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S.P.E.E.D. Hull Design

The Theory Behind The Design

Introduction

All boats, large or small, expensive or cheap, are a compromise.  The designer is given the task of knitting together the various desirable attributes forced upon him by the client specification in such a way as to produce the most efficient product.  Characteristics such as maximum draught, weight, load capacity, horsepower and overall vessel dimensions are all thrown into the mix and the end result is a best fit of all these,  as close to the ‘ideal’ as possible.

Over the last two decades, power catamarans have become increasingly popular, firstly in the commercial sector, where their inherent stability and added deck-space over a comparable length monohull were recognised at an early stage, and now ever more in the leisure market for much the same reasons.  During this time there have been various designs and designers each with differing ideas as to how to produce the best vessel for a certain market.

As with monohulls, there are two basic types of power catamaran hull form, planing and displacement.  In the case of monohulls, these two forms generally exhibit very different performance characteristics in terms of speed, whereas in catamarans they can be very similar.

In the following discussion I have attempted to compare and contrast the basic design precepts of both planing and displacement hulls with respect to both monohulls and multihulls.  As ever, there are many more factors acting on the designs than I have gone into here, but there is hopefully enough detail for the major differences in hull characteristics to be understood.  In the process, I hope to explain the reasons behind the design philosophy of BWSeaCat Ltd, and why our vessels are fundamentally different from the majority of our competitors.

Iain Worrallo MD, BWSeaCat Ltd.

  • Planing Theory

    The basic principle behind a planing hull is that of the generation of sufficient hydrodynamic lift to support the vessel weight.  This lift is created when the hull bottom is positioned with the correct angle of attack relative to the oncoming water for it to be pushed up and onto the surface. Planing hulls generally comprise of flat or concave panels in a basic ‘V’ configuration when viewed from the bow, with no rocker or curve in the keel from bow to stern.  Some designs incorporate ‘planing strakes’ along the hull whilst some have steps and other tweaks, all designed to generate more lift and allow the vessel to plane more easily.  Also the lighter a planing vessel can be built the easier it will rise onto the plane, assuming the hull or hulls have a bottom with sufficient surface area to generate the required lift when powered through the water.

    A correctly designed planing vessel is thus able to overcome the majority of the drag experienced by the hull in terms of wetted surface area (the surface area of the vessel’s hull immersed at rest), by lifting out of the water.  It is also able to dispense with much of the wave making resistance (which we will address later on), as the vessel is now essentially riding on the surface of the water, rather than having to slice through it.  This allows rapid acceleration and high top speeds.  The compromise here is that relatively large power is required in order for the vessel to achieve a planing attitude, especially in conditions of high loading which are common in commercial applications.  In simple terms a proportion of the total engine power is being used to create the lift rather than to propel the vessel forward.  Also, with the vessel essentially aquaplaning on the surface of the water, the ride can be quite harsh in rough conditions sometimes to the point of necessitating a reduction in service speed to a non planing mode, at which point this particular hull form is extremely inefficient.  There have been various advances in the design of the planing hull, most notably with the development of the ‘Deep-V’ hull form in the late fifties making for a softer ride and greater directional stability, but this came at the cost of greater wetted area, higher drag and therefore the requirement for even more power than its flatter bottomed predecessors.

    There is very little difference in performance between the majority of planing catamarans and planing monohulls in terms of speed obtained for a given power input.  In fact it could be argued that the catamaran will be at a slight disadvantage in that it is ordinarily heavier than its monohull counterpart due to having a physically larger total surface area of hull.  This, in conjunction with the fact that the catamaran will normally have less hull bottom surface area with which to generate the required lift, can have dire consequences for the vessel as a whole, if it is not properly addressed.  One advantage the planing catamaran has here though is that because it has two fairly thin hulls, it will offer a more comfortable ride in rough weather than a planing monohull.
    Ideally any planing hull should have bottom surface loadings of no more than about 50 lbs per square foot if it is to plane easily.  Clearly if the total vessel weight is more than a monohull and the bottom surface area is less, it is more likely that this value will be exceeded, resulting in a vessel which may struggle to plane at all.

