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The Multihull and the Mariner

Written by Iver P. Cooper

The conventional sailing ship has a single hull. However, multihulls—two or more hulls joined together by a deck or poles—can be found in the seventeenth century in both the Indian Ocean and the South Pacific.

The chief advantage of the multihull is extreme lateral stability, which in turn means that it can carry more sail over narrower hulls, and can forego ballast. Thus, it can be built for high speed.

The twentieth century debate over whether multihulls are better than monohulls was about as passionate as the seventeenth century theological ones of the Catholics and the Protestants. (Fortunately, monohullers aren't allowed to burn multihull advocates at the stake.)

Multihull Missions

 

First, I am going to try to whet your appetite for learning more about multihulls by talking about the missions that they conceivably could be designed to perform. After that, I will talk about the types of multihulls, their history, and the nitty-gritty of designing them. Expect this section to be optimistic in tone; I will talk about problems later.

Courier. The racing catamaran designs could be adapted for use as military couriers. Since these carry information, not goods, they can be designed with the light displacement necessary for planing. They will rely on their speed to evade enemy warships.

Landing Craft. Small Hawaiian catamarans were able to reach land despite heavy surf, when conventional ship's boats were forced to stay offshore. (Morton 63).

Passenger/Vehicle/Fast Freight Transport. In 1997, 43% of the high-speed ferry fleet were catamarans. (Sahoo). The benefits of the catamaran design include large deck areas, shallow draft, high stability, and energy-efficient high-speed power cruising (Van Leer). It's not a very large step from a passenger ferry to a troop transport. A multihull could be used, like a nineteenth century clipper, to carry perishable or high value/low density cargo.

Tankers. The high initial stability of multihulls, especially catamarans, gives them an advantage in carrying liquids, which can slop around and destabilize an ordinary ship.

Warships. Multihulls should be more stable firing platforms, and if adequately powered, they are faster than monohulls. And the cross-structure deck area provides room for additional armament. (See "Fighting Ability" below.)

Offshore Drilling. The open deck of a catamaran could be a drilling platform for exploratory drilling in lakes, rivers, and shallow coastal waters. Examples would be Caddo Lake, Louisiana; the delta region in Nigeria; Lake Maracaibo, and the waters off Long Beach, California. The catamaran could have a "well" at its center, where motion is minimized, for lowering a drill. (Van Leer).

Aircraft Carrier. The open deck of a multihull might be handy for launching a tethered balloon. This could be used for both observation and communication. In 2003, the trimaran RV Triton was used as the launch vessel for a manned balloon. (See "Air Support" below.)

Types of Multihulls

 

The catamaran has two equal-size "demihulls," whereas the proa has a main hull and a single, narrower outrigger. In the standard catamaran, the hulls are side-by-side, but hulls can be staggered (bow of one forward of the bow of the other) to reduce wave resistance. In the extreme variant, the "Weinblum," the stern of one hull is forward of the bow of the other. There has also been experimentation with asymmetric hull shapes. (Zaraphonitis).

The trimaran has three hulls. These could be equal-sized, but more usually they are a main hull and two narrower outriggers. The outrigger hulls can be shorter than the main hull, and they can also be set in an arrow configuration with their midsections aft of the main hull midsection. In an extreme version, their bows are aft of the stern of the main hull, as in Cousteau's Alcyone, so what you have is a "monocat," with a single hull fore and two hulls aft.

The four-hulled tetramaran has been the subject of some theoretical analysis. You could have, side-by-side, either four identical hulls, or two main hulls like a catamaran as well as two outriggers like a trimaran. Or a two-by-two, with two leading hulls and two trailing hulls. Or modify that into a blunt arrow ("slice") configuration, by placing the trailing hulls further apart. Or make a "diamond": four identical hulls, with the two central hulls virtually nose-to-tail, and the other two hulls flanking them.

A pentamaran has five hulls. In the Nigel Gee designs, it has one central hull and four short outriggers ("sponsons"). One pair, kayak-shaped, flanks the midsection of the main hull, and the other pair, rowboat-shaped, follows behind it. When the ship is upright, the aft sponsons kiss the water, but the forward sponsons are above it (Gee). I have seen theoretical analysis of a pentamaran with one large, two medium, and two small hulls, all side-by-side.

