Introduction: Building a Rowing Shell

Picture of Building a Rowing Shell

How to build a rowing shell using a hot-wire cutter and hand-layup fiberglass

Step 1: Cut Foam

Picture of Cut Foam

Foam cutting is an easy way to get a smooth 3D shape. Pink housing insulation foam is also very cheap. If I remember correctly, it took about 10 sheets of 2inx2ftx8ft to build this boat. The process is simple. Cut jig pieces. I used 1/4 in luan plywood. I used a bandsaw for the rough cut, and a large hand sander to get the finish shape. If the shape is accurately drawn, this method can easily get shapes to under 1mm accuracy. The edges of the jigs have to be smooth, or the wire will catch. Make sure you mark the points around each template which you want to have line up when cutting

Building hot wire:

The wire will be best if you can find 1/32" stainless steel cable to cut with. In lew of this, bicycle shift cables are OK, but leave a pattern which needs to be sanded off. Stainless steel wire also works, but it's hard to get it strung on straight. The cable should be mounted to some frame which is very springy, as the cable lengthens a lot when it heats up. The springness lets you set the tension when the wire is cold, and have the same tension when it is hot.

Using hot wire:

Put a voltage over the hot wire cutter which makes it cut smoothly through the foam, leaving little foam fibers behind it. The wire should smoke just slightly once it's cut through the foam. Don't try to force it. It is cutting with heat, so pushing hard will make the wire bend and not speed up the cut. A bent wire means you get the wrong shape. Never hot wire a piece with one person. Get one person on each side of the wire. Mark points on the shape which you want to make sure line up. Number them, so both people are going the same direction around the part. Speed up or slow down to match the other person.

I use spray adhesive to stick the foam to the templates. The propellant in the spray dissolves the foam, so spray lightly from a long distance (1 ft?). Let the propellant evaporate, and then stick the template on.

Step 2: Build a Flat Table

Picture of Build a Flat Table

If you don't have a flat table to build on, it is very hard to build the part you want without warping it. If you don't have one, build one.. It saves loads of time.

To make your table flat, build it with many legs. Then use a tight string to define 'flat', and shim the legs until the tabletop matches the string. I neglected to flatten my table, and now my boat has a downward kink in the rear.

Step 3: Mount and Sand

Picture of Mount and Sand

One of the main reasons a rowing shell is so long and skinny is to keep the wave and pressure drag down. This means it needs a very smooth and unimodal surface. Ripples will make it much slower and less efficient.

I made a centerline down my table with a taught string, and mounted all the sections along this. This worked well except that my table curved up slightly at one end, so now my boat has a bit of camber at the tail.

Sanding is equally important. Remember that bit about making a constant curvature hull shape? If you do this, you can make a giant sander to sand with, which will get rid of all of the little ripples and bumps in the hull. I made mine about 4 ft long and just walked up and down the boat with it to shape. This is way easier than sanding and sighting to get the shape.

Step 4: Fiberglass

Picture of Fiberglass

Once you are happy with the shape of your hull, fiberglass away. I wanted to be able to row in waves, so I used a lot of fiberglass. If you are fancy, putting a few layers of carbon stringer down the bottom can help with stiffness. If you do this, the carbon takes all of the load, so skimp on fiberglass (only enough for a hard shell), and make sure you have enough carbon to handle the loading.

I made my hull with four layers of glass. One (or was it two?) is unidirectional along the hull, two are bidirectional along the hull, and one is bidirectional at an angle. In hindsight, I would have ditched one layer of bidirectional cloth for another uni layer. The cloth weight was roughly 7 oz, 'rutan fabric', which is meant to be good for wet hand layups. This stuff is still my favorite fiberglass due to it's workability. It can be found at Aircraft Spruce.