    It is true that with advances in materials a catamaran or indeed any hull shape can be made extremely light if need be without unduly compromising structural integrity.  However one of the catamaran’s main advantages can actually cause a problem here. The fact is that deck area is generally greater than an equivalent length monohull, resulting in a tendency to cover it with bigger cabins, filled with more ‘home comforts’, and thus more weight.  As with any vessel, it is important at the design stage to have a clear idea of component weights and their position on board in order to arrive at a suitable hull choice for a particular application.

    Perhaps the best example of a properly designed planing power catamaran is the Arrowcat designed by Roger Hill.  Packed with creature comforts and capable of 40Kn with a pair of 175Hp outboards, she is certainly an attractive proposition and clearly shows that with proper consideration for all factors, the planing hull can be used to great effect.

    However, just as there are marked similarities between planing catamarans and monohulls, so there are marked differences between their displacement cousins.

  • Displacement Theory

    The hull speed of a displacement craft in knots can be seen to follow a relationship known as Froude’s Law of Comparison which is proven to be a good generalisation for almost every hull of the classic displacement shape.

    Froude was a scientist who worked for the admiralty in the late 1800’s and arrived at his famous hull speed equation by observing the wave trains produced by two different model hulls of varying lengths.  He noticed that similar wave patterns were produced when the models were run at speeds proportional to 1.34 times the square root of their waterline length.  It should be remembered that this work was done at a time when hull forms were fairly basic and coal fired steam engines were being used and does not take into account the extremes of modern hull design, nor the advances made in engine power to weight ratios.

    Most people’s experience of a displacement monohull is likely to be a trawler or similar fishing boat or perhaps a cross channel ferry.  Whilst these examples are obviously quite different in size, their hulls conform fairly well to the above law.

    For a fishing boat of say 40 feet in length, the hull speed as given by the equation is 1.34 times the square root of 40, or 8.47Kn.  In the case of the cross channel ferry with a typical length of 150 metres or about 500 feet, the maximum vessel speed will be 1.34 times the square root of 500, or just a shade under 30Kn.  In practice the operator will tend to run at a cruising or service speed which will be slightly less than the maximum in the interests of fuel efficiency.

    It can be seen from the above that a longer vessel can in theory be made to go faster than a shorter one.  What is actually happening is that the ratio of hull length to beam is very much greater on the ferry than the trawler and this is the key.

    What is attractive about the displacement hull form from a designer’s point of view is the motion experienced aboard this type of vessel.  The ride is generally very soft and smooth and has given us expressions such as ‘sea kindly’ and ‘good sea boat’.  What these expressions are describing is a motion that is predictable, gentle and confidence-inspiring, all of which are desirable attributes to have in a boat.

    The down-side is that as we have seen, unless the hull is very long, speeds are likely to be in the 8-12Kn range, which by modern standards is too slow for most clients.

  • Common Factors

    The maximum speed of any vessel is described as the point at which the energy being put into the water by the engines equals the energy lost to the environment by friction or ‘Drag’.  The only way that speed can be increased at this point is to add more power, or to lose less power through drag, in other words to make the vessel more efficient at moving the water out of its way.

    Drag is basically a measure of the hulls resistance to being driven through the water and is essentially comprised of two elements, Wave Making Resistance and Wetted Area. Planing hulls as we have seen are able to reduce their wetted area and wave making resistance by rising onto the surface of the water. Displacement hulls however are beholden to both of these forces, although wetted surface area is not a large factor at anything more than idling speeds.

    Of far greater importance for a displacement craft at speeds from 6 knots to about 30 knots is the wave making resistance.  Unlike a planing hull, the displacement hull is essentially stuck in the water rather than skimming on top of the surface and so hull shape and the way in which the hull interacts with its wake is of great interest.

    As a displacement vessel passes through the water, it must push the water out of the way.  In doing this, energy is lost due to the creation of waves.  The quicker the water is moved the bigger the waves will be and the more energy is used in creating them.  Larger waves are created by greater speeds and also by blunt bow shapes.  A more ‘bluff’ bow needs to move the water more rapidly than a finer one, and although ultimately the same amount of water is moved, because the fine bow moves the water more gradually, the resistance is less.

