Do you need radar on a small boat, and what does the range figure actually mean?

Radar is the one thing on a small boat that sees through fog and dark. It’s also the equipment most often sold on a number that doesn’t mean what buyers think. A dome the size of a cake tin is advertised at 48 nautical miles. The boat it’s bolted to can see maybe eight.

That isn’t false advertising. The range figure is a specification of the transmitter and the display, not a description of what reaches the screen. The gap between the two is most of what you need before spending the money.

What does a radar’s advertised maximum range actually mean?

It describes the longest range scale the transmitter and display support — not a distance at which the radar will detect anything in particular. Radar is line-of-sight. The pulse travels in a straight line and the Earth curves away underneath it, so the ceiling on detection is geometry, not power.

Furuno publishes the working formula in its radar technology explainer: D ≒ 2.2(√H1 + √H2), where D is the radar line-of-sight in nautical miles, H1 is antenna height in meters and H2 is target height in meters. Their own worked example uses an antenna 16 m up and an object 9 m high, and arrives at “about 15 nautical miles.”

Sixteen meters is a ship’s mast, not a sportfisher’s arch, and the target is three stories tall. The answer is still 15 miles — under a third of a 48-mile rating.

How far can a small boat’s radar really see another small boat?

Running Furuno’s formula with realistic small-boat numbers gives roughly seven to eight nautical miles against a target of similar height. An antenna on a four-meter arch, looking at a two-meter target, works out to about 7.5 nautical miles. Put the antenna on a two-meter rail instead and it drops to about 6.2. A one-meter buoy from that four-meter antenna is around 6.6.

Land is a different case entirely, which is why long range scales aren’t pointless. A 200-meter headland seen from a four-meter antenna comes out near 35 nautical miles. A 36-mile scale is a landfall scale, not a small-craft-detection scale.

Even those are geometric ceilings, not promises. A fiberglass hull with no radar reflector is a poor target and may not paint at all near the horizon. The US Inland Navigation Rules are blunt about it in 33 CFR 83.07: “Assumptions shall not be made on the basis of scanty information, especially scanty radar information.”

Does more transmit power buy more range?

Not in the way the watts suggest — Garmin’s own catalog publishes a 4 kW magnetron radome and a 50-watt solid-state radome with the same 48-nautical-mile maximum range. The GMR 18/24 xHD magnetron domes are rated at “4 kW peak” and 48 nm. The GMR Fantom 18x/24x solid-state domes are rated at “50 W peak” and 48 nm. Same dome sizes, same published beam widths, an eighty-fold difference in peak power, identical rated range.

That is not a contradiction. Power doesn’t move the horizon; it buys the ability to pull weak returns out of noise and rain clutter inside it. Which is a real benefit — it’s just not the benefit the range column implies. Nor do the figures scale between makers: Raymarine’s Quantum 2 publishes 20 W and 24 nm against Garmin’s 50 W and 48 nm, which rests on different internal assumptions rather than on physics a buyer can check.

Do you need radar if you already have AIS?

Radar and AIS answer different questions, and AIS has a structural gap radar doesn’t have: it only shows vessels that carry and transmit it. Under 33 CFR 164.46, the US carriage requirement reaches self-propelled commercial vessels of 65 feet or more, towing vessels of 26 feet or more above 600 horsepower, vessels certificated to carry more than 150 passengers, dredges working near channels, and certain dangerous cargo. Recreational boats are not on that list.

So AIS gives you the ship early, with a name and a closest point of approach. Radar gives you the unlit skiff, the buoy, the squall line and the shoreline, and can’t name any of them. Boats that run at night carry both — the detail is in our guide to what AIS does and doesn’t show.

The Navigation Rules treat radar as something to use properly once fitted: 33 CFR 83.07 requires that “Proper use shall be made of radar equipment if fitted and operational, including long-range scanning to obtain early warning of risk of collision.” Where the picture goes — overlaid on the plotter or on its own screen — is covered in chartplotter versus a phone. And acting on a contact means a radio call, which is the VHF and DSC side of the helm that Standard Horizon and Icom occupy.

Should you buy a magnetron or a solid-state radar?

Solid state wins clearly on warm-up and close-in detection; magnetron still buys far more raw peak power per dollar. Furuno’s Model 1715 manual describes the magnetron routine directly: “After the completion of the startup test, a timer displays the time remaining for warm up of the magnetron (the device which transmits radar pulses), counting down from 1:00 to 0:00.” The specification line reads “Warm-up Time 1-min. approx.”

