Proa cruising sailboat design, help get this idea out of my head...

Discussion in 'Boat Design' started by 360weatherbound, Mar 2, 2010.

  1. KSONeill
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    KSONeill Junior Member

    I don't think that's right, or important anyway. The big thing in a tack or shunt is control, can you complete the maneuver without blowing the tack, in a tacker, or going aback in a shunting boat? Tackers blow tacks much more often than shunters go aback. You can't blow a shunt.

    So do proas, they're exactly the same on both tacks.

    Right. Having a real stern does help.

    HA! Two sets of sheets, two rudders. Not simpler.

    And takes advantage of windward leeward asymmetry. It's nice to always have the same side to windward. Passengers, for example, don't have to shift to the other side of the boat when you come about. All the sail stuff can be in one place, and all the seats and people in another.

    I'm not sure this is true. It may be, but it hasn't been shown yet.

    It has to be a very shifty lake for this to be true. To cause trouble, you have to have large enough wind shift to 'autotack' the boat. This will put a proa aback. Ok. It will also dump a Hobie if you're out on the wire, you're almost certainly going over in an autotack. I'm not sure who's better off there, but I think the proa is.

    First generation beachcat sized Harryproas are already competitive with very evolved beachcats. A fleet of Harryproas would improve quickly. So I think you're wrong, but who knows until people start racing modern proas in a serious way?

    If you're interested in proas you should join the proa_file yahoo group and spend a month plowing through the archives with a set of keywords to plug into a google search and a wordpad doc open to take notes in. All the stuff you guys are, intelligently, trying to work out from first principles is pretty well known. You're worried about stuff like shunting that's not really a problem, and you haven't mentioned the stuff that really is a problem, like fore-aft symmetric foils or CE/CLR problems.

    Come on over! We don't bite.

    K O'N
     
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  2. marshmat
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    marshmat Senior Member

    In this context, one can think of the proa as being the limiting case of the canting-keel monohull. Lift the keel strut and the bulb completely clear of the water, and since it's not contributing to the drag now, you can make it large enough to float its own weight when the boat's at rest.

    I understand the engineering by which Rob's boats are so light, but I do think there is such a thing as "too light"- such weight savings come at the expense of reserve structural integrity for those unpredictable situations, such as running into a barely-floating container or an uncharted reef at 12 knots.

    I think the worry is largely that this is unfamiliar terrain for many, just as skeg and spade rudders were once unfamiliar terrain to those accustomed to the keel-hung variety. Pivoting a rudder through 360 degrees and having it retract or fold for shallow water is a bit more challenging than just hanging a plank on the transom, but is far from a no-win problem.

    edit- I concur with K O'N that the engineering effort would be better focused on such issues as foils and helm balance that are likely to be significantly different on proas.

    An interesting point, AK, but we should compare on equal grounds. For a cruising boat, I would consider that to mean equal cost, displacement and habitable volume for all three boats, and their linear dimensions (LOA, beam, etc.) being different in each case as appropriate for that form.

    I suspect the "well proportioned" cruising proa will come out to be somewhat longer than the cat for comparable cost and weight. (My sketches and calcs suggest that the proa equivalent to a 14 m cruising cat, displacing 7-8 tonnes fully loaded, is closer to 18 m long) That goes a long way towards making up any seakeeping or speed advantage the cat might have on account of its fore-and-aft asymmetry.
     
  3. ancient kayaker
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    ancient kayaker aka Terry Haines

    -agreed, unless tacking is permitted, but that would involve reducing performance so stress would still be kept low.

    -maybe: I don't see it as much of a problem, it is akin to boxing which was often necessary with my earlier sailing kayak due to the small sail and the large frontal area of the paddler. A long boat working upwind in a river by shunting would be a problem to itself and every other craft.

    I do a lot of boating in small lakes and around bridges so that’s perhaps an unconscious factor in my thinking. The first time I sailed I had to work my way through moored boats as I worked out the principles of sailing, based on the guideline“it’ll come naturally” - I brushed an anchor line before I could get the daggerboard down. If I had needed to shunt through that pack ...



    I must admit I hadn't even given the rudders more than a glancing thought, but I wouldn't worry about that anyway since I'm planning an experiment with "front wheel steering" this Spring.

    I have added the proa to the (rapidly getting very long) list of sailing things I want to try.
     
  4. Bruce Woods
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    Bruce Woods Junior Member


    Are you sure? Which side of the beam, top or bottom? Please explain.

    The beams must be designed for the worst case scenario, including being caught aback.
     
  5. marshmat
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    marshmat Senior Member

    Oops, I had missed that line....
    I would think a proa's crossbeams would normally be subject to a bending load, and that- for design purposes- this load would be no smaller than that of getting caught aback under full sail, ie. the same as a trimaran's crossbeams. There's no weight savings to be had in the beams by going from a tri to a proa, apart from only needing them on one side.
     
  6. KSONeill
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    KSONeill Junior Member

    That depends on the rig and the size of the ama and the sheeting arrangement. Harryproas, for example, are designed to take the full load of the rig if the boat goes aback, but I think they also have the sheeting point to windward. If they go aback the sail is not sheeted from the other side.

