MCG > Choosing a Design

i. CAT or TRI

CATS have become more popular for cruising mainly because of the accommodation space and privacy they provide and because for a given length they are faster and cheaper to build than a trimaran.

Trimarans, on the other hand, provide good performance through a wider range of conditions than catamarans, and can also be effective for cruising by utilising flare above the waterline in the mainhull, and by fitting aft cabins where additional privacy is required in the accommodation layout.

Apart from the accommodation, the trimaran provides a more forgiving motion in a seaway and better resistance to pitching if the hull shapes are well designed, and the wide beam allows it to carry more sail in fresh conditions. Against these advantages is the fact that a cat will require less berthing space and is likely to be more manoeuvrable in close quarters if twin diesels are fitted. Also, the cat will have a more steady motion than a tri while sailing downwind.

ii. TRAILABLE MULTIS (9m and under)

Multihulls in this category are generally racing boats or high performance boats suited to weekend cruising and short coastal voyages. If the boat is light enough to be easily transportable and have good performance, then the accommodation space is going to be rather limited and not really comparable to that provided on a modem wide bodied monohull of similar length.

The real advantage of the multis in this size range is their shallow draught and sparkling performance, allowing for exciting racing and the option to cruise further afield when time might be a limiting factor. In the smaller multihulls the tris will offer more cabin space for a given length, and generally better performance than the cats, at a cost of greater expense and complexity.

iii. CONSTRUCTION COST

Multihulls have more surface area than monohulls, and also require a larger number of building jigs or moulds, and so are inevitably more expensive to build, both *in time and materials.

The cost of the finished boat can be estimated from the list of surface areas and laminates, and the hardware and materials lists supplied by the designer, or more simply by judging from costs of similar boats already completed, however the finished cost can vary considerably between boats of similar design, mainly because of variations in the quality and extent of fit-out and the amount of safety gear, instrumentation and electronics fitted to the boat.

The quality of the original design and the integrity of the structure are usually the foremost considerations when assessing the resale value of a boat, and cost of materials for the bare structure is only about one third of the cost of the completed boat. It is false economy therefore to use cheap materials on the structure, as the overall saving is likely to be minimal and the resale value of the boat could be affected.

Catamaran advantages:

* Simpler and more economical to build

* Greater flexibility with accommodation arrangement

* More privacy in the cabins

* Reduced berthing space

* Better manoeuvrability (twin motors)

* Good protection in cockpit (with bridge cabin)

* More deck area

* Very steady motion downwind

* Generally higher resale value

Trimaran advantages:

* Less windage and generally better performance

* Usually more aesthetically pleasing

* Much better visibility (compared to bridgedeck cat)

* More efficient with only one motor

* Higher stability and better pitch resistance than cat

* More headroom in smaller sizes

* Less interior space to fit out

To summarise; it would be generally fair to say that a catamaran provides the accommodation facilities and comfort of a considerably larger monohull, and the trimaran provides the same in terms of performance, but in both cases the ultimate cost will also be close to that of the larger monohull.

PART 2 DESIGN FEATURES

i. KEELS

All racing boats have relatively large high aspect daggerboards because they provide the most efficient lifting force to drive a boat to windward. In many cases they are also a practical solution for cruising boats because they are easily retracted and allow the ultimate in shallow draught.

On the other hand low aspect fin keels have become popular on cruising catamarans because they provide protection for the propeller or sail drive strut, and make it easy to clean the bottom of the boat on the beach.

If good performance is to be retained it is essential that the fixed keel be well designed in both section and plan form and that the area be sufficiently large to provide good windward performance. A fixed keel which is too long and shallow, or of unsuitable section Will cause slow tacking and excessive leeway.

The fixed keels fitted to the "AZURE 37" production catamaran provide excellent windward performance and fast tacking, and have since been adapted to a number of other Grainger cruising designs

ii. AUXILIARY MOTORS

For multihulls under about 10m LOA an outboard motor is the most practical and economical solution and outboards can be employed successfully on high performance boats up to about I I or 12 metres, although motoring capability will be limited in rough conditions. For small trimarans, the most satisfactory solution is to mount the outboard on the transom on a bracket which will tilt and allow the outboard to be slid forward along the cockpit floor when not in use. Outboards fitted to wells in the cockpit are not recommended because of their complexity and the interference with the cockpit arrangement, and in particular a propensity to snag sheet lines.

