Swing Rig - What's all about?

Yes, of course you can build your kinked mast! I have built one and it seems that other people have done so befor me, hence any patent taken out for this idea is invalid. In any case, even if there is a valid patent for something you may still copy it on a non-commercial basis.

It seems to be common practice to take out patents for things which are not novel or which are obvious hence should not be the subject of patent applications. Such patents are worthless if challenged, but mostly they are never challenged and it looks good to be able to make reference to a patent application in advertising material or on a webpage, or particularly when seeking funding for development. Sorry if I sound cynical.

John

 

I can second that John, nevertheless my curiosity is clear awake and I will mail them the question to explain their claim. They are reasonably quick with answers so I might have their reply by wednesday it is now (1705 here).
I'll let you know.

 

Hello . I am currently considering options for re-rigging my freedom 40. I have owned and sailed the boat for over twenty years now,so I am familiar with its idiosyncracies. It is the center cockpit cat ketch version with wrap around sails. I have been following this thread with interest, because certain aspects of the Van De Stadt design are similar ,if a lot more sophsticated than a design I am working on. Criteria for the new rig include ,more sail area aloft ,balanced sails to reduce sheet loads and hopefully help with weather helm ,a split rig, free-standing masts ,minimal use of hardware ,and I must be able to make it myself. It wouldn't hurt if it retained the eccentric nature of the original,since God knows she will never be abeautiful boat [best described as a telephone booth chasing two winsurfers].Given that the boat has a full shallow keel and a center board ,windward performance will never be stelar [head sails not needed]. I will be happy to make anything less than 100 degrees through the compass. I have considered various swing rigs, the junk rig ,the"kinked mast rig",and James wharren's "soft wing" . My current working design incorporates features of all these rigs.I would very much like to hear more from those of you who have sailed these "kink" rigs specificaly with regard to ,sheet loads,center of effort,lead , and center of lateral resistance.Does the boat heel more or less for a given wind speed ? Does having the center of effort remain closer to the boat's for and aft axis help with weater helm? I have attempted to attach a drawing that illustrates my working concept. I know I need professional help,[possibly psychiatric] but before I get to that stage any thoughts or advise would be appreciated. Tim
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Timothy

I like your drawings.

My experience from sailing with a kinked mast is that it is very effective at reducing sheet loads but probably less effective as a means to reduce weather helm. When off the wind with the boom well off the centreline you do get some balancing effect due to having part of the sail area on the windward side of the sail's axis of rotation, but this can easily be swamped by the larger effect of changes in the angle of heel.

The factors which primarily affect weather helm are the fore and aft position of the sails relative to underwater appendages, the angle of heel and the hull shape, in that order of priority I think. I suppose that if the position and rake of your mast steps are already fixed by the structures within your Freedom hull then having a kink in the mast above deck level will effectively rake the masts forward and that will reduce weather helm regardless of the balancing effect attributable to the kink when off the wind.

With regard to reduced sheet loads, I see this as a major benefit for a small and inherantly unstable boat (like my one) where the helmsman needs to play the mainsheet continuously. It must be a less important benefit on a large keel boat where sheets are not adjusted continuously and where tackles and winches are used for sheet adjustment.

I think it is important not to overdo the kink in the mast since this could result in a sail which will not feather automaticaly when the sheet is released. You need low friction bearings to allow the rig to feather in light winds. I used bearings with sleeves turned from ultra high molecular polyethylene, the material used in artificial hips. This is a simple design which worked well on my small boat. An alternative which would possibly give even lower friction would be roller bearings, perhaps with 'Torlon' rollers as often used for large pulley blocks and rudder bearings.

I see you propose to use small gaffs at the head of the sails. An alternative would be to have square headed sails with a stiff full length upper batten which would function very much as your gaffs would. This would avoid having a short length of bare mast at the top of the rig, this bare section of mast giving only drag.

I assume that you would use zip fasteners in the sleeve luffs to allow reefing. Wharram owners may be able to comment on the effectiveness of this arrangement.

You have drawn the masts straight. Assuming that the masts are straight when unloaded they will inevitably be curved to some extent when sailing and the sail needs to be cut to allow for this. The sailmaker I used made a good job of guessing how much my mast would curve when sailing and cut the sail to suit. If you really want the mast to be straight when sailing you probably need to make it so that it is curved the 'wrong' way when unloaded.

If you design the rig to have the mast curved when sailing then the axis of rotation of the rig can interset the mast at some point towards the top of the mast and shrouds/forestay can be attached to the mast at that point of intersection. The use of shrouds would allow a lighter mast but on the other hand they are more windage and complication and they prohibit allowing the sails to swing ahead of the mast to spill wind with the wind aft. With straight masts and gaffs as you have drawn you might be able to attach shrouds to some point on the gaffs, which could help to control sail twist, however reefing could then be complicated.