Multihull History

Pre-Ring of Fire Asia

 

In 1521, Magellan encountered the single outrigger (proa) canoes of Guam, which a Spanish chronicler likened to "dolphins, jumping from wave to wave." (Levinson, 84). In 1616, Le Maire and Schouten saw a Tongan double canoe (catamaran), and another was seen by Abel Tasman in 1643. As for double outrigger canoes (trimarans), Drake saw them in 1579 in the Caroline Islands (Morton 75), and Tasman saw them near New Ireland (59). There is also an Easter Island petroglyph depicting a double canoe, possibly with one mast per hull. (Kane).

It is likely that even in this early period there were numerous variations, from one Pacific island to another, in hull shape and crossbeam design (Harvey 5; Morton 59–76).

The proas were fast; Anson thought they could make twenty knots (Morton 72). And Polynesian multhulls weren't just fishing boats; some were over 100 feet long (63), and could carry several hundred men (63). Eighteenth- and nineteenth-century pirates, from Borneo, Malaya and the Philippines, used proas much as the Barbary Coast corsairs used xebecs; to overwhelm sailing ships that were becalmed.(Morton 76; Warren, 170).

OTL Europe and America

 

The first European proa was built in 1860. Proas are capable of planing; Munroe's thirty-foot 1898 proa could travel at eighteen knots (2.5 times hull speed).

European experiments with catamaran designs date back at least to the 1660s, when Sir William Petty (1623–87) built three single-masted, double-bottomed catamarans, Invention I (1662; 30'LOA, 1.75 tons), Invention II (1662; 30 tons, carries 30 men; hulls 20' x 2'), and The Experiment (60'L, 16 guns). The Invention II won a race with the Holyhead-Dublin packet boat, and The Experiment held its own with three fast vessels of similar size. (IWHR; McMullen 24). The Experiment was able to "come within less than five points of the compass, some say very much less." (A square rigger typically couldn't sail closer than six points of the wind.)

In the 1780s, Patrick Miller constructed a large catamaran (235 tons; 100'L, 31'B, five masts with square sails and five paddle wheels between the hulls). Miller also built the first European trimaran.

Robert Fulton built a peculiar 20-odd gun steam battery, the Demologos (1814). While it has been called a "catamaran" by many popular sources, it would be more accurate to think of it as a monohull divided longitudinally into three compartments; the middle one, which was 15 feet wide, holding the paddlewheels. This was partially flooded as a result of water welling up through a 66 feet long "race" at the bottom. (Photos NH74702, NH65481, NH61883; Bauer 53). The point of the design was to protect the paddlewheels from enemy fire. However, the middle compartment would have greatly reduced the lateral stability provided by the large beam; if the ship heeled, the water would surge, shifting the center of gravity in the destabilizing direction.

Colonel Stevens used a trimaran "horseboat" on the Hoboken ferry line in 1814. In essence, the horses were on a treadmill and this turned the paddle wheels between the center and side hulls. (Baxter. 19). The same line also employed a double-ended catamaran steamboat. The hulls were 80 feet long, 10 feet across, and 5 feet deep, with a hull separation of 10 feet (so the deck across the hulls was 20 feet wide). The waterwheel was between the hulls, and there was a cabin 50 feet long and 10 feet wide. (20).

In 1862, a catamaran snagboat was converted into the ironclad river gunboat USS Benton, and served as Commodore Foote's flagship. But it appears that the builder used the old catamaran hulls "as a pair of bracers," connecting them at both top and bottom. (Konstam 10; Slagle 190).

A very large twin-hulled ship (290 feet long, 60 feet wide), the steamer Castalia, was launched in 1874. She had a draft of only six feet, which let her freely enter "tide-controlled" ports. Nonetheless, she was resistant to rolling. (Rogers 65–7).

In 1876, Nathaniel Herreshoff's Amaryllis (24'L) catamaran won the New York Yacht Club's Centennial race against over thirty monohulls, ranging up to 40' long. The racing officials disqualified it, and later barred all catamarans.

However, multihulls made a serious comeback after World War II, thanks to "new" materials such as aluminum, plywood, fiberglass and most recently carbon fiber composite. They are popular for private racing and cruising, and dominate the passenger ferry market. Nonetheless, few warships and no dedicated cargo ships are multihulls.

Will their fate be different in the new time line?

 

Multihulls in Canon

 

According to canon (1633, Chap. 4), the timberclads used in the Baltic War campaign are catamarans, with paddle wheels positioned in-between their hulls. Unfortunately, we know nothing about their dimensions. Their success will give the catamaran design a certain degree of credibility that it would not have possessed previously.

As for the ironclads, Simpson ". . . built them around what was effectively a double hull. Each of the propulsive pumps—and the tunnel in which it worked—occupied its own individual "pod," separated from the rest of the hull (and from one another) in order to prevent them from being disabled by a single hit or hull breach. It was almost a catamaran effect. . . ." (1634:The Baltic War, Chap. 44). This sounds much like the Demologos or Benton.

