Multihull Structure Thoughts

Discussion in 'Multihulls' started by oldmulti, May 27, 2019.

  1. oldmulti
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    oldmulti Senior Member

    A small one on rigs. We are often trapped into the idea a rig should be an imitation of the racing rigs we see on most boats in our area. But it depends on what you want. If you are a racer the most efficient rig is a rigid wing sail as seen on AC boats are really good. If you are a genuine cruiser you want a simple efficient rig that requires minimum effort to get maybe 80% of the efficiency of a racing rig. Also, modern fractional sloop rigs require many sail options and much deck gear to optimise performance. A good cruising rig should be moderately efficient but not require many often not used sails or expensive deck gear to get reasonable performance. If the rig can be home built it also helps lower costs. The jpegs give you many options that are being given to home builders. Each has there advantages/disadvantages but each will work with less effort and or cost than a conventional fractional rig sloop. Some of these rigs work a lot better than people think.

    One example is the Gunter rig used on a KD 650 Kohler cat. “The idea for the Gunter rig is to have a cheap and powerful rig. The mast is a Al. tube, diameter 80/75mm. The gaff is made from spruce with some UD carbon. Shape +/- the gaff of the Vrijheid, but lighter and of course flexible. The difference to other Gunter rigs is in the hoist. The halyard is direct fastened to the upper slider on the gaff. Both gaff sliders are running in a rail. The upper gaff slider is detachable from the gaff. In this way setting and striking the sails is easy done. The reefing points are just under the lower gaff part. So to reef the sail is also an easy operation. The mast rotates free. This means a low load on the gaff sliders. Together with the sleeved upper part (on the gaff) the sail will be aerodynamic clean and very powerful. The round tube is a compromise, a more aerodynamic shaped leading edge would be better, but a round tube costs less.

    Also the following link will lead you to a Dick Newick cat ketch 48 ft MONOHULL cruiser (on page 90 design review), this PDF is 35 meg.

    http://docshare04.docshare.tips/files/29373/293739446.pdf
     

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    Last edited: Jan 23, 2020
  2. oldmulti
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    oldmulti Senior Member

    Carbon fibre masts are being developed by several home builders. I do not claim to understand how to design and calculate the structure of them, I can only suggest there are several designers who will sell plans for them.

    The general advice about carbon fibre masts is provided by a retired Naval Architect Eric Spondberg: “I can give you a few guidelines if you want do some calculations on your own. An equivalent carbon fiber mast will have the same section and wall thickness as the aluminium mast. The density of aluminium is 0.096 lbs/cu.in., and the density of carbon fiber laminate is about 0.057 lbs/cu.in. If laminated well, the strength will be about twice as high, but the stiffness (modulus of elasticity) will be about the same. If done poorly, strength may be equal to aluminum, and stiffness may be half of aluminium. Most rigs are stiffness critical, so modulus of elasticity is the driving factor. For given wall thickness, 80% should be unidirectional, and the remainder off-axis, split evenly between +/-45 deg and 90 deg. The layup should be a mirror image of itself through the centerplane of the laminate. That is, the layup should go +/-45; 90; 0; 90; +/-45 in the proportions described”.

    The simplest approach to fabricating carbon fibre (CF) masts I know is to use some form of male mould. Either EG for a smaller design a windsurfer mast with additional CF on the outside or custom shaped male cores of light timber or foam if they are going to be left in the mast. Another approach is to have a shaped male mould that is removed from the mast part before mast construction. Female moulds for mast construction are the optimum but building 40 foot long half moulds especially if they have tapered tops is a long process unless you do a flop mould from an existing mast.

    With your male mould as a core, lay a 300 gsm 45/45 e glass or CF cloth the mould then lay unidirectional CF along the mast and then tightly wrapping a layer of 300 gsm CF or e glass biax 45/45 around the masts full length. Sounds simple but this requires a lot of planning. There are products that can help, you can purchase CF socks which literally are CF tubes of knitted CF that can slide over a former to build CF bulk. You can premake small “panels” of unidirectional CF (especially for flat parts for wing masts) that can be added as the unidirectional CF component between the 45/45 layers. If the parts can be vacuum bagged or done using resin infusion your mast strength will be increased.

