Un-flippable Proa

Discussion in 'Multihulls' started by Inquisitor, Jul 7, 2010.

  1. Inquisitor
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    Inquisitor BIG ENGINES: Silos today... Barn Door tomorrow!

    This is a purely academic murmuring. I have no intention of using it!

    Somewhere on the forum I read about the suggestion of using a "soft" beam on a HarryProa style Proa. As the sails load up, instead of lifting the heavy windward passenger hull, the beam would flex and allow the lee pod (with sail) to rotate till the sail de-powers.

    The idea I want to purpose in this thread has similar intent, but achieves it a different way...

    Consider an Atlantic style Proa (sails on the heavier, windward passenger hull) and a smaller, longer lee hull optimized for speed. Several have suggested that a proper design of the lee hull would size it based on it being about half way submerged when the windward hull is flying. In other words... the full gross weight is supported only by the lee hull at half its buoyancy.

    Now consider making that lee hull's buoyancy slightly less than the total weight of the boat. At the point of where the wind would normally lift the windward hull, we instead get a totally submerged lee hull. The boat sets some new balance point with the lee hull under water and the de-powered (heeled) sail. When the captain finally gets around to de-powering the sail manually, the lee hull pops back up.

    To further encourage the madness... Making the lee hull would be like an attack submarine... a surface of revolution. A light sacrificial wood skeleton frame would be set up on a rotisserie (lathe) like rig. Cover it in cheap styrene foam and shape and sand using lathe techniques. Glass or better yet... filament wind. Cut open, take out frame, foam and add beam support structure and seal back up.

    I was just thinking outside the box. I'm not eating it... let Mikey eat it...
     
  2. bearflag
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    bearflag Inventor/Fabricator

    My outrigger hull is "almost" torpedo, shaped, but it will never get fully submerged. I don't think you want to get into a situation where the amas plunges into the water to the point where it digs in and causes you to maybe pitch forward.

    Multihulls are most unstable in the diagonal direction usually.
     
  3. rob denney
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    rob denney Senior Member

    This is how trimarans used to be designed. In waves they trip over the submerged hull, as would a proa.

    A better way of making an unflipable proa is to use a buoyant mast and cant the rig to leeward and store any heavy equipment low down in the windward hull. A bouyant boom and a means of submergingit also help. When capsized, the boat very quickly swings around so the mast is pointing into the wind. The windage of the tramp and windward hull help to flip the boat up. A cant angle of 5-10 degrees is sufficient for most boats. This also helps the sails set in light air.

    Another option is to launch a sea anchor from the windward hull which would swing the boat so the mast points downwind. The windage and wave action will then right the boat a lot more gently.

    Neither works for an Atlantic proa.

    rob
     
  4. magwas
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    magwas Senior Member

    Maybe I did not take enough attention, but for me it seems that flipping of a multihul occurs when the leeward body sinks such that it will direct enough force that the other hull(s) can turn around it.
    Also from the tank experiment someone linked to the heavy weather vs multihull thread it seemed to me that more buoyancy in the ama/proa means more protection against flip.
    So for me it seems to be a bad idea to decrease the buoyancy of the lee hull.
    Correct me if I am wrong.
     
  5. boybland
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    boybland Junior Member

    A pacific proa with a strong bouyant mast that was canted slightly to leeward would be pretty hard to flip.
    The floating mast would prevent complete capsize and the cant would mean the weight of the windward hull was positioned so it pulled the craft back upright.
    I have seen youtubes of smaller catamarans that use this idea to right themselves by releasing the stays on one side.
     
  6. ThomD
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    ThomD Senior Member

    If basically what you are looking for is a proa that can't capsize, then the Brown proa can't capsize since it heels and resets somewhat like a mono. The Harry style was not supposed to capsize or fly the windward hull, but that all depends on how you design the power and stability, still seems like a good chance, though.
     
  7. rob denney
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    rob denney Senior Member


    You could just as easily add the pods to a harryproa or to a cat. The reason no one does, is because they need to be huge to work, and are then a lot of added drag when they hit the water.

    rob
     
  8. ThomD
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    ThomD Senior Member

    Drag when they hit the water doesn\t seem to be an issue. What is the evidence this is a problem. Sounds a bit like the ballast vs multihull problem where people complain that multis flip in circumstances where ballasted yachts have sunk.having an answer to the capsize problem is of merit should the need arise.

