Can I build 50' free-standing masts out of composite?

Discussion in 'Fiberglass and Composite Boat Building' started by Seafarer24, Aug 15, 2011.

  1. Seafarer24
    Joined: May 2005
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    Seafarer24 Sunset Chaser

    I have found a source for 40' (mounted height) embedded "lightpoles" with a 10" base and up to .312" wall thickness. I need to call the company and find out how deep the embedded portion is, how much they weigh, etc.

    I also need to measure the length of the mast that sits below the deck and the horizontal length available between the masts. That should happen later this week if the seller returns my call.

    Then I will need to determine what it would take to put 450 sq. ft. of sail on each one....
    I've been reading up on modern junk rigs and the more I read the more I like the idea of the cambered panel junk rig on this boat.
     
  2. Eric Sponberg
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    Eric Sponberg Senior Member

    The maximum steady-state load that any mast ever sees, whether stayed or unstayed, is always a function of the boat's maximum righting moment. For an unstayed mast, that's the direct load--max. righting moment = max. heeling moment = max. bending moment in the mast. This load occurs at the deck partners, and I typically taper the load linearly to zero toward both ends. Above the partners, the shear load is not too great, but between the partners and the heel, the shear load is very large--the closer the partner and heel are together (short bury) the higher the shear load. I typically like to see at least 10% bury (10% of the total length of the mast). On all of this, I work to a factor of safety of 3.0. There may be some higher value shock loads in certain rare conditions, but this level of safety has worked well for me and compensates for the shock loads. A round mast or a nearly round mast can be built without a shear web, but a wingmast, which is narrow for its fore/aft length, will have a tendency to buckle, and the shear web prevents that. Also, it is imperative that the wall thickness be appropriate to the overall section size--that is, the mast wall cannot be too thin or it will buckle prematurely. Based on years of designing fiberglass flagpoles for one of my clients, and testing them to destruction at full scale, I can tell you that composite tubes need to have a minimum wall thickness of 3% of their diameter. All of these conditions distill down to the fact that for any given boat, there is an ideal mast section size and wall thickness everywhere for its length and taper--the one that is not too small, nor too big.

    A stayed rig, instead of bending, undergoes pure compression. The two worries are compression strength and stiffness, and Euler column buckling. The size of the mast is again a function of the maximum righting moment of the boat, but the loading in the mast is also a function of the shroud and stay geometry. In this case, the load model is a truss--the mast experiences a distributed side load from the main sail, and point loads from the jib and spinnaker and other head sails. In Skene's Elements of Yacht Design, the distributed load is uniform all along the mast, but I like to change that so that it is uniformly increasing to the mid-height, and then uniform to the top. This tends to add load up higher in the mainsail because in fact, the top of the mainsail works harder than the lower part. The total of the loads by the mainsail and the other sails (jibs usually--spinnaker is a special case) cannot be more than the maximum righting moment of the boat. These side loads are balanced by the shrouds according to their arrangement on the mast, and these wires induce the compression in the mast that you design to. Factor of safety is the same--3.0, and wall thickness criteria is the same--3% of minor diameter.

    Eric
     
  3. Eric Sponberg
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    Eric Sponberg Senior Member

    That size of mast sounds about right--10" dia. Earlier this year, I designed a new interior for a 1980 Freedom 40 which had aluminum masts, and I measured them at 10" diameter. I am unsure of the wall thickness, but these did come from a lightpole manufacturer. I don't know who did the engineering on that size--that was determined before I joined TPI, which was at about the same time this particular boat was built. The original sailplan that I have, which shows the unbroken sheer (not the earlier "pirate ship" sheer) shows the main and mizzen sail areas of 378 sq. feet each. The mast length, by scaling the drawing (which I did in AutoCad because it was all I had to go on--no other drawings are known to exist), is 51' 8".

    I hope that helps.

    Eric
     

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  4. Steve W
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    Steve W Senior Member

    Eric,the mast tube i was enquiring about was not from TPI but from the folks that built the masts for them at the time, i dont remember who it was but it was east coast i think, this was quite a while ago,back in the days when Aero rigs were being offered and as you say,they were known to be heavy. The Cat mast i mention was from the 40ft cat Great White, i believe the boat was built by its owner and it may be that he also built the mast, i dont know,what i do know is that the weight of the mast quoted to me by the guy who was selling the wreck was way heavier than i would find acceptable, i was told later that he sold it to a Foiler Trimaran project in Florida. FWIW, i considered buying the boat without the mast but while it was a great looking cat it was not well built and while i could have spliced it back together,i would have had much more confidence in the repairs than the rest of the boat so i passed so it may be the mast was owner built. My point is that sailors seem to be impressed with carbon anything and it has been marketed to the point that at the mere mention of carbon and folks automaticlly assume its light in weight and as we both know, it aint neccesarily so. I remember the story on Sailing Anarchy about the Aerorig on a large boat that was so heavy that the boat heeled way over at the dock and they eventually cut it up and disposed of it and videoed it, i think it was the lawsuit from this case that put them out of business. A well built wood spar can be suprisingly light and very long lasting and, if owner built, very cost effective, unfortunatly,they are a lot more labor intensive to build than just rigging an extruded aluminum tube and this is the main reason why extruded aluminum masts took over the industry.
    Steve.
     
