Balsa vs. cedar

I have built a IOM out of Balsa, then I built a IOM out of just fibre glass from male mould(three layers) now as the nights get colder I am getting ready to build another. I was wondering , I have lots of cedar strips hanging around garage, so thought I could use them. 1/8 x 3/8 then sand, micro balloon then one layer of glass on the outside and one on the inside or not. That is the question because I would be using a heavyier wood can I leave out the glass not the resin to seal? I figure balsa absorbs lots of resin so the weight might be the same? All of your thoughts would help my decision.

I think you will find that the balso actually absorbs less resin than the cedar. Obviously balsa contain a lot of air, but this is mostly encapsulated in what is essentially a ‘close cell’ structure with the cells fitting quite closely. Accordingly there is not much room for rsin in the intracellular space and the resin takeup is correspondingly low.

Western red cadar has a much more open structure and hence absorbs more resin. This, in my not very humble opinion, makes it the ideal boatbuilding material. Suitably engineered, it automatically forms what I call a ‘zonewich’ or zoned sandwich. Essentialy the same material is being used as the core of the sandwich and as the reinforcement of the skins. The resin is not there to provide a seal - it and he wood into which it has soaked are providing the real strenth. No glass is needed - if you want a little local increase in pabel thickness, either increase the thickness of the core locally (efficient but tricky) or add a fow strands of carbon ON BOTH SIDES. Note one of the basic principles of sandwich enginnering - if you don’t know any better, keep it symmetrical. In extreme circumstances, adding stiffness to one skin can actually reduce the load at which the structure fails.


You are right, the cedar will not absorb any where near the amount of resin that balsa does. You could probably not use any glass or only really light glass on the outside if you wished, and only seal with resin on the inside with 1/8 cedar.


I’ve used both and would never use balsa again. The weight at the end is compareable and with an IOM it’s kind of a moot point, if you build it right you will probably need correcter weights anyway. The things that sold me are the sanding and appearance. The stiffness of the cedar allows you to sand without having to worry about sanding more on the shadows. It’s much easier to get a fair surface with cedar. So much so that with 1/8" planks you can usually sand for a while after it’s fair just to make the hull lighter. I’ve seen balsa and cedar hulls finished with clear(no paint) and if your planking is reasonable the cedar will draw oo’s and ah’s for miles while the balsa looks like an off white paint job. A well done cedar hull is so pretty that you may not want to sail it for risk of a scratch. This hull was built from a cedar 2x6 from an old hot tub.

Hi Andrew,

I built several boats of cedar back in the 70’s and my recent IOM was built in balsa.

When planking flat planks around a curved form, you need to undercut the planks to close the gap. Even so, there will be gaps and thin spots after sanding; all the more so with wider planks. So I would definately put a layer of glass and resin on the outside of the hull to seal it. Just a coat or two on thinned resin should seal the inside.

So I am suggesting that I would treat the balsa or cedar the same. Therefore the finished weight would be higher for the cedar as the wood is more dense; but it would also be stronger and more impact resistant.

As you are not yet racing your boats, use the cedar as it is already paid for.

If you do a carefull planking job you can leave the boat with a clear finish.

Wow, do you have any more pictures or details about that gorgeous boat? Also – did you resaw the 2x6 into thin strips for planks? How thick etc?


-Rick (who’s only sailboat so far is a half-finished solid oak free-sailing footy, but he hopes it will be the first step into a new hobby)

I cut cedar planks into strips for planking using my table saw. The secret is to make a zero clearance insert for the saw balde. The insert supports the thin strips as they are cut and prevents them falling into the slot.

I used a narrow kerf finishing blade to give a nice smooth finish on the cedar.

I’m going to post some pictures on Photobucket in a few days(going camping) so I’ll let you know. The planks were ripped to about 1/8"x3/8" (more sawdust than wood) Here’s a link to a little more info.

