A stone dry-dock is a masonry-walled basin cut into a waterfront, sealed by a removable caisson gate, that floods to admit a ship and then pumps dry to expose the hull for repair. The first true stone graving dock was built at Portsmouth in 1495 under Henry VII by master shipwright Robert Brygandine. The dock floods through sluices, the ship floats in over a prepared array of keel blocks, the gate closes, and pumps remove the water. The hull settles dry on the blocks, giving full access for plating, painting and propeller work — a method still used on vessels up to 500,000 tonnes deadweight.
Stone Dry-dock Interactive Calculator
Vary dock dimensions, leakage, and target pump-down time to see basin volume, required pump capacity, and an animated dry-dock dewatering diagram.
Equation Used
The calculator first estimates the water volume in a rectangular dock from length, width, and working depth. It then sizes the installed pump capacity so the net removal rate meets the target dry time while also overcoming caisson and sluice leakage.
- Dock basin is approximated as a rectangular volume.
- Pump capacity and leakage rate remain constant during dewatering.
- Minor losses, pump warm-up time, and residual sump water are ignored.
How the Stone Dry-dock Works
A stone dry-dock is a long rectangular pit lined with stepped masonry walls, called altars, with a flat floor of dressed granite or concrete and a single opening to the sea closed by a floating caisson gate. You float the ship in at high water, the gate is dropped into its sill recess, and dewatering pumps drain the basin over 3 to 6 hours. As the water falls, the keel touches down on a pre-laid line of keel blocks running down the centreline, and side bilge blocks are hauled in tight under the turn of the bilge to stop the hull rolling.
The geometry has to be right or the ship will land badly. The keel blocks must be set to the docking plan supplied by the ship's owner — block heights typically 1.2 to 1.8 m, spaced 1.2 m on centres, with the top surface laid to the rocker of the keel within ±10 mm. Get the spacing wrong and you concentrate load on a single frame and dish the bottom plating. Get the gate sill leakage above about 50 m³/hr and the pumps run continuously just to keep the dock dry, burning power and shortening pump life.
Failure modes are well known. Caisson gates leak when the rubber compression seal hardens or the sill silts up — divers clear the sill before every docking. Altar masonry spalls under freeze-thaw cycles in northern docks like Rosyth or Saint John, so you see Portland cement repointing every 15 to 20 years. Pumps cavitate if the suction wells aren't kept clear of debris. And keel blocks, usually oak capped with softwood, crush and need replacement after 30 to 50 dockings.
Key Components
- Caisson Gate: A hollow steel box-shaped vessel that floats into position at the dock entrance and is then ballasted down onto a stone sill. Modern caissons run 25 to 40 m wide and seal against rubber gaskets compressed under 6 to 10 m head of water. Leakage above 50 m³/hr signals a damaged seal or a fouled sill.
- Altars (Stepped Side Walls): Tiered stone or concrete steps rising from the dock floor to capstan level. Each step is roughly 600 to 900 mm high and 600 mm deep, providing footing for shores and side blocks against the hull. Granite is preferred for its compressive strength of around 200 MPa and resistance to salt-water erosion.
- Keel Blocks: Stacked oak or laminated timber blocks running down the centreline, capped with a softer pine pad that crushes a few millimetres to spread keel load. Standard spacing is 1.2 m on centre, height 1.2 to 1.8 m. Block top surfaces must be laid within ±10 mm of the docking plan rocker.
- Bilge Blocks and Side Shores: Wedge-topped blocks hauled inboard on rails after the keel touches down, plus angled timber shores wedged from the altars to the hull side. They prevent roll and take roughly 15 to 20% of the ship's weight.
- Dewatering Pumps and Culverts: Vertical-shaft centrifugal pumps, typically 2 to 4 units totalling 5,000 to 20,000 m³/hr capacity, draw through underground culverts feeding from sumps at the dock head. A 200 m × 30 m × 12 m basin holds about 70,000 m³, so a 4-hour pump-down needs 17,500 m³/hr installed capacity with a margin for leakage.
- Dock Sill: The dressed stone or concrete threshold the caisson seats against. It must be flat within ±5 mm across its length to give a uniform seal compression. Silting and shell growth are the most common reasons for caisson leaks after a year of service.
Who Uses the Stone Dry-dock
Stone dry-docks remain the dominant method for hull inspection, repair, repainting and propeller replacement on commercial and naval vessels too large or too valuable for a floating dock or a haul-out cradle. They are the preferred choice when you need a stable working platform, full hull access, and the ability to handle vessels above 50,000 tonnes deadweight. You see them in every major naval port and at most large commercial ship-repair yards.