    Depending on the intended wall thickness of the mast layup, you may have to do the layup in 2 parts EG the first half the CF reinforcement is vacuum bagged or resin infusioned down and within 12 hours layup the second half of the CF reinforcement and vacuum bag or resin infusion that down. If you build the mast in parts eg 2 halves you have to join the “halves” of your mast together. This takes planning and either “joiner” pieces or overlaps of the 2 halves that can be “glued” together before you do a final CF wrap of the spar. The jpegs are of CF sleeves or sock and the male mould home build of a CF mast for a Farrier 32. Mast Building https://teamvmg.weebly.com/mast-building.html Finally if you build a male mould for a mast make sure you have the room to extract the mould from the mast if required.

    But there was an interesting article in Professional Boatbuilder magazine issue 174 starting on page 65 about Carbon fibre tubes and the companies build technique. Russell Brown uses this companies product for masts and booms of small dinghies. Professional BoatBuilder - 174 - Aug-Sep 2018 https://pbbackissues.advanced-pub.com/?issueID=174&pageID=65
     

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  3. catsketcher
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    catsketcher Senior Member

    I looked heavily into gunters for my 7 metre folding cat but decided not to go that way on the end. The main reason to do so was to reduce the torque required to raise the mast.

    By going to a gunter with a 3/4 rig the mast weight was lowered and the CG of the mast was reduced in height. These two factors reduced the torque required to raise mast to about 60% of a normal mast. But I didn;t do it.

    One reason was because I was tired of innovating. Another was that the rig proportions end up old fashioned. Kohler's gunter rig has a very low jib hoist. Today's fractional rig boats typically have 7/8 rigs rather than 3/4. The gunter has much lower hoists than 3/4 so you lose a lot of genoa sail area and efficiency.

    Then there is the problem of rigging a kite or screecher and sailing a multi downwind without a kite or screecher can be okay but to never be able to fly one is a bridge way too far. So I went the full hoist set up and am happy with it. If a boat is trailerable you can usually set up the trailer winch to raise the mast. The extra cost of the gunter could go into an electric trailer winch with a remote control. Then you get some extra help when retrieving as well.

    I was also worried about the gunter gaff swishing about a bit when hoisting the main, I never built it so I can't tell how it would have worked but I don't like big booms or gaffs whacking about when I am pulling up the main. Usually the boom is already in place when hoisting the main so this is less of an issue.

    On top of all that, the archetypal gunter boats - the Holt designed Heron and Mirror have gone to a full hoist mast. So there really is no good reason to go gunter.

    As for building your own mast - I built a strip cedar mast and it was a sort of fun to make but ended up heavier and much much more expensive than an alloy stick. I don't get why people avoid stays on masts. Big bridges resolve the compression and tension loads into different load paths (suspension, stayed truss, arch, girder, truss). Only when you are really needing to reduce drag - eg planes - does it make heaps of sense to go stayless and use a cantilever beam. You can reduce load dramatically by including tension members (stays) and keeping a compression member (ally section) in column with the stays. Then you get lots of nice hand holds ( I use my lowers as hand holds all the time) when walking around and the mast is able to take the load of screechers and kites. Like the gunter idea, a free standing rig has its drawbacks that I feel preclude it from being useful to someone like me - especially a cat sailor (when cats have little bury in the main beam) who hasn't got deep wallets but loves to sail fast in a commodious boat.

    cheers

    Phil
     
  4. oldmulti
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    oldmulti Senior Member

    This entry was meant to be a simple thing about early Telstar 26 foot trimarans manufactured by Tony Smith in Britain and the US. The 300 tris manufactured were basically 26 x 15.2 foot weighing 2800 lbs and carrying 320 square foot of sail area on a 29 foot mast. Most 26 footers could fold to 8 foot wide for trailing. As you will discover these dimensional numbers vary according to production time and build location. The tri was intended to be a bay and limited coastal cruiser in good weather. It has solid side decks. The accommodation for a 26 foot tri was good. The original versions could sail reasonably well with one finishing 10th in the 1978 round Britain race and one has crossed the Atlantic.

