Underwater trim Diver trim

diver trimmed weight far towards feet: static moments of buoyancy , weight cause feet rotate downwards, , thrust finning directed downwards



diver weight , centre of buoyancy aligned level trim: static moments of buoyancy , weight keep diver horizontal, , fin thrust can aligned direction of motion best efficiency

underwater trim diver s attitude in water, in terms of balance , alignment direction of motion. accurately controlled trim reduces swimming effort, reduces sectional area of diver passing through water. effect of swimming head angle, of 15° quite common in poorly trimmed divers, can increase in drag in order of 50%. slight head down trim recommended reduce downthrust during finning, , reduces silting , fin impact bottom.

the free-swimming diver may need trim erect or inverted @ times, in general, horizontal trim has advantages both reduction of drag when swimming horizontally, , observing bottom. horizontal trim allows diver direct propulsive thrust fins directly rear, minimises disturbance of sediments on bottom, , reduces risk of striking delicate benthic organisms fins. stable horizontal trim requires diver s centre of gravity directly below centre of buoyancy (the centroid). small errors can compensated easily, large offsets may make necessary diver exert significant effort towards maintaining desired attitude, if possible.

the position of centre of buoyancy largely beyond control of diver, though cylinder(s) may shifted in harness small amount, , volume distribution of buoyancy compensator has large influence when inflated. of control of trim available diver in positioning of ballast weights. main ballast weights therefore should placed far possible provide approximately neutral trim, possible wearing weights around waist or above hips on weight belt, or in weight pockets provided in buoyancy compensator jacket or harness purpose. fine tuning of trim can done placing smaller weights along length of diver bring centre of gravity desired position. there several ways can done.

ankle weights provide large lever arm small amount of weight , effective @ correcting head-down trim problems, addition of mass feet increases work of propulsion significantly. may not noticed on relaxed dive, there no need swim far or fast, if there emergency , diver needs swim hard, ankle weights significant handicap, particularly if diver marginally fit conditions.

tank bottom weights provide shorter lever arm, need larger proportion of total ballast, not interfere propulsive efficiency way ankle weights do. there not other convenient places below weight belt add trim weights, effective option carry main weights low necessary, using suitable harness or integrated weight pocket buoyancy compensator allows weights placed correctly, there no need longitudinal trimming.

a less common problem found when rebreathers have counterlung towards top of torso. in case there may need attach weights near counterlung. not problem, , weight pockets purpose built rebreather harness or casing, , if necessary weights can attached harness shoulder straps.

buoyancy shifts

at times during dive buoyancy compensator partially filled, , air in rise highest part of bladder can reach without having flow downhill on way there. may cause air trapped in 1 side of bladder @ times, may upset trim rotating diver side more air in shifts upward. particularly prevalent horseshoe style wing bladders, there not connection between sides @ bottom of wing.

this seldom of problem, , can corrected trimming shoulders until air balances between sides. mean if diver rolls 1 side air shift upper side , tend hold diver in position. if diver trims steeply head or down, air flow higher end of bc , tend keep diver stable in position. can exacerbated similar more extreme air shifts in dry suit, main reason why dry suits should dived minimum air necessary expand undersuit.