To build a cruising catamaran, there are three main options: buying an old boat and refurbishing it, purchasing a bare hull plus deck molding for home-boat building, or starting from scratch and building everything, including the hull, on your own. The catamaran hulls were built first, consisting of internal frames, stringers, plywood, and fiberglass.
Building a catamaran boat requires patience and skill. The first step is to choose the right materials for the hull, such as fiberglass, wood, or aluminum. Then, you will need to assemble the cores, fibers, and resins to build a finished laminate hull. Each approach has strengths and limitations.
The first step to building your dream catamaran begins with a strongback, a square frame used to position temporary frames that will be used to form the hull shape. The first step of catamaran design is to decide the length of the boat and its purpose, then optimize other dimensions to give her decent performance.
If you want to build your own, look at the Hobie 16 as a guide. You can easily stitch and glue the hulls and hobie sells (or at least) the hulls together.
In summary, building a cruising catamaran requires patience, skill, and careful planning. The process of building a catamaran involves several steps, including building the mast beam, boat hull, sheathe, install bulkheads, construct the interior structure, and launch the vessel.
📹 CATAMARAN DESIGN: Hull Shape | Essential Catamaran Knowledge Ep. 1
Have you ever wondered the processes that go into Catamaran design and building a Catamaran? Maybe. Maybe not. However …
📹 2 Years in 5 Minutes – Catamaran Building Video
If you are thinking about building a boat, we seriously need to have a chat, I put this video together to highlight the work done so …
To be straight up, I was worried about getting something less than this, dare I say it im looking forward to it now. Congrats on the new boat, sure your looking forward to it. I remember saying no rush on announcing it as long as you went into all the technical details like you said you were going to. Now you have set the bar high, keeping your word, I like it. You have me looking forward to the rest of this series. Thankyou, in depth technical stuff, if you can do this on everything its bloody awesome. Everyone else does drooling over double sinks and different fabrics and bar areas. This is what I am after myself. Thankyou Terysa for putting up with him while he delivers this content :P.
I think this comes down to what are your priorities. (Not disputing the analysis, the engineer did a great job of presenting the design/performance theory). Is your priority to sail as quickly as possible? Are you buying a yacht to experience the maximum sailing performance? Are you looking for maximum comfort? Are you looking for minimum draft? If you are looking for performance, are dagger boards a requirement? Etc. This seems to suggest its like Americas Cup and the fastest boat wins. For some this may be accurate, for others its more about the experience of travel and adventure. At one time on early Ruby Rose article’s I remember hearing only a monohull was the preferred sailing vessel. We all watch several sailing websites and obviously the journey is accomplished in MANY different ways. They all seem to achieve their individual objective. I have two cars. One is a GMC Suburban and the other is 911 Porsche. Both will allow be to drive from California to New York however, (and as with everything) there will be tradeoffs. For me my goal is to travel and share an adventure of a lifetime and much less about the design of the yacht. It seems to me The Wynns, UMA, SLV, Delos, O’Kelly’s etc. are all out sailing and achieving their dreams. For me this is the most important criteria.
Please keep banging on about this stuff!! And a hearty congratulations for finding the yacht for your needs! I’m a year into research for finding a catamaran for a currently non-sailing couple retiring in a few years time. Finding opinions like ‘performance cruising catamaran’ and ‘usable living space’ is easy. But, finding any detailed facts about the science behind this is very difficult. With very little digging, I can easily know almost all of the dimensions and performance specifications regarding my laptop computer and all of its internal components. And all that info for a small machine that costs only a few thousand dollars! But, it seems catamaran manufacturers expect us to spend hundreds of thousands of dollars, if not millions, on the vessels they produce, while telling us very little about what actually makes them tick. I get the feeling they hope to sell us on the romance of the adventure without wanting us to really know what the vessel can and cannot do. With all machines, I am very, very interested in what they cannot do. That knowledge is what allows us to make decisions that keep us safely in the ‘can do’ zone. And when buying used, there seems to be even less information available. So please do keep banging on about this stuff!
Interesting info Nick and it’s good top know these things thanks. But will the Prismatic Coefficient really be a factor to discuss for anyone buying a family cruiser, It’s a bit slower but we can shower, keep the beer and wine chilled and will get less wet would be the selling point I think. I’d loved to know how they work out the position to place the mast for optimal performance, who decides that it goes further aft or toward the bow for better upwind or down wind sailing. Hopefully you will have that come up in one of your articles.
