Best rig for small catamaran circumnavigator?

A contact recently sent me this link which shows exactly what you suggest in a bi-plane configuration.

http://www.schionningdesigns.com.au/www/page.cfm?pageID=266

Eric

 

A cat schooner may be more to your liking than a cat ketch. I understand that multihulls don't like Center of sail Area too far forward. Also, the weight of the biggest mast being near the bow will effect a narrow multi water plane more than it would a wider mono one.

The old fractional sloop may be your best bet. That way the mast is more likely to end up on a cross beam. The jib can go 3/4ths the way up the mast and the main can be a gaff or a marconi. This give you very good stay geometry and it puts the biggest sail within easy reach for reefing and furling.
I know that that type of rig is so commonplace that it is boring. But maybe it is so for a good reason. Unless I had full length sail battens, I would have no battens at all. Short battens are a BIG pain in the arse. And they do cause chaffe. Full length battens that run parallel to the boom, however, may save your sail from beating itself to death every time round up into the wind.

Since this is to be a blue water/trade wind boat, I would want to keep the rig height as low as possible. Competitive upwind performance should be a distant secondary conern in view of what you want to do with this boat.

The hull design should take into account a much more generous payload capacity than is typical with the present day performance oriented catamaran. I would want at least 1,000 lbs of payload capacity minimum for a single handed boat. The size and shape of the underwater sections should reflect that reality.

Just my 2 cents.

Bob

 

You mean like this .....

Gareth
www.fourhulls.com

 

Looks good. A completely free standing and free to rotate sytem would have lots of advantages for a long distance voyager. Simple carbon or even ally poles would do it on the size of boat suggested by the original poster.

 

Biplane rig

Thomas Firth jones tried such a rig on his coastal cruising catamaran named Dandy, but after two seasons gave up on it. Apparently it works best on very fast cats because, along with the faster speed comes an aparent wind that is further forward. This may explane why it is given so much praise in the faster sailing multi community.

The boat in question will not be nearly that fast. Most likely it will be a D/L 100 boat or damn near. Especially when all the long voyaging stores are loaded aboard. Although it will most certainly be faster than a mono pocket cruiser of the same length, it will probably be too slow to benifit from the biplane rig.

Mr. Jones reports in his book MULTIHULL VOYAGING: "...On a reach, the leeward sail flopped idly about, and we were driving the boat with only half her sail area..."

And there was much more after that. And most of it wasn't flattering. The only good thing he said about the rig was how few times he had to reef it.

Its best point of sailing was dead down wind, because the two side by side mains could be flown wing and wing.

Bob

 

Wharram Soft Sail

The Soft Sail rig has certainly been shown to work well. 'Jeckells Sails' did the sails. Neville and Annie Clement built the prototype 'Tiki 46' on a farm near here, launched it at Bristol, England, and have sailed it trans-Atlantic to the Caribbean and US East Coast, where they commute with the seasons between the Bahamas and Providence, RI.

I've known and sailed on Wharram cats for decades - I was crew on the maiden voyage of his/their 65' 'Spirit of Gaia' - and I know that the merits and otherwise of his/their designs generate almost as much 'steam' as does Ould Jimmy himself.

If I were to advise a member of my family, or a close friend who asked, about JWD designs, I'd say that many sailing families and enthusiasts worldwide have built and sailed these boats, for many tens of thousands of miles. However, they are far from maintenance-free, and a thoughful read through any of the 'Wharram forums' will show that EVERY one of those designs have been modified/enhanced/improved/beefed-up by owner-builders somewhere. Sometimes, these 'mods' become incorporated by Hanneke into the new plans.....

IMHO it is important to avoid 'blind faith', and to 'think critically about everything critical'. You will note, if you read the newsletters from Annie Clement ( Letters from Peace IV ) on Wharram's website, that they've had major problems with their foremast, its support, the beam lashings, the crossbeam troughs, the 'boomless' rig, and several other matters. The point is, they managed ( with a lot of help from friends ) to keep their boat together, and improve it. A 'work in progress'.....

Look here:

http://www.wharram.com/

 

biplane rig

This rig seems appealing in theory, but as Tom Jones discovered, it has certain drawbacks in practice. I don't think faster boats can entirely sail away from the lee blanketing problems on a reach. I've read correspondence from a person in Thailand who built a Radical Bay (Schionning design) and still acknowledged that the problem occurred. They were overall very happy with the design, however, and felt that this inefficiency was a tradeoff that could be endured to get the other advantages of the rig.

One big disadvantage for a cruiser is that when the lee sail is outboard, there is very little deck underneath it to work at the boom, for reefing and sail shape controls. A great advantage of cats in general is that you can stand by the boom in the middle of a wide deck and do this work in security and comfort, and that is a big advantage to give up.

Ray

 

I'd find it very helpful if you could elaborate on this just a little, say, using a Righting Arm GZ of 11.5 feet, a max loaded displacement of 5 tonnes, and consequently a Righting Moment of 57.5 ft-tons....?

I can provide a 'bury' of 2 metres.

What do I need to consider and calculate in respect of a pair of carbon unstayed spars, in biplane configuration, and the 'well' in each hull that must take the loads?

Please, and thanks.

 

Bilbo:

You need to either learn some engineering yourself, or find someone who is not only familiar with structual engineering, but familiar with the matterial you want to make the masts out of as well.

