Marine-grade plywood is a high-quality, durable type of plywood that is designed for use in boat building and marine applications. It has several characteristics that make it suitable for withstanding outdoor exposure, moisture, and high impacts in a marine environment.

Characteristics of Marine-Grade Plywood

Marine-grade plywood has several defining features that set it apart from other plywood grades:

Water Resistance

Marine plywood is manufactured with waterproof structural adhesives that resist delaminating or decaying when exposed to high moisture environments. The glues used prevent the plies from coming apart even after prolonged water exposure.

Void-Free Cores

The core layers are made up of defect-free hardwood veneers that are free of voids, knots, and defects. This provides strength, impact resistance, and reduces points of water ingress.

Durable Face Veneers

The face and back veneers are high-quality hardwoods that are resistant to weathering, abrasion, and water damage. Species like okoumé, meranti, and Douglas fir are commonly used.

No Added Formaldehyde

Reputable marine plywood is made with non-toxic waterproof structural adhesives that do not contain urea-formaldehyde. This makes it safe for interior boat cabin use.

Designed For Exterior Use

Marine plywood is engineered specifically for exterior and humid conditions. The glues, veneers, sealing, and treatment are optimized for marine applications.

Grades of Marine Plywood

Marine plywood is commonly manufactured in the following grades, which indicate the quality of the face and back veneers:

Grade A-A

Highest grade marine ply.
Has blemish-free face and back veneers made of durable tropical hardwoods.
Most premium boatbuilding plywood.

Grade A-B

Front face has excellent appearance, back face has minor blemishes.
Commonly uses okoumé or meranti for the face.
Good strength and cost-efficient marine ply.

Grade B-B

Both sides have minor blemishes and repairs.
The most affordable marine grade.
Often uses softer woods like cedar or pine.

Within these grades, the number of plys can also vary, with 5, 7, 9 and 11 ply being common. 5 ply provides adequate stiffness for many small boat uses while 11 ply is very rigid and used on large vessels.

Uses of Marine Plywood

Due to its specialty durability, marine-grade plywood is recommended for:

Boat hulls, decks, cockpits.
Any wood surface on a boat at risk of water exposure.
Boat flooring and joinery.
Docks, piers, boat houses.
Canoe and kayak construction.
Door skins and signs placed outdoors near water.

It provides stiffness, impact resistance, and longevity in high-moisture marine applications where standard plywood would delaminate or decay rapidly.

While more expensive than regular plywood, marine ply is commonly considered one of the strongest and most durable plywood products available. When properly sealed and maintained, it can last for many decades in marine use.

Manufacturing Process

Marine plywood is manufactured through a specialized process to create a moisture resistant product:

Only select, defect-free hardwood veneers are used, sourced from sustainable forests.
Veneers are laminated together with durable, waterproof structural adhesives designed for wet environments. Phenol formaldehyde is commonly used.
The adhesive spread rate is calibrated to ensure optimal bonding between plies.
Higher numbers of thinner veneers are laminated, compared to regular plywood. This provides added strength.
Large heated presses apply high pressure to tightly bond the veneers together into a solid panel.
Factory edge sealing prevents moisture ingress through the panel ends.
Prestigious marine plywood brands undergo stringent quality control testing and are Lloyd’s Register certified.

Reputable brands use this controlled manufacturing process to create marine-grade plywood panels with complete bonding between the laminates and a high resistance to rot and delamination.

How to Identify Marine Plywood

When purchasing marine plywood, check for the following indicators of quality:

Grades Stamp: Reputable brands will stamp the grade, brand, standards certification and country of origin on panels. Grade A-B is very common.
Lack of Voids: Inspect panel edges for a solid cross-section without holes, gaps or defects.
Even Plies: The plies should be finely interlaminated without variation in density, indicating strong adhesion.
Smooth Faces: Premium panels will have defect-free, smooth veneers without patching or open defects.
Lightweight: marine ply is made from thin, dense hardwood plies so weight is moderately low. Overly heavy panels indicate excessive moisture or lower veneer quality.
Sealed Edges: Quality panels will have a colored, resin-rich sealed edge treatment for water resistance.
Scanding & Drilling: The plywood should sand cleanly without loose plies, and drill bits should not cause fraying or chipping.

Using a reputable supplier of Lloyd’s Register certified marine plywood helps ensure you are purchasing a high-performance panel suitable for boat building.

Advantages of Marine Plywood

There are numerous benefits to selecting quality marine-grade plywood:

Strong: Provides excellent stiffness, impact strength and fastener retention. High ply counts available.
Durable: Does not delaminate or decompose quickly in wet environments. Long-lasting material.
Stable: Resists warping and rot over years of water exposure. Maintains structural integrity.
Workable: Easy to cut, shape, bend, join and install with woodworking tools and fasteners. Takes paint and varnish well.
Lightweight: Weighs less than solid wood while remaining robust. Easier to transport and work with than thicker lumber boards.
Versatile: Available in large panel sizes. Can be used for hull planking, decking, cabinets, door skins and other boat components.
Cost Effective: More affordable than some solid woods and composites. Price reflects premium quality and capabilities.

