In recent years we’ve seen a surge in interest in multihulls. Thirty years ago, when my experience with cruising multihulls began, nearly all of the skippers served an apprenticeship with beach cats, learning their quirks by the seat of their pants. They hiked out on trapezes and flew head-over-heels past their pitch-pole prone Hobie 16s, until they learned the importance of keeping weight way aft on a reach and bearing off when the lee bow began to porpoise.
By contrast, the new generation of big cat buyers skipped this learning process, learning on monohulls or even choosing a big stable cat as their first boat. Heck, nobody even builds real beach cats anymore, only pumped up racing machines and rotomolded resort toys. So we’re guessing there are a few things these first-time cruising multihull sailors don’t know, even if they have sailed cruising cats before.
It is extremely hard to capsize a modern cruising cat. Either a basic disregard for seamanship or extreme weather is required. But no matter what the salesman tells you (“none of our boats have ever …”), it can happen. A strong gust with sail up or a breaking wave in a survival storm can do it. And when a multihull goes over, they don’t come back.
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.
Capsize Case Studies
Knock wood, we’ve never capsized a cruising multihull (beach cat—plenty of times), but we have pushed them to the edge of the envelope, watched bows bury, and flown multi-ton hulls to see just how the boat liked it and how fast she would go. We’re going to tell you about these experiences and what can be learned from them, so you don’t have to try it.
First, it helps to examine a few examples of some big multihull capsizes.
Techtronics 35 catamaran, John Shuttleworth design
This dramatic pitchpole occurred in a strong breeze some 30 years ago. In order to combine both great speed and reasonable accommodation, the designer incorporated considerable flare just above the waterline, resulting in hulls that were skinny and efficient in most conditions, but wide when driven under water in steep chop.
The boat was sailing fast near Nova Scotia, regularly overtaking waves. The bows plowed into a backside of a particularly steep wave, the submerged drag was huge, and the boat stopped on a dime. At the same time, the apparent wind went from about 15 knots into the high 20s, tripling the force on the sails and rapidly lifting the stern over the bow. Some crew were injured, but they all survived.
PDQ 32 Catamaran
On July 4, 2010, the boat’s new owners had scheduled time to deliver their new-to-them boat up the northern California coast. A strong gale was predicted, but against all advice, they left anyway. The boat turned sideways to the confused seas and a breaking wave on the beam capsized the boat. There were no injuries, and the boat was recovered with only moderate damage a few weeks later. Repaired, she is still sailing.
Another PDQ 32 was capsized in the Virgin Islands when a solo sailor went below to tend to something and sailed out of the lee of the island and into a reinforced trade wind.
Sustaining speed with wider tacking angles will help overcome leeway.
Cruising cats can’t go to windward. That’s the rumor, and there’s a kernel of truth to it. Most lack deep keels or dagger boards and ex-charter cats are tragically under canvassed for lighter wind areas, a nod to near universal lack of multihull experience among charter skippers. Gotta keep them safe. But there are a few tricks that make the worst pig passable and the better cats downright weatherly. Those of you that learned your craft racing Hobies and Prindles know most of this stuff, but for the rest of you cruising cat sailors, there’s some stuff the owner’s manual leaves out.
“Tune” the Mast
Having no backstay means that the forestay cannot be kept tight unless you want to turn your boat into a banana and over stress the cap shrouds. Although the spreaders are swept back, they are designed primarily for side force with just a bit of pull on the forestay. The real forestay tension comes from mainsheet tension.
Why is it so important to keep the forestay stay tight? Leeward sag forces cloth into the luff of the genoa, making it fuller and blunting the entry into the wind. The draft moves aft, the slot is pinched, and aerodynamic drag increases. Even worse, leeway (sideslip) increases, further increasing drag and sliding you away from your destination. Sailing a cruising cat to windward is about fine tuning the lift to drag ratio, not just finding more power.