The ironclads are "low, squat," "slab-sided," looking something like the CSS Virginia (although probably lacking a ram). The dimensions are 500 tons displacement, about 120 feet long, and minimum draft (with trim tanks empty and centerboard up) of 4 or 5 feet. (1633, Chap. 28; 1634: The Baltic War, Chaps. 31, 52). I would guess a maximum beam of 30–40 feet, because of river travel limitations. I suspect that they are smaller but more heavily armored versions of the Union "City-Class" ironclads.

 

Principal Dimensions

 

The basic parameters of the catamaran are its length, depth of hold, and hull width and spacing. (For a trimaran there is also the size ratio of the hulls.) Its effective beam, for stability purposes, is the sum of the hull widths and the clear space between them. The weight of the ship and its contents will determine its displacement and thus its draft.

Modern pleasure catamarans have length/overall beam ratios about 1.5–2:1 (with racers up to 3:1), and trimarans 1–1.5:1 (Shuttleworth; McMullen 56). A wide beam is advantageous from the standpoint of stability and wave interaction, and disadvantageous in terms of stress on the cross-structure. Generally speaking, the trimaran will have a greater overall beam than a catamaran of equal length, because the stresses are divided over two connecting structures. (Harvey 41, 60).

Pleasure catamaran demihulls are slim, with length/width ratios in the range of 8–16:1, with 10–13:1 being most common. For trimaran main hulls, that ratio is 8–10:1. (Harvey 58ff; White 53ff).

According to a 1990 commercial catamaran survey (two-thirds fast ferries), the ships were mainly 10–40 meters, with length/hull width 6–12:1, depth/draft 1.5–3.5:1, and centerline hull separation of 20–45% demihull length and 150–240% demihull width. (Insel 13ff). Catamaran fast ferries constructed 1994–2003 had waterline lengths of 37–106m, and beams of 9–30m. They carried 150–1200 passengers and 10–312 cars. (Soare 1184).

 

Cross-Structure

 

An open-deck (OD) multihull provides a minimal cross structure; a network of rods, springs or cables that connect the hulls, and some kind of net or trampoline surface so that the crew can scramble from one hull to another. This cross-structure is very vulnerable to the forces that drive the hulls apart (see "Structural Integrity"). Herreshoff provided joints which allowed for a certain amount of independent rolling and pitching of the hulls, thereby reducing the stress on the connections (USP189459; Kemp 353).

The next step up is a flat solid-deck (SD). This offers greater strength, and can carry deck cargo or armament. If the ship rolls over enough to "fly" a hull out of the water, the exposed underside increases the windage and thus the risk of a capsize

The most elaborate cross-structure is a higher profile "bridge-deck" (BD); it provides an enclosed area for crew or cargo and thus has at least two decks (floor and ceiling). Hence, both the roof and floor ("wet deck") of the bridge are holding the hulls together. A high profile deck adds weight and increases windage but, if it remains watertight, provides reserve buoyancy.

A full bridge-deck is one that runs the entire length of the hull and is common in catamaran ferries. A pleasure boat is more likely to have a short, highly streamlined bridge. Usually, there is just netting in the forward third of the ship, so a wave can't push the bow up.

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The so-called SWATH (small waterplane area twin hull) design, invented in 1938 (as Frederick Creed's proposal for an aircraft carrier!), combines an elaborate cross-structure with underwater hulls. This isolates the hulls from wave motion (if the hulls are deep enough), but of course it also increases draft, and you need one long (Duplus-type) or two shorter (Kaimalino-type) wing-shaped vertical struts to connect each hull to the cross-structure. In 2000, about fifty SWATH ships were in operation or construction (Dinsmore). A variable draft SWATH has ballast control, like a submarine, so it can either rise up to enter shallow harbors or sink down to minimize wave response.

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If the side hulls of a trimaran are the same height as the main hull, then a deck can be built across all three.

If the side hulls are shorter, there are three options. First, the side hulls may be suspended directly from the crossdeck, drawing less water than the main hull. (See USS Independence, 2008). Secondly, they can be mounted on vertical or curved struts that come down from the main deck so the keels are even. Finally, they can be mounted on wings that extend from one of the lower decks of the main hull.

You can also build SWA trimarans; you can put just the main hull underwater, or just the outriggers, or all three hulls. (Dubrovsky).

