Modular Cruising Catamaran

Richard Woods, I’d like to better understand why you don’t recommend bi-plane rigs? The two links I gave in a previous post were both for bi-plane cruising catamarans. Pete Hill is a very accomplished sailor, and he sailed the junk-rigged China Moon mostly single-handed thousands of miles over the course of a few years. Bertrand Fercot liked his bi-plane 30’ Wharram Tiki well enough that he is now building a larger 46’ Wharram bi-plane with Swing-Wing sails (which have some similarity to junk sails). I don’t particularly care whether I have a bi-plane rig or not. What I do want is the ease of sail handling that comes with the newer versions of the junk sail. Junk sails typically are used on free-standing masts, and I’m not sure how to mount such a mast on a catamaran’s deck – thus the choice to go with a bi-plane rig.

To gain access to the foredeck I am exploring the idea of a hatch of some sort at the front end of the cabin. I’m not sure how to do that. I know that some of the larger catamarans have front cockpits, so they have solved the problem of a forward passageway. But I don’t know the details, and don’t know if I can use that for my size boat. The problems I see are having a hatch that can withstand the force a wave breaking over the bow, and also allowing dry passage through it during a heavy rain. Because junk sails are so easy to tend I don’t expect to regularly need to go on the foredecks during heavy weather. I suppose if a forward hatch is not practical I can always go up and over the roof, or down into the hulls and back up through a hull hatch – but these are not my preferred solutions.

I also left off any details of a helm station on my drawing. Some sort of doghouse on the roof comes to mind, but that puts a small piece of the superstructure even higher.

Masalai, Thanks for inviting me to ask dumb questions. I’m doing my best not to disappoint. There is nothing I would like better than to not have a long learning curve. I’d prefer not to be doing any designing, but rather look through the existing designs, pick one that fits my needs, and start building. My reluctance to go with any of the designs I’ve found thus far resulted from my desire for an easily-heated cabin (low surface area for high interior volume; i.e. a single cabin), and a modular structure that could be mostly built away from the launch site.

Ad Hoc, The Sketchup model is to scale, though from the perspective shown in the above model the length may appear foreshortened. It really is amazing how fast one can learn the basics of Sketchup, though I am having particular trouble with intersecting curves for the hulls. Eventually I gave up and resorted to crude approximations with large triangles and rectangles.

Your suggestions of laying out floor plans and calculating CoG’s are appreciated, but as I noted to Masalai I’d rather be building than designing. The primary living quarters are in the front two modules. Together their floor space is roughly 20’ wide by 14’ long, call it 250 sq ft. Considering all the storage space that is still available in the hulls I believe that there would be plenty of space for our needs, and the big, open bridgedeck could accommodate any number of acceptable layouts.

The primary reason I’ve come to this forum for help is to see whether a couple of specific problems could be solved by the generous designers and engineers here. I know that many problems are complex, and one apparent solution may turn out to be inappropriate when other constraints are introduced. But I think enough has now been specified so that a first-approximation design can be made, if folks are interested in doing so. The stresses on the bridgedeck are not going to be appreciable different whether the berths are placed forward or aft. Nor is the windage going to change much according to where the galley is put.

 

Perhaps I’m asking too many questions at once? I’ve made another sketch of just the hulls (a bit narrower and deeper than before), with a simple, flat, three-piece deck. The forward deck has a triangular deckbeam, and the aft deck has a rectangular deckbeam. The decks are ~6” thick by ~20’ wide, and the beams are 1-1/2’ high. The fore deck and middle deck are 8’ long, and the aft deck is 5’ long.

Would anyone care to suggest a design whereby the decks are built independent of each other and the hulls, are strong enough to carry the expected loads, and can be fastened to the hulls as a secondary operation?

Thank you,
John

 

I.N.

"..I’d rather be building than designing..."
I'm sure you would, but if you don't do the basics first, you'll be on this website again, in about 1 year, asking such questions why this and that is happening and how do I cure it. Which will just point to the fact a simple CoG analysis would have shown prior to any build. As I've always said to the draughtsman and naval architects I've trained up, an extra 'hour' spent in the design office, can save endless manhours or even days/weeks in production hours, not to mention money later on. But if you choose not to do so, that is your prerogative.

"...Would anyone care to suggest a design whereby the decks are built independent of each other and the hulls, are strong enough to carry the expected loads, and can be fastened to the hulls as a secondary operation?.."

