Adjusting Boat Lift Bunks for Your Hull: Deadrise, Strakes, Pontoon Tubes, Spacing, and Height

Bunk adjustment is geometry. A V-hull is two sloped planes meeting at the keel; a pontoon is two or three round tubes; a bass boat is a nearly flat pad flanked by shallow V panels. The goal is the same in every case: spread the boat's weight across as much stiff hull as possible, at the correct angle, with the center of gravity between the lift points.

Step 1: measure your hull, not the brochure

Take these measurements with the boat on its trailer, on the lift lowered just enough to see, or floating at a dock where you can reach the bottom safely. Never measure from under a raised boat.

1. Deadrise at the transom and at mid-hull. Put a digital angle finder or phone inclinometer flat on the bottom panel just outboard of the keel, at the transom and again about halfway forward. Most hulls have more deadrise forward (variable deadrise). Typical transom figures: flat-bottom and jon boats 0 to 5 degrees, bass and pad hulls 8 to 14 degrees with a flatter pad, runabouts and bowriders 16 to 20 degrees, deep-V center consoles 20 to 24 degrees.
2. Strake positions. Measure from the keel centerline to the inner and outer edge of each lifting strake at the transom and at mid-hull. The flat panels between strakes are your candidate bunk positions.
3. Stringer positions. Look inside through hatches or the bilge and measure from the centerline to each stringer. On many runabouts the main stringers sit roughly 10 to 20 in either side of centerline, but construction varies widely, especially on boats with molded liners. If you cannot see them, ask the builder or a dealer.
4. Keel offset. Measure how far the keel (including any keel guard or strip) projects below where the two bottom panels would meet. This is often 1/2 to 2 in.
5. Where the bottom ends. Note the transom position, any notched transom or setback bracket, and where the chine ends forward.

Step 2: choose bunk spacing

Bunk spacing is a trade-off between roll stability and fit. Wider bunks resist the boat rolling off center as it lifts, but on a V-hull the further outboard a bunk goes, the taller it must be and the closer it gets to the chine, where the hull curves and gets stiffer only at the very edge. Narrow bunks sit on the steepest, most supported part of the V near the keel, but give the boat a narrow base that rocks.

In practice, installers usually place each bunk under a stringer or within a few inches of one, on the flat panel between the keel and the first strake or between the first and second strakes. On most mid-size V-hulls that lands the bunk centers roughly 12 to 24 in either side of centerline. Use the stringer measurement first and the strake measurement second: the bunk should land fully on one flat panel, with its full width in contact.

Strakes: bear on the panel, not the strip

Lifting strakes are narrow, raised strips molded into the bottom to shed spray and help the hull climb onto plane. They are not designed as bearing surfaces. A bunk that straddles a strake touches only the strake's peak, which concentrates the load on a narrow line and can crack gelcoat or stress the laminate. It also lets the boat rock on the strake. If the ideal stringer position falls on a strake, move the bunk inboard or outboard to the adjacent flat panel, or use a narrower bunk that fits between strakes. Some builders design flat landings into the bottom for lift or trailer bunks; if yours has them, use them.

Step 3: calculate bunk height and angle

For a V-hull, the bottom rises from the keel at the deadrise angle. At a distance y from the centerline, the hull bottom sits above the keel line by roughly y x tan(deadrise). Add the keel offset and the keel clearance you want above the cradle beam, and you have the bunk contact height above the beam.

Why the angle matters so much

A properly tilted bunk touches across its full width. A flat bunk under a sloped hull touches only along its inner edge.

Where the hull weight should bear

• Along stringers and bulkheads. These are the hull's internal beams. A bunk directly under or close to a stringer puts load straight into structure.
• Toward the transom. Engine, batteries, and fuel concentrate weight aft. The aft end of each bunk should reach within about 6 to 12 in of the transom so the transom is not cantilevered. On boats with a notched transom or a jack plate, support the hull bottom right up to the notch.
• Not on through-hull fittings, transducers, trim tab mounts, or intake grates. Move the bunk or cut a relief in the bunk if any of these fall on the bunk line.
• Not on the chine. The chine is stiff at its edge but the panel inboard of it often is not, and a bunk at the chine gives almost no roll support.

Keel loading versus bunk loading

Trailers often use keel rollers that carry a large share of the boat's weight on the keel, because the keel sits over the hull's strongest centerline structure and the boat must roll on and off. Boat lift cradles generally do the opposite: two (or four) bunks carry the weight, and the keel clears the cross beams. The reasons are practical. A lift cradle beam is a single point support across the keel, not a series of rollers, so a keel resting on it carries a large concentrated load at one spot. And because the cradle beams flex slightly under load, a keel that touches one beam can end up carrying far more than its share.

