How to Build a Fiberglass Boat from a Mold


The best way to build a fiberglass boat from a mold is to start with a great plug. The outside of the boat will be an exact copy of the inside of a female mold so it has to be perfect if you want it to perform well and look good.

In the article below the author will take you on a 9 month journey from getting his hands on a good plug (mold) and ending with finding the right trailer and engine for the boat.

The author of this article is a FISHING CHARTER CAPTAIN. He has been fishing the grass flats, oyster bars and mangroves all over Florida for more than 40 years. He thought it was about time to build the exact skinny water boat of his inshore fishing dreams.

In the article below, we will talk about:

• Finding a good plug (mold)
• Choosing from epoxy resins or polyester resins
• Choosing the right fiberglass cloth
• Choosing the correct building materials
• Boat design
• Rules and regulations to get your boat registered
• and much, much more......

Let's get started.


​Watch the video below to see the initial steps for rehabilitating an old boat plug and laying the fiberglass.

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​How to build a boat from a mold- Getting started


As I mentioned earlier, I am a kayak fishing guide on the Treasure Coast of Florida. I love kayak fishing BUT I wanted a boat that had more range and comfort for those dawn to dusk fishing days that I like to do.

The boats that I liked averaged from 25 to 35 THOUSAND DOLLARS and I did not want to pay that much for a flats boat. So I decided to try and build one for almost half that price.

The Hell's Bay Eldora was my favorite but that boat cost $30,000 in 2021. I also liked the Ankona Boats line of flats boats and they were the cheapest at $23,000 up to about $30,000 for their 5 or 6 models. BUT I still did not want to pay that much.

I went on many websites and found boat building kits made from wood and fiberglass for a fraction of that price but I wanted to have a boat without any wood that could rot. I don't think that wooden boats are bad but I wanted a boat that would last a lifetime.

If you want to buy a boat kit, then I would suggest Boatbuildercentral.com. I ended up buying all of my boat building materials from them and they really knew what they were talking about. They have lots of boat building kits and building materials to choose from.

Let's get back to my story because serendipity struck when my neighbor showed up one day with an old 17 foot flats boat plug that he got from a friend. This mold was exactly what I wanted to build and he told me that I could use it free of charge. He had already refurbished a few old boats and really knew what he was doing so I could pick his brain too.

He gave me lots of tips that stopped me from making lots of costly mistakes. Hopefully, I can pass on some knowledge to you and save you from making some costly mistakes with this article.

His mold was a male mold meaning that you were supposed to use the outside of the mold and place it into a female mold and then infuse it with resin and fiberglass. Of course I did not have the facilities or know how to do it this way so I used the inside of the mold because the lines were already perfect but it needed some body work.

*You have to remember that a bump in your mold will show as a divot on the outside of your boat and a divot will be a bump. You have to get it pretty much perfect for the first layer of resin and glass.

I went through 2 gallons off bondo to get the plug smooth and then I was ready to lay down my first layer of fiberglass.


How to build a boat from a mold- Should you choose Epoxy or Polyester Resins?

Well, 90% plus of all boats made today are made from polyester resins. Of course, I chose epoxy because I don't like to do things the easy way. I will go over the pros and cons of each and you can decide what you want to use.

Let's talk about the pros of polyester resins.

• They are less than half the price of epoxy resins.
• They cure in about an hour depending upon the temperature and how much hardener you use.
• They are easy to find.
• They are plenty strong for building boats.

Now let's talk about the cons.

• The fumes are horrible for your health and you should wear a respirator.
• They are not as strong as epoxy.
• You need more fiberglass cloth for a polyester resin boat.

Let's talk about the pros of epoxy resins.

• They are not nearly as harmful to breathe or for the environment.
• They are much stronger than polyester resins. Epoxy resins have a bonding strength of 2,000 pounds per square inch. Polyester resins have a bonding strength of 500 pounds per square inch.
• You can use less fiberglass when using epoxy resins.

Now let's talk about the cons of epoxy resins.

• They are expensive.
• They take 24 hours to cure.
• You have to get your mixture ratios perfect or they will never harden. You end up with pine tree sap instead of rock hard resin. Then you have to grind it all away and start over.

In the end, you will have to decide which type of resin to use.

Your next step is to choose the thickness of the fiberglass cloth that you want to use. I chose a 12 ounce biaxial fiberglass cloth for my build. There are various thicknesses from 4 ounces up to 20 ounces and higher. Whatever thickness of cloth you decide to use the key is to keep the air bubbles out. Those bubbles will grow and affect the structural integrity of the hull. If you get an air bubble you have to grind it out and fill it.

