A Fast Offshore Cruiser - 6. Parametric study.

The parametric study can be used to produce a starting point for designing a hull. By collating data of similar type and size vessels typical values and trends should be seen. The new vessel may lye on these trend lines or if the design is going to be different, an improvement maybe, it can be decided how far from the calculated norm it is wanted to be.

To produce the parametric study the data of other vessels is needed. This can be a problem as designers and manufacturers tend to only give out the basic dimensions of the vessel, but these are still enough to produce a limited amount of data. The dimensions that are given include LOA, LWL, Bmax, T, D , ballast, sail area and water ballast, if used. These dimensions can tell you the basic shape of the vessel, e.g. narrow and fine or wide and blunt, the stiffness, e.g. high or low ballast ratio, and the power of the vessel, e.g. sail area to displacement ratio.

In modern naval architecture comparative data is used in non-dimensional forms, i.e. no units, so for 5 of the 6 different items of data this has been done. The sixth item is a traditional equation that relates the displacement to the length of the vessel in imperial units and is dimensional.

The data of "7seas 1524" has been included in the study, as a comparison, though during the actual study this data would not have been available to use.

Sail area (m2) / Displacement2/3 (m3) - (non-dimensional).

This shows the power to weight ratio of the vessel, i.e. the higher the number the supposedly higher the power available.

LOA (m) / Displacement1/3 (m3) – (non-dimensional).

This relates to the shape of the hull, e.g. a high number would indicate a fine hull. This also may give an idea of the wave resistance of the hull; i.e. a fine hull will produce smaller wave resistance.

LOA (m) / Bmax (m) - (non-dimensional).

This indicates the shape of deck, in plan view, which can relate to the type of hull form it may be. The low number means a wider boat that is generally associated with performance off-wind boats, e.g. Open 60’s. This does, however, not necessarily relate to the shape of the water line.

LWL (m) / T (m) – (non-dimensional).

This will show somewhat the stiffness, so long as the vessels are similar in LWL. A vessel with a long waterline for its length will seem to be less stiff than one that’s waterlines is of shorter length for the same T.

Ballast ratio.

This will show somewhat the stiffness assuming similar keel types, e.g. fin and lead bulb or completely cast iron.

Table of parametric study data and comparison charts of in Appendix D.1.

The charts, in Appendix D.1 show that maybe the selection of yachts was not that good seeing the range of scatter that has been produced is large. But to get the typical values the linear equations of the Excel trend-lines were used.

This comparison seems to show that "7seaa 1524" will have a more efficient hull, as was required by the design brief, being finer and stiffer. There is not an increase of power available, SA/Disp, which indicates that the rig should not be any more difficult to handle, if not easier due to the smaller area. The more efficient hull will allow the vessel to go faster.