Marine Structural Drawings—Frames, Decks, and Longitudinal Structure

Marine Structural Drawings—Frames, Decks, and Longitudinal Structure

Marine structural drawings communicate how a vessel's hull is built—its bones, skin, and the connections between them. For small commercial workboats and skiffs, these drawings are the primary reference for fabricators, classification surveyors, and repair yards. Your job as a drafter is to translate scantling data into clear, dimensioned, fully labeled views that leave no room for guesswork on the shop floor or at dockside.

How a Small Workboat Hull Is Structured

A welded aluminum or steel workboat hull is a shell-and-frame system. The shell plating forms the watertight outer skin. Inside, transverse frames run athwartships (side to side) and are spaced at regular intervals—commonly 18 to 24 inches on small vessels. Running fore-and-aft along the bottom and sides are longitudinal stringers (also called longitudinals), which stiffen the plating between frames. At the very bottom centerline sits the keel bar or keel plate, the backbone of the hull. Floor plates tie the port and starboard frames together across the bottom and resist hydrostatic and impact loads. At deck level, deck beams span athwartships and support the deck plating. Where structural members intersect, brackets (gussets) and welded connections transfer loads smoothly.

For a welded aluminum skiff, typical members are aluminum angles, flat bars, tee-bars, and plate. A callout such as 3×3×¼ AL angle means a 3-inch by 3-inch equal-leg angle, ¼-inch thick, aluminum alloy. All member sizes come from the scantling data sheet, which is derived from the designer's calculations or the applicable classification rules.

The Midship Section Drawing

The midship section is a transverse cross-section cut at or near the middle of the vessel's length. It is the single most information-dense structural drawing because it shows every major structural element in one view:

• Keel bar at the centerline bottom
• Floor plate (the transverse bottom member) connecting port and starboard frames across the keel
• Bottom shell plating from keel to chine
• Side shell plating from chine to sheer
• Frames (transverse side frames) rising from the chine to the deck edge
• Longitudinal stringers shown in section as small profile shapes along the bottom and sides
• Deck beam spanning athwartships at the sheer line, supporting deck plating
• Deck plating with appropriate hatch

In AutoCAD, aluminum plate is typically shown with a specific crosshatch (often the ANSI 31 hatch rotated and scaled, or a material-specific pattern), while structural shapes such as angles and tee-bars are shown solid (filled) or with a steel/aluminum crosshatch per the symbol reference in use. Always follow the symbol standard your office or classification society specifies—for small vessels in the US, the American Bureau of Shipping (ABS) Small Vessel Guide is a common reference.

Label every member with a size callout placed on a leader line. Dimension the frame depth, beam width, deck crown, and overall molded breadth and depth. Show the waterline as a phantom line (long dash, short dash) for reference.

The Inboard Profile / Arrangement Drawing

The inboard profile is a longitudinal section cut along the vessel's centerline and viewed from one side. It shows:

• Keel running the full length at the bottom
• Chine and sheer lines
• Frame stations marked as vertical lines at their spacing dimension (e.g., FR 0 through FR 18 at 18-inch centers)
• Engine room bulkhead and forward collision bulkhead—both are full transverse watertight structures and must appear at their correct frame positions
• Frame schedule table—a tabular key listing each frame number, its distance from the baseline (AP or FP), and the member sizes at that station

Bulkhead positions are driven by design and classification requirements. The forward collision bulkhead is typically placed a minimum percentage of vessel length abaft the bow—exact rules vary by classification society and vessel type, so always reference the applicable standard. The engine room bulkhead is located by the machinery arrangement.

Notation and Callout Standards

Consistent notation prevents fabrication errors. Use the following conventions:

• Member sizes: depth × flange width × thickness for structural shapes, e.g., 3×2×3/16 AL angle
• Plate thickness: expressed as a fraction or decimal in inches, e.g., ¼" AL PL or 0.250 AL PL
• Weld symbols: per AWS A2.4 (the standard weld symbol reference); show fillet weld size and length where required
• Frame numbering: sequential from aft (Frame 0 at transom or AP) forward, or per the designer's reference—be consistent throughout the drawing set

Safety in the Lab

This is a CAD lab, but several safety points apply:

• Ergonomics: Set chair height so forearms are parallel to the desk; position the monitor at arm's length with the top of the screen at or slightly below eye level. Prolonged poor posture causes repetitive strain injury.
• Eye fatigue: Follow the 20-20-20 rule—every 20 minutes, look at something 20 feet away for 20 seconds.
• Printed reference handling: Some sample drawing sets are large-format prints. Unroll them on a clean, flat surface; do not place heavy objects on top of a borrowed classification drawing set.
• No food or drink near workstations—liquid damage to keyboards and prints is a common and preventable loss.

Workflow for This Lab

1. Review the scantling data sheet and identify every member to be drawn.

2. Set up your AutoCAD drawing with the correct units (architectural or decimal inches), layers named by member type (KEEL, FRAME, SHELL, DECK, BULKHEAD, HATCH, DIMENSIONS, TEXT), and a title block.

3. Draft the midship section first—establish the hull form geometry, then add structural members.

4. Draft the profile view, placing frame stations, bulkheads, and the frame schedule table.

5. Apply hatch, leaders, callouts, and dimensions.

6. Run a self-check against the rubric before submission.