Square rig variation

[Autodafe]

I am planning to build a variation on a square rig where yards are fixed to the mast for rotation, but raise and lower in tracks on the mast.
Mast rotation is used to control sail angle on all yards simultaneously. Reefing and furling is achieved by lowering some or all of the sail and yards. The bottom of each sail is connected to the yard below, forming in effect a single sail rig, allowing a single halyard to raise and lower all sail and yards on the mast. Refer to attached (Crayon?) sketches (not to scale).

This is to be installed as a biplane rig on a cruising cat, each sail approx 40 square metres (eventually - I've only just started building the boat...).

I am currently considering options for controlling the mast angle, so would like to be able to estimate typical and maximum torque produced by the sail about the mast.
This parameter is similar to “pitching moment” used in aircraft design, but finding a value for given windspeed and angle is taking me out of my aerodynamic depth.

Assume for the purposes of calculation all sails rectangular and identical up the mast with a foot of 3 metres. Yards are symmetrical about the mast, and sail is simple single layer fabric.

Torque will be produced by sail drag and lift forces in varying proportions as sail angle of attack changes. I can find some data for lift and drag coefficients, but very little for the location of the centre of lift and centre of drag.


My question is then, can the torque force on the mast be simply approximated and if so how? Any info most welcome.


Feel free to post comments on other design aspects as well, but be aware that I'm not aiming to produce the worlds most efficient sail here, just an effective rig thats fun to play with and cheap to build (but with less damn strings than a junk rig ;) ).

For anyone interested, I've attached a short pdf I wrote on the reasoning behind the rig.

Thanks in advance,
George

[jehardiman]

Autodafe;

To get the torque on the mast when braced hard on the wind use the moment coefficient about the 1/4 cord point, which is commonly given for most airfoils (or a constant camber section such as you have), and the lift to work out the center of effort.

On a more serious matter however, having all the yard braced to the same angle will be very inefficient due to the wind velocity gradient from the surface to the masthead. The lower yards will need to be braced further around than the upper yards to maintain a consistant AOA across the sail.

[Autodafe]

Thanks jehardiman,
I had heard the 1/4 chord was used, and it's good to have confirmation that that's for sails as well. I'll see what numbers it gives me.

I had considered the windshear issue, but looking at graphs of windshear over open water at different wind speeds it didn't seem worth a lot of effort to put twist in the sail. There is a lot of difference between 0m and 2m off the water, and not much between 5m and 15m. AoA over the top two thirds of the sail will be within +/- a few degrees in pretty much all conditions, which is close enough for me.

[apex1]

I had considered the windshear issue, but looking at graphs of windshear over open water at different wind speeds it didn't seem worth a lot of effort to put twist in the sail. There is a lot of difference between 0m and 2m off the water, and not much between 5m and 15m. AoA over the top two thirds of the sail will be within +/- a few degrees in pretty much all conditions, which is close enough for me.

Oh no, look at any tallship sailing, youll notice that is quite a lot of twist!

Regards
Richard

[Autodafe]

Oh no, look at any tallship sailing, youll notice that is quite a lot of twist!

Except Maltese Falcon :D


I'll put a spreadsheet together when I get some time to check what AoA I get up the mast on various headings.

[ancient kayaker]

Neat. The construction problems for an equivalent self-tacking fore-and-aft rig (variant of the Balestron) would be harder to solve but a square rigger is balanced laterally so it should be easier. A cross between a junk and the Maltese Falcon. Here are some additional points -

You might have problems getting the hardware you need unless you have access to a machine shop. If you change the rails on the mast to a "C" section perhaps you will be able to use standard sail track, and the "T" shaped runners for the booms will be easier to make. The hardware will have to be overbuilt as it could be disasterous if a boom were to jam.