Learning to fiberglass takes a little practice. It's basically just a process of getting some liquid in a cloth, and getting surface tension to hold it to something. If you do a lot of layers at once, surface tension may not be enough. If you do tight corners, surface tension won't cut it, so avoid both of these things, especially in combination. If you need to do sharp corners, use a touch of spray tack to stick the fiberglass cloth on, and then wet it out carefully. The spray tack holds the cloth in place, even as the epoxy is curing. I would recommend using a peel ply on the top so that the surface may be more easiy painted.

For the bottom of a hull, it should be possible to stick all layers on at once. Get someone to help you with it, as epoxy has limited pot life. Start from one end of your boat, with all of the cloth cut and ready, and work your way to the other end. By the time you are done with the far end, the near end will have set. Eat a lot before starting, as it could be 6 hours of constant work and concentration.

Wear a mask and gloves as it is very easy to become sensitized to epoxy. Many homebuilt aircrafts are never completed because their builders are not careful enough, and end up sensitized halfway though.

Step 5: Sand

Picture of Sand

If your layup is really good, you will need to do very little sanding. Mine wasn't great, so I put an extra layer of epoxy on top and sanded it smooth. Spray painting your boat so you can see dimples which don't sand and which need filling is a terrible idea. It will fill the room with pink dust.

Oh yes. Sanding fiberglass is really nasty. If you don't wear a mask, you will wheeze for the rest of your life.

Step 6: Get Distracted and Build Another Boat

Picture of Get Distracted and Build Another Boat

If your boat is taking too long, it is suggested that you build a second boat in the meantime. Pictured is the spine for a westernized westernized Proa we accidentally built one summer.

Step 7: Build Top

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the top of your boat is not particularly important. You can make it look as nice or as shitty as you like. I wanted to be able to take waves, so I built the top very strong for compression. it is made of 2 layers unidirectional fabric and one layer of bidirectional.

The top of the shell is under compressive loads, so it wants to buckle, especially around the cockpit. I added a bunch of layers of cloth to bring the load into the rails on the side of the cockpit. On the rails, I used at least 15 layers of glass to form stiff beams which could bear the compression load. It should be possible to lift the boat from bow and stern and not have it break.

Step 8: Sharpen Your Spoons

Picture of Sharpen Your Spoons

I used a sharpened spoon to carve a cockpit in the boat. The fact that it is made of many laters of 2" foam made it very easy to get a flat deck 2" down from the surface.

After cutting out the foam, bind in divinycell. I used a bunch of microbubble to make sure there were no voids. Divinycell is good for this because it can be heated and formed to make the shape.

After cutting and sanding, lay up the cockpit. The cockpit is in compression, so it needs a lot of very straight bits of fiberglass to keep it from buckling in waves. Don't try to do too much at once. Using peel ply and doing two layups in a tight space is usually a better idea than trying to get all of your thickness at once.

Step 9: Paint, Build Riggers, ...

Picture of Paint, Build Riggers, ...

Do everything else I don't have a picture of!

I made riggers out of steel. I made a seat out of fiberglass. Paint everything!

Gel-coat is a pain to use, but makes for a nice finish. I used gel-coat and followed up with spray paint to make it extra-yellow, and so I could see scratches.

Step 10: Row!

Picture of Row!

time to start building up those muscles. We started with sweep oars, which are a meter longer. This was very tiring. We then cut the sweep oars down and made them into sculling oars. Now it feels fast.

If you happened to build your boat in a room which it cannot exit from, you may be in trouble. Try removing windows.


NateS68 (author)2017-11-08

What did you guys do about the ribs? Also is there a honey comb kind of layer at all?

dbuckley6 (author)2014-07-28

Fantastic job. So a few years in, how has the boat faired? I am very interested in building one following your instructions. 3 questions, 1-any idea of overall weight , 2 - any construction mods from long term use - 3 any idea where the pattern for the plywood jigs could be found?? Cant really do anything without pattern. Thanks

AverageH (author)dbuckley62017-09-05

Everyone is looking for plans. I doubt he will give them for free.

