Introduction

Overview

This is a design case study for my sailboat, Basiliscus. Basiliscus is currently in preliminary design, and as the design proceeds I will be publishing her development in this case study. The case study will include the methods and design tools as well as the results for this particular design.

Basiliscus will be a cruising, hydrofoil trimaran, only the second of its type, since none have been built since David Keiperís Williwaw. Williwaw proved out all of the essential elements of the cruising hydrofoil sailboat. These included:

- trimaran configuration, which lets the boat heel, as opposed to a catamarn which heels very little. A multihull configuration is essential for a hydrofoil, in order to avoid lifting the weight of ballast and to allow the boat to reach a takeoff speed which does not require excessively large hydrofoils to lift off. The trimaran configuration works in concert with the foil arrangement, which was a Keiper invention.

- "diamond" foil arrangement consisting of a bow foil, stern foil, and two lateral foils, which lets the windward foil come completely out of the water. The bow and stern foils balance the boat in pitch, with the bow foil acting like a sensor to adjust the angle of attack of the lateral foil. The lateral foil carries most of the weight of the boat as it resists the side force and heeling moment of the rig. Since the lateral foil is near the center of gravity of the boat, its loading can change significantly without disturbing the pitch trim of the boat.

- surface piercing ladder foils for simplicity, strength, and robustness in the demanding offshore environment. Other foil types may be feasible, and will be investigated during the design effort.

- the ability of the hydrofoils to improve the seaworthiness of the boat in extreme conditions, even when operating hull borne.

Why has Williwaw has remained unique for 30 years? Williwaw was built in 1969, which makes it a contemporary of the Brown Searunner series of designs. It was destroyed at anchor in 1977, having cruised 20,000 miles in the Pacific, from San Francisco to New Zealand, in conditions ranging from calms to storms. Unfortunately, Keiper was not able to develop his design further because he had been unable to sell Williwaw so that he could afford to build a second-generation boat, and it was not insured when it was destroyed. These were the pioneering days of multihulls - both Keiper and Brown were Piver devotees - and one can only wonder where the state of the art of sailing craft would be today had hydrofoils become popular. But at a time when the very idea of multihull sailboats was a radical concept, the idea of hydrofoils offshore was too great a leap for general acceptance. Hydrofoils also require careful engineering using aeronautical technology which was unfamiliar to most sailboat designers of the day. Had Keiper been able to afford to develop the concept through a series of boats, as Brown was able to do, we might have many more hydrofoils sailing today.

Today, the advent of the personal computer has given the individual designer capabilities which in the days of Williwaw only existed in the biggest aerospace and naval architecture firms. The purpose of this project is to apply the principles of modern marine engineering and flight dynamics to the design of the sailing hydrofoil, to capitalize on the advances in materials and operational experience with multihulls gained over the last three decades, and build a boat that I can enjoy and take pride in having created. This case study is primarily for my own benefit in organizing the engineering of Basiliscus. I also hope that the documentation and the computer tools that result will be of value to other designers in the engineering of their craft.

The chapters to follow lay out the plan of attack for developing Basiliscus, and derive the technical details of the analysis methods. The methods are followed by the actual design results and trade studies.