BTW gang, let's limit reposting my already overlong original posts in your replies by editing them down to relevant small edited bits with the delete key, and also delete the headings in reposted stuff... I know my stuff is pretty glorious, but we only need to read it once, if that. Brian: Gary had written: > <>> If you want the PLANING hull type of power catamaran performance, >> forget sail catamaran hulls unless you are willing to invest in >> producing a hydrofoil supported boat with a proportionally huge >> motor. It would probably be easier in such a case to start from >> scratch. > ________________________________________________ > ...your quote,"forget sail catamaran hulls unless you are willing to > invest > in > producing a hydrofoil supported boat" > > Brian replied: > Not necessarily so, I have developed some ideas on this conversion of > sailing > cat hulls to power cat forms with increased HP. This came about as a > result > of > my looking at a 50 foot project overseas, and a client's desire for > greater > HP, and faster perf than the stock sailing hulls. Gary: OK, watcha got? I've pretty much showed my hand... ;-} Everybody wants greater performance and greatly reduced costs. The key difference between new and naive design clients and their designer is the training and experience needed to guide the client to practical and efficient choices in keeping with the depth of their wallets. > <>Brian: > If you are not seeking this increased perf, then don't bother. Gary: Precisely. The current run of sailing cats don't do badly under power alone. If our hypothetical redesign client would simply leave the sail rig in place, and never unfurl anything as they merrily powered about, they would look just like most of the sailing set, who seldom unfurl their sails either. <><> Brian: If you are, tell me specifically what vessel you have in mind, and what power you have in mind. Gary: I think you missed the context for my previous hydrofoil statement (I just now recapitalized the word PLANING in my original posting which you cited above). If one wants the heady speed of the narrow tunnel planing power catamarans to be provided in a converted sail cat hull, one would need to lift the entirely unsuitable hulls more or less out of the water by using lifting hydrofoils and very large motors. Or perhaps wings and small rockets mounted on deck ;-}. Otherwise I agree entirely with what you said. Of course is possible to design and build a conversion of a sail catamaran into a fairly fast power catamaran, although I strongly suspect that the costs of finally overcoming all the difficulties would certainly bring the price of the project well above the cost of a suitable new boat. Limits of the hull internal space and dimensions will still limit the size and placement of larger and heavier engines. Don't forget to leave a little room for the poor sod who will be maintaining and repairing these engines -- higher performance engines seem to always need a good deal more maintenance and more frequent repair! The external shape of the hull will adversely impact not only the speed through the water; it will present some real challenges adding adequate props and rudders as well. Perhaps a modified 'sail drive' could be mounted in a nicely protected spot just aft of the centerboards, near the bottom of the rocker in the bottom shape, where the hulls are wider and the overall center of gravity issues best compromised, but that would raise heck with the interior accomodations and still leave a pronounced stern squat. New rudders aft with protective skegs could perhaps be fitted with horizontal wings, possibly articulated trim tab fashion to remedy the squatting or as powered stabilizers to eliminate some pitching motion as well. What PDQ found out when they built their prototype powercat based on their 32 foot sailing cat was that they really needed an entirely different bottom shape to accomodate these and a couple of other issues (stern squatting at speed, protecting props and rudders for the inevitable beachings cats do so well, etc.). I know this, as I advanced them funds -- in the form of a huge down payment -- specifically to allow them to finish the new molds and shift production from the initial modified 32 foot power hull to the fully redesigned 34. One of the key modifications of this redesign was the additon of as much more flat bottom as possible from amidships aft, and particularly in the extended flat sections above and aft of the props. Generally sail hulls feature very fine exits with high prismatic coeficients, and the initial redesign had replaced that with quite a bit of flat bottom already. These extended flat bottoms replaced most of the remaining stern squat (now less than five degrees at full speed) with improved semi-displacement planing speed and performance. ------------------------------ In his next message Brian replied: I hadn't gotten to all of my email yet, but I wanted to compliment Gary on this submission It is a really good synopis of the subject of conversion. Gary: Thanks. Clearly I am giving serious consideration