Multihull Structure Thoughts

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

  1. cavalier mk2
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    cavalier mk2 Senior Member

    Finding 2 books on the school library shelf completely converted me to the idea of multihulls, Rudy Choy's "Catamarans Offshore" and Mike McMullen's "Multihull Seamanship". The third confirmation was Rob James "Multihulls Offshore". Arthur Piver's books are hilarious even if they weren't meant to be. Lots of good reads out there. Life wasn't complete without a copy of Jim Brown's "Case for the Cruising Trimaran" or the latest Wharram catalog.

    Funnily enough, as current cats keep plumping out just about any of Rudy's boats could paste them.
     
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  2. oldmulti
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    oldmulti Senior Member

    The following cats are being used as an example of the difference between production cats and possible “home built” options. The cat is the Excess 15 which is 48.5 x 26.33 foot and weighs 42,000 lbs and is capable of carrying a 20,000 lbs plus payload. Its standard rig has a 75 foot mast with upwind sail area of 1715 square foot, its “Pulse” rig has an 81 foot mast with sail area of 1850 square foot. The hull length to beam is about 8:1 at the waterline.

    The comparison boat is Grainger’s Raku 52 which is 52 x 27.25 foot. It weighs between 17,500 lbs and 22,500 lbs depending on fit out and construction materials / build technique. The Raku 52 displaces 32,500 lbs at recommended maximum displacement or 10,000 lbs payload. It has a 72 foot mast and carries upwind 1520 square foot of sail. The hull length to beam is about 14:1.

    Let’s look at the payload for people who really sail, not just charter a boat. Assume 4 people for 3 weeks sailing across the Atlantic with a 25% safety factor. Water 2000 lbs, people plus personnel gear 300 lbs/person = 1200 lbs, fuel 1000 lbs, stuff (electronic games, books, toys, surf boards canoes etc) 1000 lbs, food and supplies 1000 lbs, extra deck gear and spares 1000 lbs, batteries solar etc 500 lbs, engines 600 lbs, dingy 200 lbs and we still have 1500 lbs to spare.

    Lets look at performance. At full displacement the Excess 15 has a bruce number (BN) of 1.06 and a 240 mile/day capability (calculated from boat stats). The Raku 52 has a BN of 1.22 and a 280 mile/day capability. At built weight (no payload) the Excess 15 has a BN of 1.19 versus the Raku 52 BN of 1.38. Translation the Raku 52 is 16% faster across the board. The peak speed of the Raku 52 would be 10 knots faster than the Excess 15.

    So, if we build a Raku 52 we reduce the build weight, reduce the payload needed and carry less sail we end up with a boat 16% faster. Also, the Raku 52 uses similar build materials to the Excess 15 but just uses less of them. Translation less cost in build materials and rig. Finally, both designs have similar accommodation plans but the Raku 52 does not have walk around beds.

    The choice is yours; do you want a boat that sails to locations with minimal sea time or do you want a floating hotel room that changes views occasionally. Both boats are very capable of achieving their intended outcome. You take your choice.
     

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  3. oldmulti
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    oldmulti Senior Member

    Occasionally its useful to look at alternate boats to understand what modern boat building is being done and why. Let’s look at marlin fishing boats. These are serious powered monohulls from 50 to 90 foot long that want speed and durability to get to the fishing ground and spend days chasing the elusive fish. A little history first, most Marlin boats were built of wood and plywood then when E fiberglass and polyester was introduced boats were developed. From the late 1960’s the patent of carbon-fiber material for commercial use, technologically advanced resins and epoxies, new fabric weaves, better strength-to-weight ratios and the advent of Everett Pearson's Seemann Composites Resin Infusion Molding Process (SCRIMP) system of vacuum bagging and resin infusion etc allowed the development of today's high-tech, state-of-the-art functional fishing machines.

