Proa design

Discussion in 'Multihulls' started by Tcubed, Sep 13, 2008.

  1. rwatson
    Joined: Aug 2007
    Posts: 5,852
    Likes: 290, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Just to throw my 2 cents in re the plywood V strip planking discussion, in my next project, I was all keen to do it from plywood. I had just finished a small strip plank boat, and the process was quite laborious and took me quite a while.

    So I went out and bought a book, Sam Devlins "how to build any boat the stitch-and-glue way" Its an excellent read, and has some great tips in it re plywood building etc.

    The big eye opener to me was that fact that even in plywood, the amount of finishing, (given that glass/epoxy was used on the outside and inside)- looked to be exactly the same as I had encountered in the strip planking. It seems as soon as you start adding epoxy/glass to something - you have to sand the stuff a lot.

    The other jolt I got was something I have read a lot of - over the years, the importance of getting good, "well built" plywood. He spent quite a few pages on how to detect bad batches of ply. He even gave an example of how two new boats he just completed showed irregular condensation patches during cold weather, caused by voids in plywood. These had to be filled with epoxy.

    That took me back to many years ago when I read a newspaper article about some people had to be rescued when their old plywood trailer/sailer opened up at the bow and sank. More recently, I was reading a story of an ocean going intrepid sailor in a Norwalk island Sharpie (all plywood) having visions of the plywood hull foot giving out during a fast 24 hour run in rough seas (it didnt, it was just normal paranoia).

    I looked at the great little boat I built, and all the curves and elegant flowing hull (that could never be built out of ply), and then emailed my designer to forget about the multi-chine plywood plans, and just go with the curves that would need planking.

    The point is that I really dont want the worry of wondering how good the ply is - and how it is holding up over the years. If I lay the planks of solid wood, and glass them, I know exactly what is going on. Paranoia is just not worth it.

    The fact that the boats resale value will be higher, and the boat will be substantially lighter, are other big motivators.

    Also, the "look" of a boat is a source of pleasure long after the pain of building is over. (especially when showing it to prospective buyers)

    If a boat is lucky enough to have some developable surfaces, then frankly, laying planks is no harder than plywood. Certainly, long scarfed lengths of plywood take a quite a bit of manouvering compared to single planks. If the boat has some compound curves, its a bit more work, but I have decided to put up with that for the benefits.

    The other building method that is on the horizon for me, is the kelsall flat bench vacuum bag technique. Its not something I would try without going to a workshop - so the complexity puts me off that for the moment.

    My next boat is not a long slender multihull. If it were, I would seriously consider doing say, the bow and stern sections from fibreglass (on male sacrificial moulds) and plank over them ( glueing the planks to the hollow fibreglass shapes). This would do away with the really fiddly job of laying up glass and epoxy in narrow, hard to get at locations.

    Rob uses a form of this technique for the bow and stern of the proa ama hull. He doesnt run the planks onto the bow sections, but there is no reason that they couldnt. I am seriously considering doing the hull foot (with its permanent flotation) as a module in my next sailboat, giving really good support to the planks, while taking away the need to do any finishing on the inside of the hull.

    So - I vote for strip planking now after a lot of vacillating. Its really the epoxy sanding that is the horror in both methods, but I wouldnt go to sea without that protection.
     
  2. boat fan
    Joined: Sep 2008
    Posts: 717
    Likes: 17, Points: 18, Legacy Rep: 435
    Location: Australia

    boat fan Senior Member

    I hear what your`e saying!


    I have his ( Devlin`s ) book too , like you say it`s a good read.

    A lot of Americans build with " Fir " plywood.I personally would not go near the stuff . It`s rubbish , and that`s being kind to it.As for the other ply available over there , I cannot comment.

    I`m fairly fussy about the quality of plywood too.I would use Brunzeel if I could afford it , but the price of that stuff is absurd these days.

    Plantation Hoop Pine made here in Oz however is another matter.That plywood is really nice , at least as far as voids go . There are very , very few voids that I have ever seen.If you put a 200 watt spotlight behind a 6mm sheet you can actually see if there are voids , they show .You will not find many , if any .

