Manik - 10 June 2014 01:54 AM

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. 😛

On the other hand, the solution I propose definitely adds complexity; it is not a panacea…
Now, on the same mast, you have “double rigging” PLUS freestanding mechanism (added structure in the hull, bearings on the mast, other constrains to calculate your mast scantling against…).

We are going away from simplicity….

Dave, in that case, this comment coming from me IS ironic!!! I am not exactly the right person to promote simplicity, me the guy with the shooner rig and TWO dagger boards, one for each tack…

Laurent

I’m sorry that I killed the buzz on the Pushme-Pullyu and I didn’t mean to be defensive about Pacific proas.
The Denny misinformation propaganda stuff was all aimed at Pacific proas, and mostly aimed at my boats in particular. It has made me jumpy. Sorry Dave.

It’s a bit off an odd position for me, knowing possibly more than anyone else does about Pacific proas.. I have had a hell of a good time screwing around with these boats, but that doesn’t mean that I know it all and I certainly don’t want to come off as a jerk on this forum.
I can make a pretty good educated guess on how something will work (such as a Pushme-Pullyou), but I’m also often surprised that things can work that don’t make a lot of sense or weren’t obvious to me.
I’m hoping to be surprised by proa development. There is a lot of potential and intrigue in proas and this forum is a real hotbed of ideas. I like how off the wall ideas are thrown out there (such as some of Dave’s) because it’s often the extremely nutty ideas that lead to something useful.
My feeling is that a proa that accepts the wind from both sides is maybe not such a good idea.
A proa has a lot of compromises. On thing in it’s favor is that it only needs to accept the wind from one side and can take advantage of that fact in so many ways..
I still think that proas have huge potential. I hope to see a proa challenge the fastest offshore racers someday. I think it could happen and I think that the results could be pretty surprising (especially upwind), but until that happens I’m just happy to see experimentation in whatever form it takes.
Russell

I don’t think you killed the buzz at all Russell, probably quite the opposite 😊

One thing to be wary of is that a boat that can do everything may not do any one thing particularly well. However, it is always worth exploring the options.

Mal.

Russell Brown - 11 June 2014 07:17 PM

A proa has a lot of compromises. On thing in it’s favor is that it only needs to accept the wind from one side and can take advantage of that fact in so many ways..
I still think that proas have huge potential. I hope to see a proa challenge the fastest offshore racers someday. I think it could happen and I think that the results could be pretty surprising (especially upwind),
Russell

I think so!

[quote author=“Russell Brown” date=“1402539450”
A proa has a lot of compromises. On thing in it’s favor is that it only needs to accept the wind from one side and can take advantage of that fact in so many ways..

I could not agree more…....Taking advantage of what only proas can do….That is the part which really fascinates me.

Rob

I’ve heard several versions of “jack of all trades, master of none” in relation to Pushmi-Pullyu. I fully agree with the underlying sentiment, but it was specifically not my intent to either excel at all things, nor compromise ability with the concept. It is, rather, a study in possibilities.

As with most of the folks in this forum, I’m a Pac proa guy. I love the concept of lightly stressed multihulls quietly and efficiently doing their thing (this is an oxymoron in 99% of the multi design world; there just is no such thing). I love the idea that a stone-age society, using severely limited engineering materials and no concept of modern math or physics could put together the fastest boats on the planet, 500 to 1000 years ago

I am not pedantic, however. There is a whole other world of proas out there. These proas fundamentally different than Pac proas. How could one not want to explore their capabilities? Chief among the drivers of my curiosity is that Dick Newick believed they have great potential, never developed.  When a guy like Newick says a thing like that, sorry folks, my head is turned and my ears are open.

Arguments based on “Its never been done, so couldn’t be all that great” fall on deaf ears with me. Do I need to recite Patent Office cliches? Just as there were essentially no multihulls developed outside polynesia in antiquity, it does not fundamentally surprise me that Atlantic proas were not developed there either. Materials science alone suggests they would have been rejected immediately. It was only the genius (or happy coincidence if one prefers) of mating multiple narrow hulls to gain stability without ballast which lead to Pac proas in the first place. Pacs are optimized double canoes—not optimized for power, but for some power/material capability ratio as yet unnamed. Newick rightly pointed out that with that ratio forever altered by modern materials science, other combinations of the original double canoe paradigm might be—ought to be—explored.

