Lateral is witnessing an upward trend in the technical ambitions of refits and conversions.
Many projects are conceived with significant length increases to the original design, with associated increases in GT that pose demanding engineering challenges. Overcoming these challenges demands substantial effort in naval architecture, structural engineering, mechanical engineering, outfit engineering, and project management.
Understanding the impact of changes on a yacht’s weight and centre of gravity is a crucial aspect of naval architecture. It determines the feasibility of refit changes, considering aspects such as draught, freeboard, trim, stability, speed performance, and comfort. Refit projects often operate on narrow weight and stability margins, necessitating a heightened focus on weight engineering throughout the project lifecycle to ensure success.
The initial step involves establishing a weight budget for the required alterations. Given the absence of detailed engineering information, an overarching strategy is required, focusing primarily on significant weight changes and applying well informed margins. This strategy may utilize statistical analysis, past project data, or limited supplier information. Lateral’s extensive database aids in these initial calculations. Estimated weight changes are then applied to the lightship weight recorded from the last available weight status of the yacht, typically obtained through an inclination test. This information is crucial for evaluating project feasibility and risk, assisting clients in deciding whether to proceed with the refit, and informing early decisions regarding refit management and extent of changes.
Upon arrival at the refit shipyard, it is advisable to conduct a new inclination test. This test verifies the feasibility assumptions and serves as the baseline for weight engineering calculations moving forward. Detailed engineering brings with it the availability of construction drawings for structural modifications. Concurrently, the weight engineer further develops weight calculations and typically employs 3D structural weight models to determine the structure to be removed based on old construction drawings (if available). A similar process is undertaken for the new structure to be added. These calculations are critical as construction changes can lead to substantial shifts in the yacht’s weight and centre of gravity. In a recent significant project, the removed steel and aluminium alone accounted for 10% of the yacht’s weight, while 15% was added back on.
At this point, an updated refit weight calculation, incorporating detailed structural weight changes and other relevant calculations, is generated and compared to the original budget. If issues are identified, corrective measures can be proposed such as weight-saving options and tank plan modifications to optimize loading conditions. This will allow the project team to make informed decisions.
For weight and stability-critical projects, adhering to rigorous weighing procedures at the shipyard is advisable. This includes measuring and documenting the weight and position of all items removed from the vessel, whether for overhaul, recycling, or disposal. This is important as accurate estimates for the old vessel can be challenging due to limited information availability. Subsequently, a similar process is followed for newly installed items. By incorporating weighing data, the accuracy of the lightship weight estimate is significantly improved. During this period, the weight engineer conducts regular surveys to validate and verify the weighing data, with periodic reports keeping the project team informed.
Upon reaching a major milestone during the project, another weight verification check is recommended. This may involve an inclination test if the yacht is in dry dock or a weighing session on load cells if floating it is not practical. While the former provides a comprehensive status check, the latter will not include the vertical centre of gravity calculation. The intermediate status check determines whether any previously proposed corrective measures need to be implemented.
In the final stages of outfitting, continuous weight monitoring is practiced until the vessel’s relaunch. After relaunch, a final inclination test is performed, providing the definitive lightship details of the vessel and confirming that the project targets have been achieved. In a recent project exceeding 100 meters in length, the weight and centre of gravity results were within 1% of the calculated values. Considering the complexity of the project, where 57% of the yacht’s entire weight was removed and 66% was added back on, it becomes apparent how crucial it is to maintain precise weight control.
In conclusion, weight engineering plays a pivotal role in de-risking major refit projects. At Lateral, we recognize the paramount importance of weight engineering and provide our clients with comprehensive weight engineering services to ensure project success. With extensive experience, cutting-edge tools, and a dedicated team, we stand ready to tackle any challenge that a refit may present.
Written by Alex Meredith Hardy, Chief Naval Architect.
