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How to Build a Fiberglass Boat? – 2 Efficient DIY Methods

Fact checked by William Hart

how to build a fiberglass boat

Since the early 1960s, fiberglass boat design and construction have become popular due to the material’s outstanding strength, high stability, low production cost, and undemanding maintenance. Until now, it remains a supreme material for boat engineering.

A 16 to 25-foot fiberglass boat costs between $30,000 and $250,000, which can be rather pricey for the average adult. The good news is you can build your unique vessel at home for half the price or even less. So, how to build a fiberglass boat?

This article will guide you through two construction methods: build a fiberglass boat from a used watercraft or with a pre-made mold. Find the materials and tools you need, and see the detailed steps for each method below!

Method #1: Build a Fiberglass Boat From a Used Watercraft

The idea for this method is to cover an old watercraft with fiberglass cloth and resin to strengthen the frame, add stability, and improve its seakeeping ability.

What to prepare

This method is the perfect option if you own a used boat. So, quickly grab the following fiberglass boat building materials:

  • A used watercraft
  • Fiberglass cloth
  • Fiberglass resin and hardener (or fiberglass boat kit)
  • Microlight fairing filler
  • Boat soap and water
  • Scrub brush
  • Orbital sander
  • A foam roller or paint brush

Step 1. Prepare the watercraft

fiberglass-boat-building

To begin, remove any items you have onboard (keel, chairs, fishing gear, rail) and flip your boat over. You should work on the external hull and bottom, then move to the interior.

Now, clean the boat’s exterior to remove any grime and contaminants. Do so by running water through the hull and bottom, scrubbing the areas with boat soap, rinsing them thoroughly, and letting everything dry naturally.

When the vessel is dry, look for seams, cracks, or holes and fill them with epoxy resin. You can use a shopping card to gather the epoxy into the cracks. Afterwards, grind the whole surface with an orbital sander.

Step 2. Cut the fiberglass cloth

Take the measurements of the sides and bottom of your watercraft.

Mark the boat outline on your fiberglass cloth with a sharpie, and cut accordingly. In addition, cut a diagonal piece in the dimensions of the keel. You will layer this piece on top of the keep to strengthen the area.

Tip: Cut the cloth a little bigger than the watercraft’s dimensions to avoid coming up short later; you can always trim off the excess if necessary.

Step 3. Install the cloth

diy-fiberglass-boat

In a bucket, mix the hardener and resin in the proportion recommended by the manufacturer. Once the epoxy hardens, brush or roll it to the entire boat.

Then, put on disposable gloves and carefully install the fiberglass cloth. Press the fabric with your fingers to remove any creases before topping it with another coat of epoxy. Place the keel strip on top and apply one more epoxy coat to the keel area.

Wait until the epoxy cures completely, which might take one to two hours.

Step 4. Apply the filler coat

Blend a fairing filler with epoxy following the manufacturer’s instructions. The additional filler compound will make the epoxy lighter and easier to sand. It also allows you to sand the top layer without stripping the fiberglass.

Brush a light coat of the filler-epoxy mixture on the entire boat. Then, grind the whole surface with an orbital sander to remove any lumps.

Step 5. Fiberglass the inside of the boat

When you finish with the exterior, flip your boat over and repeat the same steps to the interior.

Finally, reinstall the boat’s furniture. And voila! Your homemade fiberglass boat is ready.

Method #2: Build a Fiberglass Boat With a Mold

In factories, fiberglass boats are made from pre-fabricated molds. First, workers cover the mold with a mold release and a gel coat. Then, they apply multiple layers of fiberglass resin, cloth, and matting, which can be up to two inches thick.

When the resin cures, workers will remove the boat frame from the mold and construct other parts of the vessel, such as the hardtop, hatch, cabin, and console.

You can use the same procedure to build a fiberglass boat hull at home.

