Do you need AIS on a boat, and what is the difference between receive and transmit?

AIS is sold as one thing and bought as two. One version draws the commercial traffic around you onto a screen. The other does that and puts your boat on theirs. Both are labeled AIS, and only the second changes what the other vessel does.

The difference is usually a few hundred dollars, which is exactly why it’s the half people skip. Here’s what the regulation requires, what each class transmits, and the two things that stop a transceiver working after you’ve paid for it.

Do you have to carry AIS on a recreational boat?

No — US federal law does not require recreational vessels to carry AIS. The carriage rule is 33 CFR 164.46, and every category in it is commercial: a self-propelled vessel of 65 feet or more “engaged in commercial service,” a towing vessel of 26 feet or more and more than 600 horsepower engaged in commercial service, a vessel certificated to carry more than 150 passengers, dredges working in or near a commercial channel, and vessels moving certain dangerous cargo or bulk flammable and combustible liquid cargo.

Recreational vessels are not named anywhere in that section. The Coast Guard Navigation Center’s summary of the same rule reads identically, and it treats Class B only as a permitted substitute for vessels already inside the requirement — certain fishing industry vessels, and smaller commercial craft operating under 14 knots outside vessel traffic service areas.

That is US federal law as the Coast Guard states it. Carriage rules differ elsewhere and under the international conventions covering vessels on international voyages; if that’s your situation, the answer comes from your flag administration.

“Not required” and “not worth having” are separate arguments. The second is the interesting one.

What’s the difference between an AIS receiver and an AIS transceiver?

A receiver shows you other vessels; a transceiver also shows your vessel to them. A receive-only unit decodes the AIS channels, draws targets on your plotter, and transmits nothing — so the bridge team on the ship you’re watching has no idea you exist.

A transceiver broadcasts your identity, position, course and speed on the same channels. Your boat appears on their display, feeds their collision alarms, and can be called by name on the VHF — which turns an ambiguous close-quarters situation into a conversation.

The asymmetry matters because of who you are in the encounter: a receiver improves what you know, a transceiver improves what they know — and they’re the vessel with the turning circle measured in miles.

A third variant transmits only on activation. ACR Electronics publishes that its AISLink personal MOB beacon alerts AIS-equipped vessels “up to 5 miles (4.3 NM) away,” and is explicit that this is not a satellite beacon’s job — it reaches nearby boats, not orbiting search-and-rescue satellites.

The targets have to display somewhere too — see chartplotter vs phone.

What do Class A, Class B and Class B SOTDMA actually transmit?

Class A transmits at 12.5 watts nominal, basic Class B at 2 watts, and Class B SOTDMA at 5 watts — figures published in IALA Guideline 1082, hosted by the Coast Guard’s Navigation Center. Power is half the story; the other half is how often a set gets to speak.

AIS divides each minute into 2,250 time slots per channel, and the two Class B schemes reach them differently. Canada’s e-Navigation portal describes the mechanism: SOTDMA sets “scan for and reserve their own slots” for the next transmission, while CSTDMA “does not permit Class B AIS devices to reserve slots on future frames” — they listen for a gap and take it. In dense traffic, a reserved slot beats a hoped-for one.

AIS type	Transmits?	Published transmit power	Position report interval	Who it’s for
Receive-only (standalone receiver, or a VHF with an AIS receiver)	No	Not applicable	Not applicable	Someone who wants to see traffic and accepts staying invisible
Class A	Yes	12.5 W nominal, 1 W low power	2–10 s underway; 3 min at anchor or moored under 3 kn	The commercial vessels listed in 33 CFR 164.46
Class B (CSTDMA)	Yes	2 W	30 s above 2 kn; 3 min below 2 kn	Small craft where cost is the deciding factor
Class B SOTDMA (sold as “Class B+”)	Yes	5 W nominal, 1 W low power	30 s at 2–14 kn; 15 s at 14–23 kn; 5 s above 23 kn	Faster or offshore recreational boats, and small commercial vessels
Personal AIS MOB beacon	Yes, on activation only	Not published on ACR’s product page	Not published	A person in the water, alerting vessels already nearby

Power and the coarse intervals come from IALA Guideline 1082; the finer breakdown by speed band is published by Raymarine, with a caveat worth repeating — reporting rates “are for reference and may not be the rate at which information is actually received,” because antenna height and interference get a vote.

