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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › Shipbuilding / Marine Fabrication › Marine Outfitting: Systems Installation and Interior Fabrication › Lesson 10
MFG-SHIP13-06 · Lesson 10 · 18 clock hours · lab

Access Hatches, Manholes, and Watertight Closures

Objective. The student lays out and installs a deck access hatch coaming and cover on a mock deck panel, with the coaming located to drawing dimensions within ±1/8 inch, dogging hardware operating without binding, and a hydrostatic/hose test showing no leakage.

Why Watertight Closures Matter

A ship's weathertight and watertight integrity depends on every hatch, manhole, and closure performing exactly as designed. A leaking deck hatch can flood a compartment, endanger crew, and cause a vessel to fail a USCG inspection. In commercial shipbuilding, closures must meet applicable class rules (American Bureau of Shipping) and federal regulations. In this lab you will install a deck access hatch the way it is done on the production floor: laying out from a drawing, cutting the opening, fitting and welding the coaming, and proving the assembly watertight by hose test.

Components of a Deck Access Hatch

Coaming – A raised collar welded to the deck that surrounds the opening. The coaming keeps water from running directly into the opening and provides the seating surface for the cover. Coaming height is specified on the drawing and by regulation; do not substitute a shorter coaming to make fitting easier.

Cover – The plate or casting that closes the opening. It bears against the coaming through a gasket.

Gasket – A compressible seal, typically neoprene or rubber, seated in a groove or channel on the cover or coaming face. The gasket must be continuous with no gaps at corners. A pinched or cut gasket will leak regardless of how tight the dogs are.

Dogs (dogging hardware) – Rotating or sliding wedge-type clamps that pull the cover down evenly against the gasket. Dogs must be adjusted so compression is uniform around the entire perimeter. Uneven compression—tight on one side, loose on the other—creates a leak path as surely as a missing gasket.

Transferring the Layout from Drawing to Deck

1. Identify the hatch centerline on the drawing and note its coordinates relative to a known reference—typically a frame line or centerline of the vessel.

2. Transfer those coordinates to the deck plate using a tape measure and combination square. Mark the centerline in both directions with a scribe line.

3. From the centerline, lay out the four sides of the cut opening at the dimensions shown on the drawing. The cut opening is smaller than the coaming outside dimension; the coaming flange will overlap the deck and be welded to it.

4. Scribe the cut line clearly. Double-check all four dimensions before you start the plasma cutter.

Accuracy target: coaming location within ±1/8 inch of drawing dimensions. Measure from the same reference points used on the drawing. A second check by a partner before cutting is good practice.

Plasma-Cutting the Opening

Plasma cutting produces a clean, fast cut in steel deck plate but also produces UV arc light, metal sparks, and hot dross. Before cutting:

After cutting, grind or file any sharp edges and verify the opening size with a tape measure before fitting the coaming.

Fitting and Tacking the Coaming

1. Center the coaming over the opening using your scribe lines as a guide. The coaming should sit flat on the deck with equal overlap on all sides.

2. Check that the coaming is square to the deck plate—use a combination square on all four sides.

3. Measure from the reference point to the coaming centerline in both directions and record. If within ±1/8 inch, proceed. If not, shift the coaming and recheck.

4. Tack-weld the coaming in place at four corners with small, equally spaced tack welds. Do not fully weld until the instructor has completed the dimensional check.

Welding the Coaming Perimeter

After the instructor signs off on the dimensional check, weld the full perimeter of the coaming to the deck plate. Use the process and filler metal specified for the plate material (typically MIG/GMAW for mild steel shipbuilding panels). Weld sequence matters: weld opposite sides alternately—one long side, then the opposite long side, then the two short sides—to minimize distortion pulling the coaming out of position.

After welding, allow the assembly to cool, then grind all external welds flush with the deck surface. The cover must seat flat; any proud weld bead will prevent the cover from seating and will cause leakage.

