Proa Shunting in Heavy Weather

Discussion in 'Multihulls' started by NewSalt, May 3, 2021.

  1. NewSalt
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    NewSalt Junior Member

    True, hopefully we have some input about techniques from the very few with experience.

    However, from what I've read, don't think that anybody has experience in the conditions that have me concerned about the leepods (breaking waves with a height of 30% of boat length), but maybe the original Jezero survived as much.

    The article also illustrates one of my design objectives (no knocking PP's or Russ's boats intended). I want a cockpit that is comfortable and the best place to spend time and pilot from regardless of the conditions rather than switching from cockpit to trampoline etc. To me, this is simpler and you can make a single station more ergonomic.

    In truth, if one of Russ's boats were for sale, that would be my first choice. Second would be designs for that type as Bieker teased us with (no knocking Rob's boats). I only don't count Madness because it is too small for a cruising, and too large for a sport/day boat (at least for my circumstances - but I did take a long hard look).

    But if I'm forced to design myself without constraints, I always start with a PP, it morphs into the ndrua variant of PP, and then ends up as what I described above. If there are reasons why it won't work out, I actually want to know - I'm not trying to defend a position.

    On a side note: @garydierking, what do you think if we started calling the Gibbons/windsurfer inspired proa rig you developed the Dierking rig, and reserved Gibbons for a teeter-totter lanteen? [Edited for clarity:] To me, a canting mast [vs. a fixed] is like the difference between a fractional and masthead, [both are still bermudan and behave the same with similar lines to pull, one just has an extra control] but your rig and Eull's [which has been used since with either mast] are controlled and behave differently. They have different capabilities and capacities, and I think that they need to evolve divergently.
    Last edited: May 14, 2021
  2. garydierking
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    garydierking Senior Member

    You can call it whatever you like. I only ever intended it as an off the beach day sailing rig because there is no elegant way to reef it. I have however sailed with it in 30 knots of wind and because increasing the halyard tension can make the sail almost perfectly flat, I was still able to control it, much like we discovered about windsurf sails many years ago.
  3. NewSalt
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    NewSalt Junior Member

    For sure. From descriptions it seems like a pretty well suited sail. I can imagine it being developed into a full soft wing and in the right conditions, nothing should be able to keep up to it.

    For cruising, not having the boom and being able to have the sail run free opens up options such as (possibly) powering completely off for a off-beam shunt, decreasing overturning power if caught aback, and it has a greater chance of developing a reasonable reefing scheme... I hope.
  4. rob denney
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    rob denney Senior Member

    The Air 40 is one of my favourite layouts. It was a design concept for a client who is still thinking about it. If he, or someone else wants one, we will draw the plans. The simpler version you describe is also viable, but as with all 'simple' designs, the devil is in the detail. ie deciding how to handle the deck space that is lost when the beam reduces, making sliding/folding beams easy to build (I am working on this with the telescoping masts for the 24m/80' cargo proa) and then all the little details.
    This seems to be against what you have previously written? Do you really want a boat that according to the CW article "a jibing mainsail in a breeze will blow the stick away," needs "ballast tuned to suit the smallest breeze variation", "is best sailed underpowered", "anchors or water jugs lugged back and forth to balance it so the overballasted ama doesn't slam into the waves, racking and shaking the whole boat", "the only (relatively) dry spot is perched on the stern".
    That article probably convinced a lot of people to go cruising in the South Pacific but provoked a lot of negative feedback about proas. It's author wrote the equivalent of another article unsuccessfully trying to explain that what he wrote was not what he meant. Ryan (new owner) has explained how he has fixed some of it, Russ can't decide whether he agrees with what was written or not.
    These boats are expensive, apparently "costing the same as equivalent space/performance/build material catamarans". Which is not surprising when you see how complicated the Bieker build is, but it is at odds with what a proa could be.

    Heavy weather shunts. On the Harryproa, you release the sheet(s), and haul them back in as the boat comes up on the wind, reducing the sheet loads. The rudders rotate automatically. On sloop rigged proas, you go out on the foredeck (narrow, usually without guard rails or pulpit), remove the headsail, carry it to the other end, rehank it, hoist it and sheet it on, as well as sheet in the main. On proas with cassette rudders you also have to raise one and lower the other. "Shunting is slow and complicated" according to the article. Little wonder Russ responded to your question by discussing power cats.

