BASIC CONCEPTS SEAWORTHINESS

BASIC CONCEPTS SEAWORTHINESS

    The seaworthiness of vessel, in broad terms, is the ability of the vessel to provide safety, and comfort for her crew in all weather conditions. The concept of seaworthiness should not only be considered in storm survival conditions, because vessels can be lost in moderate conditions a well as in storms. The effects of fatigue in construction materials and rigging could result in failure in moderate winds, and crew fatigue due to extreme motion could result in errors of judgment, or exhaustion, long before a dangerous situations need otherwise have developed. Collisions for instance can occur at any time, and accounts for the loss of a significant number of yachts, and in my opinion is a bigger danger than a storm.

   In this paper I shall explain some of the many factors that affect the seaworthiness of multihulls. Including windward ability, stability, motion in    waves and pitching and rolling. I will explain how the construction can be  designed to reduce stress concentrations, and how fatigue of materials is taken into account.

   The most informative  work on seaworthiness in modern yachts to date is Seaworthiness the Forgotten Factor by C.A Marchaj. Unfortunately the book concentrates almost exclusively on monohull design and very little is mentioned about multihulls. Since  space is limited, I will not redefine the formulae and criteria for seaworthiness, which are explained clearly in the book, instead I will just go straight on to show how a multhihull fits into the picture.

   On reading through seaworthiness the forgotten factor, I could not help constantly thinking how few  of the vices and problems attributed to monohulls were applicable to modern multhihulls. Over the past 20 years a number of distinct types of miltihull have emerged all having different sailing qualities and seaworthiness. There has been a steady improvement in the understanding of the factors required to make a multihull both safe and fast resulting in boats that are extremely seaworthy as will be demonstrated by the following analysis.

  

   A keel in the  sense used   below is a foil for resisting leeway. The keel is not ballasted as in a monohull and may be fixed, or retractable either ver tically (daggerboard) or by pivoting (centerboard).The outer hulls of a trimaran, sometimes referred to as outriggers or the smaller hull of a pros.

1.Older type of trimaran.

   Inefficient underwater and keel shape ,often with either a fixed or no keel at all. Shall sail area. Hard chine with high wetted surface. Poor pitching control. Medium buoyancy amas (around 110% of the displacement of the boat).Amas usually both in the water at the same time. Narrow beam (lengh to beam ratio = 2) Construction sometimes doubtful in sheet plywood. Low long term fatigue.

2.Second generation trimaran.

   Becoming lighter. Larger sail plans. Less accommodation. Low buoyancy (75 to 90%).Wide beam (L/B = 1.3).Considerably reduced wind age. Improvement in structural design.

3.Third generation trimaran.

   Light weight (due to the use of composite materials) (due to the use of  composite materiala).

   Large Sail Area. Wide beam (L/B <1.5 TO AS low as 1.0 in smaller boats).High buoyancy amas (up to 200% of displacement).Pitching very well controlled by use of different hull shape on main hull and ama. Sailing attitude well controlled on all points of sail. Low wind age. Dramatic improvement in structures due to use of computer aided design and better understanding of composite materials.

4.Early catamaran

   Relatively heavy. Narrow beam (L/B often over 2).High wind age. Small ail area. Inefficient underwater shape and low aspect fixed keels. Cruising cats very heavy by today’s standards. Large flat windows in coachroof causing high wind age. Often prone to hobby horsing and pitching due to rocker and symmetry of hulls.

5.Second generation cat.

   Open bridge deck design. All accommodation in the hull. Greatly reduced wind age. Keel shapes improved. Retractable dagger boards. Large sterns and fine bow burying tendencies on a reach. Greatly improved windward performance. Pitching control still poor, some attempts to reduce pitching by using bulb bows. Wider than early designs. Larger sail plans.

6.Third generation cats.

    Open bridge deck design with large accommodation in hulls .  Hobby horsing eliminated by hull shape. Bow burying on a reach eliminated. Wind age greatly reduced by rounding and streamlining deck edger. Powerful efficient rigs. Sophisticated retractable dagger boards and rudder. Minimum wetted surface hulls. Excellent windward performance. Fast easy motion through sea. Very stable with wide beam (L/B < 1.5).Similar structural design improvements taking place as for trimarans.

7.Fourth Generation cats.

   Basically as 6 above but with very streamlined bridge deck cabin for large accommodation and low wind age. Light weight maintained ,with large weight carrying ability for fast cruising.

8.Other types

    Proas (Atlantic ,ama to leeward, and Pacific. ama to windward) and trimaran foilers. In general these are development types almost exclusively for racing,as far as modern multhihulls are concerned, and they have special problems that require particular knowledge, experience and seamanship for safety at sea. Due to lack of space these types will not be dealt with in any detail in this article.

Boat Motion in a Seaway  , and the effect on the ships crew

   There are 6 basic forms of motion in a seaway:

1.Rolling.

    With the exception of type 2 above,  multihulls are virtually immune to rolling. This means that the boat sits on the water like a raft –  following the surface of the sea, giving great crew comfort while sailing, particularly down wind. When lying a hull, cats and tries exhibit different characteristics. Firstly catamarans have a very high roll moment of inertia (Ir), because the weight of the boat is primarily concentrated at the hull centerlines. The buoyancy of the boat is also concentrated at the extremity of the hull centerlines.

