Yacht Design

Sept/Oct 2003

A voyaging sailboat’s interior is the result of a balancing act by the vessel’s designer. The needs of crew, stores, and tankage all have to be taken into account. Each of these factors has to be given just the right amount of space in the right location to get the job done.

Image Credit: Eric Sponberg illustration

Take weight distribution, for example. The heaviest weights, like the engine, batteries, and tankage, should be located as near the center of the boat and as low down as possible. Weight placed centrally reduces pitching motion and increases speed and comfort; weight placed low improves stability. Proper weight placement is critical to better performance, and better performance makes for happier and more comfortable passages.

For a new boat design, you can define the number and sizes of spaces that you require and wrap a boat shape around them. But, unless you own Fort Knox, you’re certain to end up with a design you can’t afford. Almost universally, clients come to me knowing two things about their new boat design: how long it will be, and how much money they have to spend. So, we define the boat shape that will satisfy the performance and the mission of the voyage and then stuff everything into it that meets the owner’s requirements and budget. There are bound to be trade-offs among the issues of different spaces, weight, and cost.

The most important space is accommodation. We humans are a funny-shaped lotwe come in all shapes and sizesand a sailboat makes for a funny-shaped house. Humans conform pretty readily to squarish spaces and furniture, but boats do not. We design the accommodation first to fit the human form, and all other stowage and tankage becomes subservient to that.

Skene’s Elements of Yacht Design and Architectural Graphic Standards give very good dimensions of the human form. Using these guidelines combined with some practical experience, we size the accommodation spaces accordingly. Human height relates to head room and berth length, and human width to berth, seating, and door width. The height of eye both standing and sitting governs window placement, which is important for comfortably viewing the passing scene, one of the most pleasurable joys of living on a boat. Interestingly, viewed from the outside, the windows are the eyes of the vessel; they are what give the aesthetics of the boat their personality and character. So whatever windows work well for the people inside, they should be styled attractively for the outside.

Bulkhead spacing

Structure impinges on the size, shape, and placement of the accommodation spaces. Bulkheads support the hull and rig, and longitudinal girders and transverse floors support the keel, rudder, and cabin sole. Bulkheads have to be spaced far enough apart to fit berths that are long enough for humans to sit and sleep on, but not so far apart that the hull and rig are not adequately supported. A well-designed structure will fit hand-in-glove within, behind, and underneath the accommodation spaces without protruding into them in an inconvenient way. An example of this is a staysail stay coming down right through the middle of the forward V-berth.

Whereas accommodation space does not carry much physical weight at any one time, both stowage and tankage spaces carry a lot of dense weight all the time. The biggest and most convenient stowage spaces are underneath and behind settees. Frequently, mechanical equipment such as a water pump, depth or speed transducer, or even the engine, will take priority for this space. Such equipment should be protected from stowed gear shifting and falling onto it, which could cause pipe leaks or broken wire connections that interfere with the boat’s systems.

How much weight of stowed stuff should you carry and still ensure good performance? People have different habits and styles of living. If you enjoy your wine at night after a good meal, you may place emphasis on a well-equipped galley, refrigerator, food stowage, and a wine rack. A person who likes to tinker with the machinery needs space for lots of spare parts and tools.

To judge how much stuff you’ll carry, weigh it. For example, weigh your groceries every week for four to six weeks to determine food-carrying capacity for an average transoceanic voyage. Measure the size of the shopping bags (packed down) to assess the size of the space where it will all need to go. Chances are you’ll never fit it all on board. So resign yourself to carry less and eat less. Happily, almost everyone I know who has gone cruising has lost weight. Cruising is great for dieting.

Weigh your clothes, foul weather gear, linens, bedding, etc., and measure their bulk. Weigh all the other loose stuff that you want to carry, like tools, binoculars, dishes, pots and pans, etc., and note the sizes of the spaces where you keep them. It is not unusual for a family of four to carry an extra 4,000 to 6,000 lbs. of gear.

Part of this weight is for tankagefresh water to drink and wash in and fuel for the engine, preferably diesel. Sometimes people carry an outboard motor for their dinghy, and although a few diesel outboards, which can utilize the on-board diesel supply, are on the market most of them use gasoline. Since gasoline fumes are heavier than air and highly flammable, gasoline should be carried in jerry cans on deck or in separate, well-ventilated and drained lockers. The same applies to LPG gas (propane and butane) for stoves and water heaters. Gas bottle lockers must be dedicated to that single use, well ventilated, drainable, and built as close to their appliances as possible. A gas storage locker is an example of a space that cannot and must not have a double function.