For boat builders, marine ply offers an optimal combination of strength, longevity, workability and cost-efficiency. When installed and sealed correctly, it will provide many years of reliable performance.

Disadvantages of Marine Plywood

The drawbacks of marine plywood include:

Expense: Costs significantly more than exterior or interior plywood grades. Price reflects specialized manufacturing and raw materials.
Weight: Heavier than composites like fiberglass. Not ideal for vessels prioritizing very light weight.
Maintenance: Requires refinishing with varnish on a regular basis to maintain water resistance and appearance.
Appearance: Does not have a seamless grain pattern or solid wood cosmetics. May have minor veneer repairs or blemishes on lower grades.
Sealing: Edges, cuts, holes and fasteners must be thoroughly sealed to prevent moisture ingress into the wood over time. Ongoing diligence required.
Availability: A specialty product that may need to be factory ordered. Limited selection at local general hardware stores.

While marine plywood has some downsides, proper installation and sealing combined with routine refinishing helps maximize the strength and longevity of this high-quality boatbuilding material.

How to Finish and Maintain Marine Plywood

To gain full benefits from marine-grade plywood, proper finishing and maintenance is key:

Seal All Ends and Edges

Use fiberglass cloth and epoxy to encapsulate exposed plywood edges. This prevents water wicking into the wood.

Apply Protective Coating

Use a high-quality marine varnish, polyurethane or epoxy coating to waterproof the wood and protect against UV damage. 3-4 coats minimum.

Inspect for Damage

Periodically inspect for cracks, splintering, drilling damage or deterioration of the coating. Immediately re-seal any compromised areas.

Re-Apply Coating

Sand down and re-apply protective finish every 1-2 years to maintain optimal water resistance and luster.

Clean and Dry

When boat is out of water, allow wood to dry completely before applying finishes. Remove dirt or mold with mild cleaners.

Proper sealing and refinishing is imperative for marine plywood to resist water absorption and potential rotting or delamination over decades of use.

Alternatives to Marine Plywood

While marine plywood is a common boatbuilding material, some alternatives include:

Fiberglass – Composite of molded fiberglass cloth and polyester resin. Very water resistant but requires more specialized tools to work with.
Cedar or Redwood – Durable rot-resistant woods but require thick dimensions for stiffness. More prone to cracking than plywood.
Epoxies – Can be used to coat many materials. Not as strong on their own but excellent water barrier.
Aluminum – Very strong, lightweight and waterproof but requires metalworking tools and corrosion prevention.
High-Density Plastics – Materials like HDPE can be used but may lack the strength and ease of fastening.

Each comes with different pros and cons. Marine plywood offers a good balance of strength, longevity and workability at a reasonable cost for most small watercraft construction.

Key Takeaways on Marine Plywood

Marine plywood is engineered with waterproof structural adhesives and premium dense hardwood veneers to resist rot and moisture damage in marine environments.
Higher grades have defect-free face veneers while lower grades may have minor blemishes and patches.
Marine ply is commonly used for boat hulls, decks, flooring and other components that require strength, impact resistance and longevity outdoors.
Proper finishing and maintenance with varnish, epoxy and routine inspections maximizes its performance. When sealed and cared for, it can last for decades.
While stiffer, stronger and more durable than standard plywood, it is also heavier and more expensive.

Marine plywood is a smart choice when you need a robust sheet building material for a boat exposed to water, sun, and impacts. With the right marine grade, finishing, and care, it will provide many years of reliable service.

What Are the Common Thicknesses of Marine Plywood?

Marine plywood is available in a range of thicknesses to suit different construction needs. Some common dimensions are:

3mm (1/8″)

Very thin, flexible panels.
Suitable for tight bending around curving hull shapes.
Can be stacked and laminated for increased strength.
Not suitable for structural stiffness but good for overlay skins.

4mm (5/32″)

Thin, semi-flexible panels.
Adequate for paneling over framework.
Prone to vibration if not well supported.
Better for flat or gently curving surfaces.

6mm (1/4″)

The most popular and commonly used thickness.
Provides good balance of strength and weight.
Stiff enough for self-supporting furniture and cabinetry.
Easy to transport and work with using basic tools.

9mm (3/8″)

Robust panels with increased stiffness.
Suitable for flooring, shelving, and door skins.
Heavy panels requiring more reinforcement and support.
Can be used structurally but adds substantial weight.

12mm (1/2″)

Very thick, stiff panels.
Provides maximum strength for hulls, decks, and other critical boat components.
Prone to warping if not properly supported or fastened.
Difficult for one person to maneuver full sheets.

18mm (3/4″)

Extremely rigid sheets with very high fastener hold strength.
Reserved for large vessel hull planking and other critical structural elements.
Challenging to work with and very heavy. Often requires two person lifts or mechanical aids.
Mostly used on commercial, military or large recreational boats.