For cruising boats an inboard diesel installation is recommended especially for manoeuvring in close quarters where the high windage of multihulls can create problems. Sail drive auxiliaries provide a fast and simple installation but the strut and propeller will need to be protected against grounding by a skeg or fixed keel.

If only one diesel motor is to be fitted in a cat it is still preferable to fit the motor in one of the hulls rather than on the bridge for greater efficiency, reduced noise and vibration, and lower centre of gravity, however the twin engine installation will have far superior manoeuvrability.

iii. SPINNAKERS AND POLES

Multihulls carry their downwind headsails from a fixed pole set from the stem of the mainhull on a trimaran or the centre of the forward beam on a catamaran. The term "fixed pole" is used to describe a pole set from the bow (and may be removable for manna berthing) as opposed to the pole set from the mast as was once employed by multihulls and still used by some monohull classes. With the fixed pole spinnaker handling is greatly simplified. The pole can remain permanently in position while sailing and the spinnaker is simply gybed in the same manner as a headsail.

The assymetrical spinnaker is more efficient on a fast multihull which can make use of wide downwind tacking angles to maintain a higher apparent wind strength and on a cruising boat there will be little or no performance advantage, however the very simple handling characteristics encourage more frequent use of spinnakers on cruising boats, especially while short handed.

iv. MATERIALS

Foam sandwich provides the ultimate in light weight, especially for the smaller boats, its main deficiencies being a susceptibility to puncture and impact damage when being used with light layups, and high cost compared to cedar strip. High quality epoxy resins are recommended for all boats and are essential for timber boats. Polyester resins are quite adequate for foam boats, with vinylesters having better properties than the more conventional polyesters.

v. HIGH TECH MATERIALS.

There are some applications for the more expensive materials such as carbon fibre and Kevlar KevIarO has very good abrasion resistance and can be used on the bottoms of hulls, daggerboards, and keels (especially where the layups are light) to provide protection from grounding. In racing boats it is often used to keep weight to a minimum in the ends. Carbon fibre can be used to good advantage in highly stressed areas such as structural components in cross beams, and in masts and rudder assemblies where weight saving is important, however quality control and verification of material properties are crucial when using high modulus materials. Generally, for performance cruising boats, the advantages of the high tech materials are minimal in relation to their cost; especially considering they usually require special fabricating skills, and conventional E glass will normally provide adequate strength, ease of use, and good availability at reasonable cost.

vi. THE RIG

The only difference between the engineering of the rotating 3 point rig and the non-rotating 3 point rig is that of the mast base and the attachment of the shrouds to the mast at the hounds position. A variation on the 'rigid 2 3 point rig is using a more flexible non rotating spar, generally with more intermediate support from runners and an inner forestay and sometimes a baby stay as well. This type of rig allows a lighter mast section to be used and gives more control over the sail shape. It is more common on racing boats and the mast will generally have some pre-bend induced with the mainsail leech pressure assisting the prebend under sail.

iii. SPINNAKERS AND POLES

iv. MATERIALS

All Grainger designs are intended for composite (sandwich) construction. That is, using foam or timber (usually western red cedar) as a core material in combination with internal and external glass laminates. Composite construction provides good stiffness for minimal weight and reduces construction complexity by eliminating unnecessary framing in the structure. It also has the added advantage of providing good sound and heat insulation. In recent years the cedar strip plank method has become very popular with owner builders and professionals alike for its speed and ease of construction and general user-ftiendliness. Foam sandwich provides the ultimate in light weight, especially for the smaller boats, its main deficiencies being a susceptibility to puncture and impact damage when being used with light layups, and high cost compared to cedar strip. High quality epoxy resins are recommended for all boats and are essential for timber boats. Polyester resins are quite adequate for foam boats, with vinylesters having better properties than the more conventional polyesters.

v. HIGH TECH MATERIALS.