AverageH (author)2017-09-05

Where can I find plans for this?

Ironman 54 (author)2011-10-23

Hey ive been trying to find some decent cross sections to cut the foam with =, but as of yet have had no luck. do you know some place i can download them for free/ could you email me scans of the ones you used?


Did anyone ever share with you any sites with free rowing scull designs?

Thank you!

'fraid not.

boblq (author)2013-09-12

LOL, this is my main problem. Right now I am trying to force myself to finish at least a half a dozen boats before starting another.


boblq (author)boblq2013-09-12

Examples of multiple projects needing to be finished

nickscott456 (author)2013-06-24

where did you get the pink insulation foam boards and carbon , fiberglass and did you a template and if yes were from ????

lminer2 (author)2013-06-12

How long are single scull shells normally. I'm talking about one someone would practice on.

hollywolf (author)2013-04-13

Where do I get the designs for the bulk heads, outriggers, etc?

Sculptor_99 (author)2013-04-01

Great project! Did you consider using the same process to make a pair of foam molds to lay the fiberglass INTO, instead of a core to lay the fiberglass on? Then you would end up with a lighter, hollow shell (though you would probably need another layer or two of fiberglass to make up for the loss of the foam structure). OR, another option to reduce the weight: the core sections could be hollowed with the hot wire, leaving a ~1" wall. Maybe it wouldn't make that much difference. How much does the finished boat weigh?

ssaleem3 (author)2012-05-06

did you leave the foam in the boat or did you take it out and if you did take it out how did you do the ribbing to maintain the strength of the hull

damonv (author)ssaleem32012-10-21

no ribs. The strength is carried by the skin, and the foam core keeps the skin from buckling.

showlov4anerd (author)2011-12-10

Would adding some honeycombing or fiberglassing in a gunnel possibly help with stiffness? I have heard that if your shell isn't stiff, you can lose a bit of power during the stroke. I think I recall seeing carbon fiber honeycomb in Hudson shells (could be cost prohibitive though).

damonv (author)showlov4anerd2012-10-21

stiffening the top is good too. Honeycomb is more about limiting buckling or out-of-plane deflection than making the hull stiffer, so is not a big contributor to stiffness when the core is already a big block of foam. carbon at the corners of the deck in addition to on the bottom is probably the best EI increase. Making a hollow boat is probably the biggest weight savings.

Ironman 54 (author)2012-09-28

Can you post scans of the crosssections?

iMakeItHappen (author)2012-02-12

so what you want to do is sand the extra coat of epoxy, and not the fiberglass?

showlov4anerd (author)2011-12-10

Might that be autobody filler to bring up the low spots? Great ible, brings me back to my days on my college crew team.

la3l41 (author)2011-04-26

Looks great! How much does it weigh?

Ironman 54 (author)2011-01-29

Do you know any sites where you can get the designs for free?

jeffconnelly (author)2010-09-12

Do you by any chance know of a good set of plans?

casinoguru (author)jeffconnelly2010-11-03


I just bought the book Rowable Classics thru amazon. It strictly refers to wooden shells

At the back of the book there are plans with cross sections of a nice shell with similar lines to this boat.

you could probably enlarge them on a copier and come pretty close to this boat.

schkip1973 (author)2010-08-26

this is an AWESOME!! 'ible.. any chance you can tell me where to get the pink foam in Australia? Or what other brand names / chemical name it might be marketed under? I have checked at Clark Rubber with no success. Thankyou

rickharris (author)schkip19732010-09-09
The foam is extruded polystyrene foam insulation.

In general building suppliers are a good source if not DIY shops

BUT Be aware this foam melts when polystyrene resin is applied. You must use epoxy resins to avoid this.
Possibly you might be able to protect the foam surface by painting or coating with paper and PVA but experimentation needed.

randomray (author)2010-08-29

Wow , very nice job . I don't even want to build a shell and I learned a lot , thanks so much . A heads up , fiber glassing is not real expensive and not that hard to do compared to building wooden boats . Make sure you wear hand protection and face masks or you will regret it the rest of your life . Wear the right stuff and it's not a problem .