6mm marine plywood balances capability and easy workability for small craft construction, while 12mm provides maximum robustness for more demanding structural applications on bigger vessels. Consider the loads, span, and complexity of the project when selecting the ideal thickness.

What Types of Wood are Used in Marine Plywood?

Marine plywood is constructed using a range of tropical hardwood veneers that impart strength, rot-resistance and attractiveness. Some common wood species found in quality marine ply are:

Okoumé

Tropical African hardwood with a reddish-brown tone and consistent texture.
Offers excellent strength-to-weight ratio, bending properties, and weather resistance.
Frequently used for the face and back veneers on marine plywood.
Machines smoothly and has good gluing properties.

Meranti

Red/yellow tropical hardwood from Southeast Asia.
Balances durability, stability and cost effectiveness.
Used as a face veneer and a core veneer.
Machines and finishes well.

Keruing

Hard, durable Malaysian tropical wood.
Has a coarser grain texture with an orange-brown color.
Common inner ply material due to its strength.
More resistant to termites and water absorption than many other hardwoods.

Douglas Fir

Very strong softwood native to North America’s west coast.
Straight, consistent grain with pale yellow color.
Used for inner plies and also back veneers on some grades.
Provides structural reinforcement at a reasonable cost.

Teak

Esteemed tropical hardwood known for its beauty, strength and weather-resistance.
Sometimes utilized as a premium face veneer on high-end marine plywood.
Far more expensive than conventional marine ply veneers.

The optimal blend of these various wood veneers provides marine plywood with enhanced rot-resistance, dimensional stability, and longevity while maintaining affordable cost levels.

How Strong and Stiff is Marine Plywood?

Marine plywood offers impressive strength for its relatively light weight. Some key structural properties:

Density

680 – 800 kg/m3 (42 – 50 pounds per cubic foot)

The high density and hardwood construction provides rigidity and fastener retention strength. Approximately 20-30% denser than conventional plywood.

Stiffness (Modulus of Elasticity – MOE)

7 – 9 GPa (1 – 1.3 x 10^6 psi) along grain

Measure of stiffness under load. Marine ply has excellent rigidity for resisting deflection and buckling when spanning between supports.

Strength (Modulus of Rupture – MOR)

45 – 65 MPa (6,500 – 9,500 psi)

Indicates maximum load-carrying capacity before breaking. Marine plywood is very strong in tension and bending.

Hardness

3000 – 5000 N (Janka test)

Resistance to dents, abrasion and wear. Durable face veneers resist harsh marine impacts.

Screw Holding

400 – 600 N (90 – 135 lbf)

High screw retention strength allows marine ply to be securely fastened, even on vertical surfaces.

In summary, marine ply possesses robust mechanical properties on par with many softwood construction timbers, while only using 1/10th the thickness. This enables lighter and more easily worked panels without compromising strength.

What are Some Recommended Marine Plywood Brands?

When selecting marine plywood, using a reputable brand helps ensure optimal quality and performance. Some highly regarded manufacturers include:

BS-1088 Marine Plywood

Made in Malaysia to British marine standards.
Lloyd’s Register certified.
Available globally through plywood suppliers.
Graded as BS-1088-1 for hull panels.

Armourply Marine Ply

Manufactured in Australia.
Complies with marine standards AS/NZS 2272.
Graded as AS E-1, suitable for high exposure.
Used on recreational boats and military vessels.

Midwest Marine Plywood

American brand specializing in marine applications.
Uses Douglas fir or Western Larch for faces.
Exceeds US Voluntary PS 1-09 marine standards.
Custom options like ultra-smooth faces.

Sea Ply Marine

Produced by Pacific Rim Wood Ltd in Canada.
Made from Douglas fir, Western Red Cedar, Yellow Cedar.
Can be ordered with clear grain-matched faces.
Special abuse-resistant grade designed for military boats.

Marine Plywood Coosa

Manufactured in Ecuador to exceed JAS 133 standards.
Available in Coosa wood species grown in sustainable forests.
Moderate cost, imported through marine suppliers.

Choosing an established brand that adheres to formal marine ply manufacturing standards provides assurance of quality to match the premium price.

What are the Major International Marine Plywood Standards?

To regulate quality, voluntary marine plywood manufacturing standards have been developed which leading brands certify their products to meet:

European Norm EN 314 (CE Marked)

Covered under European harmonized standard EN 13986.
Sets specifications for bond durability, moisture resistance, veneer grading and thickness tolerances.
CE mark indicates compliance for export/import in the European Economic Area.

Japanese Agricultural Standard JAS 133

Requires testing of glue bond strength after boiling.
Specifies veneer grading criteria.
Most recognized standard for plywood in Asia and the Pacific.

Australian/New Zealand AS/NZS 2272

Designed for marine use in Pacific climates.

What Is Marine-Grade Plywood?