Not only is the rig not self-tacking, but the aerodynamic forces on the mast will try to rotate the sail normal to the wind, like a Raleigh disk. The torque to counter that may be quite high in a decent wind, which may make it difficult to set the sails quickly if the wind or the heading is changing, except for downwind. However, this torque is in the right direction to help with the wind velocity gradient.

I know you’re not particularly concerned with performance, but I think the sail efficiency should be high, approaching that of an aero wing. Provided the flexible leading edge does not flutter so you can set it to a low angle of attack, masking of the leeward sail by the windward sail should not be a problem on most points of sail.

The halliard tension to ensure that will be considerable. That could be a problem; I'm not sure how this rig will behave in high winds with the halliard slackened off for reefing.

When pointing close to the wind it will act like a biplane; typically the lift of the second wing is about 70% of the first, if the separation is about the same as the chord, and this improves as the separation is greater, as it is with your design.

[Autodafe]

Thanks for the kind words Terry :)

Using channel instead of rail on the mast is not a bad idea. I've been wondering for a while which is least likely to jam.

Re-reading the post I made earlier I see I was refering to velocity gradient as windshear. Whoops :rolleyes:

I hadn't thought of the problem with having a slack luff during reefing... I guess I could put a brake on the halyard and use a winched downhaul during reefing, but that would be a bit of a pain.

Maybe a stiffish boltrope would help? I'm currently putting a 15' cat together to act as a prototyping platform and I'll have to play around with that in strong wind.

[Autodafe]

Ok, I've put together a calculator for AoA twist with wind gradient.
I haven't protected the sheet so feel free to check the equations and let me know if you find any problems.

Wind gradient based on two theoretical sources, given in separate sheets in the file.

If anyone has a stash of real world data I could use, please let me know :)

The results match my expectations pretty well. Total twist over my sail span perhaps up to 10 degrees in some conditions, but less than 5 degrees over the top two thirds of the mast. A couple of degrees each side of optimal I can live with, but it would be important for anyone trying to build a competitive racer.

[Autodafe]

Back on my first question:
I'm now happy with calculating lift, drag and moment around the quarter chord point - I'm thinking of using CL 1.3, CD 0.2 and CM 0.1 at stall of 16degrees for maximum torque.

But what happens after the sail stalls? As I understand it, initially the stall moves from the back edge, and lift moves forward as total lift reduces, so quarter chord approximations are probably still ok up to the point where the foil is fully stalled.

Once the sail is fully stalled can I assume that CL=0 and centre of pressure reverts to the centre of area?

[ancient kayaker]

I don't agree with the values for apparent wind speed from which the angles are derived but I got similar shear values from my own calculation using the Ruggles formula. Thanks for that information by the way, new one to me. With the sail profile you propose most of the drive will come from the upper sails as is usually the case for a square rigger so efficiency should be optimised for that portion of the sail if it cannot be optimised throughout.

Alas, my knowledge of aerodynamic theory is insufficient to take me further with you on your journey, so I do not know how the sail will behave in the stalled condition before alpha reaches 90 deg, at which point the center of pressure is clearly centered by symmetry alone. The only practical experience I have of deliberately stalled airfoils dates back to childhood experiments with model planes which used entirely different profiles so what little I remember is of no use to you.

One thing occurs to me; the sail will be thrashing around under stall conditions, so any attempt to predict CL, and perhaps torque as well in these conditions, using theory developed to describe air flow behavior over a rigid surface, is probably doomed. Perhaps tests on scale models is a better tool.

[Autodafe]

One thing occurs to me; the sail will be thrashing around under stall conditions, so any attempt to predict CL, and perhaps torque as well in these conditions, using theory developed to describe air flow behavior over a rigid surface, is probably doomed. Perhaps tests on scale models is a better tool.

You may have a point there :)

[ancient kayaker]

I have been assuming that you’re aware of the Maltese falcon thread but here it is in case you are not- http://www.boatdesign.net/forums/sailboats/maltese-falcon-hit-miss-12459.html -it has some references to worthwhile technical sites and great pics. The depth of the yard curve for the MF is 12% I believe. This would be an included angle of about 27 deg which looks close to your sketches.

