Under-Keel Clearance Is More Than a Depth Number

"There is three metres of water" sounds simple.

It is not.

For a boat with a keel, the real question is not charted depth. It is usable clearance. How much water remains under the keel after you account for draft, tide height, wave motion, heel, squat, and uncertainty in the survey itself?

That is why under-keel clearance is a decision layer, not a map label.

Charted Depth Is a Starting Point

Depth data usually references a chart datum. Tide height adds or removes water relative to that datum. If those references are inconsistent or unclear, a route engine should become conservative instead of pretending the numbers are perfectly aligned.

That is one of the core lessons in HECTOR's routing work.

Bathymetry is useful only when its semantics are respected. A number without datum context can mislead.

The Boat Changes the Margin

Two boats can cross the same shallow patch with very different risk.

A shallow-draft day boat may have plenty of clearance. A deeper cruiser may not. A faster boat can experience squat. A boat heeled under sail effectively needs more room. Larger waves can reduce clearance dynamically as the boat moves through troughs and crests.

That means route scoring needs the boat profile, not just the route.

The path cannot be evaluated in isolation.

Tide Can Create or Remove a Route

In tidal waters, timing can matter as much as position.

A shallow pass may be unavailable now and safe in two hours. The same pass may become unavailable again later. A useful system should be able to explain whether waiting creates a safer window or whether the route is physically blocked.

That is a different product experience from simply drawing a red line over a shallow area.

It turns a warning into a decision.

Conservative by Design

Under-keel calculations should not try to sound more exact than the data allows.

If depth, tide, or chart datum information is uncertain, the output should carry that uncertainty. For recreational planning, this is advisory context, not a replacement for navigation judgment, official charts, or local responsibility. Software can support the decision; it cannot replace seamanship.

Why It Matters

Many marine apps make depth look visual and precise. The actual decision is messier.

Clearance is depth plus timing plus vessel plus motion plus uncertainty.