinking (negatively buoyant) when they try and maintain a static position without fining.
Many divers with poor buoyancy control will instinctively compensate for this lack of control by adopting a head-up/feet-down trim position.
This is a very short-sighted solution, as any buoyancy problems will become immediately apparent when the diver stops moving (i.e. on a safety stop, to take a photo, or when having to deal with a problem).
A diver should not under-estimate that amount of negative buoyancy that can be off-set by fining in a feet-down attitude.
Weighting and Trim
The first step towards achieving a flat horizontal trim is to ensure that you are not over-weighted. As mentioned earlier, this creates a push-pull effect across the divers’ horizontal centre of gravity (COG).
It is caused because weights are located on the torso below the COG, whereas the air that compensates for those weights is located above the COG. The torso rises and the legs lower. An over-weighted diver exaggerates that effect (more weight = more air to compensate).
The second step for achieving a flat horizontal trim is to simply to practice maintaining a static hover. The diver should aim to hold themselves in a position similar to that used by a freefall parachutist – with their shoulders and hips at an equal level, the back-arched, the hands clasped (to prevent sculling) and the legs and head raised.
As the diver attempts to hold this static hover they will swiftly notice whether their equipment is causing any misbalance. Having identified any tenancies to float in a non-horizontal manner, the diver can then seek to re-adjust the configuration of their equipment to mitigate this effect.
Working with a partner/observer who can give feedback on your trim practice is always beneficial. The serious practice of trim (along with other skills, such fin techniques) can very much benefit from the use of a video camera to record and assess your technique.
The diver can use ‘trim weights’ to resolve major problems in their hover. These are small weights that can be attached to the cylinder cam bands, the BCD or the ankles. They can also experiment with the location of their diving cylinder on the BCD; raising or lowering the cylinder to shift their COG upwards or downwards along their torso. More fine-tuning can be achieved through consideration of specific items of diving equipment.
Most notably, the choice of either positive or negatively buoyant fins can play a beneficial role in trim.
For very fine-tuning of trim, the diver should also be aware of their arm positioning during the dive. Extending the arms forwards in front of the diver can help to bring the upper torso lower. Holding the arms closer to the torso will negate this effect.
Next article: ‘Breathing and Buoyancy’
About the Author
Andy Davis is a RAID, PADI TecRec, ANDI, BSAC and SSI qualified independent technical diving instructor who specializes in teaching advanced sidemount, trimix and wreck exploration diving courses across South East Asia. Currently residing in ‘wreck diving heaven’ at Subic Bay, Philippines, he has amassed more than 9000 open circuit and CCR dives over 27 years of diving across the globe.
Andy has published many magazine articles on technical diving, has written course materials for dive training agency syllabus, tests and reviews diving gear for major manufacturers and consults with the Philippines Underwater Archaeology Society.
He is currently writing a series of books to be published on advanced diving topics. Prior to becoming a professional technical diving educator in 2006, Andy was a commissioned officer in the Royal Air Force and has served in Iraq, Afghanistan, Belize and Cyprus.