More thoughts on reefing: it should be possible to reef with the sails trimmed normally. They will not flutter in that condition and the lowest sail section -the one being reefed- should collapse as the wind is spilled out of it, when its upper yard is lowered. You will need to pull each yard down separately as you lower sail, but no doubt you are already aware of that.

On the design of the yards, there will likely be a lot of unbalanced vertical force on them. The Maltese Falcon has a system of bracing wires and this link to a page on the site of Brian Eiland shows the braces on another dynarig boat- http://www.runningtideyachts.com/dynarig/ -I’m not sure how much problems the use of bracing wires would cause you but if you go with an unbraced design the sliding runners will need a fair amount of vertical depth.



I was thinking about a telescopic rigid wing sail a year or so back but I couldn’t solve the design challenges. It was for a smaller boat (I’m starting to become notorious on the forum as the pesky wee tiny boat man I suspect), and you have given me another idea; it is a shameless ripoff of yours. The rotating mast will be telescopic and a yard will be attached to the top of each section forming a Tee. Like your design each sail section can be permanently attached to the yards above and below it and simply brailled when reefed. It should be quite practical, even easy for a small boat (each mast section will have to be raised by pulling up its yard manually) and it escapes many of the challenges you still face. The lower rig will be subjected to more wind shear than yours but it should be easier for me to introduce mast twist. Thank you for the idea and for helping me to get a long-stalled project going again!

[Autodafe]

Thanks,

Yes I've been through the Maltese falcon thread. As you say some good stuff there, although I always end up wanting more detail :p

Feel free to ripoff as much as you want, I'm not a huge fan of IP protection for small changes to existing ideas.
Good luck with your project, it sounds intriguing and I'd love to seem some pictures when you get to that stage :)
Designing and building a telescoping mast sounds intimidating to me, but it does avoid all the sliding yard problems I'm going to have :D And with the advantage of lowering airdraft and CoG when reefed.

With regards to my yards, I'd like to keep it as simple as possible, but if required bracing wires cause no particular grief.
I was planning to have slides with a fair amount of vertical depth, to reduce the chance of jamming as well as provide vertical rigidity to the yards. I don't expect that to be able to take the whole sail tension though. I'm hoping that the luff and leech boltropes will carry the main load to the topyard and boom. Top yard is then supported by halyard and boom heavily re-inforced. When reefed the lowered yards would be lashed to the boom.
I'll see if this theory works on my scale model.

I am aiming for the 12% mentioned for MF, but it has occurred to me that by shifting the point on the yard that I attach the sail, and having flexible battens I can adjust sail camber according to conditions if I want. This will be a little tricky on the tops'l as it is connected to the yard at more than two points, but shouldn't be too difficult.

[ancient kayaker]

Here's a sketch of what I had in mind: perhaps it can help you? It works out well for my car-topper sailboat as it folds compactly. Something I have also been looking for!

[Autodafe]

Thanks for the sketch!
Using Al tube construction should be quite straight forward.

[brian eiland]

I am planning to build a variation on a square rig where yards are fixed to the mast for rotation, but raise and lower in tracks on the mast.

I'm currently putting a 15' cat together to act as a prototyping platform and I'll have to play around with that in strong wind.....
Interesting project there Goeorge. Hope you don’t mind my entering your discussion as I’ve had quite an interest in that Dynarig style square rig myself. I’m actually surprised more folks haven’t taken the same interest as yourself.

I’ve been playing around with a few ideas myself ever since I drew up my dynarig motorsailing catamaran. But I’ve not put any great deal of work into those idle thoughts, hoping that would come as a result of a qualified client who would really want to go forward with such a project.

