Use this sailing calculation as a planning or educational cross-check for navigation, wind and current, rig loads, stability, anchoring, tides, visibility, performance, or recovery under explicitly stated conditions.

Boat speed (kt)

Boat speed through the water.

Current speed (kt)

Current speed across the course.

Relative angle (°)

Angle between the desired course and the current set, zero meaning the current opposes nothing.

Compute the signed crab angle needed to oppose a current's cross-track component. Routes whose cross-current exceeds boat speed cannot be held. The tool evaluates the supplied inputs together and returns the named outputs below; it does not infer omitted operating conditions or change the units shown.^[1]

Correction angle (°)

Signed heading correction opposite the current's cross-course component.

• Actual conditions change with vessel design and loading, sea state, wind, current, tide datum, weather, chart accuracy, equipment, crew, regulations, and safety margins. Simplified formulas do not replace seamanship, approved stability data, or current navigation information.
• Use finite inputs in the displayed units and preserve more precision than the final presentation requires. Independently verify safety-critical, financial, compliance, or production decisions.

Cross-check official charts, tide and weather services, instrument data, vessel manuals, and collision regulations; maintain conservative margins and a safe alternative plan.

• Corrected Waypoint ETA

  Compute ETA along a waypoint track while allowing for current: the boat first cancels the cross-current component, and its remaining along-track speed is combined with the current's along-track component.

• Ferry Glide Course

  Calculate the heading to steer so cross current lets the boat ferry glide onto the desired course, correcting by the arcsine of the current to boat speed ratio signed by the side the current comes from.

• Lee Bow Effect Calculator

  Project current onto the desired course as current speed times the cosine of the relative angle. This along-course proxy does not model the true lee-bow effect, which also depends on heading, boat speed, and apparent wind.