A proa and aero junk or split rig questions?

  • 07 Aug 2018 00:35
    Reply # 6418373 on 6368944
    Deleted user

    Tom Speers has got some great stuff on his site, the data for designing the leeboard I plan on using came from there, I go there and read till I get a headache and then go do something else.

    I've got to wondering if Russell Browns foil system doesn't do more than I thought, the skeg part generates lift to windward just like a daggerboard, and might be the only simple  way to get around surface piercing foils which don't work well at all at high speeds, certainly my leeboard idea will have to be modified at higher speeds.

    Check out the kickup rudder on this tacking proa

    Rob Denny is a very smart man, but he's a salesman and he doesn't spend keystrokes on anything else, once you realize that he's a wealth of great information, but you have to take everything he says about any design that's not his with a grain of salt.

    For instance, the beams on a weight to windward proa have to do much more than support the weight of the windward hull, they also have to keep all that mass stable so that the boat can be sailed, that takes a pretty resilient and rigid structure especially if the boat is being sailed at 10 knots in lumpy water.

    Also, because the windward hull is heavier it generates much higher forces on the cross beams when it hits waves then the lighter and more slender ama on the pacific proa.


    on the other hand, all the beams on a weight to leeward proa that's been backwinded have to do is not break for a few seconds until the skipper does something about it, not a lot of rigidity or structure required. If I do have to have standing rigging I'll probably come up with something that automatically douses the sail if backwinded, after all, the whole point of having a junk rig is safety :)

    Rob Denny is right about modern pacific proas, most are massively overbuilt, you've probably seen Free Radical wich is so overbuilt I think you could put the sail on either hull and sail atlantic, pacific, or weight to windward if you wanted to, awesome boat, notice the sail is always behind the mast? 


    2b

    Last modified: 07 Aug 2018 00:40 | Deleted user
  • 04 Aug 2018 03:18
    Reply # 6412834 on 6408414
    Anonymous wrote:

    The CLR moves forward because the bows of a proa are so fine the sail forces them down, not a bad thing, really helps windward performance on a proa, but I'd think that moving the rig forward would exacerbate the problem while having a heavy junk rig in the back half of the boat should help.

    It should.  There might be a second factor to the centre of lateral resistance moving forward.  Tom Speer plotted the (theoretical) pressure distribution on fore and aft symmetrical foils, and finds the centre of pressure well forward of the middle.  See Figure 20 at http://www.basiliscus.com/ProaSections/Paper/ProaSections.htm and discussion of Figures 25 - 27.

    Anonymous wrote:

    I plan on using a sliding ogive leeboard on the ama and an endless loop to move the CLR to where I need it and using an aero junk wich should keep the CE of the sail fairly far forward, between the two the loads on whatever I use for a rudder (Probably oars since I'll have a pair along anyway) should be quite low.


    I had forgotten about that.  It makes moving the centre of pressure forward redundant.  The other thing I forgot is the story Richard Woods (I think) told of his first catamaran.  It was well balanced in light and medium winds, but developed strong weather helm in strong winds.  Woods tried moving the centre of effort forward, but that didn't help.  He asked an experienced designer for advice, and was told to put the mast further aft, on the grounds that sail forward depressed the bow.  So that was the effect you described, though from sail pressure.  I have never known anyone mention that in connection with proas.  I don't know the reason.  In Russell Brown's rig, main and jib are at different angles to the vertical (looking from end to end, not from the side).  At rest, the mast is vertical, and the jib inclined to windward and lifting a little.  Perhaps that makes a difference.

    Anonymous wrote:

    PingPong really shows the difference in philosophy between the Pacific proa that evolved over millennia and the atlantic and proas that were designed around racing in the last few years, the newer proas are built around bigger sails which generate more forces and require more rigid structure and mass to carry them, which then requires even bigger sails to move the extra structure...

    while pacific proas are built around smaller sails and low forces and flexible rather than rigid structure to carry the forces so it gets lighter instead of heavier.

    Rob Denney argues that his Harryproas can have lighter beams than a modern incarnation of a Pacific proa, on the grounds that you have to design for the highest load case.  He says for the Harryproa, that is lifting c. 60% of the weight when the weather hull lifts.  For the Pacific, it's when caught aback, when the 70 - 80% of the (normally) lee hull lifts.  But if the Harryproa routinely uses its greater righting moment, the average load, and so the fatigue is greater, and the beams need more material to provide a reserve.  I don't know how that balances out.

    Denney's calculation is for a Pacific proa with a large volume weather hull, that can carry the whole weight.  There were traditional designs with voluminous weather hulls, but many had just logs, and possibly less stability caught aback then when the weather hull stayed upwind.  Your largest static load will depend on how large a volume you choose there.


























  • 01 Aug 2018 16:27
    Reply # 6408414 on 6368944
    Deleted user

    The CLR moves forward because the bows of a proa are so fine the sail forces them down, not a bad thing, really helps windward performance on a proa, but I'd think that moving the rig forward would exacerbate the problem while having a heavy junk rig in the back half of the boat should help.

