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Comparing trimarans & catamarans.
Trimarans tend to be more performance oriented than catamarans. In part, this is because it’s easier to design a folding trimaran, and as a result Farrier, Corsair, and Dragonfly trimarans had a disproportionate share of the market.
In spite of this and in spite of the fact that many are raced aggressively in windy conditions, capsizes are few, certainly fewer than in equivalent performance catamaran classes. But when they do go over, they do so in different ways.
Trimarans have greater beam than catamarans, making them considerably more resistant to capsize by wind alone, whether gusts or sustained wind. They heel sooner and more than catamaran, giving more warning that they are over powered.
Waves are a different matter. The amas are generally much finer, designed for low resistance when sailing deeply immersed to windward. As a result, trimarans are more susceptible to broach and capsize when broad reaching at high speed or when caught on the beam by a large breaking wave.
In the first case, the boat is sailing fast and overtaking waves. You surf down a nice steep one, into the backside of the next one, the ama buries up to the beam and the boat slows down. The apparent wind increases, the following wave lifts the transom, and the boat slews into a broach. If all sail is instantly eased, the boat will generally come back down, even from scary levels of heel, but not always.
In the second case a large wave breaks under the boat, pulling the leeward ama down and rolling the boat. Catamarans, on the other hand, are more likely to slide sideways when hit by a breaking wave, particularly if the keels are shallow (or raised in the case of daggerboards), because the hulls are too big to be forced under. They simply get dragged to leeward, alerting the crew that it is time to start bearing off the wind.
Another place the numbers leave us short is ama design. In the 70s and 80s, most catamarans were designed with considerable flare in the bow, like other boats of the period. This will keep the bow from burying, right? Nope. When a hull is skinny it can always be driven through a wave, and wide flare causes a rapid increase in drag once submerged, causing the boat to slow and possibly pitchpole.
Hobie Cat sailors know this well. More modern designs either eliminate or minimize this flare, making for more predictable behavior in rough conditions. A classic case is the evolution of Ian Farrier’s designs from bows that flare above the waterline to a wave-piercing shape with little flare, no deck flange, increased forward volume, and reduced rocker (see photos page 18). After more than two decades of designing multihulls, Farrier saw clear advantages of the new bow form. The F-22 is a little faster, but more importantly, it is less prone to broach or pitchpole, allowing it to be driven harder.
Beam and Stability
The stability index goes up with beam. Why isn’t more beam always better? Because as beam increases, a pitchpole off the wind becomes more likely, both under sail and under bare poles. (The optimum length-to-beam ratios is 1.7:1 – 2.2:1 for cats and 1.2:1-1.8:1 for trimarans.) Again, hull shape and buoyancy also play critical roles in averting a pitchpole, so beam alone shouldn’t be regarded as a determining factor.
Drogues and Chutes
While monohull sailors circle the globe without ever needing their drogues and sea anchors, multihulls are more likely to use them. In part, this is because strategies such as heaving to and lying a hull don’t work for multihulls. Moderate beam seas cause an uncomfortable snap-roll, and sailing or laying ahull in a multihull is poor seamanship in beam seas.
Fortunately, drogues work better with multihulls. The boats are lighter, reducing loads. They rise over the waves, like a raft. Dangerous surfing, and the risk of pitchpole and broach that comes with it, is eliminated. There’s no deep keel to trip over to the side and the broad beam increases the lever arm, reducing yawing to a bare minimum.
Speed-limiting drogues are often used by delivery skippers simply to ease the motion and take some work off the autopilot. By keeping her head down, a wind-only capsize becomes extremely unlikely, and rolling stops, making for an easy ride. A properly sized drogue will keep her moving at 4-6 knots, but will not allow surfing, and by extension, pitch poling.
For more information on speed limiting drogues, see “ How Much Drag is a Drogue? ” PS , September 2016.
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M.B. Marsh Marine Design
Length-beam ratio.
L/B = length divided by beam.
Units: Dimensionless.
Usually, the waterline dimensions L WL and B WL are used for monohulls, or for a single hull of a multihull.
What it's used for
Performance.
Larger L/B indicates a slimmer hull. This usually implies less wave-making resistance, and thus more efficient high-speed performance, but also suggests reduced load-carrying ability for a given length.