    OK now the interest. Tony Smith continuously developed the Telstar. What does this mean? The first boats were solid glass in the hulls and some parts of the underwing. All decks and cabin roof were balsa cored. In later boats the cabin roof was raised forward windows were changed. In later boats all decks (excluding the top side wing decks and cabin roof) were solid glass with stringer support. In the initial boats the folding float connectors were 6 mm mild steel, in later boats some components were stainless steel then they evolved to steel tubes with 1.5 mm walls. During this time the centre board and rudder arrangements were changed. The initial boats had the main hull go from the waterline to the wing in a straight line. Later models had a step in the main hull sides which provided more sitting headroom but caused resistance in wave conditions. Qualification. Any version of a Telstar if conservatively sailed as a bay sailor, occasional coastal cruiser and is well maintained will not cause any problems. So, if you intend to own a Telstar understand what you are buying because the next section could cause concern.

    The Telstar owners web site tells many stories. EG Advice to an owner who had a spongy foredeck. “You didn't mention where you are or which year/country 8M you own. If it is one of the nine built in the US before the factory burnt down, the ama underdecks are not a sandwich but are simply fiberglass reinforced with three large-diameter longitudinal PVC half-tubes glassed into place. Built in 1980 #207 is one of the last built in the UK and has the same deck construction and if there is no sign of actual damage, it needs a second flat layer of glass to provide the extra 2 dimensional stiffness ….”

    “I am in the process of molding new ama decks from glass and foam. The old decks balsa core is too far gone to fix. When I cut the old decks off I will take some pictures of the hull deck joints and the inner workings of the amas and post them.”

    Several owners wanted to replace or upgrade their rusted float mainhull connecting mechanisms

    On sailing “The increased main hull beam that Pat and Joe refer to is an abrupt widening amidships of the center hull found in mine other later boats. These "bulges" are a hydrodynamic abomination that make the boats slower than the earlier boats which did not have them. Fairing the fore and aft transitions of these bulges does help and I recommend it to all owners who have them.”

    And finally from an owner who had sailed from Florida to Cuba (122 miles) and started on the return journey “Winds were at 12 knots directly in our face, and the seas were 4-6 feet initially. We tacked for the first 20 miles until we hit a rainstorm. I dropped the main and pulled in the genoa. We motored for the next 5 miles through the storm in 8-10 foot seas and 20-25 knot winds. On the other side, the conditions were better for sailing. I hoisted the main and unfurled the genoa. The winds were 15 knots and I was trucking along at 6-7 under sail. It was great……..until the starboard leeward outrigger failed. The front pivot point, the large solid bolt and massive plate simply broke. This allowed the outrigger to bounce uncontrollably ripping the fiberglass from the aft part of the outrigger, separating the entire outrigger from the boat. Fiberglass was flying everywhere as I pulled the main and genoa.” He got the boat home after putting ropes around the broken hull and being very careful.
     

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  5. redreuben
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    redreuben redreuben

    Goodness me, 20-25 knots here in Freo is the afternoon sea breeze !
     
  6. Russell Brown
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    Russell Brown Senior Member

    I used to scorn those boats as a kid, but those photos show a pretty well thought-out folding system and structure. It must have been at least a decade ahead of Farrier's first boats.
     
  7. oldmulti
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    oldmulti Senior Member

    The Telstar 28 is 28 x 18 foot with a claimed 3200 lbs weight but actual built boats weighed on scales can be 4000 lbs. The T28 carries 250 square feet of canvas in the mainsail, and a total of 524 square feet when the genoa is included. Add a screacher and you get another 590 square feet of sail area. A retractable bowsprit is an option that will improve offwind performance and sail handling with the screacher. The main hull has a beam-to-length ratio of 8:1; that ratio for the new boat's amas is 15:1. The main hull new design has a semi-circular, teardrop shape, has a relatively sharp entry and flat run aft to a five-degree deadrise at the keel.