    A couple of builders of cats up north have said that the Hoop Pine Ply can get mold spots , if you are building in the open , under tarps.Not a big deal they say because a little bleach is all that`s required to remove it.Besides I would not build out doors , ever.

    Yes you are right.

    However , with strip planking you need to glass inside and out.Not with plywood.You do need to fair and sand all outside surfaces like the stripped hull .If you are careful , you can save considerable effort while you roll on epoxy on the inside of your plywood hull.You will not eliminate sanding , but with careful work you can actually minimize it on a plywood interior.

    Good for you ! Again , I hear you .I bet it looked good. :) The satisfaction of a job well done....I agree that there is nothing quite like lovely curves on a boat.well , ALMOST nothing ...;)

    I used to loathe hard chines. I think it was a throwback of images of the clunky , boxy amateur built steel boats sitting in people`s back yards :D

    I have now come to like them , especially on multihulls.I guess that`s a matter of taste .Terhohalme`s effort on that 26 ft cat is an example.I find those lines quite attractive .

    All said and done , I do agree with you. I am not afraid to take a well built boat , built of quality plywood , offshore.

    Thanks for your post rwatson , it`s appreciated.
     
  3. rob denney
    Joined: Feb 2005
    Posts: 758
    Likes: 115, Points: 43, Legacy Rep: 436
    Location: Australia

    rob denney Senior Member

    boat fan

    Bending (Flexi) ply is 3 veneers with the inner and outer ones running the same way and making up 95% of the total. 9mm dbends to something like a 50mm radius. Pacific Boatcraft used to supply it. It is also lighter than gaboon ply.

    The Gougeon's technique looks scary, but they have built tornado cats to Olympic standard, so it is pretty repeatable.

    Mast bearings are plastic, there is a ring frame, a short piece of keelson and tow around the deck bearing. It is not until you see how little materials is required to support an unstayed mast that you realise how complicated and heavy stayed masts are.

    Ply is a quick way of building, probably quicker than most other methods, but slow compared to bending a partially glassed, full size panel into shape, dropping in the bulkheads and gluing on a deck with radiussed edges.

    Looks are not my strong point, but if you like Terho's hull, it would be easy enough to make it double ended, I think.

    Look forward to seeing what you come up with using flat panels.

    Rwatson,
    I stop the planks (or panels) before the ends, then glue on a polystyrene block, shape it and glass it. Much lighter than a stem, better impact resistance, positive buoyancy and much easier to get whatever shape you require.

    Flat panels built on a table then shaped is so much easier than any other form of building, there is no comparison. With rockerless hulls, it is even easier, and quicker. I strongly recommend Derek's workshops.

    Tcubed,
    For someone who agrees with me on "most points", your design has very little in common with harrys, and almost all of the differences in the design are fundamental to what makes harrys so light, fast and easy to build and sail.

    Veneers on the outside of the mast will help, but i would advise them on the inside as well (euler and bursting). Two halves allows for internal beefing up (a 600mm chord wing mast may need a sheer web and some bulkheads) and joining them before the external veneers go on is no work at all. I worry that thin veneers at 30 degrees will not resist the large torque wing masts and gunter yards exert. Applying 4 (or even 2) veneers, then epoxy coating them is very slow compared to a layer of glass, and unlike glass, they do not reinforce the join at the trailing edge, which may fail under peeling loads. I'd be happy to comment on your weights if you want to attach them.

    How much do you pay for sitka spruce? I sell materials for carbon tube masts for about $aus25 per kg/$US44 per pound. This will be about 50% lighter than a wooden tube mast with the same properties. Wing masts are more complex to cost as a carbon one would include foam or Polycore core for the sides and sheer web, but I am pretty sure that this will make the spruce mast relatively even more expensive, as well as much heavier.

