Proa design

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

  1. Tcubed
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    Tcubed Boat Designer

    Many thanks for that valuable info Rob.

    0.8 prismatic! I would never have guessed so high.

    I am in Vieques, Puerto Rico. Right by the Virgin islands.

    Vaka .. i'm not being pretentious just lazy typing.

    I must say i am amazed you feel so strongly about unstayed masts on multis. Like i said i love the simplicity of unstayed masts on monos, whenever there is no jib of course. But the maximum righting moment of multis mean the unstayed mast needs to be impressively strong and the engineer in me starts screaming 'hang the ama off the mast' and akas and mast are now in almost pure compression don't have to be made so strong and hence can be lighter..Especially with that huge staying base, there's not even much compression on the mast. Obviously you've thought about this enough and still think that stays are not worth it..But why?
    Telescoping mast sounds very smart but i think i would worry about jams. Same reason i worry about in mast halyards or anything in the rig that is not as simple as possible. I would say the gunter is not draggy when reefed as it is a kind of telescoping mast, definitely better than the usual marconi where none of it comes down at all. The other thing i very much like about yards is that they make the sail come down, you never have to pull it down. Remember i've done the bulk of my sailing on gaffers, so sticks "swinging" around wildly is not something i'm worried about. In fact they don't swing around, the wind always blows it out to leeward unless there's a big swell and no wind, in which case i wait for it to be in the middle of a swing and drop it on the deck quickly. if you stay by the mast there's no way it can hit you.

    I include a sketch of the heavy weather sails; trysail and staysail (set on a different wire further inboard from the stay used for the jib. I think this is very efficient geometry for sailing to windward in strong wind with no, or not so much (when reefed) unused mast.

    You might well be right about the weight being too much in the hull, although it's really going to be not much more than an empty shell. There are no cockpits, just those hatches i can reach out of to steer and sailtrim. I am not a big fan of cockpits, personally, only time i've sailed in a cockpit has been on other people's boats and i must say i am of the opinion that in a lot of cases they waste a lot of space and can catch dangerous amounts of water. Although i must say multis allow for the whole cockpit concept to be completely retought... As the drawing is not yet finalized i haven't done the weight calcs. yet, but what i have in my mind for accomodation will probably weigh less than 150 Kg total. No, i'm not dreaming, when i say simple and basic it really is. Call me a masochist or whatever i don't mind

    I'll no doubt trim my sail area down a bit after comparing your numbers to mine. I LOVE lots and lots of sail (you can always reef) as there is nothing more frustrating than not moving for no other reason than lack of sail area. However 163 M^2 is probably a bit more than necessary even for a nutter like me.

    The reasoning behind the ama as long as the main hull is purely to get a very skinny shape that will have as small a drag torque effect as possible. If i make it shorter then it must be wider and deeper. I'm concerned about it affecting the steering as it slices through wavetops and in the air in between. Additionally, i've drawn in some curve to it so when it is in the water it tends to turn to leeward, ideally compensating perfectly for it's drag torque. Obviously this will be something that i'll experiment with on the model to fine tune and see how (and how well or not) it works. The neat thing with a scale model is i can see exactly how it will perform in different wind and wave conditions. You just have to scale the wind too by the correct factor, in this case 10:1 scale works out to be sqroot10 approx pi. So a five knot wind for the model simulates about 16 knt for the full size boat.

    Any idea where i might find Steve Callahan's article, which issue? This intrigues me especially. I often wonder what the steering might be like on the worst point of sail broad reach to run. At least i can bring the CLR right back.

    Your link gave me a blank page unfortunately.
    That wooden boat magazine article by russ i read when i was fourteen so it made a indelible impression. A few months later i had bought a 22' sailing scow with money i had saved, and moved out of my father's boat.

    What is imo?

    Attached Files:

  2. boat fan
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    boat fan Senior Member

    I M O ( in my opinion...)
  3. rob denney
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    rob denney Senior Member

    Nothing in Puerto Rico. Sorry.