    The amas are connected to the center hull by way of 2' 6" boxes (mainly stainless steel frame and pins with fiberglass covering) at the bow and stern, that rotate 180 degrees in a lateral plane and move the amas outward over a five-foot span with the simple pull of a line. The design is more complex and sturdy than the original Telstar 26 folding system allowing bigger floats resulting in roughly 1,000 pounds of additional buoyancy.

    The interior of the T28 has six feet of standing headroom, the width of the main salon at the back of the settee is 7' 3" and the length measured on the center line from the foot of the companionway forward is 8' 9". A galley and settees port and starboard provide seating.

    The T28 was originally conceived to be solid glass but Smiths son influenced his father to move from traditional hand-laid, solid FRP to a vacuum-bagging then to a resin-infusion system. The advantage is the correct proportions of polyester resin to e glass fiberglass in the laminate that produce lightweight consistent laminates but strong sections. The T28 includes Kevlar in high-load areas in the interior of the outriggers as well as in stress points in the hull. Vacuum-infused laminates need special attention when it comes to a secondary bond. The T28's hull and deck are bonded together using Methacrylate adhesives. The joint is a simple outward turning flange with a riveted gunwale guard for additional strength. The galley, floor panels, nav station, and furniture were fabricated using resin-infusion. "The galley weighs only eight pounds."

    During testing in gusty conditions and 2 foot chop with wind speeds in the low 20s, carried a full main and partially furled genoa. Upwind the boat speed hovered between 7 and 7.5 knots within 30 degrees of the apparent wind. With eased sheets the GPS registered 9.2 knots of boat speed in 13 knots of wind. Maximum speed was 16 knots.

    Again, I say if the T28 is used appropriately and well maintained, one of the 90 T28 built, will be a reliable comfortable cruiser with good accommodation. But again, from the International Telstar Owners Association web site “T28 starboard aft failure. After evaluating the damage, it appears that the failure began at the starboard front pivot point. The large stainless rod broke just below the acorn nut. That destabilized the ama and allowed it to flex. The flexing then broke the flat stainless plate that the rod passes through and then the fiberglass on the ama began to break up because of excess movement. The cause of the failure was primarily rusting. Both the rod and the flat plate had obvious rust penetrating the metal. The force that caused the break was 20 knot winds under sail breaking through 10 foot waves. I believe if the stainless did not have rust penetrating it, the boat would have handled the winds and waves just fine."
     

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  8. oldmulti
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    oldmulti Senior Member

    With the development of foiling “rules” like speed versus length don’t work anymore. EG an 11 foot monohull Moth can top 25 knots in good conditions. The multihull equivalent is the excellent UFO catamaran foiler. The UFO is 10.5 x 5.6 foot and weighs 110 lbs carrying a 84 square foot rig on a 18.5 foot free standing mast above deck.

    Steve Clark the designer and builder says the UFO’s are capable of taking off for foiling in 8 knots of true wind with an average size person (less than150 lbs) and then can sail at speeds up to 20 knots plus depending on skill, sea conditions and wind conditions. At 8 knots and above, you'll expect to be flying consistently on all points of sail. Upwind, downwind, reaching UFO’s tacks though about 100 to 110 degrees no matter what the breeze. For a comparison, it’s around the same tacking angles you see in moth racing.

    But the really interesting aspect of the UFO is the overall design configuration and its structure. The design configuration is a small cat that is more stable than a monohull moth which allows those who are learning to sail the boat in non foiling mode. The foils are able to be raised or lowered without interfering with the rig, easily allowing the sailor to approach or leave shallow water or trailering. Now the structure. This design is very good. The main foil mount is also the strength point for the freestanding mast. The rear rudder foil is also mounted on the same central fore and aft structure that is the strong point for the main foil and mast.