You are trying to build an outer skin on the hull of at least a 1/4 inch for a small boat like a 17 foot flats boat. You want something that can withstand a hit from a floating piece of wood without cracking open.

Watch the video below to learn more.

How to build a fiberglass boat from a mold- Reinforcing and releasing the boat from the mold


Once you have built up your fiberglass outer skin to a thickness that you like then it is time to add the stringers. I chose a main keel stringer that was 5 inches tall by 5 inches wide. The type of stringer was a high density foam covered in a 12 ounce fiberglass cloth.

This is your most important stringer that will take the brunt of any longitudinal stresses to the hull. You really need to ensure that this stringer is completely adhered to your hull as it cures. There are two 2 3/4 inch stringers about 12 inches from the center keel stringer. Ultimately, all of the stringers were adhered to the sole of the boat with 3M 5200 adhesive. 

My main concern was that the hull might fold when I tried to get it out of the mold. Adding the keel stringer took those worries away from me.

PRO TIP: Just get a hose and fill the gap between the mold and the hull with water to get it to release. We used wedges and levers to no avail and did a decent amount of damage to the spray rails of the boat. Then we tried water and it worked perfectly.

Water will lift it out easily, evenly and without harm. That was a major rookie mistake that I made so don't mess up the skin of your hull like I did.

Once I got the hull separated from the plug I could work with it more effectively. The plug weight was around 1,200 pounds. The hull's weight at this point was only about 150 pounds so I could flip it and work on the outer skin to get rid of any imperfections so that I could paint it.

To fix any imperfections in your outer hull you will want to find a good fairing compound. Fairing compound is very similar to the Bondo filler that we all used to fix our cars when we were young.

You have fairing compounds that are made specifically for polyester resins and epoxy resins so make sure that you get the correct one for your specific resin.

Just fill and sand until you get a result that you can live with. I had to do many hours of filling and sanding to get the hull to look decent.


How to choose the best paints or gel coats for your boat mold?

Gelcoats are for polyester resins. If you chose that type of resin then you have to spray the gelcoat into the mold right after your release agent cures. When you pop the hull from the mold it will already look great. You want to spray it or roll it to about the thickness of a chicken egg shell.

I used epoxy resin so I had to find a good two part polyurethane paint for my hull. After a whole bunch of internet research and talking to the guys at boatbuildercentral.com the consensus was the expensive stuff was much better. A gallon of the paint costs $350!!!! I was blown away that it was so expensive and I needed white and ice blue for my boat design. That's $700!!!!!

You will also need to find a good two part primer to seal your fiberglass and create a good platform for your paint to stick to. A gallon of primer was a little over $100 per gallon.

I am pretty proficient with a spray gun but you can roll the primer and paint on as long as you can keep a wet edge as you do it. Same thing if you use a spray gun. The wet edge is crucial.

You need to let it cure for 48 hours and then you can flip the mold and start building your new boat.


What are the best building materials to use for building a fiberglass boat from a mold?

I built my entire boat with two structural materials. Those materials where Nidaplast honeycomb core and a 1/2 inch high density transom foam material. There are other similar and popular building materials like Divinycell Starboard and Coosa boards but I chose the Nidaplast core for most of my boat build.

The honeycomb structure is nature's perfect building design already and it is covered in an absorbent cloth that sucks your epoxy or polyester resins throughout the material to make it nearly bullet proof in the end.

Nidaplast honeycomb core is a little bit less than the expensive woods that are used for boat building and probably stronger too. The problem with this material is that it does not hold a screw very well. This is why I layered my high density foam in between layers of Nidaplast. The foam will hold a screw better.
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Watch the video below to learn more.

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How to build a boat from a mold- The buildout

This is the time when you have to finish laying your stringers and start building out the actual structure of your boat. You have to decide what height you want your sole or main deck to be above your stringers. You have to decide how thick you want to build out the sides of your boat for structural integrity.

I chose to keep my center of gravity as low as possible. My theory was that some large people would be walking on the side decks and a lower center of gravity would keep the boat from listing in a major way.

My sole is 5 3/4 inches from the top side of my hull skin. In hindsight, I should have raised that by 1 or 2 inches to create a self bailing cockpit. Then I could get away without having a bilge pump but bilge pumps are only about $50 for one that can pump 900 gallons a minute. I can live with that mistake.

My floor decking is made up of Nidaplast with 3 layers of 12 ounce glass on the top and 2 layers of 12 ounce glass on the bottoms. They come in 8 foot sheets so the 17 foot boat ended up with 2 seams that needed to be filled.
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How do you make your new boat unsinkable?

