Harbour Tug vs ASD Tug

Two tugs of the same length and gross tonnage can have completely different pulling power, because the number that actually matters — bollard pull — has almost nothing to do with either figure. This explainer covers what bollard pull measures, why the ASD design has become the default for new tug orders, and how to size a tug to the job rather than the spec sheet. See our tugboat buyer’s guide for the rest of what to check before you buy.

What bollard pull actually measures

Bollard pull is the maximum continuous static pulling force a tug can exert, measured in tonnes during a bollard pull trial — the tug ties up to a fixed bollard ashore, runs its engines to full continuous power, and a load cell on the towline records the force. It’s a direct measurement of usable pulling power, independent of the tug’s length, beam, or displacement. Two tugs built to similar dimensions can differ in bollard pull by 30% or more depending on engine power, propulsion type, and propeller design — which is why port authorities and charterers specify a required bollard pull figure, not a length or tonnage, when they need a tug for a specific job.

Conventional harbour tug vs. ASD tug

The propulsion arrangement is what actually drives the bollard pull and manoeuvrability difference:

• Conventional (twin-screw) harbour tug — fixed propellers and rudders, generally the smaller, older-generation design still common at the low end of harbour work. Adequately powered and manoeuvrable for straightforward assist duty, but lacks the ability to move sideways or reposition quickly alongside a ship under way.
• ASD (Azimuth Stern Drive) tug — two independently steerable, 360°-rotating thrusters mounted at the stern. This is what most new tug orders are built to today: the same hull can generate full bollard pull in any direction without changing heading, which is the manoeuvrability conventional tugs can’t match. That flexibility is why ASD has become the standard harbour and terminal tug configuration worldwide.

A related but distinct type — the tractor tug, with azimuthing thrusters forward under the hull rather than at the stern — handles differently again, particularly in escort and indirect-towing work, but ASD is by far the more common configuration in current newbuild orders.

Tip: Don’t size a tug by comparing LOA or GT between candidates. Ask for the bollard pull trial certificate specifically — it’s the one number that tells you what the tug can actually do, and two tugs that look identical on a general arrangement plan can have meaningfully different pulling power.

Sizing bollard pull to the job

The required bollard pull scales with what you’re actually moving, not with a fixed rule of thumb — port authorities and terminal operators calculate it from vessel size, wind exposure, current, and berth layout for each job. In practice, smaller harbour tugs handle coastal and general port assist work, while the larger end of the ASD range is built for the biggest container ships, tankers, and gas carriers calling modern deep-water terminals, where wind area and current exposure demand significantly more continuous pulling force. Buying more bollard pull than a trade needs adds cost without adding useful capability; buying less than a specific terminal requires can simply disqualify the tug from that work before price is even discussed.

Escort tugs are a different standard entirely

Escort tugs — typically ASD or tractor-configured — are built and certified to a separate standard from ordinary harbour assist work. Rather than static bollard pull alone, an escort tug is rated on its indirect towing performance: the steering and braking force it can generate while running alongside a laden tanker at speed, in some cases up to around 14 knots, specifically for high-environmental-risk transits. A tug can have excellent static bollard pull and still not qualify as an escort tug if it can’t demonstrate that dynamic performance — the two ratings are not interchangeable, and a buyer looking specifically at escort duty needs to check indirect-towing certification, not just the bollard pull figure.

Reading a tug’s spec sheet

Beyond the headline bollard pull figure, a handful of details decide whether a specific tug actually fits your work:

• Trial date and conditions — bollard pull degrades over an engine’s life; a trial from build delivery may no longer reflect current performance on an older tug.
• Free-running speed — separate from bollard pull, and the limiting factor for escort work at speed or long transits between jobs.
• Fendering and towing gear — winch type (drum vs. render-recover), towline length and breaking strain, and fender layout all decide what size ship you can actually work safely.
• Class notation — tug-specific class notations (and any escort or fire-fighting notation) confirm the certification actually matches the duty, not just the hull type.

What this means for a tugboat buyer

Start from the job, not the hull. Confirm the bollard pull a specific terminal, port authority, or towage contract actually requires, then shortlist tugs with a verified trial certificate meeting or exceeding it — an ASD configuration for most modern harbour and terminal work, and specific escort certification if the job calls for it. See our tugboat buyer’s guide for the rest of the due-diligence checklist, browse current vessels for sale, or talk to a broker about a specific requirement.