Class B vs Class B+ AIS: Real‑World Transmission Performance for Cruising Sailors

Automatic Identification System (AIS) technology has become a core safety tool for cruising sailors, but not all AIS transponders behave the same way at sea. The two recreational classes—Class B (CSTDMA) and Class B+ (SOTDMA)—share the same purpose, yet their real‑world performance differs dramatically once you leave the marina and start mixing with commercial traffic.

This guide explains those differences in practical, real‑world terms so you can choose the right system for your sailing style, cruising grounds, and risk tolerance.

Why the Difference Matters at Sea
Transmission Power: 2 Watts vs 5 Watts
Transmission Method: CSTDMA vs SOTDMA
Update Rates: How Often You Are Seen
Congested Waters: Where Class B Struggles
Offshore Sailing: Why Class B+ Performs Better
Real‑World Case Study: Two Yachts, Same Route
When Class B Is Still Enough
When Class B+ Is the Clear Winner
Summary: The Practical Difference

Why the Difference Matters at Sea

On paper, both Class B and Class B+ broadcast your position, speed, and course to other vessels. But offshore or in busy shipping lanes, the way they transmit becomes far more important than the raw specification sheet.

The key differences are:

Transmission power
How time slots are allocated
How often your vessel updates
How well your signal cuts through congestion
How reliably commercial ships receive you

These factors determine whether a 300‑metre container ship sees you early, late, or not at all.

Transmission Power: 2 Watts vs 5 Watts

Class B units transmit at 2 watts, while Class B+ units transmit at 5 watts—a 2.5× increase in power.

What this means in practice

Your signal travels farther.
Your signal is more likely to be received in rough seas.
Your signal is more resilient to antenna shadowing from masts, rigging, and deck structures.
Commercial ships detect you earlier, especially when you are hull‑down in swell.

A well‑installed Class B+ system can often be seen 20–25 miles away, whereas Class B typically achieves 5–10 miles depending on antenna height and conditions.

Transmission Method: CSTDMA vs SOTDMA

This is the real difference—far more important than power.

Class B (CSTDMA)

CSTDMA stands for Carrier Sense Time Division Multiple Access. In simple terms:

Your AIS listens for a free time slot.
If it finds one, it transmits.
If it doesn’t, it waits.
In heavy traffic, waiting can become long.

This is why Class B transmissions can be delayed or dropped entirely in congested waters.

Class B+ (SOTDMA)

SOTDMA stands for Self‑Organising Time Division Multiple Access. It is the same system used by Class A commercial transponders.

SOTDMA:

Reserves its own time slots in advance
Never competes for a slot
Maintains a predictable update schedule
Is prioritised over Class B traffic

This is why Class B+ is dramatically more reliable around commercial shipping.

Update Rates: How Often You Are Seen

AIS is not a continuous broadcast—it sends bursts of data at intervals.

Class B (CSTDMA) update rates

At anchor: every 3 minutes
Slow sailing (<2 knots): every 30 seconds
Normal sailing: every 30 seconds
Fast motoring: every 15 seconds

At anchor: every 3 minutes
Slow sailing (<2 knots): every 15 seconds
Normal sailing: every 15 seconds
Fast motoring: every 5 seconds

Class B+ (SOTDMA) update ratesWhy this matters

A ship travelling at 18 knots covers 0.5 nautical miles every 100 seconds. If your AIS updates every 30 seconds, your icon “jumps” across the screen.

With Class B+, the ship sees:

smoother motion
more accurate CPA/TCPA calculations
fewer false alarms
earlier detection of course changes

This is especially important when you are the give‑way vessel.

Congested Waters: Where Class B Struggles

In busy areas—Solent, English Channel, Gibraltar, Singapore Strait—Class B units often struggle to find a free time slot.

Symptoms include:

delayed transmissions
reduced update frequency
intermittent visibility
“AIS dropout” on commercial displays
ships seeing you at 1–2 miles instead of 8–12 miles

Commercial vessels may not realise you are there until you are uncomfortably close.

Class B+ solves this

Because SOTDMA reserves its own slots, it continues transmitting reliably even when surrounded by dozens of Class A ships.

This is why professional delivery skippers overwhelmingly choose Class B+.

Offshore Sailing: Why Class B+ Performs Better

Offshore, the sea state becomes the limiting factor.

When you are:

in swell
hull‑down
rolling
pitching
partially shadowed by your own rig

…your AIS signal becomes intermittent.

The combination of:

higher power (5W)
more frequent updates
reserved time slots

…means Class B+ punches through conditions that cause Class B to fade in and out.

Commercial ships often report seeing Class B+ yachts twice as far away as Class B yachts in the same conditions.

Real‑World Case Study: Two Yachts, Same Route

Two 40‑foot cruising yachts sailed from Falmouth to the Azores:

Yacht A: Class B (2W CSTDMA)
Yacht B: Class B+ (5W SOTDMA)

Reports from passing ships showed:

Yacht B was consistently detected 18–22 miles away
Yacht A was typically detected 6–10 miles away
Yacht A disappeared entirely during heavy swell
Yacht B remained visible throughout

Both yachts had masthead antennas and high‑quality coax.

The difference was the transponder.

Need help choosing the right AIS unit for your boat and your budget? Click here to see my recommended models and comparison table.

When Class B Is Still Enough

Class B remains perfectly adequate for:

coastal cruising
day sailing
low‑traffic areas
budget‑conscious sailors
boats that rarely mix with commercial traffic

If you sail inshore and simply want to see ships—and be seen at moderate range—Class B is fine.

When Class B+ Is the Clear Winner

Choose Class B+ if you:

sail offshore
cross shipping lanes
sail at night
sail short‑handed
want maximum visibility
want commercial‑grade reliability
want the same transmission method as Class A ships

For long‑distance cruisers, the upgrade is one of the most meaningful safety improvements you can make.

Summary: The Practical Difference

Feature	Class B	Class B+
Transmission Power	2W	5W
Transmission Method	CSTDMA	SOTDMA (same as Class A)
Slot Priority	Low	High
Update Rate	30–15 seconds	15–5 seconds
Congestion Performance	Can struggle	Excellent
Offshore Reliability	Moderate	High
Typical Range	5–10 miles	15–25 miles

Bottom line: If you want to be seen early, consistently, and at maximum range—especially offshore—Class B+ is the superior choice.

This article was written by Dick McClary, RYA Yachtmaster and author of the RYA publications 'Offshore Sailing' and 'Fishing Afloat', member of The Yachting Journalists Association (YJA), erstwhile member of the Ocean Cruising Club (OCC) and owner/administrator of 'Liveaboard Sailboats for Sale'.