Contrary to what Angus has posted, I have found that there is more absorption of resin in balsa than in western red cedar. I have built two IOMs in WRC and have left each with a clear resin finish. They have been admired for their looks by quite a few of the leading IOM racers in the UK. If care is taken in edge angling the planks then a fine finish can be achieved. The complete hull was bonded with resin, no CA or white glue, and I have only glassed the inside of the hulls with a very light woven cloth and have left the outside with a resin coating which I sanded with fine emery paper and then with auto compound. This has lasted for six years and is only now being recoated with finishing resin after being sanded down to the wood again.
Another point, if you are building an IOM, then any carbon built into the hull is banned.

Happy building


This is fascinating and turns what I thought I know upside down - as ever. My statement about the resin tale-up of balsa v. western red cedar was based on ‘something I read somewhere’ followed by practical tests, which showed a weight gain 10-15% higher (which seemed the most accurate way of measuring absorption) with the cedar.

It may be of significance that, as we were looking for a way of producing simple sandwich-type structures, we were not very interested in the strength of the core and therefore used very low density aircraft balsa. Does this make a difference? Or (not for the first time), am I not very good at producing properly controlled experiments?

Any explanations?

as in most cases I’ve seen - builders get carried away by emulating the layup schedules used in real (big) boats. They tend to forget the ability shape plays in adding strength. Finally, unless they are in a habit of stepping on their hulls, there are little if any true stress issues that have significant impact on a boat hull the size we are discussing.

Folks post they have used 3 layers of 2 oz. cloth - on top of foam or wood core! If 18 feet (5.5 meter) multihulls can be built using 1/4 inch thick cedar strip, with a layer of 4 oz. glass on both sides of the composite sandwich, carry two people and powered by a 31 foot mast with 250 plus sq. feet of sail area - most of what we build is so over-engineered it is almost funny. Think - the hull is simply to keep water out. A simple balsa framework covered with coated paper or velulum would also serve that purpose. Add compound curves tot he design and it should be plenty strong enough. A mast, stepped to keel trunk, keel, bulb, and a way to attach these to the hull to keep in alignment is all that is needed - then design a "skin with one pointy end, cover with “something” to prevent water egress and …

So to use WEST system epoxy as it was originally intended (Wood Epoxy Saturation Technique) whether on balsa, cedar, pine or oak is simply overkill. The "concept was developed for BIG boats - not models. Likewise paper/tissue covering on model aircraft - not aluminum like a real airplane.

Sorry - had to jump in to put the issue into perspective. One of your countrymen managed, when I was building multihulls, to convince me that it’s the design and shape that imparts strength - not just the cloth or core material. Granted that they do too.

Considering these are just small boats and over engineered, what would you suggest i put over the cedar strips? I am glueing them together with carpenters glue, as i want to do most of the work inside and keep the smelly or toxic out in the garage when kids are not there.

Dick - that is more or less what I was trying to say: it was me who pointed out that the maximum hydraulic force a Footy is capable of exerting on its hull is approx. the same as free pouring water over the back of your hand from a heght of a foot!

What we were trying to do (and still do for fins) was to produce a ‘skinless’ sandwich in which what is generally regarded as a superior varety of paint (the sealing coat of epoxide) is in fact the major structural member and is stabilised by virtue of the trivial depth by which it sinks into the wooden core. For all practical purposes you are most unlikely to crush the core so very light balsa will do - and the fact that it is not end grain is totally unipmportant.

We gave up on the very lightweight balsa because the resin takeup was erratic. This may or may not be a piece of overkill. Cedar + resin (only) is too strong and heavy for anything on a Footy except the fin. However, if cedar is at all appropriate for an IOM, I pretty sure that it will do very nicely without any glass reinforcement, and that a ‘zonewich’ will allow you to cut down on the thickness of the cedar. This also means you can do without expensive, heavy and requently unsightly paint. :devil3:


Well - as you are probably aware - carpenter’s glue of the general variety is water based and water soluble/thinnable. Thus you have elected to use an intermediate process that requires further work/process to waterproof. Encapsulation with epoxy barriers on inside and out would prevent water migration and glue failure. Epoxy could have been used for both adhesive and sealer - but you cite small children so this wasn’t an option. Likewise, a layer of very thing glass also would protect glue from water - but adds to the overall weight more than just a thin finish.

