owlbill
September 22nd, 2008, 05:12 PM
The following is an article from the last issue of Sources magazine published by NAUI. I would like to thank the author Todd Baldi for allowing us to share this with our forum users. Please do not distribute this text in any way without permission from the original author. If you have any questions or comments about the article, please share. (NTEC = NAUI Technical Equipment Configuraton. Basically a Hogarthian/DIR style setup)
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NTEC Foundations - Buoyancy, Trim and Balance
By Todd Baldi, NAUI 13386
What separates a basic diver from an “advanced” diver?
It is an interesting question and when asked, you usually get different answers. However, it is generally agreed that one the hardest skills to master in diving is the art of buoyancy, trim and balance. Balance is perhaps one of the single most important factors in our diving and a fundamental skill for all levels from basic through technical level diving.
Balancing scuba equipment with a diver in it is a complex equation dealing with not only how much weight to use, but where to put it. Given that we all have different body types and equipment, there are no simple rules to follow to achieve perfect buoyancy and trim. Factor in a combination of different environments, exposure protection and equipment considerations, you have multitude of variables that throw a diver’s buoyancy in all different directions.
Benefits of having good buoyancy and trim are numerous. Decreased gas consumption, not fighting with equipment, streamlining in the water, less swimming effort and reduced impact on the environment are all positive attributes in a diver’s repertoire.
The secret to effortless buoyancy and trim is good weighting and were the weight is located on the diver.
First, we must focus on the aspect of weighting as it applies to balancing your dive gear.
For the better part of the last twenty years, divers typically wore a jacket style BC and separate weight belt. Jacket style BCs led to problems with diver’s trim and buoyancy. Separating the weight belt from the backpack lead to large discrepancies in the diver’s weight distribution and caused a diver’s midsection to sink while the chest area rose during the dive.
Early divers used a plastic backpack and horseshoe style wing. The original thought was to keep all buoyancy behind the diver so the hands could be freed up to focus on photography, or more commonly, hunting. Integrated weight systems were also popular at the time with the advent of to ATPAC which used marbles to weight the backpack in lieu of a weight belt. Concerns over the diver floating face down if unconscious eventually caused the back mounted BC to fall out of favor with divers. Whether this was a legitimate concern or not, jacket style BCs for single tank diving gained widespread acceptance due to this.
The recent popularity of technical diving over the past 15 years has once again caused a debate to surface about what was the best way to manage buoyancy and trim. Backplates and wings became popular in the technical diving community due to the number of stage bottles needed to be carried by the diver while maintaining adequate buoyancy during diving operations in caves and silty wreck passages.
Recreational divers have recently adopted many of the practices that technical divers regularly utilize. Single tank recreational divers have found that diving in a technical diving configuration (NTEC) allows them to experience many of the same positive attributes employed by their technical diving counterparts. Increased buoyancy control, decreased gas consumption and better body control are only a few of the benefits realized by switching to the NTEC gear configuration.
Within the NTEC system, items are placed on the diver’s body based on how we need to use them. The tanks need to be mounted up near the head for breathing accessibility and valve manipulation. The light needs to be mounted on the right hip to assist with capturing the long hose, as well as to have the ability for the diver to cut it away during an emergency and the fins need to go on your feet for propulsion. In general, this configuration works well for most anyone, but problems can arise that affect our ability to maintain horizontal trim.
Consider the diver in a proper trim position, horizontal in the water with the body within 10-20% of the midline at all times. This body position is accomplished by having the head up, the back slightly arched and knees up by flexing the glutes. In this position, the diver is very much like a see saw, with two opposing arms, and a center of gravity.
Depending on their configuration, a diver in good trim has a center of gravity based on the distribution of their weighting. The main components of establishing good trim centers on the BC, how the tank is attached to the plate and cylinder selection. Diving a separate BC and weight belt throw the diver out of trim. Using a backplate distributes the weight evenly along the divers back and also lengthwise along the main source of buoyancy, the wing. A backplate also creates a centerline around the diver’s core that allows much more precise buoyancy control.