    As for rocker, I agree it makes for a better very light air shape (less wetted surface) although if you are that concernedm, then moving crew or weight to one end of the boat will reduce the wetted area of the unrockered hull considerably. Yeah, I know this is a p[ain, bit much less so in light air than moving it to windward in a breeze. Like the section shapes, rocker is pretty irrelevant if the boat weighs twice as much as an unrockered one. Unrockered harry's with rudders on the beams turn quicker than cats and not much slower than fin keeled monos.

    regards,

    Rob
     
  4. Tcubed
    Joined: Sep 2008
    Posts: 435
    Likes: 18, Points: 0, Legacy Rep: 318
    Location: French Guyana

    Tcubed Boat Designer

    Designers agreeing on fundamental concepts does not necessarily imply that they will come up with similar design solutions.

    Right now, I'm tweaking the design, trying to create a shape that has higher prismatic the more it gets immersed, without creating too much of a strange shape for the waterflow. Light air performance is important to me, as i know light winds is very common on ocean passages, as is heavy conditions performance. So i'm always more interested in boats that will perform well in a very large range of conditions, than in peak performance.

    No doubt i'll do some comparative tow tests with some models to make a final decision. There's nothing like empirical testing. Especially for things as complex as fluid flow past an object that is on a fluid interface.
     
  5. MAINSTAY
    Joined: Feb 2005
    Posts: 60
    Likes: 2, Points: 0, Legacy Rep: 24
    Location: Lake Pontchartrain

    MAINSTAY Junior Member

    Okay, so the .xls file didn't show shunting.
    And I'm onboard with simplification.

    You are not using roller furling. So how do you get the jib from one shunt to the other?

    RHough
    The main is on a stay that is better described as a vertical backstay rather than a shroud. A shroud connotes a lateral slope to the luff, whereas the luff is in the same vertical plane as the foresail. It is the mast, not the mainsail that cants.

    That the weight of the rig is centered is a narrow view. The balance of a boat depends on all parts of the boat, not just the rig.

    Right now I have 2 rigs for a Hobie16, but no hull. While moving to Lake Ponchartrain, the there was an accident that damaged the trailer and hull beyond repair.

    Larry
     
  6. RHough
    Joined: Nov 2005
    Posts: 1,792
    Likes: 61, Points: 0, Legacy Rep: 793
    Location: BC Summers / Nayarit Winters

    RHough Retro Dude

    Larry, you are hanging a sail on a wire that has different tension on each tack. That means the luff curve of the sail will be different on each tack. You are hanging a sail on a shroud. If you took the time to compute the lateral forces on the rig, you would see how unworkable the rig is. Any gain you might get from moving the sail from the mast will be lost in extra weight and the fact that a fabric sail cannot be cut to set properly form a sagged stay on one tack and a tight stay on the other. The stiffer the sail material is the worse the rig will perform.

    If you size the stays so the sail loads are a small percentage of static tension, the size and weight of the wires required to handle that tension will be huge, the mast compression loads will be huge, and thus the entire rig will be orders of magnitude higher in weight. The hull structure to support that weight and those loads will need to be massive and heavy. I know you think you have a better mousetrap, but if you look beyond the fore and aft loads and calculate the lateral loads at maximum righting moment, you will see what I'm talking about.

    Cheers,

    Randy
     
  7. Tcubed
    Joined: Sep 2008
    Posts: 435
    Likes: 18, Points: 0, Legacy Rep: 318
    Location: French Guyana

    Tcubed Boat Designer

    The jib is shunted by having two jibs. You dump one and raise the other.

    I agree with Rhough. Any sail that hangs off a stay will impose much higher loads on the mast and therefore the boat. The mainstay rig reminds me of the mast aft rig. It could be argued that the mainstay rig won't put as much load on as the mast aft rig, but it is still much more than the loads from hanging the bulk of the sail on a spar. There was a production boat produced sometime early eighties with a mast aft rig. A few were made, until they realized they had under engineered for the monster loads and had to recall all the boats.
    Consider this, in the twelve meters the compression loads on the mast are several times the weight of the whole boat. And this is fairly typical of modern rigs with big overlapping jibs and shrouds set inboard. Imagine how much greater the compression would be if all the sail surface where on stays. Another example are the masts made for in mast roller furling or behind mast roller furling. These masts are made heavier to handle the extra compression of keeping that extra stay taut.
     
