Multihulls In A Nutshell

Posted on Wednesday 8 April 2009

Multihulls have some advantages and disadvantages compared to monohulls, and a lot of how severe those advantages/disadvantages are depends on the exact design you’re looking at. Whether one will be suitable for your purposes depends a lot on what you’re trying to do.

For instance, a lot of the charter market catamarans have trouble sailing in light air and have trouble tacking. This is because these boats were designed with the charter market in mind and are fairly low performance designs, with smaller sailplans and relatively high windage to accommodate a floating condo interior.

Compare them to a high-performance cruising catamaran like a Chris White Atlantic 48 or Gunboat 48, and you’ll see some serious differences in the design and performance characteristics of the boats.

What are the advantages of a cruising multihull?

  • They sail flatter, which means they’re more comfortable in many conditions and often safer. It is harder to fall off a 20’ wide catamaran that heels less than 10˚ than it is to fall off a monohull that is 12’ wide and heeling 25˚.
  • They’re often faster than comparable LOA monohulls
  • They often haver far more cabin space than comparable LOA monohulls, at least in the case of catamarans
  • They generally have a quicker motion than monohulls—which some people tolerate better
  • They’re often designed to be close to unsinkable, since the materials they’re made of are often lighter than water.
  • They often can gunkhole and sail/anchor in shallower waters than comparable LOA monohulls due to having relatively shallow draft.

Disadvantages of multihulls:

  • They can’t support as much food, water, cargo or equipment as monohulls of equal LOA.
  • They’re more weight sensitive than monohulls—when you’re carting around several tons of lead, what does a couple hundred more pounds of gear matter.
  • Badly designed ones don’t sail well in light winds or tack well.
  • Badly designed ones generally don’t sail faster than comparable LOA monohulls
  • They don’t self-right—however, monohulls sink—this is basically a wash, with an advantage to the multihull, since I’d rather be on a floating upside down boat, than a boat that is right-side up and sitting on the bottom of the ocean

Myths:

There is a long-standing myth that multihulls can’t sail to windward or tack well. This is obviously false. The Polynesian islanders explored most of the southern Pacific, and much of it to windward. Also, Dennis Conner’s Stars and Stripes pretty much put the whole idea that multihulls can’t point and can’t tack to bed.

There’s also a myth that catamarans or trimarans are less expensive boats than monohulls of comparable LOA. That’s basically pure crap. Think about it, you’re building two or three hulls and the structures to connect them. A multihull can often be a less expensive boat for a given performance specification, but given the same LOA, the multihulls are generally more expensive, not less.

As an example, my relatively slow cruising trimaran often passes 40’ monohull sailboats. The monohull sailboats that are as fast as my 28’ trimaran are many times the cost of it. However, compared to some of the boats with comparable LOA, it is probably slightly more expensive.

Then there’s the myth that multihulls aren’t seaworthy. This is also pure crap. A properly designed and constructed multihull is very seaworthy. If you don’t believe that, see the most recent speed records for sailing around the world and see what they were set by. Most of them are set by trimarans. This myth comes from the period when many were home-built, using cheap materials and poorly constructed. Many of the multihulls from that time were and are junk.

Different types of multihulls:

There are basically three types of common multihulls. They are the proa, the catamaran and the trimaran.

The Proa:

The least common of these is the proa. A proa is a boat that consists of a main hull with a single smaller outrigger.

There are two types of proas. The Pacific Proa, called that since it was developed and is traditional to the Polynesian Islands, has a ballasted outrigger or ama that is kept to windward. The Atlantic Proa uses a single outrigger or ama that is kept to leeward—essentially a trimaran without the windward ama.

Most do not have a bow or stern in the traditional sense, since the outrigger is kept to windward or leeward all the time. The boat is not tacked, but shunted, where what was formerly the bow becomes the stern and the rudder is moved from one end of the boat to the other.

Generally, the Pacific proa has a crab-claw sail and the mast, by design is in the center of the main hull. A crab-claw sail looks very much like an oversized lateen rig with a boom added. The Atlantic Proa often has more traditional sail plans.

