How to Fast a Tug Safely

Making fast a tugboat is an operation that, to be effective, requires some precautions. The situations are not always optimal: good weather, adequate ship speed, abundant space and time, experienced crew, etc.

We must start assuming that we do not know the acting variables, so it is always advisable to follow a checklist not to overlook critical items. All personnel involved must do carefully the following:

• A good number of heaving lines must be present at the manoeuvring stations; if the first launch is not successful, recovering and preparing new messenger means losing time;
• The head of the heaving line must be appropriate: not too light otherwise, the wind would make it unmanageable, but not even weighed down with material that could be dangerous for the tugboat crew;
• The ship’s crew at the mooring stations must always remain clear of towline in a safe area, away from the snapping danger;
• When heaving or slacking the Tug’s line, the officer at the mooring station must always control the operation to prevent sudden or unexpected loads from creating dangerous situations;
• Realising the line, officer and crew must not follow the instructions that come from the tugboat, but those that come from the bridge;
• When the Tug’s line has to let go, it must always be done in a controlled way so as not to splash in the water that could cause the towline to end up in the tugboat or ship’s propeller; never throw off the whole line in the water.

 

 

Having made these essential clarifications, let’s see how to use the available tugs. First case: “bow with bow” or “stern with bow”. The decision depends on the type of propulsion of the Tug.

We have already discussed it before, but we return to the topic here. We saw that there are many different types of tugs and that the following are the most common:

• Conventional;
• Azimuth Stern Drive (ASD);
• Tractor with Azimuth (ATD);
• Voith Schneider (VTD);

Conventional tugs.

They can have one or two propellers with fixed or variable pitch, ducted or not.

Conventional tugs develop 50% less power astern. For this reason – secured aft – they work stern to stern. A good experience/skill is required from the tugboat captain when used in this way. The ship’s speed must necessarily be slow. The “gob wire” is then used to move the tow point as far as possible abaft to minimise the risk of girting and subsequent capsizing.

A typical manoeuvre is where the conventional stern-fastened Tug works in the ship’s wake and is mainly used to help stop the headway. The stern Tug has to slack the towline and manoeuvre to get into the new working position at the turning point. All this leads to a not negligible waste of time, especially harmful in the presence of wind. For this reason, whenever possible and if the brake action is not necessary, the stern Tug does not follow in the wake but accompanies the ship more or less under the bridge on the right side with the cable looking at spring. In this way, the Tug does not suffer from the effect caused by the ship’s rejected current and quickly takes control of directionality only when the vessel stops and begins to turn.

Therefore, except for specific cases, the best solution is to secure the conventional Tug forward.

Azimuth Stern Drive tugs (ASD).

These tugs have two stern thrusters that rotate 360 ​​degrees. Usually, they have two towing points; one positioned forward, used when secured aft (‘bow to stern’) or fore (‘bow to bow’), and one in the centre used for towing forward (‘stern to bow’) conventionally. ASD reverse tractor has only a drum winch ahead and exclusively operates bow/stern – bow/bow.

ASDs provide excellent manoeuvrability, but if used aft – in ‘stern to stern’ mode – the risk of girting/capsizing is high as conventional tugs.

Tractor and Voith Schneider (ATD/VTD).

They have two azimuths or two cycloidal thrusters forward and two towing points, one in the centre and one ahead. They use modern drum towing winches for fast operation.

As we have already said, the main towing point is positioned towards the stern just over the centre of the skeg, so the propeller thrust, remaining out and forward of the tow line, guarantees optimal performance for direction changes. So they may work stern/bow to bow and stern/bow to stern. But when they use the forward winch, they suffer more from the ship/tug interaction due to the proximity of the thruster to the ship pressure fields.

The risk of capsizing is low. This kind of tug work well at high speeds and, when setting stern/bow or stern/stern, having the thrusters far from the towing point can easily control the interactions in the towed vessel’s proximity. Athwartships shifting is also accurate because the propulsion units are close to the Pivot Point. They are the most suitable tugs for manoeuvring in confined spaces for all these reasons.

Let’s take a practical example of what can happen in the manoeuvre of a 300-meter tanker heading forward with an even keel draft of 14 meters: four tugs towing on a line, two secured ahead and two aft.

For example, after crossing a channel, let’s say that the ship reaches the evolution area.

The point on which we must focus our attention is the Pivot Point of the ship. As we already know, it moves ahead in the forward and aft in the backward, and the higher the speed, the more the PP moves towards the ship’s ends. Therefore, from a practical point of view, the forward tugs, working with towline very close to the PP, will have to exert an enormous effort to obtain some result, compared to the stern ones.

Therefore, the higher the ship’s speed, the greater the bow tugs’ effort. For this reason, it is much more effective to control the ship’s steering using the stern tugs.

The situation worsens as speed increases.

The Tug’s effectiveness decreases as the ship gathers speed, so his power reserve is necessary to avoid dragging itself into a girting situation.

Often, with particular vessels and in specific situations, to increase the ship’s manoeuvrability, the pilot uses the engine ahead to exploit the turning effect produced by the propeller slipstream on the rudder and simultaneously, the stern-Tug as a brake to avoid an increase in speed.

In this case, it is necessary to pay attention to the considerable increase in Bollard Pull, the risk of breaking the towing line, and the difficulty in tug manoeuvrability when it works on a short steep line under the influence of the propeller’s wake.

For all these reasons, this particular manoeuvre is one that the shipmaster, the pilot and the tugboat captain have to discuss and plan.

Why Forward the Risks are Greater

When heading forward, the ship’s bulbous bow slows the water and causes an increase in pressure in the front area and a decrease on its sides where the water speeds up again.

The higher the speed of the ship, the greater the pressures it generates.

These pressure differences around the ship’s bow destabilise the Tug, which is very close to the ship’s bulb, ready to be secured.

If we add to the pressures generated by the ship’s movement those caused by the current rejected by the tugboat’s thrusters, the result is an increase in the interaction effects between the two hulls.

To contain the related risks in this situation, we can act in different ways:

• First, by reducing the speed, adjusting it lower as possible within the safety steering limits;
• If there is the possibility to choose between different types of Tug, it is better to make fast a Tractor forward, which, as already said, having the propellers far from the bow of the ship is less affected by the interaction effects;
• If an ASD is available will necessarily be used “bow to bow”, making it work as a Tractor with the thrusters far from the towing point. It is a significant advantage for safety: in the event of an unforeseen event ( sudden changes in a ship’s heading and speed, failure of the tugboat engine, manoeuvring error, etc.), the Tug is not in danger of being dragged across. Rotating on the towing point in line with the propellers will be trapped alongside the ship. It remains a critical situation, however less dangerous than girting.

Summary:

In principle, and considering exceptions due to particular manoeuvres/situations, we can say that it is preferable to make fast the following types of tugboat forward or aft:

Forward:

• Conventional operating with the stern hook;
• Combi-tugs working with the stern hook;
• Tractor operating with the stern winch;
• ASD tugs operating with the stern and bow winch.

Aft:

• Combi-tugs that operate with the bow hook;
• The tractor working with the bow or stern winch;
• ASD tugs operating with the bow winch.
• If the speed exceeds 7 knots – only an escort tug can work at the stern.