Vessel Stability Analysis

Stability Calculations and GZ Curve Checks Are Critical for Tug Towing Operations Ensuring tug stability and regularly checking the GZ (righting arm) curve are vital for safe towing operations due to the following reasons: Heeling Moment from Towline: During towing, the force from the towline creates a significant heeling moment (sideways tipping force) on the tug. If this force exceeds the tug’s righting ability, the tug can capsize, especially if the towline is not properly aligned or if the towing point is high or off-center. GZ Curve as a Stability Indicator: The GZ curve shows how much righting force the tug has at various angles of heel. If the GZ value drops to zero, the tug will capsize. Tugs generally have lower freeboard and less residual stability, so their GZ curves can indicate vulnerability to capsizing at smaller angles compared to larger vessels. Risk of Girting: Girting occurs when the tug is pulled sideways by the tow, potentially leading to rapid capsize. The risk increases if stability is not properly calculated and monitored, especially with improper towline arrangements or open deck fittings that allow water ingress. Regulatory and Operational Compliance: Stability calculations must account for both internal (vertical) and external (horizontal/towline) forces, as required by international regulations and classification societies. Regular checks ensure compliance and that the tug remains within safe operational limits under different towing scenarios. Prevention of Loss and Accidents: Historical incidents show that failure to update and check stability data, including GZ curves, has led to tug losses during towing, especially in adverse conditions or with improper procedures. Key Reasons for Stability and GZ Curve Checks Reason Impact on Towing Safety Heeling moment from towline : Prevents capsize due to excessive heeling GZ curve monitoring: Ensures righting ability at all heel angles Girting risk : Reduces risk of sudden, uncontrollable capsize Regulatory compliance : Meets safety standards and legal requirements Accident prevention : Minimizes incidents from overlooked instability In summary: Regularly updating and checking stability calculations and GZ curves is essential to ensure tugs can safely withstand the forces encountered during towing, avoid capsizing, and comply with safety regulations. #shipstability #safety #GZcurve #Girting

Solving USCG Stability Problems at Large Angles of Heel "Your vessel's drafts are: FWD 16'08", AFT 17'06"; and the KG is 23.8 feet to. Use the selected stability curves in the blue pages of the Stability Data Reference Book to determine the remaining righting moment at 36° inclination if the center of gravity is 1.7 feet off the centerline."   Solution: First you need to determine the Displacement from the given drafts. The drafts are assumed to be located at the FP and AP. The mean Draft will be located on the centerline at Midships, LBP/2. This should be close enough to get the USCG answer, however the exact displacement should be measured at the LCF resulting in a correction to displacement for trim. Let's do the Displacement Calculation:  (16’-08” + 17’-06”)/2 = 17’- 01” assuming the vessel is floating in seawater Using page 1 of the Blue pages of the STABILITY DATA REFERENCE BOOK we can determine a displacement of 8,500 tons. Next, we need to determine the Righting Arm, GZ, as published before we can correct it from its assumed KG of 20 feet to the given KG of 23.8 feet for 36 degrees Angle of Heel. Using page 3 of the Blue pages of the STABILITY DATA REFERENCE BOOK we can determine the GZ for 36 degrees with an assumed KG of 20 feet equal to 6.0 feet. This if from the cross-curve data. Let's correct the GZ for the actual KG for 36 degrees Angle of Heel. (Note: you do not need to draw the stability curve. Just solve for GZ at 36 degrees!) The problem gives the KG = 23.8 feet and the assumed KG is 20 feet, so we must correct for an upward vertical shift of 3.8 feet. We can do this with a sine correction. All vertical shifts are corrected with a sign correction equal to the shift multiplied by ths sine of the angle which is 36 degrees in this case. Initial GZ – (sin 36 degrees x vertical shift) = 6.0 ft – (0.5877 x 3.8ft) = 3.766 feet Then we need to correct the GZ for the "Transverse shift of G" which is given as "1.7 feet off the centerline." All transverse shifts of "G" can be corrected with the cosine correction. (If "G" shifts outboard the correction is negative.) Let's correct the GZ for the Transverse shift in "G", 1.7 feet off the centerline, CL. Corrected GZ for actual KG – cosine 36 degrees x transverse shift GZ corrected for KG of 23.8 feet with G off centerline 1.7 feet = 3.766 feet – (cos 36 x 1.7 feet off CL) GZ corrected for KG of 23.8 feet with G off centerline 1.7 feet = 3.766 – (0.809 x 1.7) = 2.390 feet Now we can solve for the Righting Moment which equals the Displacement multiplied by the GZ that was corrected to the Vertical and Transverse position of G at an "Angle of Heel" not "List". (If it was an Angle of List" GZ will equal zero.) Let's calculate the Righting Moment for the given condition:  Righting Moment at 36 degrees = Corrected GZ x Displacement Righting Moment at 36 degrees = 2.390 feet x 8,500 tons = 20,315 foot-tons   WANT TO LEARN MORE WITH PRIVATE LESSONS? Email: WilliamEGeorge@gmail.com

𝑳𝑶𝑨𝑫𝑰𝑵𝑮 𝑪𝑶𝑴𝑷𝑼𝑻𝑬𝑹 𝑺𝒀𝑺𝑻𝑬𝑴: 𝑯𝒐𝒘 𝒔𝒉𝒊𝒑𝒔 𝒎𝒐𝒏𝒊𝒕𝒐𝒓 𝑺𝑻𝑨𝑩𝑰𝑳𝑰𝑻𝒀 A loading computer system (LCS) is a specialized onboard software used by ships to calculate and monitor stability, strength, and cargo loading conditions in real-time. It helps ensure that a vessel remains safe, stable, and within structural limits during loading, unloading, and throughout the voyage. The system takes into account cargo weight distribution, ballast water levels, fuel consumption, and environmental factors to prevent excessive stress on the ship’s hull and avoid instability. The main functions of a loading computer system include calculating draft, trim, list, metacentric height (GM), and righting arm (GZ) to assess stability. It also evaluates shear forces and bending moments acting on the hull to prevent structural failures. Some advanced systems integrate with tank sensors, draft gauges, and ballast management systems to provide real-time updates and automatic adjustments. If unsafe conditions are detected, the system alerts the crew so corrective actions can be taken. A loading computer is essential for modern shipping, especially for bulk carriers, container ships, oil tankers, and LNG vessels, where precise weight distribution is critical. It ensures compliance with IMO, SOLAS, and classification society regulations, improving both safety and efficiency. By using a loading computer, crews can optimize cargo arrangements, reduce fuel consumption, and minimize the risk of accidents such as capsizing, structural damage, or excessive hull stress.