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Literature Review of Planing Craft Design - Planing Catamarans

Literature concerning the design of planing catamaran hulls is scarce. Since the hydrostatics and hydrodynamics are similar in principle to a monohull, in the past the naval architect has implemented the same design formulae to catamarans. 

By studying the documentation of experiments conducted on high speed displacement catamarans, lessons can be learned for the design of planing catamarans and these can be used to adapt the calculations for the design of planing monohulls. This, however, still leaves a degree of guesswork in the design.

A recent investigation into the resistance components of high speed displacement catamarans was conducted [Insel and Molland 1991]. The investigation was both experimental and theoretical. The aim of the study was to determine the resistance components in calm water of high speed displacement and semi-displacement round bilge catamarans with symmetric demihulls. The results of the investigation provide a better knowledge of the resistance components of high speed displacement catamarans, including the effect of hull separation and length to beam ratio over a range of Froude numbers.

Whilst these results are useful for the performance prediction of planing catamarans, they do not cover an adequate range of Froude numbers for the higher speeds expected to be reached by planing catamarans (i.e. C_v=V/ gb 3.0). Furthermore, due to the nature of the water release from the chines of a planing craft, and due to the small perturbation to the water surface made by a planing catamaran, this author suggests that the wave interference effects would be less significant and at different phases to those described by Insel and Molland [1991]. Additionally, the design of planing catamarans is such that the hull form may be symmetric, but more likely it will be asymmetric for high speed craft, giving stability in turns and minimising the flow perturbation. Again, these variations to hull form will require analysis before an accurate performance prediction model can be developed and they are discussed further in Chapter 4, Section 4.4.

Model tests have been performed in the Stevens Institute of Technology, Hoboken, New Jersey, to study the interference effects between two flat surfaces planing parallel to each other at high speed; these tests were performed, and a precis was written by Savitsky and Dingee [1954]. The data recorded was on the change in lift and wetted area experienced when planing parallel to, and at various lateral distances from another similar planing surface (i.e. a planing catamaran of block section, with parallel hulls). Unfortunately, the data was only recorded for a limited range of test conditions and therefore does not provide a comprehensive set of results.

The conditions are analogised to an aerodynamic situation of several wings of high aspect ratio. In this case the resultant upwash velocity is induced by the trailing vortices of one wing on the other, giving each wing a larger effective incidence angle, resulting in an increased efficiency of each wing. It is seen from aerodynamic theory that there is a very rapid increase in the lift as the lateral spacing between two high aspect ratio wings is reduced. The problem with the analogy for a planing craft is that they are of very low aspect ratio, and operate on a free surface of water which is deformed by the interaction effects.

Savitsky and Dingee [1954] note that the interaction effects must be due to the dynamic component of the planing force, since at rest the static component has no interaction effect.

The result of hull interactions on the length to beam ratio, , at C_V=7.5 indicates a wave rise variation starting from a value of dl = 0.3 at about 3 beams spacing and increasing to 0.6 at zero beam spacing.

Consideration of the lift coefficient as the hull spacings are varied show that the lift is always greater for two hulls planing side by side, up to a limit of about 4 beams spacing, when the interaction effects become insignificant. The increased lift of two hulls interacting is due to the influence on lift of the change in upwash velocity and also the influence of lift which results from the wetted length change, as mentioned above. By referring back to the analogy of aerofoils flying in close proximity, it is seen that the lift increase is not nearly as large for the planing hulls. However, this was somewhat expected due to the physically different flow regimes that are present, including the fact that the primary flow is only on the lower surface of the hulls.

It can be seen from these tests that if catamaran planing hulls are considered for a particular design then beneficial results can be achieved. Furthermore, by studying the effect of the air mass flowing under the deck joining the two hulls, it may be possible to optimise the design still further; this will be considered later under the heading of the ground effect.

Work by Clement and Pope [1961] presents graphs by means of which the resistance and trim of catamaran planing hulls could be determined. This work was limited in its usefulness to others since only a small selection of prediction data was published. The work was centred on empirical model tests in a similar manner to that used later by Savitsky [1964], and furthermore did not include any discussion of the hull interaction effects. A principal and important conclusion which was made by Clement was that the reduced resistances achieved by catamarans is due to the increased running trim caused by the higher length to beam ratios, rather than by the claimed aerodynamic benefits. 

It will be seen in Chapter 7, Section 7.4.1 and Figure 7.11, that current planing craft prediction methods can be applied to determine the individual hull resistances for catamarans since the experiments provide new data for the required higher length to beam ratios.

Next Chapter; The Ground Effect...

There is plenty more of this review, to be published at a later date. Contact JB if you have any particular questions.

REFERENCES

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