    Open bridge deck cruising cats benefit most from the effect and low buoyancy ama trimarans. In a the weight is concentrated closer to the center of gravity (CG),reducing Irk, and the amas take longer to pick up buoyancy as the boat heels, thereby reducing damping. In a low buoyancy am this effect can lead to capsize in waves,(when lying a hull) as will be shown later and different techniques f seamanship are required to ensure the safety of this type of multihull.

2.Pitching and Hobby horsing.

    Many early multihulls were prone to hobby horsing and pitching. This was caused by too much rocker on the hull profile and fine V sections both fore and aft. As hull shapes improved tending towards more U shaped underbodies particularly aft, pitching still remained a problem because the large width of the stern sections caused the sea to lift the sterns as the boat passed over the wave, driving the bow down. However we now know that pitching can be dramatically reduced by finer sections at the stern combined with the center of buoyancy being moved forward in the immersed hull and aft in the lifting hull. This effect can be achieved in both cats and trips, giving a very comfortable and easy motion upwind. At the time windward performance is improved, because the apparent wind direction is more stable across the sails.

   

   

3.Yawing.

    Any tendency to yaw has been virtually eliminated in the modern multihull due to the shallow draft of the hull (because of the  U share section and the light weight),and by the use of  retractable dagger boards. Once the keel is removed when sailing downwind, there is virtually no chance of broaching as long as the forefoot does not dig in. This can be prevented by firstly reducing the forefoot and by picking up buoyancy up quickly in the forward sections of the boat above the waterline. Computer simulations of the hull in different bow down trims, and at varying waterline positions.

4.Surfing

     A Multihull will suft very easily, making for fast passage making in the open ocean. Sailing downwind in winds up to 40 knots is usually quite comfortable and easy. The apparent wind being reduced by the boat speed. However particular attention has to be paid to ridder size and design to maintain good control at surfing speeds in excess of 20 knots. Elliptical balanced spade rudders of airfoil section reduce helm loads and drag at high speeds. I favour using stainless steel or titanium, rather than carbon for rudder stocks because at least the rudder will bend if overloaded, instead of shearing off.

5.Swaying.

    A modern light displacement multihull lying sideways to the seas with no sails up (i.e lying a hull),and with the dagger boards up, will surf sideways very easily in a breaking crest. It will be demonstrated later that this is a very important feature in the seaworthiness of multihulls lying a hull in a storm. Earlier multihulls with fixed keels and tris, are prone to tripping over their keels amas when struck by a breaking crest. Narrow beam increases the danger of capsize in this situation. If sideways motion of the yacht needs to be stopped,for instance because of a danger to leeward,this can be done by either deploying a  sea anchor abeam,or by putting down the upwind board (this only applies to a cat) . The upwind board can act as a brade without imparting rotational momentum to the boat.If the boat does not rotate – it will not capsize.

6.Heaving

    This is a complex problem to define clearly for a multihull, because the two immersed hulls are at different places on the wave front at any given time. Nevertheless, they heave less than monohulls because the hulls are slimmer, allowing the boats to cut through the water when sailing. Loss of apparent displacement at the wave crest and rotational momentum imparted to the boat by heaving on the upwind hull will assist in capsizing an over canvassed multihull due to wind and wave action. Heaving assisted capsize has been experienced particularly in trips. This is of particular importance and will be dealt with more fully in the next section.

STABILITY

  

   This is generals a very contentious and little understood subject when multihull seaworthiness is discussed, and is probably the biggest fear that inexperienced sailors have about this type of vessel. And while it is true that certain mutihulls have capsized, it is clear from the above that there are many different types of multihull, and indeed there are different ways in which they can capsize.

WIND AND WAVE ACTION

 

     When you combine the action of wind and waves,a catamaran is more vulnerable than a tri, because when a boat is sailing the heaving action of the wave on the windward hull imparts rolling momentum to the boat, reducing the energy reserve left under the righting moment graph. If the boat has a low static stability, and is being sailed close to the limit, with the dagger board down, it will be possible to capsize n waves in a wind speed that would be safe in flat water, Cats are more vulnerable than tris because in general the static stability of the cats is less than an equivalent tri. This is main reason that tris are considered to be safer for short handed racing – they can be sailed harder in waves with a greater margin of safety. On the other hand this is a very god reason to make cats as wide as possible to increase the static stability and there by  increase the safe sail carrying power.

SAFETY IN THE EVENT OF COLLISION OR CAPSIZE

   

     Even though it has become extreme unlikely that a properly designed multihull will capsize, the possibility still exists,  in much the same way as it exists for any monohull. The monohull’s escape valve is that there is a chance that the boat will right it self before it sinks. The multihull on the other hand can be made into a safe raft for the crew to live on in the inverted position, provided that proper provision for this eventuality has been made at the design stage.In terms of ultimate safety of the crew in the most extreme storm.

CONSTRUCTION AND FATIGUE – Integrated structure

   During the lifespan of a multihull it is subjected to many cycles of a complex array of loads, and if the boat is to survive in all conditions without damage careful attention has to be paid to avoiding stress concentrations I the structure ,and to the long term fatigue of the materials used to build it.

CONCLUSION

   In the past 20 years the level of understanding of the factors that effect the seaworthiness of multihulls has increased enormously. Accidents and failures are an inevitable part of the development of any idea, but I hope that the above discussion makes it clear that there are many different types of multihull, and that a mishap in one type does not necessarily imply that all multihull would have suffered the same fate in the same circumstances. Indeed many of the problem and vices   associated with the older design have now been eliminated, and the new generation of cruising design are very exciting boats to sail, while still offering vast accommodation, crew comfort, and most important of all – safety at sea.