Travel-Lift Sling Placement and Hull Load

How a travel lift's two slings share a boat's weight, and why half the displacement is the wrong number to check them against. The load split is simple statics -- the sling nearer the centre of gravity carries more -- and it matters because both the sling and the hull are rated. A boat whose centre of gravity sits well aft puts a disproportionate share on the aft sling, and the machine's own capacity check, which is usually made against half the displacement, passes a lift the governing sling cannot take. That is the comparison reported here: what the split actually is, and whether a half-the-weight check would have missed it. Where the slings LAND is the part that damages boats, and it is not arithmetic. A sling under a shaft, a strut, a folding propeller, a transducer, or a thruster tunnel destroys the appendage; a sling under an unsupported hull panel between frames crushes it. The correct positions are under bulkheads, frames or engine beds, and many production boats have marked or documented sling positions precisely because the right answer is not obvious from outside the hull. That constraint usually decides everything: if the only usable forward position gives an uneven split, the answer is slings rated for the actual load rather than a repositioning that puts a sling under the shaft. Sling angle is the third load path. Slings converging from the machine's beams to a narrower pickup at the hull are not vertical, so their tension rises above the vertical load they carry and they pull inward, putting compression across the hull that the vertical calculation does not show. Wider spacing is better for stability and worse for convergence, and both are subject to landing on structure. This is a two-sling static split on ENTERED geometry: it does not locate the centre of gravity, which is where the whole calculation starts and which is rarely documented; it does not evaluate the hull's local capacity at the sling positions, size the slings or their protection, or address the lift's own stability, its tyre loading, or the yard's ground bearing. The boat's documented lifting points, the yard's rigging procedure, and the lift manufacturer govern.

each sling carries the displacement times the distance from the OTHER sling to the centre of gravity, over the sling spacing; sling tension = the vertical load / sin(angle) and the inward horizontal component = the vertical load / tan(angle).

The two-sling static split as rigging practice writes it. The centre of gravity is ENTERED and is rarely documented. It does not evaluate the hull's local capacity at the sling positions, size the slings or their protection, or address the lift's own stability, tyre loading or ground bearing.

Simple statics on two supports.

Estimate. AHJ and licensed professional govern.

Field names used by the API: displacement_lb, sling_spacing_ft, cg_from_fwd_ft, sling_wll_lb, sling_angle_deg, fwd_sling_lb, aft_sling_lb, aft_share_pct, difference_lb, horizontal_component_lb, sling_tension_lb

• The centre of gravity is entered it is where the whole calculation starts and is rarely documentedthe boat's own records
• No local hull capacity a correctly loaded sling in the wrong place still damages the boatthe documented lifting points
• Two slings only a four-point or cradle lift distributes differentlythe yard's rigging procedure