Modular Cruising Catamaran

Split Junk Rig

I think the Hills would agree that Badger, with its flat-panel junk sails, didn’t sail well to windward. When Pete built the junk-rigged China Moon catamaran he attempted to address that shortcoming by using articulated battens that allowed a limited amount camber in the sail. I don’t know if he was pleased with that design or not, but he has continued experimenting with jointed battens on a subsequent boat, Shanti.

However, the newer camber-panel and split junk rigs appear to be competitive with Bermuda rigs to windward, while still retaining the junk’s easy manners. The camber-panel rig is similar to a flat-panel rig, except that a “bulge” is cut into the top and bottom of each panel (or, alternately, a “shelf” is sewn into the top and bottom of each panel). This allows the panels to form a camber, giving a better aerodynamic shape. The split junk rig, designed by Slieve McGalliard, is very similar to the camber-panel, except that the sail is “split” vertically at the mast, allowing the sail to be free of the mast on either tack.

Below is an exchange on the Yahoo junkrig group describing a Slieve’s experience in last year’s annual race around the Isle of Wight using the split junk rig.

 

I.N.

Sure no problems. If you can just sketch up, that is pen on paper, back of a fag-packet type of sketch, the basic arrangement, we can take it form there. Since we need the layout, ie length, beam of each hull and overall, displacement, distance between CL's.

 

The Flica would be a better choice for a free standing mast, but I have done it on a cuddy cat like the Romany.

One structural advantage of the free standing mast is that there are no real compression loads so you don't need a deep mastbeam.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

I was with John Hitch yesterday and posted some stuff on my build site (links in my "signature area" of all my posts), That is what I am putting on my build - I am 65 and retired, so desire something easy to manage... The mast will be raked some 10 to 15 degrees aft. The mini-keels will be set about in line with the CofE for the sails... (only three sails, a pair of fairly flat cut genoas (John has "star cut") and a small blade /storm sail running up from half way between the line of the bows and the mast base, also no roller reefing.... Do NOT use the heavy "coloured" sail protector, but use something light, (that you can easily replace?) as the sail gets too distorted when partially roller-reefed, and would be too heavy in light airs... You are cruising? - so being able to perform in "drifting situations" is an advantage, and all sail work can be done from either a forward of the mast or aft cockpit - both sheltered from the weather (sun or sleet).... John is sailing up North (inside the Barrier reef, Queensland), to return around Xmas and may drop in for another visit then... and I will get some more images for myself (and post some also)... XIT is balsa cored with hand layup of grp as preglassed panels were not available then...

Check out the ATL site carefully http://www.duflex.com.au/duflex/amateur/projects/oram_44_c_cruising_catamaran is the bigger model (44-47 ft loa & 23 - 25 beam) - mine is in my "Bob Oram link" in my signature area.... I would strongly advise, that you find a build site nearer to a launch location avoid cold and rainy areas as it is not nice to work with epoxy as the hardners need to be adjusted for ambient climate... Don't take my word, check the advice from the professionals at ATL, they deliver, and 3 boats and more will fit inside a container for a group build (mutual support and bulk discounts on other items).... Working 5 days a week project should be launched within a year with 2 novices working on the build, or about 5 years "part-time as in http://scrumbleproject.wordpress.com/table-of-contents/ .... Neil who is building by himself and occasional casual help, will build his 44C in about 14 months of 5 day week work... He will launch April 1 2010 (Neil & I are "racing" each other to get on the water first)

Why all this determination for free-standing masts and bi-pole rigs? Not a good choice, go and sail one and you will understand and remember the forces are multiplied as sixe gets scaled up.... so expect scantlings for that choice to include a one off pair of carbon fibre masts.....

 

catamaran with deck beams

Ad Hoc and all,

Attached are sketches of a plan and profile for a 36’ catamaran, with the hulls connected by box beams. Most of the dimensions are for reference only – anything can be changed as needed. I’ve stuck a shortened mast on the forward main beam. I’m not sure whether I want two free-standing masts in the hulls (my original plan), or one free-standing mast on the beam/bridgedeck. I would appreciate your walking me through the basic method of determining the expected stress on this catamaran, and then giving me some ideas of how to design appropriate beams that can be quickly fastened to the hulls.

Thank you,
i

P.S.
Fully loaded displacement is ~11,000 lbs

 

I.N.

What is the approx full laod displacement?

 

Fully loaded displacement is ~11,000 lbs

i

 

sqrt(-1),

The classification society known as "Bureau Veritas" makes their yacht rules freely available: http://erules.veristar.com/dy/app/bootstrap.html
There's also a thread on this site in which those rules are discussed:
http://www.boatdesign.net/forums/classification/bureau-veritas-rules-download-23030.html

The BV rule is by no means the be-all and end-all of design requirements, but it does include a reasonably good, easy to follow section on the load cases to which a sea-going yacht might be subjected, including catamaran-specific loads (in sec. B.4, B.6 and B.7). As you've probably noticed by now, catamarans are subject to incredible stresses in the rig and crossbeam structure, and the calculations to deal with them are not trivial. But it will give you some idea of how the boat might be stressed.

 

I.N.

