Free standing mast

Discussion in 'Sailboats' started by Paul A, Apr 28, 2010.

  1. Paul A
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    Paul A Junior Member

    I would like to put a free standing birdsmouth mast on my 22' sloop. I have this no name sloop that weighs 2200 pounds. The 24' mast is broken and I would like to make a new one without stays. The boat carries 188 square feet of sail, 96 in the main 92 in the fore triangle. The boat is shaped like a Oday 22, but the stern is flat not raked and it has the stub keel under it. 22' on deck, 18' wl, 8' beam. It has no HIN and the title has no manufacture listed, just 1971, 22' sloop.

    I'm thinking a 20% wall, spruce mast 5" at the base, tapering to 3" at the top would be stiff enough, but would be interested in hearing what you all think. It would be used in protected waters. Step on a pad glued to hull inside cabin, plywood partners at cabin roof. Add 4' - 5' length to mast to reach hull bottom. I just want to know about diameters for a free standing mast with this weight and sail area, the build seems obvious.
     
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    Get a book called "Practical Junk Rig" by H.G. "Blondie" Hasler and Jock McLeod. It describes how to build a wood free-standing mast. The fact that the book is about junk rigs is beside the point--the boat does not care what the shape of the sails are, but it's righting moment will determine the diameter and wall thickness of the mast, and the height of the sailplan will determine the length and taper of the mast. This will be good enough for your situation.

    Eric
     
  3. ancient kayaker
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    ancient kayaker aka Terry Haines

    Scaling from my (smaller) sailboat, I'd say your numbers sound about right. Not sure whether you mean 20% of diameter or radius: if diameter it's not worth the effort - might as well go solid. On the other hand 20% of radius doesn't leave a lot for rounding. My mast has 25% (of radius) wall thickness and it seemed a bit light. I left it octagonal as I did not want the wall to drop to 18% at the seams. However, because it's a 2 piece mast with a socket joint it gets a bit more stress near the joint. I did not glass it but that would make a difference. Put internal blocks at the partners as well as the fittings.
     
  4. Fanie
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    Fanie Fanie

    Hi Paul, I personally like free standing masts, some guys really invest in some stay's instead of the boat. The less cluttered the better.

    If you have any doubts you could always consult a designer like Eric above :D
    Beside's you're neighbours...
     
  5. messabout
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    messabout Senior Member

    Free stnding masts are my choice every time. There are some buts however. Among them: BUT your boat has a forestay onto which you hank the jib. You must keep the forestay tight if the jib is to function well. That implies that the mast must be pretty stiff in order to keep tha forestay tensioned well. If your foresail has a tall luff, mast stiffness will matter aplenty. A bigger but....The cabin top or wherever the mast exits the hull, must now absorb all the forces that used to be transmiited through the shrouds. Not only that but the mast is now a giant lever. If the lower part is five feet long and the sail CE is 12 feet up then you have a 2.4 to one lever ratio....and so on. If you have a mast failure it can be more than disappointing, it can be dangerous. Take great care that step at the heel of the mast is the strongest part of the whole arrangement. Imagine what would happen if that part had a failure and someone was inside the cabin. I do not wish to discourage you. I only wish to point out the structural demands of a free standing rig.

    The upsides of free standing rigs are many. A convenient feature is that you can let go the sheets when the wind is directly astern. The sail will merely weather cock. That can be a valuable attribute when you are rigging up or leaving a dock or congeted area when the wind is astern. Of course the mast must be free to rotate. The absence of all that damned wire is a blessing too. Steping the mast is a piece of cake if you build a guide box along the path from deck to heel pad. Absence of wire makes storage more pleasant too. You can go from trailer to actual sailing more quickly when all you need do is drop the mast in the hole. ( well dont just drop it unless you want to ventilate the bottom of the boat)
     
  6. jwboatdesigns
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    jwboatdesigns John Welsford

    Mast section calcs for free standing mast

    Buy, beg, borrow from your public library or steal a copy of Skenes Elements of Yacht Design, mines the 8th ed, and on page 65 there is a formula for working out mast sections for catboats ( or other unsupported masts).
    I'd put running backstays on to keep the jib luff tight.

    John Welsford
     
  7. sharpii2
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    sharpii2 Senior Member

    From the size of your fore triangle, I surmise your boat is a mast head sloop or cutter.

    A free standing mast will not work with such a rig as the luff(s) of the jib(s) will not be held tight enough for them to set right. To switch to a main only rig, the mast will most likely have to moved forward for the boat to have proper alignment of the Center of Area (CA) of the sail with the (CA) of the keel.
     
  8. Paul A
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    Paul A Junior Member

    Apparently, you're not familiar with modern sailing vessels. Have a look at Freedom yachts for one, not to forget the effort that Eric Sponberg has done in the same regard. We can have our cake and eat it too now.
     
  9. sharpii2
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    sharpii2 Senior Member

    Those boats all have carbon fiber masts, if I'm not mistaken.

    Here we were talking about a wooden mast. The wooden mast does not have the stiffness to weight ratio of a carbon fiber one. Not even close.