    I originally used stayed masts for the same reasons you quote. But without a stay to leeward the hounds are not fixed and the mast gets some peculiar bends in it. Plus, there are dramas if (when) you are caught aback. Russ has found this to be the case as well. Main reason for unstayed though is simplicity. Nothing to break, nothing to check morning and evening, nothing to replace or maintain and the ability to raise, reef and lower sails on any point of sail in any wind strength. Add in automatic depowering in gusts and you can see why I think they are the answer.

    Richard Woods, who has sailed both, reckons an unstayed ballestron rig is 25% more efficient than a conventional rig for typical 'set it and forget it' sailors. I don't rate it this highly, but it is certainly better. An unstayed wing mast rig is better again.

    The 18m.60' mast in the sailing at windspeed video weighs 120 kgs, the rm is 18 ton metres. I suspect this will be lighter than your gunter rig plus wires. I am certain it will be lighter than your rig plus the beefing up required to support it. It will also have a much lower cog.

    You have to design your stayed mast for the worst possible case. This is caught aback under full sail in wind strong enough to fly the windward hull. With only the bow and stern stays to support it, the compression loads on the mast and the loads on the ends of the boat are huge. With an unstayed rig, the boom weathercocks, the boat slows and stops, you steer back onto course, let the boom go forwards and around the mast and you are sailing again.

    Jamming (and many other things!) worry me with the telescoping. Pretty sure I have got it figured out, won't know for sure until we build one. The gunter has the mast in front of it when reefed. Very draggy. Maybe not as dangerous as I thought, if the crew know what they are doing and you evidently do.

    It is not the weight of the fitout, but the need to make a larger than necessary lee hull strong enough for the loads it will see, plus the befing up around the cut outs that adds the weight. Does the 150 kgs for the accommodation include the pod and it's beefing up, you, maybe a crew, yours and their gear, navigation, galley and safety gear, floor, toilet, doors, internal finish, somewhere to sit, two hatches and ladders, etc etc? I can't see this being less than 350 kgs, which is 10% of your proposed gross weight, all in the wrong place.

    And for what? So that you can sit inside a cramped hull with a fire hose on you every time you go outside to enjoy some fast sailing or look around because the jib blocks your forwards vision, nowhere sheltered or comfortable to sit outside and the prospect of climbing up and down 6' of ladder to get in and out of the boat. There is masochism and there is stubborn. If you put as much effort into making accommodation to windward work as you have into accommodation to leeward, you will be surprised at how much sense it makes. Same applies to unstayed masts.

    A long skinny windward hull is draggy at slow speeds when minimum drag is most required. At high speed the rudders work well and the ww hull is being lifted so drag is reduced. It will be interesting to see how assymetry on one hull works, but forget the daggerboard and try it symmetric first and you may save yourself a lot of unnecessary effort. In my experience with scaling nothing is simple enough to warrant the word "just".

    Cruising World would not send Steve's article to me, but you can try them if you like. Otherwise, try the address from his posts on the Yahoo proa-file chat group. I strongly suggest you read all his changes of story, apologies, refutations and confessions about untruths and exagerations before you waste your time reading the article, although there are some good photos of the boat in it. If he won't send you a copy, please let me know.

    I have no trouble steering harrys, with one beam mounted rudder, no daggerboard or keel, on any point of sail. The exception is sailing at speeds less than 2-3 knots in congested areas when both rudders are required.

    Sorry about the link. Go to and click on either of the Elementarry shunting videos.


  4. Tcubed
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    Tcubed Boat Designer

    Rob ,

    I know i'm stubborn but not so much so that i can't put other people's informed input into consideration and i must say you present a pretty convincing argument for both points.

    Getting caught aback with the main pressing against the stay is not a pretty picture. And these stays can't be put very far to leeward or they get in the way of the boom close hauled, unless a runner is added which is yet more complication...
    So you've given me some good points to ponder.