    I plan on using a sliding ogive leeboard on the ama and an endless loop to move the CLR to where I need it and using an aero junk wich should keep the CE of the sail fairly far forward, between the two the loads on whatever I use for a rudder (Probably oars since I'll have a pair along anyway) should be quite low.

    I'm interested in the pacific proa because it's light and simple, fast is just a nice bonus, I'm not interested in dealing with headsails or standing rigging. at most I'll have a running backstay out to the ama, might try a staysail on that, as you pointed out if you're going to be messing with a staysail the most stable part of the boat is a good place to do so.

    Ping Pong is a pretty sweet boat, lol, 240 sq ft sail on a 24' boat that weighs under 800 pounds, that's some serious get up and go, I think that's about what Jezzaro caries isn't it?

    PingPong really shows the difference in philosophy between the Pacific proa that evolved over millennia and the atlantic and proas that were designed around racing in the last few years, the newer proas are built around bigger sails which generate more forces and require more rigid structure and mass to carry them, which then requires even bigger sails to move the extra structure...

    while pacific proas are built around smaller sails and low forces and flexible rather than rigid structure to carry the forces so it gets lighter instead of heavier.

    The old saw about a boat being a collection of compromises is even more applicable to proas I guess :) 


    Bill


  • 30 Jul 2018 22:51
    Reply # 6405731 on 6402632
    Anonymous wrote:

    I'd like to keep all my canvas on one mast just for simplicity, I'm more interested in sailing than sail handling, 

    The centre of lateral resistance of a proa's hulls is always a bit forward of the midline.  To steer the boat, one must either shift the centre of lateral resistance aft, as Newick did and Denney does, shift the centre of effort forward, as in the crab claw, the Gibbons, and Rael Dobkins planned rig, or do both, like Russell Brown does.  Brown's rig and my proposal achieve the shift by raising and lowering, instead of moving sails from one end to the other.  There is always some complication.  You get to choose which, and how much.  If you want a sail attached specifically to the mast, you commit to a centre of effort aft of that mast, and having a rudder with large enough control volume (distance from turning point multiplied by area).

    There is a proa doing just that, with a junk rig: http://wikiproa.pbworks.com/w/page/14592535/Terho%20Halme%27s%20%22Ping-Pong%22  Also see the five oldest videos at https://www.youtube.com/channel/UCldKLkcQBN--TVy_eNB6SzQ  You need to scroll right a bit.

    Last modified: 30 Jul 2018 23:27 | Anonymous member
  • 28 Jul 2018 20:21
    Reply # 6402632 on 6368944
    Deleted user

    Hi Robert, thanks for the links and info :)


    I'm not surprised, first thing I thought was daggerboards+everglades = broken daggerboards :)

    I think the difference is really that Russel only builds proas for himself while Denning and CLC have to make theirs idiot proof because sooner or later an idiots going to operate  one, I'm really surprised that CLC didn't make the kickup the default on their plans, but then the system looks pretty heavy and weight is the enemy on a proa, especially out on the ends of the boat. I'd never consider any foil that couldn't kick up, but then there's a 50 acre log boom about 100 yards from the boat launch I use.

    the stay sails look interesting, I'd like to keep all my canvas on one mast just for simplicity, I'm more interested in sailing than sail handling, but the staysails would make great sense as ghosters and extra canvas downwind, they'd also help balance the sail plan downwind and weigh next to nothing.

    Don't know what to think about the AYBS, I've been kicking around something similar, I I think that if I were working on a one sided sail I'd be more inclined to go with a wing myself so I could shape the leading edge.


    Bill

  • 27 Jul 2018 19:43
    Reply # 6401642 on 6384607
    Anonymous wrote:

    Russell has said he's never had any damage to rudders an any of his proas,

    John Harris ran Mbuli aground on the first day of the 2001 Everglades challenge, and broke those rudders (https://www.clcboats.com/life-of-boats-blog/okoumefest-watertribe-ultra-marathon-challenge-2013.html and https://www.clcboats.com/shop/boats/wooden-sailboat-kits/proa/mbuli-pacific-proa-beach-cruiser-plans.html). 

    There is a proposal for kick-up versions at http://www.tackingoutrigger.com/proa_rudder.html  I don't know whether anyone ever built that design.

    Rob Denney builds rudders that do kick up in both directions.

    As for the rig, I submit for consideration the same untested idea that I proposed to Rael Dobkins, described in the attached pdf; specifically, the rig in Figure 2, minus the Bermudean mainsail.  

    A rig related to my proposal of junk-style sheeted jibs is the AYRS/Bolger rig seen on a proa in AYRS Catalyst 3, on page 43: https://www.ayrs.org/catalyst/Catalyst_N03_Jan_2001.pdf  In theory, my version should solve the balance and shunting problems listed in AYRS Catalyst 8, from page 29: https://www.ayrs.org/catalyst/Catalyst_N08_Apr_2002.pdf  The sensitivity to angle of attack could be solved by bending the battens to form a round leading edge, and possibly making the sail a double-sided asymmetrical wing.

    Of, course, only in theory are theory and practice the same, while in practice, they can be different.

