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newSchoolWorks.com › VoTechWorks › Manufacturing & Marine › Shipbuilding / Marine Fabrication › Hull Construction › Lesson 9
MFG-SHIP13-02 · Lesson 9 · 22 clock hours · lab

Shell Plating Fit-Up and Tacking

Objective. The student fits and tacks a strake of shell plating to a framed hull section, achieving a root gap and land dimension within the tolerances specified by the applicable weld procedure, and a fair exterior surface verified by batten.

Shell Plating Fit-Up and Tacking

Shell plating is the outer skin of the hull. It keeps water out, carries structural loads, and defines the hull's hydrodynamic shape. Fitting shell plates correctly—before a single full weld pass is run—is one of the most important skills in shipfitting. A poorly fitted plate that gets welded in place creates distortion, fairness problems, and weld defects that are expensive and time-consuming to correct.

What Is a Strake?

A strake is a longitudinal row of shell plates running from bow to stern. Plates within a strake are butt-welded end to end; adjacent strakes are welded along their edges (seams). Each plate must conform to the curvature of the frames underneath it, maintain the correct joint geometry at every edge, and present a fair exterior surface.

Reading the Weld Procedure Specification (WPS)

Before touching a plate, read the instructor-issued WPS for the joint. The WPS is the governing document. It specifies the root gap, the land (the flat face at the root of a bevel-groove joint), the bevel angle, the filler metal, and the preheat if required. Your fit-up must match what the WPS states. Common root gaps for hull butt joints run in the range of 0 to 3/16 inch depending on the process and joint design; the exact value is printed on your WPS. Do not guess or use a number from memory.

Fit-Up Sequence: Centerline Outward

Always begin clamping from the centerline (keel) outward toward the turn of the bilge and up the sides. This sequence distributes the plate's curvature progressively and prevents locking in unfair spots. The steps are:

1. Position the plate over the frames by crane or by hand. Rest it so its edges roughly align with the joint lines.

2. Secure one end with a hydraulic dog or clamp at the centerline end, holding the plate down to the adjacent strake or to a backing bar.

3. Work outward, placing dogs or clamps at each frame station, pulling the plate tight to the frame face. Mechanical strongbacks spanning two or more frames hold the plate against spring-back while you move to the next position.

4. Check the root gap at multiple locations along the joint using a root-gap gauge or feeler gauge. Adjust jack pressure or dog position to open or close the gap until all readings fall within the WPS tolerance.

5. Check the land if the joint is beveled. The land must be within the WPS dimension so the root pass fuses fully without burn-through.

Tack Welding

Tack welds hold the plate in position until full welding begins. The WPS (or the project quality plan) specifies minimum and maximum tack spacing, minimum tack length, and minimum tack leg size. Common practice places tacks every 12 to 18 inches along long seams, but follow your WPS. Each tack must:

If a tack cracks during fit-up or adjustment, grind it out completely before placing a replacement.

Fairing the Shell

Fairness means the hull surface follows a smooth, continuous curve with no bumps, hollows, or hard spots. After tacking, run a fairing batten—a flexible straight-edge made of wood or fiberglass—along the outside of the strake. Press it against the plate and sight along it. Any deviation greater than 1/8 inch must be corrected before the instructor signs off.

To correct a high spot (plate standing proud), place a hydraulic jack between the plate and the strongback to pull it in, or add a dog closer to the high spot and re-tack. To correct a low spot (hollow), release the nearest tack, jack the plate out, and re-tack. Never just weld over a fairness defect—heat distortion will make it worse.

Safety

Personal protective equipment (PPE): Wear safety glasses at all times in the lab. Add a welding helmet with the correct shade lens for MIG welding when placing tacks. Leather gloves protect against sharp plate edges and weld spatter. Wear hearing protection when grinding or using pneumatic tools nearby.

Hydraulic equipment: Inspect hoses and fittings before pressurizing any jack or dog. Never place any part of your body between the jack and the plate. Release pressure slowly.

Hot metal: Tacked plate and freshly welded tacks stay hot far longer than they look. Use a temperature stick or infrared thermometer if you must handle the plate shortly after tacking; otherwise wait or use insulated gloves.

