Boat Lift Bunks and Cradles: How the Part That Touches Your Hull Should Work
Quick answer
The cradle is the frame that rises with the boat, and the bunks are the padded longitudinal boards or beams on the cradle that actually touch the hull. A good setup puts the boat's weight on structurally stiff parts of the hull (along stringers and near the transom), keeps the center of gravity between the lift points, and uses bunks long enough to support the engine end. Most fit problems come from bunks that are too short, too far apart, or set at the wrong angle.
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The cradle is the moving frame of a boat lift: cross beams (sometimes called I-beams, C-channels, or cradle beams) that hang from the cables, hydraulic rams, or arms. The bunks are the long boards or extrusions bolted across those beams, running fore and aft, that the hull rests on. Everything the lift does ends up as pressure on a few square feet of hull, so bunk layout matters more to the boat than almost any other lift choice.
The load path, from hull to cable
Follow the weight of a 6,000 lb runabout through a typical 4-post lift:
- The hull bottom presses on two bunks. The hull spreads that load into its stringers and bulkheads, which is why bunk position relative to the stringers matters.
- Each bunk passes its load into bunk brackets at each cradle beam, so a bunk is a beam spanning between two or more supports.
- The cradle beams carry the bunk loads out to their ends, where they hang from cables or rest on lift arms.
- Cables carry the load up to the top beam, into the drive, and down the posts or pilings to the bottom.
A weakness anywhere in that chain shows up as a symptom somewhere else. A bunk that overhangs too far aft lets the outboard end of the boat sag and loads one cradle beam more than the other. A boat sitting forward of center overloads the front cables.
Cradle types
| Cradle type | Typical lifts | Best for | Limitations |
|---|---|---|---|
| Two cross beams with two adjustable bunks | 4-post vertical, elevator, many cantilever | Most V-hull runabouts, center consoles, deck boats | Needs correct bunk spacing and angle for each hull |
| Two cross beams with four bunks | Larger vertical lifts | Wide, heavy, or stepped hulls; large center consoles | More adjustment to get all four bearing evenly |
| Pontoon cradle (flat bunks or tube saddles) | Vertical, cantilever, and floating lifts set up for pontoons | Pontoons and tritoons | Center tube height must be matched on tritoons |
| V-cradle with fixed angled bunks | Cantilever lake lifts, PWC lifts | Hulls matching the fixed angle | Little adjustment; poor match causes point loading |
| Formed PWC cradle with V-sides | PWC lifts and drive-on ports | Jet skis and small craft | Model-specific fit |
| Floating lift tanks with bunks | Floating hydraulic and air chamber lifts | Saltwater and fluctuating water | Bunk geometry tied to tank layout |
Four rules for a good fit
1. Support the heavy end
Outboard and sterndrive boats carry a large share of their weight near the transom: engines, batteries, fuel, and often a livewell or ballast. Bunks should extend to within about 6 to 12 in of the transom (or to where the hull bottom ends, on boats with a notch or pad), so the transom and engine mounts are not cantilevered off the back of the bunks. Check the hull builder's guidance for boats with steps, jack plates, or setback brackets.
2. Bear on stiff structure
Bunks should sit under or close to the hull's longitudinal stringers, not on unsupported panels between them. A bunk under a flexible panel can leave a visible dent or a stress crack pattern in the gelcoat over time.
3. Match the hull angle
A flat bunk under a sloped hull touches only along one edge. Adjustable bunk brackets let the bunk tilt to match the deadrise at that point, spreading the load across the full bunk width. The details, including strakes and pontoon tubes, are in adjusting bunks for your hull.
4. Keep the center of gravity between the lift points
Many manufacturers rate lift capacity with the load centered. A boat that sits well forward or aft of center loads one end harder, so a lift that is adequate for the boat's weight can still be overloaded at one end. Use the capacity calculator to estimate loaded weight, and see capacity sizing for the margin rule (rated capacity at least 15 to 25 percent above loaded weight).
Worked example: is a 12 ft bunk long enough?
A 21 ft bowrider has a hull length of about 20 ft 6 in at the bottom and a 250 hp outboard. Its loaded weight, from the boat weight tables at the boat weight database and a fuel and gear allowance, is estimated at 4,400 lb, with the center of gravity roughly 7 ft forward of the transom.
- Bunks are 12 ft long and set 9 in forward of the transom. They support the hull from 0.75 ft to 12.75 ft forward of the transom.
- The CG at 7 ft forward sits 6.25 ft from the aft end of the bunk and 5.75 ft from the forward end, close to the bunk's midpoint (6.75 ft). That is a balanced setup.
- Now suppose the previous owner set the bunks 3 ft forward of the transom to clear a swim platform bracket. The bunks now span 3 ft to 15 ft. The CG at 7 ft is only 4 ft from the aft end and 8 ft from the forward end. The aft 3 ft of hull, carrying the 500 lb class outboard, is unsupported.
The second layout loads the aft bunk brackets harder, flexes the transom area, and makes the boat sit bow high. Moving the bunks aft or adding short aft extension pads fixes it. Bunk length in the range of roughly 55 to 70 percent of the hull's bottom length is a common target on two-beam cradles, with the bunk's center near the boat's CG.
