Naval Architect JB Marine Consultancy Naval Architect  

Tel / Fax +44 (0)1481 253053    Mobile Tel: 07781 100621    Email: john@jbmarine.biz

Reviews:          Lets Start with a Literature Review !

Planing Craft - Rough Water     Planing Catamarans     Planing Craft Ground Effect   Planing Craft Theoretical Models

This chapter details previous publications and information pertaining to high speed planing craft design and performance prediction. The review is divided into the following group headings; monohull design for smooth water, monohull design for rough water, catamarans and the ground effect.

Savitsky [1964] summarises the elemental hydrodynamic characteristics of prismatic planing surfaces and empirical planing equations are given which describe the lift, drag, wetted area, centre of pressure and porpoising stability limits of planing surfaces as a function of speed, trim, deadrise and loading. These empirical equations are combined to formulate a computational procedure, in the form of a table. This is used to sum the moments of the forces on the hull, and hence to determine an equilibrium trim. The calculations are used to predict the powering requirement, running trim, draft and porpoising stability of a prismatic planing hull.

This tabulated procedure is commonly used by naval architects today, and can be implemented using a computer program; the calculations requiring graphical interpolation can be performed using iterative methods. However, the principal problem of this tabulated procedure is the empirical nature of the calculations. Modern craft do not always fit into the spectrum encompassed by empirical calculations, and in addition, catamarans and other 'novel' hull forms will not fall into this category.

A similar approach was adopted by Clement [1963], [1964A] who conducted model tests on the Series 62 planing hulls. These tests were more limited, in terms of performance prediction over a range of hull forms, however Clement was more concerned with the use of stepped hulls that required a slightly different design approach than that used by Savitsky. Clement first introduced this idea of stepped hulls in 1963 and which he then substantially reinforced in later work [Clement 1964A]. In the latter paper comparisons are made between the planing surface of a boat and a hydrofoil. After a series of model tests it was found that a planing hull could be made more efficient by incorporating a hydrofoil-shaped step in the midsection of the hull, by increasing the chine width throughout the hull length, or by increasing the chine width in the forebody only, with a step at the midsection. The effect of the step is to reduce the wetted planing area by ventilating the afterbody of the hull, thus reducing the frictional resistance. Additionally, the aspect ratio of the planing surface is substantially altered, with consequential effects on the craft performance. The use of a step means that the longitudinal centre of gravity can be moved well forward, which is a particularly useful advantage since large engines and fuel tanks do not have to be crammed into the aft section of the hull.

A series of model tests of a stepped planing boat with an adjustable stern stabiliser were performed [Clement 1967A]. The 'Plum' stabiliser was designed to remain horizontal as the craft rolls, thus preventing large heeling moments occurring when one side of the stabiliser dragged in the water during a turn. The models used in the tests had a small step in the midsection which caused the after body of the craft to remain un-wetted at high speeds. The craft weight was supported 90% on the forebody and 10% on the stabiliser. The model tests were to determine the optimum use of the stabiliser, and showed that by adjusting the height and trim of the stabiliser, the forebody could be made to operate at the minimum resistance condition. Furthermore, tests were continued to investigate the use of ventilation to the step area, and also in the use of spray strips. The use of ventilating air, via air ducts from the face of the step, reduced the resistance at the 'hump speed' (transition to fully planing), and it was also shown that adequate ventilation of the step is important for minimising resistance.

The effect of length to beam ratio on the performance of a stepped planing boat with an adjustable stern stabiliser was investigated with two models [Clement 1967B]. The results showed that decreasing the length to beam ratio from 6.9 to 5.0 caused a reduced resistance at high speeds. This author suggests that more tests were necessary, and perhaps the use of several steps would alleviate the problem of an optimum length to beam ratio for a stepped hull.

These stepped planing hulls with stern stabilisers are known as dynaplanes and a report has been produced detailing the design procedure of cambered (warped) dynaplanes for small motorboats [Clement 1969]. This report is in the form of a list of calculations and procedures to follow for the design of a dynaplane.