    Even if you didn't, liftable rudders that kick up in a collision and steer when raised are what you should be aiming at. Rudders and daggerboards in cases and/or through the hull are disasters looking for a place to happen.
    Reducing the performance of the Ex40 is easy. Reduce sail or add weight.

    Proas can be long or short, narrow or wide, light or heavy.
    Long performs better, is safer and more comfortable and costs very little more.
    Narrow limits how hard you can push, results in sudden capsizes (from experience with 2.4m/8' wide, 7.5m/25' long untelescoped Elementarry) and results in dramas if/when you are caught aback.
    Light is less materials cost and smaller rig.

    Rocker is a requirement on tacking boats, it's only drag on a proa. Think of it as a low aspect aerofoil on it's side. The lift is all downwards, by about the same amount as an assymetric hull prevents leeway. The faster it goes, the further it sinks. Symmetric rocker promotes hobbyhorsing.

    Not against floating logs or whales etc near the surface.
    A schooner rig and liftable rudders allows perfect balance, of any configuration.

    The trench is a good idea. It is what was on the original Harryproa and is also on the cargo proa. To prevent caught aback capsize in real conditions, it would need to be huge.

    Move it to the ww hull, save weight and build time, gain righting moment, space and headroom.
    Harry's living space can be as small as you like. Without losing the righting moment advantages of having the crew and gear in the windward hull.

    Even if it is too small to capsize you when caught aback, any rig that does not weathercock will, based on my experience, be scary and a lot of work when it happens in a breeze. I strongly advise unstayed masts in the lee hull with sheets directly to the windward hull to avert this. Caught aback, the boat drifts until you steer it back on course.
  5. NewSalt
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    NewSalt Junior Member

    Thanks for your replies Rob.
    I would like to not have this thread devolve into bashing other boats or promoting ones own (but arguing ideas is welcome). I regret to realize that I'm responsible for considerable drift.
    I have given considerable thought to what you've written, and will give further thought to some of it.

    There are situations where you are choosing to not compare like to like. In essence, to me Harryproas are ndrua in modern materials. It's important to separate the advantages of materials/construction (that could apply to any type) from the design type itself. As I explained above, I've conceptually moved from PP, through ndrua to Atlantic because of limitations of those prior formats for what I'm looking for - even though each are excellent in their own rights.

    I know it's probably because of poor description on my part, and that I did not provide illustrations, but there are some aspects that you seem to misunderstand (for example, the platform between the hulls IS to windward, even when on the lee of the main hull in an Atlantic).

    Rocker being irrelevant on a proa is simplistic. There are good reasons for it, such as limiting the movement of CLR in waves (which is maybe why surf-running walaps had rocker), and having a rudder shallower than the hull - which is a time-honored way to prevent/limit damage. A discussion about whether the drawbacks are worth it, depending on the design criteria would be helpful, an outright dismissal is not.

    Could you point me to an explanation of your downward lift comments? I can't see how these apply when you are discussing a hull that sits across an air/water fluid interface which allows for pressure equalization; plus, racing monos have angled flats up front to provide dynamic lift, which is based on a premise directly opposed to what you described.

    On the other hand, leading edge rudders have always been problematic, and even once sorted out will remain complex, which prejudices me against reliability. I believe that they will always bear more stresses and potential for failure than the conventional trailing/lightly balanced setups in monos or trim tab rudders.

    It is for the still developing, complex rudders as well as that for a given length the Harry is a bigger boat, that I feel that of the existing proas, Russ's fit my requirements, especially simplicity, better. But please don't mistake that for a slight against Harrys - I did consider the incomplete Harry in the NE and might have gone that way if it weren't designed around outdoor spaces more than inside (but that was its design brief, not a Harry problem).

    "Do you really want a boat that according to the CW article...?" - Yes. Very much so in fact :). I would not be unhappy with a Harry either, despite the different set of compromises.

    "Rudders and daggerboards in cases and/or through the hull are disasters looking for a place to happen."

    None of the trimtab rudders to date have been in the shadow of the hull as I've described. Instead of making impactable dagger boxes as previous designs have used, I think that using shear off rudder tips, which are pretty established in production boats, in combination with a drop in spare daggerboard is hard to beat when the rudder is protected to begin with.

    "Liftable rudders that kick up in a collision and steer when raised are what you should be aiming at."
    I've always wondered if anyone has attempted oversized kayak rudders that recess into a slot in the hull when stowed, but haven't seen any; to me, I think that would be my ideal, but it is undeveloped (unless one considers the decades of use on thousands of paddle boats).