Always liked your show. I think there was a typo though. Slenderness ratio is LWL/(HVol)^1/3, the higher the faster, not the other way around. Nowadays mostly the weight+Length+sails determines boat performance. I liked seawind 1260, but the cabin ceiling too low to be comfort. Seems like Seawind 1370 improved. Looking forward to more info on pricing etc.
Nice work on this article! I was told this series is worth a look and it sure is. I believe style on a lot of new performance cruisers is still being put before cruising function quite a bit. Immersed transoms is a great example of racing style looking cool but not functional for cruising. The wider shorter sterns with immersed transoms only offer more performance once at high speed. They are slower in light air and for performance cruising light air performance is actually more important than a few % more at the top end when your likely slowing down anyway. If you have anti foul pain on your transom you have wet heels… Mick
I’m not currently not a Blow-Boater but this discussion was fascinating for me. Cat’s have become more and more popular on the Power Boat scene and I am currently thinking about “next” for us and whether to go cat or Knot… (with focus on comfort versus performance) so even for a Motor Head this was an exceptional vid… looking forward to the rest of this discussion!
Several years ago I read a book from the 1890’s on steam locomotive design and was quite surprised at how much the engineering was based on ratios. Ex. If a part for a certain function needs to be X inches long then it needs to be at least X/12 inches thick. These ratios were derived from what worked in the field and what didn’t work rather than from material science and complex maths. Although we now have computers which can perform hydrodynamic calculations in fractions of a second it is interesting to me that we still use ratios when talking about the properties of a boat.
Good start Nick. Diagrams are pretty iffy – don’t just uncover a picture of a cat and gloss over – it’s not hard to draw them properly. Look over what Hanneke did with James years ago for inspiration. Some of Tennant’s and Shuttleworth’s are good too. It’ll make things much better to voice over the diagram. Also a slenderness of 7 is pretty ridiculous for a cruising boat – that is less than 10tonne displacement for a 15m/50’ cat! Even if – huge if – the manufacturer can have it leave the yard less than 10tonne, there is no way it will stay under once you cruise. So I think the number he’s chosen doesn’t reflect raising reality in any shape or form and that makes me wonder about other points he makes (although there were no other actual numbers which itself is interesting – is the architect for Seawind going to point out that a length to beam of 8-9 is low if their boat has it?)
I think you are growing into becoming a true catamaran design analyst! I’m like probably the vast majority of your viewers became excited perusal this article about your venture into opening the pandora box of hull/boat design science. You are doing an outstanding job in educating us about the various hull design features impacting performance and comfort. Understanding and awareness of the hull features of your boat has also an undeniable positiv impact in managing the boat more safely by knowing the range and limits of its capabilities! I truly love what you do and the way you go about educating us and most important handing us the tools to understand boat design. Job very well done! A BIG THANK YOU, keep on going, excited to see what’s coming next!
Lad might not look as if he has started shaving yet, but he knows his stuff! Still not convinced hull shape would influence my future choice of Cat. Key would be not to overload even a slower cruising cat, but price will be the main factor for me, and in the used market you can get a cheaper FP, Lagoon or Leopard that is still a great safe boat that will cross oceans.(steadily not speedily) I could not get the wife to agree to live aboard a performance cat with narrow hulls. If what your research pans out, Seawind seem to be trying to find a sweet spot between cruising & performance. Hope it does not end in the middle doing neither particularly well, but don’t think it will, your future Cat looks awesome, with above average cruising & liveaboard comfort and good potential performance. Looking forward to perusal your journey & logging all the lessons learnt for mine!
Absolutely #OfTheEpic work once again Nick. You are completely filling or think bank to its brim. Much appreciated. I wish there was a way that somehow we could combine this with all of the other catamaran footage you have and come out with a finished product that is the brainchild/catalyst of everyone’s buyers guide to catamarans and cruiser life there in.
for some reason I can seem to figure out what prismatic coefficient is and how it affects performance/comfort. they showed a chart but then went on to the next aspect affecting hull performance/comfort. that being said, I’ve seen vids of people sailing cats and complaining that the waves “slap” the hulls or the bridge deck and now I understand why. they probably had it overloaded or design was so that the bow waves were crossing into the bridge deck and or hitting the opposite hull. (no idea what cat it was) very informative and interesting. Thank you.