    The weight of the accomodation may well be a bit more than i said, (i wasn't thinking about the pod) although i would be surprised if it gets anywhere near your number. As i said i have not yet done any precise final structural weight analysis. The sponson would be the single heaviest item since it needs to resist serious upload from the mainsheet and because it makes for a piece of very wide deck which will have support in the middle but still is much more engineering than merely decking a hull. The galley is all 6 mM ply, two burner stove, no fridge or any frills, just pot and pan and a few days food. The head is a couple of holes in the sponson with screw lid. There are no doors. There are steps up the side of the hull to reach the hatches. Very light and basic but none of this detracts from your points about accomodation in the ww hull being very sensible and certainly much drier!

    This does remind me though of another question. How do you feel about flare in the leeward hull?

    You see to me flare here makes a lot of sense to make this hull not get very depressed when at top performance. That was the thought that then made me consider the lifting hull with all that reserve of buoyancy the logical living (living not lifting) volume. I can't think of any performance or seaworthiness gain that can be had from reserve buoyancy in the ww hull above the bare minimum required...

    Another question. Would you consider the lee sponson worthwhile for someone who might be pushing the boat very hard on a consistent basis? I.e. might benefit from another little bump on the righting moment curve?

    Last question. I am pretty well convinced that allowing the hulls independance in pitching is the way to go as the stresses are vastly reduced and the hulls conform naturally to their particular piece of water much reducing resistance, despite the fact that most multis nowadays are as rigid as can be. Do you have any engineering preferences for the akas to do this and what do you think of this in general?

    My solution so far to make a beam which is strong but which will flex easily vertically and only vertically, is this. Picture an I beam with the top and bottom horizontal flanges quite wide, so the hulls cannot surge relative to one another (horizontally stiff). Now the vertical web which would run athwartships and which would give it vertical stiffness gets removed and replaced instead with a number of vertical webs running fore and aft. This ties the upper and lower 'boards' together so it is strong but allows for a considerable amount of vertical flexing. It also has the important bonus of being aerodynamic and 'spray-dynamic' due to all those holes. (low frontal area in other words)

    I'm still getting blank pages on all the video options apart from the first where i get a still image only, no play button or anything, but i'll try again.

  5. Bruce Woods
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    Bruce Woods Senior Member

    Facts/ experience?

    Tcubed, be warned, Denny has very little if any offshore proa experience in his designs. This has repeatedly been admitted to by Denny on other forums . His steering systems have been a disaster to date with owners having to rebuild numerous times. The man who I understand is responsible for the looks and drawings of the 50 footers Rob refers to, Mark Stevens, claimed no complete working plans are actually in existence. Try reading some of the unsubstantiated claims being uncovered at see Reply to Denney's critiques of Brown et al by Steven Callahan and
    Moderating the Proa List by Joseph Oster.
  6. rwatson
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    rwatson Senior Member

    What fun! I never realised that Proa designers got so hot under the collar.

    It appears that there will have to be a cross Pacific race between Robs design and Russels design.

    At this stage I want to go in the HarryProa - because I saw the Russel Video (in fine weather), and read this response to one of Robs criticisms,

    "Personally, I don't see what all the fuss is about getting wet. If
    you like the water, you shouldn't mind getting some of it on you now
    and then. Some of the best watches of my life have been while sitting
    in exposed cockpits in rain and/or spray. I've often left the comfy
    confines of a pilot house or dodger to roam the boat as it careens
    over large waves. That's why God invented foul weather gear. And
    maybe it should be up to the individual to decide whether to go the
    conceptual route of the Inuit who insulates and protects his/her body
    with efficient clothing, or the more wasteful Euor-American who
    prefers to burn up way more resources heating a large space so he/she
    can roam about in winter in his/her undies."

    "Had Jzerro really been designed as an
    offshore craft, perhaps it would have been prudent to build a dodger
    on her. But short of building it with unobtanium, any such shelter is
    going to involve cost, complication, and weight. And that does not
    suit Jzerro's particular purpose. "

    This puts us "cold weather" sailors out of a Russel Proa. 8 hours on deck in a cool day at the lower lattitudes is Hypothermia territory, as any number of cruising articles confirm. I consider all the "no shelter" malarcky to be the very worst kind of justification. Sure, I enjoy the romance of randrops in my hair, but not at the risk of dying of cold due to a week of inclement weather.