    The use of pods arises from situations where they have been found needed. So Cheers got knocked down on her shakeout cruise, so one was added. A Harry designed to carry significant loads in the WW hull may be uncapsizeable in normal circumstances, but possibly not when scaled to the point where the WW hull is super light. Harries do have the issues about controlling the boat from the other hull.
     
  9. rob denney
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    rob denney Senior Member

    I have never sailed a boat with a pod, but estimate that the surface area of Jzerro's when immersed is almost as much as the rest of the hull. Hence the drag. My main fear with such a pod is that it would dig sideways into a wave in big seas and hasten a capsize, the same as the small float trimarans of the 60's. A capsize in a pod boat would be terminal, the same as in a cat or tri. I also think it is weight in the wrong place on a boat that already has righting moment limits. If it is used for accommodation, there is the fact that it is a huge hole in the side of a highly stressed hull, so needs a lot of heavy beefing up.

    Cheers is totally different. The pod is on the windward side, so never gets immersed while sailing. It is an add on, so no hole was required.

    I fully expect to fly a hull on my racing harryproa, the same as on my 25'ter and all the other racing multis I have sailed. On none of them would I want a pod to prevent capsize.

    Controlling a harry from the other hull is not an "issue". A long mainsheet and either a tiller extension or steering cables work fine. Certainly much better than the joystick and lifting rudders each shunt arrangement employed on both Cheers and Jzerro.

    rob
     
  10. bjarthur
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    bjarthur Junior Member

    more accurate to say it makes it more *rightable*. it can still flip.

    canting the rig to *windward* would make it less flippable: (1) the boat's center of mass would be more to windard resulting in more righting moment, and (2) in a pacific/harry proa the sail's center of effort would point more towards the boat's center of mass thereby reducing the lever arm of the heeling force.

    taken to the extreme one would have one of bernard smith's aerohydrofoils:

    http://www.geocities.com/aerohydro/home.htm
    http://www.sailrocket.com/node/172

    would be cool to have a rig which can adjustably cant both ways. to leward in light winds to help lift the windward hull out of the water thereby reducing drag, and to windward in heavy wind to keep it from flying. does such a proa exist?
     
  11. rob denney
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    rob denney Senior Member

    Enough cant and enough weight low down in the lee hull and it would be impossible to flip without wave assistance.

    Bernard Smith's ideas are wonderful, but I would worry about a mast inclined to windward in big waves, as the exposed sail area increases as the ww hull goes over a wave. Maybe not a problem if the rig is inclined enough, but still scary.

    The mechanics of canting a rig are complex. A lot of racing trimarans do it about 20 degrees each side, mainly to keep the mast vertical when the boat heels in light air. The G32 and Orion (catamaran) used it to right boats with mast head floats.

    For me, the simplicity and the flexibility of the unstayed rig outweighs any possible advantage of being able to adjust the cant.

    rob
     
  12. ThomD
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    ThomD Senior Member

    " have never sailed a boat with a pod, but estimate that the surface area of Jzerro's when immersed is almost as much as the rest of the hull. Hence the drag. My main fear with such a pod is that it would dig sideways into a wave in big seas and hasten a capsize, the same as the small float trimarans of the 60's."

    Floats have a positive angle of attack, pods have a negative one (or should). Therefore pods tend to hydro lift when immersed, not to mension dieplacement. The First Jzerro sailed for some time on her pod in light conditions withouth anyone imediately noticing.

    "A capsize in a pod boat would be terminal, the same as in a cat or tri."

    Not sure what you mean. So far no PP has capsized, since the pod rights the boat before the boat is at 90 degrees, and 90 degrees probably doesn't happen since the sails dump the air the float and pod right. If P proa did turtle it wouldn't come back but so far so good.

    "I also think it is weight in the wrong place on a boat that already has righting moment limits."

    PPs have limits on initial stability to be sure, but it doesn't seem to hurt them:

    http://www.youtube.com/watch?v=fgydLL8Tgqw

    Where final stability is concerned they are stable beyond 90 degree which no other multi can normally claim.

    "If it is used for accommodation, there is the fact that it is a huge hole in the side of a highly stressed hull, so needs a lot of heavy beefing up."

