USCG Exam Prep · 🎯 Interactive Trainer
Radar Plotting & the Maneuvering Board
Two timed radar contacts give you everything: where the other vessel is really going, how close it will pass, and when. The board makes it geometry you can see.
Tested on the Radar Observer endorsement and the 200-Ton / MMC exams. Binnacle School has 167 Radar questions in the bank.
What it is
A maneuvering board is a polar plotting sheet used to solve relative-motion problems. From two timed observations of a radar contact (bearing and range), you draw the relative motion line, then read the closest point of approach (CPA), the time to CPA (TCPA), and — by combining your own ship's vector with the relative vector — the contact's true course and speed.
How the USCG tests it
The Radar Observer exam asks you to plot two or three contacts, find CPA and TCPA, determine each contact's true course and speed, and choose a course or speed change that opens the CPA to a safe distance. The trainer generates fresh contacts and checks your CPA, TCPA, DRM/SRM, and the contact's true course and speed against the exact geometry.
Key concepts
The line through the two plotted contact positions; its direction is the DRM and its length over time is the SRM (speed of relative movement).
Closest Point of Approach is the perpendicular distance from your ship (board center) to the RML; TCPA is how long until the contact reaches it.
Your true vector (e) plus the relative vector (r) equals the contact's true vector (m): m = e + r. That gives the contact's true course and speed.
To open the CPA to X miles, alter course (or speed) so the new relative-motion line clears a circle of radius X around your ship.
Worked example
Own ship 000°T at 5 kn. A contact is at 090°T × 6.0 nm, then 8 minutes later at 090°T × 4.0 nm. Is there risk of collision?
Constant bearing (090°) with decreasing range is the textbook CBDR — risk of collision exists. The relative motion is straight down the bearing toward you, so CPA ≈ 0. You must take early, substantial action under Rule 7/8.
Practice Radar Plotting hands-on
The interactive trainer generates a fresh problem every time and checks your work against exam-accurate math. Free with a Binnacle School account, on web and iOS.
Frequently asked questions
What is CPA and TCPA?
CPA (Closest Point of Approach) is the smallest distance a contact will pass from your vessel; TCPA (Time to CPA) is how long until that happens. Both are read off the maneuvering-board plot of two timed radar observations.
What does CBDR mean?
Constant Bearing, Decreasing Range. If a contact stays on a steady compass bearing while the range closes, risk of collision exists and you must act — it is the single most important radar-plotting cue.
Do I need a maneuvering board to pass the Radar Observer exam?
Yes — the exam is built around maneuvering-board solutions. Practicing the plot until CPA, TCPA, and true course/speed are second nature is the fastest way through it.