The Pacific Proa generally has the least cargo carrying capacity of all the multihull designs. The Atlantic Proa has a bit more cargo carrying capacity, but usually less than a trimaran due to having fewer hulls.

The Catamaran:

Catamarans are boats that have two equal size hulls and a bridgedeck connecting the two hulls. The hulls are often asymmetrical mirror images of each other and can have shallow keels, centerboards, daggerboards or a combination of keel and board of some sort. They generally have dual rudders, one on each hull, and in the case of large cruising catamarans, often have dual engines and props.

Catamarans, generally, have the most space of the common sailing designs—monohull, catamaran and trimaran. However, they are weight sensitive, and utilizing all the stowage space they provide can hinder the boat’s performance.

Many people are first introduced to catamarans in the form of sport beach cats, like the Hobie Cat. This is somewhat misleading, as the characteristics of a beach sport cat and a cruising catamaran are very, very different. A sport cat has a very high sail area to displacement ratio and is extremely easy to capsize. They go like a bat out of hell, and capsize if you look at them cross-eyed. A cruising catamaran, especially the larger ones have extremely high initial stability and righting moments and are very, very difficult to capsize if sailed properly.

There are basically two different schools of thought for cruising catamarans. The first is like those designed by James Wharram, one of the catamaran design pioneers. Most of his catamarans have little or no structure on the bridgedeck that connects the main hulls. In many cases, his boats are designed with two hulls that are connected together in a somewhat flexible manner. All of the accommodations are in the two hulls.

These boats often have fairly decent sailing characteristics, since they don’t generally have the windage created by a bridgedeck cabin. Stars & Stripes was designed much along these spartan lines, but with a very rigid bridgedeck and hull structure for performance reasons.

The other school of thought is the solid bridgedeck design with a cabin over the bridgedeck. This can be taken to extremes that result in very poor performing boats. However, most modern catamarans come from this school of thought. Compromises have to be made to balance performance and accommodation. Things like sufficient bridgedeck clearance, beam, cabin height, draft, and such are all important and need to be balanced depending on the design’s intended use.

Some boats have a solid bridgedeck from bow to stern. However, this is less common in modern designs, since having an open design for the forward third of the bridgedeck seems to have significant benefits—so you’ll generally see some form of nets or trampolines forward of the main cabin on many more modern designs.

The beam of a modern catamaran is often 50-60% of the LOA. Generally, the smaller the LOA the higher the beam to length ratio is, but there are exceptions, like the Tony Smith designed Geminis, which have a relatively narrow 14’ beam. This was done to help allow the Geminis to be kept in a single slip, rather than requiring them to use two slips or an end slip.

One major issue with catamarans is the rig. Unlike Trimarans and Proas, the catamaran doesn’t have any hull to anchor the mast to. This means that the bridgedeck has to be engineered to withstand the loads caused by the mast. On some smaller, often home-built, catamarans, this issue is avoided by equipping the catamaran with a bi-plane rig, where the boat has two masts, with one located in each hull.

Most catamarans are sloop or cutter rigged.  Some use a mast on each hull and this configuration is often called a bi-plane rig.

The Trimaran:

The trimaran consists of a large main hull and two smaller outriggers. Often, the outriggers or amas, have sufficient buoyancy to float the entire vessel in and of themselves. The hulls on a trimaran tend to be fairly long and narrow, and as such, the trimaran often has the least space of the three common designs, less than monohulls of equal LOA. They also tend to be more weight sensitive than catamarans and monohulls.

The trimarans are basically divided into two categories, IMHO. There are racing designs and cruising designs. The racing designs are often designed to have one of the amas airborne when under sail. They’re often very light and very fast, with little in the way of amenities. The cruising designs generally aren’t designed to have any of the hulls leave the water, and are a good deal slower due to the greater wetted hull surfaces.

