Dock & Boat Ramp Clearance Calculator

Towing your boat to a closed ramp or discovering that your boat lift is grounded in mud wastes weekends and risks propeller damage. Enter your boat's draft and your ramp or slip elevation to calculate exact water clearance today and find your low-water cutoff threshold.

Official Datum: IGLD85
Must use same datum (IGLD85)
Hull depth below waterline (e.g. 2.5 ft)
Under-keel buffer (default: 1.0 ft)

Clearance Analysis for Lake Michigan

Based on current water level of 568.00 ft IGLD85.

✓ Safe to Launch (9.5 ft Net Buffer)
13.0 ft
Water Over Slip / Ramp
Surface to structure floor
+9.5 ft
Net Operating Clearance
After draft & safety buffer
558.5 ft
Access Cutoff Elevation
Water level where access is lost
9.5 ft
Remaining Drop Buffer
How far lake can fall

Practical Recommendation

You have 13.0 feet of water over your ramp or slip. With a boat draft of 2.5 feet and a safety buffer of 1.0 foot, your craft has plenty of navigable room. The lake would need to drop an additional 9.5 feet before reaching your cutoff elevation of 558.5 ft.

How Boat Ramp and Slip Clearance Is Calculated

Every boat ramp and boat slip has a physical bottom elevation. For a boat ramp, this is the concrete toe where the ramp ends and natural lakebed begins. If you back a trailer beyond this toe, trailer tires drop off into soft mud or underwater drop-offs. For a boat slip or boat lift, this is the floor of the slip or the lowest beam of the cradle.

Our calculator uses the exact mathematical formula:

Water Depth = Lake Surface Elevation - Structure Elevation
Required Depth = Boat Draft + Safety Margin
Net Clearance = Water Depth - Required Depth
Access Cutoff Elevation = Structure Elevation + Required Depth

Critical Datum Warning

Important: You must enter your structure elevation using the same vertical datum as the lake gauge (e.g. IGLD85). Mixing vertical datums (such as entering a NAVD88 surveyor reading into an NGVD29 lake record) can create an error of up to 1 to 2 feet, which is easily enough to cause a propeller strike or trailer grounding. Always verify your property survey or dock engineering drawing datum before planning around low-water thresholds.

Worked Example

Imagine a boat slip on Lake Travis with a concrete bottom elevation of 645.0 feet NGVD29. The boat owner has a 24-foot bowrider with an outdrive draft of 2.5 feet, and chooses a sensible 1.0-foot safety buffer to account for wind chop and underwater silt.

The required water depth is: 2.5 ft + 1.0 ft = 3.5 ft.
The access cutoff elevation is: 645.0 ft + 3.5 ft = 648.5 ft.
If today's lake elevation is 654.2 feet, the owner has: 654.2 ft - 645.0 ft = 9.2 ft of water over the slip floor, leaving 9.2 ft - 3.5 ft = 5.7 ft of net clearance. The lake can fall another 5.7 feet before boat operations must cease.

Frequently Asked Questions

How do I find the elevation of my dock or ramp?

Check your original property survey, shoreline construction permit, or dock installation receipt. Many lakeside counties and lake authorities (such as TVA, USACE, or LCRA) record permit elevations for docks and seawalls. Alternatively, a local land surveyor can shoot an elevation bench mark on your dock in minutes.

Why is a 1-foot safety margin recommended?

Static draft measurements only reflect a boat floating calmly at rest. Passing wake, boat squat when accelerating, wind-driven surface seiche, and shifting sandbars require at least 12 inches of buffer to protect your hull, propeller, and cooling water intakes.

Can floating docks operate at lower elevations than fixed docks?

Floating docks rise and fall with water levels, but they can still run aground! If lake levels drop below the shoreline contour, floating dock sections may tilt on exposed rocks, snap anchor cables, or buckle walkways. You must know the elevation of the shallowest point underneath your floating slip.

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