BC and Backplate Selection
Wing and backplate design are currently being debated and is primarily a matter of personal opinion on which style is best. BC’s have undergone a radical departure from the early designs. Older style wings are often called the “horseshoe” or “taco bell” due to their shape. Newer designs encompass a “donut” style wing to circulate and distribute the gas bubble throughout the entire wing to increase buoyancy and eliminate a gas bubble trapped in one area of the wing. This helps control a diver’s trim and evenly distributes buoyancy over a larger surface area.
Backplates can be made using aluminum, stainless steel or ABS plastic. Aluminum is generally used for traveling or when a diver wishes to reduce weighting to distribute it elsewhere. Stainless steel is used to add additional weight and more often, to eliminate the need to wear a weight belt in the water. Divers will also switch back plate material types when switching from salt to fresh water diving environments. Plastic backplates are seldom used but some divers prefer them for traveling and other purposes.
Tank Selection
Not so many years ago tank selection was fairly easy. Your choice was usually limited to either a steel 72 or aluminum 80. Single tanks were almost universally outfitted with a yoke style K valve. Currently there are a multitude of tanks available and dual DIN outlet valves are common sights at most dive locations. More and more beginning divers are using a single tank with a dual outlet valve or a smaller set of twin cylinders with a manifold for recreational diving.
If you're in a wet suit, stay away from double steel tanks. Aluminum tanks with a weight belt that can be ditched in an emergency and are a much better option from a safety perspective. Twin steel tanks become much too heavy once the neoprene wetsuit crushes at depth. Redundant bladder BCs are typically frowned upon for numerous safety reasons.
If you're tall, a smaller tank such an HP Steel 80 is a generally a bad idea. The tank must be mounted so that you can get to the valve, but doing so places most of the weight far from the center of gravity. This can result in a head down posture due to the increased weight on the top-half of your body.
Conversely, if you are shorter, diving a large tank, such as an HP 120, is a bad idea as well. The length of the tank has the opposite effect. The tank valve will be close to your neck line and the bottom of the tank will extend past your hips. This causes two problems. One is the weight that will cause you to “pop a wheelie” while swimming and increases risk of silting out a dive site, getting your fins entangled in the line and loss of buoyancy. The other reason is that it affects your kick since the tank’s bottom is in the way of your legs while kicking.
Both options throw your body out of balance. One of the critical NTEC skills is to keep your trim within a 10-20% midline at all times. Ideally, you should choose a tank with dimensions that distribute a reasonable amount of weight over a larger surface area. Normal to longer type tanks distribute weight more evenly than short, fat tanks.
Weighting - Determining how much weight to carry
The first problem with weighting is that everyone usually carries too much weight. This problem is typically rooted back in the diver’s basic scuba course. Too often, the instructor would add weight if a student couldn't stay on their knees on the bottom of the pool or ocean. This usually leads to all sorts of problems including decreased trim and too large an air bubble in the BC, which further destroys trim and the ability to control position in the water.
Therefore, we need to reduce the weighting as much as possible. Not many divers know when to check the buoyancy. Often times, checks are conducted at the beginning of the dive, when, in fact, they ought to be done at the end of the dive. We are at our lightest at the end of the dive, with very little gas in our tanks, and this is where the buoyancy check ought to take place.
End the dive with close to 300 psi in your tank in a non-decompression situation. With your reg in your mouth, dump all the gas from your BC and drysuit, exhale, and don't kick. If you sink like a stone, take off some weight, give it to your buddy, and try again. If you don't sink, add some weight (small amounts, usually around 2 lbs or so). Get to where you sink slowly but evenly.
Proper placement of weight
Now that we have the correct amount of weight, let's consider how to position it. Single tank divers and double tank divers have different issues, so we'll address each separately.
First, consider which pieces of a diver’s equipment that are negatively buoyant. For the average NTEC diver, this includes:
Backplate
Tanks
Weight belt
Canister light
Fins
The materials and placement of each of these pieces will have a dramatic effect on the diver’s trim and buoyancy.