Installing the Cover and Gasket

1. Inspect the gasket for cuts, missing sections, and proper corner seating. Replace any damaged gasket.

2. Seat the cover on the coaming with the gasket in place.

3. Engage all dogs finger-tight, then snug them in an alternating pattern (similar to lug-nut torqueing) until the gasket is compressed uniformly around the perimeter. Use a torque wrench if a torque value is specified on the hardware data sheet.

4. Visually inspect that the gap between cover and coaming is equal on all sides—this indicates even compression.

The Hose Test

The hose test is standard USCG practice for verifying watertight integrity of closures. Procedure for this lab:

1. One student positions inside (below) the hatch to observe for leaks.

2. A second student outside directs a continuous stream of water from a garden hose at the hatch from all angles for 60 seconds of direct spray.

3. The observer calls out any location where water appears. Mark it on the inspection form.

4. If leakage occurs, determine the cause—loose dog, gasket gap, or defective weld—correct it, and repeat the full test.

5. The hatch passes only when 60 seconds of direct spray produces zero leakage. The inspector and student both sign the inspection record.

Safety Summary

Key terms8

CoamingA raised steel collar welded around a deck opening; it provides the seating surface for the hatch cover and helps prevent water intrusion into the compartment below.
Dogging hardware (dogs)Wedge-type or cam-type clamps that draw the hatch cover down against the gasket; uniform adjustment of all dogs around the perimeter is required for a watertight seal.
GasketA compressible sealing material, typically neoprene or rubber, seated between the cover and coaming that deforms under dog pressure to create a watertight seal.
WeathertightA closure that prevents water from entering under the sea conditions for which the vessel is designed; a less stringent standard than fully watertight (which must resist hydrostatic head).
Hose testA USCG-recognized acceptance test in which a direct water stream is applied to a closure for a specified period; passing requires zero leakage observed from inside the compartment.
Tack weldA small, temporary weld used to hold components in their fitted position before full perimeter welding; tacks must be strong enough to resist movement during welding but are later incorporated into or replaced by the final weld.
Weld distortionDimensional change caused by the shrinkage of weld metal as it cools; controlled by using a balanced weld sequence—alternating opposite sides—to prevent the coaming from pulling out of position.
Dimensional toleranceThe allowable variation from the drawing dimension; for this hatch installation the coaming location must be within ±1/8 inch of the drawing-specified position measured from the reference point.

Worked examples3

1. The drawing shows the hatch centerline located 18 inches from frame 10 and 24 inches from the vessel centerline. After fitting the coaming, you measure 18 3/16 inches from frame 10 and 23 7/8 inches from the vessel centerline. Is the coaming within the ±1/8-inch tolerance? By how much is each dimension off?
  1. Convert the tolerance to a fraction: ±1/8 inch = ±0.125 inch.
  2. Frame-10 direction: measured 18 3/16 in, required 18 in. Error = 18 3/16 − 18 = 3/16 in = 0.1875 in.
  3. Compare to tolerance: 0.1875 in > 0.125 in, so this dimension is OUT of tolerance by 3/16 − 1/8 = 1/16 inch.
  4. Vessel-centerline direction: measured 23 7/8 in, required 24 in. Error = 24 − 23 7/8 = 1/8 in = 0.125 in.
  5. Compare to tolerance: 0.125 in = 0.125 in, so this dimension is exactly at the tolerance limit—acceptable (just barely within ±1/8 in).
  6. Conclusion: the frame-10 direction fails; the coaming must be shifted 1/16 inch toward frame 10 before welding.
Answer: The coaming is out of tolerance in the frame-10 direction by 1/16 inch (3/16 in error vs. 1/8 in allowed). The centerline direction is exactly at the limit and acceptable. Adjustment is required before tack welding is approved.
2. The hatch opening on the drawing is 18 inches × 24 inches. The coaming has a 2-inch-wide flange that overlaps the deck on all sides. What is the outside dimension of the coaming, and what size opening do you cut in the deck plate?
  1. The cut opening equals the inside clear dimension of the coaming. The drawing calls for 18 in × 24 in opening, so cut 18 in × 24 in.
  2. The coaming flange adds 2 inches on each side: outside width = 18 + 2 + 2 = 22 inches.
  3. Outside length = 24 + 2 + 2 = 28 inches.
  4. Verify: the coaming outside dimension is 22 in × 28 in; the deck cut is 18 in × 24 in; the 2-inch flange overlaps the deck on all four sides and is the weld surface.
Answer: Cut the deck opening 18 in × 24 in. The coaming outside dimension is 22 in × 28 in.
3. You are adjusting four dogs on the short side of a rectangular hatch. The gasket manufacturer specifies that the cover gap to the coaming seating face should be between 1/16 inch and 3/32 inch when properly compressed. After initial dogging you measure the gap at each dog location: Dog A = 1/16 in, Dog B = 3/16 in, Dog C = 1/8 in, Dog D = 1/16 in. Which dogs are out of specification, and in which direction?
  1. Specified range: 1/16 in (0.0625 in) to 3/32 in (0.09375 in).
  2. Dog A = 1/16 in = 0.0625 in — at the lower limit, acceptable.
  3. Dog B = 3/16 in = 0.1875 in — above 3/32 in (0.09375 in); gap is too large, meaning the cover is not pulled down far enough; Dog B needs to be tightened.
  4. Dog C = 1/8 in = 0.125 in — above 3/32 in (0.09375 in); gap is too large; Dog C needs to be tightened.
  5. Dog D = 1/16 in = 0.0625 in — at the lower limit, acceptable.
  6. After tightening B and C, re-measure all four and repeat until all gaps are within 1/16 to 3/32 in.
Answer: Dogs B and C are out of specification—their gaps are too large (under-compressed). Both must be tightened. Dogs A and D are within specification.

