SAILING / LIFE #5
My life is going through s patch of “rough weather”, just now – so of course this means it’s time for me to say a few words about hull form, speed, stability and safety. Bear in mind, the optimum cruising boat isn’t the fastest or prettiest. It is, rather, the boat most likely to get one to one’s destination, in spite of the hazards that may be encountered en route.
So for the time being let’s focus on those characteristics of a sailboat’s hull that affect one’s cruising experience. To do this we need to define a few terms.
Length - There are a number of ways of specifying a boat’s length. Most common of these is Length Overall (LOA) that includes length extensions like pulpits, bowsprits, and pushpits. Then there is Length On Deck (LOD) whose name is self-explanatory. Still more interesting than these measures, for our purposes, is the length at the waterline (LWL) to which the hull is submerged.
Although oceans have been crossed in boats little bigger than bath tubs, the fact remains bigger boats are more comfortable than smaller ones, other things being equal. For serious ocean cruisers, I’d recommend nothing smaller than a sturdy forty footer. And at that length even four people aboard will seem like a lot, for a voyage of a week or more.
Beam - A boat’s beam is its maximum width. And again we may speak of its beam at the waterline (BWL).
Draft - A boat’s draft (D) is its depth below the waterline at its deepest point. In the case of boats with movable keels (centerboards or dagger-boards), we want to consider maximum and minimum values of draft. These numbers tell us how deep the water must be for the boat to avoid going aground. Shallower hulls can explore shallower waters – but what they gain in shallow range they lose in stability.
Displacement - A boat’s displacement is the boat’s weight, which happens to be equal to the weight of the water that would perfectly fill the portion of the boat’s hull that is submerged when the boat is floating in equilibrium with the surrounding water. The volume of that portion of the boat’s hull is called the displacement volume.
For boat comparison purposes, consider the volume (V2) of a rectangular solid of dimensions (LWL) x (BWL) x (D) = (V2). And let’s call the displacement volume described above (v1).
Now we can define the ratio R = (V1)/(V2), which is very useful when comparing boats having different values of R. For a boat of the dimensions given, (V2) represents the biggest and heaviest hull possible. And R is the fraction of that volume (or weight) a specific boat hull actually occupies.
Racing boats typically have very low values of R, because they are usually designed to “plane” on the water’s surface rather than displace their way through the water. They may be appropriately called “ultralight” hulls. I consider them totally inappropriate for long range cruising.
Keel - The size, shape, configuration (fixed, twin, centerboard, daggerboard), mounting, and weight distribution of the boat’s keel are all critical factors in a sailboat’s design. Deeper heavier keels provide a stronger righting moment in the event of a capsize. Longer keels provide more directional stability while shorter keels make it easier to execute sharp turns. A daggerboard with an “aerodynamic” cross section and weighted bottom will provide excelllent deep water performance, but will be problematic if it hits a submerged reef. A pivoting centerboard, on the other hand, adds less to performance, but can swing up out of the way when it hits an obstacle.
An excellent innovation due to trimaran designer, Jim Brown, is location of a large centerboard directly under a mid-cockpit. Its casing, the centerboard trunk, has to be massively supported from all directions – and therefor makes an ideal base for a midship mast step. By covering the top of the trunk with a suitable grating, one can use the entire trunk as a giant “scupper” to drain the cockpit rapidly when waves inevitably come aboard.
Payload
The bigger the boat the more it can carry – obviously. Not quite so obvious is the fact that the higher the boat’s R value, the more it can carry per foot of length. Heavier boats are also more comfortable and more stable when properly designed. They are also slower than their lighter counterparts, other things being equal.
Ultimately the maximum range that a boat can transport you, and the maximum time that you might be at sea without stopping to replenish supplies, are dependent on the boat’s payload capacity. Besides the weight of the boat itself and its essential equipment, the required displacement includes provision for the “crew”, their personal carry-on gear, the food, fuel, and water needed for the maximum duration of the voyage, as well as the tools, parts, and supplies that might be needed to effect repairs of any kind, and all the safety and emergency equipment that wisdom dictates you carry with you.
Speed - For a cruising sailboat, speed is not a major concern; but for planning and navigating purposes it is useful to know how the boat will behave under various wind conditions. For a computation of the practical maximum speed of a displacement hull, the following formula is a good first approximation:
K=1.34 x √(LWL)
where the speed, K, in knots is equal to 1.34 times the square root of the length on the waterline measured in feet. While this speed can be exceeded, doing so is very inefficient because, as propulsive power is increased at and beyond that point, its primary effect will be to increase the energy of the boat’s wake, especially a build up of its bow wave.
Multi-hulls - Catamarans and Trimarans are very stable, comfortable, fast, and fun to sail under conditions encountered most commonly. It is the uncommon conditions that are potentially deadly. When the waves are really big – comparable to the height of your tallest mast, it is possible for the boat to capsize by sliding down the face of a wave and burying its downwind end (bow, stern, or pontoon) into the trough between waves. Once inverted in this way, the boat is near impossible to restore to its normal upright position.
Jim Brown made trimarans significantly safer by providing a big centerboard that prevents the boat from sliding down a wave sideways. The risk of capsize is still present though when running directly downwind faster than the waves. A possible solution to this challenge might involve equipping the trimaran with a means of flooding the inverted downwind hull and then evacuating it after the next big wave flips the boat right side up. To facilitate this it would help to be able to remotely release the sheets so the sails wouldn’t prevent the roll-over back to the normal position. Until this solution is proven to work, I’d stick to selecting or building a self-righting monohull.
Seaworthiness - The most important attribute of a cruising sailboat is the quality I cal “seaworthiness”. The most important contributor to sea-worthiness of course is stability and the boat’s ability to recover from a capsize. The next part of sea-worthiness is simply the boat’s resistance to the forces that act upon it under severe weather conditions.
Even stowage of heavy items on board is critical. Think what can happen if a 200 pound battery or a 150 pound anchor were to come unmoored in a knock-down. Even the dumping of a china cabinet or a bookcase could have serious consequences. The rule of thumb is “a place for everything – and everything (held) in its place – even if the boat is upside-down.”
Still another contributor to sea-worthiness is flotation. If the underside of the deck and the interior of the upper hull structure are filled with non-porous foam, in an amount equal to or greater than the displacement volume, the boat becomes virtually unsinkable. While it significantly increases the size of the boat to provide this safeguard, it is well worth the cost in increased safety. The trick to doing this well lies in avoiding inadvertently making some portions of the boat’s interior inaccessible for maintenance and repair purposes.
Seaworthiness is also affected by vibration. As the boat is driven by the wind, everything on board is vibrating. For this reason, every nut and bolt pair must be secured or it is likely to vibrate loose with disastrous consequences. If a mast stay turnbuckle breaks loose this way, one is likely to lose the mast. Besides Locktite, that may or may not be adequate, one can drill a small hole through nut and bolt (or turnbuckle thread) and stick a piece of stainless steel wire through the hole and twist the ends together. Alternatively, one can disrupt the threads with a center punch so the nut can’t undo itself.
For an excellent technical discussion of sea-worthiness as it pertains to hull stability I can recommend no better reference than “Seaworthiness: The Forgotten Factor” by C. A. Marchaj. I’ve personally read this book and give it highest marks for comprehensiveness and clarity.