Pod cat square rigger cold weather cruiser project

Discussion in 'Multihulls' started by Autodafe, Feb 4, 2012.

  1. Autodafe
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    Autodafe Senior Member

    It is pretty much inevitable that an amateur build will end up heavier than the estimate, but as long as it comes under 1500kg (30% over calc) I will still have all the load capacity I want at DWL. I have a reasonable confidence in my structural calcs and the scantlings and weights that come out of them, but I haven't done a hugely detailed weight analysis, so my figures are only approximate. For the accurate number I'll wait for the crane driver to tell me.

    I can only assume that a lot of the weight in your formula 40's is structure to hold up a big highly tensioned rig in the middle of the boat. A 40 would have between 2 and 4 times as much sail as me, and would keep it up in higher winds, has tigher staying angles and mast compression in the middle of the beam.
    Richard Woods said of F 40 cats (on this forum) that "it was pretty easy to build under that [1800kg minimum] weight, even in wood".

    In 1974 Steve and Linda Dashew built a 38 foot plywood ocean cruising cat (Beowulf VI) that weighed under 1ton in cruising trim.

    Rob Denney's 12mm harry proa is 800kg light, and is designed for cruising.

    George
     
  2. Ilan Voyager
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    Ilan Voyager Senior Member

    About the F40. These boats had 90 m2 of sail (you won't try to move a 40 cruising cat with only 40 m2...). the rig was not highly tensioned, all the contrary, as the jib was very small 8 m2 and the mast rotating. In fact the speed and accelerations (1.65 g mesured) were the main factor of stress. The main beam, all carbon weighted 84 kg. No mistake eighty four kilogs. The boat was calculated with rather good data, obtained after a campaign of measures on "Festival de Lorient", not guessed...
    Data General is going well after 25 years. He raced heavily 3 years, and made fast cruising and charter after. No structural problems and have never needed a major refit.
    It is very reliable.
    I helped to make a strip plank/carbon F40 cat, I do know the problem. When the goal was to have a boat with ocean going capability the 1800 kg was not so easy to get if you want a good reliability. I dislike boats desintegrating somewhere between Ireland and France.
    If it's a kleenex boat just for regatta the 1800 kg was not a problem. The boat, like the IOR one toners, needs a refit after the first season, and it's garbage after the second.

    The F40 Adrenalin, made by the Gougeon Bros in strip plank/ carbon (cedar main hull, douglas fir amas) weighted around 1850 kg. Even with low volume (110%) amas the tri had structural problems and broke an ama articulation at Barcelone. And the Gougeon suffered to get the weight: after stratification of the inner carbon skin on the strips in the female mold, they had to router all the surface of the main hull from 1/2" to 3/8" thickness before strat of the carbon outer skin...

    Beowulf VI claim is inverifiable, and it's one of the impossible claims common in the 60's and 70's. Piver was a specialist of the thing, saleman's claims.

    Plus it's totally impossible to make a 38 feet cat of one ton in cruising trim if you want to survive to the Lake Geneva crossing by nice weather.
    Take a normal 38 feet design, create an Excel sheet, calculate all the involved surfaces, use normal scantlings with common techniques like plywood/glass/epoxy. Do not forget the weight of the final paint. No match: weight around 2.2 tons. The fact is there since 25 years.

    Let compare apples with apples a proa is an orange...

    Comming back to your CRUISE cat, with well proven, safe and long lasting scantlings, correct amenities and even a heating system. Correct water tanks, normal fuel tanks etc...etc...Fill the Excel sheet carefully. It's a tedious job as you have also to put the center of gravity coincidating with the center of displacement...Do not forget the pans, glasses and forks. You'll be surprised by the result. I have 35 years of experience on this exercice from prams to warships, and more the cruising boat is smaller more the job is tedious. Bow down boats are ridiculous, bow to the sky rather ugly and the water arriving 15 cm over the LWL line at the launch is disgusting.

    The problem is that your weight is irrealistic. Just work the subject. Never wait the weight from the crane driver. IT'S TOO LATE!!! All the investment thrown in case of big mistake. Do not forget: scantlings are calculated from the stresses derivated from the weight...if the weight is mistaken, the scantlings are also...
     