I would divide my ‘concepts’ into two groups; 1) as simple as possible non-automated version, 2) automation and furling of the sail sections by mechanical or electrical means. In both cases I’ve sought to eliminate the need for ‘in-mast’ furling of the sails (MF). That requires a more hi-tech mast and practically dictates a carbon fiber mast.

For your project and this discussion lets just think about the most simple version of my thoughts,
.... but be aware that I'm not aiming to produce the worlds most efficient sail here, just an effective rig thats fun to play with and cheap to build (but with less damn strings than a junk rig...

I would choose to utilize an elliptical mast section, in fact likely an exaggerated one that might be fairly long for its width. You might find this type in an old ‘mast junk yard’, as a number of early alum-masted vessels tended towards this shape to minimize the aero disturbance to their attached mainsail. As a bonus this style tended to be rather heavy-walled, an advantage for the free-standing nature of the ‘variation on the square rig’ and/or the dynarig.

My yards would differ from yours in that they would be relatively flat plate ‘sickle looking shapes’ built to the idealized 12 degree arc curve. They would be one piece across the entire span and taper at the outer ends. They would be wide enough at the mast such that an elliptical whole cut into the yard would slide down over the mast,

My yards slide up and down the mast like yours. To facilitate this sliding I would utilize several possible materials. The least expensive one would be UHMW polyethylene (http://en.wikipedia.org/wiki/Ultra_high_molecular_weight_polyethylene). In a thin sheet form it could be wrapped several inches wide around the two ‘proud sides’ of the elliptical mast section and attached with countersunk screws. Sliding against this ‘mast surface’ would be ‘blocks’ of UHMW cut and fit to the adjoining hole in the yard(arm). This UHMW material (http://www.eplastics.com/Plastic/UHMW) is cheap, workable with std wood working tools (http://forums.about.com/n/pfx/forum.aspx?nav=messages&webtag=ab-woodworking&tid=1924), tough, slick, and sea-environment friendly. Alternatively, there is a more expensive sheet and block Teflon material that could be similarly fashioned.

NOTE: The alum (or whatever) mast with thin sheets of UHMW attached at the ‘pointy ends’ would be a relatively clean structure to the wind and to other rigging lines, etc, as opposed to ‘channels’ or a ‘rail’ projecting from the mast surface.

Each sail would reach across the entire span of the yard’s forward edge and attach to the yard via a bolt rope arrangement in a track/groove on the surface of the yard. The sail material could be extremely light-weight cloth as it is supported along the full two longest edges. Aerodynamically this gives a very clean outer surface to the aerofoil…the most important of the two sides. The inner surface of the ‘sailfoil’ is interrupted by the slim side of the mast at its center, but likely the airflow will re-attach to the inner curve of the sail’s inward arc…. (and helped by the end plate effect of the flat yards top and bottom).

The ‘flat plate’ yards need to be constructed of a very stiff material that will resist bending from their ‘arched shape’, and from their tips bending up or down. Their ‘sickle shape’ (top view) should help significantly with maintaining their arch shape. There will need to be a cross wiring of their tips to attachment points at their center reinforced ‘holes’ that surround the mast. This likely could be accomplished with small tough, pre-stretched spectra line. Not only does this keep the tips from bending, but also it contributes to the yard itself riding a straighter line up the mast and resisting cocking and binding.

NOTE: There would be the temptation to build too much vertical depth into the bearing/sliding material to prevent jamming of the yards as they slide up and down the mast. This should be avoided as too much depth could actually increase the binding when the mast itself bends slightly under sail loading. It would be better to have proper cross-wiring (bracing) of the yards themselves, then a two part halyard pulling on the top yard, and a proper two part restraint on the bottom yard in a reefed configuration. Too much depth in the slide bearing would cause additional problems with stacking the stored sail rig.

