Two slings is a beam on two supports
Everything about a travelift pick reduces to one equation. Take moments about the aft sling and the forward sling carries the weight times the distance from the aft sling to the balance point, divided by the spread. With 12,000 lb, a balance point 15 ft aft of the bow and slings at 9 ft and 22 ft, the forward strap takes 12,000 times 7 over 13, which is 6,462 lb, and the aft strap takes the other 5,538.
The two have to add to 12,000, and the page prints that sum back as the only self-check available to it. If your own arithmetic ever gives two reactions that do not total the weight, the mistake is in the moment arm.
The number nobody knows is the balance point
Displacement can be weighed. Sling stations can be chalked on the rail. The longitudinal centre of gravity is found by lifting the boat and seeing which way she hangs, which means the yard discovers it about four seconds after it stops mattering.
On this boat a foot of error moves 923 lb from one strap to the other — nearly 8 percent of the whole displacement, from an error most owners would not notice. That is why the page sweeps the forward station rather than printing a single answer. Read the row you think you are on, then read the rows either side, and treat the band as the answer.
Moving a strap moves the share, not the total
The two reactions always add to the displacement, so moving a strap never changes the total — it changes the share. Move the forward strap from 9 ft to 12 ft on this boat and the forward reaction climbs from 6,462 to 8,400 lb while the aft one falls from 5,538 to 3,600. Narrowing the spread exaggerates whatever imbalance is already there; widening it pulls the split back toward even, because a wider pair is less sensitive to where the balance point sits between them. The one arrangement that gives a genuine 50/50 is the pair centred on the balance point, which on this boat means 8.5 ft and 21.5 ft rather than 9 and 22.
Where the straps land structurally is a separate question and a real one — over a bulkhead is not the same as over an unsupported panel, and a strap that fouls a transducer, a prop shaft, a strut or a folding propeller ruins somebody day regardless of the arithmetic. None of that is visible to a moment calculation.
Set down is a different problem
Once she is on the hard the load path changes completely. The keel takes most of it through the blocks, and the stands are there to stop her falling over rather than to hold her up. What that split actually is depends on how the yard blocks boats, which is why it is a field on the form rather than an assumption. Divide the remainder by the number of stands and you get a figure that is useful for thinking with and worthless as a design value, because no two stands ever end up carrying the same load.
Boats do come off stands. They come off in windstorms, when somebody works under one and shifts a stand, when the ground under a pad softens after rain, and when a chain between opposing stands is left off. None of that is arithmetic and none of it is this page to solve.
Questions people ask
How do I know where my boat centre of gravity is?
You mostly do not, until she is lifted. Some builders publish a lifting diagram with sling stations marked, which is the best source; otherwise the yard finds it on the first pick by watching how she hangs and adjusting. Because of that, the page sweeps the forward sling station across seven positions instead of giving one number, and prints what a foot of error is worth.
Does moving the slings closer together make the lift gentler?
No, the opposite. A narrower spread means a shorter lever, so both reactions go up. On the default boat, moving the forward strap from 9 ft to 12 ft raises the forward reaction from 6,462 to 8,400 lb and drops the aft one from 5,538 to 3,600 — the total is fixed at the displacement, so what changes is the share. Narrowing the spread exaggerates the imbalance and widening it pulls the split toward even, with a genuine 50/50 only when the pair is centred on the balance point.
What happens if the balance point is outside the two slings?
The arithmetic gives one sling a negative load, which would mean holding the hull down. Straps cannot do that, so the boat pivots on the nearer sling instead. The page detects this and refuses to print load figures for it, because the numbers would describe a situation the slings cannot produce.
How long do the lift straps need to be?
Girth at the station plus the reach up to the spreader beam on each side. Girth is the tape distance from toerail to toerail following the hull curve, which is a good deal more than the beam on a deep hull. The reach above the rail is a property of the machine, so the yard has that figure and you do not.
How much weight goes on each boat stand?
That depends entirely on how the yard blocks the boat, and the split between keel blocks and stands is a field on this form for exactly that reason. Whatever the arithmetic says, the per-stand figure is not a design value — stands are set by hand, no two carry the same load, and the yard owns that decision along with the blocking.