    Not particularly from what little I have seen of the bones of RB's proas they don't seem out of step with normal scantlings. HPs have accomodations in wings also, though there is arguably more nice stuff happening in load sharing with the beams. It's not as though these boats don't exist there don't appear to be structural problems and the advances in cores etc... would only make strengthening these underwing structures easier today.
     
  13. Alex.A
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    Alex.A Senior Member

    re The discussion on big waves and cats - they then turned to righting once capsized - seems to me it would be easier on a proa due to different hull lengths? One would dig in first and flip diagonally? How would the different types do this and what condition would they be in if/once achieved? Would be easier with the pac proa?
     
  14. rob denney
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    rob denney Senior Member

    "My main fear with such a pod is that it would dig _sideways_ into a wave in big seas and hasten a capsize, the same as the small float trimarans of the 60's." Pods going _sideways_ have little or no angle of attack, and once the top goes under, they are driven under very quickly. What happens in light air is irrelevant, but Jzero (the first Russ boat) would have been at 45 degrees of heel, so I suspect everyone noticed pretty quickly.

    Plenty of PP's have capsized. I think you meant to say that no Russ proas have capsized, which is not a surprise with a sailor as good as Russ sailing them. I need to see or hear about the pod in action in large breaking beam seas and the pod digging in _sideways_ before I will be convinced. I appreciate that this is not a particularly common event, but it is possible, so I design for it, same as I do for being caught aback. If they did trip sideways and capsize over the pod, they would be impossible to right, ie terminal.

    Suggest you read Steve Callaghan's article in Cruising World about their cruise to Tahiti. I can email it to you if you like. Getting wet lugging anchors and water bottles back and forth across to the windward hull in a breeze did not sound like much fun. Nor did constantly having to steer, ballast and trim the boat "to keep the ww hull from slamming back into the waves, racking and shaking the whole boat".pg 42, column 3). Easier, in my opinion, to have the weight to windward all the time, and move it to the leeward hull if you want to race in light air.

    Cool movie, great boat. But hardly windy or rough. Also no mention of how much weight was in the windward hull.

    True, but if the boat is stable at 90 degrees (harryproa with buoyant mast), this is not a requirement. Stability beyond 90 on a cat or a tri (or a harry) would be easy enough to achieve by adding a pod. Yet, no one does.

    All true, but it makes more sense to me, and will be lighter, to put the pod where it is not battered by waves when the boat heels, adds to the righting moment and is in a hull which is not stressed by any sailing loads. No doubt Russ's boats are built to "normal scantlings". My point is that a harryproa can be built just as strong, but much lighter. Compare Jzerro (38'/1.6 tonnes) with Blind Date (50', far more accommodation and comfort/2 tonnes). If Blind Date was double diagonal instead of strip planked, it would be lighter again.

    It is great to have a conversation about Russ' boats without receiving any personal abuse. Makes a pleasant change from what I used to get. Thanks for your attitude.
    Rob

    Alex,

    Harryproas have short rigs for their length because a) they are so light compared to other boats and b) the extra length costs and weighs very little. They have a very high prismatic coefficient (0.9 on my latest racing one, 0.8 on the cruisers) so have very bouyant ends. They also have nothing on the foredeck (cleats, forebeams, strikers, trampolines) to slow the boat down or hinder recovery from a nosedive. All these factors make a diagonal pitchpole less likely than on a conventional cat. However, a straight ahead pitchpole is probably as likely as there is only one hull vs two for the cat. This is a pretty rare event for a boat which does not have to sail ddw to gybe. If a harryproa did go over diagonally, it would probably land on it's side, but if it didn't, it would be a lot of work to get it back up.

    rob
     

  15. ThomD
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    ThomD Senior Member

    "Pods going _sideways_ have little or no angle of attack, and once the top goes under, they are driven under very quickly."

    What is the scenario here? The boat is backwards to the wave train, and possibly also the wind, and the wave breaks over the entire pod, forcing it under? I have to admit that doesn't sound good...

    "What happens in light air is irrelevant, but Jzero (the first Russ boat) would have been at 45 degrees of heel, so I suspect everyone noticed pretty quickly."

    They were both asleep at the time, and apparently remained so for a while.The main relevance, while not great regarding your scenario I still haven't quite grasped, is that the pod functions as a hull when working along like a boat. What may happen when being rolled like a drunk, in big waves, is another mater.



