Prototype 5.4 has an inflatable catamaran design, at a small scale (50cm in length, with 25cm between the inside of the pontoons, total prototype width of 35 cm).
Mathew constructed two pontoons from polyutherane thermoplastic. To these, we constructed and attached a ribbed skeleton structure, consisting of one solidly fixed center piece, and two pivoting ribs at the ends of the pontoons. We added a centerboard to the center cross piece, and a mast.
Catamarans have low drag due to their high length:hull width ratio, and are ideally suited for towing. However, one of the main problems with catamarans is that they have an unusually large region of "upside-down" stability, leading to difficulty with self righting.
On 16/06/2011, we tested the self-righting capabilities of a catamaran of this design. We found that it functioned extremely well, and would be able to self right itself from a completely flipped position, if the mast of our prototype had been attached more securely. However, there will be problems with scaling. In order to allow it to right, we had to use a 300 gram weight on the model. Since this prototype is 1/10 to 1/12 the size of the full scale Protei, that scales to a 200-500 kg weight, which could be unreasonably heavy. Other options that are available for a full design include putting inflatable ribs along the mast and the sail, increasing the size of the float on top, having a longer centerboard, and narrowing the distance between pontoons.
Note that it can completely be laid flat, and is completely disassembleable.
And it was built :
Gabriella was the first to jump on it! Here Roberto in the water !
If Roberto shares his weight, we'll know the buoyancy of this prototype :)
What came out of the test : A catamaran is very stable. It was also very difficult to stir this vessel.
The hydrodynamic profile wasn't really put to the test at the speed we were going.As soon as you get some speed, the front rudder gets really heavy and difficult to maneuver, flipping to one side or the other. It felt like one need to move around a huge amount of water to get somewhere. In fact, the 3 hulls together seem to behave more like a "gate" than an articulated vessel (see diagram below).
The main issue is that the force of the hulls oppose each other's dynamic : the central hull "bends" (maybe "folds" would describe better) but the lateral hull stays straight - on which most of the weight and moment would be when sailing, the boat tilting under the wind. The central hull would be much less in the water as soon as we would be sailing and tilting, minimizing the efficiency of the bending of the central hull. For this reason, most catamarans and trimarans have a rudder at the bottom of each lateral hull, not necessarily at the center.
Without ballast it is likely that this vessel would be pretty unstable since it is a very lightweight structure, and expose a large lateral surface to the wind -> lateral drift.
It was good to build this prototype to learn about stability and also to know that a multi-hull vessel behavior does not depend only of the hydrodynamics of its independent hulls, but of the interactions between the hulls.