Determining fuel tankage

The fuel supply depends on the size and fuel consumption rate of the engine and the amount of motoring the owner intends to do. Traversing the Intracoastal Waterway would likely require more fuel than taking the offshore route because of the motoring required around marinas, bridges, and passes. To plan offshore fuel requirements, figure how far you would likely have to motor if something happened to the rig. If the rig fell down in the middle of the Atlantic Ocean, you might want a 1,000 to 1,500 miles’ range at half throttle to reach a port of safe refuge. Work backwards from this scenario using the fuel consumption chart on the engine data sheet to determine the fuel tankage required. Keep at least two tanks dedicated to fuel with suitable cross-connections and piping. Contamination is to be feared, whether from microorganisms, sea water getting in through the fill pipe (keep it caulked), or from mistakes like opening the wrong fill port when taking on water. You do not want to ruin your whole supply.

Modern technology has reduced our requirements for fresh water tankage. Water-makers are compact and affordable, and you can even take handheld ones into a life rafttruly a lifesaver. Before the advent of this technology, one had to carry all fresh water for the full length of the voyage. This meant rationing consumption, and not taking water-guzzling showers. A water-maker of suitable capacity now makes it possible to have plentiful water supplies. Again, have at least two water tanks to reduce the likelihood of total contamination.

Tanks should be totally independent with no shared common boundaries. A leak or break in a boundary will contaminate the supplies in both tanks. Therefore, the boat structure and the accommodation spaces must allow for this. Tanks can be built-in for maximum available capacity or stand-alone, firmly strapped and chocked, for maximum integrity. If stand-alone, they should be relatively easy to remove from the boat for cleaning or repairs at a later date, so the accommodation joinery should be built accordingly.

Open class transoceanic racing like the BOC and Vendee Globe races has opened owners’ and designers’ eyes to the advantages of water ballast. Water ballast in tanks placed well outboard to windward makes the boat stand up straighter and, therefore, sail faster, more than compensating for the ensuing drag of heavier displacement. For light-air sailing, the tactic is the opposite. Put the water ballast on the lee side, and the sails will hang to shape better than if the boat sits upright with the sails slap, slap, slapping from the swells. You’ll make way much better, which improves morale and the enjoyment of the voyage.

Water ballast tanks may be at the sacrifice of stowage space unless the boat is designed a bit beamier to fit them in. You can use fresh water instead of sea water for water ballast, thereby getting a double benefit from a single set of tanks. With water-makers, it is easy to make enough water for both consumption and for use in ballast tanks

But then, suddenly, we got lucky. The wind grew stronger, shifted in our favor and all through the rest of the night we sailed straight at St. Maarten at 5 knots or better. Around sunrise Thursday morning we cleared the east end of Anguilla. By 0900 we had turned on the engine again and were rounding Pointe Basse Terre, at the western tip of St. Maarten. We figured we would, unfortunately, be just a few minutes late for the 0930 bridge opening into Simpson Bay Lagoon. But then, while monitoring the bridge’s radio traffic, we made an amazing discovery – it wasn’t 0900 after all. It was 0800, as St. Maarten, despite being well east of Bermuda, is an hour behind Bermuda time.

I felt like Phineas Fogg in Around the World in 80 Days, pulling victory from the clutches of defeat thanks to the unexpected gift of an extra hour. We arrived at Simpson Bay with more than half an hour to spare and, after idling around a bit, started to queue up to enter the lagoon. But then, just as suddenly, our luck ran out. Just five minutes before the bridge was to open, the engine faltered and died. Once again – literally just a few hundred yards from our final destination – we had run out of fuel.

It all came out right in the end, though. We anchored in the bay (with no help from the windlass, of course) and a tow boat came out to pull us into the lagoon when the bridge opened again at 1130. Just an hour or so after we finally got the boat secure in a marina berth, Hank arrived from the airport. He was delighted to learn that the fuel tanks were empty. This meant he could take on the very minimum amount needed to reach Antigua, and so would be carrying as little extra weight as possible when the racing began.

After our wrong-

Designing an interior

To see how an interior is developed, let’s sketch out a 40-foot LOA voyaging sailboat. The owners are a married couple with two children who want to cruise around the world on a three-year voyage, so we’ll call the boat the Globetrotter 40. They need two sleeping cabins, one head with shower, a galley, a main saloon, an aft cockpit with wheel steering, and a pilot house with a second steering station inside. Maximum draft is six feet. Twin retractable rudders are to hang on a transom that is inset from a scoop stern. For brevity, we’ll not consider water ballast at this time. The rig is to be either a cutter or a cat-ketch. See the accompanying figures as the design develops.

