Horizontal Trim 101 – Levers
The horizontal trim video below is part of the UTD Essentials DVD:
Overview
The horizontal position is often called prone, superman, or skydiver position. For a scuba diver, the horizontal trim is the most efficient way to move through the water. In addition, the horizontal trim provides vertical drag, helping divers maintain a constant depth.
However, getting into this position is often difficult for most new divers. On the various scuba boards, there are daily questions regarding: “too head heavy”, “floaty feet”, “can’t stay trim”, etc. Fortunately, this problem is solvable.
In managing trim, there are two major forces that the dive must balance – buoyancy and weight.
- Buoyancy is the upward force on the diver. This may include BCD, wetsuits, boots, fins, tanks, and of course our own personal floatation.
- Weight is the downward force on the diver. This may include weight belts, trim weights, backplates, lights, fins, tanks, regulators, and any additional item on the diver that sinks in water.
A diver’s trim pivots around a point, and this is usually around the divers midpoint. In the diagram below, this is denoted by the red triangle. Think of the diver as a see-saw across this pivot. For example, if the diver has too little buoyancy or too much weight in the upper body, he will be head heavy.
Buoyancy
The buoyancy compensator (BC) or buoyancy control device (BCD) is what scuba divers think of when required to manage their position in the water. Whether you have a jacket/vest, back-inflate, or a hybrid, it’s important to be familiar with the specific ways your BC manages air and learn to use it to its maximum potential.
Unless there is an obstruction, air will migrate to the highest point of the BC. As a consequence, the BC’s buoyancy is also concentrated at this point and this will have impact on trim. The BC’s shape, width, length, and of course location on the body can all come into play.
Also consider the buoyancy of the wing at the beginning of the dive and at the end of the dive. At the beginning of the dive, the wing will have its maximum lift – enough to offset gas in tanks and any exposure suit protection compression. At the end of the dive at 10′, the wing will be empty with 500 psi left in the tank.
The diagram below highlights the horizontal diver’s wing BC’s buoyancy in blue. Please note the position of lift compared to the pivot point.
In addition to the BC, the scuba diver’s exposure protection also provides buoyancy. For a dry suit diver in particular, the suit can be actively managed to offset trim issues and fine tune his attack in the water.
The diagram below highlights the drysuit’s buoyancy in blue.
Weight
While weight is needed for a diver to be comfortably submerged under water, the placement of weight can be used to a diver’s advantage regarding trim. Because weight is constant throughout the dive it is effective in managing trim and is often the first gear adjustment that divers will look to make.
Tanks’ buoyancy characteristics affects trim. Whether singles or doubles, moving the tanks up and down along the body will adjust trim, respectively. For example, if the diver is too head heavy, a tank can be moved lower on the body.
Hard weights are very effective in managing trim. Getting familiar with trim weights is beneficial, especially in cold water where more exposure protection and weight is used. Feature rich BCs often have trim pockets to help with adjustments. However, single weight pockets, such as the XS-Scuba weight pocket, can be used on cam bands. Some divers even thread weights into their shoulder harness.
Backplates are popular since it helps place weight in an area that promotes good trim. A stainless steel backplate will move up 6 lbs. With a heavy singles tank adapter (STA) or weight plates, even more weight can be moved up the diver’s body.
Doubles or twin tank divers also employ tail weights and v-weights to adjust their trim.
For an example of how different divers employ different weights, it’s worth skimming the drysuit weighting thread on Dive Matrix – backplates, weight belts, no weight belts, cam weights, tail weights, v-weights, STAs, etc.
Fins can also be a source of weight. Many drysuit divers prefer the negatively buoyant ScubaPro Jet Fins for this specific reason.
Lynne (TSandM) wrote a very good explanation of how to distribute weights to promote trim.
The diagram below highlights the areas of weight in green.
Leverage
To maximize the use of buoyancy and weight to affect trim, we must also discuss leverage. In addition to the obvious use of buoyancy and weights at the ends of the body, the scuba diver’s body itself can provide leverage.
It is important to note that many trim issues can be resolved by proper body position. In addition, improper body position exacerbates trim issues. Head looking down position often leads head heavy trim. As the result, the first fix to any perceived trim issues is body adjustment.
The diagram below highlights how body position (extensions and contractions) can change leverage and therefore impact trim.
From a standpoint of physics, this is a horrible analogy. The condition you describe, balancing the positive forces of buoyancy with the negative forces of weight, are more akin to a beam load. With the shifting of weight, volume and centers of gravity you describe, as well as the corresponding moments and moduli, more accurate descriptives, as well as mathematical proofing, can be provided. Your theory that the body rests on a fulcrum, while floating in the water and constantly changing weight distribution, is completely inaccurate. Unlike a body in the water, subject to the changes in forces brought on by buoyancy and pressure, a fulcurm would be a fixed, unmoving, unreactive point. While your theory may seem reasonable while trying to teach a layman, it is not what is actually happening in practice.
Thank you for feedback. The purpose of the post is for practical application and focus is explicitly on the pitch plane.
As such we can discuss this in terms of fulcrum and levers around this fixed point.
However, if you have a suggestion for making this concept easier to understand and implement, please let me know.
The best informations i ever found about TRIM, and i searched a lot, are here.
Don,
Thanks for this post. I agree that the fulcrum explanation makes the concepts accessible. What I am finding, now that I’ve added a primary light to my single-tank kit (wetsuit) is that my issue is now yaw control rather than pitch. Having removed the 6-lb tank adapter weight upon adding the light to avoid being overweighted, I found my first dive required a lot of work to maintain trim without rolling to the right. I am going to try adding 2 lbs to the left side of my top cam band and will let you know how it goes. But, I guess my point is, adding equipment can have an effect on the yaw plane as well as the pitch plane. If I were diving a stage, the light might be balanced by the stage. But, for now, I don’t see another option other than a cam weight. Have you experienced the same issue?
Jude,
Thank you for reading the post.
Adding equipment can absolutely affect yaw control. This is additional weight pressing down on one side of the body.
Most people will naturally compensate for 1# of difference by body position alone. >2# requires either adjustment of weight or use of the BC.
The best example of this is diving a stage. With a Wing style BC, the weight of the stage can be compensated placement of air (buoyancy). More gas is put into the side of the wing that the stage is on. As the stage gets lighter from use (gas is consumed), then less air is needed in the BC to compensate.
My recommendation is get familiar with using air side to side to compensate, especially for stages which change weight during the dive. For fixed items, shifting weight around the weight belt is fine.
Best of luck.
-Don
Nicely done! A great explanation – with excellent illustrations – about the basics of trim, and the only place I’ve seen an accurate explanation of the control of “angle of attack” using the “bent knees” method.