What is Marine Plywood?


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Plywood comes under the category of engineered wood. We make plywood by gluing thin layers of wood veneer together to form boards of varying thicknesses.

The grains of the successive layers of wood veneer are at 90° to each other. The combination of the glue and wood grain orientation gives plywood its tremendous strength.

Plywood comes from the category of wood that includes medium-density fiberboard (MDF), and chipboard.

We call alternating the direction of the wood grain of plywood “cross-graining.” It is the cross graining of plywood that makes it incredibly strong. You will find it next to impossible to shatter plywood with a hammer the way you can do with solid woods. The cross-graining of plywood makes it take nails and screws easily without splitting.

You will find some plywood varieties with the grain direction of a successive layer arranged at multiple angles. This is a more expensive grade of plywood because its manufacturing process is considerably more complicated. But this type of plywood also possesses a higher degree of durability and strength.

Misconceptions of Marine Plywood

plywood boards on the furniture industry

“Marine-grade” plywood is nothing but marine plywood. Let’s get some of the misconceptions surrounding marine plywood out of the way. First off, it is not waterproof. Yes, marine plywood comes from good-quality hardwood trees, and we bind the layers together with waterproof glue. But it is still susceptible to deterioration due to high exposure to water.

Another thing about marine plywood is that it is not rot-resistant because it is not treated with chemicals. However, marine plywood is strong and lightweight, and you won’t find any defects in it. These characteristics make this grade of plywood a popular choice for building boats and other wooden items and structures that will be exposed to the elements. It becomes even more durable if a suitable finish is applied to it.

Here’s a project we did using marine plywood to replace old boat seats. While it costed quite a bit more than standard plywood, it seemed worth the investment. The previous seats (also made with marine plywood) had lasted almost 40 years!

Before and after pics of new wooden boat seats. (Woodworkingtrade)

What Is Marine Plywood?

The American Plywood Association, now known as the Engineered Wood Association (APA), defines marine plywood as plywood made from Douglas fir or Western Larch.

According to the APA, marine plywood may have knots but should not have knotholes. The different grades of marine plywood are:

  • A-A
  • A-B
  • B-B
  • Medium-density overlay (MDO)
  • High-density overlay (HDO)

Marine Plywood Compared to Other Strong Types of Wood

plywood boards on the furniture industry

Many solid wood species like ipe, cedar, redwood, cypress, and shorea exhibit a natural resistance to decay. Marine plywood does not have such a natural resistance to decay, nor does it resist excessive water resistance, like many types of solid wood. If you want marine plywood to become water-resistant, you need to coat it with waterproof material.

The Reason for Waterproof Glue

You may wonder why we can’t use ordinary wood glue for binding the wood veneer layers in marine plywood. The waterproof glue is to make the layers at least resistant to high humidity or boiling water to some extent. It will ensure that the layers do not delaminate with the slightest exposure to water. This is an important characteristic that comes in hand while using marine plywood to make boat hulls.

Marine Plywood and Other Grades of Plywood

background plywood the wooden light

If you go to a hardware store, you will find plywood graded as A, B, C, and D, where A is the best and D the most inferior. However, none of these plywood grades come anywhere near the quality of marine plywood or some other high-grade types of plywood made from hardwoods.

Standard plywood will have fewer layers which will be thicker, and there will be flaws in the layers of wood veneer. This grade of plywood mostly comes from softwood like pine or Douglas fir.

On the other hand, Marine plywood comes from various types of hardwood, giving it a finer grain and making it a harder and stronger material.

You can get “void-free” plywood that gives you the guarantee that it won’t have hidden voids in the veneer wood layers. These high-grade plywood varieties will also have more thin layers, which makes the material superior to plywood that contains fewer thicker layers.

4 Unique Characteristics of Marine Plywood

Wooden isolated background texture, compound tree of different pieces

Marine plywood is the best grade of plywood you can get in the industry. We also refer to this grade of plywood as boiling waterproof (BWP). That is why we use plywood for building boats. The resilience of marine plywood makes it suitable for use in bathrooms and kitchens. Here are four unique characteristics of this grade of plywood:

High-Quality Modified Resin

Marine plywood contains synthetic plastic adhesive resin, a phenol-formaldehyde best known for its extreme adhesiveness. The high-quality resin contributes towards providing a more effective barrier to water, humidity, and moisture. A good brand will not dilute the resin used.

Marine Plywood is Suitable for Outdoor Applications

As we mentioned above, although not water-resistant, marine plywood can withstand the effects of prolonged exposure to water much better than other inferior plywood grades. These properties make it suitable for use in the boat-making industry and bathrooms, kitchens, and outdoor furniture and decking.

Marine Plywood has Tremendous Strength

The combination of how the plies are arranged and the superior glue that binds the layers imparts an incredible strength to marine plywood. It also makes marine plywood a good choice of wood for shelves and other load-bearing structures with perhaps a bit of reinforcement, if required.

Marine Plywood is the Most Expensive Variety

Compared to other types and grades of plywood, marine plywood is the most expensive. It costs roughly double even the costliest type of other varieties of plywood. However, you will see a difference in pricing among the different brands of marine plywood. The price difference is due to the variation in the quality of resin and timber that the manufacturers use. However, any project that you undertake using marine plywood will be long-lasting and durable.

Conclusion

Carpenters With Plywood

There is no doubt that marine plywood is among the strongest and most durable plywood varieties that you can find. Of course, there are many misconceptions regarding this plywood grade, which we clarified in the beginning. With the information that we have provided here, we hope you get a better insight into the different plywood grades in general and all about marine plywood in particular.

Although marine plywood is a highly superior grade of plywood, it much more expensive so it makes sense to upgrade for projects that specifically call for it.