    In general a hull will experience high water pressure at the bow, where the water is being forced away from the hull, creating a wave, low pressure at or around amidships where the surface level is lower due to some of the water having been deflected away by the bow, and high pressure at the stern where a typical displacement hull is narrowing and so the water is pulled back in again thus creating another wave.  The hull when viewed from the side essentially appears to be riding on a bow and stern wave, with a trough in the middle.  At this point the vessel is said to be moving at its ‘hull speed’.

    Wave making resistance is reliant on two factors, the length to beam ratio (the primary factor) and the beam to draught ratio.  A vessel which is very long will be faster than one which is very short if both have the same beam at the waterline.  Fundamentally, long thin things are easier to push through the water than short fat ones.  Those of you who have experience of kayaking will know that the longer general purpose boats are much faster for the same number of paddle strokes than the short white-water designs.

    As we have seen, a displacement vessel when travelling at its hull speed can be seen to be moving along in effect trapped between two waves.  The longer a ship is, the longer the distance between these waves, known in Physics as the ‘Wavelength’. Since the speed of waves in deep water is proportional to the square root of their wavelength, and the wavelength of a vessels wake is based on its waterline length, in a simplistic sense, the longer the vessel the faster it is able to travel.

    It has also been proven that deepening the draught of a ship reduces resistance. These attributes were perhaps best proven by the Japanese with their Naval ships towards the end of the Second World War, which became very long and very deep in order to gain the edge in terms of performance.

    It is however not just the overall length and maximum draught of a particular hull which is important, but also the rate at which these factors change along the vessels length, or to look at it another way, the rate of change of the underwater cross sectional area of the hull.

  • The Prismatic Coefficient

    A measure of the rate of change of cross sectional area is the ‘Prismatic Coefficient’.  This essentially compares the volume of a hull to the volume of a prism with the same length as the waterline length, the same width as the maximum hull beam at the waterline and the same depth as the maximum draught of the hull.  It is perhaps easier to imagine the boat sat in a long ‘box’ of water and then imagining how much of the box the hull actually occupies.

    Vessels with high prismatic coefficients are occupying more of the box and it is these designs which have the higher speeds.  They tend to be hulls which exhibit a fairly slow change in cross sectional area over their length, or hulls which carry their width further fore and aft.  Obviously to take this to the extreme, the best hull shape would be a box with flat ends, but this is clearly not true.  Whilst theoretically the wave making resistance along its length would be zero because the cross sectional area is not changing, the rate of change at the ‘bow’ is infinite and so unfortunately is the resistance.  In conclusion therefore it could be said that if a relatively fine bow were grafted onto a long thin box, we should in theory have a very fast hull without the need to plane.  Unfortunately, this hull would be laterally very unstable due to its slim profile and have very little internal space making it impractical for most applications.

  • How Thin is Thin?

    The answer is that the hull must be kept as slender as possible, although as has been mentioned, design requirements will dictate the final dimensions.

    It is still possible for a reasonably slender monohull to be a practical vessel, as has been demonstrated by the renowned performance multihull designer Nigel Irens with his latest few projects. These designs are reminiscent of the old ‘gentlemen’s launches’ and are exploiting good length to beam ratios and fine entry bows to achieve very good speeds from modest power.  There has been some compromise though, in that the beam of the hull has had to be made sufficient to give the vessel a usable internal volume, and so although in monohull terms the hulls are ‘skinny’, as far as multihulls go they are not, and can thus never achieve the efficiency we are looking for.