Thanks…great article explaining some of the concepts. Learned some good information and it caused me to think more about our hull design. What about the balanced load of the contents…. Like the balance in a plane? What are the effects for instance if the weight is unbalanced and you drag your stern or plow your bow? Is part of the performance also a well balanced boat? Port and starboard, bow and stern. Are there optimal balanced loads…. Also will you be selling Ruby Rose merchandise with the wording…”I like your Prismatic Coefficient.”
That was informative. We never hear anything about hull design, if you want to know you have to go looking. Other important factors that effect slamming are the distance from the bow to where the bridge deck starts and the shape of the leading edge of the bridge deck. Having the hull shape with a flat section in the stern helps reduce hobby horsing. The charter industry has turned cats into max volume to minimum length boats, at the detriment of sailing ability. What is called a performance cat today is really just a well designed cat that should sail well in all conditions. Cats with all lines led to a single helm with 2 winches and a bank of jammers would have been considered suicide 30 years ago, today it’s just a cheap option for under powered cats. If you are interested in buying a cat it’s best to sail on as many different brands/designs as possible, there is no substitute for experience. Cheers Rustie.
Great website, I enjoy it very much and learn a lot. In fact following the excellent reviews you have done I have been exposed to a seawind company and I am seriously considering the new and promising model the 1370 ( RR2 version of course…) A layman question: Does the Performance catamaran necessarily mean that it will be less stable on the water? I.e. Less stable in the sense that staying on it while sailing at a given speed will feel more bouncy than in a charter catamaran at the same speed?
LOVE the more technical information. It’s something that seems really lacking on the net in any real organized way. Everyone just says “go read The Principles of Yacht Design” which I plan on, but it would be nice if there was an organized repository of this knowledge for those of us who learn better that way than reading whole books of dry information.
Thank you for this magnificent article. In the category of fast-cruising catamaran we can assume that the GunBoats are in the top of the list . in the articles when they fly a hull out of the water, we can look the hull shape and see a gigantic rocker, can you tell what’s the advantage of this hull shape ? isn’t it a bit counter intuitive as it increases the wetted area ?
Interesting. One shape to consider is hull asymmetry. When viewed from above, a Hobie 16’s hull has an airfoil shape: the hull is flat on the outside but curved on the inside. The idea is that it co.pensates for lack of a centerboard: it generates lift for windward performance and reduces leeward drift by presenting a flat surface to the water on the outboard hull. Do modern cruising cats do this?
“Vertical dynamic lift” Is that also called ‘planing’? (Factoid: The Aleuts were planing at about 8 knots and could sustain 10 knots over distance. This was reported by Captain Cook’s navigator, probably the best technical source available in the 18th century. Maps they made of New Zealand were used into World War Two). Loading the boat. I’m a terrible backpacker. Some backpackers walk thousands of mile and their backpacks never exceed 20 lbs. Me 4-5 days 50 lbs. However, I weigh everything, make a spreadsheet, sweat every ounce – otherwise my pack would be 80 lbs. I don’t think cruisers get this extreme, but I’ll bet a couple of good scales, a spreadsheet and some obsessiveness would shave a lot of excess weight and also point out where it is. Instead of canned goods maybe another freezer might give you more and better vegetables at less weight – just maybe, I have no experience (yet). Great series. Probably going to save me a lot of money, and a lot of disappointment when I buy a multihull.
As with any product, they are designed for the customers. If the customers are more technically savvy then the catamaran companies will pay more attention to these details. articles like this are great for promoting good design. I am becoming familiar with these ratios and fluid dynamics because of a more humble project. I am trying to design and build an ultra-lightweight (<30kg) roof topable catamaran that can plane with a 2.5hp outboard. It is proving as challenging as it is interesting. It is clearly a mixture of science and art (experience as this naval architect puts it)!
One of my favorite TT articles so far. Very, very informative. I, like you, want a performance cat, but what Antoine might also be saying is that if you buy a performance cat, and you try to load it the way you would a monohull, you will seriously decrease performance AND comfort, create more slamming, and maybe even hurt stability. In that case, someone who is buying a catamaran mainly as a coastal live-aboard to mainly sit at anchor/moored/tied to a dock with short island hops, might actually want one that is less suited to performance and more tolerant of heavier loads, which might mean a lower hull volume/length number, lower coefficient, etc.? What a journey we’re all on here. Knowledge = enlightenment. Thank you Nick.
