Case Study
Designing a regulation-ready workhorse based on a proven hull.
Achieving Subchapter M Compliance for a 76 ft Modeled Bow Tug
Client
Caillou Island Towing
A family-owned towboat operator based in Houma, LA, with a fleet of approximately 20 vessels.
Project Overview
Caillou Island Towing engaged Sealy Technical Services, PLLC (STS) to design a new 76 ft line-pull tugboat that would:
- Build upon a prior successful hull and machinery arrangement.
- Fully satisfy the latest 46 CFR Subchapter M regulations for towing vessels.
Challenges:
Proven
Performance
Retain the proven attributes of the previous design (hull form, deck layout, machinery arrangement) while incorporating updated regulatory requirements, as well as increasing the beam by 30% from the previous design and adding a flying bridge / Carolina cabin.
Regulatory Compliance
Ensure the vessel met the stability, structural, electrical, and propulsion requirements of Subchapter M— covering everything from intact and stability to machinery redundancy and electrical system reliability.
Weight & Stability Control
Precisely track weights, centers of gravity, and free-surface effects across all tanks during the iterative design process.
STS Scope of Work
- Full arrangement of hull stiffeners, girders, and bulkheads.
- Beam and stiffener elements defined as solid objects, then exported for weight tracking. Global and local scantling calculations in accordance with ABS River Rules.
- Full CAD modeling for clash checking and 3D visualizations.
- Employed Orca3D for hydrostatic curves, displacement, righting arm calculations, and intact stability criteria checks.
- Post-processed Orca3D output in Excel to incorporate free-surface moment corrections for all liquid tanks.
- Utilized Orca3D’s weight-tracking module to maintain an up-to-date weight and center-of-gravity (CoG) log throughout the design iterations.
- Solicited and determined or estimated weights for vendor supplied items.
- Designed redundant power generation and distribution per Subchapter M’s electrical reliability criteria.
- Developed full set of 1-line diagrams and electrical fabrication specifications.
- Analyzed and selected propellor size and pitch.
- Design shaft details, bearing details, rudder arrangement, rudder bearings, stump jumpers, and gudgeons.
Solutions & Innovations
Regulatory Alignment
Mapped each CFR clause to a design deliverable—creating a traceability matrix that linked vessel drawings and reports directly to Subchapter M requirements.
Robust Stability Margins
Designed tank arrangements and free-surface management plans (e.g., baffling, bilge arrangement) to ensure compliance under both intact and damaged conditions.
Data-Driven Decisions
Leveraged dynamic weight-tracking dashboards to forecast CoG shifts as equipment and outfitting items were updated.
Results
- Subchapter M Compliance Achieved: Final design fully met or exceeded all applicable 46 CFR Subchapter M requirements for structural integrity, stability, machinery redundancy, and safety systems as verified by the client’s selected TPO.
- On-Time & On-Budget Delivery: Completed detailed design documentation within the client’s schedule and budget constraints.
- Fleet Enhancement: Provided Caillou Island Towing with a blueprint for future newbuilds, streamlining approval and construction of Subchapter M–compliant vessels.
Client Testimonial
— Operations Manager, Caillou Island Towing
Key Takeaway
By leveraging advanced naval architecture software and a methodical, requirements-driven approach, STS delivered a 76-foot Modeled Bow Tug design that not only honored proven performance criteria but also ensured airtight compliance with the latest Subchapter M regulations.
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