canting rotating wing mast

My 15 year old wrap around sails on my Freedom 40 sails are held together with more sail tape than original cloth and badly need replacing. In the 30 years that I have had my boat a lot has changed. I have no doubt that the cat ketch with free standing masts ,a novel concept back then has proven to be a viable cruising solution. Its a good thing for me as my financial situation is not what it once was and I won't be buying or building another boat. The last time I bought new sails I decided, unlike most other owners who chose to change to carbon fiber masts, a mast track and fully battened big roached sails, an option I was quite frankly precluded from financially , to go with the original wrap around sails and to build Hoyt style free standing booms, to replace my several times broken and fixed wooden wishbone booms . The idea was to alleviate the necessity of having to raise and lower the heavy wishbones with the sails. The booms were easy and cheap to build but I was gob struck by the prices I was quoted for the deck and step bearings( they were talking thousands not hundreds). In the end I made my own with fiber glass epoxy and graphite powder along with an uhmwpe kitchen chopping block (total cost of materials at the time less than $50.00) They have worked fine for fifteen years without a problem. The booms proved to be too flexible so I have to reef earlier than I did before but the sails are a lot easier to lower and raise. Fifteen years ago I paid $11000.00 for two new wrap around sails ( more sail cloth than four conventional sails) .Last year I got a quote of $18000.00 for Wharram style soft wing sails with zippered luff pockets that could be configured to fit my boat with its present masts in their present locations. I would have only to cut down the mizzen to adjust for the center of efforts movement aft. With the addition of the gaffs the sail area would remain the same. At about the same time I became fascinated with Rob Denny's proas and was amazed at the weights and prices he can build carbon fiber masts for. I have always been happy with my cat ketch rig but it would be nice to be able to tack trough 90 degrees or better by the compass . I can't do much about my hull but if a reduction of the weight of my rig of more than 50% would allow for an increase in aspect ratio from the present 2 to 1 to 3 to 1, the maximum that from what I have read that would be effective on a shallow full keeled vessel with a center board, I think 90 degrees or better might be achievable. To achieve that kind of aspect ratio and still keep the masts out of the accommodation the easiest thing to do it seemed to me would be to increase the hight of the main and move the mizzen aft. This would also get the mizzen and boom out of the cockpit improving vis-ability and reduce the total weight and thus material cost of the masts. I settled on a a yawl or a ketch with a very small mizzen but I was dissuaded from pursuing the luff pocket idea as impractical by members of this forum . The problem with the yawl configuration is that the mainsail becomes huge and that one of the conditions that the new rig must meet is that it is self tacking and can be trimmed by by a single crew member with a simple purchase system rather than a winch. The solution would have to be some kind of balanced sail. I looked at lug rigs of all types and the Hoyt balanced rig but finally came back to the swing rig (rotating mast with a kink in it at the partners) I had previously fooled around with the concept after seeing the Robosail "swing rig" design by Van De Stadt . This design dictates the added complexity of a rotating mast and expensive bearings ( maybe I can make them myself as I did for Hoyt booms) but has the benefit of reducing sheet loads to manageable levels and also of canting the mast to windward (greater righting moment and larger sail plan-form at 10 degrees of heel) . It also keeps the center of effort closer to the centerline of the boat off the wind ( great for cat rigged boats). A problem is that if the boom is attached rigidly to the mast to facilitate self rotation the mast is canted the least when you need it most ,upwind. Some how a method by which the mast can be made to vary its rotation relative to the boom must be contrived. Van De Stadt I believe used an electric ram between the boom and the mast which had to be adjusted when tacking. Since I want to retain the ability to self tack I decided a spanner of some sort controlled by a traveler might work. Having abandoned the luff pocket idea I needed to deal with the wind shadow on the mast, not so important for boats with head sails but perhaps critical for cat rigs. Scince the mast was to rotate any way why not make the section shape more like a wing ? Using T Spears method for geometrically developing the section shape for a teardrop mast rotated at 35 degrees I came up with a section that my computer tells me is twice as heavy as a round section of the same wall thickness and will quite readily bend to lee ward but not aft. No good. I looked a Eric Sponberg's sight (beautifull designs. I like the globetrotter 42) and have read every thing I can find on the internet that he has to say. His method of building a structural core of carbon fiber, either a tube or a box, and attaching light flexible non structural fairings for the leading and trailing edges seems to me to be the way to go. With a wing mast and no luff pocket I will need a mast track and that means I can go with a square top sail with full length battens rather than the peaked gaffs of the Wharram Soft wing. Presuming that I could build the new main mast of carbon fiber rather than aluminium with approximatively the same section diameter, wall thickness and taper, but ten feet taller and with a gradually diminishing wall thickness, and that the new square top mainsail although of twice the sail area would be single ply ,and of Cuban fiber not 10 ounceDacron,and that the wall thickness and section of the small mizzen mast could be reduced, as the mast would never be subjected to more than half the maximum righting moment, I have calculated( actually more like wildly speculated) that the entire rig would weigh approximately some four hundred to seven hundred pounds less than the current rig. Although the sail area would be increased by 30% and center of effort would be higher , the lower center of gravity and increased righting moment provided by the lighter weight canting to wind ward rig ( roughly equivalent to seven crew hanging off the windward rail) could compensate. Once I started to model the rig on the computer I found that it might be better to build a fairing from a carbon ring frame covered with fabric or extremely light flexible panels or a combination of both( lighter construction?) that would rotate independently from the mast so that with the addition of it's own spanner it could be used to adjust sail camber. It then occurred to me that I might achieve another goal on my wish list, a mast that could be lowered at anchor or underway in flat water . By inserting a hinge into the mast above the goose neck and using a sliding sleeve to reinforce the joint when the mast is raised It might be possible to devise a system that could use the anchor windlass to raise the mast( sort of like the Thames sailing barges) The entire fairing would have to be lifted along with the sleeve to free the hinge but it should not weigh more than thirty pounds .I realize that some or maybe all these changes are not workable and when I think about it this design has evolved into something a lot more complicated than I first intended but I am at least set on the yawl rig with carbon fiber masts. If in the end I decide to go with a much simpler version I am leaning towards half wishbone booms and no vang . I am at the stage I would like to consult a naval architect to spec the mast as I would like to start building soon. Before I do I would be appreciative of constructive criticism and any suggestions or alternative ideas. I apologize for the long post but its been three years since I started thinking about this . I am not getting any younger and my sails are completely gone. I have to stop conceptualizing , finalize the design and get started. . Attached is a short animation of the main that might take a while to download for those of you who have slower connections . The traveler controls the mast cant, the the main sheet controls the trim ,the fairing controls sail camber, and the vang and the out haul control sail shape. All lines are intended to run through the mast step and back to the cockpit.

