Pushmi-Pullyu; a Proa for All Oceans?
Even if it is not a complete mess up of “ooooops, I released the old rigging before I got the new one taught”, if you simply have both rigging slack for some period of time, having the mast and the sails bouncing back and forth, right and left for a few seconds will most likely be fatal due to the humongous shock loads that would generate.
And all this from a fellow who promotes, as an everyday thing, a boat-type which requires not only slack rigging, but moving the entire mast 30 feet during every shunt! Marco’s double shrouds sound like a doddle, compared to a perfectly ordinary Ndrua shunt. 😉
Your other ideas in this post are brilliant, as usual. 😊
Dave
Dave,
As much as I think that debate about proas is pointless, I must comment about a few of your opinions presented in your original post.
I haven’t sailed on all the proa types, but I think your feeling that “Atlantic proas are powerful beasts with triple the righting moment” and that Pacific proas only sail well in light winds and “suck in heavy weather” is not at all true.
Have you sailed on Atlantic proas that were being driven hard? Neither have I, but my little POS proa, Jzero sailed the 350 mile upwind leg of the Tradewinds race about 3/4 of a day faster then Azulao, Newick’s best Atlantic proa despite a 14 foot length difference and about a 50 to 1 cost difference.
Why? All I’m gonna say is that speed doesn’t have everything to do with righting moment and I don’t think that a fast boat in light airs is necessarily a slow boat in wind.
Speaking of righting moment (and having to reef early and often, as you say) using water ballast makes a Pacific proa extremely powerful and can be used as a way to avoid having to reef as often as with other types. One can pump in ballast when the wind rises, reef when it rises again, and pump the ballast in or out when the wind strength changes again.
Because a Pacific proa can be light, it doesn’t need a huge rig to sail really well in light conditions and because the outrigger hull is so far to windward, it doesn’t take much water ballast to make the boat feel really stable when the wind is up.
When I picture a “weight to windward’ proa being driven hard, what I see is lots of spray, motion, and fuss when going upwind and lots of nose diving potential when off the wind. There’s just no way that one of these boats can take advantage of it’s righting moment while reaching or running without some way to keep the longitudinal trim level.
I won’t defend my views more than I already have, but I had to add my two cents.
Russell
When I picture a “weight to windward’ proa being driven hard, what I see is lots of spray, motion, and fuss when going upwind and lots of nose diving potential when off the wind. There’s just no way that one of these boats can take advantage of it’s righting moment while reaching or running without some way to keep the longitudinal trim level.
That ran through my head last night too… You look at the evolution of the high performance tris, and one of the major issues they’ve had to deal with was increasing pitch stability so they could continue to drive the boats really hard even when sailing off the wind in heavy seas. The aft rig position with very long bow sections over which no sail is being carried, the flat bottom hull sections aft, and the banana boards, are all there to increase pitch stability / keep the bow up so the whole thing doesn’t nosedive. On a proa the pitch stability will be quite a lot lower because of the double ended nature of the boat, and you can’t use the flat-bottomed stern, or aft rig position trick either, which leaves you with banana boards or nothing at all, to help keep the bow of the ama from burying itself. None of that bodes well for the boat if we’re contemplating putting a lofty rig on it…
my little POS proa, Jzero sailed the 350 mile upwind leg of the Tradewinds race about 3/4 of a day faster then Azulao, Newick’s best Atlantic proa despite a 14 foot length difference and about a 50 to 1 cost difference.
Why? All I’m gonna say is that speed doesn’t have everything to do with righting moment and I don’t think that a fast boat in light airs is necessarily a slow boat in wind.
😊
I really wonder what it boils down to though… It’s got to be a story that numbers can tell as well. The schooner rig is going to be at a disadvantage working to windward because of the interference between the two rigs, and the lower overall L/D-ratio of having two rigs in the first place. I’d venture to guess here that Azulao wasn’t flying it’s vaka the entire time, if ever (please correct me if I’m wrong, that’d be a valuable insight!), while on Jzero it probably was more of a non-issue to keep the ama skimming. With both hulls firmly in the water and with the greater overall displacement Azualao would have had much higher overall drag, and that together with the lower L/D-ratio of the rig (= less driving force per unit sail area), could well be what did it in. That’s all idle speculation though, and not numbers. Perhaps a VPP can help us here, but probably the only conclusive way to resolve the issue would be to build a pair of boats (or one boat with an extra rig and ama) and log their performance data in a variety of conditions.
