The Slippery Slope of Deck Design

There is obviously evidence in boat magazine advertising that better training is available for the creation of computer models with realistic rendering than there is for the fundamentals of ergonomics and safe marine practices. You have to hope that the vast majority of the stuff presented as “now available” never makes it past the glossy computerized picture stage.

When designing a boat to be built of moulded fibreglass, the term “deck” becomes synonymous with the “deck moulding”. This large monolithic fabrication incorporates most of the features of the superstructure and cockpit and its design requires the integration of the deckhouse, sheerline treatment, and many details for the attachment of substructures and fittings. The deck design exercise is in practice a catch all for most of what you see of the exterior of the vessel other than the hulls and bridge deck.

Submarine style sheer edgesAlthough sales sea trials will never intentionally be made in really adverse conditions and visions of a breezy, sunny sky life aboard are forefront the minds of most buyers, it is essential for a responsible design to address the crew’s ability to safely and effectively negotiate the weather decks when exposed to severe sea conditions. Following this maxim may however result in a vessel devoid of the fashionably bulbous deckhouses, submarine style sheer edges, scenic route walkways, head bangers, shin bangers, Everest steps, etc.; in other words a boat show peculiarity. You run the risk of designing a vessel that others may label as conservative or even “out of date”, horrors.

We would never consider anything but a flat walking surface to be acceptable for any application outside the marine field. When we necessarily compromise in this regard, it is done to provide drainage, conform to the foundation vessel lines, and to serve the needs of underlying structures or accommodation spaces. It is optionally compromised to address aesthetic elements. In this regard the freedom to incorporate shape should be severely tempered by the ergonomic and safety issues. In rough seas even a normally flattish deck surface will be tilting, wet and windswept. Tripping, sliding, falling on deck or overboard, are some of the obvious hazards confronting a crewman leaving the relative safety of the cockpit on a dark night at sea. There will always be some risk but this should not be exacerbated by dangerous design.

The argument can be rightly made that rounding everything over reduces windage to some degree. This certainly works for road vehicles that always take the wind of highway speeds on the nose. In order to get it right, wind tunnel testing is carried out and often produces some non-intuitive results, things may in fact work better with a few edges. Sailboats are a little more complicated in that the wind may come from any angle and its effect may be positive or negative, depending on which way you wish to go and how the sails are set; however you will probably generally have an easier time of things if windage is no more than it needs to be. Unless you are racing, compromising identifiable safety issues in favour of unquantifiable reduced windage is a tough argument to live with.

Applying a race attuned aesthetic to the family car has had a long record of sales success with little downside as regards drivability, though plenty in regard to utility. A genuine cruising catamaran can ill afford to adopt the racing multihull aesthetic however without serious compromise in safety and especially utility. Water has a way of getting on the deck, sometimes in a fairly large quantity. Beach cats and low profile racing multihulls that routinely fly one, two, or even all hulls concede this point and their shapes tend to resemble those of very sleek submersibles with rounded sections designed to go through waves as efficiently as over them; they spend a lot of time semi-submerged anyway. At the record speeds now being achieved, there is no alternative; the crew just gets very wet.

Racing CatamaranThe cruising catamaran sailing at more moderate speeds has very little in common with a racing multihull other than the reliance on more than one floatation body. The requirement for interior accommodation dictates a full bridge deck construction and full headroom through most of the length of the hulls. For this application, you need to concede that the “through the wave” approach is perhaps not so appropriate and since we would really prefer not to get any wetter than necessary, the hulls will have more topsides, making things higher above the water and introducing a little more deck width at the bow. This is a distinct advantage to assist in getting about on the foredeck and the large floatation volume up forward will keep the bows from being driven under in extreme circumstances. When the deck and its interface with the hulls are considered, creating a weak copy of the rounded over low volume racing hull is perhaps not a good idea, cool as it may look.