    "Reducing the performance of the Ex40 is easy. Reduce sail or add weight."
    But why would you make the design compromises necessary for a peak performance design, and then handicap it? That makes no more sense than cruising on a miniTransat.
  6. rob denney
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    rob denney Senior Member

    I am not bashing anything, just pointing out what is in the article. My ideas and boats are based on 20+ years of experience, testing and analysis. There is not much point in posting if I don't 'promote' this experience.
    Part of the problem is language. An Ndrua is a traditionally rigged and sailed catamaran out of Polynesia. It has almost nothing in common with a Harryproa. Your references to 'main hulls' is confusing. It would be better to refer to windward and leeward hulls. Better still to provide a sketch of what you are talking about.
    My experience is that rocker increases the movement of clr. Rockered hulls pitch more, and sail bow down. Surf boats move significant weight (crew) aft. Sailing proas can't.
    As far as I can see, there are no benefits, only drawbacks. Look forward to discussing it if you can come up with benefits.
    Fixed under hull rudders and daggerboards are a problem with floating impacts. Knock off tips will only work in a grounding.

    Look at a Hobie 16 or early Rudy Choy hull. Lee side flattish, ww side quite cambered. They generate enough lift to windward to (just about) not need daggerboards. Turn the hull on it's side and the lift is pulling the boat down. Both are "across the air/water interface".
    The bow flats are for high speed, to facilitate planing. Harryproas also have them, both for this reason and to help sail over floating objects.

    I disagree. Leading edge rudders are just as prone to damage as aft ones. They are much less stressed as the clr is behind them. The requirement for both is the same. They should kick up in a collision.

    If you think Russ's boats are simpler than a Harryproa, you need to study both a bit more.

    Go for it, then, but there is a huge disconnect between what you say you want and the CW boats with the problems described.

    Go for it, then. Just hope you are going slowly when you hit a floating log. If not, you will also need a spare "drop in daggerboard case".

    I (and others) have done so, The drag and complexity at the end of the hull make them impractical. I look forward to seeing your solution, but suspect you will be trying to reinvent a square wheel.

    There are no "design compromises for peak performance" apart from the weight and sail area. The rest of it is about easy sailing, easy building and low cost.
  7. NewSalt
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    NewSalt Junior Member

    Hi Rob,

    I do use windward hull and leeward hull in the circumstances that the subject matters pertains to the relation to the wind. There are times where only main hull/vaka will suite the conversation (such as when it is discussing both types at the same time and what is relevant is decoupled from the spacial placement). But in this instance, I used main hull because it helps to stress that what I am describing doesn't have a leeward hull, it has a float to leeward.

    My comments regarding preference to a Russ style boat plans/used models may seem contradictory, however, as I tried explain immediately after, it is when looking at things as a whole; I'd rather a boat that has been shown to work than my own design, even if it is not my ideal. There is a lot I like about your boats, but there are a couple of dealbreakers. (I even move away from Russ style boats, towards yours when thinking from an ideals perspective, but then move on to Newick's and even beyond from the perspective of shifting weight distribution).

    Maybe I am being unfair in making generalized comments such as "stressed," and "simpler," so that you have the opportunity to correct errors I make; here my concerns:

    They are what would be traditionally called overbalanced. Spade rudders have up to around 15-20% leading edge because when you go too far above that, any over correction self reinforces. When they self-reinforce, they add considerable stress and risk losing control. Reversible rudders either have to be 50/50 or completely rotatable (which adds complexity)

    One of them is then placed ahead of the CLR where any tendency to lose control will be amplified. This can be reduced with a coupler, but again at the cost of simplicity.

    The contact point with the boat is considerably smaller, which results in greater localized stresses (and decreased tolerances for deflection if coupled). I acknowledge that by making them pivot up, the overall peak stress in a collision that you have to design around is reduced though.

    Specific to Harrproas - they are also asked to bear almost all of the lateral resistance. I have a hard time with the engineering that deliberately increases loads on one of the most complex/vulnerable parts of the boat for simplicity. An analogy would be a common rail diesel being superior in energy conversion and easier to use/maintain for the uninformed (you don't have to worry about underloading or running them up periodically), but being anything but simple - and you won't ever find one on my boat. If Harrys had a leeboard in the windward hull, I think that could accept most of the other design choices.