It surprises me that (apparently) few of us have ever sailed our proas on their “darkside” tacks, simply out of curiosity. How does YOUR proa perform with its ama to leeward? Can it even be done? (An asym vaka, for instance, might need to be assembled “inside out” to get the flat side where it belongs). Rigging will be an issue—consider purchasing 100’ of ¼” polypro for a few bucks and start from scratch—temporarily. At the very least, consider it from your armchair; what if? Where would your boat be stronger, where weaker? There’s no compromise recommended, no “jack of all trades.” We’re simply taking your existing boat and sailing it on the other tack—what changes? Please, don’t focus on what wouldn’t work; yes we know that staying is an issue when sailing “backwinded”. Yes we know your rudders are now on the wrong side, etc. It’s not necessary to solve the location and function of every bolt and screw on the boat, rather let’s paint with large strokes; what changes?

Dave

Russell Brown - 11 June 2014 07:17 PM

I’m sorry that I killed the buzz on the Pushme-Pullyu and I didn’t mean to be defensive about Pacific proas.
The Denny misinformation propaganda stuff was all aimed at Pacific proas, and mostly aimed at my boats in particular. It has made me jumpy. Sorry Dave.
Russell

No worries, Russell, and no apologies needed. I’m working on responses; just a little slammed at work. More soon.

Dave

Part 1 of 3:

Let’s look at Pushmi-Pullyu first as if it were a strictly Atlantic proa, and compare/contrast it against an offshore racing trimaran. This will be in 2 parts as Proa-File doesn’t allow more than 1200 words. In a third post, I’ll consider him as a strictly Pacific proa, again compare him to the same racing tri, then draw some conclusions about the “hybrid” boat.

We’ll use a MOD 70 to compare against. I’m not advocating going after these giant budget guys, nor am I proposing that Pushmi-Pullyu is only a good idea for 70’ers. I’m doing it because accurate numbers are available, and because, if you could beat these guys under their version of “average conditions” you could probably beat anybody, at any size.

The MOD 70 is 70’ long (no bowsprit) and all 3 hulls are the same length. Max of 55’ wide and weight a minimum of 14,000 lbs, not including crew or stores. It carries 3333 sq ft of upwind sail (more off-wind, let’s stick to upwind sail area to simplify the comparison, OK?) The mast height is limited to 100’ above the water. The boat’s have a massive 2.4 Bruce number, which means, well, they’re pretty darn powerful. BN’s above 1.5 are considered very powerful, higher than 2.0 is really rare.

Here’s a video of one capsizing. An auto-flight gadget would handily have prevented this particular accident (I’m just sayin’)  https://www.youtube.com/watch?v=mf8Clcngfvk

If the boat is 55’ wide, the amas’ centers of buoyancy are perhaps ~52’ apart. At 14000 lbs, upon just lifting the min hull out of the water, the MOD70 will have a RM(max) of 26 X 14,000 = 364,000 ft-lbs. FWIW, a half-dozen crewmen, added to the main hull, will increase this by less than 5%.

If the masthead is 100’ above the water, the boom is perhaps 10’ above, leaving the mainsail luff at around 90’. If the center of effort of main and jib is 40% of that, it might be a net 46’ above the waterline. This suggests a sail force of about 7900 lbs (364000/46) can be brought to bear at that RM(max), from her 3333 sf of sail. This is about 2.4 lbs/sq ft, so very broadly (assume a lift coefficient of ~1.0) about 26 kts of apparent wind—under full sail—will get her to fly a hull. As these boats handily do twice the wind speed, and in fact begin flying at ~12-15 kts true, my calculated numbers seem to fit reality.

Now let’s get out a chainsaw and build a proa, shall we? Hack off everything outside the main hull on one side and just chuck it away. Change the transoms into bows, substitute an unstayed balestron rig, 4 retractable rudderboards as proposed earlier (note from the capsize video that the MOD70 has 6 appendages, so we’re actually simplifying—and adding lightness—when we go to 4). For now, may we ignore curved or any other lifting foils? Categorically, anything which might help the trimaran should help the proa, so let’s focus on the straight displacement hulls if we may.

Biggest change is we’ve landfilled more than 2 tons of carbon fiber. The boat will weigh 2/3 as much as the tri (you’re welcome to disagree, but please read on a bit first), or about 9300 lbs.

In order to maintain the same Bruce number as the tri, we need 77% of the sail area, call it 2566 sq ft. We can reduce the luff to about 76’, lowering the center of effort to about 40’ above the water.

