need help in building a small catamaran

cardsinplay,most of malcolm tennants net and tube cats have the outboard mounted in a pod hung between the mast and aft beams around the pitch center.
Steve.

 

so how thin should be the fiberglass layer that covers the boat? that would keep it light and yet strong enough? and does the resin alone on the plywood (with no fiberglass mat) makes the ply much stronger or it is mainly to prevent wood rot and water leak?
another thing; how (what kind of holders with what kind of beams)would you recommend to attach the beams that hold the two canoes together (that would make it like a catamaran)?
i am reading a lot in the last days on the net about it all but still there is basic knowledge that i am missing.
thanks for your help

 

wow, that is about 3.5Kg (7 pounds) for a square meter (about 10 or 11 square feet) of fiberglass and resin. somehow i remember pieces of fiberglass as very light material. if there are about 140 layers of fiberglass in 1 cubic feet (each layer about 2 MM, 0.078 inches) so each square feet would be 96lbs/140 about 0.7 lbs which is about 0.35 kg. there are more or less about 10-11 square feet in one square meter so it would be 3.5 kg for one square meter. if i cover the whole hull, (about 3+ plywood sheets) it would be about 30Kg. sounds a bit too much or my calculations were wrong ?

 

For normal boat cloth I was told to allow roughly the weight of the cloth in resin for a hand layup. EG: 1 sqm 200gsm cloth plus 200 gm resin to wet out = 400 gsm. I don't know if this applies to more complex (like unidirectional or double bias) fabrics.

I typically add a second coat of plain resin as it starts to tack off, then a 3rd with some fairing powder mixed in. Let it go off for a couple of days and sand with orbital for a good smooth finish. The idea is to fill the weave without over doing it, and have enough thickness to sand without cutting into the weave of the cloth. The other option is to use minimum resin but lotsa high-build primer to fill the weave before you paint.

 

good, thanks.
thanks you all, guys. you have been great help. i am going to learn more on the theoretical aspect and then when i am back from my 2 months vacation away from the freezing cold in Denmark, i am planing to start building, maybe something really small to get the idea how it is done in practice and then build a canoe or something similar. i am not final with the concept of what i would build but as i said, i am gathering info and getting the feel of it,

 

Check into lighter cloths such as Dynel, Vectra, and Xynole. Some have better abrasion resistance than fiberglass which really is heavy. The resin weight stays about the same.

 

Stitch and ....

May be, I am too late for you, yoram. But here is a good example for a simple light stitch an glue canoe design with a bit of torture: http://www.bandbyachtdesigns.com/mocci.htm

Here is a picture:

 

This boat weighs 200 lbs., ready to sail, minus the 2 hp engine and the fuel. It uses 4mm marine plywood and 6 oz. glass cloth set in epoxy for its shell strength.

 

Thanks you guys. Every info helps. I went to see a small boats builder here and got more info. I am still gathering and processing the info, not knowing exactly what kind of concept i want, since i want to enjoy all worlds. To make it light and strong and cheap and easy to build and that there would be enough space on it (maybe even to spend a night) and the possibility to put a small mast and sail, that the amas would be retractable and light and above all, that it would be fun to built and to sail and not expansive that if I bang it or built it poorly, it would be wasting money more then the fun value of the building process itself. It doesn't have to be fast but at least well hydrodynamic so there would be as little waste of energy for the electric motor.
Right now I am contemplating trimaran with relatively wide, 480 long and 98 cm at beam 50-60 hight of the sides (I still do not know the proper maritime terms in English)canoe, flat bottom with two small amas, so it can be a rowing canoe by itself, and with the amas sailing. In case that board are put on the arms that hold the amas, i could have more deck space.

 

Have you seen these hulls?

http://www.marcopoloboats.com/MarcoPoloBoats/Hullz.html

Gareth

 

it is not bad but expansive and i am going to miss the fun part of building the boat.

 

Here are...

some plans for simple small canoes: http://www.bateau.com/products.php?cat=8
If you want to build a flattie it seems to me worth to think about a square stern if you will use it for a multihull. The hydrodynamics might be better with a motor or sails. Kayaks and canoes with pointed ends are optimized for 3-5kn and therefore have low cp (prismatic coefficient >50). Multihulls are better with higher cp and therefore fuller stern and bow. This might reduce wetted surface and lower skin friction and also pitching.

 

thanks Manfred, i have more questions

as i am going into the material.
could canoe or kayaks have flat bottom? some kind of trapeze shape when cut traverse from top to bottom? ( \_/ ) this will make building very easy. i would cut the plywood in the shop in strips of 40 cm that would be the side strips (boards) of the canoe, cut the bottom with jigsaw to look like that [ () ], connect the side strips at the bow and stern with stitches and stitch along the bottom of the canoe from both sides, Spread the upper part of the side strip with temporary spreader from wood and there is a canoe. Yes I know the fiberglass tape and the putty and epoxying and fiberglassing and reinforcement of the side strips and above all sanding is most of the work, but how much would it influence the performance, stability or any other kind of issue that i can not see right now?

 

here it is. i hope i did the upload right

i did it with "Paint" from "Windows".

 

Flat bottom

Your procedure seems to be O.K.. I have built a lot of boats the same way. But I would not design my first boat myself. It is a lot easier and cheaper to take a well tested design, which has been built and used over years. Bateau.com will have enough plans to choose between.
http://e-boat.net/

To understand some of the merits of flat bottoms and a square stern please have a look at the tests of Ian Smith (Australia). http://www.fionamsinclair.co.uk/yachts/smith/page23.htm