  8. MAINSTAY
    Joined: Feb 2005
    Posts: 60
    Likes: 2, Points: 0, Legacy Rep: 24
    Location: Lake Pontchartrain

    MAINSTAY Junior Member

    Mast Compression

    Guys, "huge tension," "mast compression loads will be huge," and "orders of magnitude higher in weight," "massive and heavy," "monster loads"? Very presumptive. Show me the numbers. "Those who fall out of love with practice inspite of math and science ..."

    The quotes above are applicable to aft-mast rig. Not the Mainstay rig.

    Attached are my numbers. Tell me about the numbers.

    The longitudinal loads are based on identical sail load resulting in identical sag
    Lateral loads for all three rigs are identical because the sail loads are identical.

    The stresses in the rigs are very different. To keep the stresses within the capacity of current materials, practice requires more material, usually in the form of larger size elements, or thicker walls.

    The stresses in the Mainstay are only larger if only compression is considered (as RHough points out). In the sloop rig the additional tortion and bending loads greatly increase the stress on the mast requiring a larger, heavier mast.
    Or so the numbers show.

    Larry
     

    Attached Files:

  9. RHough
    Joined: Nov 2005
    Posts: 1,792
    Likes: 61, Points: 0, Legacy Rep: 793
    Location: BC Summers / Nayarit Winters

    RHough Retro Dude

    This is not the thread to continue discussion on your rig.

    I'll be happy to continue the dialogue in a more appropriate thread.
     
  10. Tcubed
    Joined: Sep 2008
    Posts: 435
    Likes: 18, Points: 0, Legacy Rep: 318
    Location: French Guyana

    Tcubed Boat Designer

    Now that is the correct way of leading a discussion, Mainstay, by, as you have done, posting your analysis. Although it is true that a new thread is required..

    I redid the maths and i get slightly worse figures than you did but you are indeed correct in that the compression loads are not orders of magnitude greater. I'll even help you with the correct verbiage that i had myself overlooked and you should use if you don't already. Namely that you don't increase compression by having the mainsail on a stay, <because that stay (the backstay) is at that tension already>. You do however increase loads by reducing the longitudinal staying base especially when you take into account pitching forces- the sails forces do have a forward component, which in some extreme situations can challenge the boat's longitudinal righting moment which makes for asymmetry in the loads, just as in the shrouds. As for the offset mast, that invokes a whole other order of stress complexity that i won't go into right now, and less so in this thread...

    What is also missing in the analysis and something that i am personally very interested in, (and is relevant to the proa) is the method for deriving the actual physical tension force in a stay with x per cent of sag and with S amount of sail with V wind speed and operating at Cl coefficient of lift. Someone must have already done this analysis, so i can be saved the tedium of working it out myself.

    By the way, your using 600 lbs as an example loading can i think be taken to refer to a fairly light wind day!
     
  11. MAINSTAY
    Joined: Feb 2005
    Posts: 60
    Likes: 2, Points: 0, Legacy Rep: 24
    Location: Lake Pontchartrain

    MAINSTAY Junior Member

    Jib shunting and loads

    Tcubed

    Yes, 600 is low. But the spreadsheet will handle any load.

    I've never found a calc for the sail/stay sag/tension. If I started one, I'd divide the sail into slender triangles with the apex at the clew and equal bases along the luff. I can calc the wind pressure on a surface perpendicular to the wind. But don't know how to get the non-uniform pressure distrubution on a curved and angled surface. Any suggestions. anyone have any measurements of existing rigs to check the results against?