Which is a good thing since our RM has taken a huge hit. 4700 lbs of carbon fiber waving up there in the air makes a poor source of ballast, but it was ballast. Not only has its absence reduced our RM by a third as well, it’s worse than that, as the proa’s center of gravity is now closer to her center of buoyancy at RM(max). The tri’s CG was right on her centerline. The proa, assuming a pretty radical 80/20 weight distribution to match the tri’s, will have her CG moved 20%, or 5.2 ft to leeward.

Turning to weight and strength for a moment; we removed a third of the weight of the boat just by losing the windward ama and crossbeams. Everything else was/is as strong as before. That reduction in mass and RM means the remaining structure is way over-built; we could go to lighter akas, thinner hulls, lighter rigging. We’ll have less inertia, thus less pitching moment, heave, etc. The beneficial spiral lets us spin downward most everywhere, Lowering the CE of the sails, the strength of the sails themselves. All standing and running rigging, winch sizes and quantity, spiral and spiral again. On the one hand, no one’s ever built a 70’ proa anywhere near as light as 9300 lbs (4.2 tonnes), even as a Pacific proa but we can go even lighter yet, due to the spiral.

Part 2 of 3:
Going back to RM(max), would it perhaps be OK make our proa just a skosh beamier than the cut-off trimaran? Instead of the hull centerlines being 26’ apart, let’s go out to 31.2 can we, so that righting arm remains constant? Considering the beam of the ama and main hull, this might yield 36’ as overall beam. On 70’ length, this remains a markedly narrower boat than the 70’ X 50’ standard MOD70’s. Now our RM(max) is exactly 2/3 that of the trimaran’s, at 243,000 ft-lbs. Our new rig, at 77% size and 40’ above the waterline,  now creates heeling moment matching that RM(max)—thus liftout of the windward hull—at precisely the same wind speed as the heavier trimaran does. Every ounce we go lighter than 2/3, or every inch we might add in lieu of lightening ship, will only make her faster overall and fly earlier.

What about ballast? Imagine adding a “safety hull” well to windward of the main hull, yet within the class’s 50’ maximum beam—and fill that hull with 2.1 tonnes of water? The center of this ballast in now some 26’ to windward of the unballasted proa’s CG, adding an additional 123,500 ft-lbs of RM, for a total of 366,000 ft-lbs; surprisingly identical to the trimaran we began with (364,000 ft-lbs), So, with displacement and overall beam now identical, and carrying identical rigs, the un-ballasted trimaran has identical power to the ballasted proa.

OTOH, in light air we have the ability to fly that hull earlier and longer—far earlier and far longer—by jettisoning the ballast. At higher winds it’s a dead heat, performance-wise with or without designing for ballast, though a big savings in cost for un-ballasted, as we don’t need to stress the structure with this giant weight on a long stick, nor pay for the larger rig, which is only superior over a fraction of courses and wind speeds—and requires that rules permit the dumping and taking on of water ballast.

I would take it as “proven” (OK; “calculated”) that we can build an un-ballasted Atlantic proa lighter, faster and more powerful than a MOD 70 trimaran, even without leveraging the class’s extreme beam. The boat will ride with less stress and spray, with much-reduced pitching moments, rolling moments, yawing moments.

Is this the “ultimate” proa? No, it could be made much more extreme. I’m stopping here because now I want to consider this same identical boat (the un-ballasted proa) and flip it around. Running as a Pacific proa, I want to compare it, again, with the MOD 70 trimaran.

Part 3 of 3:

Running as a Pacific proa, I want to compare it, again, with the MOD 70 trimaran.

The proa now has an RM(max) of only 4.8 X 9300 lbs or 44,640 ft lbs. Using the full 2566 sf of sail area, centered 40’ above the water, we’re going to fly that ama anytime the rig develops more than about 1100 lbs of force, or .43 lbs/sf. It will take just 10 kts apparent to fly (call it 4-5 kts true wind on best courses); anything above that will require either dumping power, reefing—or adding ballast.

Turning to ballast again; adding the maximum 2.1 tonnes of water, this time to the ama, not the main hull, will give us an additional. 24 X 4660 = 111,840 ft-lbs of RM for a total of 156,500 ft-lbs at RM(max). This translates to 1.52 lbs/sf before the RM(max) is overcome, or closer to 20-21 kts apparent to fly the ama—again under full sail, set to drive at highest possible force and efficiency. Russ is precisely correct, adding ballast to a Pacific proa can dramatically increase her sail carrying ability, while retaining her ability to fly (not just “skim” but actively and stably fly) her ama under a wide range of conditions. For Pushmi-Pullyu in Atlantic mode, maybe not, but for many Pac proas, Russ’s still “da Boss!”

