Best watch a short video to get an idea of broaching – the loss of control over a ship because of a large stern wave. At best, broaching sets the ship seriously off course. At worst, the ship will end up sideways to the waves and tip over. It’s every captain’s nightmare, especially on small fast boats.
Dr Matteo Bonci (1989) was born in Genoa, Italy, and from an early age he was fascinated boats moving over and through the water. Manoeuvrability and seakeeping were still his main interests when he worked as a technical supervisor on fast motor yachts at Palumbo Superyachts in Naples.
Bonci, who now works as a researcher at Marin, saw that ships usually have optimum manoeuvrability in calm water. This made him ask how these ships behave in large waves?
Practical studies into broaching in towing tanks started in the 1970s and continued later in Japan at the turn of the century. But the question remains: why are certain ships more prone to broaching than others, and what aspects determine their sensitivity to broaching?
For his PhD research under Professor Huijsmans, Bonci decided to focus on small, fast ships of under 50 metres in length and with speeds of up to 15-20 knots (1 knot = 1.85 km/h). As a model he took the search and rescue (SAR) vessel of the Royal Netherlands Sea Rescue Institution KNRM, the NH 1816. This 20 metre fast rescue ship was built by Damen Shipyards.
He approached the problem both theoretically and practically. He used a mathematical model developed at Marin that allows the fast evaluation of design variations. It can easily simulate the behaviour of a ship in several different configurations and show which mutations are beneficial and which aren’t.
The practical measurements took place in towing tanks in Delft and at Marin, both on calm water and with following waves. A model of the NH 1816 was fixed under the carriage so that three dimensional forces and torques could be measured during the tests. These data were used to calibrate the numerical model.
Larger fins and rudder
Bonci found that sensitivity to broaching could be reduced by enlarging the directional stability (by fins or skegs) and/or the rudder. Larger directional stability helps in longer waves (over 2.5 times the boat length) and a large rudder is particularly helpful at lower speeds and shorter wavelengths. Based on his studies, Bonci recommends the combination of a larger rudder and skegs of about 25%.
Damen Shipyards was involved in the research. What do they think about Bonci’s recommendations? Jochem de Jong Manager Research from Damen’s Research & Development department says that “It’s important to have better insight in a boat’s sensitivity to broaching. Bonci has brought us a step further by studying the manoeuvrability of fast ships in waves."
Will ship designs be adapted henceforth? De Jong: "Whether or not a design’s seakeeping is improved with larger fins or rudders depends on a great number of factors that play a role in ship design. Such as the probability of encountering critical broaching conditions and the importance that the customer attaches to improved seakeeping in following waves.”
Accelerating out of trouble
One of the customers for Damen’s high-speed ships is the KNRM. Hans van der Molen, interim-head of the technical staff, says that the study isn’t really relevant to the KNRM. “The tank tests worked with 2 metre waves, scaled down. Between the islands, we ride waves of up to six metres.” The KNRM’s tactic is to accelerate out of trouble, which is why they insist on boat speeds of over 30 knots as this allows them to keep ahead of the waves.
Bonci is familiar with this technique. “One of my propositions was exactly this. In dangerous conditions, high-speed powerful craft have the chance to escape broaching by accelerating over the wave in front.” But, he says, most ships can’t do that. “Less powerful boats such as slightly bigger and slower patrol vessels or fishing trawlers do not have the chance to escape the dangerous waves, and then broaching becomes a real threat.”