    On most proas the rig falls down if you go aback; most crab claws, for example, or most Gibbons rigs. On my boat:

    http://wikiproa.pbworks.com/f/proa on beach after T200.jpg

    the masts are old windsurfer carbon masts extended at the bottom. They're wet noodles, very very flexy by catamaran or dinghy standards. They're stayed to windward, so when sailing normally they stand up fine, but if I go aback they would flex off like mad, so the beams never see the full load.

    I once had a much bigger rig on this boat, but also had a much smaller ama and much lighter and longer beams:

    http://wikiproa.pbworks.com/f/messabout2003-6.jpg

    Sometimes that rig had a windward strut. The beams were Laser topmast sections, 2" aluminum tubes. If it went aback, which it did, a lot, it would sink the ama very quickly unless I jumped to the new windward side to balance it and try to sail it around and tack. Again, very little stress on the beams.

    And for a small and not too serious boat like mine, heck, I just say I'm not going to go aback while sheeted hard, and if I do I shunt the boat and sail out of it. I've never gone aback hard since switching to a schooner. But if I do, eh, so a beam breaks, I limp in and go fix it. Not a good plan for a big cruiser, but fine for a small boat, compared to carrying heavy beams around all the time.

    K O'N
     
  7. Guest625101138

    Guest625101138 Previous Member

    Matt
    Have you been through Rob's photos on HarryProa pages. He has substantial solid foam crush zones in the hulls with foam sandwich backbones.

    Having the boat very light means there is not a lot of momentum even at high speed. The point loads on a 8t boat hitting a container at 12kts is considerable more than those on a 550kg boat. His hulls bottoms were folded after layup so they are quite plyable as well. They have ability to flex and absorb impact. Would be interesting to see what a 14lb sledge hammer could do to them.

    My concern with the low weight is keeping it in the water in high wind but then it does not have the large surface areas of a cat or tri.

    The canting keel yachts have not yet got to the extreme of Rob's proas. The lee hull is only about 400mm beam. Using ballast for countering sail moment with zero form stability would be very challenging.

    You are correct though about the windward hull being essentially outboard ballast on the lee hull. The reduced length reduces the pitching moment carried by the cross beam as well. Yes - I believe he intends to use a single beam on Solitarry. So again more structural efficiency. The two hulls serve totally different purposes and should be optimised for their role. The lee hull is long and slender to give the high speed potential with low forces and the windward hull is the accommodation pod that counterbalances the sail loads.

    Another weight saving that is not immediately apparent with the origami method is the surface finish on the outside of the infused panels. These are mirror finish surfaces and do not need any other finishing. It is better than you could get with a mould. I think he will paint them now because he had to do some unplanned bogging in the ends but the rest of the hulls are mirror smooth. So a saving in fairing bog over other methods of one-off. Lee hull sections only have one closing joint and windward two.

    Rick W
     
  8. rwatson
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    rwatson Senior Member

    Hi Rick

    I had trouble locating the 'origami' method you are talking about on their website.
    Could you include a link for me please ?

    Thanks
     
  9. Guest625101138

    Guest625101138 Previous Member

    To view it you need to log into Yahoo. If you have a log in already for another Yahoo group it will work.

    This is the Yahoo site:
    http://au.groups.yahoo.com/group/harryproa/
    Once you log in you will have access to the photos. There are two albums called 15m Solitarry build.

    Rick W
     
  10. Guest625101138

    Guest625101138 Previous Member

    Terry
    See inserted comments.

    Seeing Rob's proas is one thing that could get me back into sailing. You get a tremendously fast boat for the money. At AUD20k Solitarry is value for the money. I am not sure if he intends to make it trailerable but it would not be difficult.

    Rick W
     
  11. rwatson
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    rwatson Senior Member

    many thanks Rick - I was even a member of that group, but hadnt been back a long time.

    I would call it more the KSS method, than Origami, as Derek Kelsall ran the workshop that has prompted this build (as per Robs official web site), and Rob is using the exact same method.

    Thanks for pointing out that build, as there isnt any news of it on the http://www.harryproa.com/ site that I spotted.

    The points you make about Proas all ring a bell with me. Its a long road to acceptance as a "proper" boat, as sailors are a veeeery conservative bunch when it comes to parting with money.
     
  12. Guest625101138

    Guest625101138 Previous Member

    I have not sailed on a proa but I know Rob is a thoughtful guy who has tried a lot of things on the water. He makes much sense and I trust his observations. He does a good deal of experimenting on new ideas before he actually puts it into the design.

    From my experience the majority of people are driven by fashion and have difficulty assessing things on their merit. In my youth the RR symbol on a car could be used to sell junk. These days the BMW symbol has the same status although the cars remain good machines.

    I no longer worry about resale value on a boat. I would only spend the sort of money I am prepared to lose. Anything more creates more hassles in my life than I need.