to doing this for myself. I did leave out a whole bunch of stuff, like the difficulties adding an interior helm station to a sailboat and the minimalist electric systems generally found on them (thanks to Arild for pointing that one out). How many of the potential buyers of such boats want to stand outside to drive all the time? The crummy visibility from the saloons of such boats, and the difficulties adding all that stuff to an already efficient and thoughtfully planned interior space pretty much rules that approach out. I guess that would leave enclosing part or all of the cockpit as a pilothouse. A wide pilothouse which still allowed for an open sail handling cockpit behind (or ahead of, or raised in the middle of!) it might work out well. Hey! That would yield the very catamaran motorsailer we discussed not so long ago! Brian: There may be one other consideration in larger vessel sizes. There are some folks (and power folks in particular) who don't want to wait for a custom boat build. Maybe they suddenly decided it is time to go cruising doing the coming economic slump. They could be likely candidates for an 'upscale' sailing cat with the rig removed (frightens some non-sailers) Gary: I suppose so. Which would be easier for them: paying the very considerable costs in time and money of compensating for the incompatabilities between a fast power catamaran and the currently available sailing cat designs; accepting the already adequate speeds of existing powered sail cat performance (with or without the sail rig); or rather overcoming their anxiety about learning to sail, an activity many of us love and can practice well into our retirement years<>? Given my notion that the cost of a successful, complex and neccessarilly complete conversion (you couldn't just do half the job) would likely far exceed the cost of a new or used contemporary power catamaran<> -- the sort of client you mentioned who didn't want to wait for a custom built power catamaran might be better advised to find a suitable new or used power cat and contemplate customizing it to meet their needs. Such power catamarans are available to them right away -- although at considerably higher initial cost and much smaller availability. Their final costs would be much lower, and they would not have to wait nearly so long for the modifications and remodifications to be worked out. There are so many issues to be resolved in redesigning an existing sailing catamaran to provide planing power catamaran speeds, and these issues are guaranteed to interact with each other and with the remaining parts and performance of the boat in so many complex ways, that considerable time must be allowed to re-test, re-redesign and re-modify the initial redesigned prototype to provide even acceptable performance. A good designer might well succeed in educating them about the various aspects of these issues and convince them to modify their initial wish list in favor of more practical, and quicker options. My preference would still be for a boat which retains the sail rig and the existing tiny saildrive power plant, skipping the higher speeds under power alone. I would capitalize on the modern roller furling and powered sail handling gear available today. Admittedly that might well not be acceptable for folks who are intimidated by their inexperience sailing, and for folks who aren't interested in the extended blue water passages that sail makes available. Their loss, in my view. I am too old to indulge in worshiping the numbers: Giant engines; huge horsepower; higher speeds and ever bigger boats. I'm just not that insecure anymore. <><>I want to also point out that successful long term cruising is hardly compatable with full and tight schedules. If the schedule is more or less vague, then the journey trumps the destination, producing happiness and contentment. For my retirement cruising (in that spirit) I have little lust for high speeds at all, save outrunning unexpected heavy weather on a deepwater passage. In such a case there should be plenty of wind already available to achieve a fine turn of speed under sail alone, relieving the anxiety about digging too deeply into fuel reserves for an extended high speed run. This is fun... Gary

> So what kind of speed would we get from the tiny diesel engines, Gary? Rupert. p.s. Great post (which I cut as advised, otherwise....) > <><>I want to also point out that successful long term cruising is > hardly compatable with full and tight schedules. If the schedule is > more > or less vague, then the journey trumps the destination, producing > happiness and contentment. For my retirement cruising (in that > spirit) I > have little lust for high speeds at all, save outrunning unexpected > heavy weather on a deepwater passage. In such a case there should be > plenty of wind already available to achieve a fine turn of speed under > sail alone, relieving the anxiety about digging too deeply into fuel > reserves for an extended high speed run. > > This is fun... > Gary > _______________________________________________ > Power-Catamaran Mailing List