There appears to be considerable variation between what different people think is ‘adequately strong’ and what materials are most appropriate.

I would not for one second wish to suggest that anyone was wrong in absolute terms or to deny their reasons for using the materials and scantlings that they do. However, we seem to be labouring under a considerable lack of objective data (I certainly am - my thinking changes every time a seagull goes past the window).

Let us look first at hull structures. A very good start would be to establish the panel stiffnesses of the types of material and treatment people actually use.

Is anyone who is a better structural engineeer than me be interested in layting down a specification for testing the stiffness, etc of small panels?

Is anyone interested in making up a small sample (I guess a few inches square) of their favourite hull structure - just flat - and subjecting it to possibly destructive testing? If they are, are they prepared to post the results.

I am prepared to maintain and post a database of any findings.

If that goes well, I guess the next candidates are fins, rudder stocks and masts.

Information is king!


Unfortunately, panel stiffness by and in itself - except for perhaps a deck (and that is questionable) tends to pressure one to use heavier materials “just in case”. We all know/remember the comparison of an egg shell and strength induced by a compound bend. So too plywood - and now aramid or carbon composites. Once a flat panel is distorted in a compound way, inherent strength and stiffness seems to increase until a fracture point of the material is reached.

It was once pointed out to me by a UK friend who builds very lightweight multihulls - that in many cases our hulls are well overbuilt - some exceeding the ability to keep out water, and becoming as thick - or thicker - than full size power boat hull thickness.

To that end, I am reminded of kayak designs using carbon fiber rods/tubes, and covered in thin cloth or paper - properly finished to prevent water intrusion. Yes - strength is built-in where needed, to allow stepping/sitting/storage - but if one takes a common radio control yacht, and builds it with aircraft weight fabrics - is that overkill? I had great concerns about my old catamaran (full size) whose sides could be compressed with hand pressure. It sure looked “eggshell” fragile, but it never broke or leaked. Strength of materials (my opinion) should vary depending on it’s intended final use. After all, a sailboat is merely a mast/sail, attached to a keel with rudder, and with space for radio equipment. The rest of the hull is designed to reduce drag, and keep the contents (radio gear) dry.

A rather simplistic explanation, but it seems everything else is secondary. Does one really need decks of carbon with teak/mahogany strips attached? Only for looks would be my thought.

I have a large r/c multihull. It is built of one layer of 4 oz. glass cloth, and while flexible, it is still strong and light.

Just my observations, here. :wink:

You are going along my line of thought - I think. The tendency of the modeller is to make things that feel ‘full size’ strong rather than ones that will stand up to the actual loads imposed upon them.

Why not a Footy built to model aircraft standards? - and please people, do not come up with the ‘kids’ argument: kids play model aircraft quite successfully and IN THE VOLUMES INVOLVED IN A FOOTY the high-tech materials are certainly within the reach of a UK teenager who wants to do it. I presume that their American equivalents are richer.

One of the reasons why not is that we do not seem to have any serious ideas of what the strength of the materials actually is and what forces are actually involved.

Hence the proposed project.

Any takers?


Keep building them lighter and lighter until they break - then build the next one just a tiny bit stronger.

This is how we wind up with “A” Class full size catamarans, 18 feet (5.5 Meters) in length - 30 foot (10 Meter) mast and sail that weigh in at an unbelievable 168 lbs. (about 76+ kg) all up sailing weight ! True - it’s mostly carbon construction and Kevlar/Carbon sail material - but no titainium, or other “un-obtainium” parts. Harken blocks, aluminum fittings, plastic mesh trampoline, stainless wire rigging, and of course, the obligatory “extra” cost of a mere $21,000 plus trialer and delivery - gulp.

While we in the US may be able to afford higher prices for our materials - the retailers are more than happy to compensate for the differences as well - and charge us higher accordingly! :rolleyes:

I will see if I can find my e-mail correspondence regarding weight and over-building.

I agree in general. We don’t just want the hull to be watertight, but we also want to prevent tin canning - which potentially slows the boat down and eventually leads to fatigue. However in our boats, once the thing is on the water, and assuming that the load-bearing areas are strong enough, for most building materials this does not require much of a hull thickness at all.