    So a proa like Solitarry is one way to get an ocean capable boat that has real speed at a cost I would not be concerned about. It could be made trailerable so can be stored safely and transported by road to cruising grounds around the country. If I was looking at a sailing boat it would be my current first choice. I would like to shape the hulls a bit differently to Rob but they could be done with the origami method.

    One of the main things I have learnt with my pedal boats is that length does not correlate in any way to cost but it has a huge bearing on the speed. So I like hulls that are long and slender.

    I expect the yard that is doing the infusion for Rob could make flatpacks for Solitarry and transport them economically in Australia. I estimate the infusion table is about 6m long X 2.5m wide. Rob will get smarter with the layup to overcome some of the unplanned effort on the first build as well.

    Being so light and with little windage the auxiliary can be quite small. A little electric outboard with solar cells on the cabin top could do the job of getting into open water.

    Rick
     
  13. ancient kayaker
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    ancient kayaker aka Terry Haines

    Rick: thanks for your comments and information. As a canoe builder I appreciate the advantages of long narrow hulls with minimum wetted area. The improvement in drag with the mathematically-derived shaped hulls I have moved towards largely by trial and eror, compared with several factory-built molded plastic boats is surprising, at least at cruising speed.

    I found it advantageous to make them slightly asymmetrical fore-and-aft but that is more of a practical issue getting the smallest and lightest boat to support a given weight with a narrow entry without increasing the wetted area by moving the max beam slightly aft of center.

    However I wasn't aware that fore-and-aft symmetry actually was the lowest drag configuration, I assumed since sailboats are rarely if ever designed like that it was not beneficial: just hadn't really thought about I guess. Still learning!

    I have followed your posts on human powered craft with interest. You have used the cat format to reduce wetted area and maintain adequate stability. Has the discussion on proas changed your thinking towards an asymmetrical HP craft?
     
  14. Guest625101138

    Guest625101138 Previous Member

    I only have one catamaran pedal boat. Most are stabilised monuhulls of essentially unconstrained optimised length. That cats actually increase wetted surface over a monohull of equivalent total displacement and hullform by 26%. Actual power increase for my displacement is about 40%. So cats are not very desirable for a fast HPBs.

    The proa makes sense for a boat where you want a righting moment to counter sail loads but this is not the case with a prop powered vessel. The stabilised monohull is the best choice where there is no length constraint.

    Simple observation of the various rowing shells give a good indication of the easiest driven hulls for the various power levels available and the displacement. When someone wins an Olympic race in a shell with a transom I might be convinced otherwise.

    I do not know if the transom on a typical cruising cat actually offers any performance benefit. They provide a means of easily boarding from a dinghy or the shore if rudders fold. Beyond this I guess they encourage the stern to squat thus giving a bow up trim to counter the pitching moment from the sail. Light proas can achieve this by shifting the movable ballast aft but I suggest lifting flat sections on the bows would be another way.

    A 15m long hull displacing 550kg requires around 300N to drive it at 12kts. So the pitching moment from the sail required to drive it is not huge when it is countered by a 15m long hull.

    Rick
     

  15. sharpii2
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    sharpii2 Senior Member

    Tacking Proa concept calcs

    I have played with the concept of a 'Tacking Proa' (Single Outrigger).

    With the Pacific proa's, the windward float was floating ballast. It appears little effort was made to make such floats light. Weight was an advantage. The beauty of floating ballast is that it doesn't have to be very dense, unlike submerged ballast. Anything that gave the float sufficient mass would do. So a solid wood float worked quite nicely.

    With a 'Tacking Proa', or Single Outrigger, the situation is quite different. This is because the float changes sides every time the boat changes tacks. So, on one side, it is floating ballast. On the other it is buoyancy. To simplify the situation, I thought of the float as half of a small catamaran which had most of its weight in its two hulls.

    This works quite well for calculating the rig size. Say my float is to displace 300 lbs and have, say, 700 lbs of total volume, giving 400 lbs of reserve buoyancy. I take the lesser figure, the 300 lbs of weight, and double it. Then I take the maximum Beam of the boat and subtract half the Beam of the float and half the beam of the main hull from it.

    I then divide the remainder by two. I then multiply that by the 600 lbs to get my maximum righting moment. This divided by the height of the Center of Area of the sails dictates my sail plan.

    The weight of the main hull and the reserve buoyancy of the float, since it is greater than its weight, play no part in this calculation.

    So, compared to a catamaran with two hulls the same size, I am going to have to use a smaller rig per Beam.

    The rig size could be close to the same as for a trimaran with two 700 lb buoyancy floats if the over all Beam is near the same. This trimaran would be, in effect, a double outrigger in that The total volume of each float would most likely be less than the displacement of of the main hull. Most modern trimarans are not designed this way.

    The main advantage of a Single Outrigger would be that it would be a relatively inexpensive sea going boat where lower cost per payload and complexity trump higher performance. I can imagine storing water or food in the float to get its weight up, to be replaced with water ballast from the sea, as these stores are consumed. Since the weight (or the reserve buoyancy, if it is less) of the float will be the limiting factor in stability, I could probably get away with an unstayed rig, and not have to worry about overloading the boat, like I would with a more conventional multihull.
     
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