Thanks for the data.
Ok, the first thing is that you have 3 transverse beams. In terms of taking transverse pitch connecting moment loads, ie twisting, only 2 are doing the work, the other is just adding weight. So, which two do you want/need?

The fwd most is doing the most work, so it sounds sensible to keep this. However you need to support the mast. If you keep the beam to support the mast and this beam ends up being as stiff as the fwd most beam, then you're in trouble. Because the beam supporting the mast and its loads will attract the main transverse loads owing to it stiffness, yet not be designed for this torsional load.

Therefore, it is best to ignore the beam up fwd and use the beam for the mast as the fwd beam and the aft beam, as the main beams to take the torsional loads.

Do you concur this is ok, unless you ahve some other function for the fwd most beam?

 

marshmat
Thanks for the Bureau Veritas link. That will take awhile to digest!

Ad Hoc
No, Thank You! I appreciate the explaination you gave for how the beams are loaded and where they can do the most good. The forwardmost beam I showed smaller because I imagined it's primary function to be supporting an anchor, trampoline, and perhaps a boarding ramp. Perhaps it could be retained, but sort of free-floating, taking minimal loads. So I agree with you that the mast beam and the aft beam should be the primary load carriers.

i

 

I.N.

Ok, understood...i'll put something togther later today, assuming wife doesn't hassle me too much, and we can take it from there.

 

Here's one way to think about it. Consider a wave that has a wavelength larger than twice the beam, and the boat is straddling the crest. The wider the boat, the higher the crest will be above the waterplane of the hulls. For a very narrow catamaran, the curvature of the crest will not be very important. But very wide cat, the curvature is definitely a factor.

So there are really two ways for the wave to impact the bridge deck. One is for the boat to be immersed in the wave crest, especially at the bow, by the amount of the bridgedeck clearance. This would be the same for both narrow and wide catamarans. This case would be most relevant to hitting a wave crest head on, so the crest passed from bow to stern.

The other way is for the waterline on each hull to be near its normal position, but for the wave to mound up between the hulls. The wider catamaran needs more physical bridgedeck clearance to handle this case. This would be most relevant to a beam sea, with the wave crest running parallel to the boat. In reality, of course, waves from all directions are present and the sea is composed of a lot of lumps instead of being pure ridges. But the same principle would apply to the lumps.

This argues for a lower limit to the aspect ratio of the tunnel (under deck, between hulls) as well as a minimum height to the tunnel. The height of the tunnel is commonly cited, but I've not heard anyone talk about the aspect ratio of the tunnel.

 

Hi I.M., interesting thread,

Hear Hear! I have been on the lookout for some years for a cruising oriented "Pod cat" design. I am now also in the process of (slowly) designing my own, and quite enjoying the task, but it would be much simpler if some stock designs were already out there.

In regards to bridgedeck clearance, the maximum slope of non-breaking waves is typically regarded as 1:7, but I have seen transient peaks in confused swell that look much steeper than that. The required height is also affected by boat inertia (translational and rotational), hull immersion rate and reserve buoyancy. My own conclusion is that the problem is too complex for a feasible solution, and rules of thumb vary significantly between designers. On the bright side, with a modular design it may well be possible to change the clearance retrospectively if required by modifying just the cross beams.


Daiquiri,
On the subject of a biplane rig, I'm no expert, but one simple point always seems to me to be lacking from debates on this topic: multiple masts allow a boat (of given righting moment) to carry more sail area in safety. (Given a fixed sail aspect ratio, twice as many sails means ~40% more sail area to the same heeling moment). A single sail is certainly the pinnacle of efficiency, but as a cruiser (untroubled by sail area handicaps and rating rules) I can live with a less "efficient" boat that sails faster and more safely.

You make a very valid point about air draft however - for an area where very light winds were the prevailing feature a shorter rig is at a definite disadvantage.

 

Autodafe, the lower the sail the less wind available as power especially in light wind situations... I consider it is more significant to have a robust pair of hulls (for a cat - as a tri has different engineering issues that I have not even looked at)... - - Warram designs - in traditional form allow a fair amount of flex between the hulls, - - mine has some flex - but not much, - - in the overall structure, the John Hitch X-IT is as stiff as all get-out, the hulls could be called wave piercing as only 2 ft wl beam - images in a recent post on "my little piece of peace" thread... I am not in favour of trying to make a silk purse out of a sows ear, or, scaling up from a toy or little day sail "lake" boat - it just does not work from an engineering point of view....

 

Hi Mas,
I agree windshear is a consideration against low sailplans, but by the same token it favors sails such as gaffs and junks which get more of the sail near the top of the mast.
It would be great to get some more info on windshear, if anyone has it, perhaps a series of windspeed vs height for a variety of weather conditions.
The data I have seen suggest that the difference between 5m and 10 is more significant than that between 10 and 20m. But I'm prepared to be corrected on that one.

Loved the pics of X-IT too, it is much along the lines of what I would like, but I think I'm too lazy to go for a highly streamlined bridgedeck, as I plan to build in plywood.

My personal preference is for a stiff structure - it simplifies the calculations, and also prevents shock loads due to "flexible" joints reaching the end of their travel, but it is less important with an unstayed rig.