    A lot is asked of these rudders:. 1) To control yaw as their primary function 2) to limit leeway 3) to control the CLR, 4) to pivot up in collision and continue to function while pivoted 5) to remain in sync with their partner.

    1,2,3, and 5 are all discrete functions in trim tab rudders with a separate leeboard and are generally fault tolerant as a result - which is why they fit with my desired simple. They have their issues, most surrounding fault mode #4 - but everyone involved with them acknowledges that and are ready to jump on to a solution / alternative. I would too, and likely will use something other than initially proposed. But I'm not looking for a Swiss army knife rudder.

    I do respect your experience, as well as others who have chimed in here, but I often feel (wrongly or rightly) that you are generalizing from the context of the boats you know, when some of the compromise that I'm making (in theory at this point) are specifically to address those factors and that is what I'm looking for feedback on. Can I ask if your opinion of rocker would be the same when specifically used in the context of the windward/lifting hull exclusively, as I described? (And I get that if you depend on the float to counteract bow down attitude, then you need to provide more buoyancy and account for the torsional loads). Would you think that the kayak rudders would have more success on the hull that is not being depressed, therefore no likely to be submerged? Etc. (I know it is hard to convey tone - these are meant as legitimate questions, not rhetorical).

    Re: rocker= asymmetric hull on edge, there are vastly different mechanics involved in lateral vs. vertical effects when you have a denser, incompressible fluid on the bottom of the boundary, so your comparison is not really a proof, but I will do some reading on the matter.

    Can you explain how Harrys are distinct from ndrua to me? The reason I equate them is because they are both shunting boats with the longer hull to leeward. They both have their rigs, lateral resistance, and steerage in the lee hull. Compared with PP, they have more weight to windward, necessitating longer hulls and allowing for narrower beams. To me, the mechanics determine the type, not where you live on the boat etc. Proportionally how much weight goes to windward and the length of the hulls is a spectrum which can be illustrated by the 80' cargo Harry approaching Russ's boat to my guesstimate.

    This is already too long, and discussing CLR with rocker will be even longer as it is complex, so I'll have to generalize. Yes, rocker adds to hobby-horsing which may briefly change CLR dynamically, and sailing bow down, which statically changes CLR, and allows for weight trim of CLR. But by having the bulk of the CLR concentrated in the middle, variations from waves can have less of an effect dynamically. CLR and centre of buoyancy are correlated, but can be varied from one another and a lot of the effects are from the centre of buoyancy, not CLR.

    Ultimately, if the rudder issue can be solved simply, then a relatively narrow main/ww hull with a pivoting (fore/aft) centerboard [edit: I meant lee board] would gain the upper hand over a rockered assymetric. (everything is connected :/ )

    Last edited: May 23, 2021
  8. Dejay
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    Dejay Senior Newbie

    I'd be curious to learn more about the design of these rudders and potential downsides too.

    But I think it would be more productive to ask for empirical data on these questions (balance / oversteering, structural engineering). Personal or aesthetic preference are fine but can't really be discussed. And it may be less productive to argue from personal incredulity ("Swiss army knife").

    So the question should be "how well balanced and sensitive do they steer?". Or "do they take the loads in extreme cases?" or "how complicated or expensive are they to build?".

    The choice to make collision safety paramount by using kick up rudders and avoiding centerboards means you'd have to argue a better alternative that still satisfies that requirement. Your dealbreaker is his hullbreaker ;)

    To me they do seem simple. Or rather they seem "elegant" because you just have an empty hull with a mast and two rudders and the design and positioning becomes more flexible. Easy to replace or change their design without affecting other parts of the boat.
    Concentrating multiple functions into one assemblage instead of spreading them out doesn't mean added complexity. You have a board sliding in a case, the case on a steering axis and a pivot. I'd love to know how the steering axis is balanced.
    But they are still discreet parts just close by. Separating them over the whole length inside the hulls adds a different kind of complexity.
    And the complexity of a daggerboard case in the hull is avoided.
    If carbon fiber in this one assemblage can satisfy the increased structural requirements then that is a simple solution.
    I suspect concentrating the forces and structural engineering into two points (mast steps and rudders and crossbeam connects) is a reason why the harry proas are so light.

    These are just my uneducated opinions and thoughts but this is a really interesting thread for me as a novice. I wish there would be more independent reviews or studies on the harry proas! Including a test of sailing over 10 submerged logs haha.
  9. NewSalt
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    NewSalt Junior Member

    I'd like to see that too. And I agree that I should have asked for specifics about Rob's specs and feedback from clients/experiments - although I did presume that he would reply with some.

