Modular Cruising Catamaran

[ImaginaryNumber]

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.

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. Richard Woods

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.

Posted by: Slieve, Sat Nov 8, 2008
Hi Wally
The report on the Split Junk gives the basic details, but not the race result. We were in the top third of the handicap fleet of over 800 boats, but that does not tell the story. Turning onto the last beat we found the boat dead in the water, and discovered that the bilge water was up to the floor boards, so we pulled out into the adverse tidal stream to get elbow room to pump out and look for the source of the problem. We didn't find it so returned to racing and achieved the published result. In practice, from earlier calculations and from examination of the log on the plotter after the race it was clear that we had lost over 40 minutes, and that was not counting the time lost carrying an extra half ton of water. If we subtract 40 minutes from our finishing time we would have been in the top 10% of the fleet, and possibly in the prizes.

The real point of the exercise was to compare the rig against a range of boats and the results seem to indicate that the split junk rig, even in its undeveloped form can outperform the average Bermudan cruiser without the crew having to work hard. We tacked 36 times and gibed twice and changed reef twice, yet we only adjusted the halyard and downhauls twice and the main sheet 4 times during the whole 50 miles of the race, and that was with a force 5 to force 6. Add in the fact that we only left the cockpit for domestic reasons and that we did not suffer from water over the deck and it becomes clear (to me) that the rig seem to have every advantage over the more conventional boat.

Since then I have sailed at the Stavanger Rally and the performance of the boats with Arne's sails confirm that the modern junk rig is a real contender.

Two days ago I did a delivery on a rundown Bermudan sloop. The fully battened main sail stuck in the mast track so we had to sail under genoa alone. The engine lost all its oil so we had to beat into a narrow estuary in a light wind. The baby stay caught the clew of the sail each time we tacked so it was difficult to sheet in quickly after each tack. After beating a couple of miles up the narrow river I was exhausted, especially when aware that it would have been a pleasure to sail the same trip with my split junk rig. I really cannot see why anyone would want anything other than a good cambered junk or wing sail.

Cheers,
Slieve.

Posted by: Jon
I am interested to read Slieve's account of his performance in this years Round the Island race. Normally I sail a 28' Sunbird and find it an excellent rig. In this years race however I was crewing on a Bermudan rigged Westerly Konsort. I was amazed at how tiring the sailing was. Every tack took 3 people's full effort (there were only 3 of us), and even when not tacking we had to post one crew member on lookout who had to peer out under the Genoa and call to the helmsman whether to stand on or give way. Quite terrifying under the windy and very busy circumstances.

However my favourite memory of the day was when we were halfway between Needles and St. Catherines point - enjoying a fast beam reach. I looked behind and saw Poppy with her beautiful white Junk rig just coming around the Needles. She flew through the pack and within 20 minutes had overtaken us, and everyone else it seemed. 20 mins later she had disappeared into the distance ahead. It seemed that no-one could touch her for speed on that leg. We caught up with her much later in the day - and now I know why- but she still crossed the line well ahead of us.

Jon

Posted by: Slieve
Thanks for the chat on the 'phone this evening Jon. I also spoke to Colin, the owner of the Konsort and he agreed that we overtook you on the last beat when we started racing again.

For the record, I was concerned that the Konsort was ahead of us at the end of the first 13.5 mile beat at the Needles, but you were able to tell me that you started at 07:30 where our start was at 07:40. Even ignoring the handicaps used in the race as you didn't (couldn't in the conditions) use your spinnaker and using the WOA handicap which would make my boat 1% faster, on elapse time you took 9:45:50 and we took 9:28:27. Subtract the conservative estimate of 40 minutes we lost pumping out and checking for leaks and I think a corrected time of 8:48:27 suggests that we took about 10% less time for the 50 mile course.

Apart from the difference in time, we were not tired at the end and went out for a meal before the (60+ year old) boys drove over 70 miles home. (Our combined age for the 3 of us was over 190 years)

The reason for labouring these points is to show that a good home made Junk rig can beat an equivalent cruising Bermudan rig when sailed on equal terms without too much effort on the part of the crew.

Cheers,
Slieve

[Ad Hoc]

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.

[Richard Woods]

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

[masalai]

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/amat...sing_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.....

[ImaginaryNumber]

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

[Ad Hoc]

I.N.

What is the approx full laod displacement?

[ImaginaryNumber]

Fully loaded displacement is ~11,000 lbs

i

[marshmat]

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:
Bureau Veritas rules download

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.

[Ad Hoc]

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?

[ImaginaryNumber]

sqrt(-1)

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

Thanks for the data.

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

[Ad Hoc]

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.

[tspeer]

Originally Posted by Inquisitor

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. ...

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.

[Autodafe]

Hi I.M., interesting thread,

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.

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.

[masalai]

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....

[Autodafe]

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.