Try a scenario, or make your own. Predict who gives way, then reveal.
Dashed lines follow the current headings. A hollow cross is where paths meet. A filled cross marks a projected close pass. The shaded circle represents the close-pass margin. Click a cross for timing and closest approach.
Auto matches arrival times where forward tracks meet. Editing speed switches Auto off; turn it back on in either boat’s controls. Parallel overtaking is supported. With several boats, it targets one meeting area; other encounters may remain. Speed alone cannot join diverging paths. Auto uses 0.1–20 kn; these limits can leave a near pass instead of an exact meeting.
Boat icons are enlarged. Wind arrows point in the direction the wind blows; the compass names where it comes from.
For keyboard control, focus a boat or rotation handle and use the arrows. Enter opens boat settings. Wind also responds to arrows.
Open water, vessels in sight, steady motion. Each edit sets a fresh encounter. Earlier overtaking obligations and the need for stand-on intervention require history and context.
The model uses a 35° sailing no-go angle and 6° head-on aspect tolerance. These are teaching assumptions. Channels, traffic schemes, current, leeway, restricted visibility and local rules need additional context.
The margin is a practice setting, not a regulatory safe distance. With several boats, check the whole encounter before choosing a manoeuvre.