    To be fair, I don't think that even if you can sort out the engineering etc. to minimize the downsides that the generalities no longer exist, so it would not be inappropriate to discuss the inherent merits/drawbacks, and I invited Rob to point out flaws in my thinking of what held me back wrt to his style.

    For example, there are some excellent planes made with canards; they have some very positive attributes but they also have drawbacks ergo you don't see many and almost all of those are only for professional pilots. Someone should be able to talk about the merits/drawbacks without having to consult a Saab engineer about how theirs were optimized.

    Right from the start, I've acknowledged that there are different types of simple and that I had a bias towards one of them. I think it perfectly fair that I talk about my "aesthetic" when defending my own preference which I simply stated without trying to convince anybody else or get them to follow. During the ... ellipsis, I switched gears between discussing attributes and my current personal preference - one follows the other, but the argument was never that my opinion/preference affected objective reality.

    I think my statement where this wouldn't apply (where I did not delineate between personal preference and judgement regarding 'best') would be 'having trouble with the engineering that would deliberately increase the loads on a delicate structure.'

    So, let me redact that and ask: Rob, what are the differences in the load on this style of rudder compared to if the proa had a separate leeboard? If a potential client liked your boats, but wanted a leeboard separate from your style of rudders because they believed that this made a more robust system, would you be able to accommodate that and if not, what are the considerations?

    I think what is lost here is that I am not set on any particular type. I don't like the hazard of trim tabs, and if I went that route would focus a lot of attention on minimizing that risk, and then would have to continue to do so while sailing (it was the single greatest compromise from the design goals which included shallow water sailing).

    But, I feel like someone who has a gaff rig and wants to point a little higher and reduce chafe/weight aloft etc. and the only other options being pushed by the pros are rotating, carbon fiber wing-masts. Sure they have been engineered to near perfection, but they also have drawbacks and a lot more things are critical to be just right for them to do their jobs. Under those parameters, I'd stick with the gaff and work around the drawbacks. What I'd really like is a middle ground where you don't need to specialized materials to compensate for limitations inherent in the design - I'd still use the materials, but only to make it more robust, not to make it possible.
    Last edited: May 23, 2021
  10. rob denney
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    rob denney Senior Member