Here, I will offer what little help I can.

First off, you have not provided enough imformation. You need to know your mast height. You now have to take this load (57.5 ft/tons) and multiply it, expressed in pounds, by the height of the mast in inches then divide that by 2.0. As you see, this will come out to a huge number. This is your 'Bending Moment'.

Once you've done that, you now need to find your minimal 'Sectional Modulus'. This is done by dividing your 'Bending Moment' by the by the 'Allowable Stress' of your matterial, which is usually expressed in pounds per square inch. If it is expressed in kg per square centimeter, make sure the weight and the mast lengh are also expressed in kg and cm. This will give you your Sectional Modulus (SM) expressed in either inches cubed or cm cubed.

Now you need to draw a mast section that has at least that value.

As I hope you can see by now, there is a lot involved here.

And that is not the half of it. This is just the preliminary stage.

I hope you follow my original suggestion. This stuff is learnable by just about anybody, but it takes a considerable amount of time to learn (at least it did for me).

Best of luck.

Bob

 

50 years experience ...

Take inspiration from others and then go beyond.

as Isaac Newton said .... " if I have seen further than others, it is because I have stood on the shoulders of giants." J. Wharram may well be one of those giants.

 

Sharpii,

You have an error there. Bilbo's maximum righting moment is 57.5 Ft-tons. You do not multiply that by the mast height, and you do not divide it by 2 or the depth of bury. 57.5 Ft-tons is the live load bending moment on the mast at deck level. You have to express this in inch-lbs, and then divide by the allowable stress (in psi) of the material you are using to get the required section modulus, which comes out in units of inches^3. Allowable stress should be one third the yield stress (as for aluminum) or one third the ultimate stress if there is no yield stress (as in wood or composites).

For example, assuming long tons = 2,240 lbs: 57.5 Ft-tons = 128,800 Ft-lbs = 1,545,600 inch-lbs.

If high quality carbon fiber laminate is used (standard grade, vacuum cured epoxy), ultimate stress is going to be about 70,000 psi (could be 100,000 to 120,000 psi if made in an autoclave), so one third of 70,000 psi is 23,333 psi.

Required SM = 1,545,600 inch-lbs/23,333 = 66.24 inches^3

Then you have to calculate what the section size should be to have the section modulus that achieves the required value. Section modulus is a function of the geometry of the section. You need to calculate moment of inertia first. For a round tube, for example, the moment of inertia is I = PI/64 x (OD^4-ID^4). Then Section Modulus SM = (2 x I)/OD. This actual section modulus, as I said, has to be equal to or greater than the required section modulus. This I and SM for a round section are the same in every direction. For a non-round section, I and SM are different longitudinally and transversely, and that has to be taken into account depending on which direction the mast is loaded (usually some in each directtion, but certainly to the maximum transversely). Calculate SM by trial and error until you get to a reasonable solution.

You have to be careful, too, that you do not make the wall thickness too thin. If it is too thin, then the mast section can buckle prematurely before the laminate reaches full strength. So in every design, there is an optimum section size for the design at hand.

Finally, one has to consider deflection, which is dependent on the modulus of elasticity of the material times the moment of inertia of the section, ExI. the larger the ExI, the smaller the deflection. This is a whole separate engineering problem that is done at the same time as the strength calculations. It is easy to have too much deflection, and you hardly ever have too little deflection. One strives to design for just the right amount.

The equations are more complicated for non-round sections, but they can be figured out. This is why I like to use elliptical sections for my wingmasts because the equations for I and SM are nearly as simple as for a round section. For an aerofoil section, the method for determining the I and SM requires a complicated procedure using Simpson's rule, and the engineering process gets really complicated and drawn out. Elliptical sections have very favorable aerodynamic characteristics, so the whole package of design and engineering is doable in a reasonable amount of time and results in easy construction and very good boat performance.

Eric

 

For a multihull, especially a small multihull, there is nothing more economic than a stayed, rotating aluminum mast. It will still be efficient and light-weight. At these sizes you could save equal weight by using modern fiber rigging instead of wire rigging, instead of going with a free-standing, rotating, carbon-fiber rig.

For cruising conditions a gaff rig could work very well, though I would go with a fractional (fully-battened, square-top) marconi sloop rig with a roller-furling reacher set on a pole ahead of the jib.

 

Something like this:

 

Unstayed aluminum masts

Unstayed alloy can fatigue over time. Unlike steel, aluminum accumulates sub-critical stresses over time, and suddenly breaks. If you want aluminum, you need staying to reduce the whipping. This is what Tom Colvin does with aluminum junk masts. Actually, Douglas Fir is good for unstayed masts, with its bending strength of 8000 psi.

As far as bi-plane rigs go, bi-plane sailors let the weather sail out a bit more than usual when beam reaching, and haul the lee sail in a bit more than usual. In overview, it would look a bit like one sail, with the different angles combining to make camber.

 

Randy i read your question with much interest,ive wondered the same thing,for a mast in your neck of the woods i would research using nipa or mabe bamboo for a mast,mabe wrapped in fiberglass n epoxy.i like lateen style sails and kinda like a junk rig.mabe a rig thats not so tall so less load.low tech and low cost,consider using rope to hold the mast up,some of the new high tech stuff,stronger than steel stuff.All the magazines ive read over the years always talk of people fixing wire rigging thats failing in the middle of nowhere.