I’m not near my scanner at the moment so I can’t post sketches (rough ones)


....Torque will be produced by sail drag and lift forces in varying proportions as sail angle of attack changes. I can find some data for lift and drag coefficients, but very little for the location of the centre of lift and centre of drag...
Have a look at this posting and referenced PDF. Surprisingly the CE is very near the geometric center of the sail:

"The rig geometry illustration shows the effect of the wind gradient on the angle of attack. Fortunately, with the sails trimmed for optimum performance the CE of the sails sits just forward of - and near the centreline of the mast. Hence twist of the mast is low and has no great negative influence on the angle of attack. The aerodynamic forces on the rig cause the rig to twist the wrong way, closing the “leech” instead of opening it. However, the complication of making the yards hinged was not thought worthwhile for the performance gain expected."
http://www.hiswasymposium.com/pdf/2004/Gerard%20Dijkstra.pdf

...might have a look here as well
http://syr.stanford.edu/HISWA_Tyler_2002.pdf

On a more serious matter however, having all the yard braced to the same angle will be very inefficient due to the wind velocity gradient from the surface to the masthead. The lower yards will need to be braced further around than the upper yards to maintain a consistant AOA across the sail.
I raised this question as well over on the Maltese Falcon thread (http://www.yachtforums.com/forums/31736-post108.html). As Someone said it just complicates things to much to try and account for this twist,,,so forget it.

Hope I’ve added some new points for discussion. I’d like to see a small model built and tested.

[ancient kayaker]

Brian: I agree this is an interesting and thought-provoking thread, and it's nice to see it getting more attention. I have had an interest in wing sails for some time but the practical difficulties have stopped me going ahead so far. George has shown the way ahead here. I definitely want to try something along these lines. George: per your analysis the wind shear does not seem to be enough to worry about but in my case it will be an order of magnitude greater due to the much lower mast (~4m); not sure how to deal with it at present but I have some ideas.

[Autodafe]

Hi Brian,

Thanks for your thoughts and for the links.
I had the Tyler paper but the Dijkstra is new to me; Its good to see that they feel centre of effort will lie close to centre of area. I think I'll stick to my conservative estimate to work out the worst case effort required to "sheet" the mast - I'm using that to calculate size and reduction for mast drive systems.

Since I started researching a few years ago I've been surprised at the total lack of any reference to square rigs with "dipping" yards, as it appears to me to solve most of the problems with making a useable square rig:
-Reduces weight and windage aloft in heavy weather and at anchor and
-Avoids the requirement to have a reliable furling system on each yard.
It's good to know I'm not alone in seeing some possibility here, thanks to Terry and yourself.

Like you, I've been thinking elliptical mast section.
I don't consider the reduced sail turbulence would be highly significant for my application, but the reduced windage when reefed and at anchor I consider an important consideration, particularly as I plan to use a very large (hence stiff) mast section: 300mmx600mm hollow plywood.

The reason I have gone for slides in tracks rather than bearing the yard on the mast is that I feel this will make reducing free play of the the yards easier. Any movement of the yards around the mast is likely to be sail twist in the wrong direction. Machining mast-shaped slide bearings for each yard with adequately small tolerance sounds beyond my level of expertise. It would also restrict my somewhat carefree approach to mast construction to have to ensure a uniform section shape - easier if extruded section is used :D

The "deep" slides I'm talking about are not all that deep, only around 70mm on my current plan, precisely to avoid the issues you mention. :)
I'll see how it goes on the model.

My thinking on the straight yard sections is twofold, both simplifying construction and reducing the chance of sail chafe on the yards. As with all aspects of the rig it's going to be a suck-it-and-see design process.

