Why a Catamaran - Summer 2000

January 14, 2008

Efficiency

A properly designed catamaran, with the proper length to beam ratio of the hulls and the longitudinal center of gravity (LCG) in the proper place, is a very efficient boat.  The secret of efficiency in a displacement hull is a long and narrow hull.  Nothing new here.  If you look at designs of old ships, a Roman trireme or a Viking boat, they were long and thin because the available "people power" to propel them were limited, inefficient and even in the case of slaves, expensive.  

Today's displacement yachts have become gradually wider to get more room, at the expense of a bigger engine to plow the hull through the water.  This is not as bad as it sounds, since the limiting speed in a conventional displacement hull is given by the length of the boat at water level (Froude's law).  Once the hull speed has been reached (8.48 knots for a 15.24 (50 ft) boat at water length), additional power will have very little effect, unless the boat starts planning, which is a completely different (and very power hungry) story.  A properly designed displacement catamaran can go at speeds well above the hull speed of an equivalent length displacement boat with an acceptable power (e.g. fuel) requirement.  Typically, a cat with a water level length of 47 ft can have a 18 knots top speed and an economical long range speed of 12 knots, something a monohull of 47 LWL can only dream about, since its hull speed is only about 8 knots.

For more information read the excellent article "A Case for the Displacement Power Cat" written by Malcolm Tennant in the Power of Multihulls magazine. (www.powermultihulls.com) .  Another good source is the web site of Malcolm Tennant (www.tennantdesign.co.nz).

Safety

Due to its width, it is much harder to roll over a catamaran than a displacement boat.  When a displacement boat is rolled beyond a certain degree, (the amount varies from boat to boat), the self righting capability diminishes enormously, especially should in the process some windows get broken and water start coming in.  Monstrous waves of the perfect storm will eventually flip over even a catamaran, but by that time a conventional boat will long be gone.

A conventional boat in the 15 m (50 ft) range has at the most 3 watertight compartments.  The hulls of a catamaran have at least 3 or even 4 compartments in each hull, making a cat harder to sink.

Redundancy

When I was considering to build a conventional passagemaker, I had to address the problem of a get home capability in case of an engine or drive shaft-propeller failure.

Let's face it.  All the solutions offered for the get home capability of a single engine, single screw vessel are not 100% satisfactory.  Sure, nothing is perfect, but I know that I would not feel happy in the middle of an ocean, depending on a single engine.  The only solution is to have two engines, two drive shafts and two propellers.  Unfortunately this causes another problem in a conventional, single keel vessel:  either the drive shafts, propellers and rudders are very exposed or the bottom of the boat has to be specially designed with two keels to protect them.  Nothing wrong with that arrangement, most big passagemaker yachts are thus equipped.  But in a displacement hull of 15, 16 meters or thereabouts with a hull speed of 8 knots, two engines just seem a waste.  I feel that two engines are justified when you zip across an ocean at 12 knots or cruise around home base at 16 knots or more.

The catamaran makes the choice easy:  you must have two of everything, giving you a superb redundancy.  You can even have a small fire or flooding in one engine room and still have a spare one.

Roll Dampening

Some roll dampening equipment, both at anchor and underway, is essential for the comfort passengers and crew of a displacement boat.

Flopper stoppers apparently work fairly well, but it is not my idea of fun and safety to handle two 40 - 50 kg fish.  Stabilizing fins, although very expensive, seem to work well when under way but are useless when anchored in a bay with a constant swell.  If you think that you can live with a passagemaker boat without any roll dampening equipment, you are a better man than I am or have a masochistic and sadistic streak in you. 

The catamaran eliminates the problem, not with some artificial contraption, but by design.  Roll dampening is not even an issue, thanks to the width of the vessel.

Living Space

It is difficult to compare the amount of living space of a monohull versus a catamaran.

The length of an oceangoing power catamaran has nothing to do directly with it's "size", where size is meant  the length of a boat, usually expressed in meters or feet. We sort of know what to expect of a  42 ft trawler or a 18 m motoryacht but, if you want to get a feeling of the size of a catamaran, you must consider the length of the covered living space.  If we take the PH8 as an example, living accommodations are about 9 m long and 5 m wide. I then suggest that you multiply the accommodations length with the factor 1,5 and that gives you a rough conventional "size" value. I keep the factor relatively small on purpose, because  not all the space on a catamaran can be cramped full of furniture and junk. You would end up with LCG and weight distribution problems (see the Technical article Power Catamarans and the LCG).

Then as is the case of the PH8, the three areas (kitchen, dinnet, bridge, passenger cabins) are clearly delimited by the basic architecture of the boat and cannot be change at will. It is unfortunate that the taxman and the marinas  charge the catamaran by its length.

The seagoing capability of an oceangoing power catamaran is different from the one of a sailing catamaran. On a sailing catamaran the deck  (the part between the two hulls) is fairly low over the water and is flat on the bottom. That's because a sailing catamaran can only point a certain degree against the wind, but cannot sail against the wind (and waves). If you power against the waves, after certain height they slap hard against the underside of the deck, a teeth-rattling experience, I am told. The oceangoing power catamaran must be able to navigate against the waves and therefore the deck is much higher, taking away space for the side cabins. Incidentally, to go more comfortably against waves all power catamarans have an "anti slam nacelle" in the middle of the deck, to break the big waves.

The hulls of the power catamaran must be thin for efficiency. A ratio of length to width of a hull of 1:14 is optimal. If you reduce the ratio down to 1:8, you have two conventional trawlers. The PH8 has a length to bean ratio of 1:13.8.

Most sailing catamarans are made of composite, which uses up less space inside. The frames of aluminum are big,  because the hulls and deck are subject to a high stress in heavy seas and one cannot compromise. (This is not the place for discussing along aluminum versus composite. I have made my choice, right or wrongly). Not putting living space into  the hulls lets you use them for engines and  various systems, which should make maintenance (except for the engines) easy. One thing I must say in all honesty is a big minus in catamarans:  the engine rooms are miserable.  Granted, there are two of them, giving real redundancy, but gone is the sumptuous engine room with a king size work bench.

 

Marinas

The main problem of a catamaran, (but at the same time also its big plus point) is the width.  Marinas and boat yards in the Mediterranean are simply not equipped to handle such a wide boat.  Or if they do, at a hefty premium.  To get a feeling of the width of a catamaran go to a marina, find a 16 meter long boat and look at its bow or stern.  Now imagine a width of 7 meters.  More than likely it will not fit where it is now.  Or next time you go to a boat yard look around for a travel lift that can take a boat 7 meters wide.  

On the plus side is that once the cat is out of the water it does not need a cradle nor anything else to prop it up safely.

If you are looking for a boat to keep moored in a marina to "see and be seen" (a favorite pastime of Italian boaters) then don't contemplate a catamaran, which is made to navigate.

Price

A catamaran is more expensive to build than a monohull.  After all, everything is double, including the hulls.  A small cost saving can be probably achieved in the interior finish, where a catamaran can use more household items, due to the regular shape of the floor plan.

 

That's the theory.  In practice we'll see how it works out.  If you have your own opinion or experience for or against catamarans, let me know.  You can always e-mail me at the following address: photz@caracolyachts.com

 

2008-01-14
cat_why a cat.htm