    Some examples, Jarrett Bay boats meet an owner who desire for speed and low decibel levels needed research. The research indicated a full carbon-fiber package would yield the best speed, with the level of sound-dampening material needed to achieve the desired decibel levels, the weight savings would be eliminated, and the boat would actually be heavier. “We came up with a hybrid method for the hull skin, sides and transom by building them out of cold-molded product made out of plywood and epoxy,” Davis notes. “The wood acts as a fairly good sound insulator too.” But, all the stringers, bulkheads, decks and everything from there on are foam-cored, with carbon-fiber-infused skins. “In the case of all the topside stuff, we’ve taken the E-glass out and replaced it with carbon fiber,” Davis says. “On the hull itself, we do have the E-glass laminate on the hull, inside and out, but after that, and with a few exceptions, everything else is a carbon-infused lamination.” As far as mounting those new high-horsepower engines, one of the headaches with carbon fiber is the absolute need to isolate it from metals to prevent galvanic corrosion. The main stringers where the engines sit have a higher-density foam core and an overlay of carbon fiber. Over that is a welded aluminum cap that is properly sized to accept the motor mounts. To isolate it all, a layer of E-glass was applied before the paint. And to make sure the through-bolts would not come in contact with the carbon fiber, a compression sleeve was inserted. “Everything you see mounted on that boat, and everywhere that particular material is used across the board, we use a compression or grip sleeve to make sure there is no chance of any contact with a screw, bolt or metal component to set up galvanic corrosion,” Davis says. “This does complicate the build, but we made it happen.”

    Bayliss Boatworks also think triple-ply Okoume hulls have the best sound-deadening capability. The hulls are glassed inside and out, and use epoxy resin throughout the build, the construction process will vary with the building materials used. During wood construction, for example, a specific West System epoxy will be applied. "When we get to the cabin, superstructure, flybridge and console, we use Core-Cell foam that is glassed on both sides," Bayliss notes. "We still use Philippine mahogany for the framework in the overhead of the cabin, which provides room for air ducts, wiring bundles, lighting and so on." To further weight savings, Bayliss has replaced his decking material with a very durable 3M honeycomb core, and all bulkheads are a combination of marine plywood and Core-Cell, glassed on both sides. “Our boats are really quiet, ride very well and fish like they are expected to,”

    Spencer first began with composite construction, the company engaged structural engineers to supply most of the data. In addition, with the all-important resin companies, the builder also consulted on which were the best formulas to use for resin infusion, given the higher temperatures. “We use post-curing for our builds, and will actually cook the whole hull, cabin and bridge in an autoclave oven in order to gain the full benefit of the product,” Spencer says. “When we cure at 200 to 250 degrees, the result is a harder, more stable outcome, without any of the post-cure issues like print-through. This way, we are able to take full advantage of these high-tech resins.” And while carbon fiber does have its drawbacks — the aforementioned need for isolation, for example — there are many other uses where strength and weight savings are needed. “We don’t do the hull, but with the bulkheads and decks where you would need, let’s say, 20 ounces of E-glass, it only takes 10 ounces of carbon fiber and half the amount of resin to saturate it,” he points out. “The weight savings can be significant.”

    ACY yachts. Back in the 1980s, LaCombe began using carbon fiber to reinforce the deck beams, as well as replacing the fiberglass fabric with Kevlar for added strength and reduced weight. “The Kevlar was really good for collision bulkheads,” he notes. LaCombe’s take on working with carbon fiber echoes everyone else’s concern about the material: “Think about it as being a sheet of steel,” he says. “If you put a screw in it, you’re going to have to isolate those two metals.” And with that in mind, ACY makes absolutely sure that whenever carbon fiber is used, special attention is given to protection and isolation.

    Translation: Think about the total build and its intended use. Some times “saving weight at all costs” can lead to a noisy expensive hull structure that requires special isolation tactics to prevent corrosion etc. One of the great joys of constant camber wood building techniques is that you end up with an easy to build hull, that is quite and pretty bullet proof especially if done in WEST epoxies etc.

