Actually, it is incomprehensible that two-hulled motor yachts are so rare to find on our waters. Catamarans are usually extremely fast and are therefore often the first choice when it comes to top speeds, as in racing. Union Internationale Motonautique (UIM) Formula 2 is just one example. XCATS have established themselves internationally. These are “Xtreme Catamarans” from 7.6 to 10 m in length that compete in races in many countries. Another remarkable example of high-speed use at sea is the “Adastra”, a futuristic trimaran (three-hull boat) 42.5 m long and 16 m wide.
Is there a global market for double-hull motor yachts? Catamarans offered on the market mostly have larger dimensions and come from abroad. Motor catamarans that are chartered on our lakes and waterways are often relatively heavy and have little power. Above all, they should offer space for as many guests as possible, neglecting their sporty character.
Successful examples of the use of catamarans are solar boats with e-drives for inland waters, but also for use at sea. Large panel areas and low driving resistance are a must here. The “Turanor PlanetSolar”, built in 2010 with an area for solar cells of 537 square meters, shows what this looks like if you do it right. The catamaran achieved a constant speed of seven knots over long distances with electric motors alone. The “PlanetSolar” crossed the Atlantic and the South Seas and at the same time proved the seaworthiness of machine-driven catamarans.
In recent years, slow, but unfortunately not very maneuverable platforms with two floating bodies with a length-to-width ratio of almost 1 to 1 have spread on inland waters. Not only on the Havel are houseboats or less attractive platforms with shelter, powered by a low-power outboard motor.
Catamaran ferries are no longer a rarity in commercial shipping. They are much faster than conventional monohull boats with low energy consumption. In addition, many catamarans or SWATH (Small Waterplane Area Twin Hull) are currently used as work ships because of their great heeling stability and favorable sea behavior. An interesting example of this is the 25 m long and 13 m wide research vessel “Estonia”.
In sailing, catamarans or trimarans keep setting new speed records over long stretches of the sea. They can reach speeds of up to 50 knots. When the New Zealanders competed for the first time at the America’s Cup in 1988 with a 90-foot mega-sloop against a smaller US catamaran, the inferiority of the monohull was almost embarrassing and caused a stir.
The most important advantage of the catamaran is the low flow resistance of the individual submerged hulls with a large length-to-width ratio; and this is especially true for high boat speeds. As is well known, the wave resistance is decisive for the hull speed of a boat. The portion of the wave resistance in the vicinity of the hull speed is around 70% in the monohull and reaches its maximum when the plane moves on. The longer, narrower and slimmer the individual hull of the catamaran, the lower the wave resistance. Theoretically, a thin plate pulled through the water would have zero wave resistance!
The ratio of the length to the width of a monohull planing boat is between 2.5 and 3.5. For good gliding properties, you need wide rear ribs with little tail rise. In contrast to this, values of L: B up to 15 (!) Are achieved for the individual boat hull or swimmer of the catamaran, which largely minimizes the wave resistance. The displacement of the yacht should be as small as possible. With a few exceptions, catamarans are also used as displacement hulls beyond the “hull speed” – which is of minor importance for cats with a very slim underwater hull.
Another advantage of catamarans is that, in contrast to the glider, the stern is very slim and guarantees a low-resistance outflow without, or with very little, detachment. Catamaran motor yachts of this type are extremely energy efficient even at high speeds.
Of course, restrictions with regard to flow resistance should not go unmentioned. The swimmers of cruiser catamarans should often be “furnished”, whereby the choice of a larger waterline width is inevitable. Although in these cases the wave resistance increases compared to a catamaran with slim swimmers, the aspect ratio L: B of 8 to 9 is still much more favorable than with a monohull.
Really heavy catamarans require large-volume floats, which also have a negative effect on the drag. With the same displacement of the catamaran and monohull, the surfaces wetted by the water and thus the proportion of frictional resistance are greater on the catamaran. However, this feature is particularly important at low speeds and has only a moderate influence on the energy efficiency of double-hulled motor boats with a smooth underwater hull.