Housekeeping: Keep the area around the mock-up clear of scrap plate, cables, and hoses. Tripping while carrying a hydraulic jack or while leaning over a plate edge is a serious hazard.

Lockout/Tagout (LOTO): If you must adjust or unjam any powered positioning equipment, follow the shop's LOTO procedure before reaching into the pinch zone.

Instructor Check

Your instructor will measure the root gap at five equally spaced points along the joint with a feeler gauge or root-gap gauge. All five readings must fall within the WPS tolerance. The batten fairness check must show no deviation greater than 1/8 inch along the full length of the strake. Both checks must pass before you proceed to full welding.

Key terms9

StrakeA longitudinal row of shell plates running the length of the hull; plates within a strake are butt-welded end to end.
Root GapThe distance between the unwelded root faces of two plates at a joint, specified by the WPS to ensure proper root fusion.
Land (Root Face)The flat, unbeveled portion of the groove face at the root of a joint; controls burn-through and fusion at the root pass.
Weld Procedure Specification (WPS)A written document that prescribes all variables for a weld joint—process, filler metal, joint geometry, root gap, preheat, and more—that welders and fitters must follow exactly.
Fairing BattenA flexible, straight-grained wood or fiberglass strip used to check the smoothness and continuity of a hull surface by pressing it against the plating and sighting along it.
Hydraulic DogA clamping device that hooks over the edge of a plate and is tightened by hydraulic pressure to pull adjacent plates together or down to frames.
StrongbackA structural bar or beam temporarily welded or clamped across a plate to hold it flat or to the correct curvature against spring-back during fit-up.
Tack WeldA short, temporary weld placed to hold parts in position for fit-up; must be sound and feathered at the ends so it can be fused through during production welding.
FairnessThe condition of a hull surface that follows a smooth, uninterrupted curve with no abrupt high or low spots when checked with a batten.

Worked examples4

1. Your WPS specifies a root gap of 3/32 inch ± 1/16 inch. After fitting the plate, your feeler gauge reads 1/8 inch at the midpoint of the joint. Is this reading within tolerance, and by how much does it deviate from the target?
  1. Convert all values to a common unit. Use thirty-seconds: 3/32 inch = 3/32; 1/16 inch = 2/32 inch; 1/8 inch = 4/32 inch.
  2. Find the allowable range: lower limit = 3/32 − 2/32 = 1/32 inch; upper limit = 3/32 + 2/32 = 5/32 inch.
  3. Compare the measured value (4/32 inch) to the range (1/32 to 5/32 inch): 4/32 falls between 1/32 and 5/32.
  4. Calculate deviation from target: 4/32 − 3/32 = 1/32 inch above nominal.
Answer: The 1/8-inch (4/32-inch) reading is within tolerance. It is 1/32 inch above the nominal target but still inside the ± 1/16-inch band. No adjustment is required at this point.
2. The instructor requires tack welds at intervals not to exceed 18 inches. Your shell plate joint is 7 feet 6 inches long. What is the minimum number of tack welds needed, and what is the resulting uniform spacing?
  1. Convert the joint length to inches: 7 feet 6 inches = (7 × 12) + 6 = 90 inches.
  2. Divide by the maximum allowable spacing to find the minimum number of intervals: 90 ÷ 18 = 5.0 intervals exactly.
  3. Number of tacks = number of intervals = 5. (Five intervals means five tacks, assuming tacks are placed at the end of each interval rather than at both ends plus between; confirm with your instructor whether end tacks count, but the standard approach for a run is one tack per interval, giving 5 tacks for 5 intervals of 18 inches.)
  4. Uniform spacing = 90 ÷ 5 = 18 inches. This meets the maximum spacing requirement exactly.
Answer: A minimum of 5 tack welds placed at 18-inch uniform spacing satisfies the WPS requirement for this 90-inch joint.
3. While running the fairing batten, you find the plate stands 3/16 inch proud of the adjacent strake at one frame station. The acceptance limit is 1/8 inch. How much must you reduce the deviation, expressed as a fraction of an inch?
  1. Express the measured deviation in thirty-seconds: 3/16 inch = 6/32 inch.
  2. Express the acceptance limit in thirty-seconds: 1/8 inch = 4/32 inch.
  3. Calculate the excess: 6/32 − 4/32 = 2/32 inch = 1/16 inch.
  4. You must move the plate inward by at least 1/16 inch to bring the high spot within the 1/8-inch limit.
Answer: The plate is 1/16 inch out of tolerance. The plate must be jacked inward at least 1/16 inch, the tack ground out and replaced, and the batten re-run to confirm the correction before sign-off.
4. The WPS calls for a 1/8-inch land on a single-V groove. After beveling, you measure the land at 3/16 inch. Assuming the WPS allows ± 1/16 inch on the land dimension, is the joint acceptable?
  1. Convert to a common unit in thirty-seconds: specified land = 1/8 = 4/32 inch; tolerance = 1/16 = 2/32 inch; measured land = 3/16 = 6/32 inch.
  2. Allowable range: 4/32 − 2/32 = 2/32 inch (1/16 inch) minimum; 4/32 + 2/32 = 6/32 inch (3/16 inch) maximum.
  3. Compare measured value (6/32 inch) to the range (2/32 to 6/32 inch): 6/32 is exactly at the upper limit.
Answer: The 3/16-inch land is exactly at the upper tolerance limit and is therefore acceptable. However, because a land that is too large can cause incomplete root fusion, the fitter should note it and the production welder should be informed to use appropriate parameters.