A good example of the two different design philosophies are the Corsair 28 and the Telstar 28. The Corsair 28 is about the same LOA as the Telstar, but has a much smaller and lighter cabin. The Corsair 28 doesn’t have standing headroom in its cabin, and is designed with a porta-potty and camping stove as standard accommodations. The Telstar 28 on the other hand has almost 6’ of headroom throughout most of the cabin, a head with holding tank, and a proper propane stove/broiler, optional refrigerator and sink. The Telstar also has a bit less sail area than the lighter Corsair 28. One is clearly a racer and the other a cruiser.

Of the cruising trimarans, there are basically two different schools of thought. One, like the older Jim Brown designed Searunners, has accommodations in the wing-decks that connect the main hull to the amas. These boats are far heavier and have much more windage than the more modern designs, like the Chris White Hammerhead 54, which has all the accommodations in the main hull. There are some boats that bridge these two designs, like the Dick Newick designs, which have much smaller solid wing-decks with very limited accommodations and space contained within.

Trimarans, often have better sailing and performance characteristics than do catamarans. They generally sail a lot more like monohulls, since they heel a bit more and can often pivot on their main hull.

Many of the smaller sport designs, like the Corsairs and the Telstar, fold to allow them to be trailerable without disassembly. However, the folding design generally sacrifices interior space.

The beam on most trimarans is about 60-75% or so of the LOA, but there are extreme examples, like BMW-Oracle’s new trimaran which has essentially a square footprint, with a beam to length ratio of 100%.

Most trimarans are sloop or cutter rigged. Some use rotating wing-masts, but many do not. Some are ketch or yawl rigged. Most use stayed masts, and take advantage of the wider staying base of the multihull design. Some use an unstayed free-standing mast design, but these are not that common.

Multihull Construction:

One thing I’d point out is that the scantlings on multihulls is very different from that of monohulls. Multihulls are often lighter in construction in some ways, due to the lack of need to haul around several tons of ballast. There are basically two different methods of constructing modern multihulls generally.

Cold Molded Wood Composite

The first is what Chris White and the Gougeon brothers have done. That is cold-molded laminated wood composite construction. This generally consists of laying up multiple, very thin layers of wood and laminating them together using epoxy. The wood is often finished off by a covering of fiberglass to give it some added durability, but the bulk of the strength is in the cold molded lamination of the wood, not the fiberglass skin, which is effectively just a surface treatment.

Cold-molded wood composite boats tend to be very rigid and fairly light. One major advantage of cold-molded wood composite is the very high fatigue resistance that the wood construction provides. Properly built, these hulls are almost as low maintenance as a fiberglass hull.

Cored Fiberglass Laminate

The second is cored laminate construction. The most common core materials are PVC foams like Divinylcell and Airex, and end-grain balsa, like Contourcore. In this construction, the strength of the material is the fiberglass or composite skins, and the core adds to the strength and lightness of the laminate.

This is not the same thing as the cold-molded wood composite that I mentioned previously—the main difference being the thin veneers of wood used in the previous method are essentially encapsulated and thoroughly saturated in epoxy, which is not the case with cored fiberglass construction—where the resin is only used to bind the skin to the core material, but does not generally saturate the core material.

Highly loaded areas are often given additional strength via the use of carbon fiber or kevlar. The hulls are often given an inner layer of kevlar to increase the puncture resistance.

Unsinkability

Most modern multihulls, due to the two methods of construction mentioned above, are fairly buoyant and because of the lack of heavy ballast, generally lighter than water over all. If you combine the overall buoyant materials used in constructing the modern multihull with multiple hulls, often with multiple, independent water-tight compartments—you basically end up with an nearly unsinkable boat. While very few multihull designs are self-righting from a knockdown, the fact that most are nearly unsinkable and have enormous initial stability makes them relatively safe vessels.


1 Comment for 'Multihulls In A Nutshell'

  1.  
    March 13, 2010 | 5:26 am
     

    […] recommend you read the post I wrote a while back on multihulls as a starting point. It is located here. Trimarans are typically less roomy than a monohull of equal LOA, while catamarans are typically […]

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