Single Tank Divers
Historically, most single tank divers wore a weight belt around their waist. Depending on their exposure protection and their tank selection, this one piece of equipment can cause the most problems in terms of balance. Requiring 20 pounds or more in one location means you'll have serious issues with balance underwater. This often leads to issues with trim as the weight tends to become an anchor. Add 40 pounds of lift in the form of a BC surrounding the diver’s chest and the diver will be seriously out of trim and balance underwater. Visualize a 200+ pound adult sitting on one end of a teeter totter while a 40 pound child sits on the other and you will get the idea. It is seriously out of balance and probably not much fun for either person.
There are several different solutions for transferring weight evenly along the diver’s body for the single tank diver:
A weighted single tank adapter can be attached along the axis of the tank and places an additional 6 pounds of lead over the plate to offset that weight on the belt.
Trim weight pockets are small pockets of weight that are attached to the tank bands.
If diving a backplate and wing, bolt on weight plates are made to attach directly to the backplate and can add an additional eight pounds of weight with a very low profile.
Adjusting trim using these tools can be tricky. The higher you mount the weight, the more likely you are to roll forward. The weight also needs to be balanced left and right. The items above are great for tweaking the amount of weight you'd like to shift and also good for travel.
Keep in mind, as you move your weight off your belt or waist, it often becomes non-ditchable. If you place too much weight in a non-ditchable location, you may place yourself in a dangerous scenario.
Double tank divers
Double tank divers have the opposite problem. The combination of loss of weight belt due to steel tanks and drysuits leads to them feeling as though they are going to go “heels over head.”
How can you alleviate the “heads down” feeling?
Switch to an aluminum backplate - The AL plate will take 4-6 pounds out of the plate and allow you to shift weight to wherever you feel it would lead to better trim.
Use different tank configurations - Tank selection dramatically changes a diver’s weighting and trim. Steel HP 100's are a common choice as are HP 130’s but there are other alternatives in cylinder choices. Each tank choice has it’s own buoyancy characteristics and experimentation is needed to determine which one is best for you. Some critical aspects to look for are which tank allows you to swim in a head’s up position to maintain good trim and buoyancy and buoyancy characteristics of the tanks when diving them at a low operating pressure. Experimentation is key to determine what tank setup is the correct one.
Tail weight – A 4-6 pound weight can be attached to the lower bolt of the doubles to help a diver maintain a heads up position throughout the dive.
Twin tank divers can also utilize strategies to get rid of the weight belt entirely.
A V-weight can add between 8 to 11 pounds of weight between your doubles. This puts the weight in a place where it can help establish better trim but can make it more difficult to haul doubles up the side of a small boat.
Bolt-on weight plates – These small weight plates can be bolted directly onto your backplate. The lower profile will also help reduce side to side rolling tendencies.
Lights
Canister lights are favored by technical divers and rapidly gaining acceptance by recreational divers. Technical divers find the increased light output helpful not only to use for signaling but also to increase their view while exploring deeper sights due to the low light level found at these depths. A canister light can add weight to the diver’s rig depending on the size and materials the light is made of. However, due to superior battery technology, lights are getting smaller and lighter so the weight of a battery canister does not have as much an affect on a diver’s weighting scheme as it once did. A common question is whether the additional weight of a battery canister will tip the diver to one side. For recreational divers, the weight of the canister is typically negligible since lighting requirements are usually minimal. For extended dives, stage bottles are placed on the left side of the diver in the NTEC gear configuration to help balance a larger battery canister pack.
Fins
Fins also affect the divers weighting strategy. Typically solid rubber fins are used by divers in an NTEC gear configuration. The fins are negatively buoyant and can assist in an overall weighting strategy. Plastic fins are generally frowned upon because they are generally too buoyant and cause a divers feet to float too high. This leads to problems with controlling the amount of air in the legs of the drysuit and can cause lack of buoyancy control and trim problems. Some divers choose ankle weights to get around this problem. Conversely, this destroys a diver’s trim by causing the legs to drop and the diver’s body to “pop a wheelie.”
Summary
Equipment is only half the equation to establishing proper buoyancy and trim underwater. Proper training is a must, to provide an experienced eye that can develop diver trim, balance, breathing and buoyancy. For a single tank or recreational diver, the NTEC workshop offered by many NAUI Tec instructors, offers a fundamentals based diving course focused on developing pin point buoyancy control, trim, balance and team diving approach utilized by many technical divers. NAUI Intro to Tech can help more experienced divers and those that wish to transition to double tank diving and eventually progress to technical level dive profiles.