I've design several catamarans in the past, (commercial) where the two hulls ahve been bolted together via box beams. It is not a difficult task in the engineering sense. All you need to do, is calculate your full/max load and then establish the sea conditions and hence the applied loads that shall be experienced. From this, you can design the structure to be "bolt-able", or any other quick means of fastening, having the ability to "take out" and sherk the applied loads to the surrounding structure. It is all about the applied loads, the rest is child's play after that.

 

Some random thoughts - hopefully of value:

1. You should be able to master Delftship in about half the time as it has taken you to get to grips with Sketch-up. Once you come to grips with the idea of surface modeling it is very easy to use. It has more capability than Sketch-up for boat rendering and design data. Cannot remember if Shecth-up has layers but this adds tremendously as you get further into the design process to easily get weights of the main components.

2. What you are aiming to build is more like a pontoon boat. Accommodation separate to the hulls. You may get design and construction cues from these.

3. Joining cat hulls requires structurally competent beams. I would look to separate these from the accommodation, at least in the first instance. So the boat would be like a big beach cat until the accommodation pod was added. This gets large structural loads out of the accommodation pods. Large beams could be socketed into the hull on tapers. Pulled up for assembly and jacked off for disassembly.

4. Constructability on an engineering project usually follows the design. It IS iterative but the method of construction does not dictate the outcome. I would look for the boat that came closest to suit the needs then look at constructability. If there are simple design changes that made it easier to build then these are incorporated.

5. The design effort that goes into a complete boat of these proportions from scratch is significant. The more you can take from an existing design the less work involved. If you know of a boat that has the basic proportions then this might be a better starting point. A NA will charge around $100/hr and you do not get a lot of design for a days work. You can make your own estimates but taking your ideas and getting detail design for a completely new boat would be say 10 weeks work at a guess. So before you start building you have spent $40k on a unique design. Can you save this on the construction?

Rick W

 

Miss Cindy

Here is a pint-sized version of the style of catamaran I am wanting - sans modularity. Large single cabin, free-standing masts, easily managed sails, fun to use. It doesn't have to be car-toppable! Don't any of you catamaran designers see the same vision that I see?

http://turtleislands.net/mc/default.html

 

Scaling up is not really successful, the diagonal twisting forces are the killer... - - why does it cost a lot less to build 4 boats 20 ft by 10 ft than one boat 40 ft by 20 ft? the same reason the 4 boats weigh less than the one... I can't be bothered, DO SOME practical RESEARCH... GO SAILING... Try boats about the size you want... WHY reinvent the wheel - it probably has been done and found not to be very successful at the size / scale you have expressed interest....

Have a look at this sailing rig, It is called a "Hitch-hiker" rig after John Hitch... if you want an easily handled sailplan - - NO forward or any-where else work all done from within...

 

Would it be too much to ask for you to elaborate on bridge deck clearance? I’ve probably read about every word you have on your website and know the reasons for bridge deck clearance... wave pounding the underside. Obviously there is always going to be a point where pounding occurs even at 3+ feet clearance and I understand why hulls wider apart need even more clearance.

To the discussion at hand… say ImaginaryNumber’s size 36’x 20’. What would the differences be that you would recommend based on mission (costal cruiser, racer, blue water)? And with your recommendation for blue water, what has been your experience of how much you have still experienced pounding?

 

I don’t have any credentials… (I’m an amateur) but I’ve found that Delftship is very easy to learn as long as you don’t ask it to go beyond doing the hull shape. Forget using it for interior structure and bridge deck. You can export the hull bottom using File/Export/DXF 3D Mesh… This is easily imported into Google SketchUp. I’ve found building with it to be a lot easier. However, I would not want to try building the hull in SketchUp.

e.g. – Building a 90 degree staircase on the back of a Cat hull took nearly a half week with Delfship. Doing the same thing with Sketup takes less than 5 minutes.

But, for the stage you’re in now… throwing boxes around… you’re doing fine.

 

http://freeship-plus.pisem.su/indexEN.html is the Russian version, I used it and mostly sorted and satisfactory within 6 months, Delftship is the European ? version and has an upgrade path to its commercial stuff. If you are not brain dead or afflicted by encroaching Alzhimer's then either path is the way to go as the others cannot give displacement and weight analysis which is necessary to determine if the stuff you want aboard will fit (space wise as well as weight carrying ability) DO NOT WORRY ABOUT bridgedeck refinements yet... Until you realise what is achievable and realistic in cost benefit analysis, the technical stuff can wait... Then sort out the structural engineering issues, then and only then, bridgedeck clearance and hull lines for cruising efficiency...

