Multihull Structure Thoughts

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

  1. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    The following is about Spirited Designs Stow Away 8.5 sailing power catamaran and especially the material build options. The Stow Away 8.5 is 27.9 x 13.1 foot and displaces 7700 lbs with a payload capacity of 2200 lbs. The fractional rigged of a 35.6 foot fixed aluminium mast carries 452 square foot of sail. Being a power sailor there are two 50 HP outboards that drive the cat at 20 knots peak speed. Under sail the cat can cruise at 7 to 8 knots. The hulls are semi-displacement Tennant canoe sterns type which are efficient under power and sail. The underwing has 1.65 foot clearance. This cat can be seriously cruised around any coast or bay. The fuel supply would limit its ocean capability if you turn engines on in anything below 10 knots of winds but if treated very carefully under sail this boat could travel far.

    The accommodation in this cat is good with one double berth and 2 singles. The main saloon has and hulls have 6 foot headroom. The main saloon has the dinette and galley with a loo in a hull. 2 single berths are in the other hull.

    But the real interest is in the build options. The standard build is either PVC foam e-glass vinylester or Duflex (balsa or foam) e-glass vinylester or epoxy. There are plywood components. There is kit build options to simplify the build.

    Now there is a build variation available in the Stow Away 8.5. You can build the hull in HDPE Polyethylene plastic sheet (think kitchen plastic cutting board material). There are boat building companies that use HDPE in power boats up to about 30 foot. Why? The tensile and compressive strength of HDPE is about 4000 PSI (pounds per square inch), marine plywood tensile and compressive strength is about 4500 PSI. The Flexural Modulus (ability to handle bending before failing) of HDPE is about 20% of marine plywood. The weight of HDPE is 60 lbs/ cubic foot, marine plywood is about 36 lbs/cubic foot. HDPE or lower strength Polyethylene plastics are used in “plastic” kayaks etc construction.

    In simple terms the major advantages and disadvantages of polyethylene as opposed to fiberglass or aluminum:

    ADVANTAGE: Polyethylene is extremely impact-resistant. If the HDPE is UV stabilized it can be obtained in many colours and does not require any painting or other finishing material. HDPE can be plastic welded together not requiring any tape seam strengthening. Translation you can buy a panel cut it to shape place it over a frame and weld the flat panel hull parts together. Work done.

    DISADVANTAGE: Polyethylene isn’t as structurally stiff as fiberglass or aluminum, so HDPE boats are limited in size. There are few over 30 foot and many of the relatively large models require structural hull supports built from different materials. As an example there is US Bass 20 foot power boats that have 25 mm HDPE bottoms with internal framing that if built in plywood would have the same framing and 18 mm plywood. The HDPE bottom weighs twice a plywood bottom.

    Yes, a HDPE hull skin saves a lot of finishing work but comes at a weight penalty and requires the same internal framing structure as a plywood build. HDPE is not suitable for structural bulkheads etc which should be either plywood or designed foam glass.

    There are many variations HDPE from standard to marine with solid, lightweight and foam cored options. The material is available in 8 x 4.5 foot to 13 x 6.5 panels in thicknesses from 3 mm to 25 mm. The cost is about the same as good quality plywood. In the case of the HDPE Stow Away 8.5 the displacement increase required the hulls to be redesigned to handle the extra structural weight.

    Investigate and understand what is possible with a designer. The jpegs show a Duflex build Stow Away 8.5.
     

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  2. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Today is about a foil supported design designed by Uniworks Boats in Finland and was done for a client in about 2014. No idea if the tri was built. The tri is 40.1 x 36 foot that displaces 5600 lbs. The 55 foot carbon fibre mast carries a 516 square foot mainsail, 333 square foot jib and a 775 square foot genoa. The length to beam on the main hull is 12.5 to 1 and the floats length to beam is 14.3 to 1. Even without foiling this boat will be fast but if it can be built to the specified displacement and have a good set of foils it will be very fast.

    The foiling system shows 5 foils but the main support foils are in the floats with the rudder foils on the floats and main hull the stabilising foils. The foil profile is not specified but with the constantly evolving knowledge of foil shape the performance potential of this tri would be increasing with the latest shapes. The reason the AC 50 cats can sail as fast as the AC 72 cats is mainly due to improved foil shape knowledge with less drag and improved lift characteristics. The sectional shape of the foils varies along their length to suit the speed at foiling depth in the water.