  3. spidennis
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    spidennis Chief Sawdust Sweeper

    I came across this photos saved to a folder for my reference, and I believe it's similar to what you're doing? Not quite but is is a center pod, biplane rig so maybe there's something for ideas?
     

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  4. Autodafe
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    Autodafe Senior Member

    Ilan Voyager, I appreciate your concern. If there is a fundamental flaw then now is the time to deal with it.
    I'm looking to build a simple, reliable cruiser, not a one season lightweight. However, I also want to understand the forces acting on a cat at sea, and how to minimise them through design. If I simply copy the designs of others I've learnt nothing.

    Thank you for the 1.65g - measured numbers are very hard to find. I doubt a cruising vessel will push hard enough to ever see that, but I assumed 2g in my calcs, plus material strength factor of 3-8 (depending on the material).

    I was exaggerating my sail area down a bit - it won't be 40m2, it will be 50m2. SA/wetted area and SA/Disp are then 2.7 and 26 respectively at full load, which seem fine to me. Downwind I can improve the SA/WA a bit by lifting the boards.

    Your comment on engines and fuel allows me to clear up one area - I'm not planning to carry either, except for heating.
    Electric maneuvering thrusters running of the house battery bank is the current plan. If that doesn't work, a 10hp outboard with a few hours of petrol.

    Comparing to existing proven designs is something I have done many times. Consider a very approximate case: Take a standard performance cruising 40' cat, say 5t empty. Chuck out the engines, fuel tanks and associated paraphernalia, 1t lighter. Reduce the plating area of the hulls by 60% by removing all in hull accommodation - another ton saved. Reduce the full bridgedeck to a 30% bridgedeck pod, with associated reduction in surface area. 700kg in the bank. Now consider the weight of all the bunks, heads, shelves, benches, sinks, washing machines etc. you no longer have room for. Another ~500kg. Because the boat is now less than half the weight, both rig and beams can be lighter. Say 500kg between them. (Even more if you make the effort to use a low-stress rig mounted on the hulls.)
    Final weight 1400kg, and that without reducing any local scantlings to take advantage of the new low weight.

    Possible flies in the ointment:
    - if the low profile hulls are no longer up to the global loads. As far as my maths can tell they're fine, particularly with the pod taking the lions share of the torsion load.
    - if you want to carry a lot of stuff and or crew and need the podgy hulls and extra space.

    I don't quite follow your closing remark. From a structural point of view, a vessel with a 2.5ton maximum load displacement that weighs 1200kg empty is very similar to a 2.5ton maximum load displacement vessel that weighs 1500kg empty.

    Assuming I'm completely wrong, and the boat ends up sailing like a limp noodle, then worst case scenario I've learnt a lot about boat design and building and I'm $15k out of pocket for non-reusable materials. Sad, but not tragic.

    George
     
  5. Autodafe
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    Autodafe Senior Member

    Spidennis,
    Thanks. Those look like the "Future" series Pogo referred to earlier.
    Cheers,
    George
     
  6. Ilan Voyager
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    Ilan Voyager Senior Member

    A bank battery able to feed engines is very heavy. Forget it, and you'll need a genset.
    A 10 HP outboard is not enough, it will die of exhaustion. You'll need or 2 10 HP or a 25 HP. Multis have a lot of windage and it's the main factor.

    Remake your calculations, your assumptions are, lets say, very simplified and simple hard guessing...The 2.2 tons is for a boat without diesels and big things... A part the financial loss (15K is very optimistic) you'll loose at least 2000 hours of hard work...

    Make the sheets carefully. Simply because lifes are implied...A factor of 3 to 8 over the 2g is simply exagerated if the calculations are done correctly.

    On a light cruise cat the accelerations range from 0.65 to 0.8. A total coefficient of 3 is largely enough if you are able to calculate correctly the static stresses. For small boats it's high school maths. Many good books on the subject.

    About my former post: you said 1200 Kg, I say more probable 2200Kg that's almost twice...There is a fundamental contradiction if you use a coefficient from 6 to 16 ( 2g multiplied by 3 to 8) how do you obtain 1200 Kg. The use of so high coefficients would lead to a overbuilt and heavy boat. May we know the materials and the scantlings?

