Chart Datum, Tides, and Under-Keel Clearance Explained

Under-keel clearance is the amount of space between the deepest part of your boat and the seabed. Knowing how much room you have beneath your boat is especially important when navigating shallow water, unfamiliar areas, or changing tides. Determining what your under-keel clearance is takes more than looking at a single depth on a chart. Charted depth, tide or water level, your boat’s actual draft and load, waves, speed, bottom conditions, and other factors can all affect how much water you really have to work with.

Even with careful planning, depth calculations are only estimates. Give yourself an appropriate safety margin and, when using tide or water-level information with charted depths, make sure both are based on compatible datums.

What Is Chart Datum?

A chart datum is the reference point used to measure the depths shown on a nautical chart. Think of it as a baseline for those depth numbers. When you combine a charted depth with a predicted tide or water level, both need to use compatible datums for your calculation to be accurate.

Along much of the U.S. coastline, NOAA charted depths and tide predictions are referenced to Mean Lower Low Water, or MLLW. The Great Lakes and certain other waterways use different reference systems, so always check the datum listed on the chart, tide station, or electronic display before using the numbers together.

What Does a Charted Depth Represent?

A sounding is the depth shown on a nautical chart relative to the chart’s depth datum at the location that was surveyed. If a chart shows 8 feet, that doesn’t mean the water will always be exactly 8 feet deep.

The actual water level may be higher or lower depending on tides and other variables. Charted depths may also be based on older surveys or areas with limited survey coverage. NOAA chart products can include information about the age and quality of the underlying survey, which can help you understand how much confidence to place in the charted depth.

Depth Contours and Spot Soundings

Contours show general depth bands, while spot soundings show measured depths at specific locations. Pay close attention to the shallowest depths along your planned route, but remember that conditions may differ from what’s shown on the chart, particularly if the bottom has changed since the area was last surveyed.

Maintained Channels

Even maintained channels may not be as deep as you expect. Many things can change and/or reduce available depth between dredging cycles or surveys. When channel depth is critical to your route, check the latest U.S. Army Corps of Engineers information, Coast Pilot guidance, and Local Notices to Mariners.

How Tide Height Changes Available Water

As the tide rises and falls, so will the amount of water available beneath your boat. When the predicted tide height and charted depth use the same vertical reference, you can use the tide height to help estimate the water depth at a particular time.

For example, if the charted depth is 6 feet and the predicted high tide is 2 feet above the same chart datum, you can estimate approximately 8 feet of water before accounting for other factors. At low tide, the estimated depth is reduced.

Keep in mind that tide predictions are just that, predictions. Wind, atmospheric pressure, runoff, and other varying conditions can cause actual water levels to differ. When available, check nearby real-time water-level observations and always leave an appropriate safety margin.

What Is Vessel Draft?

Vessel draft is the vertical distance from the waterline to the deepest part of the boat. While your boat’s specs are a good starting point, the listed draft may not reflect how deep your boat is actually sitting in the water.

Fuel, water, passengers, gear, and other added weight can cause your boat to sit deeper. The way the boat is loaded can also change its trim, lowering the bow or stern. Depending on the type of boat you own, the keel, outboard, sterndrive, rudder, propeller, or another component may be its deepest point.

Know your boat’s Current Draft

When determining your boat’s draft:

• Check whether the published specification is for the drive up or down, or the centerboard up or down.
• Account for fuel, water, passengers, gear, and any modifications that add weight or change how the boat sits in the water.
• Consider how your boat’s trim may change when loaded or underway.
• Know which fixed or movable component represents the boat’s deepest point.
• Don’t assume your depth sounder’s transducer is located at the deepest point of the boat.

What Is Under-Keel Clearance?

Once you know the estimated water depth and your boat’s current draft, you can estimate your under-keel clearance. Start with the available water depth and subtract the deepest part of your boat’s draft. The amount remaining is your estimated under-keel clearance.

• Estimated available depth = charted depth adjusted for compatible tide or water-level information.
• Estimated under-keel clearance = estimated available depth – your boat’s current deepest draft.

For example, if you estimate 8 feet of available water and your boat draws 3 feet (vessel draft) at its deepest point, your estimated under-keel clearance is 5 feet.

Do know, a positive number doesn’t automatically mean you have enough room to safely pass. Waves, boat speed and movement, an uneven bottom, and changing or uncertain conditions can all reduce the amount of clearance you actually have. Always factor in a safety margin.

Factors That Reduce Effective Clearance

Even after you calculate your under-keel clearance, conditions on the water can change the amount of space you actually have beneath your boat. Keep these factors in mind, especially when navigating shallow or unfamiliar water.

Squat

As a boat moves through the water, it can sit slightly deeper and change trim, particularly at higher speeds or in shallow, confined water. This effect, known as squat, varies based on the boat, speed, water depth and channel conditions. Slowing down can help reduce squat, but always follow guidance specific to your boat.

