SAILco Press

One of the anomalies of our time is the elusive answer to the question of why so many of those beautiful big catamarans do not sail anywhere near their expectations. In fact, the irony is that in so many cases ordinary monohulls sail right past them. Why? Why are these sleek looking catamarans with massive sail plans, allegedly favorable hull-beam ratios, some as high as 10:1, just do not sail beyond monohull speeds, and when some do, they often have all sorts of dangerous proclivities?

Certain boats are terrible sailors. They wallow, pound, drive their bows under water, throw enormous wakes, tear the gelcoat off the protruding bottom sections and have all sorts of mischief associated with being grossly out of proportion in LOA/BMAX. Some boats from other manufacturers appear to have sufficient underdeck clearance but when you really examine them closely, they only have that clearance at the center. Along the inside edges may be bulges, humps, shelves, steps, curves, etc.

There are two basic reasons for this poor performance. The first being the most obvious, I will not discuss in this article; that being drag from propellers and sail drive units. Any boat that has a pair of three-blade props, is dragging two drogues which not only slows it down, but creates excessive turbulence and possible cavitation in front of the rudders thus causing erratic steering and still more drag. (in some models, there is insufficient clearance between the prop and the rudder to use folding props. This subsumes yet another problem, that is rudder erosion.)

The second, most important and most subtle is waterplane shape. Surface drag is highest at the air/water interface. The waterplane shape or "footprint" is the shape the water actually "sees" during sailing. Anything that disturbs the uniform symmetrical shape of the waterplane will create drag. This is especially true if it happens between the hulls! Because water is not compressible, irregularities in waterplane shape set up waves that clash in the center.

Often you will hear the feeble excuse for overly wide beam as preventing hull wave interference. In this article I will prove that the phenomenon of hull wave interference is caused by protrusions into the waterplane shape and these protrusions are the cause of the hull waves. A simple proof of that is all the day sailing boats that do not have protrusions, have properly proportioned LOA/BMAX ratios and have ample bridgedeck clearance do not have the problem of creating waves between the hulls. (You never see waves between the hulls on a Tornado!)

Notice, look carefully at brochure photos any of the popular modern charter catamarans. At any speed they cause no excessive wave on the outside of the hulls. If the hulls were symmetrical, and bridgedeck clearance sufficient, that pattern would be repeated inside (between) the hulls, but too often it is not.

Therefore, reason number two, by far the most important, clearly stated is: anything placed between the hulls that interferes with a clean symmetrical waterplane shape at the actual waterplane in use while sailing, will create sufficient drag to destroy the performance of the boat. This also includes insufficient bridgedeck clearance which often turns a catamaran into a barge in a seaway.

Now, here is the kicker. When you look at the boat plans, everything looks proper. When you examine the boat out of the water and check its lines, it seems to match the paper representation. But does it? I submit to you that in too many cases it does not because the tunnel between the hulls on many of the boats is filled with little bumps, curves, steps, transoms and other protrusions that create the actual waterplane shape when the boat is underway.

A third vital generator of drag is a transom submerged below the waterline. As in any aero-dynamic or hydro-dynamic form, most often described in NACA foil shapes, it is the exit that causes the most drag. Therefore, you can tell immediately if a catamaran sails with its transoms submerged and creates a wake, that wake represents the amount of drag created.

Photo 1. While this vessel appears to have great underdeck clearance when looking at it from the dock, as you can see by this photo, it really does not. . The white arrows point to the obvious humps and bumps at the waterline. The purple arrow points to a huge protrusion that ends in an additional transom. Note the distance between the boot top and the waterline. When fully loaded, and especially when under power, that area is underwater.

In photo number one, (1) I show the inside tail end of a very popular 42 foot charter cat. The buttocks of this boat have been enlarged to accommodate greater interior space. Note in photo number 1, the subtle protrusion that destroys waterplane shape. In photo number two you can see the results of that protrusion while underway. I took that photo during a survey. The boat was completely empty. No water, precious little fuel, no beer and only four people on board, yet it created numerous mini waves at the stern and created a serious wake. All that in smooth water. Just imagine this boat in a seaway with a full charter load or weighted down for an extended cruise?

In photo 2, you will note the sterns depressed to the boot top, further exaggerating the anomaly of the waterplane. Even at rest, unladed, this vessel has poor prognosis for good performance. Looking forward, you can see the escape hatch and the turbulence it will cause. The at-rest waterplane is not the sailing waterplane which takes into account waves and loads. Note the results of the shapes detailed in photo 1. Strange little wavelets, splashing underneath, excessive wake. The photo 2 and MPEG movie was taken under sail in calm waters from the opposite transom. Note how far the stern of this almost empty boat is depressed. The speed robbing attributes are obvious.

Figure 1, Essential theoretical waterplane

The accompanying diagrams, figure 1 a shows the paper representation of that waterplane. Figure 2 shows what the actual waterplane shape would look like while sailing in a loaded condition. That diagram makes it obvious that there will be hull wave generation and interference and that the real hull/beam ratio is almost the same as a monohull of that length, somewhere around 5:1. Driving the stern down to have a partially submerged transom adds yet more drag. No wonder it is a slowpoke!

Figure 2. The waterplane as

the water sees it. Note the

humps that create the

A-symmetry

In every case of arrested speed development I was asked to explore, I found roughly the identical problem. That problem, loud, clear and unequivocal is "stuff" in the tunnel between the hulls. That "stuff" can be a big oversized nacelle, steps and flare just above the waterline, bulges,

"plane-ing wedges" and those sweeping curves that begin right at the waterline. In every case they create drag in direct proportion to how they affect the waterplane shape while sailing.