Alec Bittner (author)2009-03-28

Were can one purchase the foam blocks you guys used for the boat. Thanks And thanks so much for posting this!

randomray (author)Alec Bittner2010-08-29

It's housing insulation found in Home Depot and Lowes and most other lumber/home centers in the States . It comes 2" thicknesses , 2ft x 4ft ? , Owens Corning .Probably other brands in other countries and areas .

JamesRPatrick (author)2010-08-26

This is great for a first instructable!

lacrosseboy459 (author)2010-07-01

Im really interested in building a shell for offseason long was your boat, and about how much did it cost you to make, also what were the other dimensions, thanks

ethanbradek (author)2010-06-18

I'm interested in this project, I just have a few questions. How much did the over-all build cost? Also, Is it built without a keel? That's one of the parts that I was curious about when considering building my own shell. Thanks and great job!

uphrsn (author)2010-01-21
I am very new using this web site so maybe I am in the wrong location for this request. I have a couple of questions for the builder of this rowing shell and I would be much appriciated if they could be answered.
  1. After the foam is sanded did the fiberglass resin go directly on to of the foam? My experiance is that resin will disolve the foam similar to the spray on glue aresol.
  2. How did you decide on the plywood temptate dimensions and were they 24 inches apart? Also were they left attached to the foam pieces and glasses over?

Wade Tarzia (author)uphrsn2010-05-07

I don't think epoxy melts that Dow pink foam stuff.  Perhaps you are thinking of Polyester resin? 

Jobar007 (author)2010-04-22

The only problem that I can see with this is using the pink foam. It will delaminate if it is impacted. The fiberglass will pull from the foam and create and air pocket that will fill with water. You could instead go with boat building foam instead to avoid that.

moneymaker (author)2010-04-01

What's an estimate for the cost of this project?

Walter Martindale (author)2009-12-19

 Have you considered a set of real sculling oars? You'd get away with smaller oarlocks.  Boat looks unsinkable thanks to the foam remaining inside the hull.  good for offshore sculling.  Good project.

hinkle_adam (author)2009-07-13

This is a great process. Where did you get the pattern for your jig pieces? I can't find anything like it on the internet and I want my boat to cut the water properly.

JoshuaTerrell (author)2009-06-26

Hardcore. Take that Vespoli and your $8000 fussy shells! What city is that?

dwc230 (author)2009-06-08

I love this plan. It is so interesting, and I learned a lot from reading through it. I want to build a boat too, partcularly, a skull. I don't know anything about so many things, like the hot wire for cutting (shaving) the styrofoam. I guess I'll have to do some research. Thanks!

Hoaxinmi (author)2009-05-10

Holy Cow. I cant believe you did this. Nice Work!

engineerable (author)2008-11-23

I was the random visitor on Sunday Sailing a few weeks ago when you brought out the rowing shell. That's the first time I've attempted to use one and although I was always on the edge of tipping over and going for a cold swim in the SF bay, it was great fun! Next time I need to put the wetsuit on before boarding the rowing shell so I won't be as worried about capsizing. Thanks for letting me use it and giving me vital tips like "never let go of the handles".

shooby (author)2008-11-01

Looks good. Where's the first photo taken?

damonv (author)shooby2008-11-01

77 mass ave, cambridge MA, right in front of the entrance to MIT (can't you tell from the quantity of glasses?)

shooby (author)damonv2008-11-02

Haha should have known, right outside the architecture building. I live in Boston, the Peter Pan buses are pretty characteristic.

hedgiehog (author)2008-11-01

cool instructable, needs more detail, maybe add templates, measurements etc

GorillazMiko (author)2008-11-01

That's amazing! It looks complicated; and expensive. +5/5 stars.

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