AIS has changed offshore navigation in ways that are genuinely significant, and the argument that radar is redundant once all the ships are broadcasting electronically has a superficial logic to it. That argument falls apart the first time you find yourself in fog off Nantucket, closing on a lobster boat running without AIS, with a string of pots somewhere ahead. Or approaching an anchorage at night after a 600-mile passage with squalls on the radar and the shore lights washed out in rain. Or sailing north of 50 degrees latitude where ice — growlers especially, barely above the waterline — shows on radar and nowhere else.
Radar sees the physical world. AIS reports what vessels choose to transmit. These are different things, and for offshore sailing, you want both.
Why Radar Still Matters in the AIS Era
The population of non-AIS targets in coastal and offshore waters is larger than most sailors appreciate. Fishing boats under 300 GT are not required to carry AIS and many don't. Small commercial vessels — tugs, supply boats, harbor launches — often run without AIS or with units that are intermittently on. Recreational boats carry AIS only when the owner has chosen to install it. Log debris, fishing gear, and unlit marks are not broadcasting anything.
Then there is the weather dimension. A squall line shows on radar as a solid band of precipitation return. The gap between cells — a navigable slot — is visible on radar before it's visible to the eye. Watching the radar picture build in the hour before a system arrives gives you actionable information about timing, intensity, and where the lanes are. This is one of the most underappreciated uses of radar for offshore sailors: not collision avoidance, but weather reading.
Dome vs. Open Array for Sailboats
An open array radar — the flat, rotating beam antenna seen on most ships and larger powerboats — provides better range, better target discrimination, and a narrower horizontal beam width that resolves closely spaced targets more clearly. It also rotates, which creates hazards on a sailboat. A boom or a crewmember who moves at the wrong moment can make contact with a rotating array, and the radar itself should be positioned clear of the mast, boom, and any sails — not always achievable on a 40-foot cruiser with limited deck real estate. Open arrays also present meaningful windage on a sailboat rig, particularly the 4-foot and 6-foot versions used on performance cruisers.
A dome (radome) radar encloses the rotating antenna inside a fixed housing. Nothing rotates on the outside. The hazard profile is essentially zero, the windage is dramatically lower, and the sealed housing is more weather-resistant. The 18" dome — the Garmin Fantom 18x, Simrad Halo20, Furuno DRS4DNXT, or Raymarine Quantum 2 — is the standard choice for cruising sailboats up to about 50 feet. Range to 40 nautical miles is achievable; target discrimination in the inner ranges (under 3nm) is what matters for fog navigation, and the dome handles that adequately.
The 24" dome steps up range performance and beam width without adding the open-array complexity. On a boat over 45 feet where the radar can be mounted high on the mast or a sturdy arch, a 24" dome is worth considering. Beyond that, the step to an open array is a significant jump in cost, windage, and installation complexity — and on most sailboats under 55 feet, it is not necessary.
Solid-State vs. Magnetron
Traditional radar uses a magnetron transmitter — the same technology used in microwave ovens. Magnetron radar is reliable, well understood, and produces strong returns at all ranges, but it has a blind zone at close range (typically under 50-100 meters) and takes time to warm up. The magnetron itself degrades over time and eventually requires replacement.
Solid-state radar — the Garmin Fantom series, Simrad Halo series, Furuno NXT series — uses a semiconductor transmitter. It starts instantly, has no moving parts in the transmitter, and uses pulse compression to achieve better range resolution than traditional magnetron units at equivalent power levels. The key advantage for sailors is the close-range performance: solid-state radar can see targets at 20-30 meters, which matters enormously when you're threading into a marina entrance in fog. The Garmin Fantom series uses Doppler processing to highlight moving targets in green (approaching) or red (departing), which dramatically speeds up the visual assessment of a busy radar picture.
Radar Settings: What Most Sailors Get Wrong
The most common radar mistake is running gain too high. Gain controls receiver sensitivity — too high, and the display fills with noise that looks like targets. Every white pixel is evaluated by the brain as potential collision risk. The correct gain setting shows a slight amount of speckle across the display in open water; if you see nothing, you've set it too low. If you see solid rings of noise, you've set it too high. The automatic gain mode on modern units is generally reliable for initial use, but experienced radar operators adjust it manually based on conditions.
Sea clutter control (sometimes labeled "sea" or "STC" — Sensitivity Time Control) suppresses returns from wave tops near the radar. Run it too high and you eliminate small targets in the near range. Run it too low and the center of the display is a solid blob of wave return that hides anything close aboard. The correct setting clears the wave clutter while leaving genuine targets visible. This requires experimentation on a day with actual sea state — not on a flat calm practice day.