Sven Stevens has reported that Pacific Bee just walks away from the Atlantic proas as well on anything but a reach as well.—I wonder if all of them keep their vakas in the water, and if any of them have a sloop rig…
Thanks for sharing Russell!
Cheers,
Marco
Even if it is not a complete mess up of “ooooops, I released the old rigging before I got the new one taught”, if you simply have both rigging slack for some period of time, having the mast and the sails bouncing back and forth, right and left for a few seconds will most likely be fatal due to the humongous shock loads that would generate.
And all this from a fellow who promotes, as an everyday thing, a boat-type which requires not only slack rigging, but moving the entire mast 30 feet during every shunt! Marco’s double shrouds sound like a doddle, compared to a perfectly ordinary Ndrua shunt. 😉
Your other ideas in this post are brilliant, as usual. 😊
Dave
Dave,
You got me confused… Or is it you who is confused?
Who are you referring to, when talking about a fellow who promotes moving the entire mast 30 ft during a shunt?
Me, who you quoted?
Or Marco, who I was answering?
I told you, you got me confused…
Laurent
Well he isn’t referring to me…
It’s a good point you make Laurent where human error is concerned. There’s probably about as much or more that can go wrong with tbis setup as when you are shunting a large crab claw rig without rehoisting (I think that’s Dave’s point). If the rig is semi-freestanding like you said, meaning it can at least take the loads of the sails flogging around when in the unstayed state, then that would make things a bit easier when tacking and remove that risk of having the rig come down due to human error during the tacking procedure. 😛
Every boat has its Achilles’ heel and for Atlantic proas, that’s light wind performance. They are no better at ghosting than cats or tris, both notorious for poor performance in lighter wind
To some extent I have to disagree with this, although it will depend on the design details, as with any boat. My experience with the Mi6 was quite the opposite.
The Mi6 had a fairly generous but not excessively large sail plan. The vaka had enough rocker so that the ends were not deeply immersed, keeping the wetted surface low. The ama, to leeward, could be lightly loaded because the vaka had sponsons which allowed the crew weight to be placed well to windward, so the the ama barely touched the water in ghosting conditions. The result of these features was that the boat was very quick in light winds and a real pleasure to sail in those conditions.
While the Mi6 was designed to be able to sail in both Atlantic and Pacific modes, we found no benefit to using Pacific mode and the boat was generally more comfortable to sail in Atlantic mode due to the rig location on the leeward side of the vaka (again a design detail, which could be changed if you wished to sail more in pacific mode).
Incidentally, if I was to design an Atlantic proa now, particularly on a larger boat than the Mi6, I would position the rig on a sponson on the windward side of the vaka. This is for a few reasons:
- The weight of the rig to windward would help with the ama unweighting in light conditions as mentioned above.
- Placing the rig further to windward helps with helm balance.
- In the event of a backward capsize (the real Achilles heel of Atlantic proas), a buoyant mast mounted in the sponson will prevent a complete capsize and most likely the boat will recover easily.
Here’s the link to the old Mi6 web page: http://www.users.on.net/~malcolmandjane/mi6.html
Mal.
Given that Atlantic proas are scary and dangerous things with big performance potential, maybe it would make sense to do the opposite of Dave’s suggestion. In good conditions sail in Atlantic mode and take advantage of the raw power. Then, when the going gets tough, turn the boat around and utilise the more benign characteristics of the Pacific proa!
I really wonder what it boils down to though… It’s got to be a story that numbers can tell as well.
Herein lies a problem. Science is populated by formulas derived to explain data. The raw data comes first and someone develops formulas to explain the data. After peer review and sufficient sample data, formulas make the jump from explainer of data to predictor of future performance.
Proas were developed by people uninterested in theoretical physics, mathematical proofs and data collection. They were interested in fast, trouble avoiding transport from point A to point B. Proa development was iterative and evolutionary. Things that worked were kept and things that didn’t were dropped. Materials (island vegetation) available very much defined this evolution.