I love the notion of free standing rigs, but as my plans involve a pod-cat and biplane rig I have elected to go with a stayed design (ref. my sail plan sketch). It would be possible to engineer mast and bearings for a bury of 800mm, but not easy. The cross beam at the mastheads gives me a good place for nav lights and wind instruments anyway :)

George

[Autodafe]

Hi Terry,

Those bottom few metres are the real killer for wind gradient!
I'm glad I'm not worrying about them, but I look forward to hearing how you progress :)

George

[ancient kayaker]

George: from your first post I gather you are still in the early stages of building the boat so the rig is some way off yet. I recently completed a small (10 ft/3 m) sailboat that I can try this idea out on, but it will still have to wait for warmer weather unless I want to try it on an iceboat :)

For me the rig poses a problem, when changing tack there is no way to feather the rig, it must be hauled round on the other tack and be backwinded, stopping my lightweight boat in its tracks, unless I wear ship. I still want to try it out, but I am also thinking of having a double skin forming a full aerofoil profile which would symmetrical on either tack and self-tacking - i.e., the wing-sail that I wanted for Christmas!

In my earlier adaptation of your concept I planned a one-piece sail with pockets for each yard instead of separate sail panels. I don’t know if that is a usable idea for you; it cuts down on rigging but may only be suited for light duty use, and the yards must be threaded through the pockets. Alternatively, I see no reason why you can’t use sail tracks on the yards which would make for easy maintenance and replacement, and even provide the opportunity to have a heavy weather suit with less height per sail to reduce the strain on the yards. For light winds, the square riggers of old extended the width of the yards to increase sail area, although a spinnaker is probably simpler. I’ll just throw those ideas into the melting pot!

In a strong wind a square rigger would sail under topsails alone; it always seemed counter intuitive to me but it kept the sails high where they were less disrupted by surface effects. With separate sails you have the option to remove or brail the lower sails and send the upper sails back aloft. It's a thought in case you plan on blue water sailing. *

I think your concept of reefing from the bottom is more practical than the in-mast system used by MF, although it may have been the best solution for MF since that is a push-button operation. For the design shown in your first post, you need to consider how you will access the full width of the yard in a storm to brail the sail. I think lazy jacks were mentioned earier but they will disrupt the flow of air slightly.

Addressing the design of the uphaul; I think that your design will need more tension than a typical fore-and-aft sail of similar size, as you want to force it into a given profile whereas a fore-and-aft sail can take a natural shape between the mast and clew, and much of the tension is at the clew. I don’t remember if you addressed it before but using the downhaul to apply full tension might be easiest.

Having read both your comments and thought a lot more I agree with you and Brian on the depth of the slides. The upper sails and their yards will be hauled down by the lower sails and the whole thing will tend to resist any tendency to cock except at the bottom, where the yard is withing reach. Providing the uphaul is carefully designed or the top yard has extra slider depth I see no problems with this. However, if you plan to sail under topsails per * above additional slider depth may be wise unless the yards are wired to each other at their ends.

[brian eiland]

Thought I might make a cross reference link to this other discussion as it may get even more folks and opinions involved:
http://www.boatdesign.net/forums/motorsailers/dynarig-motorsailer-ala-maltese-falcon-16276.html

I may not participant in these discussions for about a week as I will be out of town. I posted this message over there, "But this other subject thread has me thinking about a 'bare bones' version....buildable in back yard. Started writing and sketching a bit yesterday, but now headed to Florida to visit an old friend for a week. So probably won't finish that until I return."

[Autodafe]

Just think how much an iceboat would extend your sailing season :)

My boat is still a good way off, but the rig testing dinghy is just about ready to go in the water.

A enclosed wing sail is an interesting possibility. Speeding up tacking is a strong attraction, but I prefer the aesthetics of simple sails, where yard and slide can be inspected easily.
I may try out both on my test dinghy and see how I feel about it then. Thanks for that idea Terry :)

I have been considering electric or pneumatic assistance for mast rotation to speed up tacking, but I would prefer to avoid the extra complexity of mechanical assistance. An enclosed wing could help here.
I am proposing to use a pulley drive system to control mast rotation rather than sheeting to the yard, and my early calculations suggest a reduction ratio of 40 or 50:1 may be required if I get caught out with full sail in a gale, which would mean perhaps 30 seconds to tack each mast manually through 160 degrees. Faster would definitely be nice.