 

Stability and Seaworthiness

 

A ship can heel over (tilt to one side) as a result of crosswind pressure on sails, waves coming against the beam, recoil from firing a broadside, and turning. A self-powered catamaran doesn't have to worry about the first of these, but the others still apply.

The great advantage of the multihull is lateral stability (resistance to heeling). Imagine a monohull ship in the form of a rectangular block that floats in water. If you cut the original hull in half lengthwise, and connect the halves with some lightweight crossbeam, the resulting catamaran has the same length, displacement and draft as the original multihull, yet the metacentric height (initial stability is metacentric height times displacement) is increased. If the space between the halves is equal to the total widths of the hulls, thus doubling the effective beam, the increase in metacentric radius is 4.8-fold (Biran 65), and, if depth is half-breadth, density 0.5, and center of gravity at half-draft, the increase in metacentric height is seven-fold. The improvement in stability is such that a multihull doesn't have to carry ballast.

The Navy conducted a study of the effect of prolonged wave-induced rolling. Only 10 degrees roll cut crew efficiency by 50%; 20 degrees by 80%. (White 23).

On the other hand, a multihull does have some seakeeping weaknesses. If the freeboard (height of main deck above water) is unchanged, then simple geometry dictates that because of the greater beam, the deck is immersed at a lesser angle, and stability decreases after that immersion angle is passed. The heel angle of maximum stability might be 6 degrees for a catamaran, 20 for a trimaran, and 60 for a monohull (Shuttleworth).

Likewise, multihulls are likely to have a lower angle of vanishing stability (heel angle at which the ship no longer has a tendency to right itself) than a corresponding monohull. Nonetheless, the dynamic stability (total work which a wind or wave must do in order to capsize the ship might be 50% more for a multihull than a monohull.

Monohull warships have a relatively high center of gravity because of armor and armament. This would reduce their range of stability if they weren't given a large metacentric height to compensate. This makes them "stiff"; if they heel over, they will snap back too fast and this can cause discomfort for those on board, or even dismast a sailing ship. Having a short roll period also means that they are more likely to encounter sea conditions in which the wave period matches the roll period, and that exacerbates the rolling motion. (Atwood, 69ff).

Multihulls naturally have a large metacentric height. However, the roll period is lengthened if there's a lot of weight distant from the roll axis. That's certainly true of an open-deck catamaran, which has almost all its weight in the hulls (Shuttleworth), and it's true to a lesser degree of trimarans and catamarans with elaborate crossdecks. (A tall mast helps, too. White 194).

A ship pitches as well as rolls. Because ships are longer than they are broad, they have more resistance to pitching. Because multihulls are squatter than monohulls, their pitch period is closer to their roll period, and this can result in a very unpleasant motion called "corkscrewing." This can be alleviated by redistribution of weight.

A multihull running with too much sail up can pitchpole (somersault forwards). Pitchpoling can also be caused by the bow being buried in rough seas, which is why you don't want to have a simple solid crossdeck extend all the way to the ends.

Monohull proponents complain that if a multihull capsizes, its multihull's lateral stability becomes a disadvantage, as it's just as stable upside-down as rightside-up. Multihull fans retort that a monohull is just as stable on the ocean floor as on the surface.

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SWATH designs will have a larger beam than a conventional catamaran of the same displacement to compensate for their relatively small waterplane area, which determines their initial resistance to wind-induced heeling. (swath.com). However, because the SWATH hulls are deep underwater, they are virtually immune to wave action.

 

Propulsion

 

Multihulls can be powered by the wind (as captured by sails) or be self-powered by combustion of fossil fuel.

Trimarans are essentially monohulls with outriggers, and the mast(s), if any, will be placed on the main hull.

For sailing catamarans, there are two choices. First, to place the mast in the center, i.e., on the cross-structure. If the multihull is open-deck, the main beam, on which the mast rests, must be strong enough to support it. Indeed, the Hawaiians, working with wood, stepped the mast on a longitudinal beam that distributed the downward thrust over three or more crossbeams. (Kane).

The second option is to put the mast(s) on the hulls. In the "biplane" (parallel) mast array (DUO 425), there's one mast on each main hull. The problem is that the masts can only be stayed on the "inboard" side. A variant on this are A-shaped "bipod" masts (SMG50), in which the masts are mounted on the hulls but meet above the crossdeck.

It is worth noting that by putting two short masts on the hulls, rather one long one on the cross-deck, you lower the center of effort and therefore reduce the "heeling" action of the wind (see Stability).

