Work out how far past the waterline the trailer has to go. Enter the ramp slope and the depth you need at the back of the trailer; get the distance down the ramp and a check on the tow vehicle’s rear axle.
Use this before launching at an unfamiliar or shallow ramp, when you want to know whether the trailer can get deep enough without putting the tow vehicle’s wheels or exhaust in the water.
Ramp slope comes from the ramp’s published grade, a phone inclinometer on the concrete, or a level and tape (rise over run). The depth needed at the trailer’s rear is roughly the boat’s draft at the transom over the bunks or rollers, plus a little clearance — measure it on your rig.
The vehicle check uses the distance from the trailer’s rear end forward to the tow vehicle’s rear axle, measured with the rig hitched and straight.
The ramp is a sloped plane. To put the back of the trailer at a given depth, it has to travel down the slope until the vertical drop below the waterline equals that depth: distance along the ramp = depth ÷ sin(angle). A percent grade is converted to an angle with the arctangent of grade ÷ 100.
Everything ahead of the trailer’s rear is higher by the distance forward times sin(angle). The page uses that to find the water depth at the tow vehicle’s rear axle; a negative number means the axle is still above the water.
angle = atan(grade % / 100) (or degrees as entered)
distance down ramp = depth needed / sin(angle)
horizontal distance = depth needed / tan(angle)
water at rear axle = depth needed − reach × sin(angle)
Each row below starts from the default inputs and changes only the columns shown. Recompute any row by hand with the formula above.
| Slope | Unit | Depth | Reach | Angle | Down ramp ft | Rear axle ft |
|---|---|---|---|---|---|---|
| 12 | % | 2 | 22 | 6.84° | 16.8 | 0.62 dry |
| 15 | % | 2 | 22 | 8.53° | 13.5 | 1.26 dry |
| 8 | % | 2 | 22 | 4.57° | 25.1 | 0.25 wet |
| 10 | ° | 2 | 22 | 10.00° | 11.5 | 1.82 dry |
| 12 | % | 2.75 | 26 | 6.84° | 23.1 | 0.35 dry |
| 10 | % | 3 | 20 | 5.71° | 30.1 | 1.01 wet |
With the calculator defaults (Slope given as = %; Ramp slope = 12; Depth needed at trailer rear (ft) = 2; Trailer rear to vehicle rear axle (ft) = 22), the arithmetic runs like this:
angle = atan(12 / 100) = 6.843°
distance down ramp = depth / sin(angle) = 2 / 0.11915 = 16.79 ft
water at vehicle rear axle = depth − reach × sin(angle) = 2 − 22 × 0.11915 = -0.62 ft
Result: 16.8 ft down the ramp. The trailer’s rear sits 16.8 ft past the waterline, measured along a 6.84° (12.0%) ramp — 16.7 ft horizontally. The vehicle’s rear axle stays 0.62 ft above the water (5.2 ft of dry ramp).
Now change Ramp slope to 8 and leave everything else alone:
angle = atan(8 / 100) = 4.574°
distance down ramp = depth / sin(angle) = 2 / 0.07975 = 25.08 ft
water at vehicle rear axle = depth − reach × sin(angle) = 2 − 22 × 0.07975 = 0.25 ft
Result: 25.1 ft down the ramp. The trailer’s rear sits 25.1 ft past the waterline, measured along a 4.57° (8.0%) ramp — 25.0 ft horizontally.
On an 8% ramp the trailer has to go 25 ft past the waterline instead of 16.8 ft, and the tow vehicle’s rear axle ends up in the water. Shallow ramps are where tongue extensions earn their keep.
Ramp slopes vary along their length; many have a gentler upper section and a steeper apron, or a drop-off at the end of the concrete. Look at the ramp at low water if you can, and never back the trailer wheels off the end of the pad.
Depth needed depends on the boat and the trailer. Roller trailers often let a boat roll off with the transom only partly afloat; bunk trailers usually need the stern floating. Find the depth that works on your rig once, write it down, and use it here.
Keep the tow vehicle’s exhaust, rear hubs, and wheel bearings out of the water when you can. If the numbers say the axle goes under, a tongue extension or a different ramp is a better plan than a wet launch.
Units: Feet for distances and depths; percent grade or degrees for slope.
Where the model stops: A uniform ramp slope and calm water. Waves, wakes, current, ramp drop-offs, and changing slope are not modeled.
Not a substitute: Follow posted ramp rules, keep the parking brake set and wheels chocked, and use a spotter.
| Grade | 1.5 ft | 2 ft | 2.5 ft | 3 ft |
|---|---|---|---|---|
| 8% | 18.8 | 25.1 | 31.3 | 37.6 |
| 10% | 15.1 | 20.1 | 25.1 | 30.1 |
| 12% | 12.6 | 16.8 | 21.0 | 25.2 |
| 14% | 10.8 | 14.4 | 18.0 | 21.6 |
| 15% | 10.1 | 13.5 | 16.9 | 20.2 |
Most public ramps are built at about 12 to 15% grade. A shallow 8% ramp needs almost twice as far to float the same boat, which is when rear wheels end up in the water.
A phone inclinometer laid on the concrete gives degrees. With a level and tape, measure the rise over a known run: 1 ft of rise over 8 ft of run is 12.5%.
No. 12% is about 6.8°, and 15% is about 8.5°. The page converts for you.
That is the distance the trailer actually travels, and it is easy to judge from marks or the waterline on the trailer.
The ramp is too shallow for your rig at this water level. A tongue extension adds reach, or try a steeper ramp or higher water.