 

Fun project, but starting to get complex.
Couple of things:
1) Joining an unstayed mast at deck level is heavy and complex. We are engineering one at the moment and it is not trivial. Easier, unless you are going to be raising/lowering it regularly (ie each time you go sailing) to have a one piece mast and a removable gin pole beside the mast.
2) If you go with a wing masts with fairing, the fairing has to be strong enough to support the battens and the track. May be easier to incorporate it in the structure of the mast.
3) Easiest way to get a raked mast is to use an oversize deck bearing and offset the mast in it. Use epoxy/graphite on the mast and it is easy enough to change if the rake is wrong.
4) Wing masts do not have to have solid carbon walls. There is usually a core for most of it, with solid carbon strips down the sides, leading and trailing edges. This makes them of comparable weight to a tube of same max width.
5) Unstayed wing masts can have a square section at the base which makes them much stiffer than round ones of the same width and wall.
6) Sails from Aus seem to be cheaper than what you have been quoted. Try Neil Tasker at Barracouta in Sydney.
7) Our engineer charges $Aus1,000 for the basic unstayed mast design and an hourly rate for refinements.

Happy to answer any specific questions.

rob

 

Rob I tied to reinforce the mast at and above the partners by using the arms that rotate the boom, the conical section that covers the flange containing the over sized bearing , as well as a box section bonded to the mast ,The box section as well as holding the two gudgeons ,that support the free standing boom allows for a hole in the mast for the reefing and control lines to run into the mast above the partners and exit the mast heel. I thought that in fact the whole kinked section of the mast from the step to the hinge above the boom could be treated as a rotating tabernacle and.that the sleeve securing the hinge as the step and partners of the mast section above the hinge. Initially when I was thinking of using fixed masts and Wharram style soft wing sails with luff pockets I had intended to use the mizzen mast which could easily be lifted by hand as a gin pole to remove the main mast. If a wing shaped mast can be made that is as light as a circular sectioned mast with a fairing and the bending issues could be addressed It would certainly make things a lot easier. On another thread you asked me why I would want the mast to bend for and aft when if the mast bends to lee ward the same depowering effect occurs. I have been thinking about that and perhaps your right but when I watch my own masts in gusts they bend aft and to lee ward and it seems to me that the aft bend contributes the most to flattening the sails and pulling the center of effort forward. The two ply wrap around sails each about 750 sq. ft. I bought fifteen years From Ullman in the States for $11000.00. They were well made . The quote for $18,000.00 I got for the Wharram type soft wings sails was from Jeckells in the U.k. I live In Thailand for six months of the year and have been told that Rolly Tasker in Phucket can make them for considerably less. When I decide on the final design ( its been my experience in the past that changes are expensive) I would certainly consider using your firm.