I am hoping that with a large number of yards to hold sail camber and reduce sail span the halyard tension will not need to be high for good sail shape, that is the yards will act in the same way that battens keep junk sails under control.

I'm planning to use lazyjacks for sail control, and hopefully will not have to access the entire boom to reef.
Interesting point about using tops'l only in heavy weather. Perhaps I should put dual tracks in the mast and hoist a small storm sail on the opposite side of the mast to the main sail when required. It could have a downhaul to hold it at any desired height up the mast and I wouldn't have to go to the trouble of removing the main sail from its yards in strong winds.

I agree with your assessment that the top yard will require extra care in slider and uphaul design to avoid being jammed.

Bluewater (high latitude) cruising is part of the long term aim, but I expect to spend a couple of years working the bugs out in coastal cruising before that, as there are many aspects in addition to the rig that are rather experimental.

[ancient kayaker]

I am wondering how the power of the wind can be used to turn the mast. May be too violent in a storm, but it would have more power when you needed more power. I have no idea how it could be done at this point, just the seed of an idea.

[Autodafe]

The idea certainly has potential.
I've occasionally wondered on the same lines, either an adjustable trim tab arrangement on the top yard to move the CoE or a mechanical wind turbine drive where clutches can be used to drive the mast either way.
Due to the potential complexity I think I'll try the manual control first then add external power if required :)

[sharpii2]

I am planning to build a variation on a square rig where yards are fixed to the mast for rotation, but raise and lower in tracks on the mast.
Mast rotation is used to control sail angle on all yards simultaneously. Reefing and furling is achieved by lowering some or all of the sail and yards. The bottom of each sail is connected to the yard below, forming in effect a single sail rig, allowing a single halyard to raise and lower all sail and yards on the mast. Refer to attached (Crayon?) sketches (not to scale).

This is to be installed as a biplane rig on a cruising cat, each sail approx 40 square metres (eventually - I've only just started building the boat...).

I am currently considering options for controlling the mast angle, so would like to be able to estimate typical and maximum torque produced by the sail about the mast.
This parameter is similar to “pitching moment” used in aircraft design, but finding a value for given windspeed and angle is taking me out of my aerodynamic depth.

Assume for the purposes of calculation all sails rectangular and identical up the mast with a foot of 3 metres. Yards are symmetrical about the mast, and sail is simple single layer fabric.

Torque will be produced by sail drag and lift forces in varying proportions as sail angle of attack changes. I can find some data for lift and drag coefficients, but very little for the location of the centre of lift and centre of drag.


My question is then, can the torque force on the mast be simply approximated and if so how? Any info most welcome.


Feel free to post comments on other design aspects as well, but be aware that I'm not aiming to produce the worlds most efficient sail here, just an effective rig thats fun to play with and cheap to build (but with less damn strings than a junk rig ;) ).

For anyone interested, I've attached a short pdf I wrote on the reasoning behind the rig.

Thanks in advance,
George

Why the biplane rig.

Why not just make a single, larger rig that could be controlled from a more central location.

Really, the big advantage of a square rigger, and the main reason they were used for large ships, was that the sails always stayed on the same side of the spars.

This enabled crew to get behind the sails to furl or set them.

I don't see any reason why a rig like you want to set up cannot work. The mast could be allowed to twist somewhat for the sails to set at a slight wind variant.

The real problem with this set up is weight and co$t.

It would certainly be cheaper, simpler, and probably more reliable to set up a fore and aft rig. Especially on a multi hull, where the inital stability is quite awe inspiring.

If I were to go with a square rig on such a boat, I would use sheet lines to control the yards, even if I went with an unstayed mast.

This way, I would put less strain on the yards, allowing me to make them of smaller and lighter sections.

An unstayed mast, however, offers too much benifit to a square rigger to pass up, as there is a lot more freedom of movement for the yards, allowing them to be sheeted tighter to the wind.