    The construction of such a tri needs to be very well-done carbon foam epoxy. The structural design of a tri like this requires an excellent understanding of point loads and how to distribute those point loads through out the rest of the structure. Example image foiling along and the float foils hit an object. Not only do you break the foil but you potentially pitch pole as you lose the lift. If the foil is to “strong” you have the potential to split the hull open. This is no small design job. If you break a normal daggerboard you slow but don’t pitch forward much or are you likely to split open a hull.

    Only a few jpegs.
     

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  3. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    You need a transportable cat for shifting from one location to another, how about a cat that folds to reduce its beam, here is the Uniwork design cat for you. The cat is 50 x 29.5 foot that weighs 15,700 lbs. Hey, you said its transportable, it can fold etc. The original design was for a foldable 50 foot cat. Every component in this design is a maximum width of 8.2 foot. The main cabin is 8.2 foot fore and aft. Organize a few semi trailers and you can transport it from one location to another. The mast height is 82 foot and carries an 860 square foot mainsail, a jib of 645 square foot, a genoa of 915 square foot and a gennaker of 1610 square foot. The rudders are underslung folding spade rudders and daggerboards.

    The designers claim the cat will move at 20 knots plus. A claim I do not doubt especially with the non folding carbon fibre build option which is claimed to be 3,000 lbs lighter (a weight of 12,700 lbs). The hulls have a chine above the waterline which indicates narrow waterline beams. My guess would be 11 or 12 to 1 length to beam with a minimum wetted surface design.

    The accommodation even in the hull only version is quite good and with the removable deck cabin you can get a very roomy galley and dinette. You can seriously cruise in this boat in any configuration.

    The structure of the standard build is a sandwich of PVC (Divinycell, Corecell), multiaxial fabrics and epoxy resin. A carbon fibre foam epoxy version is available, at customer request, if you want a lighter very high performance racer cruiser. The cross beam structures fore and aft are substantial which will minimize any torque movements. Mast options are aluminum or carbon fibre tubes or wing masts.

    A nice design if you want the maxi transportable cat. The jpegs give the idea.
     

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  4. waynemarlow
    Joined: Nov 2006
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    waynemarlow Senior Member

    And you could build it in a narrow shed, miles from any port.
     
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  5. garydierking
    Joined: Sep 2004
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    garydierking Senior Member

    Something I would like to see in catamaran specifications is the under wing clearance with a cruising load.
     
  6. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Gary. A discussion on catamaran underwing clearance will be soon. Today a nice tri from Uniworks Boats in Finland. The designers were requested to design a trimaran for sailing races in open ocean. This is not a small request. The tri “Open-30” is 30 x 22.3 foot with a weight of 2,700 lbs. The 46 foot (assuming) carbon fibre mast carries a 415 square foot main, a 127 square foot self-tacking jib, a 300 square foot genoa and a asymmetric gennaker of 1240 square foot.

    Now let’s look at the weight of the Open 30 tri. The Seacart 30 x 21.6 foot carries 600 square foot of sail, weighs 2600 lbs and is Autoclaved carbon fibre. The Grainger APC2 31 x 28 foot carries 650 square foot of sail, weighs 3300 lbs and is a full carbon build. Both the Seacart and Grainger can fold for transport. The Open 30 can be disassembled.

    The accommodation inside a Seacart is minimal, the Grainger 30 is good but the accommodation plan for the Open 30 is spacious compared to the other 2. But the real function of these tris is racing speed not cruising accommodation.

    Now we get to the real point of the above. The structure of these tris. I will bet the Open 30 is a carbon fibre pvc foam epoxy build to build to the weight specified. A Seacart main hull has 3 layers of 200 gsm unidirectional carbon on the bottom outside, 12 mm Nomex and a 285 gsm biax carbon on the inside all done in epoxy cooked inside an autoclave oven at 4 atmospheres. The Seacart is painted to save weight. The Grainger 30 has a main hull of 200 gsm, Kevlar 12 mm PVC foam, 200 gsm Kevlar and floats of 193 gsm carbon fibre, 9 mm PVC foam, 193 gsm carbon fibre all done in epoxy. The folding cross arms are carbon flanges and foam carbon webs and are a work of art.