The owners are going to be diligent about not carrying too much excess weight, so we shoot for a displacement/length ratio of about 200 at lightship weight, which is the dry boat with no gear on board. A slightly raked stem and scoop stern give a waterline length of 35 feet. Therefore, lightship displacement is 19,208 lbs. Anticipated personal gear, effects, and consumables is about 6,000 lbs., giving a full-load displacement of about 25,000 lbs., which sinks the boat about four inches, and we scribe this full-load waterline on the profile. An overall beam of 12 feet allows for a nicely proportioned hull for sailing, and works well for this many people on board. Head room is to be six feet, three inches, so we set the top of the coach roof six feet, five inches above the cabin sole, which allows for deck thickness and headliner. Height of eye while standing is five feet to five feet, four inches. At about 12 feet below eye level amidships we can set down the sheer line, the most important line in the aesthetics of the design. The scoop stern takes up a lot of boat length, to the detriment of the rest of the accommodation. What I’ve done lately is bring the deck and hull volume aft and above the sloping scoop sheer, set inboard just a bit, which allows for a nicely sized cockpit with lots of stowage space underneath.

The forepeak bulkhead, set five feet back from the stem, gives adequate space for the anchor windlass and a crash forefoot to protect the stem against collisions. Anchor chains will be two lengths at 200 feet each, but we don’t want all that heavy chain housed so far forward in the stem. Therefore, we’ll slide them down the top of the crash forefoot into the space below the forward cabin seat to get them aft a little bit.

Forward cabins are notoriously noisy; the sound of water rushing along the bow and pounding in heavy seas makes sleeping there difficult at best. So for added comfort, we provide two pilot berths high and outboard by flipping up the backs of the settees in the main saloon. The settees also convert to sleeping berths (example of dual function) so, near the middle of the boat where motion is least, we have four good sleeping berths.

In the cockpit, a T-shaped arrangement always works well, but we don’t want to make it too large. When a wave poops the boat, we want to minimize the amount of water we take on. Also, we want to keep the bulkhead at the forward end as far aft as possible to maximize the space for the pilot house, aft cabin, galley, and head. The side seats in the cockpit can fit two people on a sidethe forward one of which will be protected by the extended sides and roof of the pilot house.

We set the pilot house sole so that a three-cylinder 35- to 50-hp diesel engine fits underneath. Being forward of the cockpit, this engine room will be a very dry space great for engine life. Note that the engine space is offset a little to starboard because we need standing space in the aft cabin to port. We also set the height of eye again at five feet to five feet, four inches, and the pilot house top for six-foot-one-inch head room so that the boat does not look too top heavy. The steering helm is to starboard, and a chart table big enough to spread out an entire chart is to port. To see over the pilot house, the camber of its top at the outboard corner is five feet above the cockpit solejust at chin level.

The head with separate shower is to starboard opposite the L-shaped galley to port in the area between the pilot house and the main saloon. Finally, the aft cabin is under the pilot house to port. To gain standing head room in this cabin, we use some of the space under the chart table and drop the sole about six inches. This completes the initial layout of the major accommodation spaces in the boat.

You can see how the human form determines the sizes of the spaces. We’ve left spaces for lockers that will hold clothes and linens. Under the cabin soles we have space for sumps, keel attachment, and depth and speed transducers. Heavier items are placed toward the center of the hull, lighter items toward the ends.

A straight traffic pattern

All walking traffic right through the boat has a straight line just to starboard of the centerline. Usually, the straighter the traffic pattern, the safer the boat, the easier it is to build, and the less it costs. With more turns in traffic flow, you have to build more corners in the furniture and add more hand-holds to brace yourself while underway. All this adds to cost.

Now we can fit in the tankage and major stowage items. Underneath the cockpit sole just behind the main pilot house bulkhead is where the fuel tanks go, one each port and starboard. Two 70-gallon tanks provide a cruising range of 1,000 miles at 7.25 knots. The next space aft is for sail and rope stowage, which is relatively light gear.

The two deep-cycle gel cell batteries fit under the main companionway steps next to the head, close to the engine and the electrical panel at the inside helm console. We put two water tanks well to port, one under the berth in the aft cabin along with the water-maker, the other under the port settee in the main saloon. A holding tank fits under the aft end of the starboard settee in the main saloon. The forward ends of both settees can hold dry goods. We’ve kept the heaviest equipment and stowage items pretty much in the middle 60% of the boat length and down low. The next step is to sketch in the windows, rig, keel, and rudders. The exact position of the keel depends on the center of effort of the rig and on the weight balance of everything else about the boat. Throughout this discussion you can see the interplay of the three factors of space, weight, and cost. We need to make them all fit together for the boat to be a success.

Eric Sponberg is a naval architect who designs ocean voyaging sailboats in Newport, R.I. On the web at: www3. edgenet.netsponbergyachtdesign.

y delivery, it seemed oddly appropriate for this last detail to come out right!

By Ocean Navigator