Manufacturers publish the power figures plainly. Garmin’s AIS 800 specifications list “5 W Class B, SOTDMA,” with a 2 W CSTDMA variant for Japan only; Raymarine’s AIS700 lists “5 W SOTDMA”; and Furuno’s FA-70 is switchable, stating “5 W or 1 W (SO-TDMA), 2 W (CS-TDMA)” on one unit. Icom’s MA-510TR publishes 2 W, and Furuno’s Class A FA-170 publishes “1 W or 12.5 W selectable.”

One figure in circulation needs scoping. The Navigation Center’s AIS overview says AIS “updates as often as every two seconds” — true, of Class A at speed. A 2-watt Class B at 6 knots reports every 30 seconds, and a fast ship covers ground between updates.

Is being visible on AIS the same as being visible on radar?

No, and the difference is what AIS can’t see. IALA’s guideline puts it in five words: “AIS only shows vessels fitted with AIS.” A kayak, an unlit skiff, a half-submerged container, a fishing boat with the set switched off — none of them exist on an AIS display. Radar detects objects; AIS receives broadcasts.

Neither discharges the underlying duty. Navigation Rule 5, in the Coast Guard’s amalgamated Navigation Rules, requires that “every vessel shall at all times maintain a proper look-out by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions.” Rule 7 adds that “proper use shall be made of radar equipment if fitted and operational,” and warns that “assumptions shall not be made on the basis of scanty information, especially scanty radar information.”

The Coast Guard’s AIS FAQ is blunter: AIS “should never be solely relied upon for collision avoidance or navigational decision-making.” And 33 CFR 164.46(d)(1) states that carrying AIS “does not relieve the vessel of the requirements to sound whistle signals or display lights or shapes” under the collision regulations. Read as a stack: AIS is identity, radar is detection, and eyes are the layer neither replaces. Whether radar belongs on the same boat is a separate question — see marine radar basics.

Do you need an MMSI to transmit AIS?

Yes — an AIS transceiver without a programmed MMSI is a receiver with extra steps. The Navigation Center defines MMSIs as “nine-digit numbers used by maritime digital selective calling (DSC), automatic identification systems (AIS), and certain other equipment to uniquely identify a ship,” and its AIS FAQ states that AIS devices “must be operated with an official 9-digit Maritime Mobile Service Identity (MMSI) number assigned to the vessel and its owner.” US-assigned numbers begin 338, 366, 367, 368 or 369.

Three practical consequences follow.

You may get one attempt. The same FAQ notes that “U.S. sold AIS Class B devices are not user configurable,” while Class A sets are but keep static data password protected. Getting the number wrong on a Class B is a service call, not a menu.

Where the MMSI came from matters if you leave the country. For recreational vessels staying in US waters, the Navigation Center lists approved organizations — BoatUS, US Power Squadrons and Shine Micro — that issue MMSIs without an FCC license. But “if you plan to sail to a foreign country, you will need to apply for an FCC ship station license,” and that means deleting the existing number and replacing it with the FCC-issued one.

One MMSI, one transmitter. Every radio aboard must use the same identical MMSI, excepting AIS-SARTs and handhelds not dedicated to the vessel, and 33 CFR 164.46(f) permits portable AIS only “such that only one AIS device may be transmitting on board a vessel at any one time.” Two transceivers running together is a fault, not redundancy. It’s the same trap that catches people on DSC radios — see VHF, DSC and MMSI.

Does your VHF already do half of this?

Some fixed-mount VHF sets include an AIS receiver, but it’s a model-number question rather than a brand one. Icom publishes the clearest example: the IC-M510 ships in two configurations, one with an integrated AIS receiver and one without, and Icom notes that the “AIS receiver is not included depending on the transceiver version.” Same model name, different capability.

Standard Horizon, Garmin and Uniden all build fixed DSC sets across a similar range, and the check is the same every time: read the part number on the box, not the family name in the review. And remember what a built-in AIS receiver buys — traffic on your screen, nothing on theirs.

Where a boat ends up genuinely visible, it’s a Class B SOTDMA transceiver, an MMSI programmed correctly on the first attempt, and — for the person rather than the hull — an AIS man-overboard beacon on the lifejacket: ACR’s AISLink, or McMurdo’s FastFind CREW1.

The short version

Under US federal law, recreational boats aren’t required to carry AIS; 33 CFR 164.46 is a commercial rule from top to bottom. If you’re buying anyway, the split that matters is receive versus transmit — a receiver improves what you know, a transceiver puts you on the screen of the vessel that can’t stop. Class B SOTDMA transmits at 5 watts with reserved slots against basic Class B’s 2 watts and best-effort access, both well under Class A’s 12.5 watts. Nothing transmits without a programmed MMSI, and on a US Class B you generally get one attempt at entering it. None of it sees the kayak, or discharges Rule 5.