    One of the nosiest boats I have been on was a 36 foot power cat anchored. It had a step in the solid glass hull 50 mm above the water. Ripples hit the step whilst parked and the resulting sound echoed through the hulls. A new bang every 20 seconds is not good. The jpegs show a cold molded wood hull with glass bulkheads.
     

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  4. oldmulti
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    oldmulti Senior Member

    The following is a book that is aimed at people who wish to design or understand the design of yachts and small boats in general. The Elements of Boat Strength for Builders Designers and Owners. The best description of the book is provided by Dave Gerr the author on his website at Books https://www.gerrmarine.com/Books.html and look at ELEMENTS OF BOAT STRENGTH https://www.gerrmarine.com/ELEMENTS_OF_BOAT_STRENGTH.html

    As well as the 43 page presentation made to 2012 IBEX by Dave Gerr https://www.gerrmarine.com/ELEMENTS_OF_BOAT_STRENGTH/BoatStrengthIBEX.pdf

    This is a good document that is worth chasing up. It has some references to multi’s but its mainly about mono’s but the design information is applicable to all. The cover off the book is shown below IT WILL NOT connect to any source of the book.
     

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  5. oldmulti
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    oldmulti Senior Member

    The Euro 1460 is a new custom Schionning design commissioned by a client wanting to think outside the box and utilise the bi-plane rig on a commercial vessel. The boat is 48.5 x 26 foot weighing 22500 lbs, the displacement has not been defined but at a guess is 33,000 lbs. The bi-plane rig means easy sailing and sail handling with minimal sail controls cluttering the deck space. The mainsails are 590 square foot and the jibs 225 square foot. As the biplane rig with a total sail area upwind that could be 1630 square foot. The hulls L:B 12:1. The wing deck clearance is 2.65 foot. This boat’s performance will be reasonably good if its kept light but the bridge deck cabin and the biplane rig will mean its upwind performance will need to be developed to get the best out of the cat. Reaching and running especially will be good.

    The concept suits commercial work well due to it's simplicity and the outward situated masts mean that the interior bridge deck space can be utilised more effectively. The interior layout of the Euro 1460 features a large main cabin with ensuite situated forward on the bridgedeck, with a walk-around galley behind leading out to a large cockpit with plenty of seating for guests. Two queen cabins are located down in each hull, with their own toilet and shower and a smaller berth forward. This layout would suit skippered chartering with crew occupying one cabin and guests in the main cabin, or any desired combination. Now this cat could be modified to have cruising accommodation of a 60 footer in a 48 foot package judging by the main cabin size.

    The construction will be standard Schionning foam or duflex glass flat panel approach with with carbon fibre in cross beams and CF around the edges of bulkheads. The biplane rig will have carbon fibre masts. This is an initial design and more detail will be coming from Schionning. This could be interesting.
     

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  6. oldmulti
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    oldmulti Senior Member

    The following graphs are from 2 multihull designers showing build time from scratch versus size of boat. The lower limit of hours is for a kit built or flat panel construction technique and simple fit out. The upper hours for a given size is for a shapely hull with full fit out. Notice the hour difference for the same size of boat, often the full fit out shapely boat takes 50 % plus more tome. Do not underestimate the work involved.

    Also, a quote from HH cats about one aspect of their cats: “For example - the HH66 has incredible 270kg (600lb) x 6m (20’) long daggerboards that are built from aerospace technology carbon and cost over $200,000 each. We need to know they are built perfectly and will withstand whatever they may encounter. To test each dagger board, we designed an impressive test jig that applies a massive 17,000kg (37,500lb) force – equal to twice the maximum working load.”

