How to Build Your Own Hull - the Lost Foam Method

There are a lot of kits out there, and more are cropping up every day. Having said that, the odds that a kit of the exact boat you're looking to add to your fleet being available every time is slim. You can chalk up one to the small hobby and move on to another boat, or you can get up and do something about it yourself.

Building your own hull is actually a pretty straightforward exercise and the shape of most submarines lend themselves well to a technique called the Lost Foam Method. In a nutshell, you build your model out of foam, lay up epoxy and cloth on the exterior, cut it in half, pull out the foam, and you're left with a perfect (well... perfect-ish) fiberglass shell.

With the help of some excellent videos put together by my good friend Dwayne Hill up in Quebec, Canada, I wanted to take a few minutes and run through the process, start to finish. The boat in the pictures and video is a completely scratch-built model of the Russian Kilo Class in 1/48 scale.

View the videos at the links below, and read on for a description of each step:

Video One: Plans, templates, foam and stacking

Video Two: Epoxy cloth, hollowing out the boat

Video Three: Control surfaces, rudders

Video Four: Inlets, grates, scribing

Video Five: Sail and periscopes

Materials and Tools Recommended:

-Two-part epoxy resin (West System)

-Fiberglass cloth (West System)

-Large scissors

-Disposable brushes

-Closed cell construction foam (Styrofoam brand by Dow Corning)

-Steel or fiberglass rod - 3/8" diameter and 6" longer than your model

-Razor saw or hacksaw


-Pencil or fine marker

-Masking tape

-Safety glasses

-Utility Knife

-Access to a band saw or jig saw (optional)

-Rubber gloves

-Hot glue gun

-White glue

-Automotive body filler

-Automotive scratch-filler

Section One: Do Your Research

Any project requires planning, and building your sub is no exception. The biggest asset you'll have at this point is a good set of blueprints, ideally one that includes loftings, or cross-sections of the hull at given stations along the hull. This will allow you to create the foam block outlines when you start cutting foam. Also ensure that you have the rights to use the plans from the original owner.

Now that you have plans, you should decide what scale you're going to build in. Big boats are easier to work on and behave better on and under the water, while they also are harder to transport and store. Larger components such as props, pumps and seals may be harder to find and cost more as well. I'd highly recommend going with a standard scale such as 1:32, 1:48, 1:72, or 1:96. If you're lucky, you'll be able to find off-the-shelf detail parts that will make your life easier down the road during detailing.

So, plans in hand, now you need to take them to the local print shop and get them blown up to 1:1 scale for the boat you're building. This will make things much easier when you need to measure out distances and sizes.

Section Two: The Foam Part of Lost Foam:

What you'll want next is to find your foam. Closed-cell construction foam is ideal for this purpose, cheap, easy to find, and easy to work with. It is typically sold in 4'x8' sheets in thicknesses of 1" and 2". Ideally, your stations from your drawing will coincide nicely with the thickness of your foam sheet. If not, you'll need to cut sections of foam to match your drawing or else extrapolate at a mid-point. Styrofoam product by Dow Corning is what I use.

Now that you have your foam, you need to transfer your drawing templates to them. You take your drawing and make one copy of each of the station outlines. Number them from #1 at the bow through all of the sections until you have all of the templates marked out clearly. That done, you secure the templates to the foam with spray adhesive or white glue.

The next step involves cutting the foam along the outline of the templates, using them as a guide. Modern subs are a great project to start with due to how easy the basic shapes are. Most are circular in cross-section, and you can mark and drill the center of your circle to press a rod through for perfect alignment and strength in the next steps.

To cut the styrofoam, you can use a very sharp utility knife, but if you have one, a scroll saw or band saw makes quick work of this stage.

Beginning at either the bow or stern, begin stacking the discs of styrofoam onto your alignment rod, adhering them together as you go. I find that hot glue works well for this, setting up quickly and with a nice, strong bond.

This is where things start getting exciting, as your project now looks like a submarine and not just a pile of foam sheet! What you have at this point is a submarine-ish shaped object that consists of stepped discs. What we need to do now is knock the edges down and blend them one into the other.

If you're working on a boat that has a perfectly cylindrical cross-section, a great time-saver is to rig up a cheap lathe using a power drill and the rod that you previously inserted down the length of your boat for alignment. You'll need to build a simple mount for the drill using pipe clamps and 2x4's. By powering up the drill, you end up with a lathe that turns your boat quickly about its center-axis. You can use your utility knife to shape the edges together roughly, then progress to a rasp or other coarse file to further blend the edges. As you get closer and closer to the correct shape, progress to finer and finer files, ending with sandpaper for a smooth finish. If your boat is not perfectly circular, you'll be stuck doing things manually, unfortunately.

Section Three: Laying Up your Shell

If you've managed to come this far, you're probably pretty happy. You've got a submarine-ish looking thing on your bench and you can already envision what it will be like to run under the waves of your local pond. Congratulations! You've likely come further than 75% of the dreamers who undertake these projects.

The next step is to create your hard shell that will become the hull of your submarine.

You'll need to get some good quality two-part epoxy such as that from West System. You'll also need some fiberglass cloth (not the matte, the cloth!). Cloth is specifically designed for use with epoxy. Also note that I've stipulated epoxy and not polyester resin, which is what most people are used to seeing and using. Polyester resins will attack your styrofoam plug, melting it into a pile of goo and effectively killing all of your hard work. Don't use it. Epoxy is more expensive, but much easier to work with and it smells much better as well!

