DIY Styrofoam Catamaran




Introduction: DIY Styrofoam Catamaran

About: Here i feature various projects from my YouTube channel and website. i do lots of small office supply weapons and some small vehicles too and some stuff that might be used for science experiments. i try to d...

This is a Styrofoam catamaran i built. I had been wanting to build this for a while, then in the last few days of summer i decided to build it. This only took 2 full days to build and one day to test. The catamaran costed me around 200 dollars (sail not included) and this is comparable to a store bought catamaran which would cost 2000 dollars

In this instructable i have included a few schematics (located on the last page), a materials list, and various pictures and descriptions for all steps.

If you plan to build this project i suggest you check out the video and if you like this project be sure to check out my channel or my website for more projects like this.

Step 1: Materials

The materials i needed for this project were:

2"x4"x8' pieces of wood - 2

2"x3"x8' pieces of wood - 1

2"x2"x8'pieces of wood - 6

1”x2’x8’ Styrofoam sheet - 5

1.5”x14”x4’ Styrofoam sheet - 5

½’’x2’x4’ piece of plywood - 2

½’’x4’x6’ piece of plywood - 1

Sail and mast - 1

hinge - 1

Epoxy (dollar store epoxy is cheapest and works well) - 7

Step 2: Assembling a Pontoon

The catamaran will be composed of 2 pontoons. These pontoons will have a frame filled with the white, less durable Styrofoam in the center; then we will later add the pink Styrofoam to the outside.

To start off i lay out two 2x2 pieces of wood with the white Styrofoam pieces in the center. This will be the inside of the pontoon. Next i add a front triangle onto the pontoon. As you can see in the third picture i added 2 struts near either end of the pontoon. Once that is finished i fill the pontoon with white Styrofoam and cut it to fit in the gaps.

Step 3: Finishing the Pontoon

to finish off the pontoons we must first cut 4 pieces of Styrofoam to size (2 for each pontoon). To cut them all at once i clam them together and use a reciprocating saw to cut the height from 2 feet to 17 inches (the height of the pontoon).

After that i use 1 - 1.5 tubes of epoxy as adhesive. I spread the epoxy on the wooden frame and then lay the Styrofoam down onto the wooden frame and add weight to it so the Styrofoam stays tight to the frame as the epoxy hardens. After that i use the saw to taper the bottom edge. I also add Styrofoam to the front triangle and taper that as well. Once complete you should go ahead and do that for both pontoons.

Note: The strut layout in the second pic is different then the layout of the of the struts in the previous page. The layout in the previous page showed the correct positioning of the struts.

Step 4: Completing the Pontoons

Now you can see both pontoons side by side. To complete them i add a wooden triangle positioned at the top, and lined up with each strut, this will be used for mounting the frame. To add some more buoyancy i also cut a full 2' x 8' piece of Styrofoam in half lengthwise (giving us two 1' x 8' pieces). I use epoxy to mount those onto the inside of the pontoons.

Now both pontoons combined give a buoyancy of 460 lbs when fully submerged. This seems like a lot but when you consider you wouldn't want more then 60 percent of the pontoon submerged (then you would be uncomfortably close to the water) and you might also have you weight towards one side of the pontoon then it seems this 460 lbs is just enough for one person to float comfortably. (i fit 2 full size people on but it sits very low). If you wish to be able to make this a 2 person boat you can add more Styrofoam sheets to the outside. Each 1"x 2' x 8' sheet once cut to a height of 17" provides 62 lbs of buoyancy when fully submerged.

These calculations are fairly simple to do on your own. First calculate the volume of the object (length in meters x width in meters x height in meters), this will give you volume in meters cubed. Then multiply the value you get by the density of water (1000 kg/m cubed) this will give you how many KG the object can float when fully submerged.

Step 5: Making the Frame

To make the frame i cut my 2x4 s to 4.75 feet long and attached them to the pontoons using screws. I chose that width because it was just small enough that i would be able to fit the frame (when separated from pontoons) in the back of my van. Next i add the 2x3 piece down the middle of the boat. This is both for structure and to mount other parts off of.

Step 6: Adding the Keel

Now i cut out the keel out of a piece of plywood. Using the plywood, i also glue a slot so that the keel can be slid in and will be able to slide up and down within the slot.

The keel is an essential part of the sailboat and helps to turn the force provided by the wind into thrust in the forward direction. This is also the pivot point when you turn your boat

Step 7: Adding the Rudder

The rudder will need to rotate so i decide to mount it off of a hinge. I add a hinge to the back of the boat and then cut the rudder out of the piece of plywood. The rudder should be fastened in place with 2 bolts and the bolts should have a washer in between the nut and the wood of the rudder. The bottom bolt will have to be taken while the boat is out of the water so that the rudder can pivot upwards when it is on dry land.

To finish off i add a handle made of plywood. The handle is to only have one bolt through it so that it can pivot up and down.

Step 8: Finishing Touches

To finish up i cut a piece of plywood to size and fit it on top of the catamaran to use as a seat.

