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How to Work with Marine Plastic Lumber

The author made an outboard-engine mount out of aluminum and marine plastic lumber, seen here, along with a furling lead stanchion clamp (top right), a jib sheet sheave (middle right) and a fuel port spacer (below right)

The author made an outboard-engine mount out of aluminum and marine plastic lumber, seen here, along with a furling lead stanchion clamp (top right), a jib sheet sheave (middle right) and a fuel port spacer (below right)

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Marine plastic lumber (MPL), more commonly known by the proprietary name, StarBoard, has replaced wood for many projects on new boats or during a refurbishing project—items such as instrument panel mounts, cup holders, rod holders and cockpit seats can be made with MPL, just to name a few. We have used MPL on our boat for a wide range of projects.

MPL is made of high-density polyethylene (HDPE) plastic. It is a very common type of plastic and is used in a range of applications, from making water jugs and plastic bags to computer parts and geo-textiles.

The specific material properties mean that it will not rot, is impervious to saltwater and does not require finishing, while additives in the formula make it UV stable and color-fast. It can also be bent or thermoformed to create a variety of shapes and effects. It is hard enough to resist damage from most blunt impacts, but soft enough to be worked on with common wood-working tools, like jigsaws, circular saws, drills, sanders and routers. For all intents and purposes, you can regard it as one would regard high-quality marine grade plywood.

Limitations

MPL should not be used for structural purposes because it's flexural and tensile strength-to-weight ratio is much lower than wood. For example, with a ½in piece of MPL you should provide support every 15in in all directions to avoid bending or sagging. It is definitely not an appropriate material for re-planking a hull, or for use in bulkheads or engine stringers.

Most common marine adhesives, such as
SikaFlex and 3M 5200, do not bond well to MPL. They may be used to caulk joints to provide a degree of water tightness, but only if those joints are already fixed in place with another type adhesive or mechanical fasteners.

MPL is also affected by temperature, expanding 1/32 in per foot for every 40-degree change in temperature, and does not stand up well to temperatures over 180F. It should, therefore, be kept out of locations where temperatures vary significantly or where they can get hot, like and engine compartment.

To ensure clean cuts with minimal furring, jigsaw blades should have a minimum of 10 teeth per inch (I have found blades used for cutting aluminum to work quite well) and an 8in circular saw should have blades with 70 to 80 carbide-tipped teeth. Sharp edges are best rounded over with a router, using a flush trimming bit. If a router is unavailable, hand sanding or an orbital sander with 120-200 grit sand paper will also work to knock down or round over the edges.

Drilling and Fastening

MPL can be drilled with common drill bits. Almost all fastener holes should be drilled 1/16 into 1/in larger than the fastener diameter due to MPL’s expansion and contraction with temperature. If MPL is being fastened to marine plywood, solid wood or fiberglass, drill the correct size hole in the substrate and oversize the hole in the MPL. Drill bits have a tendency to heat up and can even melt the plastic. As a result, the drill bit exit may not be as clean as the entry. A countersink bit can be used to clean up the hole. As fastener heads should be countersunk anyway to minimize deformation of the material, it is a good idea, where possible, to drill the holes in the reverse direction of the intended fastener. That way the counter sink bit can be used to both clean the hole and countersink the fastener head.

Another option for countersinking the fastener heads is to counter-bore the holes and hide the fasteners with plugs made of MPL. Do not counter bore more than one-third the thickness of the material you are using and undersize the holes by 1/32in. Use a plug-cutting bit for the plugs and freeze the plugs to make them contract. While the plugs are still at freezing temperature insert them into the undersized holes. When temperature the equalizer, the friction between the counter-bored hole and the plug will be enough to keep the plug in place: though for extra insurance, a little cyanoacrylate adhesive or a two-part plastic bonding system can be used to ensure the plugs never move.

Hole-saws can be a little trickier to use with MPL than with wood because the teeth of the saw often become gummed with melted plastic while cutting. To correct the problem, simply remove the saw bit from the material, wait a couple of seconds for it to cool and harden, and then pull off the plastic.

A typical pile of off-cut MPL found at a boatyard (right); when cutting MPL with a jigsaw, a fine-toothed blade works best (far right); typical glues and bonding agents don’t work with MPL, so make sure you use a two-part activator and bonder combination, like Loctite Plastics Bonder system (inset)

A typical pile of off-cut MPL found at a boatyard (right); when cutting MPL with a jigsaw, a fine-toothed blade works best (far right); typical glues and bonding agents don’t work with MPL, so make sure you use a two-part activator and bonder combination, like Loctite Plastics Bonder system (inset)

Bending

MPL can be bent, or thermoformed, quite easily. The material is softened at temperatures between 210F and 240F, which can be attained using the “low” setting of any standard heat gun. Simple bends can be made by clamping the MPL to a flat surface and allowing it to overhang the surface at the point it will be bent. Pass a heat gun set on low back and forth across the bend line, while gently applying pressure to the material. To make a 90-degree bend with tight radius corners some material must be removed prior to bending. Using a router with a ¾in 90-degree “V” bit, cut half-way through the material at the inside of the bend line. Then follow the same steps outlined above for the simple bend.

Gluing and Welding

The molecular structure of HDPE, the material that makes up MPL, makes adhesion with conventional glues very poor. SikaFlex or 3M 5200 will provide a tenuous bond that will suffice for caulking joints or in some cases as a bedding compound, but will not provide enough bond strength for any structural purposes. The only adhesive that will reliably work on MPL is a two-part activator and bonder combination, like Loctite Plastics Bonder system. The activator works like a solvent to denature the molecular surface of the plastic and allow the bonder to insinuate itself. Another option is to weld the MPL using standard HDPE welding equipment and practices, though it should be known that very few DIY-aficionados have HDPE welding equipment, and the fumes released from the work are very toxic, so if you’re going this route, you may want to consult a professional.

Sources for MPL

MPL, and especially StarBoard, does not come cheap, and most hardware stores don’t sell it. Even at marine chandleries, it often has to be specially ordered. However, due to the ubiquity of MPL in modern boats, most boatyards have large amounts of it on hand, since the client pays for MPL by its square dimension, from which the required pieces are cut. The leftovers and miscuts then are often discarded, as the material has already been paid for and it is not economically efficient for a boatyard to keep the scraps. Most boatyard owners are happy to allow you to sort through the scrap pile and take anything you want.

Another source of cheap or free MPL is boatbuilders. Similar to boatyards, boatbuilders use MPL for a variety of projects. We sourced almost all of our MPL, including pieces up to 2ft by 3ft, from the bin outside of a kayak manufacturer. Check with boatbuilders and other manufacturers that use HDPE or MPL for their processes to see if they have scraps for sale or scavenge. Remember that not all HDPE has UV stabilizers added to the formula. It is, therefore, important to check with the source of the plastic before installing it in a location that will be regularly exposed to sunlight.

So, what will you make?

With the versatility of MPL and all of the things that need to be replaced or fixed on almost every boat, it is really an embarrassment of riches as far as project options go. If you haven’t used MPL before, try experimenting on smaller projects, such as a cutting board or cupboard latch, then move onto the more complicated jobs. When it comes to the world of MPL, your imagination is the only real constraint.

Robin Urquhart and his partner, Fiona, departed September 2015 from their home port of Vancouver, British Columbia, on a multi-year voyage that will end with landfall in Australia

July 2016

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