-----------------------------------------------------------------------
NTEC Foundations - Buoyancy, Trim and Balance
By Todd Baldi, NAUI 13386
What separates a basic diver from an “advanced” diver?
It is an interesting question and when asked, you usually get different answers. However, it is generally agreed that one the hardest skills to master in diving is the art of buoyancy, trim and balance. Balance is perhaps one of the single most important factors in our diving and a fundamental skill for all levels from basic through technical level diving.
Balancing scuba equipment with a diver in it is a complex equation dealing with not only how much weight to use, but where to put it. Given that we all have different body types and equipment, there are no simple rules to follow to achieve perfect buoyancy and trim. Factor in a combination of different environments, exposure protection and equipment considerations, you have multitude of variables that throw a diver’s buoyancy in all different directions.
Benefits of having good buoyancy and trim are numerous. Decreased gas consumption, not fighting with equipment, streamlining in the water, less swimming effort and reduced impact on the environment are all positive attributes in a diver’s repertoire.
The secret to effortless buoyancy and trim is good weighting and were the weight is located on the diver.
First, we must focus on the aspect of weighting as it applies to balancing your dive gear.
For the better part of the last twenty years, divers typically wore a jacket style BC and separate weight belt. Jacket style BCs led to problems with diver’s trim and buoyancy. Separating the weight belt from the backpack lead to large discrepancies in the diver’s weight distribution and caused a diver’s midsection to sink while the chest area rose during the dive.
Early divers used a plastic backpack and horseshoe style wing. The original thought was to keep all buoyancy behind the diver so the hands could be freed up to focus on photography, or more commonly, hunting. Integrated weight systems were also popular at the time with the advent of to ATPAC which used marbles to weight the backpack in lieu of a weight belt. Concerns over the diver floating face down if unconscious eventually caused the back mounted BC to fall out of favor with divers. Whether this was a legitimate concern or not, jacket style BCs for single tank diving gained widespread acceptance due to this.
The recent popularity of technical diving over the past 15 years has once again caused a debate to surface about what was the best way to manage buoyancy and trim. Backplates and wings became popular in the technical diving community due to the number of stage bottles needed to be carried by the diver while maintaining adequate buoyancy during diving operations in caves and silty wreck passages.
Recreational divers have recently adopted many of the practices that technical divers regularly utilize. Single tank recreational divers have found that diving in a technical diving configuration (NTEC) allows them to experience many of the same positive attributes employed by their technical diving counterparts. Increased buoyancy control, decreased gas consumption and better body control are only a few of the benefits realized by switching to the NTEC gear configuration.
Within the NTEC system, items are placed on the diver’s body based on how we need to use them. The tanks need to be mounted up near the head for breathing accessibility and valve manipulation. The light needs to be mounted on the right hip to assist with capturing the long hose, as well as to have the ability for the diver to cut it away during an emergency and the fins need to go on your feet for propulsion. In general, this configuration works well for most anyone, but problems can arise that affect our ability to maintain horizontal trim.
Consider the diver in a proper trim position, horizontal in the water with the body within 10-20% of the midline at all times. This body position is accomplished by having the head up, the back slightly arched and knees up by flexing the glutes. In this position, the diver is very much like a see saw, with two opposing arms, and a center of gravity.
Depending on their configuration, a diver in good trim has a center of gravity based on the distribution of their weighting. The main components of establishing good trim centers on the BC, how the tank is attached to the plate and cylinder selection. Diving a separate BC and weight belt throw the diver out of trim. Using a backplate distributes the weight evenly along the divers back and also lengthwise along the main source of buoyancy, the wing. A backplate also creates a centerline around the diver’s core that allows much more precise buoyancy control.
BC and Backplate Selection
Wing and backplate design are currently being debated and is primarily a matter of personal opinion on which style is best. BC’s have undergone a radical departure from the early designs. Older style wings are often called the “horseshoe” or “taco bell” due to their shape. Newer designs encompass a “donut” style wing to circulate and distribute the gas bubble throughout the entire wing to increase buoyancy and eliminate a gas bubble trapped in one area of the wing. This helps control a diver’s trim and evenly distributes buoyancy over a larger surface area.