    "Plenty of PP's have capsized."

    I wasn't thinking of the traditional ones or the cruise ships, just the modern sailing vessels.

    "I think you meant to say that no Russ proas have capsized, which is not a surprise with a sailor as good as Russ sailing them. I need to see or hear about the pod in action in large breaking beam seas and the pod digging in _sideways_ before I will be convinced. I appreciate that this is not a particularly common event, but it is possible, so I design for it, same as I do for being caught aback. If they did trip sideways and capsize over the pod, they would be impossible to right, ie terminal."

    Ok, but designing for something and eliminating it is not always the same thing. RB proas are designed to resist capsize in that circumstance, the sails dump the wind, and the pod creates a righting moment, this is pure monohull stuff. I understand that your gut tells you this is all going to turn out badly, but there are gut views of your boats also, in the absence of a lot more sea trials and visible sailing by more people in more circumstances, the rest of us are just going to have to wait and see.


    "Suggest you read Steve Callaghan's article in Cruising World about their cruise to Tahiti. I can email it to you if you like. Getting wet lugging anchors and water bottles back and forth across to the windward hull in a breeze did not sound like much fun. Nor did constantly having to steer, ballast and trim the boat "to keep the ww hull from slamming back into the waves, racking and shaking the whole boat".pg 42, column 3). Easier, in my opinion, to have the weight to windward all the time, and move it to the leeward hull if you want to race in light air."

    Steve has rejected your characterization of these events. A lot of work can be required on a lot of different designs when the going gets tough.


    "Cool movie, great boat. But hardly windy or rough. Also no mention of how much weight was in the windward hull."

    I'd like to know that also. It is certainly true that the windward hull is not a multifunctional platform on these boats. Of course whatever the weight, Russ could put some accoms there if he wanted to and keep the simpler elements of his boat intact. Not waiting for it...

    "True, but if the boat is stable at 90 degrees (harryproa with buoyant mast), this is not a requirement. Stability beyond 90 on a cat or a tri (or a harry) would be easy enough to achieve by adding a pod. Yet, no one does."

    It's more than stable, it self-rights, as I should have said. I'm not sure it would be easy, the pods required on two sides, on non-accom hulls of say a square kurt hughes tri, when the structure is already out there, does not seem feasible. I think you have to fall in one or the other camp. PP self rights but doesn't have hard stability like the tri. You have to choose. If I had to set out across the pacific tomorrow on a PP proa, or a hughes tri as wide as it is long, I would have a greater comfort level on the tri, with a reasonable size rig.

    "My point is that a harryproa can be built just as strong, but much lighter. Compare Jzerro (38'/1.6 tonnes) with Blind Date (50', far more accommodation and comfort/2 tonnes). If Blind Date was double diagonal instead of strip planked, it would be lighter again."

    Really, I doubt it on the DD. But either way I don't think there is any available magic here. To hold up the same sail the boats need the same platform and the same weight distribution, rig efficiency also plays a part - along with everything else :). While it is an efficient use of space to build like an HP, it isn't low wind resistance. You are for the most part dragging two hulls. The Ball rig and freespar, have not yielded good sail shape. The rudders have end plate issues, and look vulnerable, and high tech. It may all work out great, and I hope more than anything it does.


    "It is great to have a conversation about Russ' boats without receiving any personal abuse. Makes a pleasant change from what I used to get. Thanks for your attitude."

    As fully paid up member of the HP patrons society, I don't have anything but best wishes for HPs. If what you say is right about them, then it is a better world. The same is true of RB PPs, but as impressed as many people are with them, they are never going to take off, at least that is what it looks like at the moment to me. One can talk about this stuff, but at some point it has to be judged by results, and the only game in town is the HP. If anyone knows where i can get a set of plans for an RBish PP I would like to know. I did think of buying Kauri, at one point, but I just didn't have the chops for that kind of thing.

    I would also say that whatever the real world personality mix would be, who has more in common that you and Russ (and a few creative French builder/sailers)? Not only are you both proa innovators, and serial builders, but you both have a great ability with simple wood epoxy strategies and boat building in general. While I asume it isn't actually personal amoung the principals here, I am reminded of a story Jim Brown at one point told about how he really had it in for either Cross or Horstman, at one point. But when the competition slackened he came to realize what a neat guy he was and they became friends.
     
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