You could have asked once you have arrived at your final design what is the additional ‘TPI’ (tonnes per inch immersion) if at DWL. To be honest the basic waterline is an academic (although necessary design datum) I would insist that the DWL includes – All tanks including holding tanks at 95% full, specified dinghy and outboard weight, life raft, specified anchor cable and anchor (otherwise they will ‘short chain’ you😉), solar panels and associated control gear and invertors, an allowance for engineering stores (fluids) and spare gear, Weight of passengers- if boat licence for 6 or 8 so be it – with luggage allowance (say 40 kilos each), food and provisions for certified number of passengers for 3 weeks. If the DWL then compromises the key numbers including bridge deck clearance then the NA has to ‘go round the buoy’ and iterate hull form to achieve the objective. Good vid.😀👍⛵️
That makes so much sense now, in fact it`s why I binned my first missus all those years ago! Over time, her slenderness ratio decreased to the point it dramatically reduced performance and as her wetted surface area increased it had a negative impact on speed, comfort etc and I had to have her bottom scraped more often. Seriously, thanks Nick, I did enjoy this first foray into the design process and look forward to more of the same, cheers easy.
Interesting information. Do mfg’s publish the empty weight (leaving the factory) and the useful load? This is done with airplanes. 2 tons is not a lot of weight especially if the boat is not fully outfitted with cruising gear. Generators, dingy (with motor ) water makers, various sails. The list goes on… then there is the Pabst Blue Ribbon too!
For customers that will spend big money on a Catamaran, I would think they spend a lot of time going over this type of information. But I think everyone knows most buyers just go for the boat that has a nice finish and a great sound system. The science behind fluid dynamics regarding lift and resistance in the water is the most important. Thank’s for a great look into a small but complex part of Catamaran design.
This is great .. thanks so much ! (Don’t take me wrong, but I would like to hear more from the naval architect itself, probably was the way of editing but it looks like Nick is always interrupting the explanation) but this totally great and I’m with you Nick, I prefer to listen to this characteristics rather the color of the galley or sofas… cheers 🍻
Interesting. But, one important consideration that was addressed but passed over very quickly, is the law of buoyancy or Archimedes principal. Regardless of the ratios that impact performance and lift at speed (i.e. dynamic performance), one has to understand that for any given weight of boat and load, the volumetric displacement of the hull will be the same. Archimedes Principal basically states that the weight of, in this case, a fully loaded boat, will have to displace the volume of water of that same weight. In other words, a cruising boat or a performance boat of the same weight, will have to have the same volume of hull below the waterline (i.e. the volumetric displacement will be equal). That is why, I am assuming that performance boats have to be lighter which is intuitive, but also required, because narrower hull for any given length of boat will have has less volumetric displacement therefore, will only support a lighter fully loaded boat. Therefore hull DISPLACEMENT will always be directly related to the boat weight.
Fascinating information. My question why can we not put a underwater wing on the hulls to create lift out the water? I can do it with my serf board. Surly being able to even a just the angle of attack of this wing with electric mothers would give optimum displacement to wetted serf ace for a specific speed for a cat? I have never seen it on a carousing cat…why not?
my preferred concept…how fast can I make this thing go, comfort be damned! These engineering concepts and detailed analysis, is also applicable to automotive design. I want to know spring rates, alignment specs, tire specs, and all the pedantic details. As an enthusiast, I love this analysis series!
Brilliant. I’ve only ever listened to one other presentation from a marine architect… It fascinates me… Odd really, they do understand what goes on. I’m looking forward to the numbers of the advantages of dagger boards over keels. There is so many myths about both, that a design expert can clarify so much…
I assume this was recorded some time ago, so my question is probably going to go unanswered, but I want to know what the benefit of assymetric hull design is. A good example is the Outremer that SLV are on. From top down, there is a pronounced opposing camber similar to a pair of aircraft wings. This should create a low pressure area between the hulls with a corresponding increase in water speed. As a byproduct of this, one would assume the hulls are then in compression which I suppose is OK???