The above critique is based upon the proven science that the hull-speed limitation factor is a result of the ratio of hull waterline length (WL) to hull waterline beam (HWB). The customary hull speed limitation formula being VMAX=1.4 WL. In reality, the 1.4 is a variable, not a constant. That number increases based upon the WL/HWB ratio. Catamarans are theoretically proportionately capable of higher speeds than monohulls simply because their hulls are narrower, therefore, they generate bow waves at much higher speeds. Those theoretical speed limits are generally higher than the vessel can go anyway for other limitations, so hull speed limitation factor becomes a moot point in large catamarans.

Since Polynesian times, certain fundamental ratios have been observed, the single most important one being overall length/beam ratio (BMAX) That ratio is beam equals one half the waterline length. (BMAX=WL/2). This is occasionally stretched to overall length (LOA) in some cases but any further and you begin to get into difficulty. A fifty foot boat with a thirty-two foot beam may be close to unmanageable. On one such boat, a good client of mine who is an experienced cruising catamaran sailor, brought such a boat back from South Africa to Florida. On the trip, they stuffed the bows so badly it threw everyone forward; Fortunately, with only minor injuries. He complained that the boat pounded so badly it actually hurt his feet. There were numerous other complaints. The boat now sits unused tied to the dock. Proof of this can be found in the saga of Play Station which was originally designed at 100 feet but nose dived so badly it was considered dangerous. Lengthened to 125 feet (which narrowed the BMAX/WL ratio) it became the record breaker.

Photo 3. While admittedly a sport boat, this boat shows the characteristics required for unimpeded realization of performance. Note the way buoyancy is carried forward to the bows, the fact that there are no appendages of any kind in the tunnel and bridgedeck clearance is not an issue. The single center mounted outboard retracts completely into the bridgedeck. Hull beam ratio is correct and not an issue.

Photo 4 Playstation

Proof of this can be found in the saga of Play Station which was originally designed at 95 feet but nose dived so badly it was considered dangerous. Lengthened to 110 feet (which narrowed the BMAX/WL ratio) it became the record breaker.

Steve Fosset, owner skipper and Morrelli & Melvin designers.

Play Station is a perfect example to use. In this dramatic cover photo it seems as if the boat is creating a wake. Further inspection also reveals that it has just come down off the top of a wave and given the helicopter pilot the dramatic photo he was looking for. As you all know, drama sells magazines.

Photo 5

An other 47 foot boat I recently surveyed with sea trials in the harbor at Isla Mujeres, Mexico, pounded and hobby-horsed in the harbor! The accompanying photo of the underbody revels there is only six (6) inches of clearance under the protrusions. This 47 foot by 27 foot cat has only six (6) inches of clearance under the hump shown in the photo. There is a mere 18 inches in the balance of the area. As expected, the noise from slapping is considerable, even in a minor harbor chop.

Photo 6

This very popular 34 foot catamaran has squeezed every available inch out of the underdeck tunnel. Note the extreme protrusions that assure turbulence underneath. I suspect this boat would be half again faster without that intrusion into already cramped space. There is only eight (8) inches of underdeck clearance as measured in this photo. Note that the waterline has been raised about eight inches. If the normal ballpark figure of 1000 pounds per person for food, water and clothing is considered, and the ballpark figure of one inch of immersion per 1000 pounds is applied, bridgedeck clearance, hmmm?

Photo 7

Evolution of what was once a terrific hull form that never buried its bows or left a wake into a poor performing caricature of its former self. Note the "plane-ing wedges" and the oversize nacelle. Between them, they occupy almost all the space in the tunnel. One owner of such a boat complained he never could get the boat over five knots on his transatlantic from England to the United States. If you think this looks bad now, just imagine what it will look like with a dinghy in stern davits and four people in the cockpit?

Photo 8

Photo eight shows a full curved arrangement. While better than most with appendages, steps or nacelles, it still carries some performance penalty. This vessel is an interesting case. Originally thirty-eight feet, big improvements were made when it was lengthened to forty feet. That made a big performance difference. Again, the vessel was increased to forty-two feet. That made a major difference. It is more than just four feet of waterline, it also changes the BMAX/LOA ratio to be much more favorable. Now there is virtually no wake.

Photo 9

Forward facing flat panels have a history of gelcoat damage as well as noise. This configuration certainly cannot help performance.

Photo 10

One example of steps protruding into the tunnel space. There are many other variations on this theme. Even the high performance boats that are designed like this seem to suffer the same fate as the cruising or charter boats. A quick check of the race records will confirm my assumption.

This generates a plea from me to the industry. It is possible to satisfy the demands of the purchasing public with vessels that actually will perform up to the claims being made for them. As a long-term enthusiast and booster of multihull technology, especially for cruising boats, I see only minor design features that have crept in where other methods of answering to problem would be more appropriate.

For instance.

The first Seawind 1000 catamaran imported into the United States was a big disappointment to its owner. It had mediocre performance at best and even twenty-five foot monohulls were passing it to windward. The boat was transformed from an also-ran to a winner by building a three foot extension on the sterns. Some minor adjustments were made to sails, but the basic change was the LOA. (This, of course, also changed the LOA/BMAX ratio) The point is, that certain changes that allow a cruising boat to achieve much more of its potential performance may not be that difficult to achieve and may increase the value of the vessel without have a major impact on the cost of production.

Photo 11 Photo 12

Extended stern Original stern

Based upon the success of the Miami modifications, designed and engineered by Tom Mestris of The Multihull Association of South Florida, Seawind came out with a new model, the Seawind 1000XL which incorporated most of the changes engineered and proven in Miami a photo of the new model is below.

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