Lay out your cloth and cut it to shape, aiming to get the upper or lower half of the hull in one sheet, if possible. Note that curved sections like the bow or stern may require you to slit the cloth so that it can be overlapped without wrinkling. Preparation at this point will save hassles as you're wetting out the cloth. Cut enough to cover the half of your model in 3 layers.

Begin by wetting down the entire upper (or lower) half of your plug with properly mixed resin. A 2" disposable brush is perfect for this. Once you have a layer of epoxy on the plug, carefully lay out your cloth over it and press it down firmly to keep it in place. Saturate the cloth with more resin, watching closely for any trapped air beneath it. If you find a pocket of trapped air, dabbing at it with the brush will force it out through the cloth.

Ultimately, a hull thickness of around 1/8" is a good target. With a mid-weight cloth, this will end up being around 3 layers. Once you have three layers of cloth laid up, put one last layer of epoxy over everything to smooth out the final finish.

Let the resin cure for at least 24 hours. The longer your leave your resin to cure, the less chance it will have to warp once you remove the core inside. Going on vacation in a few weeks? Leave that thing alone until you get back. That way, you'll be sure of a strong cure with little to no warping down the line.

Once the resin is fully cured, you can begin to sand and fill the hull to eliminate imperfections and get a perfectly smooth finish. Large imperfections can be filled with a good automotive body filler while smaller ones can be addressed with automotive scratch filler or glazing putty.

Section Four: Splitting the Hull

We are now ready to split the hull in half in order to gain access to the interior of your boat.

The best way to start is to mark out the location of your sail and cut out that area of epoxy shell. This done, you will have exposed the styrofoam core. What you will need for this next step is your jug of acetone.

Acetone dissolves styrofoam quite handily, but it makes a stinky, goopey mess. A properly ventilated work area, safety glasses and protective gloves are a necessity for safety. Pouring acetone into the opening for your sail will dissolve your core and make splitting the hull much easier than if you were to attempt it with the core in place. If you get a glimpse inside your model after you dissolve most of the styrofoam, you'll see that there is still quite a bit of it left inside. Don't fret. You only need to get rid of enough to make it possible to split the hull.

What you want to do next is secure your model on your bench so that it sits perfectly level. Run a strip of masking tape around the perimeter to provide a clean surface to transfer your mid-point via a pencil or fine-tipped marker.

Another alternative to scribing at the mid-point is to scribe at the waterline. This works well for boats with a lot of freeboard (high surfaced trim). Cutting many modern nuclear boats at the waterline will not allow adequate access to the interior for the installation of linkages, and splitting at the center is recommended. This also makes painting it easier, as the lower hull is typically anti-foul red and the upper is flat black. Splitting at the half-way mark saves masking.

As you can see from the photo above, you will want to build a jig to hold your pencil at a perfectly uniform height as you mark out your cut. Two pieces of plywood screwed together at 90 degrees works perfectly for this. Drill a hole for the pencil, secure it in place, and you're good to go.

Mark out the centerline of your hull around the entire perimeter.

Use your razor saw or hacksaw blade to cut the hull around the entire boat. If things go well, your boat will come apart easily and you'll end up with two empty-ish hull halves coated with some half-dissolved styrofoam on the inside.

Using a rag and more acetone will allow you to scrub out the remaining styrofoam and leave a clean inner surface for you to work with.

What you'll notice at this point is that the hull is somewhat flexible along its long sides. A good idea before you get too much further is to add some internal bracing to help maintain perfect alignment.

If you've created a perfectly flush cut on both halves (check by placing it cut face down on your workbench and looking for gaps), one method that works well is to lay out a layer of wax paper on your bench followed by a layer of fiberglass cloth over that. Trace out the contour of your boat onto the cloth. Rough up the inner 1/2" of your boat about 1/2" away from the cut and then saturate it well with epoxy. Saturate the cloth outline of your boat very generously as well. Drop your hull half onto the cloth and press it down firmly with weights, allowing it to cure overnight.

Once you trim off the excess, cut out access to the interior of your boat with the razor saw or rotary tool. Leave a 1/2 lip around the entire perimeter. This will provide a lot of strength and rigidity to the hull and will also create a perfectly flush face for the other hull half to mate to. Repeat the process for the other half of your boat.


The above processes can also be used to create the sail for your boat, or you can use Dwayne's method of constructing it with plastic sheet.

The lost foam method is a quick and dirty way to get a hull. The entire process takes very little time. The quality of the finished product depends mainly on your ability to source good blueprints with the hull profiles detailed at many stations along its length, and your ability to follow through on the construction processes.

By watching Dwayne's videos (linked above) you can also learn how to finish up the remainder of your boat, including the creation of your control surfaces such as dive planes and rudders, and building your own Watertight Cylinder.

For those of you with the mechanical aptitude, patience and skills needed to build a hull like this, it can be an exceptionally fulfilling process.

As a word of caution, however, it is my experience that the percentage of people who successfully finish a project like this and bring it to completion is exceptionally low. If you're unsure, or if you can self-reflect honestly enough to know that you might not have the patience to see it through, I'd recommend against building your own hull. Breaking down the amount of time and materials needed to build these hulls, you'll see that spending a few hundred dollars on a professionally-built product is a worthwhile investment.

Questions? Comments? I'd love to hear from you. Feel free to email me at any time at

Bob Martin

the RCSubGuy

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