    Well said. Thanks. If you want more details on harryproas, join the chat group at | Topics and read Rick Willoughby's posts. Rick is probably the most knowledgable aero/hydro dynamicist on, has designed world record human powered boats and analysed, optimised and sailed on the Melbourne 18m/60' Harryproa which is currently for sale.
    Harryproa rudders are in oversize cases so they can be raked forwards (one way rudders which rotate 240 degrees around the leading edge) or aft (2 way rudders which only rotate 60, enough to steer). Rake happens automatically when you shunt, can be set for particular balance requirements and is easily altered.
    Couplers are ok on small boats in smooth water, not so good in waves. They also limit your steering options. Being able to use either or both rudders, together or in opposition, at any depth the boat will float in, is a handy thing.
    The contact point with the boat is via a large diameter drum supported by multiple bulkheads and shelves. Far more surface area than a daggerboard case and above the waterline so no fouling or leaks.
    It's a lousy analogy. Making the rudders and their attachments strong enough is the same as making the masts, beams, daggerboards, bunk bases and galley shelves strong enough: Analyse the loads, decide on a safety factor (much easier for rudders if they kick up), build and test them. Pultruded carbon strips and simple cnc cut nose and tail moulds in the Harryproa plans make rudder building an easy job. Making the mounts simple is a challenge, but far less of one than making daggercases and in hulls rudders collision proof.
    Shunting works best with large rudders and no fixed foils to stall, which may be why there are so few videos of this type shunting. If the fixed foils are on the windward hull, the difficulties they cause are magnified.
    I have tried several leeboards, they tend to misbehave at speed, but on the lee hull they are the best option if you want to start with small rudders. My advice would be not to make them too pretty or permanent as they will be ditched pretty quickly. Unless you have a death wish, they and the rudders should be mounted on the hull, not in it.
    Correct. Not easy, but well worth it once you make it happen.
    Without the first part of #4 you have an unsafe boat. If this is ok, and you don't mind all the hassle and limitations of the trim tabs in cases, go for it. For me, if it is not as safe as I can make it, I am not interested in designing, building or sailing it.
    The second part of #4 is not a requirement, but would be nice. #5 is nothing to do with the rudders, their structure or attachment.
    No one who has used them on a proa would call trim tab rudders "fault tolerant". Quite the opposite. Make a mistake in the raising/lowering/sheeting on routine and something will break.
    I would be remiss if I generalized from boats I have no experience of. ;-) Ask specific questions, I will answer them, with the reasons why. You might like to note that my generalisations are based on probably building and sailing more different proas and arrangements than everybody else on this forum combined. That includes pretty much every variation you have suggested.
    You can ask and I will answer, but I don't know what you are describing. Please reference or repeat it.
    I consider them legitimate, but was confused/cranky when you wanted a boat which answered none of your requirements.
    I think the kayak rudders will be more effort than they are worth, except on a kayak sized boat where performance is not critical. They will be less drag (slow and causing the nose to depress) if they kick up on top of the hull, but there are collision issues with this. Build a model, you will see what I mean.
    I didn't say it was a proof, just seems logical, and another reason not to have rocker. To save yourself some reading time, hold the back of a spoon under a tap and see whether it is sucked towards the denser, incompressible fluid or pushed away from it.
    If you can see similarities, no problem. But it is the same as saying modern Pacific proas are the same as traditional ones. They aren't, in almost every relevant way. Materials, righting moment, performance, hull shapes, steering, leeway resistance, rigs, comfort, seaworthiness, watertightness, platform stiffness, skill required, build time, value, weight, payload, etc.
    I design to requirements, not formulae. Bucket List is/was even more extreme. There is nothing wrong with the weight distribution in modern pacific proas if you can accept the lower righting moment or have an active crew. My problem with them is sitting in the shower to leeward and pumping water/moving ballast to stop them falling over.
    I suggest you build 2 models and compare this. If that doesn't convince you, build a one or 2 person version of what you are planning. You will find your "yes" points outweigh the rest of it. You will probably also find that it does not make a whole lot of difference to the sailing unless you optimise everything else. However, you will also find that it is harder to build, heavier and more expensive which, added to the performance and comfort loss is why i don't use it.
    True, but toss the leeboard and make the rudders bigger and it will be even upperer. Move the rig and the rudders to the leeward hull so you have some living space, safety when (not if) you are caught aback and don't get a sore neck trimming and you will be closer to the 'ultimate'.
    The rudder attachment on the cargo proa is simpler than on any multi I know of apart from the canoes steered with paddles. They also add #6 to your list: vertical lift. If they work. Stay tuned.

  11. NewSalt
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    NewSalt Junior Member

    "To save yourself some reading time, hold the back of a spoon under a tap and see whether it is sucked towards the denser, incompressible fluid or pushed away from it."

    Perfect, thanks. This does verify your position. As this is on my main/windward hull, this is a positive effect! (I'm kidding).

    I'll try to rephrase my follow up question wrt to rocker. My intent was to have a rockered walap profiled main/ww hull, but on an Atlantic styled proa. (A prime motivation was to reduce the exposure of dagger boards). The ama/lw hull would not be rockered. Do you feel that the objections you brought up still apply?

    Let's presume that I'm eventually satisfied with a rudder style different than dagger boards. This would modify my plans and allow me to do away with the rocker. This would allow me to move the rudders to anywhere. Although I'd prefer them to ww, I'd send them to lw because it would then allow the rig to move to leeward where I'd ideally like it (for capsize resistance, not craning your neck, all the same reasons HP have them) and that hull has to get a bit bigger to accomodate. We're not so far apart then.

    Can you elaborate on why you feel that a leeboard to ww is not as effective? All boards stall in a shunt, so I don't get it.

    Do you mean that HP users constantly angle both boards to prevent separation right up until there is no way on through a shunt? I thought one of your objections to trim tabs was needing to tend them during a shunt while HP were passive.

    Other than separating the leeway resistance from the steering, there are two situations where I feel that this arrangement is safer.

    1) Would be in an unexpected gust where you are on the verge of capsize. If the lateral resistance is on the ww hull, as the hull lifts, the proa is allowed to dissipate energy by skidding sideways.

    In the case of lateral resistance being in the lw hull, their lift being directly under the centre of heel will decrease the effective righting moment when the boat does start heeling (this effect is admittedly small compared to the previous)

    2) similarly, when hit by a breaking wave, a board on the ww side lifts preventing rotational energy from building up.

