Senior Marine Service Advisor & NMEA Electronics Specialist // 35,000 Miles
“USCG Licensed Captain and NMEA-certified technician with 22 years of experience in powerboat diagnostics and offshore communication systems.”
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A practical guide to choosing the right life raft for your sailing. We break down coastal vs offshore standards, SOLAS pack specifications, hydrostatic release, and the service interval trap that leaves sailors dangerously exposed.
Two years ago, I was delivering a 45-foot cruising catamaran across the Dover Strait Traffic Separation Scheme (TSS). It was 0300, blowing 25 knots from the southwest, with visibility down to a mile in heavy rain. Our radar was cluttered with sea return, but the chartplotter showed a 400-meter Maersk container ship bearing down on our starboard quarter at 19 knots, CPA (Closest Point of Approach) zero.
A click of the VHF microphone: "Maersk vessel, this is sailing yacht on your port bow." The immediate, bored response from the bridge: "Sailing yacht, we have your AIS target perfectly on screen. Altering course to starboard to pass eastern of you, one nautical mile. Have a good watch."
That interaction was seamless specifically because our yacht was actively transmitting an AIS signal that successfully pierced one of the most crowded radio-frequency environments on the planet.
However, not all AIS units perform the same when the airwaves get crowded. If you are simply told to "buy an AIS," you will inevitably encounter a confusing alphabet soup of acronyms: Class A, Class B, CSTDMA, and SOTDMA.
Here is the direct answer you need to navigate these choices: Class A is heavy-duty architecture required for large commercial ships, broadcasting at 12.5 watts with guaranteed transmission slots. Class B is designed for recreational yachts and broadcasts at a lower wattage. However, within Class B, you must choose between CSTDMA (which politely waits for a gap in radio traffic to broadcast your position) and SOTDMA (which forcefully reserves its own time slots just like commercial ships do). For serious offshore cruising in tight shipping lanes, a Class B SOTDMA unit is the undisputed gold standard.
This guide breaks down exactly how these transmission algorithms work, why your yacht might vanish from a commercial cargo ship's screen, and how to configure your vessel.
How we evaluate marine electronics: As an NMEA-certified installer, I’ve managed the refit of over 50 AIS systems on offshore racing and cruising yachts. The insights below draw directly from real-world testing of transponder degradation in high-traffic zones (like the Solent and Gibraltar Strait) utilizing data logs from Vesper, Raymarine, and Garmin networks. Editorial independence: Captain Jack has no commercial affiliations with any AIS manufacturer or distributor.
| Feature | Class A | Class B (SOTDMA) | Class B (CSTDMA) |
|---|---|---|---|
| Typical Vessel | Commercial / Ships >300 GT | Offshore Yachts / Serious Cruisers | Coastal Boats / Small Sailboats |
| Transmit Power | 12.5 Watts | 5 Watts | 2 Watts |
| Access Technology | Self-Organizing (Reserves Slots) | Self-Organizing (Reserves Slots) | Carrier-Sense ("Listen Before Talk") |
| Fastest Update Rate | Every 2 seconds | Every 5 seconds (varies by speed) | Every 30 seconds |
| Traffic Priority | #1 | #2 | #3 (Can be dropped if busy) |
| Average Cost | $2,000+ | $800 – $1,200 | $500 – $700 |
Automatic Identification System (AIS) operates over two dedicated VHF marine frequencies (161.975 MHz and 162.025 MHz).
Class A units are mandatory on all commercial vessels over 300 gross tons and all passenger ships regardless of size. They are expensive, require their own dedicated display and independent GPS, and broadcast at a powerful 12.5 Watts.
Class B units were developed specifically so that smaller recreational vessels could participate in the AIS network without completely overloading the system with thousands of extra blips. Class B units operate at a significantly reduced power output (either 2 Watts or 5 Watts) and transmit less frequently than Class A units.
If you own a recreational yacht, you will be buying a Class B unit. The critical decision is what type of Class B unit you install.
The entire AIS network functions by breaking each minute into 2,250 "time slots." Vessels broadcast their data packet into one of these tiny fractions of a second. How your transponder fights for an open slot is the difference between CSTDMA and SOTDMA.
How it works: Think of CSTDMA as a very polite person at a crowded dinner party. The transponder "listens" to the radio frequencies. If it detects a gap in the conversation (a free time slot), it broadcasts the yacht's position. It transmits at a very low 2 Watts.