Because of the large overall width of catamarans (L: B ≈ 1.7 to 2.5: 1), the moments of stability increase sharply with increasing heel and prevent larger heeling angles and rolling movements in the sea. The same applies to the trim. However, at higher speeds on deck it can get really wet. Nodding movements (pitching) cannot always be prevented if the wave frequency is unfavorable. A prerequisite for good seaworthiness is that the decks and the beams connecting the hulls ensure sufficient clearance to the water surface. Not only “PlanetSolar” and “Adastra”, but also many sailing catamarans on their spectacular circumnavigations of the world have proven that motor catamarans and trimarans are capable of high seas.
While monohull motor boats often stay in the harbor from 5 to 6 wind strengths, since heavy rolling, pitching and yawing movements of the boat are unavoidable at sea depending on the direction of the sea and all accessories are mixed up, the catamaran lies largely calm in the sea.
Catamarans have excellent steering behavior. They run straight ahead as if on rails, provided that the rudders in the starting position are adjusted by one to two degrees, depending on the direction of rotation. There is hardly any yaw movement or breaking off course. However, this has the consequence that the turning circle will be larger compared to the monohull at full speed. Whether this is really a major disadvantage remains to be seen.
On the other hand, catamarans are quite maneuverable in the harbor and can turn on the plate. The two-screw drive with two oars is indispensable for this. The flow to the propeller and rudder couldn’t be better behind the slim float. The result is a very low-vibration drive. The prerequisite is that the speed and direction of rotation of the respective propellers can be controlled separately by hand or can be determined with the aid of a joystick and appropriate program control for the respective properties of a catamaran.
If the catamaran is equipped with very slim, streamlined hulls, the space available in the submerged part of the swimmer is very limited due to the lack of width. After all, there is enough space for all the units, the propeller drive, tanks and storage spaces.
This is compensated for by huge deck areas. In general, in cruiser catamarans, the outer sides of the hulls extend to deck level, so that the entire width and length of the catamaran can be used for cabins, control stand and a lot of free space. With an average length-to-width ratio of 2.5 for double-hulled boats, a deck area of 40 (!) Square meters results for a 10 meter long catamaran. A monohull boat of the same length cannot nearly match that.
The question arises, why are monohull boats preferred to catamarans despite the favorable properties? First of all, there are the acquisition costs, which are certainly higher compared to a monohull boat of the same length and quality. The effort involved in making two hulls and the connecting beams requires more costs. In addition, the two-screw drive with two engines is a fundamental requirement for a good motor catamaran. On the other hand, the comfort offered on the large deck area is a good argument for the higher price. Unfortunately, small cruiser catamarans are difficult to implement under these conditions. Another sore point for very wide catamarans is the mooring problem. Quite often there are only anchorages available for cats in the harbors.
The external appearance of catamarans, the overall picture, takes getting used to and often does not correspond to the ideas of elegance of a sleek motor yacht. Perhaps this is the reason that catamarans have become popular as work ships and ferries in recent years.
Finally, another argument should be addressed, which speaks for the planing boat and not for the catamaran, which is always on the move as a displacer. The transition to planing, the lifting of the bow area, the clearly visible bow and then stern wave, the tight turning circle at full throttle, all of these are experiences that especially fast planing boats offer. Desires that the catamaran cannot fulfill, whereby energy efficiency is completely disregarded. If you look at the advertisements for motor yachts, many are shown at maximum speed. Often, despite a speed of 30 knots of airflow, a young, lightly clad lady should not be missing on deck. Motor boating should be fun.
Perhaps knowing the peculiarities of catamarans and their qualities can help improve the image of catamaran motor yachts. If you exclude racing and high-speed boats as well as platforms for houseboats and professional ships, the only catamarans that remain are motor yachts from around 10 meters in length. Concepts for smaller catamaran-like motor boats are also conceivable, which, like the monohull, also promise driving pleasure. The main advantages of catamarans are their energy efficiency, which is also noticeable at higher speeds, the good seaworthiness and the huge amount of space. In the negative balance, the high costs must first be mentioned.
The elegance of a large motor yacht as a monohull is currently undisputed. However, since the sense of style and fashions are constantly changing, it cannot be ruled out that catamarans will meet the expectations of an attractive motor yacht in the future and will become increasingly popular.