But the mast section would certainly have to be large, even if it is made of carbon fiber. And TWO masts would each have to be the same section as ONE mast, so one of the masts could be bare to shorten sail.

The truss rigging you show between the two masts would certainly be a proplem, as it would restrict the freedom of movement for the yards.

[Autodafe]

Why the biplane rig.
Why not just make a single, larger rig that could be controlled from a more central location.
Mainly, why not? It is slightly less efficient than a single rig, true, but the difference is smaller than many other refinements that cruising boats don't bother with. It will be slightly heavier than a single rig, but not much, and the weight is lower offsetting the loss of stability.
The main reason for me is that I am starting from the premise of a pod cat, and I have no desire to have to climb on the roof every time I want to inspect or adjust sail. Solution: put the masts down at deck level. There are quite a few other benefits to biplane rigs, discussed in depth in some other threads.
With a square rigger biplane also means that I could sail one hull forward and the other backward. This is pretty pointless in these days of motors, but could be highly amusing in tight spots with onlookers :)
If I didn't think it would be too difficult to control lots of masts simultaneously I would have four, one at each corner :)


Really, the big advantage of a square rigger, and the main reason they were used for large ships, was that the sails always stayed on the same side of the spars.

This enabled crew to get behind the sails to furl or set them.

It also means the mast and yards are nicely on the windward side of the sail where turbulence does the least damage.


I don't see any reason why a rig like you want to set up cannot work. The mast could be allowed to twist somewhat for the sails to set at a slight wind variant.

Sadly the twist is in the wrong direction for induced windshear assuming the mast is sheeted by the base.


The real problem with this set up is weight and co$t.
It would certainly be cheaper, simpler, and probably more reliable to set up a fore and aft rig. Especially on a multi hull, where the inital stability is quite awe inspiring.
If you're right on this one then the rig isn't getting built :) but I've done some fairly detailed materials breakdowns and I think I can get 80sqm of sail in the air for under 6kAUD and total weight (including sails and rigging) of just over 300kg. Not light, but not particularly heavy either, and definitely cheap.
Because the sails have almost no cut shape I can buy cloth and sew it myself. Mast and yards are to be out of epoxy timber with some glass, built at home. I may add some CF if it's too flexible at testing, which could add another 2kAUD and ~15kg.

If I were to go with a square rig on such a boat, I would use sheet lines to control the yards, even if I went with an unstayed mast.

This way, I would put less strain on the yards, allowing me to make them of smaller and lighter sections.
The load on each yard isn't very high, even in 60kts, as the supported area isn't large, so the yards are surprisingly light (at least on paper) to cope with the engineering loads. I suspect that sheeting every yard would add more weight in running rigging than it would save in yard weight.

The reason I am thinking pulley-cable system to drive the mast rotation rather than sheeting the boom is that I don't want to have to move the sheeting point of both sails every time I tack.
This would be less of a problem with a single mast rig, but it would still require changing sheet and sheeting point every tack.

An unstayed mast, however, offers too much benifit to a square rigger to pass up, as there is a lot more freedom of movement for the yards, allowing them to be sheeted tighter to the wind.

But the mast section would certainly have to be large, even if it is made of carbon fiber. And TWO masts would each have to be the same section as ONE mast, so one of the masts could be bare to shorten sail.

The truss rigging you show between the two masts would certainly be a proplem, as it would restrict the freedom of movement for the yards.

I agree that the ability to sheet the yards hard on the wind is vital, and the main reason traditional square riggers couldn't sail to windward.

It is not entirely clear from my sketches, but my rig does have this feature. The stays are connected to the mast top-strut (not the mast) and chainplates are on the bow and stern of each hull. This keeps the stays well clear of the yards, allowing them spin freely to any angle, while still giving me a staying base nearly the full width of the vessel and consequently small compression loads in the masts. This is another benefit of the biplane rig :)