How do you avoid easing the mainsheet in strong winds? First, ease the traveler instead. To avoid pinching the slot, keep the main outhaul tight to flatten the lower portion of the main. Use a smaller jib or roll up some genoa; overlap closes the slot. Reef if need be; it is better to keep a smaller mainsail tight than to drag a loose mainsail upwind, with the resultant loose forestay and clogged slot. You will see monos with the main twisted off in a blow. Ignore them, they are not cruising cats. It is also physically much easier to play the traveler than the main sheet. Be glad you have a wide one.
Check Sheeting Angles
Very likely you do not have enough keel area to support large headsails. As a result, you don’t want the tight genoa lead angles of a deep keeled monohull. All you’ll do is sail sideways. Too loose, on the other hand, and you can’t point. In general, 7-10 degrees is discussed for monos that want to pinch up to 40 degrees true, but 14-16 degrees makes more sense for cruising cats that will sail at no less than 50 degrees true. Rig up some temporary barber haulers and experiment. Then install a permanent Barber-hauler; see “Try a Barber Hauler for Better Sail Trim,” Practical Sailor, September 2019.
The result will be slightly wider tacking angles, perhaps 105 degrees including leeway, but this will be faster for you. You don’t have the same hull speed limit, so let that work for you. Just don’t get tempted off onto a reach; you need to steer with the jib not far from luffing.
Watch the fore/aft lead position as well. You want the jib to twist off to match the main. Typically it should be right on the spreaders, but that depends on the spreaders. If you have aft swept shrouds, you may need to roll up a little genoa, to 110% max.
Use your Tell-Tales
On the jib there can be tell-tale ribbons all over, but on the main the only ones that count are on the leech. Keep all but the top one streaming aft. Telltales on the body of sail are confused by either mast turbulence (windward side) or pasted down by jib flow (leeward side) and won’t tell you much. But if the leach telltales suck around to leeward you are over sheeted.
Keep Your Bottom Clean
It’s not just about speed, it’s also pointing angle. Anything that robs speed also makes you go sideways, since with less flow over the foil there will be less lift. Flow over the foils themselves will be turbulent. Nothing slows you down like a dirty bottom.
Push hard, but reef when you need to. You will have the greatest lift vs. windage ratio when you are driving hard. That said, it’s smart to reef most cruising cats well before they lift a hull to avoid overloading the keels. If you are feathering in the lulls or allowing sails to twist off, it’s time to reef.
Pinching (pointing to high) doesn’t work for cats. Get them moving, let the helm get a little lighter (the result of good flow over the rudder and keel), and then head up until the feeling begins to falter. How do you know when it’s right? Experiment with tacking angles (GPS not compass, because you want to include leeway in your figuring) and speed until the pair feel optimized. With a genoa and full main trimmed in well, inside tracks and modified keels, and relatively smooth water, our test PDQ can tack through 100 degrees with the boat on autopilot. Hand steering can do a little better, though it’s not actually faster to windward. If we reef or use the self-tacking jib, that might open up to 110-115 degrees, depending on wave conditions. Reefing the main works better than rolling up jib.
Boats with daggerboards or centerboards. The comments about keeping a tight forestay and importance of a clean bottom are universal. But the reduction in leeway will allow you to point up a little higher, as high as monohulls if you want to. But if you point as high as you can, you won’t go any faster than similar monohulls, and quite probably slower. As a general rule, tacking through less than 90 degrees, even though possible, is not the best strategy. A slightly wider angle, such as 100 degrees, will give a big jump in boat speed with very little leeway.
Chris White Custom 57
In November 2016, winds had been blowing 25-30 knots in stormy conditions about 400 miles north of the Dominican Republic. The main had two reefs in, and the boat was reaching under control at moderate speed when a microburst hit, causing the boat to capsize on its beam. There were no serious injuries.
Another Chris White 57 capsized on July 31, 2010. It had been blowing 18-20 knots and the main had a single reef. The autopilot steered. The wind jumped to 62 knots in a squall and changed direction so quickly that no autopilot could be expected to correct in time.