    There is one kind of jib that might work on a free standing wooden spar. That is a 'balanced jib', where the foot of the sail is attached to a boom and the boom has a hold down strap set significantly back from the leading end (anywhere from 10% to as much as 40%). I had a boat with such a sail and it was tremendously effective. The thing is, this boat had a stayed mast.

    Smaller versions of this kind of sail were used on racing sharpies at the end of the 19th century. These sharpies, due to their mimiscule beams, had free standing wooden masts. But on these boats, the jibs were tiny in proportion to the mainsails. It would be an interesting experiment to try a larger jib of that type with a free standing wooden mast. It might just work.

    One thing that can be counted on is that the top of the mast will bend forward causing the luff of the jib to sag. The way the balanced jib gets around this is the leach sags as well due to the placement of the hold down strap. To get the luff to sag less and the leach to sag more, put the hold down strap further aft on the boom.

    If this is tried and fails, the mast might be able to get by with just an upper and lower shroud on each side and a pair of running back stays. This, though not as simple as an unstayed mast, is probably simpler than the original rig.

    Or go to a carbon mast and have someone like Eric design it for you.
     
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  10. Paul A
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    Paul A Junior Member

    Your argument is weak and certainly not based in sound engineering principle. Free standing rigs preceded carbon fiber masts by an order of magnitude in time. You clearly have some knowledge, maybe sticking to your base would be best. I'm not trying to insult you, but I'm a structural engineer and material choices and substitutions are commonly utilized, especially when exotics are used.
     
  11. ancient kayaker
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    ancient kayaker aka Terry Haines

    Paul, I don't think Bob was suggesting that only carbon masts can be made without standing rigging; there are hundreds of boat designs using free-standing wood and aluminum masts and he is well aware of that. You asked for advice and he is trying to give it to you.

    As a new member you are very welcome to the forum, and are no doubt aware that there are some very experienced and knowledgeable people who will lend you their time willingly. I have obtained invaluable support from many of them in pursuit of my boating desires. As a new member, also you may not be aware that we have lost several of our top experts recently as a consequence of "internet attitude" or plain bad manners. We do not want to lose any more; so I beg you to be respectful until you know who doesn't deserve it.

    The sailing rig would certainly have an impact on mast design and it would help the membership if you were to provide a sketch or at least dimensional information of your sail plan.

    I'm not by any means an expert in sailing rig design. However, as an engineer I can say that there are several alternatives to a very stiff mast that have not been explored yet in this thread. There are things that can be done without resorting to shrouds, such as an oval mast section to optimise fore-aft stiffness, a back stay, closing forestay tension into a triangle of forces using vang and boom lift and so forth. Without more insight into your boats layout it is difficult for the experts to be specific.
     
  12. troy2000
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    troy2000 Senior Member

    For starters: if you already have all the answers, why are you asking for advice?

    Instead of throwing generalized insults at the gentleman, why not tell us specifically what was wrong with his reply? It seems to me that if stayed rigs didn't perform noticeably better as a general rule, no one would have gone to the trouble of festooning traditional boats with them.

    Unstayed masts in this country were normally used for specific purposes. For example, on sharpies -- because they didn't have jibs, because the masts could be dropped to go under bridges, because they could let the sails blow free while they tonged, and because flexible unstayed masts would spill gusts of wind instead of letting the narrow, shallow-draft boats be knocked down.

    They certainly weren't used on sharpies because their overall performance was equal to stayed rigs of the same size.
     
  13. Paul A
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    Paul A Junior Member

    Where did I imply any insult? Did I not go on to insist it wasn't in case it was misinterpretation? I believe I've supplied sufficient information for the data requested. With the exception of a reference to a book I've been met with:

    This is your contribution to the discussion?

    I'll assume, possibly incorrectly, this implies a wooden, freestanding mast in a masthead sloop application. Oh please. I just fell of the turnip truck?

    Well at least an attempt, but an incorrect one, fishing sloops, one in particular, the smack, the direct descendant of the Friendship sloop carried a free standing sloop rig 150 years ago. I'd think by now we'd have managed some material and engineering substitutions and modifications to improve the solid, heavy spruce masts they used.

    I've done the beam and column calculations I think relevant, but will double check with a borrowed copy of the Jester book. I don't think I started the negatives here, nor do I think I offered more then what was dished out, though it would have been nice to have the figures pushed through dedicated software, by folks more commonly used to making these types of calculations. Clearly it was my error in thinking this might be so here.
     
  14. troy2000
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    troy2000 Senior Member

    OK, I'm done. Good luck with your project.
     

  15. ancient kayaker
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    ancient kayaker aka Terry Haines

    You didn't imply. It was right on the chin!

    It might have been nice, has often been so in the past and will be again in the future. As I noted in my previous post we have had some problems with bad manners and tolerance of anything approaching that is probably at an all time low on the forum.

    Paul, I honestly think a straightforward unconditional apology would have served you better. We all make mistakes after all. But it's up to you to judge such things as you see best.

    I join Troy2000 in wishing you sucess in your endevours.
     
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