How it works
On a sloped ramp, water gets deeper as you back down. On a grade of θ, every foot you travel along the ramp adds sin θ feet of depth. The boat floats once the water at the back of the bunks reaches the depth the hull needs.
Ramp angle θ = atan(grade % ÷ 100)
Depth needed D = hull draft + bunk height + clearance
Bunks below the waterline = D ÷ sin θ (measured along the ramp)
Water depth at a point a further up = D − a × sin θ
Worked example
A 13% ramp, 18 in hull draft, 14 in bunk height, 2 in clearance; trailer axle 3 ft ahead of the bunk ends; tow vehicle rear axle 22 ft ahead.
- Angle: atan(0.13) = 7.4°, sin = 0.129
- Depth needed: 18 + 14 + 2 = 34 in; the bunk ends go 34 ÷ 0.129 ÷ 12 ≈ 22 ft past the waterline
- Water at the trailer axle: 34 − 36 × 0.129 ≈ 29 in (hubs under). At the tow vehicle’s rear wheels: 34 − 264 × 0.129 ≈ 0 in: right at the water’s edge.
How far the bunks go under
Distance along the ramp from the waterline to the rear of the bunks, by ramp grade and depth needed.
| Ramp grade | 24 in deep | 30 in deep | 36 in deep |
|---|---|---|---|
| 8% | 25.1 ft | 31.3 ft | 37.6 ft |
| 10% | 20.1 ft | 25.1 ft | 30.1 ft |
| 12% | 16.8 ft | 21.0 ft | 25.2 ft |
| 14% | 14.4 ft | 18.0 ft | 21.6 ft |
| 16% | 12.7 ft | 15.8 ft | 19.0 ft |
Launching and retrieving tips
Most boats load more easily with the bunks slightly less submerged than for launching, so the hull centers itself on the bunks. Back in straight, set the parking brake and keep someone at the bow line. On shallow ramps, note where your rear wheels sit at the waterline: that mark tells you where you will be next time.
Before heading to the water, check the rig’s total weight with the boat trailer weight calculatorand the trailer tires with the trailer tire load calculator. Hitch height matters on ramps too: a level trailer is easier to back straight (see the hitch drop calculator).
Common mistakes
- Using the brochure draft. Measure from the hull bottom at the transom to the waterline, with the engine tilted up.
- Forgetting the bunk height. Water must cover the bunks as well as the hull’s draft.
- Backing too deep. Deeper water doesn’t help once the boat floats; it only soaks the tow vehicle.
- Dunking hot hubs. Let bearings cool first to avoid drawing water in.