Waves, Wake, and Pitch

Waves and wakes cause your boat to rise, fall, pitch and roll. That movement can bring the keel, propeller, rudder, or drive closer to the bottom, even when your calculated under-keel clearance appears sufficient.

Bottom Type and Irregularity

The bottom isn’t always flat or consistent. Mud, sand, rocks, coral, debris, and other obstructions can create very different risks, and rocks or other hazards may rise between charted soundings. Your boat should never make contact with the bottom, even if it’s soft mud or sand.

Survey and Position Uncertainty

The depths shown on your chart aren’t exact measurements of every point along your route. Survey age and coverage, GPS and transducer accuracy, and changes to the bottom due to varying conditions can all affect your estimate.

Water-Level Uncertainty

Predicted tides and actual water levels don’t always match. Always monitor before and during your trip.

How to Set a Safety Margin

There is no single under-keel clearance safety margin that works for every boat or situation. The amount of room you leave beneath your boat should depend on the conditions and level of risk. An unfamiliar route, rocky or uneven bottom, older survey data, higher speeds, waves, or poor visibility are all good reasons to give yourself more room.

Some marinas, waterways, vessel manufacturers, or other operators may also have their own minimum clearance requirements. Consider those requirements when planning your route, but never reduce your safety margin simply to make the numbers work.

A Practical Planning Process

When planning a route through shallow or unfamiliar water, work through these steps to estimate your under-keel clearance and determine an appropriate safety margin:

• Use the most detailed, up-to-date chart available for your route.
• Confirm the charted depth units and datum.
• Find the appropriate tide or water-level station and make sure its datum is compatible with the chart.
• Use the shallowest reliable depth along your planned route rather than an average depth.
• Know your boat’s draft with the expected fuel, passengers, and gear aboard.
• Apply compatible tide or water-level information to estimate the available depth.
• Account for factors such as squat, waves, trim, bottom conditions and the age and quality of survey data.
• Build in an appropriate safety margin.
• When possible, plan your route for a time when water levels provide more clearance.
• If you’re unsure whether you have enough clearance, wait for more favorable conditions or choose another route.

Using a Depth Sounder

A depth sounder provides valuable real-time information about the water beneath your boat, but it’s important to know what the number on the display actually represents. Depending on how your sounder is configured, the depth may be measured from the transducer, the waterline, or adjusted to show the depth below the keel. Check your unit’s settings and, when possible, compare the reading with a known depth.

Sounders can lose bottom for various reasons. Do not wait for the display to show zero before reducing speed or changing course.

Tide Predictions vs. Observed Water Levels

A tide prediction tells you what the water level is expected to be at a certain time, while an observed water level tells you what has actually been measured. Observed levels can give you a better picture of current conditions, but they may still differ from the water level at your boat because of distance, timing, and local conditions.

When planning or navigating shallow water, compare tide predictions with available real-time observations, weather conditions, and local knowledge. For more information about how tides affect your time on the water, use the Sea Tow app.

When Clearance Becomes Uncertain Underway

If you become unsure about the depth or your available clearance while underway, slow down and reassess your route. Pay attention to depth trends rather than reacting to a single reading, and only move toward deeper, known safe water when you’re confident of the direction.

Don’t assume an area is safe to enter just because you see another boat pass through it. That boat may have a shallower draft, and its operator may have more local knowledge of the area.

If you’re unsure how to safely proceed, contact your local Sea Tow Captain for navigational assistance and local on-water knowledge.

Frequently Asked Questions

Do I add the tide height to the charted depth?

Only when the charted depth and tide height use compatible datums and the same units of measurement. A positive tide height above the chart datum can increase your estimated water depth, while a negative tide height will reduce it.

Is MLLW the same as mean sea level?

No. Mean Lower Low Water (MLLW) and mean sea level are two different reference points. MLLW is based on the average height of the lower of the two daily low tides over a specific period and is commonly used as the chart datum along much of the U.S. coastline.

Does a charted depth include my boat’s draft?

No. A charted depth tells you the estimated water depth relative to the chart datum. To determine your under-keel clearance, you still need to subtract your boat’s current deepest draft and account for an appropriate safety margin.

Why does my depth sounder differ from the chart?

Your depth sounder and chart may show different depths for several reasons, including changing tides or water levels, your sounder’s transducer offset, your exact location, the age of the chart survey, changes to the bottom, and how your sounder is performing. Make sure you understand what your depth sounder is measuring and use both sources of information when navigating.

Can I pass through water that is exactly equal to my draft?

No. If the water depth is equal to your boat’s draft, you have no practical safety margin beneath the boat. Waves, boat movement, an uneven bottom or uncertainty in the depth information could cause you to make contact. Choose deeper water, wait for a more favorable water level, or simply take another route.

Sea Tow Team

Sea Tow has been the premier leader in on-water boating assistance since 1983. We want to share news, press, tips and all things boating.

Updated on: September 14, 2026 - Created on: September 14, 2026