    Last question: how many boats have you built?
     
  7. Richard Woods
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    Richard Woods Woods Designs

    Actually not quite true. Worst case scenario is you're dead. Maybe sad and tragic.

    I think you're being very brave to try something no professional designer has done.

    It seems odd that the only time you say you'll go outside is to reef. Most people want to do that from a protected area. Similarly having a sliding opening door at the front is asking for trouble

    I assume you aren't going to ever go into marinas, and that you will be singlehanded.

    I remember asking Pete Goss whether they would make a smaller version of Team Phillips as a test bed. No need he said. Years ago I raced against a monohull with a rig like yours (only one mast though). Before the event the owner boasted that it was a break though design. He finished last in the first leg, retired in the second and the mast broke in the third.

    Windage isn't just frontal area. Streamlining helps. look at the drag for a round wire. One reason why the clipper ships were a breakthough design was because they started using wire not rope shrouds.

    Phil Morrison, who designed QAB2, told me the boat was way under weight. That boat made Atlantic crossings. I think it used 8mm cedar with thin glass skins. Not sure if he meant as launched or as raced. Otherwise I agree with you, Ilanvoyager

    Sorry to be negative

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  8. Ilan Voyager
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    Ilan Voyager Senior Member

    "I remember asking Pete Goss whether they would make a smaller version of Team Phillips as a test bed. No need he said." Laughs. We know how it desintegrated in 2000, after disastrous trials where they lost a bow and had to add a lot of structure. Classical example of faulty engineering.
    NA Adrian Thomson has a too long list of "super high advanced" boats that ended in pieces. The most incredible is that he finds always a client to risk his money. How does he do? hypnotising them? Goss is an experienced guy not a little boy...Thomson must have the Khaa ( the snake in The Jungle's Book) secret technique...Last victim Alinghi.
    Sebago was properly terrifying and Team Phillips was a bad horror movie... All very very very light...and extremely under-engineered.
    Phil Morrison is in another league, but more in dinghies. None of his multis has been a major. Irens has a long, very long list of highly successfull multis and none of them has been extremely light...A broken boat does not win the race.
    Yes QAB2 was a high sea boat; very thin skins can work. But such thin skins lack durability, with a possibility of water migration in the core, and are fragile to any shock.
    Date General could have been made at 1600 kg, but the boat having to be dismounted and to travel on truck it has been preferred to give an extra strength to the outer skin. Also Irens made the choice of relatively high freeboards (ie more surface and more weight) as at the beginning high sea races were in the program of the F40. The cat is "dry" compared to the tri Biscuits Cantreau where you are permantly in a diluvian shower. The boat being in good shape after 25 years demonstrates that it was a good choice.
    The 2560 kg of "Renaulac" show the result when 2 NA (Finot and Conq) and a shipyard Beneteau, without experience of multis, try to design and build one...

    I'm also deeply sorry to be so negative about the boat of Autodafe, but he can be burnt in the affair. And I'm not the Inquisition, but such a thing like a 1200 kg 40' cruise cat can't be let wandering without reaction.
     
  9. Richard Woods
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    Richard Woods Woods Designs

    I agree entirely with your Adrian Thomson comments. Alices Mirror, Paragon and the F40 foiler can be added to the list

    Not sure you're quite right about Phil's designs. Didn't one win an OSTAR? and another come second/third in a RBR??

    I also agree with your last sentence. You reckon 2200kgs, I say more

    Richard Woods
     
  10. Autodafe
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    Autodafe Senior Member

    Hi Richard
    Glad to have your voice of experience, no matter how negative.

    If I die, you have my permission to say "I told you so" :)

    Seriously, there are many static load and inshore tests that I can do that will show up design or construction weaknesses.
    I assure you all that I'm not going to sail into the blue yonder the day after I finish building a boat that I started off by describing as an "odd prototype".

    Your correct about usage - mostly singlehanded, very few marinas.

    Most of the cruising boats I've looked at still require a person to go to the mast to reef, not just mine. Running reefing lines inside seems like complexity looking to fail. Am I missing something?