Berths in the hulls aft will give the most comfortable movement for anchored up at night - feet aft/head forward... Toilet and shower in the forward end of one of the hulls, washing machine in the "toilet space" in the other hull... consider hitch-hiker option (no main and all the controls associated with it and no head banging boom for 'old farts' to worry about / forget is there - - OUCH)...

Got that? sensible layout first

Engineering issues (split and join) next

Cruising design on hulls and bridgedeck clearance

Find an existing design that meets most of your needs / desires and buy build that - saying, "So close to what I am after, why re-invent the wheel"

A well trod, tried and proven path...

Sailing wing on wing (both sails set outside) is bloody silly as a minor change in wind or whilst climbing some swell could force a catastrophic jibe... Never do that, and cats are better tacking downwind (better VMG) That is also why windvane autopilots are stupid, cats often make their own wind (the apparent wind seems to come from more foreward than the true wind) so windvanes fail to do anything useful... (windvanes seem to work beautifly on those heavier canoe stern cruising mono's happily holding a steady 4 knots through the water)

 

Like everything in yacht design, bridgedeck height is a compromise. Higher is better, but adds to windage and overall boat weight. A wider hull spacing does not mean the bridgedeck needs to be higher. It is best to start the bridgedeck as far aft as possible and have a gently sloping front. Don't carry the deck low aft. That is because as the boat pitches the aft end of the bridgedeck can hit the water. Furthermore, the wave interactions between the hulls usually mean there is a high wave under the cockpit.

I write more on my website, but as I said, as a starter try for 18in - 24in clearance.

Just because one sailor used a bi plane junk rig doesn't make it a good efficient rig. I have seen Badger sailing to windward in the Caribbean. It was pitiful to watch.

As I understand it, the Chinese developed the junk rig because they only had woven bamboo for sail cloth. A material that couldn't take a load. Hence the full battens.

A 35ft catamaran with a modern full battened mainsail fitted with lazyjacks and singleline reefing with good deck gear can be reefed single handed in a minute or less. I doubt if a junk rig can be reefed quicker. But the fully battened sail will get the boat to windward far better otherwise race boats (and indeed the vast majority of cruising boats) wouldn't use a conventional rig.

Do you base your junk rig comments on personal experience, or just from your reading?? If the latter I strongly advise that you try a junk rig, or a biplane rig yourself before deciding.

it is quite possible to have a centrally mounted unstayed mast on a bridgedeck catamaran.

If you sail in the PNW you need lots of sailarea and an efficient rig. The average wind speed is about 5 knots (in the UK it is 13 knots). Believe me, I have sailed in the PNW for the last 5 years and not yet had to reef.

(I am late replying to this thread as I've just come back from a few days cruising, despite the rain.)

Hope this helps the debate

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Bertrand Fercot's Pha is not junk-rigged in the conventional sense. The biplane rig is a sort of soft wing sail, with the shape of the sail defined by sets of doubled battens, the mast running up inside the battens. I don't know what sort of performance he gets to windward, though he did sail the boat across the Atlantic from west to east. He's an accessible guy, if you want to ask him. His Tiki 30 looks as though it were built more for comfort than speed, with a big pilot house on the center deck. It could be his standards for performance are more relaxed than some. On the other hand, he likes Pha well enough that he's building a Tiki 46 with a similar biplane rig.

There are a couple of junk-rigged Wharrams here:

http://wharrambuilders.ning.com/group/junkrigged/forum/topics/wharram-cats-rigged-with-one

I'm a big fan of junk rig, and have some experience with it, though only in monohulls. Windward performance has traditionally been a problem for the rig, but in late years much experimentation has been done to improve this, with cambered panels and gurney flaps. In fact, I'm such a fan of the rig that I've put it on unsuitable boats, like this 11 foot dinghy:



However, I don't think that it is a good idea for a cat with any pretense to speed, though I could be wrong. A real naval architect once told me that he'd sailed on several junk-rigged Wharrams, and he called them death traps. I don't know if I'd go that far, but there are definite problems. Because fast boats are generally operating with apparent wind well forward, it behooves the rig to be weatherly. The weight aloft is another problem. Even monohulls will need a stout stick if it's unstayed, and because of the great initial stability of cats, rigs must be much stronger than similar monohull rigs. It's not just the stick-- the battens, sheetlets, euphroes, and so forth add up to a considerable weight up high, and this weight exacerbates the tendency of fine hulls to pitch. And because of the balanced nature of a junk rig, tacking is not as crisp as with a rig where all the mainsail is behind the mast, and cats sometimes are a bit sluggish to tack, anyway.