    Infusions at Fulcrum (builder of the UFO) take place in the morning, so the two molds—hull and deck—were fully loaded with gelcoat and laminate stack the day before ready for the morning infusion. “Globally, the laminate is 10-oz (330 gsm) glass, 2mm Soric coremat (designed for infusion), 10-oz (300 gsm) glass, and that’s it,” Clark said. “In the beams, we increase thickness of core with a 1mm layer of core mat and reinforcements of carbon fiber, 300-g unidirectional carbon. In the deck there’s a double layer of glass where you sit.” An added complexity for the infusion is a thick mast pocket of 55-oz/sq-yd (1,851-g/m2) triaxial. More details at A Cleaner, Faster Infusion Shop - Professional BoatBuilder Magazine https://www.proboat.com/2020/01/a-cleaner-faster-infusion-shop/

    But the real issue with foiling boats are the foils. The hull structure needs to be appropriate but the foils need to be strong, consistent in shape and be able to take knocks etc. The vertical components of the foil/strut is an aluminium foil shape with 4.8 mm walls. The foils themselves are carbon fibre structures with the main foil having a flap attached. The joint between the vertical aluminium strut and the foils is critical to the success of the design. The connection between the height sensing wand and the foil flap also take time to develop to be reliable strong and function across a wind /sea state range. This is an excellent well thought out design.
     

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  9. oldmulti
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    oldmulti Senior Member

    The TS3 is a demountable catamaran that is 36 x 18 foot that weighs 4200 lbs and displaces 6000 lbs. The sail area on the wing mast is from 700 square foot to 1100 square foot with screacher. The 46.5 foot wing mast is in the centre of the boat. TS is short for Tres Simple (Very Simple in French). The boat is basically 2 hulls with minimal internal accommodation, 2 carbon fibre main crossbeams, a fore aft central spare for the forestay attachment and an accommodation nacelle that contains 2 double berths, seating and a basic galley. The accommodation cell has sitting headroom. The accommodation cell is a maximum size of 18 x 8 foot. The result is the total boat can be disassembled and towed on a trailer. It would need a crane to assemble and disassemble it.

    The TS3 is fast, in the initial tests in Lorient the TS3 reached 18 knots in 12 knots of wind speed. I could accept this with new sails and an empty boat with smooth water. The TS3 is built by resin infusion of glass, PVC foam and vinylester resin. The Carbon fibre box beams are full width of the boat and are strapped to the hulls at the gunnels. The jpegs give the concept etc.

    Now the reason for discussing this boat. This a good design that does sail well. The designers intended purpose for the boat is the problem. The architect Christophe Barreau wanted to build a 36 foot catamaran to pass through the Northwest Passage in a one-design sailboat race! “An old dream of traveling to Japan has been in my head for a while and as I like sailing in the polar regions, the obvious route is to go through the north. If we look at the maps, we see that the longest distance to cover without shelter on this route is around 250 NM. Given the accuracy of the weather forecast over 48 hours, it seems possible to envisage such a trip with a small fast boat without taking too many risks.”

    If you study the jpegs you see it has exposed steering seats, the “cockpit” floor is exposed netting and you have to move between eg the loo and accommodation nacelle via external paths. Not my idea of fun racing in the polar regions in an ice filled environment partially above the Arctic circle. I think this is a great design for warmer climates.
     

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  10. Adrian Culda
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    Adrian Culda New Member

    Any blueprints or info on larger tri's aroung the 75'ish mark ?
     
  11. oldmulti
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    oldmulti Senior Member

    Adrian. I just want to be sure we are talking about a boat similar to the jpegs below. The Neel 65 EVO. This boat costs about $US1.7 million in its basic format and my guess about $US2 million setup for full cruising. Assuming you will be building in fiberglass composite construction I will talk about production boats and what they take to build. Production (as in almost car production line) 45 to 48 foot cats take about 2000 man hours from start to water. These boats are built with full female moulds using resin infusion and preprepared standard furniture, electric wiring looms, known mechanical systems and very standard layouts. Very little customization. Dolphin 48 foot cats are built in semi production and allow customization of many components but still have standard hull and deck shapes from female and males moulds take on average 10,000 hours from start to water. Home built 48 foot cats vary enormously but 10,000 hours or 4 years full time would be a realistic minimum and you would be using a lot of premanufactured flat panels to make building easier. A 65 foot home built tri like the Neel 65 would take at least double that time and likely to be double the cost. Now I can give you some positive thoughts. There is a guy in Australia who built a 100 foot sailing cruiser cat in steel in 11 years from start to water. It took another few years to fit out fully. He was a school teacher and could work 60 hours a week on it and had 10 weeks a year holidays. Another guy built a 60 foot Crowther aluminium charter cat by himself. It took 4 years of which he had 6 days off during the entire time and he worked 12 hours/day. Some 65 foot plus Harryproa’s have been built in a reasonable time but they have less accommodation that a NEEL 65. If you have the resources and people that can cheaply help you build a 75 foot boat I fully support your efforts but please understand what you are taking on.
     