Hi Nick, good article and information. Please can you consider creating and leaving a record behind that can be downloaded, of this and the other key facts, that you are indicating will come in the next article’s, or do so at the end as a “Summary of things to consider cheat sheet”? That would be super useful . Kind regards Gavin
Really appreciate this article, as it is a rare day you get to hear from a naval architect. Thank you, Ruby Rose Team! As an aside, have you heard of whether a closed cell foam core is preferable to balsa core? I thought it was (if for no other reason than things shouldn’t degrade as easily) but so many are building with balsa, I’m beginning to wonder?
great discussion, I would however suggest you have (like any of us would after making a boat buying decision) a bias towards a certain design which is steering the discussion. The several times said “there are no bad boat designs just different intentions” (paraphrased a bit) should be talked about much further because it is all compromise and we are just looking for our ideal compromise (not just yours). Objectivity would show for each feature that might compromise speed could add comfort so in a so forth through all of your excellent talking points. It is also important to frame this discussion with the scale of actual differences in sailing performance. My point being that is you watch the ARC races (for example) there are plenty of well skippered “condomarans” finishing well up the order every single year. Yes it’s won by a Gunboat but there are plenty of Lagoons and FP’s in front of and intermingled with the “performance” cats. That is because other factors have a bigger bearing on performance than simply hull design. It would be cool to talk about these real world influences at some point with objectivity and holistically look at the realities of getting from point A to point B (circumnavigation: Point A to Point A!) with the perfect blend of speed AND comfort for different types of sailers (and sometimes queasy reluctant crew). Your Naval Architect wants to say it but you won’t let him because you keep steering the conversation towards performance being the answer
What you need to know about a boat to make a well-informed buying decision are all the things you can NOT change. So yes, the hull ratios and shapes are way more relevant than the number of fabrics for the saloon, the size of the winches or the controller of the electrical power system (just to name a few).
I would be interested in center of gravity, both vertically and horizontally. For instance, Could you extend the interior bridge deck forward? How do the catamarans with high booms and lounge areas perform? Also, the design of the transom of mono-hulls has shifted in the last few years to retractable swim platforms; will catamarans ever incorporate that design?
Two Comments 1) Love the intro. I was waiting for Duran Duran to start singing and Nick to start dancing in parachute pants! 2) What a great job working with an engineer that is immersed in the complexities of physics, and simplifying the concepts. As an engineer I run into folks everyday that do not appreciate your questions are NOT simple to answer (in our complex world of design). Nick showed respect for him and his knowledge, while trying to simplify his answers for the audience. Brilliant.
Hi Nick, thanks for the great article, I learned a lot! I think your slenderness ratio is upside down, it should be length / cube root of displaced volume. It seems that a catamaran should only use half the displaced volume since there are two hulls. In this case, assuming 25% over the published light displacement, I get 7.3 for the Seawind 1370, 8.4 for the Outremer 45, and 6.4 for a Lagoon 450S. I wonder how much difference this makes in the real world. Unfortunately, we won’t be test sailing the 1370 any time soon.
I’m not getting the slenderness ratio (7:52 ) what units are we talking? Not only that, higher ratio number means faster boat? but as the volume increases the ratio increases (More displacement, is heavier, so slower…) but as the length increases the ratio goes down (but more length = faster boat!). Is that ratio the right way around?
So, as a follow up question, could you ask him how he manages the balance between the variables to create a Performance Cruiser? There are true performance boats (pre-foiling racing cats), there are cruising cats (Leopard/FP/etc), and there is this wide range in the middle called performance-cruisers. Is it all decided by the hull design, or can you take a perf cat and make it a perf cruiser? My guess is it is designed in, and you work within the design parameters (weight loading/balancing) to maintain that perf part of the perf/cruiser. Great series – looking forward to the rest of it!
Interesting interview, thanks for putting it together. These concepts are of course interesting, but in the end the most useful tool to assess the performance of a boat are its polars right? They factor in all the aspects including water and air drag, the size of the sails, their interactions, the static and dynamic righting moment (for boats that generate some), the impact of heeling (less relevant for catamarans), the ability to plane,… It’s next to impossible to anticipate the resulting performance from considering isolated aspects even if yes, a more narrow hull will typically mean more speed. This being said, when you watch articles of the recent Vendee Arctique, it seems clear that performance is just part of the story for passage mating. I would argue that comfort is also key and that is probably even harder to assess from isolated boat characteristics than performance. Speaking about monohulls, one rather unintuitive thing I learned recently is that boats with lifting centerboards are often more comfortable at anchor (less motion) compared to those with lifting keels, because the centerboarders are designed to have balast independantly of the weight of the keel, so they remain stable even with the centerboard up. This to say that considering only the performance value of a keel would result in overlooking this essential aspect of comfort at anchor.