    Now as far as the harryproa's ocean going abilities V the Russel Proa - I have an open mind. Thats why I want to see a race organised.

    The proof of the pudding is in the eating. Any ocean going HarryProas out there ?
  7. RHough
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    RHough Retro Dude

    There needs to be a qualifying series. Open to all boats of a certain weight. The designer/skipper of each takes supermodels out daysailing in SF bay, until each boat has a volunteer crew.

    The boats that can't get a supermodel to crew, are eliminated. The others get to race to Hawaii. ;)
  8. rob denney
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    rob denney Senior Member


    It is a pleasure explaining things to people who are prepared to consider them.

    My rigs (ballestron and una) do not have any down force on the mainsheet, the boom is rigidly attached to bearings on the mast. This is easy enough with an unstayed rig as it is so solid in this area. The sheet leads directly to the windward hull so as well as no vertical loads, it can be caught aback without putting any loads on the rig.

    No more from me about your weight until I see the detailed analysis. But no matter how light it is, it is still in the wrong place, particularly if you intend to overcanvas the boat and/or pump ballast.

    Flare is drag. And it is drag when you least want it. I much prefer to have a longer hull and get the additional buoyancy this way. The 50’ proas have less sail than most 40’ cats. The additional length makes them much less prone to pitching and nose diving and increases top speed. It is very cheap (except for marina fees) and adds very little weight. The same cannot be said for flared bows with forestays attached which add considerable weight. Wide flat decks are also scary when you go through a wave or nosedive as they make it slower to surface.

    I agree about buoyancy in the ww hull, but while it does not do any good, it does not do much harm either. And any harm is far outweighed by having the low drag lee hull.

    Lee sponsons do give you a little more safety when sailing beyond the limit, but the limit is much higher when they are to windward. They are also increased drag in waves and a large hole in the side of a highly loaded hull. However, my main complaint about them is that they behave like the small hulls on old trimarans, and in extreme sea conditions, they may dig in while the boat is sliding down a wave and cause it to capsize. I would also worry that if the boat does capsize past the sponson, it will be impossible to right.

    If self righting is important to you, you can cant the buoyant mast to leeward about 10 degrees (depends on the weight and it’s distribution in the windward hull) and the boat will have positive righting moment at 90 degrees of heel. The canted mast also helps a lot in light air.

    Re independent pitch. I designed and built a 40’ cat in NZ a few years ago. It had a single beam in bearings so the hulls could pitch independently. Worked well and was very fast, but it was also very light (600 kgs/1,320 lbs) so we never really found out how much speed was because of what. It certainly had a very nice motion. I tried the same idea on U, an early 7.5m/25’ prototype proa. Did not make much speed difference, except if you inadvertently moved forward, the bow would pitch down, which was slow. I have since found that with the short ww hull, the motion is very pleasant, partly because upwind the bows hit the waves at the same time, so there is none of the corkscrewing you get in cats. Consequently, I would not bother on a harry. If you do go with free to pitch, let me know, there are plenty of pitfalls.

    Our beams are all box section with large radius corners as they are so easy to build, light and cheap. They are also relatively low drag. I would be very wary about relying on the section shape to give you pitch freedom as the engineering on twisted I beams is pretty tricky. Much safer to mount the ends in bearings or slots. Yet another advantage of an unstayed rig is that the boat does not have to be rigid. This is another weight and money saver.

    Sorry about the video. Have you downloaded the player?

    Weird about the proa designers, although it is not Russ and I so much as the fans of Russ who got upset because I used quotes from the articles in Wooden Boat and Cruising World to compare the two types. None of the fans have ever seen or sailed a harry, but consider they know all about them. In the early days some of the criticism was worthwhile and made me think about solutions. Now that harrys are out there doing what the prototypes suggested they would, the criticism is mostly personal and tedious. This is why it is great when someone like tcubed comes along with an open mind.