I'm even more dubious about biplane rig than junk rig. Thomas Firth Jones built a cat with two sticks and sailed her for three years in increasing dissatisfaction, finally replacing the rig with a conventional fractional sloop rig. Even the high-priced versions, like the Radical Bay have not been entirely successful. On the other hand, there's Tony Bigras' Miss Cindy, a nanocat which just completed a successful voyage of substantial length. His ideas for sheeting the rig (which appears to be more of a battened lug than a junk) are very interesting. Again Tony is an accessible guy, so you could talk to him about your ideas and see what he says.

All that said, I'd love to be proven wrong, because junk rig is such a pleasure to use, and the biplane rig is simple to engineer.

 

A slight update.

There should be no need to go on the foredeck, or even out of the cockpit, to do any sail handling on a well designed cruising catamaran except to hoist/lower the spinnaker. And maybe a sea anchor/parachute when you will sure want to get to the bows in safety.

However you WILL spend a lot of time on the foredeck when coming alongside, anchoring etc. And chances are that at some stage you will need to get there FAST

To add to Ray's comment. James Wharram was a great advocate of junk rigs. He tried one on his Tehini. It lasted a year, then he took it off and replaced it with a conventional ketch rig.

And as an aside. It seems that many people here think that a computer designs good boats.

It doesn't.

All it is is a glorified pencil. And to continue the analogy, some people are asking how to sharpen their pencil.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Biplane rigs are less efficient when compared to a single-masted rig. It is their inherent characteristics, can't do nothing to change it.
For a same total surface and aspect-ratio, a biplane rig's height is 70% of it's single-masted correspondant. It means that it can catch less airflow (the wind speed increases with height above water surface) and therefore will produce less thrust.
Furthermore, two sails means two sources of induced drag, which again directly decrease the overall rig performance, and the VMG in particular.
Less thrust, more drag. Make your own conclusions.
And then, there are the obvious constructive and handling complications...

 

I think Richard was probably referring to the Badger owned by Annie Hill and her then-husband Pete Hill, which is a junk-rigged monohull dory yacht designed by Benford. Because of Annie Hill's terrific books, it's one of the more famous junks. In her books, she sort of glances over the issue of windward performance, which supports Richard's observation.

Back to biplane rig, Thomas Firth Jones sailed with one on his Dandy for 3 years before finally giving up on it. He cited the usual lee blanketing issues, that biplane rigs are prone to, and said that the rig was both slow as well as disappointing to windward. H also pointed out that reefing could be a trial, since the leeward sail was further out over the water than it would be in the skinniest of monohulls.

But then there's Tony Bigras, who overcame the efficiency issue by putting a lot of area into his rig. 200 sq. ft. of sail is a lot for a 16 foot cruising cat with a beam of 8 feet. He says he solved the lee blanketing issue when reaching by sheeting the windward boom to the bow, hauling the clew forward of the mast-- in effect, converting two sails into one foil. He could do this because Miss indy has an unstayed rig, whereas Jones' Dandy was stayed.

I remain unconvinced but it's hard to argue with Miss Cindy.

 

I’m not an engineer. Would you be willing to guide me through a back-of-an-envelope calculation for determining the full/max load, establishing the sea conditions, and then designing a quick-assembly structure? We could pick a pre-existing catamaran about 36’ long so that we wouldn’t have to spend time on construction details, except as it applies to the hull-connecting structures.

I’m encouraged to try Delftship. What CAD software do commercial designers of catamarans use that allows them to design both the hulls as well as bridgedecks and interiors?

I’m open to either the beams being separate from the accommodations, or the accommodations being directly incorporated into what are in effect very large deck beams. Can you point me in the direction of a design that uses sockets and tapers for attaching the beams to the hulls?

Yes, much, much easier. I don’t have the resources to hire a naval architect to design from scratch. One of my hopes in starting this thread was to find out if I had overlooked a design that was close to what I wanted. Although some of the respondents have also indicated an interest in a modular catamaran, no actual design has been presented. FlatCat and Miss Cindy have the single interior space. Wharrams (and others) build their hulls independent of each other, then assemble latter. There are a number of monos and multis having free-standing masts, some using junk rigs. I see most of the pieces of what I want already developed. They just aren’t configured together quite like I want them.

That does look like an interesting sail plan. Can you give me a link to a description of how it is configured, and what its sailing characteristics are?

Could one of your designs, say the Romany 34 or the Flicka 34, readily be converted to support a free-standing wood mast?

I agree, though they do have the advantage of lowering the center of effort, which some cruisers appreciate. And junk rigs, because they are squarish, lower the CoE even further.