On 12 Mar 2008, at 13:31, Roger Jacob wrote: >> I know this is na over discussed topic, but can I get a 46'sailing cat >> and make of it a power cat? (changing engines from 4 to 6 cilinders, >> biggers fuel tanks...)Will it be a safe and seaworth boat? What are >> the downsides and upsides? >> >> Roger > > Is it over-discussed? I would be interested in reading some of the comments myself. rupert. Part One: OK, here goes. These are my thoughts. I encourage a lively discussion of them, particularly by folks who disagree in general or specific ways. Disclaimer One: I am speaking in broad generalities here, and in no way am I baiting advocates for any specific boat design to loudly proclaim their loyalties or the unique ways in which they exploit the advantages or dodge the shortcomings of their type of design. There are probably a number of boats which do not entirely fit my general types in one way or another, although I can't think of any which are clearly midway between them. Disclaimer Two: I own and love my PDQ 34 (described here as my second type), and claim that I have some experience and insight into its design and the ways it contrasts with others. I am not a naval architect, and have no association with any commercial boating interest, apart from being a happy consumer. First, even a casual survey of the power catamarans available today will show two distinctly different sorts of boats. Give particular attention to pictures of them underway. One throws big splashy wakes typical of a power boat, and the other throws much smaller and less splashy wakes associated with a sailing catamaran. This first one is derived from planing monohull designs, and gains power catamaran status with the addition of a relatively narrow and shallow tunnel down the middle of an otherwise familiar hull. This provides significant improvements in handling, comfort, stability and internal space while retaining the basic planing boat hull shapes and large lusty (also means thirsty) engines. The majority of modern power catamaran designs available today (under about fifty feet) fit this general pattern. These boats trade off cruising range for speed, and exhibit typical planing behavior, namely a range of less useful speeds where the boat is climbing up and over its bow wake (getting 'up on the step') before it succeeds in skimming over the surface of the water supported largely by lift. This planing lift is achieved by providing the hull with large flat bottom surfaces aft, and the aforementioned lusty engines. Thanks to the typical 'deep vee' style bows, among other factors, these boats handle rough water rather well, although their limited range means that they dare not go far enough offshore for ocean crossing passages. They are excellent coastal cruisers, although by virtue of their higher top speeds they probably are more used for shorter and faster trips typical of planing monohulls -- like a run out to the hottest fishing hole. They have achieved long coastal passages, although their general use is the shorter faster cruising. I don't think there are very many of this design among the over fifty foot sized boats. The other sort is derived from sailing catamarans and is recognized by its slender hulls, its dramatically wider and taller 'tunnel' between the hulls (where water seldom contacts the bridgedeck -- ideally!), and its much smaller engines - by a factor of three or more. These boats generally have beams around half their length, and the individual hulls width in the water compared to their length is on the order of one to ten or more. These boats feature somewhat lower speeds, depending upon the slender hulls making a correspondingly smaller bow wake, and what is loosley called 'semi-displacement' behavior where a small flat bottom section aft gives just enough lift to to allow the much smaller engines to push through the bow wake. Note that I said through in place of over. The large bridgedeck provides abundant space for very comfortable interiors, and the various designs cleverly use the narrow hull space to compliment that, again derived directly from sailing catamaran practice. The huge beam means means wonderful freedom from harmonic rolling while the large separation of the props, provides fabulous maneuvering. The slender fine entry hulls unhappily are more sensitive to harmonic pitching, although that is generally easilly remedied by changing speed and angle of attack to the offending waves. This pitch sensitivity, with a couple of other common design choices limit their heavy weather performance, while their roll resistance enhances it. We don't need no steeenking bow/stern thrusters, paravanes, flopper stoppers, active or passive stabilizer fins. The small engines provide excellent range and superior fuel economy. Overall the rough water tolerance is similar to that of the better planing hull's, and the comfort levels significantly better. Designers recognize that these boats are also poorly suited to cross oceans, and provide fuel storage suited for coastal cruising. They too are excellent coastal cruisers, with differences of speeds available, boat motion, and such, and are generally used in this way -- more cruising and less fishing. They have also achieved long coastal type passages, and are arguably better suited to do so. Only the largest power catamarans of this second type can easilly manage long ocean passages. Among today's designs I feel that there are rather few of these second sort of power catamarans under fifty feet or so in length, and that virtually all the power catamarans over fifty feet conform to this approach. check out part two in my next message