The design of a hydrofoil system for sailing catamarans
Thesis (MScEng (Mechanical and Mechatronic Engineering))--University of Stellenbosch, 2006.
The main objective of this thesis was to design a hydrofoil system without a trim and ride height control system and investigate the change in resistance of a representative hull across a typical speed range as a result of the addition of the hydrofoil system, while retaining adequate stability. The secondary objectives were as follows: Find a representative hull of sailing catamarans produced in South Africa, and to establish an appropriate speed range for that hull across which it is to be tested. Test and explain the drag characteristics of this hull. Find a suitable configuration of lifting foils for this hull that would not require any form of trim or ride height control to maintain stability throughout the speed range. Test and compare the resistance characteristics with and without the assistance of lifting foils. Test and explain the effects of leeway and heel on the total hydrodynamic resistance both with and without lifting foils. A representative hull (RH1), based on a statistical analysis of sailing catamarans produced in South Africa and an existing hull design of suitable size, was designed. A speed range was then established (0 – 25 knots) based on the statistics of the original (existing) design. A scaled model (of RH1) of practical and suitable dimensions was designed and manufactured, and its characteristics determined through towing tank testing. A hydrofoil system was then designed and during testing, was adjusted until a stable configuration was found. This resulted in a canard type configuration, with the front foil at the bow and the main foil between the daggerboards. Although a stable configuration was achieved, it was noted that any significant perturbation in the trim of the boat would result in instability and some form of trim control is recommended. The main objective was achieved. The experimental results concluded that a canard configuration was found to be stable for the RH1 (foil positioning already mentioned) and the addition of the hydrofoils provided a significant improvement only above a displacement Froude number of 2, which for our full scale prototype, is equivalent to approximately 14.2 knots. This is in agreement with the results of several other research projects that investigated hydrofoil supported catamarans with semi‐displacement type demi‐hulls. Below displacement Froude number of 2, a significant increase in total hydrodynamic resistance was observed. Since the speed of sailing craft is dependent on wind speed, there will often be conditions of relatively low boat speed (below displacement Froude number of 2). So it was recommended that a prototype design would have a retractable hydrofoil system which could be engaged in suitable conditions (sufficient boat speed). The effects of leeway and heel on the total hydrodynamic resistance were determined experimentally, but it was found that these trends were affected by the resulting changes in wave interference resistance. Since wave interference depended strongly on the hull shape, it was therefore concluded that no universal trends can be determined regarding the effects of heel and leeway on the total hydrodynamic resistance. These effects were determined for RH1 and it was shown that these effects are drastically altered by the addition of the lifting foils.