The design dilemma occurs when the speculative benefits of Ferrari’s aerodynamic sensibilities encroach upon the real requirement of getting about safely on the decks of a sailboat. Curvaceous vertical surfaces can be incorporated with relatively little effect on the ergonomics but even apparently mild slopes and curves in the horizontals will have a noticeable negative effect on walking stability. The highly rounded deckhouse tops and hardtops that seem to be prevailing in the general market are really not defendable as potential working deck surfaces. They are in fact no-go zones and seem to go hand in hand with other rig access compromises. Telling an owner that he is not supposed (or expected) to walk on a particular part of the deck is a prohibition difficult to observe when he may find himself staggering backward a few steps in the dark while gazing skyward, trying to clear a line aloft. Presumably, providing the practical necessities of a sailboat does not trump peculiar aesthetics in some marketing studies.

TronThe rules governing good step design are widely recognized and published. The ratio of rise to run that can be comfortably negotiated is no mystery. Unfortunately catamarans demand a kind of Escher-like stair treatment that is difficult to reconcile with the Tron Light Cycle aesthetics recognized as a design comfort sop, but deviating from correct stair ratios creates perilous installations. Over-height transom step risers look OK in the photos but will be no fun to negotiate. Rounded step edges inevitably result in an ass-over-teakettle pratfall, comedic on stage but potentially fatal at sea, making it unfair to expect anyone to step up or down while straddling a rounded cabin top perimeter or deck edge, thump – splash. In an emergency, attempting to clamber up any obstruction may appear to be preferable to following a circuitous and presumably safer way so steps installations need to be provided along the direct routes from one work station to another, precluding the necessity.

In the same way that step ergonomics are well defined, the head banging envelope is also no mystery. At least one well known design requires you to access the cockpit from one side only, as there is no headroom under the hardtop edge if you try the other. The excuse theory presented by the sales pitch is that you will learn, like one of Pavlov’s dogs, the hard way, eventually. I am a slow learner; I think this is untenable design.

The decks of a sailboat are swept by rigging lines and sails from time to time, inadvertently or necessarily. The design of projecting structures and hardware should take this into consideration. Projections can also be trippers and even apparently minor ones can be toe-stubbers; it’s tough to design for hopping on one foot.

The old rule “One hand for the ship, one for yourself” only works if there is something for your hand to hold on to. On the huge expanses of catamaran deck, there will inevitably be sections that will be between grasps, but there should be ready handholds wherever possible. Perimeter rails may be only 24” height for some inshore vessels but if you expect to go in water over your head, 30” is the recognized minimum. Even this height is a compromise between the ideal and what is practical, a passenger vessel would be 42”. Substantial foot stops should be provided to keep all or part of you from slipping overboard under the rail. These can be in the form of a toe rail or moulded projections in the deck structure. The design and provision of these items is prescribed in recognized ISO Standards that are part of CE approval. It is remarkable to observe how many infractions may be on display at any boat show.

TrampolineAll trampolines are not created equally. There are a number of options ranging from light weight and very elastic fishing net derivations to heavier web sailmaker’s fabrications with relatively little flex. For racing applications, where weight saving, reduced windage and an agile crew may excuse a significant loss of walking stability and durability, the lightest stuff is obviously desirable. For a cruising boat, unless cost is critical, it is hard to justify using anything from the lighter end of the scale; the significant area covered by the trampoline should not be a surprise test of nerves or physical fitness. The transition from hard deck to flexible trampoline should be flush, other arrangements are disconcerting and potential knee wrenchers, even at the dock, fun though they may be.

It is not necessary to study these parameters in any technical way when contemplating a vessel design. Just walk the decks, deckhouse top, hardtop, trampoline, transom steps; anywhere you may expect to traverse while sailing. Imagine getting quickly and safely about on a dark night. If the design is successful, you can move between any likely standing places on board without being conscious of ever having negotiated the obstacle course or taken any undue risks.


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