Clement [2003] has continued the development of the dynaplane configuration and has established a method for the design of a planing boat using an adjustable stern-stabiliser. Model tests indicated that there is the potential for a 50% reduction in resistance between optimised stepped and un-stepped planing craft. This 'airplane' configuration (so named because the LCG is positioned close to the Centre of pressure of the lifting surface, with the aft-step acting as a tailplane) runs with the aft-body entirely clear of the water at planing speeds; the aft-body is designed with a different longitudinal trim specifically to achieve this - actual design is important to assist in clearing the hump-speed. It is further noted that best results for the airplane configuration are achieved when the deadrise angle does not exceed 15 degrees, incorporating some sweepback in the step configuration. Clement also incorporates a vent-pipe for aerating the step. 

Figure 2.02 (After Clement [2003]) Configuration of a planing boat having minimum drag.

Blount and his colleagues have been interested in determining the dynamic stability of planing craft. In an early paper [Blount and Hankley 1976] detailed experiments and analysis for a variety of high-performance, hard chine craft are reported. Initially this work was concerned with the need for full scale correlation data to determine the applicability of prediction techniques. Particularly, this applied to propeller cavitation effects which prevented accurate speed-power correlation between model and ship (mainly at high speeds). Later, attention was directed to the dynamic instability of small high speed craft [Cohen and Blount 1986]; here details are given concerning the elemental losses of stability of high speed craft underway. Instability is reported in both transverse and longitudinal directions, with motions ranging from a rapid loss in running trim, progressive heeling, or a sudden roll-yaw motion. A long term research plan is offered for the assessment of dynamic stability of high speed craft, so that a set of guidelines can be implemented when designing high speed planing craft.

The latest developments of the aforementioned research plan and a summary of previous work done, including the following points, is given by Blount and Codega [1992]:

· Fundamentally, stability depends solely on the location of the crafts centre of gravity and on the forces and moments resulting from the orientation of the boat. At low speeds, these forces and moments are essentially the same as for the hydrostatic case, but at higher speeds these forces and moments differ significantly. The instabilities which occur most often are known as chine walking, bow steering, bow diving, chine riding and porpoising, and are all speed dependant.

· Dynamic instabilities can be categorised as non-oscillatory and oscillatory. Non-oscillatory instabilities usually occur at speeds lower than those associated with oscillatory instabilities and are generally found on heavily laden craft travelling at moderately high speeds. These instabilities are typified by a loss in running trim, progressive heeling, bow steering, or a combination of rotations and may result in a new stable orientation. The onset of these instabilities may be rapid and without warning. Oscillatory instabilities include roll oscillations (chine walking) and pitch and heave oscillations (porpoising). In both cases the amplitude of oscillations is related to boat speed, and the oscillations occur without any excitation from the environment or the operator. Whilst design guidelines exist for predicting and avoiding porpoising [Savitsky 1964], there are no reliable guidelines currently available for predicting the conditions of chine walking.

It is possible to analogise the buttock lines of a high speed craft to an aerofoil [Wellicome and Jahangeer 1978], [Cohen and Blount 1986], [Bate 1991], [Blount and Codega 1992], and from this analogy it is possible to develop a theory for prediction of the pressures at given points on the hull. This has been performed to different, limited extents by different people, for example an empirical calculation for this analogy is given by Cohen and Blount [1986]. This method suffers from a very restricted application, with the problem of equating each buttock of the hull to an aerofoil section and then applying an empirical equation to each section. The extension of the aerofoil analogy suggests that highly curved underwater buttocks are more prone to developing low pressure areas with the accompanying destabilising moments than are less curved buttocks. The most accurate method for predicting powering, using fluid dynamic theory, should involve a modular model encompassing the velocity distributions over the hull. The theory is substantially more complicated for a high speed craft due to the high energy losses associated with the aerodynamic and hydrodynamic fluid separation from the craft, which breaks down to very turbulent flow.

One of the prime results of Cohen and Blount [1986] is a preliminary design guideline for the prediction of transverse stability, in terms of the planing area and displacement. This prediction technique is suitable for application with Savitsky's prediction of porpoising stability, but is limited in its application. Small changes in the hullform (that would not be registered by the prediction technique) can have very significant effects on the dynamic stability, such as the variation of spray rails.

Whilst the dynamic stability should be seen as an important factor, it is generally assumed that the primary design objective is for minimum resistance, whilst simultaneously incorporating dynamic stability. Although as the speed increases, the relative importance of the dynamic stability increases. Therefore, on the subject of minimising the resistance of a given hull, experiments have been carried out to investigate the effects of longitudinal bottom spray strips on planing boat performance [Clement 1964B]. It was found that such strips extending aft from the bow, about 70 percent of the hull length, decreased the resistance somewhat (2.5 per cent) at high speed, but increased the resistance at low speed. The performance was noticeably improved by sharpening the edges of the spray strips. The position of the spray strips is said to be most important in the spray area [Savitsky 1964], [Latorre 1983] and an optimum configuration would incorporate spray strips from the bow which merge into the chines just aft of the designed operating stagnation line (which introduces further complications such as varying displacements and powering requirements).

