Balance a hollow crankbait by comparing the mass of the complete, sealed lure with the water it displaces. Include the printed structure, bill, hooks, rings, ballast and finish. Use the calculation to choose a starting weight; weigh and float-test the assembled prototype before treating it as floating, suspending or sinking.
1. Define the finished lure before adding weight
Choose the body dimensions, shell material, wall thickness, bill, hooks, split rings, attachment hardware and coating. Generate the print parts and use the as-built mass estimate, which accounts for modeled internal structures. Changing the print setup or hardware after balancing changes the result.
2. Compare displaced water and total mass
For the fully submerged model, net buoyancy expressed as a gram-force equivalent is displaced-water mass minus finished-lure mass. At the example water density of 998 kg/m³, 1 mL of displaced water has a mass of 0.998 g. The value is a force-equivalent readout in grams, not extra ballast already installed.
Net buoyancy (gram-force equivalent) = displaced volume (mL) × water density (g/mL) − finished mass (g)
A positive value indicates upward force when fully submerged; a negative value indicates downward force. Near zero is a starting point for a suspending prototype. The studio’s ±0.1 g suspending label is a numerical classification, not a claim about manufacturing tolerance or how long a real lure will suspend.
As a first estimate, additional ballast mass is the current net-buoyancy equivalent minus the desired remaining upward margin. Regenerate the parts afterward: changing a ballast ball can also change its pocket or supports and therefore the finished mass. The final as-built readout matters more than a ball-only subtraction.
3. Keep weight amount separate from placement
Total weight determines only part of the behavior. Record the ballast’s fore-aft station and height, and inspect the centers of mass and buoyancy. Moving a weight can change rest attitude and righting behavior without changing its own mass. Keep it clear of the wire path, hook attachments and shell walls, then regenerate and inspect the parts.
4. Compare one documented change
Keep a baseline source file. Compare variants with the same body, materials, hardware, print options and water density. Our public square-bill example changes one tungsten-alloy ball diameter while rebuilding every variant’s print structures. It reports mass and fully submerged buoyancy estimates; it does not report a swimming trial or physical float test.
5. Weigh, seal and float-test the prototype
Weigh the printed parts and actual hardware before final assembly. Account for glue, paint and clear coat that differ from the modeled finish. After sealing, record the complete weight, check for leaks and observe rest attitude in the intended water. Make small, recorded corrections rather than assuming nominal filament density predicts the finished lure exactly.
Why does the lure sink or sit nose-down?
If it sinks unexpectedly, compare the actual dry weight with the as-built estimate, verify hooks and rings, and check for water ingress. A heavier finish, different hardware or unmodeled infill can consume the planned buoyancy margin. Recheck the water-density input rather than treating freshwater and saltwater as interchangeable.
If it floats but sits nose-down, inspect weight placement, the bill and front hardware as well as the total mass. A positive fully submerged buoyancy value does not prescribe a level waterline. Check the actual rest attitude before changing the line tie or expecting a swimming adjustment to solve a static-balance problem.
What the calculation cannot tell you
Mass and fully submerged displacement do not establish the exact waterline, rise or sink speed, maximum diving depth, swimming stability, print strength or catch performance. Those require additional models or physical measurements. After a float check, compare swimming behavior under recorded line and retrieve conditions, keeping simulated and measured results separate.