The math in this lesson1

Learners add flange widths to an 18 by 24 inch opening and check hatch location deviations.
7.NS.3
In class: Instructor demonstration: transferring hatch centerline from drawing to deck, scribing the cut line, plasma-cutting the opening, fitting and tacking the coaming, welding the full perimeter, grinding flush, and fitting the cover with gasket and dogs. · Student hatch installation: each student plasma-cuts an opening in a mock deck plate, fits and tacks a coaming to dimension, presents for instructor dimensional check, welds the coaming, grinds welds to flush with deck, installs the gasketed cover, and adjusts dogs for uniform compression around the perimeter. · Leak test exercise: the installed hatch is subjected to a controlled hose test (simulating USCG hose test practice), with a second student inside observing for leaks; any leakage is marked, gasket or dog adjustment is made, and the test is repeated until the hatch passes; results are recorded on an inspection form.
Materials: Mock deck plate panels, access hatch kit (coaming, cover, gasket, dogs, hardware), plasma cutter, MIG welder, angle grinder, combination square, tape measure, scribe, torque wrench, garden hose with nozzle for hose test, inspection record forms, full welding PPE, hearing protection
Check: Instructor measures coaming location (±1/8 inch), checks dog operation (no binding, uniform gasket compression), and witnesses hose test with zero leakage for 60 seconds of direct spray; all three criteria must pass; inspection record must be signed by student and instructor.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

NCCER Shipfitting – Layout and Fabrication
1. When laying out a hatch coaming on a deck plate, measurements should always be taken from:
NCCER Shipfitting – Dimensional Inspection
2. The tack-welded coaming measures 3/16 inch from the required position. The tolerance on the drawing is ±1/8 inch. The correct action is:
NCCER Shipfitting – Marine Outfitting
3. After welding the coaming, external welds must be ground flush with the deck surface primarily because:
ABS/USCG Weld Test – Watertight Integrity
4. During a hose test, a student inside the compartment sees water dripping at the corner of the hatch on one side. The most likely cause is:
NCCER Maritime Welding – Welding Sequence and Distortion Control
5. The recommended welding sequence for a rectangular coaming is to weld:
NCCER Shipfitting – Safety
6. Which personal protective equipment is specifically required for plasma cutting that may not be needed for routine hand-tool work?
ABS/USCG Weld Test – Acceptance Testing
7. In the USCG hose test as practiced in this lab, the hatch is considered to have passed when:
NCCER Shipfitting – Marine Outfitting
8. A gasket that appears continuous but has a small pinch cut at one corner should be:
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