If a boat can plane, smaller L/B often suggests more efficient performance at low planing speeds. The balance generally tilts in favour of high L/B for fast boats.
Typical ranges of L/B are:
2 to 4 - Small to mid-size planing powerboats.
3 to 4 - Most small to mid-size sailboats and motor yachts, the longer ones generally having higher L/B. Some "skimming dish" racing sailboats also have L/B in this range; their wide beam gives them more initial stability so that they can fly larger sails.
4 to 6 - Fairly long and lean for a monohull. Some large, efficient long-range cruisers fall in this range, along with many racing monohulls.
6 to 10 - Large freighters; main hulls of cruising trimarans; a few very portly cruising catamarans; the lightest and slimmest of large sailing monohulls.
10 to 16 - Fast cruising cats and tris; a few racing multihulls.
Over 16 - Racing multihulls. Such high L/B is conducive to very light, low-drag hulls for race boats, but makes it very hard to get enough room inside the hulls for equipment or living space.
Living Space
If a boat is going to spend most of its time in a marina or at anchor, relatively low L/B implies a larger, more spacious interior and increased carrying capacity when compared to slimmer competitors of the same length. For a boat that must entertain guests at the dock but will rarely be used in rough weather or at high speeds, this may be a significant advantage. The slimmer boat, though, will generally have the advantage when fuel is expensive or when the weather picks up.
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trimaran proportions
Discussion in ' Multihulls ' started by raedy07 , Feb 23, 2016 .
raedy07 New Member
hello, i wonder if there are established 'total lenght to overall width' proportions for trimarans. reason is i would like to decrease total width as it can be very hard to enter our river, it can be really crowded with other boats. the fact that high-tides are at night doesn't help. i would compensate the decrease in width with increase of flotation in the outriggers so the righting moment would not change. is there a problem with this approach? the boat in question is a plywood deepV philippino-type outrigger motor 'banca'. essentially i tri i guess. cheers, Radek
Tom.151 Best boat so far? Crowther Twiggy (32')
raedy07 said: ↑ hello, i wonder if there are established 'total lenght to overall width' proportions for trimarans. reason is i would like to decrease total width as it can be very hard to enter our river, it can be really crowded with other boats. the fact that high-tides are at night doesn't help. i would compensate the decrease in width with increase of flotation in the outriggers so the righting moment would not change. is there a problem with this approach? the boat in question is a plywood deepV philippino-type outrigger motor 'banca'. essentially i tri i guess. cheers, Radek Click to expand...
introduction RAINBOW'S END: LOA 27m, width 16m, draught 1.5m, displacement 8-10tons (?), motor 230hp Mitsubishi 6D22. mainly used for scuba-diving day-trips and also longer excursions. we bought her 2years ago, when prior owner gave up using her as coastal cargo&passenger ship. since then, slowly-slowly, we're converting her to best suit her new destiny i next want to turn my attention to the outriggers; i think they're too low (should be just 'kissing' the surface?) , too buoyant (the number of bamboos was increased from 6 to 10 without anybody being able to explain why) and arrangement should b more V-shape - now they're slapping d waves, doesn't feel right. i suppose that's why 1 cross-beam cracked, as the wood was sound & no matchstick (20x25cm)
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rxcomposite Senior Member
Raedy- This is going to be a steep learning curve for you if you are not a naval architect. The optimum spacings and displacement of the hulls can be found in the library of Cyberiad. Search Leo Lazauskas, a member of this forum. Also in this forum is a free software Michlet, also by Leo. Bamboo outrigers works for the small banca because it is very pliable but when this big banca are made, they use wood which makes it very stiff and prone to cracking. Unfortunately, no rule exist in the Philippines regarding the design/strength of the outrigger. MARINA, the governing body doesn't have it. What you can refer to for the design are the international Class Societies rule. DNV and Lloyd's Register has design standards for crossbeam. You might also find the study conducted in Japan by a Filipino naval architect Dr. Glen Aguilar regarding the forces on the outriggers. This has been extensively discussed in the local forum of Filipino naval architects. PM me so I can show you the leads or how to become a member.
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oldsailor7 Senior Member
Sounds like you should simply beef up your crossbeams. :idea:
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