OFFSHORE CATAMARAN FEATURES

• Length/Beam Ratio... European builders who must sail their boats transatlantic to their bases in the Caribbean, have settled on a minimum length to beam ratio (L/B) of approximately 50%
• Static Stability ... A bad cat (for offshore) might have a static stability in the 25 knot range. A good Cat in the 50-60 knot range
• Pitching is caused by too much weight in the ends of the boat. (SEA-SAW example)Accommodations and storage areas that extend much forward of the mast.Netting forward rather than solid decking is crucial for an offshore cat--for comfort and for safety!

Good Cat, ... Long overhangs fore and aft. Accommodations concentrated in center of boat (weight kept out of the ends). Beam/Length ratio 58%, Static stability about 55 knots.

• Bridge deck clearance...
  • The highest possible bridge deck clearance
  • Adequate beam between the hulls--but not too much distance
  • and smooth transitions between the nacelle and hulls...
• Load Carrying Capacity... This may be the most important point of all. It's not just that the boat goes slower, when you immerse the extra hull depth, the boat gets sluggish. This hull submersion also decreases the bridge deck clearance which promotes hull slamming as well! When you sail offshore you will carry 1,000's of pounds of extra water, fuel, stores, safety equipment and amenities. At a boat show, look at the lower transom step--especially when there are a number of people in the cockpit-- is the step awash (actually underwater?) Not enough load carrying. Is the waterline at the water (or below it) at either end or entirely? Not enough load carrying.
• Hull design and construction;

Monoblock hull and top cover

Underwater level solid GRP

Fixed keel

Reserve buoyancy

Too much deck clearance increases risk of nose diving

Consider the torsional strength

Mast pressure should be diverted to the hulls with A frame to prevent platform colapse.

Beam width is important for mooring and lifting at remote places.

Water tight compartments at the bow and bottom is good for collosions

Tree point rigging gives a possibility for a big main sail. But for open sea sailing it is a big risk. This give more performance but to consider small main and big cenoa is safer with back stay rigging.

Bridle is a must!

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Now, imagine an older design catamaran with narrower hulls (The waterline beam of each hull being narrow.) The hulls don't have the buoyancy to give enough stability , so the designer is forced to gain stability the only way he can--he increases the overall catamaran beam. The trade off? Several and all bad:

• The two bow waves come together under the nacelle as they angle back from the bow and the distance is longer (with the wider hulls) so the wave is bigger--the result is excessive pounding under the bridge deck! In other words, the self generated waves combine with even a modest chop causing pounding in relatively moderate conditions.
• The narrow hulls don't give you the load carrying ability a serious cruiser needs.
• The narrow hulls don't allow the berths to nestle comfortably (and low) in the hulls, forcing berths to be uncomfortably high and overlap the bridge deck in some way in order to make them full size (or allow the charter company to advertise king sized beds!.
• Being so far apart, the hulls sometimes sail in two different wave systems imparting a very uncomfortable motion.
• The extremely fine bows leave insufficient buoyancy forward to prevent nose diving.

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The truth is, a well designed catamaran behaves much like a light displacement mono hull.

The Fountaine Pajot 42, recently delivered transatlantic, rode the front edge of the most recent hurricane with 30 knot winds and averaged almost 16 for 24 hours! (Because storms normally move at 10-12 knots, a fast catamaran has the capability to run away from them-an important safety feature!)

In the Ocean, you need bridge deck clearance. That is, the height of the center part of the boat between the hulls must be as high as possible to avoid wave pounding.

if you point as high as a mono hull, your speed will also be about like a mono hull. The great increases, and better VMG’s are achieved by footing off a bit. Get a good set of polars. Experiment. You will gain a 20% advantage overall, and as much as 50% or more on a reach!

Sailing principal of VMG. That is, in a Cat, your Velocity Made Good to windward is better when you don't point up as high. You sail a little further, but a lot faster and get there faster than a comparable mono hull.

In the ocean with storm conditions you must slow the boat down. If you don’t, the boat will surf faster than the wave system and literally plunge into a wave front, tripping the boat and causing it to flip over-not capsize.