Savitsky and Brown [1976] conducted many investigations, including studies of the use of trim tabs, the effect of hull warp, re-entrant vee-transom hulls, and the performance of a hull in the preplaning range. Savitsky and Brown show that trim tabs can help optimise performance by minimising drag at a given speed and loading conditions. Equations are given for the inclusion of trim tabs in the design process, and it is shown that these calculations can be included in Savitsky's tabulated procedure. Conclusions show that it is more efficient to generate flap lift by means of flap area rather than flap deflection. The tests conducted into warped hull forms were somewhat brief, however, conclusions given by Savitsky and Brown suggest that the drag of a warped hull is markedly increased compared to a hull with parallel buttocks, although there is only a small increase in lift. Furthermore, it is stated that problems arise since power boats are forced to operate in a low aspect-ratio configuration (which is less efficient). This author will show, in Chapter 4, Section 4.2, that warped hull forms can be beneficial when designed correctly.

Savitsky and Brown [1976] furthered the tests performed by Clement (on re-entrant transoms) and concluded that while the re-entrant configuration can reach higher aspect ratios, there is a non-compensatory loss in lift and drag due to the re-entrant vee. Thus it was concluded that the re-entrant transom is not beneficial, although these tests were performed over a limited range of low deadrise hullforms.

A regression analysis of the smooth water data of seven transom-stern hull series was conducted [Mercier and Savitsky 1973]. An analytical procedure was developed for predicting the resistance of transom hulls in the preplaning range, specifically for volume Froude numbers less than 2.0. This analytical procedure is in agreement with similar model tests, however the range of applicability is limited to craft of similar form to the series 62 hull.

Work has been presented on the development of a small craft power prediction method which allows the designer to select, with improved confidence, hull proportions, engine power, reduction gears and propellers [Blount and Fox 1976]. Firstly, this paper discusses resistance prediction for the hull. It is acknowledged that Savitsky's tabulated method is the standard approach to the problem, however since Savitsky's method is for prismatic craft, a correction factor is required for non-prismatic (warped) hulls.

The establishment of the effective beam as the maximum chine beam, and the effective deadrise as the deadrise at mid-chine length, allows the development of an 'engineering factor' for modifying the Savitsky prediction method. It is commented that hull warp can be used as a designers tool to control dynamic trim in a similar way that a wedge can be used, and developed a 'modification factor' (M) for non-prismatic hulls. This resistance multiplying factor enables more accurate resistance prediction in the pre-planing range, for non-prismatic planing hulls, and is most suited to heavier hulls such as to be expected for normal commercial or military loading:

Modifying Factor:        [2.01]

The limits of applicability of this equation are:  and

Also:                                                        [2.02]

A prediction method is detailed for the resistance of appendages, initially based on a study by Hadler [1966]. The calculations are laborious but are not complex and are important when considering the conditions for which the final propeller is selected. For preliminary design studies, these calculations are unnecessary and Blount and Fox [1976] offer a simpler approximation for an appendage drag factor, given by the equations below:

Appendage Drag Factor                                               [2.03]

and:                                                                                [2.04]

A propeller selection technique is offered for cavitating Gawn-Burrill propellers, including a detailed set of design charts and efficiency characteristics.

A discussion of hull proportions for smooth water minimum powering states that displacement, chine beam, deadrise, and longitudinal centre of gravity are all significant factors affecting speed and power. An iterative series of calculations were made for a range of these significant hull factors for a range of speeds and displacements. This optimisation process showed that a locus of all the minimum power requirements can be plotted to offer the designer a design aiming point, depending on the hull form, this is shown by Figure 2.03. This type of study is investigated in greater detail throughout Chapter 6.

Fig 2.03: Design chart for minimum P_E. (After [Blount and Fox 1976])

Blount and Fox [1976] concluded with a summary of problems causing low trial speeds (under predicted resistance) of constructed hulls:

• Stock propellers with blunt or thick leading edges.

