The Pepsi Torpedo has been redesigned, using a 3-liter bottle. The main objective is to improve the submarining characteristic. Pictures have been posted on the Photo section of this website. Improvements to the existing Pepsi Torpedo have made it almost competitive with the local V-12, but it still skidded along on its nose in a strong wind, sometimes pitched forward at a 45 degree angle. This new fat hull should improve that problem.
It was difficult to find discarded 3-liter bottles at the town dump, so several full bottles were purchased at 87 cents each. The bottle is exactly the right size, and doesn’t need to be extended like the 2-liter bottle. The nose was plugged with a ping-pong ball, just like the previos design. The bottom of the bottle was left as-is.
Drag measurements (see a separate thread on this subject) have shown that this hull has a very high form drag. I have asked for some guidance on this from the hull design experts at the Stevens Institute of Technology, in Hoboken, NJ. Some hull design tests were done there on the Americas Cup boats. Dr Savitsky, of the Towing Tank, was kind enough to send me some useful information. He suggested a trip wire around the hull, about 5% to 10% aft of the water line entry, to trip the laminar flow into turbulent flow. I have tried this on the Pepsi Torpoedo, and believe it improved the speed. The new boat has a similar feature, but a double layer of tape was used instead, because it is easier to hold it in place. It can be seen in the pictures.
The hull material necessitates some unusual building methods. It is thin, and flexible, and glue adheres poorly. So everything is mounted to the deck, which is 1/16 plywood. The rudder is hung off the stern with an aluminum bracket, which is bolted to the stern. This is possible because the bottom of the bottle is significantly thicker than the rest. The hull material wants to curl inward after it is cut, so it naturally grabs onto a balsa lip on the underside of the deck.
The keel is made from aluminum flashing material, which comes pre-bent at a right angle, so it is easy to fold it down to a tear-drop shape while bending it around a thin aluminum tube. The hull is reinforced with wood strips at the interface with the keel.
The whole assembly is held together by a threaded rod from the bottom of the keel to the top of the deck. Aluminum tubing is used as spacers. A lot of tape is also used at the hull/deck interface. It was found that any wrinkle in the tape will cause a leak, so it was very important to take great care when applying the tape. Postal tape was very useful, except it would not stay flat on compound curves. A more stretchy tape was used on those areas.
Total weight, including rig, is 17.7 ounces, which is partly due to the keel being a little heavier than intended. The soda-bottle hull is light, but a balsa hull is probably even lighter.