Concluding, it appears that either a water-ballasted (Pac mode) unballasted (Atl mode) or fully un-ballasted Pushmi-Pullyu, at 2/3s the mass and 2/3 the beam of a MOD 70 trimaran could have identical Bruce numbers, identical sail drive/RM(max) ratios,  identical hull lengths, and could fly her weather hull in true winds from 4-40 kts (reefing to make the last) Such a boat would be a “walkaway” in light conditions; pretty much anything under 10 true, plus precisely as powerful yet more comfortable and easier on her gear even in extremely heavy conditions—thus could be driven even harder than these guys do now.

As cost generally relates linearly to mass, Pushmi-Pullyu, in 100% carbon fibre, spectra sails and all the unobtanium the “bug boys” use, would cost a third less then their best—and would beat them, under all conditions.

Dave

The only question I would have regarding your demonstration is: how do you do the rigging, knowing that you lost half of the base structure for your shrouds? Is the “ballasted safety ama” far enough from main hull to create enough angle for the shrouds?

That point put aside (there must be a way to take care of it), now that you have “proven” (by calculation only, but still, it definitely look sound to me) that an Atlantic Proa or a Pacific Proa (with ballast) could be as good as one of the best racing trimaran out there, why not choose only ONE configuration, and optimize it even further?

For instance, having the wind coming from both sides like any tacker, your daggerboard has to be symetrical, your main hull has to be symetrical. If you receive the wind always from the same side (be it Pacific or Atlantic side), you can optimize boards, or even hull accordingly.

Cheers,

Laurent

If you start with all the carbon fiber and epoxy and stuff needed to make a MOD 70 trimaran, i guesstimate that you could build a 100 X 60 foot pacific proa instead, once you recalculate the needed structure to take the much lower stresses. A 100 x 60 foot pacific proa with the same rig and sails, and some meat ballast and water-tanks in the ama would be a lot faster then the 70 foot atlantic proa.
You should not limit the one thing that the pacific proa is good at (L/D). Use the budget/amout of CF/epoxy/foam-core for the one aspect that really makes the boat fast.

Cheers,
Johannes

Additionally, “Pushmi-Pullyu” needs a balestron rig, which is a slow rig in the MOD70 world…...Surely as Johannes says, it is better to put all the saved material and weight into a much bigger pacific proa AND as Russell and Laurent say, take full (unique) advantage of shunting asymmetry AND have a decent stayed rig????? Best of all worlds???

Rob

First off, thanks for the really great posts Dave! Coming up with a set of examples and corresponding numbers that work out so nicely is not easy. 😊

As I pointed out some posts back, you can have a fully stayed rig on an atlantic proa or hybrid proa as well (somehow I like ‘hybird’ more than pushme-pullyu), you just have to offset the mast toward the ama a bit, and hoist the atlantic rigging while lowering the pacific rigging, when you do a tack from pacific to atlantic mode. If you are sailing close-hauled you could probably do the rerigging and the tack without even stopping the boat. The staying base in atlantic mode is narrower than it could be for a pacific proa, so you will have higher loads in the shroud(s), but I think that’s not SO dramatic from a weight standpoint. I would much rather opt for a stayed rig than anything else, lots of weight aloft is not a good idea if you are packing such an enormous rig on the poor boat. 😉

On the pacific/atlantic part of the discussion, I think you can see quite clearly that even with the 2 tons of water ballast, the pacific proa reaches it’s RM(max) of 156,500 ft-lbs at 21 knots apparent instead of the unballasted atlantics 243,000 ft-lbs and MOD70s 364,000 ft-lbs at 26 knots apparent. That is a huge difference in the sail carrying ability between the pacific and atlantic proas: the atlantic proa can carry a whopping 55% more sail area at the same apparent windspeed as the pacific proa, which is already loaded up with 2 tons of ballast water at this point. Sure a pacific proa can be built lighter (or larger for the same displacement) than an atlantic, but the pacific proa (empty) would have to be a whole 2 tons lighter than the atlantic proa (so 2.2 instead of 4.2 tons) just so that the water ballast is already accounted for, and then we haven’t even started making inroads into making up for the colossal difference in sail area yet…

I think you simply can’t make up for it, so I disagree with Dave’s assessment that a pacific proa would have a performance advantage over the MOD70; I think it would be at a significant disadvantage to the atlantic proa or MOD70 at the higher windspeeds. At lower windspeeds that’s probably the opposite; if the pacific proa is the only one flying a hull and the comparative lack of RM doesn’t inihibt putting up lots of sail (the wind just isn’t strong enough for it to be a problem), then the pacific proa will walk away from the others.