There are some applications for the more expensive materials such as carbon fibre and Kevlarg. Kevlarg has very good abrasion resistance and can be used on the bottoms of hulls, daggerboards, and keels (especially where the layups are light) to provide protection from grounding. In racing boats it is often used to keep weight to a Minimum in the ends. Carbon fibre can be used to good advantage in highly stressed areas such as structural components in cross beams, and in masts and rudder assemblies where weight saving is important, however quality control and verification of material properties are crucial when using high modulus materials. Generally, for performance cruising boats, the advantages of the high tech materials are minimal in relation to their cost; especially considering they usually require special fabricating skills, and conventional E glass will normally provide adequate strength, ease of use, and good availability at reasonable cost.

vi. THE RIG

The majority of modem performance cruising multihulls now carry a simple 3 point fractional rig. That is, two shrouds and a forestay, with all additional wires required to keep the mast in column being contained on the mast itself in the form of diamond wires or jumper wires, although sometimes a removable inner forestay will be taken to the foredeck to prevent the mainsail pressure pulling aft bend into the mast below the hounds position. Apart from the simplicity and economy of this type of rig, it also allows the option of rotating the mast to improve airflow over the leeward leading edge. Rotating rigs are also popular with some cruising people because they allow the mast to be feathered into the wind while raising or lowering the sail. The only difference between the engineering of the rotating 3 point rig and the non-rotating 3 point rig is that of the mast base and the attachment of the shrouds to the mast at the hounds position. A variation on the 3rigid2 point rig is using a more flexible non rotating spar, generally with more intermediate support from runners and an inner forestay and sometimes a baby stay as well. This type of rig allows a lighter mast section to be used and gives more control over the sail shape. It is more common on racing boats and the mast will generally have some pre-bend induced with the mainsail leech pressure assisting the prebend under sail.

PART 3. HULL DESIGN

i. BEAM TO LENGTH RATIO

Apart from the overall length of the boat the Beam to Length Ratio, perhaps more than any other factor, best describes the suitability of the hull for the intended purpose figure is one of the most important considerations for a designer in commencing work on a new hull design. Like everything else in yacht design, selecting the beam to length ratio is the art of finding a workable compromise between two design extremes. Long skinny hulls have low resistance and accelerate easily under increased power, but fat hulls have the ability to carry weight, have reduced wetted area, and are easier to tack.

ii. SECTION SHAPES

The hull shapes will determine the handling characteristics of the boat, and have been the subject of a great deal of development and evolution over the last 25 or 30 years. In the last 12 years or so a number of the world's better designers have built on this development to provide a new breed of hull shapes which, in my opinion, have been a major factor in bringing multiliull performance into a new realm. A good example of this development has been in the fore and aft distribution of buoyancy in trimaran hulls (and later in catamaran hulls) to dampen pitching and provide reserve buoyancy forward for offwind stability. The principles behind this effect were described in detail by John Shuttleworth in the mid 1980's.

iii. STEM PROFILE

To rake or not to rake? Plumb bows have become popular on some catamarans to maximise waterline length, however, waterline length is most important in fresh conditions, and in these conditions the leeward hull of a cat will be at least partly depressed and therefore close to its maximum length anyway. A shorter waterline in light conditions will reduce wetted area and allow faster tacking, and the raked stem has the distinct advantage of deflecting more water and making for a drier boat.

iv. KEEL ROCKER

The trend toward flatter rocker in recent years has largely been induced by the desire to increase the prismatic coefficient (buoyancy in the ends) and thereby dampen pitching and optimise reaching performance. Designing rocker into a new hull is a delicate balancing act as it has to be considered in respect to the flatness of the underwater sections and the required buoyancy.

For example if the sections forward are fairly flat or rounded (as opposed to a IV' section) then it is important to ensure there is adequate rocker forward to avoid hard landings while beating in a seaway. The run aft should remain reasonably flat especially if some planing lift is being sought downwind. An excessively flat rocker line might also require that lost buoyancy be compensated by the beam to length ratio.

v. BRIDGEDECK CLEARANCE (Cats)

Many cruising cats have low bridgedeck clearance to provide maximum possible headroom in the bridgedeck saloon without creating an excessively high structure with poor appearance and high windage. In some conditions this is fine but there are occasions, especially beating to windward in steep seas, when the slamming under the bridgedeck will become a limiting factor on how hard the boat can be driven and could severely impede performance in some conditions. Slamming under the bridgedeck is also affected by the shape of the hulls and their separation, and also by the length of the bridgedeck, and could be expected to be worse if the hulls are excessively wide or the width between the hull centrelines is unusually narrow, thereby causing the bow waves to converge under the bridgedeck. Because of the complex factors involved in determining bridge clearance no specific rule can be provided to cover all designs, however, in regarding to seakeeping, the more clearance the better.

vi. WEIGHT AND DISPLACEMENT

Displacement is the amount of water displaced by a hull (or hulls), that amount of water being equal to the weight of the boat in any given loading state.