Good to know just like the coefficient in the Sounders-Brown equation but my arthritic bones will better tell me when a rain drop will precipitate. Seriously I’d like to get your opinion at the end of the day a performance hull will get me from Panama to Marquesas in 24 days versus 37 days best case scenario. In the former I need to carry much less than ideal and have less space to hangout. Ratio of time spent circumnavigating versus under anchor. A fair optimum is between performance and the floating apartment. In slightly more cramped conditions it will take me 2 weeks shorter than lumbering in palatial luxury (loaded those words). BTW I had decided a particular trimaran until I saw your review late and the speed space equation came up again.
Great article. A lot of different terms like drag and displacement and such were thrown around a lot without a lot of context on what they actually are and how important they are to the overall performance. I’m less clear what drag is on a hull and how much it effects performance but I can define displacement. Everything knows what it is, but they don’t understand why it’s so important. If you had a boat sitting in the water and you froze time and removed the boat, the volume of the hole left in the water is the displacement. Multiplied by the weight of water is the weight of the boat. Displacement is important when you start moving because you have to push water out of the way to make a “new” hole in the water. How much effort this takes depends on how much water and how far you need to move it to make the hole per second. This is why cats are faster than mono hulls. If they have the same displacement, the cat has to move the water half the distance to make the hole it needs.
I asked the builder of my Whitehaven 11.7 (designed Mark Pescott) what the Prismatic Coefficient was and he informed me that he could tell me but then he’d have to shoot me! (I never did find out what it was). This was a boat that had hull length/beam ratio of around 12.7 (displacement of 4 tonne) and was capable of outsailing most other cats both upwind and downwind.
That diagram for calculating the volume of semi-circular prism was not correct (which should have shown area of one end times length). Hey ho! Enjoyed it all the same! (Surprising that displace this deviled by length needs to be higher for better performance – feels counter intuitive.) Thanks for another good technical Tuesday.
hmm… and here i was trying to draw up the concept of two narrow Submersible bodies built to harbor a wider squat submersible body above the waterline… idea was to use hydrofoils while traveling and to submerge the vessel to wait out storms my fear was that the upright floats/ballasts would rest too deep to go over reefs.
Id like a muliti maybe 4×4 hydrofoil type ie on skis with turbine power below the water but maybe above the hydrofoil shelving like how a plane works stealth wise, also safety and electricity ie using alternators bring more enwrgy from the sea also not just from tue motors and solar but wave power wind power etc but definitaly keey biofuel on board and built in diesel engine just incase…
D/L > 7? Can’t be correct. No matter the Units of Measurement, which aren’t given, the result would increase as displacement increases, and decrease as length increases, the opposite of the stated objective that a higher value would mean better performance. Standard formula is (displacement in long tons (2240lbs)) / (0.01 * LWL(ft))^3. Curious to see what the formula Antoine uses actually is.
Key question is “Fit for purpose”, for me permanent live aboard anywhere in Australian, Papua New Guinea, Solomon Islands, Vanuatu, New Caledonia, Fiji, Samoa, Tonga, New Zealand and passages anywhere or direction in between, (this means a good knowledge and sea states to be found and if at a certain time you must go, or, wait for a weather window and what specifications for the weather are allowed to be included and what speed performance do you expect for any weather option… Now you may consider the technical qualities of the boat and number of people/crew fittings for comfort etc you specify… Life is a compromise… In Australian designs & USA & many other nations, Only one comes near to my definition of “FIT FOR PURPOSE”, is Bob Oram’s design in his 39 C… But I cannot work on “Duflex” as I an highly and adversely sensitive to Epoxy hardener… More needs to be considered sail style… John Hitch is one sailor/builder/racer/explorer I admire for his “Hitch-Hiker” rig and his final sailing design was EXIT with an 80ft mast and huge light weight flat cut genoa to each bow on roller furling (All in or all out) and a blade sail also roller furled used for heavy weather… He sailed as the only crew with his wife as a loving passenger… I am for less sail on a 12M skyhook… I like the feeling of squeezing all the performance in comfort, surfing when the sea state allows, using weather windows sensibly… (With my wife as a passenger and feeling safe with my skill keeping us safe, yet having fun surfing).