The United States Coast Guard requires that all boats over 16 feet need to be filled under the decking with 2 pound foam. This foam is a 2 part mixture that you have to get to fill the entire cavity under the sole of your boat. You can tilt your hull up or down and it will flow to fill the empty spaces.

You can also finish your sole decking and drill holes and pour the foam in until it fills the empty spaces and comes out of the hole. I did a combination of both and I am about 98% sure that there are no unfilled areas under the deck.

This foam is great for dampening the noises of your boat for more stealthy fishing too.

Once you get your decking down it is time to start building out the sides, bow and stern of your boat. The transom is very important because it will have to take the torque of your engine. The last thing that you want is to have your new engine snap the transom and fall into the sea.

My transom is made of two layers of 1/2 inch transom foam and roughly 6 layers of 12 ounce fiberglass. My boat is definitely over engineered but I am okay with it weighing an extra 150 pounds or so. I might give up an inch of draft but I will have a rock solid boat.

Watch the video below to learn more.
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How to build a fiberglass boat from a mold- Structural Integrity

It is very important that your boat have longitudinal and lateral strength to withstand the rigors of waves and the twisting stresses that can occur when you are using your boat.

My boat design does not have ribs going perpendicular to the stringers underneath the sole. I did this for two reasons.

Number one- There was not any room for ribs because I lowered the sole so close to the bottom. The 3 stringers are adhered to the sole for the entire length of the boat. The sole is acting like on huge lateral rib.

Number two- I beefed up the bow and stern lateral walls to act as super heavy duty ribs once they were attached to the upper decking by epoxy glue that I made from epoxy and silica.

I had some extra 5 by 5 inch stringer material left so I used that as support for the rear deck to tie the sole, with the sides and the top rear deck. I also doubled up the 1/2 inch foam to make rod holders/structural supports down the length of the cockpit attached to the side decks.

Once I had all of those in place, I cut pieces of Nidaplast and stuck them to the sides with 3M 5200 marine adhesive.

*I should have just used silica and epoxy to make a paste because it would have been stronger BUT 5200 bonds at 700 pounds per square inch so I am not worried about it failing. It is almost $20 a tube though. OUCH!!

I used my left over 2 pound expandable foam to fill any voids between the hull and the Nidaplast walls, trimmed the excess and fiberglassed over them. This is a great way to deaden boat noise and increase the structural integrity of the boat.

Watch the video below to learn more.
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How do you make the decking for a boat?

I really over engineered this boat of mine. The decking is make up of a layer of Nidaplast covered in 2 layers of 12 ounce fiberglass on both sides. Then I added a layer of the high density transom foam (to hold screws). Then I added another layer of Nidaplast covered in 2 layers of 12 ounce fiberglass on both sides. MY DECKS ARE STRONG!!!!

I created the decks this way because they play a crucial roll in keeping the entire boat together. They are adhered to the hull with epoxy paste made from epoxy and silica. You have to get this part just right because they are on there forever.

* Make sure that you give your self access points to the bow compartment and stern compartment because everything is permanent.

You have to make yourself a lot of epoxy paste to make sure that you have an unbroken seal around your entire boat so no water can come in between the hull and the decks. Make sure that you have a couple dozen clamps too. You really have to get this part right or your upper decks won't be level and securely attached to your hull. 


Watch the video below to learn more.
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How to build a boat from a mold- Putting it all together

I ended up getting a 52 mm rub rail to cover the imperfections in the layers of deck material. It was the thickest rail that I could find but I needed one that thick because the sides of my overbuilt decks needed to be covered up.

WOW is that stuff expensive. The smallest roll was 70 feet long and all that I needed was 40 feet. That stuff is $10 a foot. So I had to spend $700 for it but only needed $400 worth. The key to putting the rub rail on is getting the tracks for it straight and screwed down tightly. If you get the tracks right, then it is a breeze to snap the rub rail onto it.

Now it is time to make your boat look nice. You can do a lot of sanding to get your decking looking perfect or cover all the bad spots with Seadek foam coverings. I chose the 6mm foam because it was the thickest one that I saw.

I plan on being on this new boat a lot and want a nice soft floor to stand on all day. I found a place online called Castawaycustoms.com that sold full sheets of the foam. The sheets were 80 inches by 40 inches. 

This stuff is expensive too!!! Two sheets were $400 after shipping. I got a 48 inch ruler and some other pieces so the decking ended up costing a little over $500 for the entire boat. BUT it looks really cool and is worth the money in my opinion.