    Also, the jpegs indicate the volume of items in a kit of all sizes. If you are building from scratch you have to create all these components to start with. Schoinning are now suggesting you cannot build a lot of their boats from plans unless you have a kit to start with. From Schoinning site: “PLEASE BE AWARE THAT THE MAJORITY OF OUR DESIGNS REQUIRE THE PRE-CUT KIT AND CANNOT BE CONSTRUCTED USING THE PLANS ALONE.”

    Please do not underestimate the cost and time to build large boats or overestimate your abilities.
     

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  7. rob denney
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    rob denney Senior Member

    What they should be thinking about is the effects of impact with an immovable object at 20+ knots. There are 2 possibilities: 1) the board breaks, and if the object is floating (whale, shark, container, log) it then removes the rudder and the saildrive/propellor. 2, (even scarier), the board doesn't break, in which case, the boat comes to a halt. Anyone not strapped in flies forward until they hit something or go overboard. Google car crash tests 25 mph to see the result. Remove the airbags, seat belts and crumple zones, to get an idea of the result on a 66' cat.
    Daggerboards on high speed cats should be illegal, especially as it is so easy to make them kick up.

    Intelligent Infusion is a far simpler, cheaper and quicker way to build boats than paying someone else to build kit components, which then have to be laboriously wet laminated together.
     
  8. oldmulti
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    oldmulti Senior Member

    Kurt Diekmann is an air plane constructor and yacht designer from Munich Germany. His tri Archeopterix was built in 1999 and has crossed the Atlantic with its good sailing performance. The tri is 32.75 x 20.35 foot capable of folding to 7.5 foot wide (whilst on the water). The draft ranges from 5.5 foot to 1.5 foot. The tri displaces (probably weighs) 4500 lbs. The sail is a sloop with a mainsail of 290 square foot (has 3 reefing points) and a furling jib of 215 square foot sail area. A 33 square foot storm jib is available with a sea anchor: a 280 square foot parachute is used as a sea anchor and holds the bow into the wind in any condition. There is no traveller in the cockpit to take up space, instead the mainsail is controlled by 2 sheets, each with a triple pulley setup for perfect control. The engine is a Yamaha F9.9 AEML high-thrust outboard in a center well location.

    The construction is Marine Plywood with Epoxy West System, and a light fiberglass external finish painted in white epoxy paint. But the absolute joy of this tri the simplicity of construction. Look at the internal jpegs. Minimal framing and stringers depending on bulkheads and internal furniture for reinforcing of the “monocoque” hull structure. The hull shape does a lot of the longitudinal hull reinforcing with its fiberglass taped chine lines. My GUESS is the hulls are from good quality 9 mm plywood. The cross arms are simple aluminium mast sections with a simple to construct folding system. Judging from the jpegs its effective as well. This keeps things simple to build and maintain allowing lots of time for exploring and sailing. The initial tri was built in 2 years part time, a second boat was done in 1 year 4 months from start to finish by mainly one guy.

    The boat has two cabins divided by a center cockpit. The main cabin has a v-berth with a spacious double berth. The aft cabin has two single bunks for visitors. So, the boat sleeps 4 persons comfortable plus 3 more if necessary. The galley has a LPG stove with 2 burners. A real household sink makes cleaning dishes a simple task.

    A Buegel-anchor (16kg) with 20 meters of 8mm chain and 40 meters of 14mm Nylon line make anchoring easy in almost any ground. A 10kg Britany-anchor with 10 meters of chain. Anchoring line 50 meters, 14mm Nylon. Third anchor Danforth-folding-aluminium.

    This tri has been designed by a person who understands light structures and effective build techniques to achieve those structures. I wish there were more designers had a similar approach to design to achieve light simple “cheap” boats that can be built in a reasonable time.
     