• Overweight hull construction relative to preliminary accepted weight estimates. 

• Incorrect allowance for drag during performance predictions, partly due to craft becoming rapidly covered with a heavy coat of marine growth (fouling allowance).

The effect of wedges on the performance characteristics of two planing hulls was investigated by Millward [1976]; the intention was to determine the optimum wedge configuration and range of effectiveness of a wedge.

Results of tests with a series of wedges on two models of the David Taylor Model Basin (DTMB) series 62 planing hulls in the water channel confirmed that a wedge or trim tab does increase the dynamic lift on the hull. However, since the induced resistance is also increased, the wedge only reduces the total resistance if the change in effective displacement leads to a reduction in the other components of resistance.

In the cases where the total resistance was reduced, it was seen to be an advantage to use a wedge as a method of reducing resistance and also for trim control, as opposed to longitudinal movement of weight. It was found that reductions in resistance of up to 25 per cent were obtainable, depending on the LCG position, in the range 1<F<4. The optimum wedge length was found to be in the range 0.05 to 0.10 of the projected chine length, tending to the lower value for lighter displacements and forward LCG positions.

The optimum wedge angle was also found to be a function of displacement, LCG position and speed, reaching a maximum of about 10 degrees at the heaviest displacement. It was concluded that for overall performance it was necessary to use an adjustable wedge for trim control, although the practicality of this is questionable (a distinction is made between these wedges and trim tabs, which can be adjustable).

In recent years the increases in speed of planing craft have led to a situation where the aerodynamics of the hull can be expected to have a significant influence on the overall resistance, and hence performance, of the craft. Since the designer is interested in minimising the resistance of the craft, it is necessary to develop a performance prediction model that includes calculations for the aerodynamics of the hull, as discussed in Bate [1991]. Experimental tests on an offshore racing monohull showed that the aerodynamic lift at sixty knots comprised 27 per cent of the total lift [Wikeby 1990]. Furthermore, this reference offers graphs for aerodynamic lift and drag coefficients against angles of attack for the aforementioned monohull, and for a tunnel boat hull of similar dimensions. These aerodynamic lift and drag coefficients can be applied to other planing monohulls of similar form, hence allowing the inclusion of these characteristics during the computation of high speed planing craft performance [Bate 1991]. The subject of aerodynamics of tunnel boat hulls is dealt with in more detail later in Sections 2.5, 2.6 and 2.7 .

A subject of further interest and importance is the spray area of a planing hull. Whilst this subject was well documented [Savitsky 1964], [Clement 1929] and [Clement 1964B], further detailed experiments were performed [Latorre 1983]. These substantial experiments produced the results of prismatic planing model spray and resistance components, including a detailed analysis of the spray thickness. The features of the whisker spray and spray blister were also discussed, and it was seen that in addition to causing the spray blister formation, the intersection of the chine with the stagnation or spray root line also formed a 'bow wave', similar to a gravity wave from a typical displacement hull. This wave effect may not be relevant at very high speeds. The whisker spray was identified as the droplets formed from the spray sheet occurring forward of the main spray blister, the thickness of which was found to decrease in thickness with increased speed. The spray blister occurred in the form of a sheet of water; the thickness of the spray blister was also measured experimentally and calculated (both methods produced agreeable results). The spray components are shown in Figure 2.04.

Fig 2.04: Spray elements. (After [Latorre 1983])

Besides performing extensive tests on the spray formation on prismatic planing models, Latorre [1983] also included some experiments to determine the types of flow on the hulls, i.e. whether the flow was laminar or turbulent, and to determine the position of the transition point. These tests resulted from tests and calculations of the frictional drag component which are dependant on the type of flow, and which were in excellent agreement with Savitsky's empirical calculations. These latter experiments were performed using acetanilid film patterns on the planing model bottom, which being of a greyish-white colour showed the laminar/turbulent transition line, as the turbulent flow removed the acetanilid film to show the black hull bottom underneath.

The results of these experiments showed that extensive laminar flow was present on the planing hull bottom, in  the region of 50 per cent of the total planing area. As Figure 2.05 shows, the transition line tended to start well forward on the centreline and to traverse diagonally out and aft to the chine, near the transom.

Fig 2.05: Laminar and turbulent flow on a planing hull. (After [Latorre 1983])

Next Chapter; The Theoretical Model...

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

REFERENCES

<<<<<<  Back to JB Home Page  >>>>>>