What this shows as well is that the hybrid proa is confronted with a range of apparent windspeeds (the band from 21-26knots) where it is neither possible to carry the full sail are in pacific mode, nor fly a hull if you’d tack over to atlantic mode. If you did want to destabilize the atlantic mode by putting those 4600lbs of water to leeward of the ama, you’d have to put them a whooping 18ft to leeward, to bridge the RM(max) gap between the unreefed ballasted pacific proa, the atlantic proa. In other words: it’s not gonna happen. You will have to reef in pacific mode and wait for the wind to go up enough so you can tack, shake out your reefs and fly a hull in atlantic mode.

That’s all well and good, but I think Dave left out something really important, and that’s the question of pitch stability. If we were on a broad reach with all that sail area up, then we would have that same moment of 246,000 ft-lbs trying to bury the bow. That means that on our 9300lb atlantic proa, the effective center of buoyancy CB (potential C-foils included) would have to be 26.4 feet infront of the center of gravity CG, to provide balance in pitch. On a trimaran that’s much easier to do than on a proa, since you can design the boat in such a way that the heavy parts of the boat, that is to say the heavily loaded parts like the iakos, mast, maststep, center- and daggerboard cases, and stuff like room for accomodations and all your sails, are all at the back of the boat, while at the front of the boat, forward of the front iakos, you put a long bowsection which doesn’t weigh much to really help bring the CB well forward of the CG when on a reach.

Now we’re building a proa… Out of the box our CB and CG are both at the midplane of the boat, and we have to come up with something to get the two 26.4 feet apart if we want to be able to be able to carry as much sail on a reach as we can when we are close-hauled. In my eyes the easiest way to start making significant inroads into producing the enormous bow up moment we need, is to place a C-foil (banana board) well forward of the midplane. Say we design it to carry 70% of the displacement of the boat, so 6500 lbs of force, then it would have to be 37.8 feet forward of the midplane, which is 2.8 feet forward of the bow, to provide our needed bow up moment. So that simply doesn’t work, even with the balance issues aside (the lateral forces produced by the rudder and daggerboard have to be in balance to ensure the boat is balanced in yaw). We could try to give the hull some rocker, so that as the boat leans forward the CB moves forward more rapidly (less buoyancy aft), but even with rocker, C-foils placed at a reasonable location, and with the addition of horizontal stabilizers on the rudders to produce a doward force aft, which the MOD70s don’t need, I’m skeptical as to whether we can produce the required bow up moment. Large quanities of water ballast in the rear of the boat would move the CG further aft (remember it’s the distance between the CB and CG that counts), but my guess is that in terms of performance we’d be paying a high price, while the MOD70 where the forward CB and aft CG is the natural state of affairs, is smoking along, business as usual.

Anyone have an idea how to fix this? Or am I mistaken in some part of that calculation/consideration? If this calculation is correct, then I think we’ve perhaps found the reason why we only see large offshore racing trimarans and not atlantic proas.

Cheers,
Marco

Rob Zabukovec - 15 June 2014 11:57 PM

Additionally, “Pushmi-Pullyu” needs a balestron rig, which is a slow rig in the MOD70 world

Perhaps. The extreme pace of experimentation in the offshore tri world is both a joy and a curse; a joy as it pushes the boats into most of the corners of the design envelope quickly even if expensively. A curse because “second place” concepts aren’t explored in any depth in the race for speed, quickly.

“No one likes unstayed rigs on multihulls because they are not powerful enough.” (Not quoting you Rob, but rather as a cliche.) This is not a scientific conclusion but rather a prejudice. It may be true, but that truth is not proven, in fact has counter-examples.

Pushmi-Pullyu does not require unstayed mast(s), as Marco so aptly points out (and also Mal? You Rob? Can’t recall) I chose to represent him as unstayed in order to avoid limiting discussion of the hulls and staying base solely in order to support someone’s preferred rig.

Dave