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  9. catsketcher
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    catsketcher Senior Member

    Thanks for that Old Multi

    I like the use of of ply with no stringers. It is such a fun material to work with (I am using it at the moment). The chined hull shape gives rigidity and some useful interior room. There does not seem to be any redundancy in the beams, they seem totally dependent on the underwires, so I hope these are very sturdy and large in diameter. Underwires should be sized for stretch, not tensile strength (IMHO). I never really understood why Lock Crowther put an arc in the Twiggy beams, or why Schionning has curved forebeams. A compression strut should be straight to take more load, as these beams are. I would have liked the designer to get the French curves out instead of the ruler, some people like Hereschoff, Frers and Kantola can make such sweet sheer and cabin lines that require no more build time.
     
  10. cavalier mk2
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    cavalier mk2 Senior Member

    Shades of a Newick Argonauta or Trice 3, without the curves... seems practical though.
     
  11. oldmulti
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    oldmulti Senior Member

    Timberwolf is one of those tri’s you would like to own for a short time. Why? Top speed 31 knots so far. Boat speed equals or exceeds wind speed across most of the wind range. OK. The tri started as a Formula 28 but ended up in its initial configuration being 30 x 25 foot weighing 1500 lbs without any eg sails etc on board. Its sailing displacement is 2400 lbs. It carries 600 square foot of sail up wind on a ply timber CF reinforced wing mast. The initial Bruce number was 1.85. This is a very fast boat.

    Dr Chris Cochrane, who is GP, designed and built the tri in 18 months part time from 1999. He claims he has a little experience in boat building. The initial build is mainly ply with timber frames and stringers. The main hull is 2 layers of 3 mm covered with a light external glass, over close set stringers and ply bulkheads. The deck is reinforced ply with glass. The floats were tortured ply again covered with light glass. The cross arms are 6 mm ply box over a timber structure. Carbon fibre is used to reinforce the cross arms to support up to 9000 kgs. The wing mast is a ply/timber/glass structure. Now you have a very fast tri that only cost $24,000 NZ. Pity, 20 knots plus is not fast enough. So, you have to improve its performance.

    The upgrade of Timberwolf started in 2011. The floats were replaced with 35 foot long carbon fibre foam structure with reverse bows. The cross beams were beefed up. The mast was changed to a bigger carbon fibre wing mast with new fibre rigging. New bigger sails etc. New carbon fibre foils. Now we have a 30 knot plus boat.

    I don’t know what NZ people are taught at school but they all seem to be multiskilled in some industry and boat design/building. Interesting boat. The jpegs are of the latest version. The mag pages are of the initial build. The PDF is of the later version and has many photo’s.
     

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  12. Slingshot
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    Slingshot Junior Member


    Here are two examples one I have seen 1st hand and the 2nd from the Facebook post.

    The looping 49 (Patrick Luscher design) sistership to this I went on in French polynesia the owner built and launched (mostly by himself) the boat in 14 months. The boat had not been faired anywhere, but everything was in place and he sailed from France and was enjoying Tahiti very much for 160K Euros. In the trade winds he said the boat went down wind at an easy 8 knots with the jib only. The main thing the owner indicated he would change is not to round over the hull at the shearline as it took away head room.

    The 2nd boat is from Schionning's website. I found the owners Facebook page where he describes the build. " 20,000 hours and the budget out the window", but happy was the main takeaway.
     
  13. Slingshot
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    Slingshot Junior Member

    Oldmulti do you have any information on the Looping catamarans from Patrick Luscher? They are popular with the French.
     
  14. Russell Brown
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    Russell Brown Senior Member

    I found that tri pretty interesting too. What it lacks in aesthetics it makes up for with function. The folding system looks great from a structural standpoint with wide angles between the sea stays and beams and how much simpler could a folding trimaran possibly be? The interior looks nice too and plenty of volume. Couldn't agree more about plywood.
    I'm glad I got to see photos of this boat. Definitely worth studying. Thanks for posting it, Oldmulti.
     
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  15. oldmulti
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    oldmulti Senior Member

    Slingshot. Please look at page 36 item number 539 on this thread about a Looping 50 with a few web sites about that cat. I will do further research on other models, it may take a few days.
     
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