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| Figure 5. Oakley 37 self-righting sequence capsizes. |
Figure 6. Oakley 36 self-righting sequence capsizes to port. Boat rolls to approx. 180 degrees and returns. |
- 3.3 Moveable Ballast
Although moveable ballast method largely superseded by inherent buoyancy method, it can still be applicable to small crafts which large capacity of buoyancy above waterline may not be achievable. In this method, low volumes of seawater located in the lower ballast tanks, which relocate during capsize. Relocation of this amount of weight creates enough moment to continue roll movement even after reaches the point of vanishing stability. Required moving ballast is most likely 10% of the ship’s lightship weight. Thus, this method is mainly used for displacement ships where additional weight is no harmful to ship’s performance. The best example of this method could be seen in RNLI Oakley 37 ft. and 48 ft. 6 in. lifeboats, produced during the 1950s and 1960s. Both of the boats traditionally manufactured to their time with built in wood with a double ended hull form having a low superstructure and an iron ballast keel.[1]
Benefits of the ballast transfer method include:
- Can be fitted to small open boats where buoyant superstructure is not a feasible option.
- Ballast is plentiful and readily available.
- Can be designed to be self-operating.
- ↑ K. C. Thatcher, 2013. Self-Righting Craft Basic Principles and Design Requirements. The Masthead Vol. 7, 12–21.