    But to the real question. What do you mean by drop the jib? Do you have two permanent stays (Forestays(?), jibstays(?), ?) or do they come down, too? Do you need to go on deck or have some way to secure the sail? Details! Details!
    Larry Modes
     
  12. Tcubed
    Joined: Sep 2008
    Posts: 435
    Likes: 18, Points: 0, Legacy Rep: 318
    Location: French Guyana

    Tcubed Boat Designer

    Yes there are two stays. There are also two identical jibs. One is lowered and lashed down, the other is set. Yes, that's more work than other setups and i'm not saying it's perfect, but it has already been done that way. In my own case, where most of my sailing will be for the long stretch, shunting effort is a somewhat lesser consideration than if the design were intended for coastal sailing. In fact, for short tacks there is the option of dropping the mainsail and sailing with the jib only, shifting it across the mainhull on the tacks with an extra removable sheet. This would mean that the proa would then be tacking like a regular boat, but half the boards would be sailed with the wind blowing from the wrong side of the proa, which is not efficient but it's just another option for maneuvering in certain situations which might not allow for shunting.

    In fact, i have drawn in another pair of stays inboard of the forestays for a staysail to have more options yet. You can compare looking at the regular rig and the heavy weather rig pictures that are posted earlier. So i guess some might call it a cutter.

    The basic formula for aerodynamic lift force is

    F = 0.5*rho*S*Cl*V^2

    where rho is fluid density, S is surface area, Cl is coefficient of lift, V is fluid velocity past the foil. Use all S.I. units so F will be in Newtons.
    Integrating triangles as you say is the way to go, i think, but one must keep in mind that the force the sail exerts on the stay is not the aerodynamic force per se , but rather the force required to keep the sail stretched in position with said aerodynamic force acting on it. Essentially it is something like a a continuous modified catenary acting in turn on a regular modified catenary.
    It is essential here to identify possible mathematical simplifications for solveability.
     
  13. MAINSTAY
    Joined: Feb 2005
    Posts: 60
    Likes: 2, Points: 0, Legacy Rep: 24
    Location: Lake Pontchartrain

    MAINSTAY Junior Member

    Jib Shunting

    Tcubed
    Thanks for the formula. Let me work with it for a while.

    Your proa uses an unstayed mast, except for the jib stays. Are the stays a necessary part of the design? If not, there may be two other options for the jib.

    First. Put the jib on a free-standing mast. Imagine a large wind surfing rig at each end, one acting as an overlapping jib and the other as a mizzen, and then on the other shunt the opposite. When you shunt, you only have to handle 3 sheets and no need to go on deck to secure a jib. The jib masts do not have to clear the boom, but can be close to the mast. Right? My understanding is that the boom is always alee the main hull.

    Second. Put the non-overlapping jib tack to a forward extension of the boom. There’s only 1 sheet to handle. Or 2 sheets, if you have an overlapping jib. One form is AeroRig, but there are other forms in the public domain that are free to use.

    Whether you use the jib-stay or wind-surfing or boom-extension, your CE shifts. I like your idea of a fixed fin on the windard hull, with a pair of centerboards in the lee hull to adjust the CLR.

    There is a patent that uses a large circular bearing to pivot the rig. A familiar analogy is a 6’ or 8’ lazy-susan. In this way the mast shifts around the CE and there is one less thing to do when you shunt. Your widened deck in this area seems well suited to this type of base for the rig. The design is firmly in the public domain.
    Larry
     
  14. Tcubed
    Joined: Sep 2008
    Posts: 435
    Likes: 18, Points: 0, Legacy Rep: 318
    Location: French Guyana

    Tcubed Boat Designer

    proa rigs and directiona stability

    So far in my design i have a deck stepped freely rotating wing mast stayed in all directions. The outer forestays are set to leeward so they would be the load carriers in the case of an accidental backwinding. The inner forestays are set on the leeward hull centerline, so less to leeward than the outer forestays. Then there is the shroud to the windward hull in an inverted Y arrangement, which allows the hulls free independant movement.

    The unstayed mast is attractive, especially for not being able to be caught aback, but i have yet to be convinced that the unstayed mast can be engineered to be lighter for the same strength.