Backplates can be made using aluminum, stainless steel or ABS plastic. Aluminum is generally used for traveling or when a diver wishes to reduce weighting to distribute it elsewhere. Stainless steel is used to add additional weight and more often, to eliminate the need to wear a weight belt in the water. Divers will also switch back plate material types when switching from salt to fresh water diving environments. Plastic backplates are seldom used but some divers prefer them for traveling and other purposes.
Tank Selection
Not so many years ago tank selection was fairly easy. Your choice was usually limited to either a steel 72 or aluminum 80. Single tanks were almost universally outfitted with a yoke style K valve. Currently there are a multitude of tanks available and dual DIN outlet valves are common sights at most dive locations. More and more beginning divers are using a single tank with a dual outlet valve or a smaller set of twin cylinders with a manifold for recreational diving.
If you're in a wet suit, stay away from double steel tanks. Aluminum tanks with a weight belt that can be ditched in an emergency and are a much better option from a safety perspective. Twin steel tanks become much too heavy once the neoprene wetsuit crushes at depth. Redundant bladder BCs are typically frowned upon for numerous safety reasons.
If you're tall, a smaller tank such an HP Steel 80 is a generally a bad idea. The tank must be mounted so that you can get to the valve, but doing so places most of the weight far from the center of gravity. This can result in a head down posture due to the increased weight on the top-half of your body.
Conversely, if you are shorter, diving a large tank, such as an HP 120, is a bad idea as well. The length of the tank has the opposite effect. The tank valve will be close to your neck line and the bottom of the tank will extend past your hips. This causes two problems. One is the weight that will cause you to “pop a wheelie” while swimming and increases risk of silting out a dive site, getting your fins entangled in the line and loss of buoyancy. The other reason is that it affects your kick since the tank’s bottom is in the way of your legs while kicking.
Both options throw your body out of balance. One of the critical NTEC skills is to keep your trim within a 10-20% midline at all times. Ideally, you should choose a tank with dimensions that distribute a reasonable amount of weight over a larger surface area. Normal to longer type tanks distribute weight more evenly than short, fat tanks.
Weighting - Determining how much weight to carry
The first problem with weighting is that everyone usually carries too much weight. This problem is typically rooted back in the diver’s basic scuba course. Too often, the instructor would add weight if a student couldn't stay on their knees on the bottom of the pool or ocean. This usually leads to all sorts of problems including decreased trim and too large an air bubble in the BC, which further destroys trim and the ability to control position in the water.
Therefore, we need to reduce the weighting as much as possible. Not many divers know when to check the buoyancy. Often times, checks are conducted at the beginning of the dive, when, in fact, they ought to be done at the end of the dive. We are at our lightest at the end of the dive, with very little gas in our tanks, and this is where the buoyancy check ought to take place.
End the dive with close to 300 psi in your tank in a non-decompression situation. With your reg in your mouth, dump all the gas from your BC and drysuit, exhale, and don't kick. If you sink like a stone, take off some weight, give it to your buddy, and try again. If you don't sink, add some weight (small amounts, usually around 2 lbs or so). Get to where you sink slowly but evenly.
Proper placement of weight
Now that we have the correct amount of weight, let's consider how to position it. Single tank divers and double tank divers have different issues, so we'll address each separately.
First, consider which pieces of a diver’s equipment that are negatively buoyant. For the average NTEC diver, this includes:
Backplate
Tanks
Weight belt
Canister light
Fins
The materials and placement of each of these pieces will have a dramatic effect on the diver’s trim and buoyancy.
Single Tank Divers
Historically, most single tank divers wore a weight belt around their waist. Depending on their exposure protection and their tank selection, this one piece of equipment can cause the most problems in terms of balance. Requiring 20 pounds or more in one location means you'll have serious issues with balance underwater. This often leads to issues with trim as the weight tends to become an anchor. Add 40 pounds of lift in the form of a BC surrounding the diver’s chest and the diver will be seriously out of trim and balance underwater. Visualize a 200+ pound adult sitting on one end of a teeter totter while a 40 pound child sits on the other and you will get the idea. It is seriously out of balance and probably not much fun for either person.