- Naval Ship Repair: HMNB Devonport's 14 and 15 Docks are the only UK docks rated to refit Vanguard-class ballistic missile submarines, with stone-lined basins dating to the 19th century and modernised pumping plant.
- Commercial Ship Repair: Damen Shiprepair Rotterdam operates Dock 8, a 405 m × 90 m graving dock, used for VLCC tanker hull recoating and propeller work on vessels up to 500,000 dwt.
- Heritage Vessel Conservation: The Brunel-designed Great Western Dockyard in Bristol holds SS Great Britain in the same dry-dock built for her launch in 1843, now glassed over for permanent dehumidified display.
- Cruise Ship Maintenance: Grand Bahama Shipyard's Dock 2 takes Royal Caribbean Oasis-class hulls — 360 m long, 47 m beam — for biennial bottom paint and stabiliser overhauls.
- Offshore and Heavy-Lift Construction: Hyundai Heavy Industries Ulsan uses graving docks up to 700 m long for serial construction and float-out of FPSO hulls and ULCC tankers.
- Submarine Refit: Norfolk Naval Shipyard's Dry Dock 8, opened 1942 in granite-faced concrete, refits Nimitz and Ford-class carriers and Virginia-class submarines.
The Formula Behind the Stone Dry-dock
The single most useful calculation in dry-dock planning is pump-down time. It tells you whether your installed pumping plant can drain the basin within the tide cycle and the contracted docking window. At the low end of typical operating range — a small heritage dock of around 20,000 m³ with one 5,000 m³/hr pump — you'll see pump-down in 4 hours and the operator has plenty of margin. At nominal commercial scale, around 70,000 m³ with 17,500 m³/hr installed, you target 4 hours but lose 20 to 30 minutes to leakage and pump warm-up. Push to the high end — a 700 m supertanker dock at 250,000 m³ — and you need 60,000 m³/hr or more, often staged across 4 pumps, and any single pump failure pushes you outside the tide window.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| tpump | Time to dewater the basin from full to dry | hours | hours |
| Vbasin | Volume of water in the basin at high tide above the dock floor | m³ | ft³ |
| Qpump | Total installed pump capacity | m³/hr | ft³/hr |
| Qleak | Combined caisson seal and sluice leakage rate | m³/hr | ft³/hr |
Worked Example: Stone Dry-dock in a mid-size commercial graving dock
Your shipyard operations team in A Coruña Spain is sizing the dewatering plant for a refurbished stone graving dock 220 m long, 32 m wide, with a working depth of 11 m above the dock floor at high water. The contract requires the dock to be dry within 4 hours of caisson seating so the survey team can begin block inspection on the ebb tide.
Given
- L = 220 m
- W = 32 m
- H = 11 m
- Qleak = 300 m³/hr
- ttarget = 4 hr
Solution
Step 1 — calculate the basin volume of water to be removed:
Step 2 — at the nominal target of 4 hours pump-down, solve for required pump capacity including leakage:
You'd specify 4 vertical-shaft pumps of 5,000 m³/hr each — total 20,000 m³/hr installed, giving a small margin and N+1 redundancy if one trips.
Step 3 — at the low end of typical operating range, with only 3 pumps online (15,000 m³/hr) after a single unit fails:
That is 1 hour 16 minutes outside the 4-hour contract window. The survey team will be standing on a wet floor at the start of the ebb and you'll lose the working tide. At the high end of leakage — say a degraded caisson seal pushing Qleak up to 1,500 m³/hr with all 4 pumps running:
You're still close to spec with all pumps but a single failure now puts you well over 5.5 hours. The sweet spot for this dock is a 20,000 m³/hr plant with seal leakage held below 500 m³/hr — that gives you 4 hours nominal and one-pump-out resilience.
Result
Nominal installed pump capacity needed is 19,660 m³/hr, which rounds up to 20,000 m³/hr across 4 pumps. In practice that means the dock floor is walkable around 4 hours after caisson seating — survey crews can step off the altar stairs onto dry granite and begin block inspection on schedule. At the low end with one pump out you stretch to 5.3 hours and miss the tide window; at the high end with a failed seal but all pumps running you hold 4.2 hours. If your measured pump-down time runs longer than predicted, the three most common causes are: (1) a hardened caisson rubber gasket letting sill leakage climb above 1,000 m³/hr — divers should pressure-test the seal before the next docking, (2) a fouled pump suction screen dropping individual pump output by 15 to 20%, often visible as drawdown in the sump well, or (3) sluice valve poppets not fully seated, allowing harbour water to refill from the wrong side of the gate.