Some builders, particularly of certain fishing and offshore hulls, do specify keel support for storage. Where the boat manual says so, add a keel pad (a carpeted block on the cradle beam set at the keel line) and set it to touch lightly when the bunks are loaded, not to carry the boat by itself. When in doubt, ask the builder; it is their structure.

Pad hulls, bass boats, and flat bottoms

Low-deadrise hulls are more forgiving on angle but less forgiving on spacing. With little V to center the boat, bunks need to be wider apart (often well outboard of the pad) to stop the boat rocking. Make sure the pad itself is not resting on a cradle beam. On bass boats, the heavy outboard, jack plate, and batteries at the stern call for bunks that run fully aft.

Pontoon tubes and tritoons

Pontoon tubes are round, commonly in the range of about 23 to 27 in diameter on recreational pontoons, and many have a keel strip or lifting strakes along the bottom. Two common cradle approaches:

• One bunk centered under each tube. Simple and common. The bunk should be wide enough to support the bottom curve; carpeted 2x8 or wider bunks spread the load better than 2x6. If the tube has a center keel strip, a bunk with a center groove or two narrow bunks either side of the strip avoids point loading on the strip.
• Two bunks per tube (saddle). The tube nests between two bunks set roughly symmetrically either side of its bottom. This centers the tube and reduces rocking. Saddle spacing must match tube diameter.

Measure tube center-to-center spacing on your boat; it varies by model and beam. On tritoons, the center tube may be a different diameter or mounted at a different height than the outer tubes, and on performance packages it often carries the engine.

More on lifts for pontoons is in pontoon boat lifts.

Fore and aft: centering the weight on the lift

Many manufacturers rate lift capacity with the load centered. A boat with its center of gravity well aft of the lift's midpoint overloads the rear cables, motor, and posts. To check, find your boat's approximate CG (for most outboard boats, about 30 to 40 percent of hull length forward of the transom; sterndrives somewhat further forward), then position the boat so that point sits close to the middle of the cradle. Use bow stops or guide posts to make the position repeatable. Loaded weight and capacity margin are covered in capacity sizing and the capacity calculator.

The adjustment procedure at the dock

1. Level the empty cradle side to side and end to end first. Bunk adjustment cannot fix an out-of-level cradle (see lift raising unevenly).
2. Mark the lift centerline on each cradle beam.
3. Set each bunk's lateral position from the centerline using your stringer and strake measurements.
4. Set bracket angle with an angle finder and height with a tape from the beam top.
5. Snug, do not fully tighten. Drive the boat on slowly, raise just clear of the water, and observe from the dock and from a boat alongside.
6. Check: level side to side, expected bow attitude, full carpet contact visible along the hull, keel paper test.
7. Lower, fine-tune, then torque all bracket bolts. Recheck after a week.

If the boat still sits crooked, work through boat sits crooked on lift.

Frequently asked questions

How far apart should boat lift bunks be?

There is no single number. Place each bunk under or close to a hull stringer, on a flat bottom panel and not on a strake. On many mid-size V-hulls that puts bunk centers roughly 12 to 24 inches either side of centerline. Low-deadrise bass boats usually need wider spacing for stability. Measure your boat's stringers and strakes rather than copying another boat's setup.

How high should boat lift bunks be set?

High enough that the bunks carry the boat and the keel clears the cradle beams by about 1/2 to 2 inches. A good estimate for bunk height above the beam is the bunk's distance from centerline times the tangent of the deadrise angle, adjusted for keel offset and the clearance you want. Both bunks should match within about 1/4 inch.

Should boat lift bunks be angled?

Yes, on any V-hull. The bunk should tilt to match the hull's deadrise where it makes contact so the full bunk width bears on the hull. A flat bunk under a 20 degree hull touches only along one edge, which can raise bearing pressure five times or more and leave dents, carpet wear lines, and gelcoat stress cracks.

Can bunks sit on the lifting strakes?

They should not. Strakes are narrow raised strips designed for performance, not bearing. A bunk straddling a strake touches only its peak, concentrating load on a thin line and letting the boat rock. Shift the bunk to the flat panel inboard or outboard of the strake, or use a narrower bunk that fits between strakes.

How do I set up a boat lift for a tritoon?

Use a pontoon cradle with a bunk or saddle for each tube. Measure the bottom of each tube relative to the others with the boat level; the center tube is often larger or set lower. Set the center bunk to match so all three tubes contact together, and confirm the center tube is bearing if it carries the engine. Check tube spacing against the cradle before buying.

Sources and further reading

• Boat builder owner's manuals (support points, stringer layout, lifting and storage guidance)
• Boat lift manufacturer cradle and bunk setup instructions (bracket adjustment ranges, centered-load capacity ratings)
• ABYC standards, abycinc.org
• U.S. Coast Guard boating safety resources, uscgboating.org
• NFPA 70, National Electrical Code, Article 555, nfpa.org