    Last modified: 27 Jul 2018 20:05 | Anonymous member
  • 22 Jul 2018 16:57
    Reply # 6392462 on 6368944
    Deleted user

    What a great idea David, that would work really well :)

    Funny part is, I've decided I want a rotating mast :) I've opted for some version or combination of the split or aero junks for the proa I'm building, but I think now that whichever way I go I'll run the mast through the battens and set it on a teardrop shaped mast that rotates with the sail. It just seems to make the best sense to me to take as much mast turbulence out of the equation as possible.

    I think a flat bottom might be a better way to go on a cruising proa, I can see several advantages and no cons, which doesn't mean there aren't any cons :) Although I'd want to round at least round a couple feet at the bows, be easy to do in strip plank.

    Just a thought, the stressed ply flared V russell uses should add a lot more strength and stiffness to the hull than a flat bottom, might have to add a bit of extra wood to make up for it, especially if using standing rigging.

    Jezero was a pretty little boat, looked really cramped for a 30 footer though, the owner of the marina where he rebuilt her after he lost the ama and mast described it as an inspired design but badly built and thought that stretched to 45' she would be a fantastic boat.

    Rob Denning seems pretty serious about keeping weight and stress centered in his proas as well.

    Bill


    PS, I've been working on that steering oar design we were talking about earlier :)

    used it for about 8 hours yesterday, worked pretty good.




    Last modified: 23 Jul 2018 02:18 | Deleted user
  • 22 Jul 2018 00:56
    Reply # 6391911 on 6368944

    I think David Webb's suggestion of a rotating mastcap is worth considering, if things need to rotate, though you'd have to work out where to attach the tails since they could not go to fixed spots on the deck.  Maybe another rotating collar below the boom? (BTW I am visiting David and Rosemary at Russell Island at the moment).  I'd favour keeping it all fixed for simplicity if you can.  The dinghy/model will soon show you what is possible.  Models are great development tools.

    Russell Brown and I discussed flat bottoms.  He did not like the idea.  He thought a chined V would be better, though favoured his flared V sections.  They are still built out of sheet ply, 9mm from memory, stressed to take the shape.  He did say that the boat he had the most affection for was his original 30ft Jzero, with its sharp, angular lines and raw simplicity.  However, he conceded that was more nostalgia for youth than a reflection on seaworthiness.

    His boats are lightly (though exquisitely) built and he was ruthless in keeping weight to a minimum and concentrated amidships.  A more sedate, cruising proa could relax those parameters somewhat.  It would still be a fast boat, given that the ama is lifting out and you are just driving one long, relatively skinny hull!  Makes my heart skip a beat just thinking about it.

  • 21 Jul 2018 23:27
    Reply # 6391872 on 6368944

    Hi,

    regarding rotating masts, would a rotating mast cap be a simpler solution. Something like a trailer stub axle could be epoxied into the top of the mast with a plywood disc bolted to the wheel studs. This would allow the halyards, topping lifts etc to rotate as needed while having a fixed mast.

    Just a thought.

    David.

  • 21 Jul 2018 20:57
    Reply # 6391787 on 6368944
    Deleted user

    Hi Graham :)


    Well, I got started on a model, then thought wait a minute, I have a full sized rig I can play with, so I went out and turned the mast on my dingy 90 degrees and mucked about with it for a bit, it still looks to me like the chimney would still be usable, unless it's something to do with the jibs that's the problem, I really don't know much about them, with my ADD looking after one sail is about my limit :)

    With a mast that rotates with the sails action I don't see how you can get away without attaching lazyjacks/halyard/YHP to the mast so everything moves together, I'd really like to avoid a rotating mast, just more things to go wrong, more cost, more weight, but if that's what it takes :) .

    I favor the v bottom wave piercing amma as well, not just quieter, but with less initial buoyancy it's much less likely to shake the wind off the sail every time it hits a wave. the pacific proa lets you use lighter crossbeams which means that stays run to the ama will put more wave action into the rig than on a Cat.

    you know, I'd never actually looked at the aero-rig before, mostly because they look like bermuda rigs and I'm interested in sailing, not sail handling :) The aero junk doesn't have the strut although I suppose the double battens are just about as much weight and work as the strut would be.

    Does the areo-rig look like a bermuda rig trying desperately to be a junk rig to anybody else?

    I'm not sure about the flat bottom, initial buoyancy is a serious problem on most proas because  the bows are so fine to keep wavemaking down, a flat bottom might help a bit there. Also, a flat bottom has much less WSA than a V bottom and long slender hulls have a lot of WSA to begin with. 

    from the construction pics I can find I'd guess that half or more of the overall length of Russell Browns proas are empty space to keep weight out of the bows, wouldn't be as much of a problem with the slightly broader hull we're thinking on and a flat bottom might help there as well. Might have enough extra wave making that you'd have to move the ama out further though. I'd dearly love to know Russells thoughts on the matter :)

    Fortunately the proa I'm working on is flat bottomed so I can get a better idea once I get it done, it'll also have 3 mast steps so I can play around with different sail plans.


    Fair winds on your cruise


    time to go sailing :)


    Bill

       " ...there is nothing - absolutely nothing - half so much worth doing as simply messing about in junk-rigged boats" 
                                                               - the Chinese Water Rat

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