However there are two other practical considerations:

  1. Ensure that the hull can take a few knocks from other boats on the lake without excessive damage. Sure, Footy v Footy may not be an issue, but Footy v EC12 at speed may make a bit of a mess before the Footy is pushed aside. For larger boats, which are less likley to be merely pushed aside, this is even more of an issue.

  2. Ensure that the hull can take a little bit of rough handling out to the water. If my hulls were all wafer thin, I’d be terrified and paranoid of every other person at the lakeside everytime my boat was out of the water. I think I’d be enforcing “no walk zones” around my boat.

In some classes this is probably unavoidable. In true development classes (including the Footy) yeah - push it to the limits as Dick suggests.

fwiw, I’ve seen a few 1M hulls built of nothing but balsa brames (the usual 8 frames) & plastic aircraft covering! I also saw one of balsa & paper tissue ( soaked with resin.) The latter being of delicate design, was able to sail as well as other designs, but was only used in calmer waters.

For something small, I’d use an egg as an example of how strong it it is (in compression) but as it has been said, it can still be punctured. I think grain direction, careful use of bracing, and a glass/resin skin will be important. Soaking the wood ( like porous balsa) with resin will help too. All used well and in a trade-off with weight.

OK then. There don’t seem to be any immediate takers for a collaborative project, so here’s what I’ll do.

My current half-built Footy is strip planked and will be sheathed inside abd out with very light carbon tissue + epoxide.

I’ll put my time and money (hopefully got much of it!) where my mouth is and before I take her off the building frame I’ll use her as a plug for an ultra-thin shell single skin carbon/epoxide hull (basiically the same structure as her own outside skin). I’ll beef the thin shell hull up with some carbon reinforcement until I get something that seems likely to resist practical sailing loads (and the odd knock). And I’ll post the results.

Sorry guys, I’m not actually prepared to carry out (deliberate) destructive testing on a whole boat!

Would this be useful to people?


As a long time aeromodeller Angus I am finding this very interesting. I especially like ‘vintage’ models and built many so appreciate the strength of ‘sticks and tissue’. Putting that together with an article way back in 'Wooden ‘Boat’ mag. about a guy building very lightweight canoes using stringers and modern heatshrink fabric…

‘Solarspan’ over balsa stringers creating a multi chine hull.

You will find my foam core method horrendously strong I fear Angus, but taking the skin down to 1/64 ply on a thinned down core I believe the strength would still be sufficient-plus and the weight would be getting pretty low. Still a chine hull, but that can have it’s advantages too.


Grahams foam blanks are 13g.
you could simply paint one and sail it.
thats hull/deck everything.
don’t want hard chines?
simply take a peice of insulation foam and carve your hull,coat it with a thin slurry of epoxy bog,sand and paint.dig out as much of the foam as you dare.
I bet I could get under 10g easy and bulletproof to boot
The little 4 inch hull in my avatar is built this way,the whole boat incl rig weighs less than 1 g
This method will easily best any attempts at a real lightweight moulded structure.
my bowsprit would make short work of any hollow superlightweight moulded boat…take my 720 turn and on with the job:)

I can of course supply hull cores without making the internal cutout which would make a good starting point from which to round the corners and do as Brett suggests.

Sounds like we need a harsher penalty than a 720 for ‘ram & sink’ tactics!!!


Accidents happen,I wasn’t serious.
Just pointing out the folly of going to a regatta with a boat to fragile to do the job in all conditions without missing a beat.
In order to finnish first…first you must finnish.


Your carbon hull won’t need any bracing, except for holding the keel in place. With the deck on, the hull should be strong enough. Then, if you will, add some extra bracing after you try fit the deck and gove it a few squeezes.

I will probably cut out some of the bulkheads in my hulls, as I think the sides & bottom will keep their shape by now.

there are 2 schools of thought on model yacht scantlings as far as I can see.
1 make the hull skin also the main part of the structure.maybe only 1 internal bulkhead in the keel/mast area.

2 A much thinner skin on a considerable internal structure.

Over the years most builders seem to have settled on option 1,
especially production buiders.
I think that it is a better option in a Footy sized model as well,as the skin could be so thin on a model with an internal structure.
I don’t put any internal structure in my Footy models as all.I simply glue the fin into the hull and attach the servos to either side of the fin.glue the mast tubes in…nothing else at all.
A layup with cloth totals of about 100gm2 seems enough,you could go lighter sure…but you are going to put in 100g of batteries and perhaps 50-70g of servos inside this hull,so if the hull weighs 10g or 30 g is not a major issue to me at this stage.If of course the stakes get raised further down the line then I will take another look at my stance.