Furuno’s own pitch for its solid-state DRS-NXT series is the mirror image: “No preheating time, low energy consumption (no use of a magnetron).”

Minimum range splits the same way, and buyers rarely hear about it. Furuno’s magnetron 1715 publishes 30 m. Garmin’s magnetron xHD domes publish “20 m (66 ft.)”; its solid-state Fantom domes publish “6 m (20 ft.)”, and Raymarine’s Quantum 2 publishes “18 feet (6m)”. A radar blind inside 30 meters is no help in a crowded anchorage.

The mechanism is pulse length. Furuno explains the trade plainly: “When the pulse width is short, the detection power is low but the distance resolution is good. Conversely, when the pulse width is long, the detection power is greater but the distance resolution is reduced.” A magnetron picks one pulse width per range scale and lives with it. Pulse compression sends a long coded chirp and squeezes it on receive, which is how a 20-watt set reaches out at all — Raymarine publishes “Chirp lengths” of “400 ns to 22 us” across bandwidth of “Up to 32 MHz” for its “X-band solid-state transmitter with pulse compression technology.”

Radar	Type	Published max range	Transmit power	Horizontal beam width	Warm-up
Furuno 1715 radome	Magnetron	36 nm	2.2 kW peak	5.2° typical	1 min. approx.
Garmin GMR 18 xHD dome	Magnetron	48 nm	4 kW peak	5.2°	not published
Garmin GMR 24 xHD dome	Magnetron	48 nm	4 kW peak	3.7°	not published
Garmin GMR Fantom 18x dome	Solid state	48 nm	50 W peak	5.2°	not published
Garmin GMR Fantom 24x dome	Solid state	48 nm	50 W peak	3.7°	not published
Raymarine Quantum 2 Q24D dome	Solid state	24 nm	20 W	4.9°	“powers up instantly”
Raymarine Cyclone, 4 ft. array	Solid state	72 nm	55 W peak	1.99°	40 s boot; under 5 s to transmit
Furuno DRS4D-NXT dome	Solid state	see note	see note	2°–3.9° adjustable	“No preheating time”

Two caveats. Furuno publishes conflicting figures for the DRS4D-NXT: the global product page lists a 610 mm radome at “100 W” and “0.0625 to 72 NM”, while Furuno USA’s page for the same model lists “25 W” and “48 nm”. We won’t guess which is current. And the max-range column is the least comparable number here — no maker publishes the target size or antenna height its figure assumes, so these aren’t measurements of the same thing.

Does beam width matter more than range?

Horizontal beam width decides whether two targets show as two targets, and it’s set by antenna width — not power. Furuno puts it directly: “the narrower the beam width of an antenna, the higher the resolution of the radar.” They give 0.75 degrees for X-band antennas up to 3 meters wide, against “an average horizontal beam width of 5.7°” for radomes around 40 cm.

Multiply that out. At one nautical mile a 5.2° beam smears every target across about 168 meters of arc; the 1.99° of Raymarine’s 4-foot Cyclone array covers about 64. Two boats a hundred meters apart are one blob on the dome and two contacts on the array.

Within domes, diameter is the lever: Garmin’s 24-inch housing reaches 3.7° where the 18-inch manages 5.2°, on identical electronics. Furuno’s RezBoost processing claims 2° to 3.9° for the DRS4D-NXT. An open array wants mast height and deck space a 28-foot boat rarely has, so most small craft are really choosing a dome diameter.

Height is the other lever, and the cheapest. Furuno again: “The range of a radar can be increased by simply installing the antenna higher…” There is a limit. Vertical beam width is deliberately generous — Raymarine publishes 20° for the Quantum 2 and 25° for the Cyclone — so targets stay painted while the boat rolls, and mounting too high pushes the near field under the beam.

The short version

Radar’s advertised range is a transmitter and display specification, and on a small boat with a low antenna the real ceiling against another small boat is seven or eight miles — by the maker’s own published formula. Power isn’t what sets that ceiling, which is why one maker sells a 4 kW radar and a 50-watt radar with the same rated range. Buy for what changes the picture: beam width, which comes from antenna width, and antenna height, which is free. Solid state earns its money on instant transmit and a minimum range in meters rather than tens of meters. Keep AIS alongside it, because the boats most likely to worry you transmit nothing at all.