Simple: Performance==Start with the hull+rig and fit the interior inside Cruising==Start with the interior and wrap the hull around it with big engines and a smaller rig. More seriously, its interesting how the priorities of hull design change Mono vs Cat. You spend a lot of time on a monohull looking at the righting moment and how that affects the shape (and prismatic coefficient) of the hull, obviously that is not thought of for a cat, These guys are concentrating on hull drag & whetted area, I would have thought a lot more thought would go in to the leverage that the rig can exert before you start lifting a hull and how to compensate for slip. Aerodynamically you often don’t worry about the ‘whetted’ area, it’s all about the frontal shape/size. I suppose water creates more drag through contact? So the balance has to be taken between LAW for top speed and drag. Something else to remember is that big heavy boats move more gently = less seasickness for the crew 🙂
The ratios you used and values that are more indicative of a performance catamaran hull seem to me to be backward in the article. What am I misunderstanding? The statements regarding distance of the hull from centerline (also increasing stability to a point), hull shape and rocker make sense. . Statements from article 1. Slenderness ratio: > 7, performance; < 7 more typical of cruiser 2. Prismatic coefficient: the higher, the better for performance Equations from article: 1. Slenderness ratio = Hull Volume / Length. The term "Displacement" was also stated in lieu of "Hull Volume" 2. Prismatic coefficient = Hull Volume / (Area of cylinder form which encompasses the the entire Hull Volume) Testing equations 1. Slenderness ratio - A cruising cat would likely have a higher (larger) hull volume than a performance cat for a given hull length. Thus this ratio gets larger as the design is geared towards cruising, and smaller for performance. Replace the term "Hull Volume" with "Displacement", the conclusion is the same as a cruising cat will have a high displacement than a performance cat. 2. Prismatic coefficient - When the Hull Volume is a smaller portion of the cylinder, less exposed wetted surface area, thus less drag. The more the hull looks like the cylinder (approaching a coefficient of 1), the more the wetted surface area and drag.
I love a good technical episode. I think there was a little language barrier/confusion because you both kept flipping between ‘volume’ and ‘displacement’. If I’m not mistaken, the slenderness ratio should be flipped around to be: Length at waterline divided by displacement (SR = Lwl/Vol) . The way it’s written is the DLR – Displacement Length Ratio, which is the inverse. The way it’s written is saying that “above 7 for performance” is meaning you get higher performance by making the hull shorter and fatter/adding more volume – which is like saying you’d go faster by turning the boat sideways :p (IMO it’s a little like the confusion when American’s and Aussie talk car fuel efficiency – one tend to says miles per gallon, whereas the other says litres per 100km stat, the former is better as it gets bigger, the latter better as it goes lower)
I don’t think that the majority or people buying an owners version catamaran are that concerned about performance, they would not sacrifice load carrying capacity for all of their toys to gain a couple of knots sailing speed. Catamarans are coming out with hulls flared outwards above the waterline now as standard, so you get the best of both worlds anyway.
If your interested in boat data and ratio’s there are a couple of websites that provide numbers for Mono’s & Cats. Numbers include Comfort ratios, Displacement/length ratios, capsize ratios, Length to beam ratios etc. sailboatdata.com/ multihulldynamics.com/default.asp multihulldynamics.com/news_article.asp?articleID=255 – This is a link the the FP Helia 44 as an example. Proving she is a heavy slow cruising cat, but is relatively stable
As a boat designer and builder, I don’t agree with the naval architect. You can have long slender hulls for catamarans. The issue with cats and multihulls is all about payload / displacement / bridge deck. I believe design to sea state 3 as a rule of thumb. It just becomes less practical. We are talking displacement, the higher the slenderness ratio the better can go to 10 or even 12. Do not confuse foiling with these just yet. That is a whole other HUGE subject and not related to slenderness (I specifically focus on foiling boats and am not interested in displacement, semi displacement, or planing hulls, but need to understand the fundamentals of these first). Foiling boats are in its infancy. One issue to discuss is fouling potential – and design has a part to play (fewer sharp angles!) Generally though, as a starting article on design, to focus on hull is crucial so slenderness ratio and Prismatic coefficients is a great place to start. Well done! Rocker. There is also a aft rocker for both planing and displacement hulls – which affects trim angle through speed variances and hull resistance. Then look at transom design as well.