If I’ve read things right, Russell Brown followed a modern version of the same kind of proa evolution as the Pacific islanders. He had some much better materials (even on his $1000 Jzero) and was able to design a boat that worked really, really well. I’m fascinated that he consciously chose to NOT follow Newick’s example of the Atlantic proa - which would have been expected.
Obviously, Mr. Brown chose “simplicate and add lightness” over brute strength and power. He consciously chose a design where an expert operator could deal with the higher potential of capsize (Pacific versus Atlantic). His well known reluctance to encourage others to follow the same choices is very telling. He’s been pretty clear about why he doesn’t engage in these discussions - and if I had been through some of the idiotic forum/mailing list crapola he has I probably wouldn’t either.
There really aren’t well proven formulas out there that explain the “whys” of proas definitively, I think you have to analyse the evidence we do have really carefully. Weight. Drag. Efficiency. Power. Operator skill. Risk. You can’t expect the numbers to accurately tell the tale, because there isn’t enough of a data set from which to create the formulas.
Formulas do not account for all the important factors in design. Modern military stealth aircraft are designed to be unstable and unflyable - but superior fly-by-wire computer control systems mitigate these conditions and make the unflyable ... flyable.
I’m pretty convinced that Mr. Brown’s designs expect and require “superior fly-by-wire computing” in the operator. Reefing early, not getting caught aback, actively adjusting ballast, keeping weight out of the boat are operator responsibilities.
I’ve observed a lot of product design over the years and there are few products that are the products of real thinking and objective decisions. There are a lot of products that are the result of duct taping popular features together - the though being that if you check off the most features people want, you win. Features don’t make products - delivered real benefits do. Trying to add the features of both Atlantic and Pacific proas together doesn’t just get the benefits of both - it also gets the drawbacks of each.
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Bill in Ottawa
It’s a good point you make Laurent where human error is concerned. There’s probably about as much or more that can go wrong with tbis setup as when you are shunting a large crab claw rig without rehoisting (I think that’s Dave’s point).
That was precisely my point Marco. Laurent, my apologies for using the perjorative “you” in my earlier post. It just struck me as funny that the objection to Marco’s idea of swapping out stays occurs in a forum dedicated to a class of boats (I believe I said “boat-type”) which routinely (and successfully!) virtually dismember the entire rig every time it shunts.
I meant no criticism, of anyone, and humbly apologize if I offended. It just struck me as funny. 😊
Dave
Wise words Bill! Speculation like what we are doing here (some might say we are constructing a hypothesis), is generally the precursor of the experimentation necessary to gather data though. At the age of 14 Russell just hit it bang on and built a boat with an absolutely ingenious layout. He didn’t have a proafile forum to discuss his ideas with other like-minded people that shared his interest, he just built the thing and really gave a few really good boats at the time a run for their money. I’m not that ingenious, but it’s not going to stop me from speculating about the possibilities, to see if perhaps there’s a configuration hiding amongst all those endless possibilities, which is a bit better than the pacific proa as exemplified by Jzerro.—Maybe there is a still better alternative, and maybe there isn’t. 😉
Ultimately any configuration which we speculate may be better than a pacific proa will have to be put through its paces on the water, against a boat which represents the best that pacific proas have to offer at the same pricepoint. Too bad there isn’t a team of proa enthusiasts around in the area; if there were I’d suggest just getting together at the north sea for a couple of weeks with some proper data-logging equipment (would be a cool electronics project! 😊 ), and build a couple of quick to build 5-7m proas and try out every hull/scantling/rig/board configuration we can think of. Speculate, build ‘em, race ‘em, test ‘em until they break, and afterwards sit down and extract what could become a first rough VPP from the mountains of collected data, and some guidelines for scantlings. In a group that would probably be an enormously fun exercise, but alone that’s just way too much work to take on, because we’re probably talking about at least half a dozen hulls here… Either that, or we draft some rules for an open proa class and organize some long races. 😉
Marco
Well, I didn’t get as far as I’d hoped this morning, so there’ll be more later…
James:
… the windward ama and akas aren’t entirely wasted if they are providing extra RM if that is what you are trying to maximize (more sail area, more speed, more strength needed, more weight and around we go!) . So is it a case of diminishing returns or is it part of the reason that trimarans still dominate ocean racing? Or have I missed the point entirely?