The total cost for my hull build was a little over $8,000 and I put just under 400 hours into the build over a period of nine months.


How do you choose the right trailer for your boat?

There are a lot of boat trailer manufacturers in Florida. From what I can tell they are all very similar. You have to choose between getting a galvanized steel or aluminum frames. Here are the pros and cons of each:

The pros of galvanized steel are:

• They are about $500 cheaper than aluminum trailers for a small boat like mine.
• It is easy for manufacturers to get so the building process is sometimes faster.

Let's talk about the cons.

• Galvanized steel trailers will rust.
• They weigh almost twice as much increasing your towing fuel expenses.

Now let's talk about the pros of an aluminum trailer. 

• They don't rust.
• They don't weigh as much as galvanized steel trailers.

Now let's talk about the one con.

They cost more.


What is the best axle type for a boat trailer?

You really don't want to have a trailer with leaf springs any more. The new torsion axles are much, much more efficient than the older leaf spring trailer types.

Torsion axles use some type of strong, tightly wound elastic cords that give you all of the shock absorbing power of the leaf springs without the maintenance.

I ended up using a company called Palm Beach Trailers to build my custom aluminum trailer. I needed a very specific sized trailer to fit in my garage. It had to be no longer than 20 feet 6 inches from the tip of the trailer to engine propeller or it wouldn't fit in the garage.

A standard trailer for a 17 to 18 foot boat just wouldn't work for my situation so I needed a custom one with a collapsible tongue.

The total cost with a spare tire and extra hub and bearing kit was about $2750.


How do you choose the right engine for your boat?

My boat is built for stealth in skinny water scenarios and not speed and comfort while driving. With this in mind I had to figure out how much my boat would weigh with 3 people and accessories in it.

The hull weighs 800 pounds. Three large people weigh 750 pounds. Six gallons of gas weighs 40 pounds. Ice, beer, tackle, cast net........ 60 pounds.

I read somewhere that the minimum amount of horsepower that a boat with a relatively flat bottom would require to get on plane effectively was the total weight divided by 50.

So here is my math: 800+750+40+60= 1650 pounds. I checked the weights of tiller sized engines and a 40 horse powered 4 stroke Suzuki engine weighs 240 pounds.

1890 lbs. divided by 50 = 37.8. So I ordered a 40 horsepower Suzuki 4 stroke engine.

*I wanted to go with the bare minimum engine because my portable gas tank is a 6 gallon one. I hope to fish all day and be able to use less than 6 gallons of gas.

I called all over Florida and the best price that I found was $4800 for a 2021 Suzuki 4 stroke 40 HP. The tiller kit was just under $1,000 installed. Nick's Creative Marine in Riviera Beach is the company that I chose to buy my engine through.  This brings my total cost for the engine to $5,800.

I chose Suzuki for my engine because I noticed that most of the commercial fishermen that I see when I am on the water have Suzukis. They need the most reliable engines to do what they do so that was the deciding factor for me.

Suzuki has a great warranty too. It comes with a 3 + 3 warranty. The first 3 years they will fix whatever breaks. The next 3 years you are covered for any upper unit issues but not any lower unit issues. That's how I interpreted the warranty anyways.

So now my total cost of the boat build is $8,000 (hull) + $2,750 (trailer) + $5,800 (engine) = $16,550.

BUT I still have to get a poling platform, push pole, tiller extension........ So I figure the all in cost for my boat will come in under $20,000. That puts me about $3,000 cheaper than the cheapest Ankona Flats boat and $10,000 cheaper than the Hell's Bay Eldora that was my top pick.

All in all I am glad that I built this boat on my own exactly the way that I wanted it and it is definitely over engineered.

Watch the video below to see the final buildout and the finished product.

How do you get a home made fiberglass boat registered and inspected?

Once you have your boat finished you will have to get it registered. I imagine all states are different, but in Florida you will have to have the Florida Wildlife Commission (FWC) come by and inspect your vessel. They are mostly trying to make sure that you didn't steal it after a hurricane.

That is what the wildlife officer told me anyways. I guess after hurricanes there are people that steal damaged boats and try to re-register them once they have fixed them up.

He gave it a visual inspection and asked for photos of the different stages of the building process. ie. 2 pound foam under the sole of the boat; high grade gas lines; proper ventilation for the gas tanks; fire extinguishers......

Then he gave me a signed inspection document.

After you get the inspection you can do to the DMV and get it registered.

I hope this helps you if you decide to build a fiberglass boat from a mold. I probably won't ever do it again. I was itchy for 9 months straight.

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