John

 

Hi John. Thanks for your thoughts I rendered the drawings showing the boat without regard for dynamic loads. THe masts will bend aft and to leeward as they are meant to be free standing The idea is that they would rotate on stub masts raked at about 15 degrees to the deck. the mast it self would have about a 10 degree kink.There would then be about 5 degrees of rake while sailing to windward.The booms would be fixed to the mast .The center of rotation would intersect a point just forward of the gaff peak.This would result in about 25 percent or less of the sail being before the axis of rotation and should make it posible to rig a self adjusting vang [sheet?] to the peak of the gaff on the mainsail with a line lead through the mizzen mast . The freedom in its original configuration has two equaly sized masts and sails ,one stepted in the bows and the other just aft of the cabin on the center board trunk.When I began thinking of using the Wharram type soft wing sail ,because of the gaff and the desired additional sail area aloft it became apparent to me that the center of effort would move way aft if I used equaly sized sails and left the masts in there present locations I chose to increse the aspect and area ofthe main and move the mizzen aft where there is already a substantial bulkhead which hopefully will be capable of withstanding the loads imposed by the greatly reduced mizzen. My experience has been that the mizzen is in the wind shadow of the main and thus has to be very flat and produces very little drive and a lot of drag on a beat ,and that in practice it is much more usefull to me for ajusting trim and for keeping my nose into the wind when raising sail or anchoring. This setup also allows that the masts be raked 5 degrees and the overall center of effort retain its for and aft position relative to the original rig albiet slightly higher.The idea is to use conventional slab reefing and because of mast rake each reef will move the center of effort further forward keeping the boat in trim.The two main reasons I am thinking of using a kinked mast are as the mast rotates the boom which would be much longer lifts clear of the water reducing the danger of tripping and the balanced sail would allow me to still use a simple four to one purchase for the main sheet despite the increase in sail area. I have heard that tripping is a problem with the nonsuch 36 because of the legth of the boom. If the main effectivly becomes an asymetrical spiniker downwind as Van Der Stadt implies,then,thats great too . I agree with you that the mast area above the gaff is a problem and a head board would be better but I can't figure how design one that will work unless I discard the Wharram zip luff pocket and go with a mast track and full battens and that means hardware. Concerning mast bend and sail shape; my current wrap around sails [clew to clew] are cut flat .The amount of draft is controled by the out haul. As the mast bends the sail flattens and the leech opens spilling wind. I think your right ,that the sail would be improved with a certain amount of luft round depending on where and how much the mast bends.I think that the wylie cat uses a very flexible mast and lots of luff round in the sail. I recently looked at the Tom Wylie design and saw the design for the wylie cat 65 and was very impressed. It also made me more confident that I can handle the over nine hundred sq. ft. total area of my proposed mainsail as even the sail of the wylie 30 has that much area. Thanks for the tips on fabrication Great ideas. I am no where near at the stage of figuring out how to build it .My main concern now is if using the wharram soft wing sail and/ or the kinked mast is there a reason I should alter the lead for the for the center of effort ,and should I use the total area of the sail to determine the center of effort or just the triangle as defined by the throat the clew and the tack? I sould also say that if I had the money I would abandon all this and just commision a Sponberg Globtrotter Tim

 

Cantingrigg/ modelboat/ swing

hi all,
thanks for the infos.will give it a go , including a pocket luff like the laser and a small jib, the model will not be rc, so its got to be well balanced. the canting mechnism can be manually through a simple set of shrouds , that i can ease and tighten on either side.for stay tension sould be maintained.
whats erics sponberg contact.
cheers anyway for your help, will keep you posted , how its going on
j

 

Timothy,

I still dont see why you should not have a bit of sail area above the gaffs, I would have thought this is just a matter of filling in the triangle between the gaff and top of mast with a bit of cloth, with the luff sleeve continuing to the top wide enough so that it will slide down the thicker lower part of the mast when reefing/stowing sail.

The Wharram style sleeve luff rig does not seem to need mast track and battens but even so I wonder whether the addition of these admittedly expensive items would not be an improvement. Full battens and a suitable free sliding track would allow the sail to have roach and better control of sail camber and would take much of the load off the luff sleeve and zipper. The luff sleeve would become just a means of streamlining an otherwise conventional fully battened sail. My own kinked mast rig, which is much smaller than the one you propose, does have full length battens but since it is not intended to be reefed it does not need a mast track.

I have heard befor about a mizzen sail needing to be sheeted tight and set flat. A couple of decades ago AYRS member James Byam Shaw carried out a series of tests to try to optimise a ketch rig on a 20' daysailing boat and he ended up with a conventional mainsail but a wing masted fully battened mizen.

I agree that the kinked mast will help avoid boom dip when off the wind. Do you think this is particularly a problem with your boat? Your main boom looks to be a fairly normal length but perhaps it is set lower than normal. I have an idea that in Europe at least there are regulations for minimum boom height due to health and safety issues.

John

 

Swing the cat rig

Can anyone explain to me why the disadvantages of the cat rig discussed so widely on this forum do not apply to the swing rig?

The swing rig is higher in aspect ratio than traditional cat rig solving some of the problems associated with the cat rig are there more considerations?