Matching cradles to boat types
| Boat type | Typical bunk setup | Common mistake |
|---|---|---|
| Bass boat (low deadrise, pad hull) | Two bunks set wide, near flat, long enough to support the pad and transom | Bunks too narrow, letting the hull rock; bunks ending short of a heavy transom |
| Bowrider or runabout (moderate V) | Two bunks angled to match deadrise, under stringers | Flat bunks touching only the inner edge |
| Deep-V center console | Two or four angled bunks, sometimes taller to keep the keel clear | Keel resting on a cradle beam and carrying the load |
| Wake boat (heavy, ballast) | Longer bunks, often four on larger hulls; verify tower and canopy clearance | Ignoring ballast weight in the capacity check |
| Pontoon or tritoon | Bunks or saddles under each tube | Center tube on a tritoon floating above or bearing too hard |
| PWC | Formed or V-side cradle matched to hull | Hull sitting on a strake or intake grate |
For boat-specific advice, including towers and ballast, see lifts by boat type and wake boat lifts.
Field check: signs your bunks and cradle are wrong
Safety: Look at the hull from the dock or from a boat alongside. Never stand, swim, or reach under a raised boat or cradle to inspect bunks. To look closely, lower the cradle so the boat floats free, or lower the empty cradle onto its stops. Keep people out of the water near a powered lift; shock risk from faulty dock wiring is real, and lift circuits should have GFCI or ELCI protection.
- Boat lists to one side when raised: bunks at different heights or one bracket slipped. See boat sits crooked on lift.
- Bow high or stern low: bunks too far forward, or boat not driven far enough on.
- Carpet worn through at one end only: point loading at that bracket; the bunk is twisted or the angle is wrong.
- Hull marks that follow the bunk edge: the bunk is not tilted to match the hull.
- Keel touching a cradle beam before the bunks take load: bunks too low.
- Boat drifts off-center while lifting: bunks too far apart or no guide posts. See guide posts and V-sides.
Freshwater, brackish, and saltwater differences
In freshwater lakes, carpeted treated-wood bunks often last 8 to 15 years. In brackish and saltwater, bunk brackets and fasteners usually set the life: dissimilar metals (aluminum cradle, steel bolts, copper-treated wood) corrode quickly unless isolated. Floating lifts in salt water often use aluminum or composite bunks for that reason. The trade-offs are covered in bunk materials, and when the boards are tired, bunk board replacement walks through the job.
What most guides get wrong
- "Any cradle fits any boat of the right weight." Capacity is necessary but not sufficient. A pontoon cradle under a deep-V or a flat cradle under a stepped hull can damage the boat at half the rated load.
- "The keel should carry the weight, like on a trailer." On most lift cradles, bunks are designed to carry the hull weight and the keel should clear the cross beams. Some hull builders call for keel support; follow their guidance where it exists.
- "Set it once and forget it." Bunk brackets slip, carpet compresses, and wood warps. Check bunk position every season, and again whenever you change boats.
Frequently asked questions
What is the difference between a boat lift bunk and a cradle?
The cradle is the whole moving frame that rises and lowers with the boat, usually two or more cross beams hanging from cables or attached to arms or rams. Bunks are the padded longitudinal boards or extrusions mounted on that frame that actually touch the hull. You adjust the bunks to fit a boat; the cradle is part of the lift's structure.
How long should bunks be on a boat lift?
A common target is roughly 55 to 70 percent of the hull bottom length on a two-beam cradle, positioned so the bunk's middle sits near the boat's center of gravity and the aft end reaches within about 6 to 12 inches of the transom. Heavier outboard boats need the aft support most. Check the hull builder's guidance for stepped or notched hulls.
Should the keel touch the boat lift cradle?
On most lift cradles, no. The bunks are designed to carry the hull weight, and the keel should clear the cross beams by a small margin so it does not become a single concentrated support point. Some hull builders call for light keel support, so check the boat's manual. A keel bearing hard on a beam usually means the bunks are set too low.
Can I put a pontoon boat on a regular boat lift?
Only if the lift has, or is converted to, a pontoon cradle with bunks or saddles under each tube. A V-hull cradle will not support pontoon tubes properly and can dent them. Many lift makers offer pontoon conversion kits. Confirm the lift's width, capacity, and the tube spacing of your boat before buying or converting.
Why does my boat sit crooked on the lift?
Usually the bunks are at different heights, one bunk bracket has slipped, the boat was driven on off-center, or the cables are out of level. Lower the boat, check that both bunks measure the same height above the cradle beams and the same distance from the lift centerline, and level the empty cradle before reloading.
Sources and further reading
- Boat lift manufacturer owner's manuals and cradle setup instructions (bunk spacing, bracket adjustment, centered-load ratings)
- Boat builder owner's manuals (support points, lifting and storage guidance for the hull)
- ABYC standards, abycinc.org
- U.S. Coast Guard boating safety resources, uscgboating.org
- NFPA 70, National Electrical Code, Article 555, nfpa.org