Many marinas have space up near the bulkhead that is gradually being filled in by erosion. Often you see small power boats docked here. We have had excellent success placing catamarans, with their shoal draft, in these spaces--often for lower rates than normal slips!

Everyone hates the heat build up in the main saloon. A boat with more vertical shielded windows enjoys a 10 degree temperature advantage when the sun is high. It also protects the interior from fading and sun exposure.

On the water, it is reserve buoyancy which provides the support and stability...

The question is whether in severe conditions, the extra width of the trimaran counters this obvious

shortcoming? I think not. The short coming is further added to by the very low buoyancy in the

outrigger ends.

Most cats will capsize if heeled to about 70 degrees. The difference on this

design is that the side of the hull is angled outward at 15 degrees. As the cat rotates, the center of

buoyancy moves towards the sheer line, retaining positive righting moment to a higher angle of heel.

The designer has a choice. For the same overall beam, he can provide maximum initial stability with

vertical hull sides or even inwardly canted hulls or he can have maximum ultimate stability by using a

side angle or a knuckle. I believe the later is preferable for ultimate seaworthiness.

There is another safety issue which comes in at hull design stage. The plumb stemmed bow is a

common feature. On the race cats it provides maximum waterline length for that little extra

performance. I like the style of the bow overhang and can quote one instance where the 40 ft cat ran up onto an obstruction, with barely a scratch, where a plumb stem would have been disastrous.

Overall stability, the cruiser, I find the best motto is to stay away from the extremes

unless for a particular owner. One example of this is the high bridge-deck clearance on some cats.

Slamming is something that can never be avoided entirely. Most of the time, the noise and the impact

are the only issue. Cats started with too little clearance.

The rig and style of rig is clearly very important in difficult conditions.This again is where I diverge from the typical cat of today. The fully battened main, small headsail is supported by just the three stays. In order to maintain some fore-stay tension, the shrouds are set well back, restricting the boom angle. When sailing down wind this can cause problems.I prefer the ability to let the boom out a lot further, which we achieve by using runners.

A situation often found in trade wind sailing is a series of squalls passing through at intervals. The

typical cat rig cannot be reefed on a run and needs to round up to do so. This is not what you want to do at the time - and those well aft shrouds make the manoeuvre more hazardous. A number of owners of the three stay arrangement have described how they have hung onto full sail in conditions when they would have been happier reefed but they were not happy to attempt to round up either.

This is where any of the un-stayed or the Freewing Twins rigs come into their own. Firstly, letting the

main out beyond 90 degrees takes much of the pressure off the steering, allows feathering of the sail till the squall has passed or reefing if necessary without change of course. It removes the danger of the unintentional gybe.

The ideal is a rig where the boom is not restricted by stays, which can be used as an anti broach devise.

Most foam or timber cats cannot sink. There are still a few solid glass cats around that can.

The boat itself cannot sink. The life raft is all too often taken to mean just that. Get aboard and it will save your life. Not necessarily so. Stay with the ship if at all possible, should be the rule. The case for some means of leaving the ship, as I see, will only apply in the case of fire or when the boat is drifting out of control into dangerous waters. In the second situation, the life raft is not likely to be of too much use. Can the tender fill the requirements of both situations. I do not believe it is too difficult to provide a suitable design to fulfil this purpose. Some races and some situations do require life rafts on board.

It does not take too much imagination to see that a cat tender would be more efficient in every roll and could be equipped as a life raft.

Structural Loads;

The primary global loads that are being investigated in this research, under guidance from industry, are listed

below in order of priority.

1. Longitudinal bending moment

2. Transverse vertical bending moment in cross-structure,

3. Torsional moment in cross-structure (Pitch connecting moment),

For the transverse vertical bending moment the most severe condition has been found to be at zero speed in

beam seas.

The numerical model shows that the transverse vertical bending moments and shear force are generally largest

in beam seas, while the largest value for pitch connecting moment occurs at a wave heading angle of 60 degrees

for most wave periods.

Why are catamarans so weight sensitive?