    I feel #2 to be of lesser importance because in ultimate conditions you can always pull up all boards and lie ahull or to an anchor/drogues, but it does allow you to continue sailing with an extra margin of safety in rougher conditions.

    LR to windward also reinforces maintaining the correct wind orientation at rest.

  12. rob denney
    Joined: Feb 2005
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    Location: Australia

    rob denney Senior Member

    Until it lets go!

    Yes. Hulls are pretty hopeless foils. If you want upwind performance, use deep narrow foils. Enough rocker to protect (for drying out, not against hitting floating objects) the rudders and boards will make a very inefficient hull.

    Pretty close. Let me know if you want a set of plans. ;-)

    Fixed boards are far more prone to stalling in a shunt. Stalling is draggy. If they are on the windward hull, the boat will have more tendency to round up when they do.

    Yeah. We call it steering. ;-)
    Not at all. They need to be steered so they don't stall. My objections to trim tab rudders are: they need to be lifted and lowered each shunt, so are twice as big as they could be and add time to the process. It they aren't lifted, something breaks. If they are in cases, they destroy the hull when they kick up at speed. They cannot be steered partly raised, making an unnecessarily deep draft boat. The cases need to be stronger/heavier/harder to build in case of impact.

    Correct, according to the theory, but I have never noticed any skidding, or even much movement sideways in cats with the lee board up and a hull flying. Or in Harrys when the heel angle is sufficient for the lee hull rudders to leave the water. Regardless, if you need an automatic device to prevent capsize, there are far more reliable ones than a board coming out of the water at high heel angles.

    Maybe, but I doubt it will make any difference. Pretty easy to test on 2 models tied together and pulled along the breaking wave front at the beach, if you think it is worth exploring. Even if it does, the wave needs to exceed the height of the beam to capsize the boat. In waves this big, you should be hove to or sailing carefully. This is one of the biggest advantages of a proa. In big waves/wind, you only need enough sail area up to give steering control, whereas on a cat or tri, you need enough to be able to tack.


    Not a problem I have encountered on Harryproas.

    Fixed boards anywhere, through hull foils and excess drag on the windward hull should be avoided if you want to maximise safety, performance and ease of sailing.
  13. popobowa
    Joined: Apr 2012
    Posts: 14
    Likes: 3, Points: 3, Legacy Rep: 10
    Location: Tanzania

    popobowa Junior Member

    Hi, I am new here, have read nearly this entire thread.. I have been studying Proas and esp. HPs a while ago...I do like the spacy ufo looks of the Denny HPs. Now i am once again looking for a world cruiser.. was delayed by C19 and other unforeseens...
    I have put aside my reservation about bad resale value.. cause if I go for an HP .. i will keep it!
    Now I have to figure out how it works for me.
    Ive gone thru this looong thread and come to the conclusion that I like 90% of HPs characteristics ... but dont quite get my head around 2 points.
    1. the rudder/leeboard issue.
    A_ it can be raised in a steerable box and swiveled(kickup) away from bridgedeck. So impactsafety only for one direction when sailing upwind with both down?
    Further if both can be used for steering they must have their rot. axis central which at speed may leed to flutter if not (as I read somewhere) they tilt back , rearwards of motion thru water? which would make them stable ...
    My Q: how do you control the steering input as the front blade will exert more steering force on hull than the rear? needing less input. When on the other tack (dir.) this balance must be inverted.
    Or is one forced to steer both rudders independent with different input?
    How would an autopilot cope?
    Maybe using hydraulic sys. with valveswitch for fwd/aft?

    2. where on a proa would you fit the 2 drives? I would go for serial hybrid in a proa with lots of PV on a large bimini.. maybe 15m²/ 3kW? assuming a 18m to 20m HP plus small 25_35kW rangextender (rotax/bombardier 35 to 55kg.) Powering 5 to 10kW pod motor on each rudder, one facing eithr way.. best with a symetrical bladeprofile, can also generate power and shld give app 8kt, or 1 to 2 azimuth drives motor in boat or pod. Regen option.

    I believe the rudder/leeward blade issue with differential steering , kickup /rake poss., lift up, is a complex issue. If the trimming of both blades is done manually by helmsman.. he needs to know what hes doing or wont get best performance windward.... cant let a novice shunt?
    There was the Q posed how to get best windward without too much drag, and if 2 or 1 ½ rudder immersed or 1 only...blade drag , as remarked, will be very much less (high aspect ratio) than hulldrag if latter shld create ww lift. I believe tht it will show in the wake's angle to boat heading?