  5. Jeremy Clarkson
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    Jeremy Clarkson Junior Member

    yes you could make a lot of them your self for 20k

    What I would do is get core of something maybe foam or pvc or wood, then rub it really hard with wax.

    Then buy lbs and lbs or spooled thick carbon roving and just wind it around the pole then put a finishing layer of twill weave on the top and slide it off the pole
     
  6. Seafarer24
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    Seafarer24 Sunset Chaser

    Eric,

    Your posts are always very helpful!

    I realize that I am aiming for considerably more sail area with the junk rig than the original rig. I'm doing this for a number of reasons:

    1) I've heard quite a few F40 sailors are not so pleased with the light-air performance of their boats and a common modification to the rig is to lengthen the booms to fit larger sails.

    2) The junk rig is a much lower-aspect sail and will benefit from more sail area while still not creating as much healing moment due to its lower height.

    3) The junk rig is very easy and quick to reef, so in effect I am designing the sail area around light-wind conditions. I would ideally like to get a SA/D between 15 and 20.

    The boat I am looking at is in FL, the light pole manufacturer, HAPCO, is in VA. I would be interested to know where the owner of the boat you redesigned the interior for got his masts / poles.
     
  7. Steve W
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    Steve W Senior Member

    Jeremy, building the mast the way you describe is putting the carbon in exactly the wrong direction and it will break as soon as you hang a sail on it, a mast needs the bulk of its fibers running the length of it, a wood mast for example has pretty much 100% running lengthwise which is why they can be built fairly light, with aluminum you have no choice so they have equal strength in both directions by default so they are not particularly light, an aluminum lightpole as an unstayed mast will be rather heavy but have been used for this application, they may be thicker walled than a typical mast but taper fairly radically so may not be bad.
    Steve.
     