In 2018, the 34-foot catamaran was sailing in the Gulf of Mexico under full sail at about 6 knots in a 10-15 knot breeze. Squalls had been reported on the VHF. The crew could see a squall line, and decided to run for cover. Before they could get the sails down, the gust front hit, the wind shifted 180 degrees, and the boat quickly went over.
38-foot Roger Simpson Design
The catamaran Ramtha was hit head-on by the infamous Queen’s Birthday storm in 1994. The mainsail was blown out, and steering was lost. Lacking any control the crew was taken off the boat, and the boat was recovered basically unharmed 2 weeks later. A Catalac catamaran caught in the same storm trailed a drogue and came through unharmed. Of the eight vessels that called for help, two were multihulls. Twenty-one sailors were rescued, three aboard the monohull Quartermaster were lost at sea.
15 meter Marsaudon Ts
Hallucine capsized off Portugal on November 11 of this year. This is a high performance cat, in the same general category as the familiar Gunboat series. It was well reefed and the winds were only 16-20 knots. According to crew, it struck a submerged object, and the sudden deceleration caused the boat to capsize.
Multihulls We’ve Sailed
Clearly seamanship is a factor in all of our the previous examples. The watch needs to be vigilant and active. Keeping up any sail during squally weather can be risky. Even in the generally benign tropics, nature quickly can whip up a fury. But it is also true that design choices can impact risk of capsize. Let’s see what the numbers can tell us, and what requires a deeper look.
We’ve experienced a number of capsizes both racing and while driving hard in these popular 23-foot catamarans. The combination of light displacement and full bow sections make pitchpoling unlikely, and the result is very high speed potential when broad reaching. Unfortunately, a narrow beam, light weight, and powerful rig result in a low stability factor. The potential for capsize is real when too much sail is up and apparent wind is directly on the beam. The boat can lift a hull in 12 knots true. This makes for exciting sailing when you bring your A-game, but limits the boat to coastal sailing.
Corsair F-24 MK I trimaran
Small and well canvased, these boats can capsize if driven hard (which they often are), but they are broad beamed, short-masted, and designed for windy sailing areas. F-24s are slower off wind than the Stiletto, in part because of greater weight and reduced sail area, but also because the main hull has more rocker and does not plane as well. They are faster to weather and point considerably higher than a Stiletto (90-degree tacking angle vs. 110 degrees). This is the result of greater beam, a more efficient centerboard design, and slender amas that are easily driven in displacement mode. The boat is quite forgiving if reefed.
Going purely by the numbers, this boat seems nearly identical to the F-24. In practice, they sail quite differently. The Dash uses a dagger board instead of centerboard, which is both more hydrodynamic and faster, but more vulnerable to damage if grounded at high speed.
The rotating mast adds power that is not reflected in the numbers. The bridgedeck clearance is higher above the waterline, reducing water drag from wave strikes. The wave-piercing amas create greater stability up wind and off the wind. The result is a boat that is slightly faster than the original F-24 and can be driven much harder off the wind without fear of pitchpole or broach.
Without proper testing, calculating stability yields only a rough picture.
Evaluating multihull performance based on design numbers is a bit more complicated than it is with ballasted, displacement monohulls, whose speed is generally limited by hull form. [Editor’s note: The formula for Performance Index, PI has been updated from the one that originally appeared in the February 2021 issue of Practical Sailor.
The following definitions of units apply to the adjacent table:
SA = sail area in square feet
D (displacement) = weight in pounds
LWL = length of waterline in feet
HCOE = height of sail center of effort above the waterline in feet
B = beam in feet
BCL = beam at the centerline of the hulls in feet.
Since a multihull pivots around the centerline beam, the overall beam is off the point and is not used in formulas. Calculate by subtracting the individual hull beam from the overall beam.
SD ratio = SA/(D/64)^0.66
This ratio gives a measure of relative speed potential on flat water for monohulls, but it doesn’t really work for multihulls.
Bruce number = (SA)^0.5/(D)^0.333
Basically this is the SD ratio for multihulls, it gives a better fit.