    For a displacement catamaran to be effective, hull length to beam ratios of at least 10:1 should be aimed for and if possible even higher.  From this it can be seen that it is not really practical to build displacement catamarans at less than 8m in length as this will translate to an 800mm beam for each hull at the waterline.  Much less than this and certainly inboard propulsion is ruled out as the hull will not be wide enough to accommodate the engines.  Having said this, our own 6m design has been proven to work well with inboards and outboards, but this is down to each hull having the same beam as the 8m and 10m design.  This is not the last word in efficiency as the hull length to beam ratio is only about 7:1, but we can still achieve a respectable 24Kn from twin 60Hp outboards and 12Kn from twin 25Hp diesel inboards, which whilst not setting any records is certainly no worse than a 6m monohull of similar weight and ability.

    The advantage that the catamaran has over the monohull here is that the hulls can be made much more slender because they are spanned by the Bridge deck which ties them together making the whole boat laterally extremely stable.  In theory this opens up a whole new avenue of potential performance to the designer in that length to beam ratios of 15:1 and more are easily achievable, albeit on hulls of about 15m or greater in length.  The problem of reduced internal space in the slender monohull scenario is overcome by the massive available on-deck space that the catamaran affords, regardless of hull shape.

    It would appear then that a displacement power catamaran with slender hulls, fine entry and a high prismatic coefficient, has the potential to be the ideal platform on which to design a range of fuel efficient, stable and safe working vessels.

  • Design Requirements

    I am not and never will be a Naval Architect.  Mathematics was never my strong point and the thought of performing endless theoretical calculations fills me with dread!  I was lucky enough early on in my career to be given the opportunity to carry forward the designs of the late Keith Bennett.  Keith pioneered the design of small displacement outboard powered catamarans in the UK from his tiny boat shed in the Scilly Isles producing some 200 vessels throughout his 40 year career.

    I met Keith in 2000 having restored one of his early designs and taken her by sea from Padstow in North Cornwall to the Scillies.  I had already become interested in boat building during this initial project and through further talks with Keith about hull design and the potential of the displacement form of which he was a firm believer; it was clear what I would be doing with my life.
    In designing our new range of 8, 10 and 11m Catamarans, I have taken great care to remain as true to Keith’s original philosophy as possible, whilst attempting to incorporate as many of the efficient features known to modern Naval Architecture, in order to produce what I believe is the best compromise for our particular requirements.

    The majority of our past, present and hopefully future customers have come from the commercial sector where key issues are maximising free deck space and minimising running costs, and the new hull was designed with these two factors primarily in mind.

    Clearly the catamaran is at an obvious advantage from the start with respect to the first of these requirements and it is really only left for us to ensure that all hatches in the deck are flush and that the general arrangement is as uncluttered as possible to ensure a safe working environment.

    The second issue of running costs is the main reason we have chosen the displacement catamaran as the basis for our endeavours.  As we have seen, there is a clear theoretical advantage to this approach, mainly manifested in the need for far less horsepower.  This translates to lower initial engine purchase costs, lower ongoing maintenance costs and lower fuel bills.  Planing hulls can be efficient also, but they really only have two conditions, planing or not planing.

    A planing hull when planing is very efficient as the throttles can be eased back to reduce fuel throughput.  However, as mentioned previously, in rough weather, riding on the surface of the water and over every wave can become uncomfortable.  This can mean that speeds must be reduced to such a degree that planing is no longer possible. When this happens the hull is not acting as designed and will generally be very inefficient.  Another factor is that commercial boats generally have to carry reasonably heavy loads made up of either their catch or their clients.  This can result in the vessel struggling to plane at all, even at wide open throttle.

    Clearly a displacement design which could slice though the waves rather than riding over them and which was efficient right through the speed range by virtue of not having to plane was the sensible choice.

    As a rule, there are many more planing catamaran designs being produced in the world today than displacement ones.  The reason for this is not clear and seems to leave a gaping hole in the market especially in the 6 to 12 metre bracket.  This has made the job of producing a brand new displacement design at this size quite difficult in that there are only a limited number of production vessels to compare in order to see which features work best.

  • Design Specifics, the S.P.E.E.D. hull

    I have given this design the acronym S.P.E.E.D., standing for Small Power Extended Efficiency Displacement.