    Waynemarlow, who appears on this web occasionally, has the skill to quickly produce a 20 foot carbon fibre tri main hull at home part time. But for a serious 30 foot carbon fibre tri plan on a long very careful build and/or the need for some very exotic equipment.

    The performance of the Open 30, if built to weight and with a good rig, will be very good with the right crew. The jpegs give the idea.
     

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  7. Eric ruttan
    Joined: Jul 2018
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    Eric ruttan Senior Member

    Will the underwing clearence be in a new thread, or here?
     
  8. oldmulti
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    oldmulti Senior Member

    Eric. Underwing clearance will be here. It is not just a number, there are many approaches that need to be discussed.
     
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  9. peterbike
    Joined: Dec 2017
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    Location: melbourne

    peterbike Junior Member

    Thanks oldmulti,
    I hope you get all the xmas presents that you wished for... ;)
     
  10. waynemarlow
    Joined: Nov 2006
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    waynemarlow Senior Member

    Loving the write ups, this thread is a real reference book.

    Happy Xmas to all from our locked down Tier 4 UK ( no family allowed to visit unlike the large Xmas dinners we normally have )
     
  11. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    The underwing clearance stuff tomorrow but today the final Uniworks design for now for discussion. The 9 meter open bridge deck catamaran is designed for racing and fast cruising. The cat is 30.7 x 22.3 foot that weighs 3,100 lbs and 3,800 lbs loaded. The 41.3 foot mast carries a 388 square foot mainsail, a 162 square foot self-tacking jib and a 410 square foot genoa. The draft varies from 1.8 foot and 8 foot with its daggerboards. The rudders are transom hung with kick up rudder blades. The cat is transportable as it can be disassembled.

    Compared to the Open-30 tri designed by the same group this cat is slightly heavier but has more sail area on a shorter mast. Translation this cat will be seriously fast and it will depend on the course and crew as to if the cat or tri wins.

    The structure of the cat could be similar to the tris mentioned in the Open 30 tri post above. The cross beams on the cat appear to be foam glass. A very aggressive option for a home builder could be EG “Turning Point” hull specifications a 32 foot Grainger design that had 200 gsm unidirectional 8 mm Western Red Cedar 200 gsm unidirectional done in epoxy. Turning Point IMOR rated weight was 2800 lbs. If I built a WRC version, I would put 400 gsm 45/45 biax outside. If doing strip plank cedar, please be a good builder with good timber scarfing skills, know how to apply thin glass skins without any pin holes and have a very fair hull prior to any “sanding”. One false sanding move and you may not have much glass left and finally do not run a boat like this aground.

    The accommodation is limited with space for a few single berths, a loo and a small galley. The tri would have better accommodation.

    The jpegs give the idea with the last jpeg of the Grainger cat Turning Point. Merry holidays to all and I hope we are all here by the end of 2021 after having a fun year of sailing. Wayne, I have lived in 5 million person Tier 4 plus lock down for 100 days with face mask wearing. After lockdown, no community Covid for 50 days if everyone plays the game.
     

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  12. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Gary requested the underwing clearance of cats to be added. I will do so if they are listed. But we have a few issues to deal with. There are many factors that dictate underwing clearance whilst sailing. This is not a simple subject.

    I have sailed on fat hulled cats with low wing decks that had constant bumps but no real pounding and I have sailed on high wing deck thin hulled cats that didn’t bump but if a wave hit it felt like hitting a wall. Wharram cats high wing deck clearances have had deck sections broken by wave action yet 26 foot Heavenly Twins full bridge deck cats with 300 mm of wing deck clearance have sailed around the world.

    Many owners of Heavenly Twins, medium sized Prout catamarans, Catalacs, Bobcat etc owners complain about going to windward and underwing pounding. Low wing decks to obtain 6 foot headroom in main saloons do not work well. Yes, in flat water or long ocean swells without wind blown waves these boats will sail OK without much pounding. Get them in a short chop and sleep will not be easy, especially if they have bunks forward of centre. So, lets look at the factors of underwing pounding.