It all comes down to drag caused by wetted surface. The most efficient shape for a hull in terms of wetted surface is a hemisphere. The hulls of a catamaran, however, require a long, skinny shape. It is advantageous to avoid excess weight in cats because as weight increases, the long, skinny hulls sink relatively lower into the water than broad-beamed hulls. This results in a disproportionate increase in wetted-surface area, which increases surface drag. Monohull performance will not suffer from overloading as much as a cat will because the hull is closer in shape to a hemisphere. However, it’s important to point out that surface drag affects a cat’s speed only in lighter winds; the adverse affects dissipate as wind speed increases.

The transverse structure of a catamaran encounters some loads that are not present in a monohull. The two hulls need to be held together and resist a variety of forces that are trying to move them relative to each other. The main forces to contend with are those created by mast compression, plus the bending and torsional loads created by one hull trying to rotate relative to the other when one hull is on a different wave than the other. Designers normally incorporate a set of transverse beams or bulkheads that span across the catamaran and are firmly anchored to both hulls. On a cruising catamaran, there is typically one bulkhead under the mast and at least one other that spans across near the aft end of the boat.

How important is bridgedeck clearance?

It all depends on how you plan to use the boat. Bridgedeck clearance is not as critical on smaller coastal-cruising cats that don’t go too far out into the big bad ocean, but on offshore boats that will be subjected to big seas, higher bridgedeck clearance is essential to minimize pounding. We look for bridgedeck clearances on cruising catamarans to be between 4 percent and 6 percent of the length of the boat off the water. This height simply makes it easier for waves to pass harmlessly between the hulls rather than pounding up underneath the bridgedeck. As well as having suitable bridgedeck clearance, a cat should have underbody surfaces that are relatively smooth.

All cats will pound from time to time, especially in steep chop. The critical design factors to consider to reduce bridgedeck slamming are bridgedeck clearance, rig weight and height, bow shape, and keeping the weight out of the ends of the boat. The combination of these factors in correct balance will give a much smoother running boat and give far greater levels of comfort. As anyone who’s experienced catamaran pounding can attest, it’s an unpleasant, nerve-wracking experience. Higher bridgedecks reduce pounding. Less pounding is faster, minimizes structural stress, and, most important, makes for a much more comfortable ride. It should be realized that the designed height of the deckhouse and topsides will be relative to the bridgedeck height, so it is important to balance “sleekness” against the consequences of insufficient bridgedeck clearance.

What factors are involved in designing a cruising cat’s underwater profile?
Start with a length-to-beam ratio for the hull of at least 11 to 1. This is tougher to do in smaller cats, but becomes increasingly easy to achieve as a cat gets bigger. From that point a shape that looks elegant will, in all likelihood, perform well. Because water is so dense, changing the boat’s course quickly uses large amounts of energy. In other words, when designing any shape that runs through the water, the water should not be surprised at the direction that it is being asked to take. I believe that keeping the transoms relatively narrow and maintaining as flat a run aft as possible are important factors in design when I am considering this “no-surprise” rule.

Loads on a monohul is normally greatest on the whole below the waterline where keel attached. The greatest loads on a catamaran are imposed above the waterline on the structure holding the two hulls together. These tortional or twisting loads are created by the different motion of the hulls and can be large

Cats also experience much larger rig loads, on a monohull the heeling of the boat spills wind and deceases the load. Both tortional loads and rig loads only increase as a cat's beam increases. Since trimarans heels more than Cat's less load produces. So hulls must be more strongly reinforced around these areas to withstand the tortional and rig loads.

The helm placement at the cockpit’s aft outboard corner is a compromise between the bulkhead-mounted steering stations of a Prout, Fountaine Pajot, or Perry and Catana’s fully exposed helm station at the aft end of the hulls. From the Lagoon’s helm, you have a good view of the main and some protection from the cockpit bimini, although the cabin top obscures part of the view directly ahead

Judjıng Production Boat quality,

See more than monkey fur such as carpets, liners etc. such material is effective to trap salt mist and as a result mildew. Stick your head to the places where manufacturer would not bother any body would not bother to look. Laminate is neat? free of glass splinters or puddles of resin. Cracked jelcoat is not a good sign. Check highly stresed areas, mast steps, chain plates,