It may be useful to differentiate righting moment (RM) from moment arm A lighter hull, flying at the end of a longer arm, may have more—or may have less righting moment than a heavier hull on a shorter arm. I understand that’s what you’re saying, James, it just helps me think sometimes if I take them apart.
MItchellhay:
… A combo-proa would have to be built as sturdily (and as heavily) as an atlantic proa, so in light air (even in Pacific mode) it seems there would be a weight and speed penalty. Lifting a hull would help, but displacement is displacement.
You’re comparing Pushmi-Pullyu to a one-way proa, I believe, and under favorable conditions. I’m trying to compare him to a trimaran or a catamaran, under extremely variable conditions. Any specific boat is no more than the sum of its compromises or put another way, all will have strengths—and weaknesses. My proposal with Pushmi-Pullyu is that gains made when in one mode may (and may not!) overcome losses when in the other. Having the ability to switch from one to the other gives the pilot more “arrows in his quiver” to more closely match a boat’s abilities to the conditions he finds himself in.
Marco:
… you’re going to have to be able to fly a hull regardless of whether you’re in pacific or atlantic mode, and that makes things a bit more complicated; that smells like two sets of rudders in each hull to me, which would be pretty crazy…
Yes, optimizing this concept requires flying a hull, if not every minute, then at minimum more often than your competition. I think the auto-flight gadget’s a good idea, because it’s simple and retro-fittable. Yay for me. Much more deeply, I believe that automatically flying the windward hull or hulls of any multihull capable of flight will increase its performance. The more I think about it, I ask why do modern racing multihulls spend millions on sail shape, carbon spars, and unobtainium rigging, yet do not spend a few thousand developing auto-flight mechanisms—mechanical, electrical or hydraulic? Of course I ask the same question about using “multi” hulls at all, when developing kites for offshore “mono-marans” would render them obsolete at a stroke, but I digress…
As to the 4 rudderboards: (let’s put a stake in the sand here and suggest that this boat has 4 Jzerro-style rudderboards, one in each corner of the boat). Virtually all catamarans have 4 foiled appendages, Some even more—some trimarans have 6. Racers routinely withdraw half or more of these while both shorthanded and racing, and think nothing of it. The two “extra” rudderboards need only add a few tens of pounds of mass to the boat, and no additional wetted surface at all.
Marco:
As for the weight distribution, I’m inclined to say that making the ama as light as structurally possible would be the best course of action. When in atlantic mode the weight only hurts performance (higher total displacement), and in pacific mode it helps you get the ama up in the air more easily, increasing the envelope for good light-wind performance. There’s no benefit in deliberately making anything heavier than it absolutely needs to be (ama included)
There are compelling argument both ways, Marco. Compare, for instance a 90/10 Pushmi-Pullyu and a 60/40 one. The 90/10 will have the broadest performance window, flying her hull in lightest winds (pac mode) and having the absolute highest righting moment in heaviest (atl mode). Problem is—is middle performance. Such a boat may be unable to maintain pac mode (overpowered) in winds too light to fly her vaka in Atlantic mode. The 60/40, OTOH, will have all sorts of middle performance—there will likely be a range of wind/water conditions where she could efficiently sail in either mode. Problem now—as you point out—is that extreme condition abilities (light wind and heavy) are compromised—in addition to structural considerations as you approach a simple asym catamaran design. Best is likely somewhere in the middle and I chose to talk about 75/25 because that’s a fairly popular compromise—and as Russ points out, adding ballast to the equation is a method of covering that “middle ground” by essentially changing the weight distribution ratio on the fly. Smart guy. Logic seems to dictate that one would want an ama-ballast tank filled when the ama is to leeward under some conditions—like where we can no longer carry full sail in Pac mode, but can’t—quite—fly the vaka in Atl mode. Ballast to leeward, if it brings the vaka out of the water, would be beneficial.
Dave
I was thinking about that weight distribution issue a lot too. You really have to keep a hull in the air for optimum performance. For a smaller and lighter boat that’s bound to be easier than for a large craft due to the large contribution of crew weight to the total, which allows the use of sponsons for your crew to great effect, and that without increasing overall displacement. I think on a craft like what I have in mind for Firstborne, with a target of 500kg displacement (of which 1/3rd is crew) the envelopes for hull flying should be enormous, especially when you add the option of say another 100kg of water ballast to the mix. If you find yourself in atlantic mode and aren’t flying the vaka, then you should probably be in pacific mode even if it means adding water ballast, I think it’s as simple as that.