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  12. DGreenwood
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    DGreenwood Senior Member

    As a Project manager and, at times, a production manager, I have tried, through many rants on these pages, to get the reality of these numbers through to amateur builders. Build a kayak, or a dinghy first. Try to get a handle on how much difference having skills makes in the quality and speed of your build.
    Try to take in what multiple thousands of hours means in terms of working on weekends and evenings.
    Calculate what the realistic costs for materials and equipment mean in terms of your income and life style.
    The fact is, very few get past the talking about it stage. Of the ones that do, very few succeed. Of those that succeed, almost all start out with a cogent plan.

    Great thread by the way.
     
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  13. oldmulti
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    oldmulti Senior Member

    Whilst we are the theme of building boats, there are many ways to make it simpler and easier if you follow a few basic rules. One, learn to live in a smaller space. Two, buy a second hand boat of good quality. Three, understand there is a hell of a difference between a really good fitout and a basic fitout. I know one 60 ft cat that has $60,000 of crocodile skin upholstery. Also appreciate there are fast build designs and build techniques versus slow techniques.

    Now let’s break this down. A Neel 47 production trimaran (jpeg below) costs 30% ($500,000) of a Neel 65 Evo. Or if you need more room a Neel 51 costs 42% ($710,000) of Neel 65 Evo. Those extra 4 double berth cabins cost a lot of money. So does the bigger rig, winches, bigger ropes, anchors and longer chains, electronic, multi steering stations, extra heads, larger water and fuel tanks, bigger engines, more safety gear oh and also in the 65 ft boat probably a full-time crew to help maintain it.

    But you go faster in a bigger boat. Yes, often you can, but rarely. I sailed a lot on boats capable of 20 knots averages with full crews. When cruising with less crew we averaged 8 to 10 knots maximum. Fast boats in a seaway are just uncomfortable. When you slam into the back of a sea and the boat goes from 18 knots to 3 knots in a second you better be sleeping with your feet facing forward. Sore ankles are a lot better than a sore head.

    Be very assured, if you can get a good second hand boat you will be sailing a lot sooner and often at less cost than a home-built boat. I know there are many reasons to build your own boat but saving money is not the best reason. If you worked for 10 years extra and saved the cash you would go a long way to paying for a good second hand boat.

    The killer for most people is the fitout of a boat. One famous designer told me all he needed was a dry place to sleep, a good compass, a camp stove and good ice box and he could cruise the Australian coast if he had a good sailing boat. My partner wants a microwave, gas stove, 160 litre plus fridge, queen double berth, full dinette covered with non water absorbent material that doesn’t stick to your skin, full internet connectivity in all circumstances and if possible airconditioning. The boats sailing capability is my problem. The cost difference between these 2 approaches for the same size boat is massive. A full blown fitout can double your hull material costs and it WILL be a maintenance nightmare especially the more remote you get from major locations.

    Now we get to the basic design and material build. The less surface area you have to build the faster the build. The simpler the shape the easier to build. The larger the sheets of build material the faster and simpler to build. The less “parts” required to build the boat the faster the build. If you can “surface finish” the majority of the material panel structure prior to building you will save an enormous amount off fairing work. Aluminium boats are fast builds in larger sizes especially if you don’t fair or paint them before sailing (this is almost heretical to most home build people). Ply boats are also fairly fast if you have access to good quality timber and ply, but now a days they often are not cheap to build and have lousy resale value. Constant camber boats can be fast if they are simple shapes and not over loaded with accommodation. Male mould foam glass building is slow! You literally fair a boat 3 times. Flat panels resin infusion foam glass can be fast if you are well organised. If you have access to or have built a female mould for a fiberglass foam build the shell will be fast. But the real issue is the shell is often only 33% of the time of a build. The “good” internal fitout is the next 33% and the rig and mechanical systems etc is the final 33%.
     

