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SELF-RIGHTING BOAT DESIGN 49

Figure 5. Oakley 37 self-righting Figure 6. Oakley 37 self-righting
sequence capsizes. 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Although moveable ballast method largely superseded by inherent buoyancy method, it can still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. 6in 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
[6].
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.

Sayı 6, 2016 GiDB|DERGi