    Wing deck clearance. If you have a 300 mm underwing clearance it will pound/bounce in seaway irrespective of the underwing shape, bulges etc. My preferred minimum wing deck clearance is 65 mm per meter of length. That means 395 mm underwing clearance for a 20 foot long cat, 590 mm for 30 foot, 790 mm for 40 foot and 980 mm for 50 foot or above.

    Hull width at the waterline effects the clearance required. Slim hulls (eg above 12 to 1 length to beam) require more clearance than fat hulls (eg 8 to 1 length to beam) as slim hulls go deeper into the water the more the hulls are loaded. Now we get to the tricky part. The hull shape above the waterline can effect this. A hull that has a lot of flair like a Wharram or a hull that has a step/chine just above the waterline of a slim hull can get away with less clearance if done properly. But the Summer Twins 25 jpegs show hulls that are meant to have a 9 to 1 length to beam ratio below the chine. People loaded the 25 foot cats up to where the chines went below the waterline and the hulls now have about 7 to 1 length to beam an less than 300 mm underwing clearances. The builder’s solution, add 3 foot to the boats length and build it out of lighter material. Translation, actual loaded sailing hull shape is what is needed to be understood.

    Overall beam. The narrower the overall beam and hull spacing should require less underwing clearance. Some designers work on a 20% of the hull’s lateral clearance between the hulls as the wingdeck height. OK, but less beam, less stability and if your underwing goes to low, it will still pound.

    Overloading. If a designer says a payload of 3000 lbs and you load a cat with 6000 lbs it’s not the designer’s fault if the wing deck pounds or breaks. I was on a 37 foot cat who’s 12 mm ply well framed forward under wing had a 300 mm square hole punched through it by wave action going upwind letting water into the hulls.

    The length of a bridge deck can have a big impact on pounding. Full length bridge decks have weight and solid underwings in the wrong places and will pound. If the first thing that meets a wave is your bridge deck you are going to pound in a seaway. Let the hulls do the initial lifting of the bows or sterns before the under wing is exposed to the sea. I think a wing deck no longer that 70% of the length overall is a good compromise.

    High center of gravity causes pitching, which in turn creates pounding on under wings. Big central cabins with all mod cons and heavy rigs can lead to a lot of pitching especially if they are matched to fine ended hulls. Some designs depended on their underwings to minimize pitching. Find another design if this is your boat choice.

    Do not underestimate the value of a good set of daggerboards or low aspect ratio keels. This combined with good rudders will allow you to optimize your sailing angles upwind to find a course that minimizes pitching and underwing ponding.

    Some sample modern wing deck clearances. Grainger Raku 32; clearance 650 mm LB (length to beam of hull) 12:1. Hughes 30; clearance 900 mm LB 11:1. Hughes 37; clearance 775 mm LB 10.8:1. Grainger Raku 40; clearance 755 mm LB 13:1. Schoinning Arrow 1200; clearance 750 mm LB 14.5:1. Schoinning GForce 1200; clearance 700 mm LB 13:1. Hughes 45; clearance 880 mm LB 12.4:1. Grainger Raku 48; clearance 880 mm LB 13:1. Schoinning Arrow 1500; clearance 900 mm LB 14.5:1. Schoinning G-Force 1500; clearance 950 mm LB 14.5:1.

    Some older cats: Outremer 50; clearance 770 mm LB 12:1. Catana 411; clearance 800 mm LB 12:1. Catana 471; clearance 900 mm LB 12:1. Lagoon 410; clearance 800 mm LB 8:1. White Atlantic 42; clearance 730 mm LB 11:1. Gemini 105M; clearance 500 mm LB 9:1. Seawind 1000; clearance 800 mm LB 10:1. Stow-Away 8.5; clearance 500 mm LB 10:1. Shuttlecat 31; clearance 920 mm LB 11:1. Shuttlecat 40; clearance 900 mm LB 12:1. Tiki 26; clearance 550 mm LB 11:1. Tiki 30; clearance 630 mm LB 11:1.

    Smaller cats with low clearance: Hirondelle 22 ft; clearance 300 mm LB 8:1. Patterson Heavenly Twins 26 ft; clearance 300 mm LB 6.7:1. Patterson Summer Twins 25; clearance 300 mm LB 9:1 (theory). Catalac 8 and 9 mtr; clearance 400 mm LB 7.3:1.