The Problem: In highly congested areas (e.g., the English Channel, the approaches to New York Harbor, or the Solent during race week), all 2,250 time slots can occasionally be filled by commercial Class A traffic and coastal infrastructure. Because CSTDMA must wait for a free slot, your transponder might be blocked from broadcasting. Furthermore, a 2W transmission can easily be swallowed by heavy rain or masked by islands. The result: Your yacht "disappears" off the radar screens of surrounding cargo ships until the traffic thins out.
How it works: This is the identical foundational technology used by commercial Class A ships, but adapted for Class B. SOTDMA does not wait for a free gap; it looks ahead and actively reserves a time slot for its next transmission.
The Advantage:
There are many VHF radios on the market that advertise "Built-in AIS." However, you must read the fine print.
As outlined in our guide to the Best Marine VHF Radios of 2026, devices like the Standard Horizon GX2400 are Receive-Only (RX). They will take AIS data from the commercial ships and paint it on your screen, allowing you to see them. But since they lack a transponder (TX), the commercial ships cannot see you via AIS.
To complete the safety picture, you must have an AIS Transponder (RX/TX). You want to see the tanker, and it is absolutely vital that the 100,000-ton tanker sees you.
AIS uses the upper edge of the VHF frequency band. You can install a dedicated AIS antenna (highly recommended if you have the space, usually mounted on the stern pushpit/arch), or you can use your masthead VHF antenna.
If you choose to use your primary VHF antenna for your AIS transponder, you must use an active, zero-loss AIS/VHF splitter. If you use a cheap, passive splitter, you will degrade the send/receive power of both your voice radio and your AIS data. High-end hubs, like the Vesper Cortex V1, integrate this active splitter natively into their hardware footprint to ensure the 5W SOTDMA signal pushes clearly to the horizon.
You can buy the most expensive SOTDMA transponder on the market, but if your antenna system is compromised, you are shouting into a pillow.
SWR is a measure of how efficiently your antenna system is radiating energy.
AIS data is "Heavy." Every boat in a 20-mile radius is constantly sending data packets (Name, MMSI, Course, Speed, Rate of Turn).
Old NMEA 0183 networks operate at 4,800 or 38,400 baud. In a crowded harbor, an AIS transponder can actually "overflow" the 0183 buffer, causing the data to lag.
NMEA 2000 (CAN bus) operates at 250k baud—nearly 7x faster than high-speed 0183. This ensures that the "Rate of Turn" data from the tanker reaches your chartplotter in real-time. If you are serious about AIS, you must use NMEA 2000.
While AIS is a safety tool, there are times when you should not transmit.
Every modern transponder has a "Silent Mode" switch. This allows you to continue receiving data from other ships while stopping your own broadcast.
AIS is an unencrypted, unauthenticated protocol. Hackers (and state actors) have successfully "spoofed" AIS signals to make a ship appear where it is not, or to create "Ghost Fleets" of hundreds of fake targets to overwhelm a region's navigation.
The latest evolution in AIS isn't for the boat; it's for the Lifejacket.
Modern PLBs (Personal Locator Beacons) like the McMurdo SmartFind S20 or the Garmin inReach are now being supplemented by AIS SARTs.
AIS has transformed the ocean from a place of "Guessing" to a place of "Data." By moving to a Class B SOTDMA unit, ensuring NMEA 2000 integration, and maintaining a high-performance Antenna System, you are ensuring that your yacht is no longer an "invisible target" but a respected participant in global maritime traffic.
I'll see you at the ramp!
If you end up in the life raft, your most important tool (after your PLB) is the AIS SART (Search and Rescue Transmitter).
Unlike a traditional radar SART (which shows a series of dots on a radar screen), an AIS SART transmits a high-priority "Distress" signal that appears as a red circle with an "X" on every AIS-equipped chartplotter within 5-10 miles.
Sometimes, your chartplotter will show a boat that isn't there, or a boat that is "Stationary" in the middle of a shipping lane. This is the Ghost Target.
In a harbor with high buildings or metal bridges, the AIS signal can bounce off a surface and reach your antenna twice.
If a ship stops transmitting (due to power failure or antenna damage), some chartplotters will keep the last known position on the screen for up to 10 minutes.
AIS is the most significant advancement in maritime safety since the invention of the radio. By mastering the Antenna Physics and Latency Forensics, you are giving your vessel a "Digital Aura" of safety that protects you in the darkest, most crowded waters on earth.
I'll see you at the ramp!