I’m actually more worried about the pitch instability than the other problems now, because like I mentioned earlier the options to increase pitch stability are much more limited on a proa than one-way craft… To me it looks like you’d want pacific mode for light airs on all courses, and for downwind at all windspeeds, while the atlantic mode would be for windward work when there’s a lot of wind. Reaching would probably be split between the two modes, the exact crossover point being highly dependent on the exact masses, beam, sail area, and windspeed. Personally I’m wondering if the case of having so much sail area up on a reach that you’d run out of righting moment on a pacific proa, is even realistic or not. Maybe with a really big gennaker, but I’d guess that the pitch stabilty will be primary factor limiting sail area on a reach, long before the heeling moment becomes an issue. If I’m correct in that guess, then you’d be better served if it was the vaka that was providing the pitch stability…
Also, overall it seems kind of ironic that the whole point of this hybrid proa would be to increase max RM per unit of boat displacement, that is chiefly to attempt to improve windward performance, when the greatest strength of Russell’s boats seems to be their exceptional windward performance as it is. It would be somewhat of a surprising finding if there’s still significantly more to be had there… Not impossible, but surprising.
Even if there is room for improvement there, then this hybrid proa seems like an awful lot of extra hassle and extra structural weight for a performance gain on windward courses. For smaller boats where the crew makes up a large amount of the total displacement, I doubt that there’s much to be had there because by moving the crew to the ama and pumping in some water ballast you can probably get close to 50/50 anyway—and at the moment I’m interested in small boats. 😉 For a larger boat the windward benefits of the hybrid proa may be more pronounced, provided you can provide the required pitch stability and the total displacement doesn’t increase too much…
Marco
Herein lies a problem. Science is populated by formulas derived to explain data. The raw data comes first and someone develops formulas to explain the data. After peer review and sufficient sample data, formulas make the jump from explainer of data to predictor of future performance.
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Bill in Ottawa
Bill,
I have to disagree with that general statement. You put all “formulas” in the same “basket”, where, truly, all formulas do not belong.
There are formulas which are ***Models***, and they fall in the category that you describe above. You derive a model from data and you check its accuracy and enveloppe limits (how wide a range of value for key parameters can you go and still predict what is going to happen with your formula).
Economical “science” fall in that category (you will notice the ” ” I used…).
I would argue that some models and their formulas have “promoted” to hard science; in other words, the have never been proven wrong or even inaccurate ***in their acceptable range of validity***
Newtonian physics fall in that basket.
As long as you are considering the physics of objects that are not too small or not too fast, Newtonian Physics apply; all the time, every time.
An as far as I know, we are not talking of quantum proas or lightspeed proas… 😉
So, as one of my collegue engineer used to say: “you can’t beat Gravity”
A righting moment formula which expresses the effect of gravity on an off-centered mass portion of an object is not a Model that you may consider applicable or not. It is physics. It happens. It is not questionable.
Now, the impact or importance of the righting moment in the overall boat performance is another story… because your design choices that affect the righting moment do not impact only that but also an array of other physical characteristics of the craft, such as resistance in water, inertia, etc…
Cheers,
Laurent
It’s a good point you make Laurent where human error is concerned. There’s probably about as much or more that can go wrong with tbis setup as when you are shunting a large crab claw rig without rehoisting (I think that’s Dave’s point).
That was precisely my point Marco. Laurent, my apologies for using the perjorative “you” in my earlier post. It just struck me as funny that the objection to Marco’s idea of swapping out stays occurs in a forum dedicated to a class of boats (I believe I said “boat-type”) which routinely (and successfully!) virtually dismember the entire rig every time it shunts.
I meant no criticism, of anyone, and humbly apologize if I offended. It just struck me as funny. 😊
Dave
Dave,
No offense! You used a generic “you” where I understood a personal “you”...
And as you know, I am not usually “accused” of promoting crab-claw rigs… therefore my surprise.
But I see your point… shunting a crab claw rig would be considered as normal practice for most members of this forum, but slacking a western style mast rigging is not.
Cheers,
Laurent