Bollard pull ? the engine?s contribution

At next year?s ITS Conference in Miami, bollard pull will be the subject of a paper to be presented by John Noble, deputy chairman of The Salvage Association. Here, he provides a personal introduction to some of the issues surrounding this often controversial topic.

Ask a tug operator what bollard pull his tug has and he is likely to quote you the immediate post-building maximum figure. The same question to a surveyor is likely to obtain a completely different answer on the basis that bollard pull at the time of survey is the figure that the tug itself can actually achieve! Defining bollard pull itself is really quite simple. It is the amount of force that a tug can exert on a towing rope. Bollard pull is regarded widely as the best benchmark with which to judge the capability of a tug whose principle role is towage. This approach differs from defining the capability of harbour tractors and some escort tugs, where manoeuvrability and stability are often seen as more important factors in judging a tug?s capability. Nonetheless the traditional bollard pull figure carries a lot of weight.

To determine a bollard pull figure the procedure is to measure the force exerted by the tug by using a simple meter that is placed between the tug?s line and the connection made fast to a strong point on a dockside. The meter is used to gauge the force acting on the tug?s line. Of course, to achieve a maximum figure, factors aside from the engine power must be introduced. These influence the achievable bollard pull at a particular test and include water depth, trim of tug, propulsion system and make up of the dock wall. However, tug operators and builders conducting the initial test will aim for the highest figure possible for the sake of the publicity literature! So, is the result of a controlled test valid and is this a fair assessment on measuring a tugs bollard pull? The answer is usually ?no? because the bollard pull test is usually conducted in artificial circumstances and is not a measure of sustained capability. But it is an indication from which a surveyor can start.

Readers will be all too familiar with tug propulsion systems and the arguments that go on between the relative advantages of Azimuth and Voith units. The relative merits of the different propulsion methods are always a source of keen discussion. The configuration of the propulsion units is as likely to be taken into account, especially when water depth may be an issue, for example in shallow water tow off manoeuvres.

In Miami I will be addressing the broader issues and guiding delegates on what the surveyor is looking for in determining the required bollard pull for a specific job. Also, while engine power alone is not the sole parameter for gauging what bollard pull is required, it is a very good starting point.

Perhaps one of the most contentious issues is determining the required bollard pull before making a judgement regarding the suitability of any tugs offered. Too many tug operators still rely on the new-build figure and the tug?s age and condition and throw doubt on the reliability of the ?new? figure on the day of survey. One question on which this readership may care to comment is: is it fair that the engine power on build is used as a rating for an engine that ages?

Clearly it is impractical to conduct a bollard pull test every time a tug is required for a warranty tow. Equally, it is imprudent to rely on information published at a time when a tug was new, or if a test was conducted sometime prior to the tow under survey. Fortunately, in most instances the experience and commonsense of the surveyor combines with the pragmatisms of the tug owner and tug master to allow most tows to proceed normally and without incident.

Surveyors will often suggest that tug owners are too subjective in their judgement on a tug?s performance. Equally, owners often suggest that surveyors are too objective on bollard pull issues. So, who has the right approach and who is in the best position to judge what a tug is capable of achieving?

I suspect the answer lies onboard the tug itself, where the experienced tug?s master is well aware of the capability of his craft in all sorts of weather types with different tows at the end of the line. In reality it is the dialogue between the tug master and the surveyor that should determine the outcome.

So, is bollard pull such an issue after all? I welcome reader?s comments or opinions on this issue as I prepare to tackle the subject in more depth.