    The basic form is one of a fine entry bow and a flat box like stern section with 850mm maximum demi-hull beam, giving a length to beam ratio of 12.35:1 for our 11m hull.  This is as close to the ‘ideal’ displacement hull shape as possible for our applications, whilst still affording enough internal space for inboard propulsion and or accommodation if necessary.  The hull shape has been designed to work from 8m to 14m, by either blanking off a section of the mould or adding a further section onto the moulded hull.  There is also the option to widen the beam of the hull, thus the maximum number of vessel options can be accommodated with a single mould tool.

    The essentially square transom section continues forward from the stern, gradually forming a Chine which blends into the curved sections of the bow with a gradual upward sweep.  This Chine shows typically a 50mm section and is designed to generate lift in the bows by deflecting the bow wash.  This lift in conjunction with the dynamic lift generated by the flat stern section results in a smooth steady ride with none of the pitching motion which can be the only disadvantage of a fine bow in some cases.  Examples of this approach can be seen in the designs of Craig Loomes of New Zealand with his 11 and 14m catamarans and also in designer Larry Graf’s range of ‘Glacier Bay’ catamarans from the USA. Since leaving Glacier Bay, Larry has continued with his ground-breaking work in the powered multihull field with his extremely interesting Aspen 26, based on the Proa concept.

    Essentially this feature is borrowed from planing hulls.  The effect of wetted area on the overall drag of a displacement vessel is of significance at speeds higher than around 30 Kn, and whilst the majority of our clients would be happy in the 20 to 30 Kn range, we felt that to offer a hull capable of up to 40 Kn would give us a market advantage.  Clearly to achieve these speeds, a certain amount of lift is necessary in order to reduce wetted area.  The Chine as designed will serve to lift the bow slightly at speed, which in turn will cant the flat aft sections of the hull to the water flow, thus generating dynamic lift at the stern also.  The overall effect will be a general lifting of the whole vessel, although not to a fully planing state.  The hull will therefore be capable of achieving the speed of a planing vessel, whilst retaining the soft predictable motion of a displacement round-bilge hull.

    Above the waterline a second Chine, which I have dubbed the ‘Storm Knuckle’, with typically a 100mm section, follows a similar stern to bow upward sweep as the first chine.  This feature is only designed to be immersed in rough weather and serves to lift the vessel over oncoming waves and also to pick the stern up in heavy following seas.  This is a design attribute championed by the late Malcolm Tennant, also of New Zealand.  A further ‘knuckle’ is visible above this on the sides of the vessel which represents the deck level and is used as a shelf to support the outer edges of the deck.

    In the tunnel I have added a large angled ‘wing’ connecting the Storm Knuckle and the Bridge Deck.  This, if immersed affords a rapid increase in buoyancy in order to help pick the vessel up if extreme conditions are encountered.  This is coupled with a full length ‘Wave Breaker’, which is essentially a triangular girder running the whole length of the Bridge Deck.  The Wave Breaker is there to do just that.  If the going gets rough and all the other features have failed to lift the vessel over the waves, the Breaker will minimise the effect of the sea hitting the Bridge deck.  These attributes can best be seen in the designs of Scott Jutson with his range of ocean capable displacement catamarans.

    Another feature borrowed from Malcolm Tennant is the relatively high Bridge Deck which he believed to be the cornerstone of any power catamaran design if it were to have true offshore potential.  The result of this increased clearance is that the long fine hulls can do their job of cutting through the oncoming waves without being impeded by these waves impacting on the wing deck structure with the characteristic ‘slamming’ associated with low deck catamarans.  A happy coincidence of this is a higher load carrying capacity as there is still sufficient clearance under the deck in the loaded condition for the structure not to be compromised through excessive impacts from the sea below.

    By keeping construction weights to a minimum through extensive use of honey-comb core materials, and ensuring that every component contributes to the overall structure of the vessel we have been able to develop our craft into some of the stiffest and lightest on the market.

    There has always been and will always be a place for more conventional hull designs in all sectors of our industry, but with attention turning ever more to the finite nature of  fossil fuels, there is an increasing argument that designers should ‘step up’ and offer alternatives.  Necessity as they say is the mother of invention.