  8. BATAAN
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    BATAAN Senior Member

    I don't have much experience with hollow masts other than big steel ones on square rig, but have fiddled with Chinese balanced lug since the early 70s on quite a few different boats, stayed and unstayed, up to 70 feet and three masts, and since the poster seems to like the idea, I'm going to weigh in here.
    He speaks of a 'cambered' lug sail and I'm not sure what the meaning is but I take it to mean built-in 3d curved shape instead of dead flat like the inventors did it.
    Many have tried to improve or standardize the battened lug but few have been successful. Tom Colvin's designs stick pretty close to the original geometry and techniques, while Hasler's book shows an all-around approach that seems to work pretty well, but gives a small sail area by design, has parallel battens for neat reefing but less shape control, and so is more suited to yachts, which typically have less initial stability or need for raw horsepower than your average junk.
    The advantage of the flat cut and long yard and non-parallel battens of the Chinese working sail is the ability to change the shape from loose and baggy to flat as a piece of plywood with a slight aerodynamic bend in the right place, all by tensioning the luff parrals and sheet.
    Unstayed masts were widely used before the availability of wire rope, and the masts were very stout, built of hardwood usually, banded with iron and stepped in a curious way between two tabernacle planks and a bulkhead with the heel not even touching a mast step, but very well supported laterally and fore and aft below deck and quite unlikely to rot.
    The photo of the Foochow Pole Junk loading shows one of these masts well.
    The sail is difficult to make light and still have the battens not fold up. I used 6061t6 recommended by Colvin and still had the upper ones bend so far when reefed I quickly went to the pipe store and slid a reinforcing pipe of the next smaller size inside of the upper 3 battens, and still have broken 2 upper battens in a bad knockdown when the sail went in the water to the peak. Pipe is easily fixed with a sleeve if a piece of the next size bigger is carried to make repair fishes after finishing the break and pounding the ends round, and we did so in about an hour in that case and the repair is considered permanent now. It shows on the second batten in the photo, just behind the plastic pipe chafe piece.
    Speaking of chafe, whatever mast material you use, it must be very smooth, as on the off tack the sail cloth is being rubbed against the surface, and the parrals are always digging in and trying to chafe as it moves. Due to the weight, this is especially galling in a chop and light air, when the mass tries to slam around and must be controlled by tightening things, causing more chafe.
    My mast is 46' overall, solid douglas fir, cut to 10" at the partners, 8" at heel and 6" at head, from a monster 24"x 55' Class A piling I cherry-picked from about 600 of the same in a big pole yard, and the finish is not varnish, but cooked-in Vaseline on the working area, which takes about 1 hour per year to maintain, does not chafe the sail, wood or ropes, makes water recoil in horror instead of penetrating the wood and is just generally satisfactory, but a little dirty on the sail. Lube really helps the rig last, and should be considered when choosing mast material and finish.
    Solid wood is too heavy for your vessel.
    Laminated wood is unsuitable to being greased instead of varnished, fiberglass or carbon mast might not take the chafe and banging of the yard and battens well, aluminum light pole may be rough and need a polish etc.
    I think the AL mast could drive you batty with the noise because the rig does repeatedly tap tap tap sometimes.
    Also, since there is a single halyard and it is a direct reversal of load if single part to a winch which I recommend for a big sail, the halyard block is very highly loaded and subject to failure. I'm on my 4th or 5th attempt and it says "capacity 3 tonnes" on the $40 steel logging block that I haven't managed to break yet, unlike the $400 Harken hi-load which was the last one to fail and send bits crashing down along with the sail.
    My mast is stayed just like a working gaff rig with the head sprung pretty hard forward and a good bend in the mast. Like a big gaff schooner, this bend straightens out when the sail is hoisted from the halyard load and side thrust of the large and heavy sail. If the mast was rigged so it was straight when not under load, it would bend backwards when the sail was up.
    When deciding on the Chinese lug, look inside and ask why and really try to separate engineering from romance.
    For me and BERTIE it was essential due to a need for enormous sail area to push her blunt trader's hull that I could handle by myself as I got older, and her ability to shrug off the weight aloft and stand up to the huge sail in a lot of wind, so minimizing the drawbacks.
    Any other craft, designed for a lighter rig and lighter conditions (BERTIE loves force 5-7), must be very carefully analyzed as to its suitability.
    Most sailing is in pretty light air, so needs a large sail area to make a boat go well, unless it is a very slim and slippery one.
    Doing this with a Chinese lug gets heavy very quickly and greatly increases inertia loads on any mast system.
    These loads can form a rhythm in the wrong sea conditions and make the mast whip enough to overload it and break it off at the partner if unstayed, but are usually countered by taking the halyard aft and to windward to stop the 'pumping'. I had a fully set Chinese rig disappear over the bow once (not on my present boat) due to this effect and it was most dramatic.
    Experience sailing the rig would be a good thing to get before spending a lot of money on something that may not work as well as you would hope on this boat, which was not designed for it.
    One photo is a 70 foot 1917 slim ocean tug made into a 3 mast junk and a 32' very cheap lapstrake SPRAY type built in a few weeks by a couple of drunks as a Junk sloop. These were done as experiments using the cheapest ways possible consistent with reasonably long life and worked out surprisingly well and are both still around.
    Here is an interesting link to a junk rigged cat that seemed to work well and I've included a photo of her.
    http://www.duckworksmagazine.com/04/s/vintage/multihulls/index.cfm
    And general junk rig stuff.
    http://www.thecheappages.com/junk/tutorial.html
    For me, Chinese lug rig is for a big sail that must be handled by a small crew on a suitable hull, and seems to work best when sticking closely to traditional geometry and rigging on a very stiff hull that can take it.
    Anything else, different geometry, simplified rigging, bendy battens etc is not 'Chinese lug', but an adaptation of that, and a new design as a result, which may be quite a bit better for some boats, but needs thought.
    For any sail under 500 square feet there may be superior options for a yacht hull.
     

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  9. BATAAN
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    BATAAN Senior Member

    Some of the photos did not attach.
     

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  10. Steve W
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    Steve W Senior Member

    Just as a matter of interest a couple of years ago i did a 3 week cruise up the gulf coast of florida with a friend in his 28ft Egret sharpie with an unstayed gaff schooner rig, the masts were solid wood glued up from selected 2x6s from a big box lumberyard in Minnesota,(im not sure the wood species), they were about 26ft long. Anyway when we were done with the cruise we put the boat away in storage for the summer so we had to remove the masts, his method for removing them was to strip off everything except the halyard which he would tie a stop knot in and pull through until the knot stopped in the sheave at the masthead, he would then tie the other end to the boat,then,by himself (70yrs old)stand at the mast and lifting with his legs,lift it straight up until the butt was sitting on the deck,then, adjust position and toss the mast into the canal like a scottsman tossing the caber and haul it back in with the halyard, each mast weighed about 55lbs on a bathroom scale,they would have been a lot lighter if built by the birdsmouth method or with staves, wood makes great masts. 1sqft of spruce at 3/4" thick weighs about the same as 1 sq ft of 1/8" thick aluminum.
    Steve.
     