What to prepare

For this method, you will need:

  • Fiberglass boat mold
  • Building plans (include the keel and transom dimensions)
  • Mold release
  • Gel coat
  • Epoxy resin & hardener
  • Thickener or filler
  • Fiberglass woven roving cloth
  • Fiberglass chopped strand matting
  • Marine-grade plywood
  • Sander

Step 1. Apply two coats of mold release

A mold release substance helps with easy hull removal later on. Before you apply it, make sure the mold is free of dirt and grime.

Then, dip a cloth into the solution, wipe it on the entire mold, wait until it dries, and apply a second coat.

Step 2. Apply gel coat and the first fiberglass layer

Follow the manufacturer’s instructions to mix your gel coat and apply it to the mold with a paintbrush. The gel coat will provide a resilient exterior layer that protects the fiberglass from external elements.

Before the gel coat cures, apply a thin coat of epoxy resin. This lightweight resin will capture the contours of the mold and create a flexible base layer.

At this point, you should put the first fiberglass layer over the resin.

Step 3. Reinforce the keel and transom

construction-methods

Now, move on to creating the keel and transom. Firstly, you need to obtain their dimensions. You can search online for free plans or measure the mold and calculate the dimensions yourself.

Once you have the measurements, cut two pieces of plywood – one for the transom and one for the keel.

Then, coat the wood with a layer of mixed resin and hardener. Make enough of this mixture to have some leftover, which we will combine with a thickening agent.

Take the thickened blend you just created, apply it onto the wood, and follow up with some epoxy. At this point, you can attach the wood to the fiberglass cloth.

Step 4. Apply three more fiberglass layers

homemade-fiberglass-boat

The next layer is epoxy resin, which will strengthen the structure of your DIY fiberglass boat. Brush it on, wait until it cures, and grind the surface.

Then, apply two layers of fiberglass woven roving cloth to the boat. Install the cloth as follows:

  • Cut the fabric following the mold’s dimensions
  • Apply epoxy resin and carefully glue the fiberglass to the mold
  • Use a foam roller to cover them with another coat of epoxy resin. Remember to flatten any kinks or creases.

The final layer is fiberglass matting. The matting comes in small strands, so you don’t have to cut it. All you need to do is apply epoxy resin, glue one strand on the mold, and cover it with more epoxy. Repeat for the entire mold.

Then, sand the surface.

Step 5. Remove the boat from the mold

Let the epoxy resin cure for at least 48 hours, then flip the mold and gently pry the edge to remove the boat from it.

Now that you have the hull, you can proceed to build a fiberglass skiff to your liking. Feel free to paint it your favorite exterior color, decorate it with your desired furniture, add a teak deck, or build sails to turn it into a sailboat.

This Youtube video will help you visualize the whole process Making a fibreglass rowing boat hull at home from a boat mould

How Thick Should a Fiberglass Boat Hull Be?

In general, you should aim for a hull of ¼ to 1-inch thickness.

When it comes to fiberglass boat building, thicker doesn’t mean better. The material is resilient, strong, and durable; therefore, a standard fiberglass thickness is sufficient to withstand the impact of waves and wind.

Over-engineering a fiberglass boat only leads to a waste of money and resources.

There are various types of fiberglass: E-glass, S-glass, mat, Uni-Directional Fibers, Bi-Axial Fibers, and Tri-Axial Fibers. Some have much higher strength than others, thus requiring fewer layers to reach the desired toughness.

To ensure the accurate thickness of each coat you apply, you can use a mil gauge to measure the coats and make necessary adjustments.

Conclusion

Wrapping it up, how to build a fiberglass boat? There are two ways: installing fiberglass cloth on a used watercraft or layer fiberglass resin and cloth on a mold to form the hull. The former method is more practical to execute at home.

We hope our guide inspired you to get your DIY project off the ground. It can take months to finish the boat, but your hard work will pay off splendidly.

So, when do you plan to start? What method do you choose? Share your thoughts with us in the comment section.

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