 

The kinked area can be strengthened, as you point out. However, because it sees the highest load, more strengthening is required. I prefer the offset bearing.

Bending a mast fore and aft flattens it if there is any luff round in the sail. The luff round adds a kink to the section when the mast is not bent, which shows up as the draft in a tube mast sail. In a wing mast sail, it shows up as a kink behind the wing section, which is not good. Hence, I prefer a flat cut sail and using the mast rotation to get the shape.

rob

 

Rob I would like to thank you for taking the time to look at my proposal and for your thoughtful response. I don't know why I did not think to offset the mast in the bearings before. Perhaps my mind was fixated with the Van De Stad kinked mast. It certainly would be much easier all round to build a straight mast and angle it trough the bearings which would be subject to the same loads in any case. I think the arrangement shown in my attached sketch is what you are suggesting. It has the added attraction that I could have the mast speced now based on RM and decide at any time weather or not to incorporate the balanced canting feature. I like the way my current rig automatically responds to gusts. My wrap around sails (think luff pocket) are cut flat. If the mast straightens in light winds and a kink develops in the sail behind the mast it is because I have failed to ease the halyard and the out haul. I trim them essentially the same way I do my windsurfer. Yesterday I paid close attention to the bend in my windsufer mast as I was planing at full speed. In the gusts it responded exactly the way the rig on my boat does. The mast bends aft in line with the pull of the sail and the tip deflects a little more to lee ward. The leech goes slack the square top twists off the draft is pulled forward as the sail flattens. It still seems to me that the the bend of the mast in line with the pull of the sail is the most effective at automatically depowering the sail. In the case of a rotated wing mast would not the pull be along the long axis and would it not be necessary to under rotate the mast manually in the gusts so that it would more readily bend to leeward and spill wind.?

 

Your sketch is correct. You can offset the bottom bearing in the opposite direction if you want smaller bearings.
Agree about the tube mast bend. Also about the wing needing to be under rotated before it will bend to leeward. This is not automatic, so may not be ideal on a cruising multi, less of a problem on a leadmine. I still think it is better than a bending fore and aft wing mast of large chord, but not of small chord. Not sure where the cross over is from large to small but my rig is 600mm of mast and 3m of sail, which is definitely large. On big boats there is the added problem of getting the trailing edge of the mast stiff enough to support the track and the battens, while still bending.

rob

 

There was a french 40 footer (?) that was put together as a proof of design for a big mother unstayed 90 footer(?) a few years back- the mast had a bend in it about 25% of the way up that rsulted in the mast tip moving to windward when downwind sailing, so besides the mast rotating in the deck, the rest of the the idea was geometric- that is, no other moving parts. It's not on my favorites still, but I could look for it, if you haven't done anything yet. It's an elegant solution to the problem. At least that's what I thought a few years ago when I saw it.

Paul

 

Paul I read about this boat some where on the internet but I could not access the article I was directed to. I would be very interested In any information that you have regarding the prototype or the full size version if it was ever built. I have just got back to my boat here in Canada after my annual six month stay in Thailand to find that my expensive and for me absolutely necessary battery bank (I live off the hook) needs to be replaced. This represents a hefty chunk of my mast building budget but I am still anxious to proceed. I have decided to abandon the kinked mast idea for Rob's suggestion of the offset bearings. Using this method would allow me to build the mast conventionaly without the kink and to use it without modification if I decide to abandon the canting concept. It also means I can secure a tubular mast in the bearings, with a fairing rotating about it, or I can build a wing sectioned mast rotating in bearings within the main bearings. The offset bearings have the benefit of moving the entire mast to windward and make it possible for all the lines to be led to blocks above the deck and outside the mast at the rotation point of the main bearing, reducing their number by half, with no holes in the mast necessary. Having the blocks outside the mast would also allow me to dispense with the control arm quadrant thingys , run the mast rotation control lines from a tang on the boom to the mast, and then through blocks on the main bearing to the traveler, hopefully acting like a vang. I am still debating the pros and cons of wing section vs tubular section with a fairing and I can see that on a multi hull the gust response a tubular section provides is probably not necessary ,as a fast light boat would be more likely to accelerate rather than heel. But in a lead mine like mine I don't know that a stiffer for and aft wing section is the way to go. Once I can get that sorted hopefully I can get Rob to advise on the best building method ,have his engineer specify the mast lamination layup and have plans made up for me.

 

I'm sure Rob would be able to figure it out- heck, he probably knows about it.

Paul