Performance index = (SA/HCOE)^0.5 x (D/1000)^0.166
By including the height of the COE and displacement, this ratio reflects the ability of the boat to use that power to sail fast, but it understates the importance of stability to the cruiser.
Stability factor = 9.8*((0.5*BCL*D)/(SA*HCOE))^0.5
This approximates the wind strength in knots required to lift a hull and includes a 40% gust factor. In the adjacent data sheet, we compare the formula’s predicted stability to observed behavior. Based on our experience on the boats represented, the results are roughly accurate.
Ama buoyancy = expressed as a % of total displacement.
Look for ama buoyancy greater than 150% of displacement, and 200 is better. Some early trimaran designs had less than 100 percent buoyancy and would capsize well before flying the center hull. They exhibited high submerged drag when pressed hard and were prone to capsize in breaking waves.
Modern tris have ama buoyancy between 150 and 200 percent of displacement and can fly the center hull, though even racing boats try to keep the center hull still touching. In addition, as a trimaran heels, the downward pressure of wind on the sail increases, increasing the risk of capsize. The initial heel on a trimaran is more than it is on catamarans, and all of that downward force pushes the ama even deeper in the water. Thus, like monohulls, it usually makes sense to keep heel moderate.
These numbers can only be used to predict the rough characteristics of a boat and must be supplemented by experience.
This is the first real cruising multihull in our lineup. A few have capsized. One was the result of the skipper pushing too hard in very gusty conditions with no one on watch. The other occurred when a crew unfamiliar with the boat ignored local wisdom and set sail into near gale conditions.
Although the speed potential of the PDQ 32 and the F-24 are very similar, and the stability index is not very different, the feel in rough conditions is more stable, the result of much greater weight and fuller hull sections.
Like most cruising cats, the PDQs hulls are relatively full in order to provide accommodation space, and as a result, driving them under is difficult. The increased weight slows the motion and damps the impact of gusts. Yes, you can fly a hull in about 25 knots apparent wind (we proved this during testing on flat water with steady winds), and she’ll go 8-9 knots to weather doing it, but this is not something you should ever do with a cruising cat.
Stability by the Numbers
The “stability factor” in the table above (row 14) is based on flatwater conditions, and ignores two additional factors. Unlike monohulls, the wind will press on the underside of the bridgedeck of a multihull once it passes about 25 degrees of heel, pushing it up and over. This can happen quite suddenly when the boat flies off a wave and the underside is suddenly exposed to wind blowing up the slope of the wave. A breaking wave also adds rotational momentum, pitching the windward hull upwards.
Multihulls by the Numbers
|CORSAIR F-24 MKI
|336. sq. ft.
|340 sq. ft
|540 sq. ft
|850 sq. ft
|1,150 sq. ft
|1,370 sq. ft
|2,850 sq. ft
|HEIGHT OF CENTER OF EFFORT (HCOE)
|OBSERVED HULL LIFT (TRUE WIND SPEED)
|WINDWARD SPEEDAT HULL LIFT
|REACHING BOAT SPEED
|Capsized in wind
|Capsized in wind
|Capsized in gale
|Capsized in Squall
Autopilot is a common thread in many capsizes. The gust “came out of no place…” No it didn’t. A beach cat sailor never trusts gusty winds. The autopilot should be disengaged windspeeds and a constant sheet watch is mandatory when gusts reach 30-40 percent of those required to fly a hull, and even sooner if there are tall clouds in the neighborhood. Reef early if a helm watch is too much trouble.
“But surely the sails will blow first, before the boat can capsize?” That would be an expensive lesson, but more to the point, history tells us that well-built sails won’t blow.
“Surely the rig will fail before I can lift a hull?” Again, that could only be the result of appallingly poor design, since a rig that weak will not last offshore and could not be depended on in a storm. Furthermore, good seamanship requires that you be able to put the full power of the rig to work if beating off a lee shore becomes necessary.
Keeping both hulls in the water is up to you. Fortunately, under bare poles and on relatively flat water even smaller cruising cats can take 70 knots on the beam without lifting … but we don’t set out to test that theory, because once it blows for a while over even 40 knots, the real risk is waves.