As ever, that is very sensible and very pragmatic.

Just 3 points.

  1. At the moment we’re just looking at the hull itself. Apply the same weight cuttin logic to everything else and we start to make a difference.

  2. OK, obviously the thing needs to be able to take the odd knock in the real world. However… Say the maximum speed of Footy is about 1.4 knots (0.7 m/s). Go find yourself some water flowing at that sort of rate. Dip ypur hand in. Get an idea what it feels like. Alternatively, pour water over your hand from a height of about a foot (35 cm). That’s the kind of force its hull is subject to when sailing. Now squeeze your Footy until you can just feel it give. Which force is bigger?

  3. In a lot of models yachts, the need for rigidity is the result of having to cope with the rigging loadings of very tall narrow stayed rigs (and high ballast ratios). Generally speakimg a Footy does not have to handle any of these.

Over to you, ladies and gentlemen.


No, there’s no stays used on a Footy, if you use Brett’s stayless rig, but then you are using the hull & deck in a different way to support the mast. More in the way of deck compression sideways. In that case, I thgink it’s more of a matter of the wind pressuer you want to handle (heeling forces.)

I have made up 4 test peices.
A 2x100g loose weave cloth
B 1x100g loose weave cloth
C 2 x60g tight weave cloth(fine threads)
D 1x 60g tight weave cloth.

panel A is my normal layup,I use it mainly because I get a strong hull which is easier to demould than a lighter structure and I am able to get a good pinhole free surface.
I have performed some bending tests on the samples by placing them across the jaws of a open vice and placing a weight in the centre.
D and B are both single layer layups and were much less stiff than the other 2
A is the stiffest closely followed by C
C would be my pick of these 4 samples for the best stiffness/weight ratio whilst still being practical to use.I estimate a 15-20 g hull could be built with this layup.


If you haven’t thrown the test panls away, could you tell uis what the distance between the jaws of the vice, the width of the panel ‘along’ the jaws, the weight applied and the deflection.

I have two carbon samples curing.


Just a quick note to add to Brett’s comment about carving a hull out of insulation foam, covering with glass and resin and carving some of the foam out afterwards.
The gentleman who won the World M Class Championships in Durban, South Africa (in '78 if I recall) was Lenart Atkison. His method for creating his winning boat was to carve the shape out of styrofoam, cover the whole thing with glass and epoxy and fair the outside after it had cured. The hull was what we called “turtle shell” in those days, that is there was no delineation between the hull and deck. And no seam between them either because it was done as a complete shell from the get-go.
Anyway, it was cast with its stern open. Once the shell was faired out the styrofoam plug was melted out of the inside with acetone. I made an M Class hull this way not long after meeting Mr. Atkison at the race, and I can attest to how unpleasant this method can be. The shell however was lighter and stronger than most of the hulls (without integral decks) that were available at the time (pre-carbon kevlar era).
For a Footy I would think that this method might have some value for those intrepid individuals who want to make a hull with round bilges but don’t intend to go into production. The mass of gooey, viscous melted styrofoam should be manageable for a Footy’s volume (for my M Class hull I was left with trying to extract a grapefruit size glop of the stuff).
Please note: only do the melting process outdoors on a day with wind, downwind of anything important (home, family, pets and neighbors). Wear a respirator, this is a stinky process. Don’t use a lot of acetone and be patient, it takes a while. You may have to roll your hull around to get the acetone to melt the foam on the sides. Also, do not substitute polyester or vinylester for epoxy when laying up your hull with this method as these resins will melt the styrofoam.
If you try this method, the best of luck to you. I will never do it again but it can produce a superior hull if you are careful and tenacious.

Would it not be a better idea to leave the foam inside the hull as flotation?
The added weight would be worth it IMHO. Also keep the strength should you use a lighter skinning method.

Has enyone had experience with a material called “Depron”?
I believe it is a foam that comes in sheet form.
It could be most useful for Footy hull construstion.:stuck_out_tongue: