Calculator
Formula Used
Load sinkage = added mass ÷ (water density × waterplane area)
Heel loss = half bridge deck beam × sin(heel angle)
Trim loss = distance to critical point × sin(trim angle)
Dynamic squat = squat coefficient × speed² ÷ (2 × gravity)
Effective clearance = static clearance − load sinkage − heel loss − trim loss − tide rise − wave allowance − dynamic squat
Safety margin = effective clearance − safety reserve
How to Use This Calculator
Select the unit system first. Enter the measured static bridge deck clearance of the Lagoon 440. Add the reference and current loaded displacement. Enter the combined waterplane area for both hulls. Then add heel, trim, tide, wave, speed, squat, and reserve values. Press Calculate. The result appears above the form.
Example Data Table
| Case | Static Clearance | Current Displacement | Wave Allowance | Speed | Estimated Use |
|---|---|---|---|---|---|
| Light marina check | 0.72 m | 12,500 kg | 0.05 m | 3 kn | Dockside estimate |
| Cruising load | 0.72 m | 13,500 kg | 0.18 m | 6 kn | Normal passage |
| Heavy rough case | 0.72 m | 14,500 kg | 0.30 m | 7 kn | Conservative check |
Why bridgedeck clearance matters
A Lagoon 440 carries its bridge deck between two hulls. The space under that deck affects comfort, speed, and safety. When waves rise into the tunnel, the deck can slam. Slamming feels harsh. It also adds shock loads to fittings, furniture, and crew. This calculator turns several small effects into one clearance margin.
Physics behind the estimate
Clearance is not fixed after the boat leaves the dock. Extra stores, water, fuel, crew, tools, and dinghy weight push the hulls deeper. The added immersion depends on seawater density and the waterplane area. Heel lowers one side of the bridge deck. Trim lowers the forward or aft end. Tide, wave crest height, and speed related squat reduce the usable gap further.
Planning with real numbers
Use measured values whenever possible. Measure static clearance in calm water. Enter the present displacement, not only brochure displacement. Add a reserve because tanks move, guests shift, and waves are uneven. For a conservative passage, use the highest expected wave crest in the bridge deck tunnel. Use the largest tide rise above your reference waterline.
Reading the result
The final clearance is the estimated free gap at the critical point. The safety margin subtracts your reserve. A positive margin means the case has room left. A negative margin means the setup may slam or touch water under the chosen assumptions. The warning text should be treated as a planning aid, not a survey certificate.
Best practice
Repeat the calculation for several loading cases. Compare a light delivery case, a cruising case, and a fully provisioned case. Change heel and trim to match the route. Sheltered motoring can use small waves. Open water should use larger values. Keep notes after each trip. Real observations make the next estimate better and safer.
Limits of this tool
Hull shape is complex, so this model stays practical. It assumes the added load sinks both hulls evenly through the entered waterplane area. It also treats heel, trim, waves, tide, and squat as independent deductions. Survey measurements, naval architecture data, and sea trials are better for final decisions. Still, a clear margin gives a useful early warning before loading the boat heavily. Use it before every serious cruising plan.
FAQs
What is bridge deck clearance?
It is the vertical distance between the underside of the bridge deck and the water surface. On a catamaran, this space affects wave slap, comfort, and safe movement through chop.
Is this calculator only for the Lagoon 440?
It is tuned for Lagoon 440 style inputs, but the same physics can help with other cruising catamarans. Change the dimensions and displacement values to match the vessel.
Why does extra load reduce clearance?
Extra mass displaces more water. The hulls sink until buoyancy balances the added weight. The calculator estimates that sinkage from water density and waterplane area.
Why does heel affect bridgedeck clearance?
Heel rotates the boat. One side of the bridge deck moves closer to the water. Wider bridge deck spacing can create a larger vertical loss for the same heel angle.
What is dynamic squat?
Dynamic squat is a speed related downward allowance. It is stronger in shallow or restricted water. The coefficient lets you choose a conservative value for planning.
Should tide be entered as a deduction?
Use tide rise when your clearance reference is lower than the expected water level. Higher water reduces the free gap between the deck underside and the surface.
What safety reserve should I use?
Use a reserve that matches risk. Calm marina checks can use a small reserve. Open water, heavy load, or unknown waves should use a larger reserve.
Can this replace a marine survey?
No. It is a planning calculator. Use real measurements, manufacturer data, and professional inspection when clearance is critical for safety or vessel operation.