    Streamlining is good, but it's a compromise too. The classic diagram of two cross-sections having the same drag, but the streamlined one having 40 times the frontal area is great in theory, but shaky in practice. If you want to keep the same level of comfort inside it seems impossible to streamline without increasing weight (bigger surface area) and frontal area. If the streamlining doesn't work for any reason (eg someone mounts window shades, or stows a dinghy on the foredeck) then you've just taken two big steps backwards for little gain.
    My approach has been to keep the shapes as compact as possible and then just round off the corners, aiming for a Cd of ~0.5 with low frontal area and minimum weight.

    IlanV,
    I've never previously designed or built a boat over 16'. So what? I'm not selling my advice or design, and I'm not claiming any unique insight.

    Basic scantlings:
    Hull 9.5mm gaboon marine ply with 450g dbl bias on each side. 4 ply bulkheads and 4 ring frames per hull, but (due to the high number of "chines" around the hull) no stringers. Chines composite epoxy radius with most re-enforcing from the 450gsm sheath.
    Pod (subject to change depending on insulation design) 9.5mm ply front and floor, 200gsm cloth both sides. Sides and roof 6mm ply, 200gsm cloth outside. Frames 50x12mm timber at 400mm centers.
    Beams x2 solid laminated timber arranged in triangulated planar truss as shown in previously posted diagram - 70kg/beam, average 50cm2 of timber for each individual strut.

    As far as the 2g, I'm talking total, not over and above the static 1g - you seem to have a different convention otherwise the 0.6-0.85 you mention for cruisers makes no sense.
    Another point is where the force is applied. Design requirements for straight vertical acceleration with both hulls in full contact with the water are quite low. If the acceleration is being produced by an unbalanced force acting on the bows (as it almost always will be) the the relation between magnitude of force and acceleration depends on many other factors including bow shape, pitch and roll inertia etc...

    If you can provide me with a good way to approximate these accurately then I'm in your debt. No other person I've asked or book I've read can.

    The best I've managed is a crude approximation:
    bending the hulls are supported
    a) at midpoint only (hogging)
    b) at ends only (sagging)
    c) by one bow and one stern (wracking)
    d) by one hull "clamped" to some fixed object (lateral bending)
    In each case I use the vessel displacement x2 (2g acceleration) and add rigging loads based on RM.
    In cases b and c I assumed the forces are applied over some length of hull required for buoyancy and hydrodynamic forces to be applied, with a net result that the applied force was ~1m from the ends of the hull in these cases.

    Anyone who has a better approach, or just wants to redo my calculations please feel free to share.

    Regarding batteries and aux power - the weight of batteries depends on the required range.
    If there is enough wind to overpower a 10hp outboard (or similar total wattage of electric motors) then I should be either sailing or at anchor.

    This post is getting long. I'd better leave it here.


    Given all the concern I'll re-do my structural calcs from scratch in case I was hallucinating the week I did them.
    George
     
  11. Ilan Voyager
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    Ilan Voyager Senior Member

    I recognize that I had a memory hole about Morrison. He got results. But Nigel and the duo LPVP are above the others. Nigel has been designing winners for almost 30 years.

    2200 kg if made by an experienced guy, along with a very good choice of materials.
    A careful amateur, who respects the plans and do not add extra layers and do not mess with the epoxy will be around 2400. After....
     
  12. Timothy
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    Timothy Senior Member

    Jan Gougeons new boat Project x "Sticks" is a 40' folding pod cat with basic cruising amenities and weighs less than 2000 lbs. LOA 39' 7"
    Beam 14'
    Draft - boards up 9"
    Draft - boards down 4'9"
    Displacement 2,000 lbs
    Sail area upwind 575 sq ft
    Sail area downwind 1,000 sq ft
     
  13. Autodafe
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    Autodafe Senior Member

    Nice boat.
    I saw a picture of it at launch but I hadn't seen any specs before. Thanks
     
  14. spidennis
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    spidennis Chief Sawdust Sweeper

    Strings .......
     

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  15. Autodafe
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    Autodafe Senior Member

    Jan's use of planar truss cross beams gives me a lot more confidence that mine will work.
    I've always wondered why no multi's seemed to use them. For pod cats at least they seem to make excellent sense.
     
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