Quite often a displacement figure lighter than the realistic sailing weight will be provided in sales brochures or specifications, however if a displacement figure is unrealistic, or is based on the weight of the unladen vessel then any performance or structural calculations based on this figure will give unrealistic data.

A high designed displacement (Disp. DWL) figure does not necessarily mean a boat has inherently heavy construction, but may indicate that the boat has been given a higher payload allowance in the design stage. There is a advantage in the designer being conservative in the initial weight estimates to allow for realistic payload and possible excessive construction weight. Some degree of payload should always be incorporated into the designed displacement and a generous displacement figure quoted on a specification sheet might be more of an indication of the designer's concern that the boat float correctly on its lines when loaded, than an indication of unusually heavy construction. If inadequate buoyancy is provided in the design stage the boat may be adversely affected by poor hydrodynamics, such as dragging the transom for example, or unusually low bridge clearance in the case of a cruising catamaran.

vii. OUTRIGGER BUOYANCY (trimarans)

The buoyancy ratio is a measure of the float buoyancy compared to the total sailing weight of the boat and provides a good indication of the sail carrying and seakeeping capabilities of the boat (in combination with the width between the centrelines of the floats). Unfortunately it is difficult to always compare buoyancy figures as a percentage on a boat for boat basis because the boat 'weight2 figure used to calculate the buoyancy ratio does not always correspond with the actual sailing displacement of the boat and a realistic displacement should be verified before making specific comparisons. The best guide to the sailing weight of a particular vessel is the weight on the IOMR rating certificate if the boat has one, or a good estimate based on boats of similar configuration and construction. This figure can then be compared to the outrigger displacement if the figure is provided.

viii. THE C14AWER PANEL

In 1986 we integrated what we call the Ichamfer panel2 into the design work for a 10.6m (35') LOA open bridgedeck cat which we were designing at the time and which was to become the Alfresco 1060. The chamfer is quite simply a 45s bevel between the bridgedeck and the inboard hull side and our original intention was to create a more uniformly stressed structure by minimising the loadings on the hull to bridge join but it had a surprisingly beneficial effect on the motion of the boat in a seaway and significantly improved access into the hulls when applied to bri'dgedeck catamarans. BOAT MOTION At the time there were two of the Alfresco 1060's building side by side and one of the owners elected to employ the straight bevel (chamfer) as designed while the other rounded it out, effectively creating a large diameter radius between the hull and the bridge. When the two boats were sailing side by side it was possible to detect a distinct difference in the motion of the two boats in short lumpy conditions, with the boat which employed the chamfer as designed demonstrating a more even motion as it encountered wave action. The chamfer panel effectively has a dampening effect on the motion of the boat and this is especially noticeable in quartering and beam seas. When the rising wave encounters the chamfer, the wave acts to lift the boat slightly and if the wave is large or steep enough to strike the underwing it will eventually do so with less force and create a minimum of vertical acceleration to the boat. A steep beam sea is potentially the most uncomfortable sailing environment for a catamaran, with wave forces at 90s to the hull side often creating a sharp jerky motion. The inboard hull side causes most of the problem here as waves are effectively trapped in the comer created by the hull side and the bridge. In this situation the horizontal accelerations are dampened in a similar f ashion to the vertical accelerations with the chamfer panel lifting the hull and easing the wave under the leeward hull.

STRUCTURE

The structural form created by the use of the chamfer panel was ideally suited to the integration of composite technology to produce an evenly stressed structure which was also simple and economical to build. Unidirectional glass fibres are laid across the bridedeck and splay out down the inboard hull side, thereby removing the highly stressed comer join which is otherwise formed, and effectively distributing the loads from the main bulkhead into the adjoining structure.

ACCOMMODATION

In 1989 we were commissioned to design the Azure 37 production catamaran (also known as the G37) and in this case the chamfer was a major attribute to the design quite apart from its' structural and seakeeping advantages. The chamfer panel allows the steps into the hull to be moved closer to the hull centreline, thereby making access from the hull into the bridge a reality without having to stoop and without having the upper coach house extend too far across the boat thereby stealing valuable