After that we need to make somewhere to mount the mast of the sail. To do this i obtain a small piece of pvc pipe and heat the bottom of it so i can squeeze it down and make it thin. Once that is done i simply mount a few screws in it to attach it to the frame of the boat.

Step 9: Setting Up the Sail

Now i use a mast and sail that i had laying around and slide it into the pvc pipe. I secure the mast with metal cables that attach to various points on the boat. Once the mast is in place i rig up the main sail and the jib and it is now ready to sail

Step 10: Testing Out the Boat

Now that the boat is complete you can go ahead and take it out on the water and test it out. Be sure to wear a lifejacket and bring a paddle just in case you have a hard time getting it in using the wind.

To see the test you can also check out the video

Step 11: Schematics

Here are a few schematics of the boat, they are not to scale. Beside the schematic picture i also attached a real picture of the boat

In order we have a top view of the boat, a front view, a side view, and a picture of the rudder and keel. All measurements of the schematics are done in meters.



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    18 Discussions

    Awesome little catamaran. I've probably going to build this for fun. My dream boat is a catamaran (not quite as small as this) but I've been having a hard time finding the right plans. I came across a package online that, according to this guy ( is supposed to provide hundreds of blueprints and how-to resources for boatbuilding. Hm, I'll have to do some more research in the meantime. Thanks for the content!

    Not exactly sure, the pink foam is fairly durable plus wood sticks out further on the bottom then the foam so the foam doesn't even take much wear. After i tested it and dragged it along the beach the foam was still in perfect condition. I also don't think the epoxy will fail over time so i figure if i don't leave it outside in the elements ill probably be able to get a good 4 years out it

    if you check out my Sawfish kayak instructable, you'll see that I made my kayak out of the same kind of foam, and used no wood for the structure at all.

    The original Sawfish is now three seasons old, and I keep her strapped to the roof of my car most of the summer, paddling often. Coating the foam with fabric as shown in my poor man's fiberglass instructable keeps the foam from being worn or damaged, and makes it much stronger.

    I'm still exploring the different ways to build with foam, your catamaran is great!

    Congratulations for your enthusiasm. Really you want to sail...

    However, I am afraid that the structure and materials used are not strong enough to resist the forces that you will find in something stronger that very soft winds.

    In Cats, the waves hit the leeward bow first, while the sail is pulling the windward hull. That creates tension diagonally. And when the windward hull start to elevate, it will be worse.

    I recommend using it with oars or a small electric engine, not with sails.

    Remember that the weather can change suddenly and be prudent .

    Good luck

    For the size of this boat, a triangular sail isn't as efficient or powerful as a Leg-O-Mutton type. try Duckworks or check polytarp sails as a keyword, these kinds of sails put more working area up higher in the air while making it easy to duck under the boom. A Lateen type sail like those used on the Sunfish could also work.

    The side area of the pontoon "amas" is large enough that you may not need to add a dagger/centerboard. You won;t ever be fast in this design because of the profile drag and wetted area of the pontoons, but that's okay. Use a surform sanding block to round off all the foam and wood edges. The lettering on the ink foam can be removed with alcohol, or sometimes the printing is on a removable clear shrink plastic layer. Nice simple design, could be stored as flat components.


    1 year ago

    Tyvek or similar house wrap makes good sailcloth and can sometimes be found at salvage places.

    What are the dimensions of your sails? How tall is your mast?

    1 reply

    ill have to get back to you on that. Sail area was probably somewhere between 5-7 meters square

    This is awesome!! It is by far the most doable self build sailboat I've seen. I have been wanting a buy a boat of this size to sail around on in some of the lakes close to me, but $2,000 is a bit hard to spend on one. This I will definitely be making. Some improvements I might do is adding some kind of solid wrap around the pontoons as an added protection and for looks. Amazing build though! Thanks so much for sharing! I'll post mine when I get around to it.

    1 reply

    1 year ago

    Cool idea. But DO NOT use the white foam unless you want ballast in your floats, it is open cell foam and acts like a sponge in water. Just ask someone who does flat roof leak repairs and replacements like I have done for 5 years. The colored foams are generally very water resistant due to being closed cell.

    For longevity use dry green treated lumber, I say dry because I have seen some at the lumberyard that appeared to have come straight from pressure treating and was not dried, a 8 foot 2x4 should not weigh 30 pounds and more ;-( And because wood will also act like a sponge. Cut and lay your wood out the way you want it then water proof it before you assemble it. because the wood in the floats is going to be mostly covered by the foam it will take a very long time to dry out again and will start rotting before it dries.

    If the craft can not be stored inside when not in use, the foam will need to be coated for UV protection. In a year or so the foams color will start to fade, after that it will start shedding when rubbered.


    1 year ago

    Great job! And very well laid out instructable.

    Thanks, you can check out the video where i test it out a bit, but it sailed pretty nice. Very stable and could do essentially anything a normal sailboat could do. Unfortunately i havnt had a chance to test it in heavier winds yet