    No ocean going harrys yet. First ocean capable one launched, the owner died, second one has been up and down the East Australian coast and seen breaking waves big enough to fill the open box boom (3m/10' off the water) with no problems, 3rd one has been too busy taking blind people sailing in Holland, 4th one still has not got a rig.

    Not sure what a tradewind slide to Hawaii will prove (a race back, up the trades would be more interesting), but would love to do it.

    When Steve was giving me (not harry's, he showed very early in his tirade that he knew nothing about them) a hard time on the proa list, he made a $2,000 wager that Russ' boat would beat mine in an ocean race. Half a dozen people who knew about harry's immediately jumped on the offer and put up $5,000. Steve and a few other previously vocal people dropped off the list and have not been heard from since. No one offered to match the increased bet, and most of the sniping stopped. There are still a few people following me around the forums, but i ignore them except on the very rare occasions they have something new or interesting to say.

    The race idea sounds good to me. My 50' entry for the Single Handed Transpac (hulls, rudders and beams are built, "just" the rig to go) weighs less than a tonne/2,200 lbs. Russ' 38' boat weighs about twice as much. They have similar accommodation, mine is much cheaper.

    There is no reason to put any limits on who can compete. If I ever get organised enough to campaign my ocean racer, I will gladly race anyone.


  9. terhohalme
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    terhohalme BEng Boat Technology

    Nicely put.

  10. keith66
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    keith66 Senior Member

    Back in 1984 i and a group of 5 friends built a 36ft flying proa to compete in a sailing class of the Southend raft race. he was built from 3 ex USAF grp drop tanks cut & shut together. We put in 3" of rocker and a gracefull clipper bow each end. His hull was 2' in diameter and the Ama was made from a written of unicorn cat with both bow sections glued back to back. The whole contraption was 16' wide and then we stuck a 170sq ft standing lug on him from an admiralty whaler, roller furling jibs were fitted each end and served mainly to balance the ferocious weather helm.
    "Voodoo child" was built in three weeks with a total outlay of £200 between us all. I would never have taken him far out to sea but in the estuary he was fine and sailed like the clappers. To this day i have never had so much fun with my clothes on!
  11. Tcubed
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    Tcubed Boat Designer

    Flare is resistance,.. How so?
    It's not really appropriate to say making it longer is a better way to add buoyancy, as it then ceases to be comparable. Longer is automatically faster unless you keep the same sail area and you look at ghosting conditions. Longer also means you can increase beam whilst keeping the decided Max Longitudinal Righting Moment/Max Transversal Righting Moment the same, which implies more sail to take advantage of the extra righting moment (even if weight is kept the same) i.e. faster and faster . It's not for nothing these racing tris, when not limited by a length rule become longer and longer.
    In practice length is a constraint that one decides upon when creating a design (more so even than when designing a mono) which reflects the available budget, the variety of harbours one might visit (maneuverability) and space available for the build. In my personal case i've decided 16 M is about as long as i want to go, and it still gives plenty of speed potential and a reasonable amount of internal volume to move around in.

    You're certainly right about wider flat end decks being slow to rise if submerged, compared to the semi-wave piercing bow. On the other hand, a tall bow with flare has enough volume, or reserve buoyancy to be very difficult to submerge in the first place. Question of preference i think.. Apart from running in high young waves where the faces typically exhibit a maximum of concavity. In this situation, i think i would prefer a shape with enough sheer to keep the ens out of the water, no matter what, over the hogged sheerline style where both ends would be submerging at once which if it does not cause a pitchpole, at least creates a very strong yaw moment, which could easily lead to a broach. Remember that water particles are compressing on the wave face, which in wave lengths around twice to four times the boatlength means the bow is experiencing considerably higher water speed than the stern when on the wave face.