Stone Dry-dock vs Alternatives
Stone dry-docks aren't the only way to get a hull out of water. The main alternatives are floating docks, which are themselves ships, and marine railways or syncrolifts, which haul vessels onto land on cradles. Each has a clear application range and the choice usually comes down to vessel size, location, and how much capital the yard can sink into civil works.
| Property | Stone Dry-dock (Graving Dock) | Floating Dock | Marine Railway / Syncrolift |
|---|---|---|---|
| Maximum vessel size | Up to 500,000+ dwt (700 m × 100 m basins exist) | Typically up to 100,000 dwt, limited by pontoon strength | Up to ~25,000 tonnes for syncrolift, less for railway |
| Capital cost | Very high — $200M to $1B+ for civil works | High — $50M to $300M for the dock vessel | Moderate — $10M to $100M |
| Pump-down / lift time | 3 to 6 hours typical | 4 to 8 hours typical | 30 minutes to 2 hours |
| Working stability | Excellent — solid masonry, no motion | Good but pontoon flexes under load | Cradle creep and rail wear introduce small movements |
| Lifespan | 100+ years with repointing every 15-20 years | 40 to 60 years before major refit | 30 to 50 years; rails and cables wear out |
| Best application fit | Large naval refits, VLCC repair, new-build serial construction | Mid-size commercial repair, mobile naval support | Small craft, fishing fleets, fast haul-outs |
Frequently Asked Questions About Stone Dry-dock
Calculated pump-down assumes a single rectangular volume, but real basins have stepped altars and side culverts that hold extra water you don't account for in L×W×H. On a typical 200 m dock the altars alone add 8 to 12% to the effective volume below the last 2 m of head.
The other big factor is pump head rising as the basin empties. Centrifugal pumps lose flow when static lift increases — at the last 1 m of water, flow can drop 20 to 30% versus the rated nameplate figure. Check your pump curves against the actual lift at the sump, not the average.
Widening an existing graving dock means cutting back the altar masonry on both sides, which is rarely cheaper than 40% of new-build cost and forces a 12 to 24 month outage. The decision usually swings on whether the existing sill and caisson rebate are reusable.
Rule of thumb — if the customer beam is within 2 m of your current entrance and you have a 30+ year backlog of similar work, widen. If beam is 5 m or more over, or your caisson is at end of life anyway, build new and keep the old dock for smaller hulls. Belfast's H&W Building Dock was built rather than widened for exactly this reason.
Docking plans show the keel rocker as built, not as the ship floats today. Vessels in service hog or sag — typically 50 to 150 mm over a 200 m hull depending on cargo history and age. An engine room high spot usually means the hull has sagged amidships and the blocks under the engine room are now too tall relative to the ends.
The fix is to take a soundings survey at the last waterline before docking and compare to the as-built. Crush pads on top of each block (softwood, 50 mm thick) absorb up to 15 mm of mismatch. Anything beyond that needs the blocks re-set, not just re-padded.
Visual inspection misses the two most common culprits. First, the gasket compression set — rubber under sustained compression hardens and loses its rebound, so even an undamaged seal stops conforming to small sill irregularities after about 8 to 10 years. Press a fingernail into the gasket; if it doesn't recover within a second, replace it.
Second, sill silting is sneakier than it looks. A 5 mm layer of mud across the sill width is invisible from the dock head but creates a continuous leak path because the gasket bridges over it rather than sealing into it. Have divers water-jet the sill before the next docking and re-measure leakage.
Below 5,000 tonnes the economics almost always favour a syncrolift or marine railway. A graving dock costs 5 to 10× more in civil works and the pump-down time eats into your turnaround on small jobs that should clear in 48 hours.
The exception is if you handle vessels with deep draft relative to length — some fishing trawlers and small naval craft sit too low for cradle support and need block-and-shore docking. In that narrow case a small graving dock or a small floating dock makes sense even at 2,000 to 5,000 tonnes.
For a typical merchant hull form, the keel blocks take roughly 80 to 85% of the displacement and the bilge blocks the remaining 15 to 20%. For fine-lined naval hulls or submarines the keel share rises to 90%+ because there's almost no flat bilge to bear on.
It matters because oak keel blocks crush at about 8 to 10 MPa across the grain. A 50,000 tonne ship on 150 keel blocks of 600 × 600 mm contact area puts roughly 7.7 MPa per block — already at the edge. Underestimate the keel share or skip the softwood crush pad and you'll see splits radiating from the block tops after a single docking.
References & Further Reading
- Wikipedia contributors. Dry dock. Wikipedia
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