    This Q to those of you with Proa experience:
    > how do you best deal with steering the boat when beating upwind? broadreaching and downwind only the rear blade is needed.. thats clear to me.
    And again .. where do you best mount the drive(s)? or 1 drive and thruster? or 2 pods at end of rudderblades?

    looking fwd to an echo
  14. NewSalt
    Joined: Apr 2021
    Posts: 24
    Likes: 2, Points: 3
    Location: Canada

    NewSalt Junior Member

    "Pretty easy to test on 2 models tied together and pulled along the breaking wave front at the beach, if you think it is worth exploring. Even if it does, the wave needs to exceed the height of the beam to capsize the boat. In waves this big, you should be hove to or sailing carefully."

    These studies have been done without the boards and suggested that wave induced capsized are possible at breaking wave heights of 30% of length, and likely at 50% (in catamarans). So, =to beam is a roughly equivalent, sure. Iirc the authors recommendations included a f/u looking at boards, although I doubt that I would be able to control the variables enough to have any statistical relevance if I perform them myself.

    Since, 1 in 100 waves will be 150% of significant wave height though, and those are the most likely ones to be breaking, you're at risk even with a significant wave height of closer to 13'-15' waves on an average 40' boat (and yes, those are still very large). Heaving to/lying ahull doesn't protect against wave capsize, continuous skillful active sailing may protect some, but only preventing rotational energy really does fully, which would involve a sea anchor/drogue or theoretically having minimal lateral resistance in the leeward hull in multis, and shallow draft/high topsides in monos for skidding purposes.

    "Pretty close. Let me know if you want a set of plans. ;-)"
    That would depend on if you're flexible with appendages. I don't see why the rudders shouldn't be able to be reduced and provision made for a leeboard. If the drum interfaces are made to the original plan, the customer would bear the cost of reverting to the plan, but it would be simple enough to to - and then you'd have empirical evidence for your preference. That is not to say n any way that there is any onus on you to do so, or that choosing to maintain your design as is implies anything with regards to your character.

    Alternatively, if you actually showed your system in an open format so that it could be assessed and if you discussed the compromises needed objectively, I might then be persuaded. (I don't mean the following as an insult so please don't infer that tone). Cult leaders have demonstrated repeatedly that if you cut people off from discussion with others, have a strongman personality at the top, and repeat the same thing over and over, then people will believe just about anything. I'm open to be persuaded, but it would only be by open discussion, which I personally don't count the harryproa forums as due to the structure following aforementioned patterns that are known to be subversable. The problem is not that there is not some very interesting discussion happening amongst intelligent people there, but rather that it is happening only within a narrative (the same problem is seen with polarization in politics).

    I raised some concerns about the generalized inherent obstacles to what diagrams of your rudders would face, and while I have been enlightened on a few points, most are unanswered (and some now reasked by another member's post). Sidestepping (such as me discussing the reduced reliability from complexity resulting in changing the narrative to crash resilience, which is admittedly the trim tabs worst feature and brought up ad nauseum), misdirecting (such as discussing the drum in the hull solely as the attachement point and ignoring the pivot), and applying 'just engineer it to withstand,' to your preference but denying that same argument for alternatives that you've decided not to use won't win me over either (but that is a preference, it may be enough for others).

    So, Rob, I am sincerely open to being informed, please show us your rudder system and explain how it has overcome the issues involved with being placed ahead of the CLR, how you maintain control over the CLR while still allowing for kicking up and the variable drag of steering, how the well documented issues of a centrepivot foil design are addressed, and why you wouldn't choose a lower stress option, even if you can engineer for something more criticsl etc. Etc.

  15. Dejay
    Joined: Mar 2018
    Posts: 721
    Likes: 138, Points: 43
    Location: Europe

    Dejay Senior Newbie

    What I've read on the harryproa website the simple solution is to just use the oversized tender with an electric outboarder and that goes only in one direction.
    I'd prefer a separate more optimized outboarder or trolling motor or pod drive, but there is no need for the proa to motor in both directions. Right?

    I'm also considering solar proa hybrid because the rig and sails are to one side while the other side could be a huge cabin with flat solar panel roof and no accommodation in the hulls. Maybe even same length hulls.
    But I believe you need a larger surface of at least 36m² but preferably 50m² to get reasonable power like 3kW with horizontal solar panels (see this thread for some calculations).
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