    Which brings me back to flare, given a certain leeward hull length.
    If there is no flare and one designs for underwater semicircular sections, then at low rig pressure conditions the lee hull is on a section of circle which is no longer minimal wetted surface shape, when it matters most. If on the other hand, one designs for semicircle at low pressure conditions then at max heeling moment the underwater sections of the lee hull look like a 'U', which is not so optimal either......Also the lee hull will experience greater variations in immersion if it does not have flare.
    That is the thinking that made me opt for a parabolic cross section which immerses relatively little as the rig loads up to maximum allowable force and which is a fairly close to achieving minimum underwater girth, whilst retaining a pretty similar underwater geometry irrespectve of where in the power range one is operating. It is a compromise as well, i acknowledge, but in my opinion so far, somewhat preferable to not having flare.

    On lee sponsons. It is certainly true they could trip a capsize in a big breaking cross sea. But i'm inclined to think that if it is high enough and the lee hull has generous reserve buoyancy it would be quite difficult to get the wing deck underwater such that it could trip. I think by the time waves were breaking that monstrously, one would be avoiding beam on seas already and no doubt be under not much more than bare poles. All of which makes me think it is probably a worthwhile safety feature, although it probably comes down to personal judgment as to the pros and cons in one's own particular requirements.

    On volume in the windward hull. I applaud the innovative design solutions you've created for the proa. They will certainly make the proa type more appealing to a large majority of sailors who have been hesitant of the proa for the obvious reasons..However i do think that the harryproa, as is so far, does not represent an oceangoing performance optimization solution. I think you underplay the role of windage. This is not a problem for coastal cruising, but in my particular case where my main objective is fast transoceanic passage making, it might well be.
    I have always been very concerned with windage in all my designs because apart from it lowering pointing angles to windward (by lowering the total L/D ratio of the entire above water structure) it is the single most important factor determining just how much wind is needed to prevent being able to make good to windward, a very important safety issue, i feel.

    So, it seems that we will have to organize an ocean crossing race to determine once and for all the merits of the different proa options. I take it yours have not done ocean crossings yet ( due to lack of storm stories)..? It would certainly be a lot of fun. Proas barred from multihull races? No problem, we'll just organize our own! Hey Russ! you listening to any of this? Or anybody?

    As for the funny idea of the ladies for qualifiers, i'm all for it. So far , in all the races i've competed in i only allow female crew. They're easier to train, and they're much easier on the eyeballs. Besides i might be prequalified: My wife is quite a looker, i'de post a picture of her but i don't want to upset admin.


    P.S. The race should be the sum of the times of a round trip so there is unavoidable upwind work to insure testing is for all points of sail.
  12. RHough
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    RHough Retro Dude

    Oh my ... dear chap, gentlemen don't sail to weather. That is why yachts have engines, for upwind work and to run the ice maker. ;)
  13. Tcubed
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    Tcubed Boat Designer

    Are you taking the piss, mate?
  14. RHough
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    RHough Retro Dude

    Just having fun. :)

    Of course windward ability in heavy air is a primary seaworthiness consideration. Once you have a vessel that is capable of boat speed > wind speed you are always 'sailing upwind'.

    Joined: Feb 2005
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    MAINSTAY Junior Member

    Proa Rig


    Your Proa is a great project. Congratulations. With such a large sail area and single handling. the choics of rigging is so extrordonarily important.

    Might I suggest a rig that can be handled entirely from the hatchway?

    The primary feature is that the mainsail is not set on the mast, but on a vertical stay. The mast for a proa is stepped near the third point of the forward cross arm and canted so the head is over the centerline of the hull and raked so the head is at the head of the mainsail. It is supported by one shroud to the end of the crossarm, the stay for the mainsail, and the forestay. For a boat of this size another stay between the main and fore stays is suggested to divide the sail area into smaller pieces, and to help keep balance when reefed. All sails, including the main, are on stays and can be roller-reefed with ease. The mast is not in bending or torsion since no sail is set on it. The mast is primarily in compression.

    With the mast out of the cabin you would have more room below. But adaption at this late date depends on the stage of construction.
    Larry Modes

    The attached .xls sketchs may hwlp visualization.

    Attached Files:

    • Proa.xls
      File size:
      30.5 KB
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