Multi-hulls crafts

Catamaran:The catamaran or twin-hull vessel consists

of two hull arranged in the same direction

separated from each other and attached

by a common deck.

For commercial purpose semi-planing type

catamarans are predominant.

The component hulls are of the planing type,

featuring V-type sections and a cut-off

transom stern.

The division of displacement andwaterplane area between tworelatively slender hulls results in alarge deck area, good stabilityqualities and consequently a smallrate and angle of roll.

It is found that seakeeping qualities interms of angle and rate of pitch arepoor compared to a conventionalhull.

Active control of pitching motion bymeans of fins may eliminate thisproblem.

Design of catamaran hulls:The normal arrangement of catamaran

hulls relative to each other is parallel

and abreast.

Very frequently, hull form is such that

the outboard surfaces of the hulls are

vertical planes.

The curved surfaces are then inboard and opposite each other.

The principal advantage of this arrangement is to further reduce the wave-making component of resistance on both outboard sides of the assemblage.

A further extension of this reasoning is a staggered arrangement of the hulls in their longitudinal placement, allowing one hull to overlap or lead the other by a distance that is preconceived to produce a wave cancellation on the lagging hull.

In this arrangement, the apparent design fault is the lack of transverse symmetrical support and stability caused by the resulting imbalance of heeling moments.

Performance of catamaran vessels:The resistance of a catamaran is mainly affected by:

1- The wetted surface ratio WS/V2/3

2- The slenderness ratio L/V1/3

3- The hull spacing S/L

The wetted surface ratio is relatively high

compared with planing mono-hulls of

the same displacement.

Consequently, catamarans show poor

performance at low speeds (Fn<0.35)

where skin friction is predominant.

At high speeds (in hump region, Fn≈0.5)

the low trim angles associated with the slender demihulls of the catamaran lead to a favorable performance.

No pronounced hump being present in the resistance curve.

At the planing speeds (Fn around 1.0) the

equivalent mono-hull (of equal displacement)

will show an advantages

As the hydrodynamic performance decreases

with decreasing the aspect ratio (the ratio of

the wetted breadth of the demihull to its length).

Concerning the performance at high plqaning speeds, the catamaran had less resistance at speeds in excess of Fnv=4.6 than a mono-hull.

At Fnv=6.0, the catamaran had some 30% less resistance, this reduction increasing to about 45% at Fnv=7.0

This advantage is due to the fact that at such high speeds the conventional boat is operating at a very small trim angle and high resistance

While the catamaran operates at a higher trim angle nearer to that for minimum resistance.

The hull spacing ratio is associated with interference effects between the component hulls.

These effects consist of:

1- Wave interference effects

2- Body interference effects

Body interference effects:The body interference effects are caused by the change of flow around one demi-hull due to the presence of the other demi-hull.

The main component of the changed velocity field associated with body interference effects results from the induced flow of one demi-hull at the location of the other one.

Wave interference effects:Wave interference effects are due to the superposition of the two wave systems, each associated with a component hull in isolation.

The wave interference may influence the resistance to a large extent.

From a wave pattern analysis, it may be

concluded that the beneficial wave

interference is achieved by the

cancellation of part of the divergent

wave systems of each demi-hull.

Adverse wave interference arises on

interaction of the transverse wave system.

The wave interference factor Ifwp=(Rwp-2Rwpd)/2Rwpd

Where Rwp is the wave pattern resistance of the catamaran and Rwpd is the wave pattern resistance of one demi-hull.

Theoretical and experimental data for

symmetrical demi-hull indicates that

wave interference becomes significant

at Fn values of 0.2.

Maximum beneficial effects occur at

Fn≈0.32 and S/L≈0.3, whereas adverse

effects are most pronounced around Fn=0.4.

Small Waterplane Area Twin Hull ships “SWATH”:The SWATH concept is a type of catamaran that features two fully submerged hulls, each connected to an above water, box-like deck structure by one or more relatively thin struts.

Usually control surfaces are employed

to reduce pitching motions of the vessel.

The benefits of the SWATH concept are

derived from their low motion responses

and sustained speed capability in waves.

A SWATH ship has about a 75% larger wetted surface area compared to a single screw mono-hull of equal displacement.

Consequently SWATH ships incur a substantial frictional resistance penalty relative to conventional surface ships.

This has to be compensated for (at least partly) by a reduced wave-making resistance, which can be obtained on account of the small waterplane area and the slenderness of the struts.

However, carful attention has tobe directed to the required depthof submergence of the hulls andto possible unfavorableinteractions between the wavesystems produced by the struts.

At speeds above the primary resistance hump, the spray resistance of struts becomes significant.

SWATH ships, because of their very small TPC, are very sensitive to changes in load and its distribution.

Triamaran:Triamaran is a symmetrical arrangement for staggered hulls.

It would appear then that to take the most advantage of wave cancellation, the triamaran is more adaptable than the catamaran.

However, the triamaran has considerably more wetted surface than either the catamaran or a single hull of comparable total displacement.

Consequently any advantage in powering do not excite except at very high speeds.

Multi-hull may be used for the following vehicles:

1- Oceanographic research.

2- Ferries.

3- Oil drilling plate forms.

4- Submarine rescue ships.

5- Mine counter measure vessels.

6- sailing catamarans.

7- Off-shore rigs.

8- Diving support vessels.