There are several different solutions for transferring weight evenly along the diver’s body for the single tank diver:
A weighted single tank adapter can be attached along the axis of the tank and places an additional 6 pounds of lead over the plate to offset that weight on the belt.
Trim weight pockets are small pockets of weight that are attached to the tank bands.
If diving a backplate and wing, bolt on weight plates are made to attach directly to the backplate and can add an additional eight pounds of weight with a very low profile.
Adjusting trim using these tools can be tricky. The higher you mount the weight, the more likely you are to roll forward. The weight also needs to be balanced left and right. The items above are great for tweaking the amount of weight you'd like to shift and also good for travel.
Keep in mind, as you move your weight off your belt or waist, it often becomes non-ditchable. If you place too much weight in a non-ditchable location, you may place yourself in a dangerous scenario.
Double tank divers
Double tank divers have the opposite problem. The combination of loss of weight belt due to steel tanks and drysuits leads to them feeling as though they are going to go “heels over head.”
How can you alleviate the “heads down” feeling?
Switch to an aluminum backplate - The AL plate will take 4-6 pounds out of the plate and allow you to shift weight to wherever you feel it would lead to better trim.
Use different tank configurations - Tank selection dramatically changes a diver’s weighting and trim. Steel HP 100's are a common choice as are HP 130’s but there are other alternatives in cylinder choices. Each tank choice has it’s own buoyancy characteristics and experimentation is needed to determine which one is best for you. Some critical aspects to look for are which tank allows you to swim in a head’s up position to maintain good trim and buoyancy and buoyancy characteristics of the tanks when diving them at a low operating pressure. Experimentation is key to determine what tank setup is the correct one.
Tail weight – A 4-6 pound weight can be attached to the lower bolt of the doubles to help a diver maintain a heads up position throughout the dive.
Twin tank divers can also utilize strategies to get rid of the weight belt entirely.
A V-weight can add between 8 to 11 pounds of weight between your doubles. This puts the weight in a place where it can help establish better trim but can make it more difficult to haul doubles up the side of a small boat.
Bolt-on weight plates – These small weight plates can be bolted directly onto your backplate. The lower profile will also help reduce side to side rolling tendencies.
Lights
Canister lights are favored by technical divers and rapidly gaining acceptance by recreational divers. Technical divers find the increased light output helpful not only to use for signaling but also to increase their view while exploring deeper sights due to the low light level found at these depths. A canister light can add weight to the diver’s rig depending on the size and materials the light is made of. However, due to superior battery technology, lights are getting smaller and lighter so the weight of a battery canister does not have as much an affect on a diver’s weighting scheme as it once did. A common question is whether the additional weight of a battery canister will tip the diver to one side. For recreational divers, the weight of the canister is typically negligible since lighting requirements are usually minimal. For extended dives, stage bottles are placed on the left side of the diver in the NTEC gear configuration to help balance a larger battery canister pack.
Fins
Fins also affect the divers weighting strategy. Typically solid rubber fins are used by divers in an NTEC gear configuration. The fins are negatively buoyant and can assist in an overall weighting strategy. Plastic fins are generally frowned upon because they are generally too buoyant and cause a divers feet to float too high. This leads to problems with controlling the amount of air in the legs of the drysuit and can cause lack of buoyancy control and trim problems. Some divers choose ankle weights to get around this problem. Conversely, this destroys a diver’s trim by causing the legs to drop and the diver’s body to “pop a wheelie.”
Summary
Equipment is only half the equation to establishing proper buoyancy and trim underwater. Proper training is a must, to provide an experienced eye that can develop diver trim, balance, breathing and buoyancy. For a single tank or recreational diver, the NTEC workshop offered by many NAUI Tec instructors, offers a fundamentals based diving course focused on developing pin point buoyancy control, trim, balance and team diving approach utilized by many technical divers. NAUI Intro to Tech can help more experienced divers and those that wish to transition to double tank diving and eventually progress to technical level dive profiles.