Since carpenter’s glue can easily be thinned, and it’s of low toxicity for your children, you might try a thinned down, almost water consistency and brush on the cedar to act as a sealer. Not to prevent water migration, simply to limit amount of epoxy that is absorbed by the wood. Once dry, you can take epoxy, and thin with alcohol to again water consistency and paint on over the glued sealed wood. This might limit amount of epoxy taken on and limit the added weight. In the case of the glue, expect grain to be raised and some intermediate sanding will be necessary. I think you will have this grain issue regardless of what you use - neat (unthinned) epoxy, polyester resin, varnish or polyurethane.

I am currently about to embark on a water “clear” polyurethane seal/finish for my balsa hulled RG-65 (Hull #4) and trials on scrap balsa seem to indicate a very limited amount of grain raising that has taken place. My desire to use this was prompted by a clear wood finish withhout the golden/amber color added by varnish or regular polyurethane wood finishes. It doesn’t have the UV protection, but that seems to be what adds the yellow into most other finishes.

If you plan to paint the hull, then any good sanding-sealer developed for wood finishes could be used as well, since it will be covered by your paint scheme.

Just a few ideas

I know you answered the question? but I am kind slow ! I know you said to just seal it but… if I have used carpenters glue, should i not put at least one layer of fibreglass over the out side then seal inside with resin? Can anything else besides fibreglass, (how about rice paper?)

I think that both Dick and I are ultimately saying the same thing. Unless you have so soaked the exterior in carpenter’s flue, the limited amount of resin that is taken up by the grain of the wood is quite enough - epoxide rersins are very strong things - and you don’t need much strength. Where you do need it (keel, rigging loads) build in a sub-frame: treat the hull skin as a monocoque).

AB - in line with Angus’ recent post - view a cross section of your hull side. Disregard the glue used to hold the strips together, but that is all that you are protecting from water.

INSIDE: Coating of some sort - but must be waterproof
CORE: piece of wood (solid cedar strips)
OUTSIDE: Coating of some sort - but must be waterproof

All you are doing is protecting the glue joints from being desolved by the water. Really no need to seal wood since you are sailing minimal hours … but …

We seal the wood to prevent water being soaked up instead of resin. Water will not replace the resin. There is really no benefit to spending time and effort to cover the hull with paper, cloth glass, carbon fiber, etc. since the stresses are so minimal. That said, having a rough outer hull isn’t desireable, so we sand it down and seal the wood grain from being raised by the water. Eventually the water will dry up, and grain will lay down until next trip to the pond. The glued strps in their final shape is what gives strength to the hull… the glass adds strength, but in some cases merely is used to resist scratches from scrubbing against rocks or being dragged across the sand (big boats). More for “wear resistance” than strength.

Not sure how else to explain this - but if I had to post a quick and dirty, then I’d say use water based polyurethane, and after several coats and sandinging spray with clear Krylon or similar spray… to protect from water - not add strength!

Don’t walk away disgusted, keep asking in a different way and eventually we all (and you) will be able to understand.

Again - the suggestion is for waterproofing, not for strength. In a round about way - if you HAD used epoxy for the glue of the strips, then it wouldn’t matter what finish you used as anything would be cosmetic - our suggestions are primarily to keep the glue holding the strips together.

Here are a couple of pictures of the Kite IOM that I described on a previous post #9.

Note the fine finish created by closely fitted planks and the use of only epoxy for construction adhesive and finish coat.

The biggest cost is in time. The total material cost to complete the hull alone was just £15.

Small note about the thin woven glass coat on the inside. It did not fully cover the planks up near the gunwhale. It was about 1.5cm below.
I was ‘T’-boned by a broaching boat in a head to head situation, and the only plank that cracked was ABOVE the glass layer. The thin reinforcement had spread the impact.



Ralph, your boat looks great. I now wish I had started glueing with epoxy from the start.
Dick and Angus thank you for your insight. I think I will glass the outside with a fine a glass as possible, I worry if the epoxy and finish will crack and then the glue will let go then the boat will go to Davy Jones locker, not worth the little extra weight of the cloth.


Can I use polyester resin with cedar. I just found full can of polyester in the garage I forgot I had, not even opened. I do like epoxy but I also like to use up the stuff I have to keep costs down?