How strong is your hull?

Hi there,

I am totally new to rc sailing (have flown foamies before) and I am building my first hulls. The thing that puzzles me a bit is the reported weight of other hulls compared to my initial efforts. Mine come out at 42 (2mm depron, ricepaper + wbpu skin) and 53 gram (2mm depron, balsa kingplank and garboard, wbpu + ricepaper). Compared to other numbers I can find that is very light.
So that makes me wonder: do I underestimate the strength required (could be, I still need to marry a 600 gram keel + rig to this foam sandwich and I have no idea how :). Or do most people overestimate the required strength and end up with overbuilt boats? So how often does it happen that a boat break from some mechanical stress? (loose keelbox, torn forestay etc?) If the incidence of broken parts is low, isn’t the part too heavy & strong?


My theory learned from some wonderfully talented “big” boat designers/builders …

If it doesn’t break, it is overbuilt! Build the next one just slightly stronger. :smiley:

I have some veneer ready to start on RG-65 hull #5 and it is 1/20 inch thick (about .5" or 1.27 mm) and I feel it might also be a bit “heavy” since it is from a hardwood species and not from balsa. In reality, you are simply keeping the water out and supporting the deck and keel.

Another indicator that things don’t go wrong often enough might be the lack of ‘safety’ equipment. It appears to be unusual to carry extra floatation material (like a blown up plastic bag in the bow), and I think that is not because people don’t mind the dive when their boat sinks…

Jelle, go for it my friend, most boats are overbuilt.
The application of some of your building skills acquired from the model aircraft hobby will help in the advancement of constructing these tiny craft.
As Dick has already written, “if it hasent broken, it`s built too heavy.”

bought a CF tube today, thinking to use it as a mast. They didn’t have the size I wanted, so I went with 8mm. I put it on the scale which made me change my mind. I will not be using a mast that nearly is heavier than the hull!
So, has anyone ever tried to make a mast out of balsa and carbon tow? :slight_smile:

Probably - but somewhere time versus effort versus cost begin to take hold, along with some builders’ impatience.

Many simply elect to purchase some items to get on the water as quickly as possible, while others enjoy the minute detail and building process that can be rewarding in itself.

Whatever you enjoy is the final answer, and be sure to look to many of the kite shops for a variety of round, rectangular and square tubing. A lot of weight will be added strictly for visual appeal - so don’t let the mast weight hold you back, unless you are planning a super, ultra-lightweight build. In that case you will be looking at servo weights, line versus wire standing rigging, batteries (type and size) and a whole host of other “normally overlooked” issues. Add in paint, resin, metal fabrications, etc. and it all really adds up quickly.

Good luck - and a suggestion: If you are the first to try it - document with photos as we all enjoy technical information, even if it is critical to get something on the water as soon as possible.


I am not in a hurry. Building it myself is just a solution to the problem of lack of suitable supplies. Kiteshops are not on every streetcorner (none in my town AFAIK), the one I was this afternoon did not have the size i wanted. I had ordered some carbon tow anyway to make a keel with, so I might as wel try it out on a mast too. But first on some scrap to test the method and see how strong it is (I fear compression of the balsa will be the weak point).

Jelle, there is no balsa, simply carbon tow and resin wrapped around a metal mandrel. You can wrap your own mast but in this size, it is unlikely that you will quickly get better results than a bought one.
Standard cheap pulltruded carbon fibre tubes come in at least two wall thicknesses (.5 and 1mm). A 5 - 5.5mm should be sufficient diameter.
Wrapped tubes are much lighter. Kite booms are very light and can also be tapered. EG a Avia Skinny ULT-40 or Skyshark 2PTL are 10gms for a metre and US$10! Caution - I have not used these, so someone with experience may advise!
My kite supplier gets his from Holland.

Regarding the hull - are you creating a stressed skin with paper on both sides of the foam? Will the Polyurethane waterproof? What is the small dent resistance (hanger rash). How does it compare to .5oz fibreglass? Does the weight include the deck? What design are you using? Is it a low displacement? Good to see some experimentation. Have you thought of a very light, bendy McCormack Unarig? See also Claudio’s rear mast proposal…

That Unarig looks like a good and simple idea. Thanks! But to make it work on a bigger boat would require some calculations + experiments. Assuming both boats have the same beam, a rg65 would be twice as stiff as a footy (rg65 is ~2 times as long). With a higher mast (not sure about mast height for footies) that would mean a moment that is 4 times as big (2 * sail area, 2 * mast height). That would keep the required stiffness or the wire bottom still within limits.
As for my depropn hulls, yes they are a sandwich of depron between two layers of wbpu soaked rice paper. The addition of the wbpu-paper skin gives it a lot more stiffnes. I have not yet dented them while handling, but that may become an issue when the keel is attached and forces are bigger. I guess that is what those boat stands are for.

I tried depron once for an RG65 and found it to be much less inherently stiff than 1.5 mm (about 1/16") balsa. The lightest skinning with epoxy glass I could manage more or less doubled the weight of a balsa hull. One of the most popular designs is the Palo de Agua and the original 10-year old prototype is still sailing 10 years on. It is all balsa construction sealed with repeated coats of aircraft dope and sanding. I guess your water based PU could substitute the nitrocellulose dope.
Btw, 6 mm carbon tube is generally sufficient for the mast. A little flex on a una rig may not be such a bad thing if full sized Lasers are anything to go by.

it is not the depron that made your experiment heavier then a balsa hull, is it? Without a skin it is very floppy as you found, but glass + epoxy may be overkill. The total weight contribution of the depron in my hulls is 10~15 gram, so it really countr how much skin you put on, be it epoxy or wbpu. WBPU has a solvent in it (water) so it is easier to put on a thinner coat (the solvent evaporates).