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The first transatlantic record-setting self-powered ships used a steam engine to drive a paddle wheel (Great Western 1838, 14 knots); later ones powered a propeller (Napoleon 1857, 20 knots). The steam engine was replaced, first with a steam turbine (Mauretania 1907, 25 knots), then with a diesel engine (Normandie 1935, 30 knots). Large warships in which the was driven by a gas turbine (which has a high power to weight ratio) appeared on the scene in the Sixties. Finally, the propeller was replaced with a hydrojet, powered by a diesel engine (catamaran Hoverspeed Great Britain 1990, 37 knots) or a gas turbine (monohull Destriero 1992, 53 knots). (Pinder 143).

Paddlewheels can be placed between the hulls for protection, but propellers and hydrojets would probably be mounted on the hulls themselves. This is problematic in the case of SWATH designs; "until very recently it has been very difficult to package much power in the submerged bodies." (Friedman).

Simpson's timberclads had paddlewheels, whereas the ironclads "used powerful diesel-driven pumps scavenged from the Grantville coal mine to provide hydro-jet propulsion."

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On a hybrid (sail/steam) ship, it's advantageous to be able to lift the paddlewheel or propeller out of the water when sails are in use, to reduce resistance. Patrick Miller's Edinburgh trimaran had paddlewheels whose immersion could be varied. And the CSS Alabama had liftable screws.

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On a hydrofoil-supported catamaran ("hysucat"), foils generate lift, like airplane wings, once the ship is in motion (Hoppe). Foils can be mounted below more conventional underwater hulls (see sailing biplane catamarans Techniques Avancées and Spitfire), or suspended ("midfoil") from the cross-structure. To maintain pitch stability, two foils are better than one (Loveday 5).

 

Speed

 

In light winds, sailing multihulls are slower than monohulls of equal displacement because the two hulls experience greater frictional resistance. As winds pick up, multihulls can surpass the monohulls, because they can safely carry more sail (if the hulls are spaced far enough apart). Because of the great beam, the trimaran can carry more sail without capsizing, and thus has a higher maximum speed than a catamaran of equal length. However, three hulls cost more than two, and on a fixed budget, you may need to choose between a longer catamaran and a shorter trimaran.

For racing sail, the trimarans are superior in light winds (the outriggers are barely in the water and resistance is like that of a monohull) and the catamarans beat them when winds strengthen. (Howard 5).

In 1905, the three masted 56m schooner Atlantic (300 tonnes, 41.18 m LWL, 8.85m beam, 1720m² sail; Ancko) established a record run of 12 days, four hours (10 knots) for the 2950 mile run from New York to Falmouth. The record, first broken by a trimaran in 1980, has been held exclusively by catamarans since 1988, and is now (2006) held by the catamaran Orange II, which made the passage in 4 days, 8 hours (28 knots). The same ship also holds the round-the-world record (50 days, 16 hours) and the 24 hour record (706 miles). Orange II has length 36.8m, beam 18m, mast height 45m, and 700 m² upwind and 1000 m² downwind sail area. (multiplast; Bernard Gallay Yacht Brokerage).

Of course, Orange II is strictly a racer. Her displacement was 28 tons light, 30 tons fully loaded, for her 2005 round-the-world record journey (50 days, 16 hours). And to achieve that low displacement, her hull, deck and sparring are all carbon composite. A catamaran intended to carry cannon or cargo would have a much lower sail power-to-displacement ratio, and wouldn't have this turn of speed.

That said, any monohull whose maximum speed is limited by its ability to safely carry sail rather than its hull speed could be improved on by "slicing it in half" and redesigning it as a multihull.

The problem of frictional resistance can be alleviated by using a hull of semicircular cross-section (McMullen 35), the circle being the shape that provides the lowest ratio of perimeter to area and thus the least wetted surface for a given carrying capacity. (For stability reasons, a monohull can't have that shape unless it's heavily ballasted.) Even then, two hulls will experience about 40% more frictional resistance than a monohull of the same displacement and length (Stevens). The world's largest sailing catamaran had about 50% more wetted surface area than an equal length monohull, despite having a smaller draft. (Lawless).

The disparity is smaller with low-displacement racing designs. If you take into account both elimination of ballast and beam/draft ratios, a catamaran equivalent of a YD40 yacht (ballast is 40% displacement) might have an increase in wetted surface area of only 8%.

On the other hand, the hulls of a catamaran might be made very narrow, to reduce wave resistance further, and this increases the ratio of surface to enclosed volume. This is taken to an extreme in the SWATH designs, where the wavemaking is by very narrow struts, reducing wavemaking ...