  11. Eric Sponberg
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    Eric Sponberg Senior Member

    An example of a perfect waste of money, for masts that will likely break the first time you take them sailing, for the reasons Steve W stated above--the fiber runs in the wrong direction.
     
  12. Eric Sponberg
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    Eric Sponberg Senior Member

    Personally, I am not a fan of the junk rig. What you gain in sail area and ease of handling you lose in windward performance because junk rigs just don't go to weather very well. You know, it's that big blob of a big round mast on one side of the sail that destroys all the aerodynamics. But that's just me.

    The design displacement of the Freedom 40 is 20,000 lbs, and the SA/Vol^2/3 = 17.02 for the designed sail area of 784 sq.ft.

    The boat that I redesigned the interior for had the original aluminum masts as built from 1980. For the life of me, I cannot remember where TPI bought them. The wishbone booms and sails were severely damaged/aged a long time ago, and the owners salvaged replacement Douglas Fir wishbone booms and sails from another Freedom 40.

    Eric
     
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  13. BATAAN
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    BATAAN Senior Member

    Junk and gaff are roughly the same in windward ability when properly set up and adjusted, and the blob of a mast on one side seems to make little difference. I say this having sailed against many traditional boats of fairly good performance like tall gaff cutters and big schooners.
     
  14. Seafarer24
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    Seafarer24 Sunset Chaser

    I'm taking a shot in the dark here that none of you have spent much time reading the Junk Rig Association website? There are quite a few people working at modernizing the junk rig to keep pace with other modern rigs while still being easier to use and build. By build, I mean that I have a better shot at designing and sewing a decent-functioning junk sail than designing and sewing a modern triangular sail, or square-topped main, or what-have-you.

    The bendy or hinged battens are not really much in use. The way the sail is cut with the batten pockets "offset" from it allows them to create curvature in the sail while retaining the straight and lightly loaded battens.

    [​IMG] [​IMG] [​IMG]

    The "ribs" created by this technique make tip vortices a thing of the past.
    There is no mast disturbing the airflow at the leading edge of the sail.
    Since the Freedom 40 is an equal-height "ketch" (could just as well be called a schooner) rig, I can build the sails with one sail on either side of the masts, so one is always drawing perfectly cleanly.

    These are some of the files made available by one of the leaders of this technique: Arne Kverneland's files

    There have been plenty of boats converted to an unstayed junk rig that weren't even designed for unstayed masts and work just fine.

    [​IMG]
     

  15. BATAAN
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    BATAAN Senior Member

    The vessel with blue sail in picture seems to be heeling in excess of 30 degrees and is carrying too much wind. Boats sail best if you try to keep them at their design waterline or as close as you can.
    If he had a reef in he'd sail and look much better.
    The batten pockets are very interesting concept and will be cool to see how they work out.
    In my experience one of the strengths of the CL is how the flat panels never "shiver" in a strong blast of wind, but remain calm.
    It seems (pure conjecture) that those baggy pockets which give good shape for light air are very poor for when it's stinky and blowing strong gusts, when a flat panel would be better, and there is no way to adjust the amount of shape except when building the sail, and of course they are only truly effective on one tack.
    I think they are a faddish novelty but time will tell.
    Cat schooner/ketch with unstayed masts and equal size Hasler type sails has logged many hundreds of thousands of miles on boats like BADGER and many others and proven the concept thoroughly.
    In 1974 I rigged a 36' steel lifeboat for a couple this way. They had put on a stubby fin keel and a nice low cabin and made solid masts of fir. We made the sails laid out in the park on the grass and sewed them on a heavy home machine in two days, and the rigging only took a few hours. It sailed amazingly well and they took off for the So. Pac.
    Remember that the bending load on the mast is not relieved by heeling if overpowered when going dead down wind, so maximum beam stability is insufficient in load considerations for unstayed masts.
    This can happen in conditions where it is not possible to round up to relieve the pressure, such as in bad seas or a rocky inlet or confined by whatever, and must be thought of.
    My own broken unstayed CL mast went over the bow of this heavy 18' boat that was going downwind in a narrow channel in 1975 between some pilings on one side and a sunken barge wreck on the other and I could not turn, and a big gust hit, the mast bent, the bow depressed, the boat accelerated and the 4 1/2" spruce pole broke clean at the thwart.
    This one had plastic pipe battens but they were too bendy.
     

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