Rain clutter control suppresses precipitation returns. Unlike sea clutter, which is a physical time-gating function, rain clutter is typically an FTC (Fast Time Constant) differentiation filter. Running it on clear days is wrong — it degrades target detection. Running it in heavy rain makes the picture usable. Off in clear conditions, on in precipitation: that is the rule most sailors forget.
MARPA: Target Tracking for Non-AIS Vessels
MARPA (Mini Automatic Radar Plotting Aid) is a feature on most modern chartplotter-radar combinations that allows you to manually designate radar targets and have the system calculate their CPA and TCPA automatically. You touch a target on the radar display, tell the system to track it, and within 3-5 minutes (the time required to establish a motion track) you get a CPA/TCPA readout for that target — exactly like AIS, but for vessels that are not broadcasting.
Most MARPA implementations track 10 targets simultaneously. The Furuno system is widely regarded as the most reliable MARPA implementation in the recreational market; their NavNet TZTouch3 displays are used on serious offshore boats partly for this reason. Garmin and Simrad units handle MARPA adequately for most situations. The limitation is that MARPA requires radar contact — the target must produce a consistent return. Small wooden boats, heavily loaded inflatables, and wave-obscured targets can be intermittent.
Radar Overlay on Chartplotter
Modern chartplotter/radar integration allows radar returns to be overlaid on the chart display. Properly calibrated, this shows you exactly what the radar is seeing relative to your charted position — the coastline return aligns with the chart, buoys appear where they should, and unknown returns can be evaluated against the chart for context.
Calibration is the critical step. The radar heading must be aligned with the vessel's heading, and the radar and chartplotter must share a GPS position source. If these are misaligned, the overlay is actively misleading — chart features and radar returns will be offset, and a sailor trusting the overlay could plot a course into a shoal that looks clear on the combined display.
In fog especially, radar overlay allows a navigator to cross-reference what they see on radar with what the chart says is there. The approach to a marina entrance in zero visibility: the chartplotter overlay shows the entrance markers on the chart, the radar shows the physical channel markers returning signal, and the correlation between them confirms the approach is on track.
Range Selection for Fog Navigation
Fog navigation is the radar use case that offshore sailors encounter most often and handle most poorly. The instinct when visibility drops is to extend radar range — to see farther. The opposite is usually correct. In coastal waters in fog, targets of interest are typically within 2nm. Running a 12nm or 24nm range compresses the detail in the close range where you need it most.
The standard fog navigation approach: primary display at 1.5-3nm range, a secondary display (if you have a multi-display setup) watching out to 6-12nm for faster-closing traffic. Most collisions in fog happen when vessels fail to see each other at 1nm or less — the close-range display is where the work is done.
Reading Squall Lines and Precipitation
Precipitation returns on radar appear as irregular, often moving blobs distinct from the fixed returns of land and vessels. The leading edge of a squall line shows as a bright, dense band; the heaviest cells are the brightest. Gaps between cells appear as clear areas in the precipitation band.
Watching a squall line develop over 30 minutes tells you its motion vector, its density, and where the navigable gaps are. A line moving at 25 knots will close 12nm in 30 minutes — easily calculable from watching two or three radar sweeps. If you're 8nm from a gap in the line, you may be able to pass through it. If the line is solid, getting to a harbor or bearing off to run parallel to the line may be better options.
The Garmin Fantom's Doppler coloring is particularly useful here: precipitation returns are colored the same way moving targets are, which can create some confusion. Understanding your specific unit's display logic before you need it in a squall is not optional reading.
Radar Reflectors
GRP (fiberglass) sailboats are poor radar targets. The hull returns almost nothing; the mast and boom provide some metallic cross-section but at an angle-dependent level. In poor visibility, a typical 40-foot fiberglass sailboat presents a radar cross-section comparable to a wave crest — easily missed on a ship's radar.
Passive radar reflectors (the octahedral aluminum corner-reflectors you see hoisted in the rigging) provide modest improvement. The Davis Echomaster, mounted at 10 meters height, improves target returns at close range but provides no reliable enhancement at the ranges that matter for large ship collision avoidance. Testing by the Danish Maritime Institute and others has consistently shown that passive reflectors underperform their manufacturers' claims in real sea conditions.
The Echomax Active-X and Echotec active radar enhancers receive a radar signal and retransmit an amplified reply — effectively a SART that operates continuously. These provide dramatically better target enhancement than passive reflectors. If radar visibility matters to you (and it should for offshore sailing), an active enhancer is worth the $400-600 investment. Combined with a Class B AIS transponder, you are genuinely visible to ships that are looking for you.