    Your suggestion sounds similar to the arrangement used in 'Cheers', which had two unstayed masts, each with a regular bermudan sail. This arrangement is easy to shunt and does not move the CE around all that much.
    Yes the boom is always to leeward.
    I think that a stay hung rig is feasible on a proa but only with rig to windward configurations.

    The foil in my windward hull is not fixed. It would be adjusted dependant on the heading relative to the apparent wind, with maximum immersion at the close hauled angle and progressively raised at freer angles. The end foils are fully assymetric so the bow foil is not used at all. CLR is adjusted with the windward foil. The stern foil in the present configuration is not adjustable, it is either being used- all the way down- or fully up.
    My idea is that by adjusting the CLR the boat can be directionally stable at all headings. This follows experimentation i have done with non radio control model sailboats with adjustable foils which could be made to self steer at any desired relative wind angle. The only complication here is the extreme effect of apparent wind to the very high performance capacity of this type of boat. I'm pretty sure that i can get it to work with the working model though.

    I include three images of the latest incarnation of the design, with wishbone boom, reduced leeward hull freeboard and volume and reduced sponson. The shroud is just visible. Two of the images are from the water surface and all represent the boat at design wind with the windward hull just out of the water.
     

    Attached Files:


  15. MAINSTAY
    Joined: Feb 2005
    Posts: 60
    Likes: 2, Points: 0, Legacy Rep: 24
    Location: Lake Pontchartrain

    MAINSTAY Junior Member

    Tcubed

    I imagine myself single-handing your proa in the South Atlantic at sunset with a rising wind with the coast of Nambia 20 leagues ahead. I imagine I need to shunt or reef.

    What do I do to shunt? I imagine I would:
    1. Loose the jib sheet and go on deck to the present bow to lower and secure the jib. I imagine the halyard is cleated near the tack so I may control the rate it drops and simultaneously keep it on deck. I imagine with the change in CE hull will head up, but to what degree I can’t imagine.
    2. Go to the main sheet and reverse the sail. I imagine the proa will pick up speed on the new heading quickly.
    3. Go on deck to the new bow to unsecure and raise the jib. I imagine the halyard is cleated near the tack so I may control the hoist and keep the sail from dipping into the sea.
    4. Go below to sheet in the jib and adjust the main sheet and wing angle.
    In heavy seas or cold weather I would be reluctant to walk to both ends of the proa and back.

    What do I do to reef? I imagine I would:
    1. Loose the main sheet and go on deck.
    2. Loose the halyard, fasten the tack.
    3. Adjust the outhaul, hopefully without having to go to the end of the boom.
    4. Go below and readjust the main sheet and wing angle.
    That’s not as bad in heavy seas or cold weather, or any time.

    Your original idea was a gunter rig with a short mast. Let’s revisit the short mast idea.

    Class C has experimented with wing sails that pivoted in the center on a short mast and on each tack the wing set at an abeam angle. This may work even better for shunting. A wingmast-like yard pivoted as its center on a short mast like a see-saw or teeter-totter. For the proa, the pivot may be raised and lowered.

    To shunt , I imagine I would:
    1. Loose the main sheet.
    2. Keep tension in one brace(sheet to end of yard) while gathering other
    3. Secure both braces and wing angle.
    4. Adjust other main sheet. (Like a tacking jib, this shunting main has two sheets.)
    Since it does not require going on deck, I can imagine doing this in any weather and single handed.

    To reef, I imagine I would:
    1. Lower the pivot, and secure the loose brace.
    2. The main covers the cockpit or hatch, come up under the mainsail.
    3. Loose the right outhaul, and gather the sail to the center of the yard,
    4. Repeat for the left half, and stuff sail below.
    5. Bring up new sail, “raise” right half, and then left half of sail.
    6. Raise pivot with halyard (which unlike a sloop, is actually hauling the yard).
    7. Adjust main sheet and wing angle.
    8. Get a cup of hot java.
    Since it does not require going on deck, I can imagine doing this in any weather and single handed.

    Of course, this is only imagination. How are shunting and reefing going to be done on your proa?
    Larry

    PS Still working on sag.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.