The math in this lesson2

Learners divide a 7 ft 6 in joint by an 18-inch maximum interval to count tacks.
6.RP.37.NS.3
In class: Instructor demonstrates shell plate fit-up on a curved section using jacks, clamps, and hydraulic dogs to pull the plate fair; students observe the sequence of clamping from centerline outward and the use of a strongback to hold plates in position. · Each student fits one shell plate to the bottom section mock-up, achieving the root gap specified on the weld procedure specification (WPS) used for the joint, and places tack welds at intervals per the WPS. · Students run a fairing batten along the outside of the tacked shell strake and mark any high or low spots; they then adjust tacks or use a hydraulic jack to correct fairness defects before instructor final check.
Materials: Shell plate blanks (cut in Lesson 6); hydraulic or mechanical clamps, dogs, jacks, strongbacks; fairing batten; root-gap gauges; WPS (instructor-issued, matching the joint type); MIG welding setup; soapstone for marking; safety glasses, gloves, hearing protection
Check: Instructor measures root gap at five equally spaced points along the joint using a feeler gauge or root-gap gauge; all five readings must be within the WPS tolerance; batten fairness check must show no deviation greater than 1/8 inch. Mapped to NCCER Shipfitting: shell plating; supports ABS/USCG weld test fit-up requirements.

Practice check8

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

NCCER Shipfitting – Shell Plating Fit-Up
1. When fitting a shell plate to a curved hull section, clamping should begin at which location and proceed in which direction?
NCCER Shipfitting – Shell Plating Fairness
2. A fairing batten is used after tacking to:
ABS/USCG Weld Test – Fit-Up Requirements
3. What document specifies the required root gap, land dimension, and tack weld spacing for a hull shell joint?
AWS D1.1 Certified Welder – Joint Fit-Up
4. Your WPS specifies a root gap of 1/8 inch ± 1/16 inch. A feeler gauge reads 7/32 inch at one location. What should you do?
ABS/USCG Weld Test – Tack Weld Quality
5. A tack weld placed during shell plate fit-up cracks when you release one of the dogs. The correct response is to:
NCCER Shipfitting – Safety
6. Which minimum PPE combination is required when placing MIG tack welds on shell plating?
NCCER Shipfitting – Shell Plating Fit-Up
7. A shell plate strake is 10 feet long. The WPS allows a maximum tack spacing of 15 inches. What is the minimum number of tack welds required?
NCCER Shipfitting – Shell Plating Fairness
8. The batten check reveals the shell plate has a low spot (hollow) at one frame station that exceeds 1/8 inch. The correct corrective action is to:
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