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  14. rob denney
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    rob denney Senior Member

    That is to because of your materials and method choice. A satisfactory carbon mast can be easily built by anyone who can use a $150 vacuum pump and a pair of scissors. No other tools are required. It will be about 60-70% the weight of an alloy extrusion of similar strength and stiffness and cost about $Aus30/kg, $US9/pound. These are standard on Harryproas.

    1) Lighter overall as the entire boat does not need to be strong enough to take the rigging loads.
    2) The sail(s) can be raised, reefed and lowered on any point of sail. Hence they can be considerably larger, negating the need for extras.
    3) The main can be eased out to 90 degrees without wrapping around the stays. This allows sailing safely at much deeper angles.
    4) Nothing to adjust, break, wear out, maintain, replace or most importantly, worry about. The mast is inserted and forgotten about until it needs a repaint.
    5) The sail can be fully depowered, on any point of sail in any breeze. There is no need to think about luffing or bearing away in a squall, just ease, or dump, the sheet. This eliminates sailing under reduced sail at night and in squally weather., significantly improving daily averages.
    6) The mast bends under high loads making the first reef automatic.
    7) Gybing in a gale by allowing the sail to weathercock around the front of the mast is simple and safe. Compared to winching in the main, bearing away and having it crash across into the stays.
    8) No need to go on the foredeck, except to anchor and relax. Hence much less need for "handholds"
    9) Unimpeded vision all round the horizon. No blind spot behind the headsail, the direction from which another boat is most likely to collide with you.
    10)Extras can be flown, but not ones that require tight luffs, which are pretty rare on cruising multis for cost, weight and handling reasons.

    On the down side, there is no headsail to back to get the boat to tack. And no handholds, but far less need for them if you don't have to visit the foredeck to handle headsails.

    This is a fallacy. 15 years ago we built a mast for a Simpson 11m cat with 750mm/30" between the bearings. It is still standing.

    The Harryproa unstayed masts we are currently playing with have no sail track or sail cars, boom or deck bearings. They also have an optional removable auto aligning wing section foil. Fun times. Bit different to the traditional Marshall Islands canoe I have been sailing for the last couple of days! Description and video at Mini Cargo Ferry Prototype – HARRYPROA http://harryproa.com/?p=3155 in the next couple of days. Fascinating comparing it with the mini cargo proa prototype.
     
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  15. catsketcher
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    Location: Australia

    catsketcher Senior Member

    I don't need to get in a flame war with Rob but for those who are reading you can do a simple test on the difference between cantilever stress and compressive stress with a piece of stick - try and break it.

    No one tries to break a reasonable sized stick by pushing down along the length of it. Yet with little force you can easily break a stick by bending it between your fingers, or if it is bigger by sticking it under a foot and pulling upwards. This is cantilever beam bending. It puts a large stress within the structure of the cantilever - in this case the stick but it could be a mast or a bridge or a wing of a plane.

    So for most engineers the least stress in the structure occurs when the mast is put into compression and the tensile loads are taken by stays. Or you can insert a shear web into the cantilever and the shear loads within this will be high. This is one reason why planes have thickish wings - to get a tall enough shear web within the wing.

    But sailboats go much much slower than planes and so the drag penalty for stays is vastly reduced (Drag increases by the square of velocity so reducing velocity gives a much lower drag). Bridge designers know these things and so large bridges - subject to large forces are never simple beams or cantilevers but stayed trusses, suspension bridges or arch bridges where the tensile and compression loads are resolved into different pieces. Although it may look more complex, it is safer, cheaper and stronger to design a bridge with hundreds of wires (look at say the Anzac bridge) and attachments than to try to make a large bridge with a cantilever or simply supported beam. Simplicity is not always the best option.
     
    bajansailor likes this.
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