I had to watch twice before writing this comment, because something doesn’t make sense. If Slenderness ratio is diplaced volume divided by waterline length, then a slender hulls which has lower volume and longer WL length should have a lower ratio not higher as said in the article (decreasing the numerator and increasing the denominator)
What it really boils down to is “cruisers” vs those who plan to circumnavigate. Personally, when crossing an ocean on a 1,500 NM plus passage, where there is literally nothing to see, would you rather get to your destination in three weeks or 1-1.5? People who say, “we aren’t in any hurry” fall into one of two categories, if not both… 1. They are simply island hoppers who spend 90%+ of their time on the hook or docked. These are the epitome of the term “cruiser.” And that is PERFECTLY fine if that is their preference. 2. Those who cross oceans on a slow boat simply did not research and/or could not afford anything else. And that is perfectly fine for them as well. But, I have yet to see any sailing website who is not miserable around day 10 of a 20 day passage…. Food for thought, and as always, it comes down to preference and compromise ❤️🤙
The reality of a cruising catamaran is that 2 tons of weight is just not enough in the real world. If you take 800l of water 400- 500l of diesel another 100l of dinghy fuel. Then you add a diving compressor and 4 sets of dive gear and 4 humans you have maxed the weight without even the spares needed the extra gas cylinders, paddle boards, kite boards and kites, cameras computers and all the food and drinks etc. I think that 4000kgs is a more realistic figure to aim to be under.
Fineness Ratio is the word you were looking for. A cruising cat is generally 7:1 to 12:1. That’s hull beam to length below the waterline. Modern performance cruisers step way out above the chine as living space floor is usually above the waterline. This requires a balancing act and will be like using speed brakes if you freight it enough to submerge the chines. Catamaran language is absolute garbage which is a great benefit to manufacturers and dealers. Hull beam to length should be a straightforward answer. Hull to hull width or total beam is something else entirely. Getting the fineness ratio at or above 11:1 with good hull living space takes use of exotic material construction to run narrow hulls below the chine and maintain shallow draft unless you exceed 60ft. That’s part of the reason the bigguns are so much faster.
🙄😳 Your father in-law makes surf 🏄♂️ boards!? So, why not disseminate his info’ so that we can buy boards from him? Plus you can probably receive some money for brandishing his info’ in your plug… 🤷🏽♂️Just saying…P.S; maybe you can have a Ruby Rose 🌹 Board with its logo made up for you guys and sell that…
Well, thank you very much. This explains to me why it is ridiculous to look for a performance catamaran as a liveaboard plattform. If the hulls are designed for perfomance, they work best empty and lightweight, narrow and shallow. Same for a cruising condomaran with beamy round hulls btw. But then you move on board, want a generator, lots of fuel and water, diving gear, compressor and A/C, huge solar array, a good dinghy with a proper outboard on the davids, fridges, freezer and a lot of provisioning, space for your clothes and toys. A performance hull will drown itself and the cat will drastically suffer performance, a cruising beamy hull will swallow it and barely get deeper into the water, it has a higher carrying capacity. You pay a lot of money for lightweight performance, sacrifice living room inside and in the end a fat heavy Lagoon or Leopard sail faster and more comfy, and is cheaper and safer, it will not flip that easy. So make an educated decision, will you race or will you live aboard and cruise, and then chose your hull design and brand. It is a no brainer, that a performance vessel is a expensive but poor cruising vessel.
It’s not the rocket science!!! People who buy catamarans don’t race them against other catamarans. Unless you buy a proper racing catamaran and use it in the race only, it doesn’t matter how fast you sail. All other catamarans are cruising catamarans only. People who buy them, buy them for pleasure of sailing and living onboard. They are not in the hurry to go anywhere. The most important thing for buyers is comfort! That’s all. Most of the people don’t know how to properly trim the sails. Make sure you keep the bottom clean and that you boat is safe. It doesn’t matter if you sail 1 knot slower. French Polynesia is not going anywhere. You have plenty of time to get there. I guess I’m going to play an idiot and ask all those fancy terms and questions at the next boat show.
The best Cats are amazing to sail on, and so awesome cruise on but don’t try and tell me that they are incredibly well built and well engineered because they are not the best boats are almost tolerable its just a Tupperware shell with accessories bolted into the shell somewhere . a silly engine arrangement with a prop hanging in the water, And they can sink !OK maybe anything could sink but for a million plus there should be some counter measures to sinking above and beyond seamanship. There are some amazing French Yacht gurus from skippers to designers and crew…this -person is none of them sorry seems nice but puh-leez …
I’ve been perusal Cats on YouTube and I’ve seen some things. Like any boat, Cats are high maintenance. I’ve watched Zatara and it is a big slow Cat with room for a Family. La Vagabonde seems fast enough for me, about twice as fast as Zatara. 20 years ago writers were very afraid of them Pitchpoling. That no longer seems a problem. Perhaps they made the rigs smaller? La Vagabonde withstood a rush voyage across the Atlantic, and they seem to have suffered some damage from the storms. I owned a mono hull and I wouldn’t do it again.