Everything critical to safety in a blow we learned on beach cats. Like riding a bike, or—better yet—riding a bike off-road, there are lessons learned the hard way, and those lessons stay learned. If you’ve been launched into a pitchpole a few times, the feeling you get just before things go wrong becomes ingrained.
Perhaps you are of a mature age and believe you monohull skills are more than enough to see you through. If you never sail aggressively or get caught in serious weather, you’re probably right.
However, if there’s a cruising cat in your future, a season spent dialing in a beach cat will be time well spent. Certainly, such experience should be a prerequisite for anyone buying a performance multihull. The statement might be a little pointed, but it just makes sense.
Capsize by Wind Alone
Capsizing by wind alone is uncommon on cruising multihulls. Occasionally a performance boat will go over in squally weather. The crew could easily have reefed down or gone to bare poles, but they clung to the idea that they are a sail boat, and a big cat feels so stable under sail—right up until a hull lifts.
Because a multihull cannot risk a knockdown (since that is a capsize), if a squall line is tall and dark, the smart multihull sailors drops all sail. Yes, you could feather up wind, but if the wind shifts suddenly, as gusts often do, the boat may not turn fast enough. Off the wind, few multihulls that can take a violent microburst and not risk a pitchpole. When a squall threatens, why risk a torn sail for a few moments of fast sailing?
You can’t go by angle of heel alone because of wave action. Cat instability begins with the position of the windward hull. Is it flying off waves?
A trimaran’s telltale is submersion of leeward ama. Is the leeward ama more than 30-40 percent under water? The maximum righting angles is a 12-15 degrees for cats and 25-30 degrees for trimarans, but that is on flat water. Once the weather is up, observation of motion becomes far more important. Is the boat falling into a deep trough, or is at about to launch off a steep wave and fly?
Just as monohulls can surprise a new sailor by rounding up and broaching in a breeze, multihulls have a few odd habits that only present themselves just before things go wrong. Excuse the repetition, but the best way to learn to instinctively recognize these signs is by sailing small multihulls.
Because of the great beam, instead of developing weather helm as they begin to fly a hull, multihulls can suddenly develop lee helm, causing the boat to bear away and power up at the worst possible moment. This is because the center of drag moves to the lee hull, while the center of drive remains in the center, causing the boat to bear away.
If the boat is a trimaran, with only a center rudder, this rounding up occurs just as steering goes away. This video of a MOD 70 capsize shows how subtle the early warning signs can be (www.youtube.com/watch?v=CI2iIY61Lc8).
Off the wind, the effect can be the reverse. The lee hull begins to bury, and you decide it is time to bear off, but the submerged lee bow acts like a forward rudder. It moves the center of effort far forward and prevents any turn to leeward. Nearly all trimarans will do this, because the amas are so fine. The solution is to bear away early, before the ama buries—or better yet, to reef.
We’re not trying to scare you off multi-hulls. Far from it. As you can probably tell, I am truly addicted. Modern designs have well-established reputation seaworthiness.
But multihull seaworthiness and seamanship are different from monohulls, and some of those differences are only apparent when you press the boat very hard, harder than will ever experience in normal weather and outside of hard racing. These subtle differences have caught experienced sailors by surprise, especially if their prior experience involved only monohulls or cruising multihulls that were never pressed to the limit.
Although the numbers only tell part of the story, pay attention to a boat’s stability index. You really don’t want an offshore cruising boat that needs to be reefed below 22-25 knots apparent. Faster boats can be enjoyable, but they require earlier reefing and a more active sailing style.
When squalls threaten or the waves get big, take the appropriate actions and take them early, understanding that things happen faster. And don’t forget: knockdowns are not recoverable. It is satisfying to have a boat that has a liferaft-like stability, as long as you understand how to use it.
Technical Editor Drew Frye is the author of “Rigging Modern Anchors.” He blogs at www.blogspot/sail-delmarva.com