    Over time good cruiser racer cats have higher under wing clearances resulting in less pounding in a seaway. Older cats often had accommodation prioritized over actual sailing performance and comfort in a seaway. When serious cruising you need rest and structural integrity, high wing deck clearance helps with both. Jpegs give an idea.
    Another view on the topic can be found at: https://catamaranguru.com/wp-content/uploads/2020/03/catamaran_bridgedeck_clearance.pdf
     

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    Last edited: Dec 25, 2020
  13. Bruce Woods
    Joined: Sep 2007
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    Bruce Woods Senior Member

    Firstly merry xmas to all.

    This raises another subject.
    "Misleading advertising in multihull sales"

    Picking one design that I am reasonably familiar with,I notice the Seawind 10 is quoted at 800mm.
    This doesn't take into account that the Seawind 1o has two engine nacelles and a central wave breaking nacelle, that renders the effective clearance to approximately half of the adverised. Add to that the fact that this clearance is at design minimum weight, you can see that advertised figures are meaningless.

    Like most things multihulls reducing design considerations to a set of figures is generally a waste of time. The proof is in the sailing, as illustated by the ocean miles heavenly twins, catalacs and prouts have to their designs.

    We seem a litte precious about our multihull design considerations. Multihulls are different things to different people. We should not let our own requirements cloud our discussions.
    Who would have thought a modern multi would weigh north of 20 ton and have a cockpit on the saloon roof. The reality is that most boats need to perform at anchor as shown by the sales data.
    Light weight , , well powered sailing catamarans have proven dangerous time and time again in either inexperienced hands or inattentive hands. Its time to stop talking inflated averge cruising speeds, accept that the sport of sailing is dieing, so few are getting a good grounding in sailing skills before running out and putting a downpayment on a boat and accept that the french cattle marans are a valid compomise.
     
    Last edited: Dec 25, 2020
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  14. oldmulti
    Joined: May 2019
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    oldmulti Senior Member

    Bruce your comments are valid, most people who sail floating apartments have little experience of a really good sailing boat, but even the French multi manufacturers are now succumbing to a latent demand for more performance. A Lagoon 50 mast is 90 foot high and carries 1600 square foot of sail to drive its 20 tons. This what a 10 ton racing 50 footers were carrying 20 years ago. By the way, a Lagoon 50 will capsize if it has full sail in 37 knots of apparent wind and not sail any faster than a Grainger Raku 32 foot cat. The Raku 32 will capsize if carrying full sail in 28 knots of apparent wind. In Australia these numbers matter as this afternoon a 40 knot wind change is promised at 3 pm after a 10 knot wind day. Not an unusual event. Also, the 24 foot cat below will sail faster than a Lagoon 50.

    Today, a style of boat I really like. A fun trailer sailor with camping accommodation. Turning Point Design built the cat for the 750 mile Race to Alaska and general fun. The trailable cat is 24 x 10 foot weighing 750 lbs and when ready for the water in racing trim with 3 crew displaces about 1400 lbs. The carbon fibre rotating wing mast is 34 foot (ex Farrier 25 C) carrying 350 square foot mainsail area and a jib/genoa upwind. With the crew racks for the overall “beam” is 16 foot. These racks with 3 crew sitting on them and 300 lbs of water ballast increase the righting moment of this design in racing mode by 75%. Translation, with crew sitting in the cockpit and no water ballast the boat will capsize in 14 knots of wind, with crew sitting out on racks and water ballast, the cat will capsize in 19 knots of wind. The mast has a 2 lbs carbon fibre mast head float to prevent a full capsize and allow self righting if required. The length to beam on the hulls is about 16 to 1.

    The foils started as standard daggerboards and rudders, but were upgraded to Z foils and T rudders which helped keep the bows up. The accommodation is a double berth space with room for a “small galley” to warm up the freeze-dried food and store the 40 litres of drinking water. The motive power is a peddle arrangement in the cockpit that can provide up to 3.5 knots.

    Now lets get to the structure. Buy a truckload of out-of-date Nomex from Boeing for $100, then use your prepreg carbon fibre skills to build a very light weight cat. The shell of the cat weighs 450 lbs, a hull half weights 52 lbs for about 100 square foot of surface area. Translation about 400 gsm biax carbon fibre outside and either 400 or 200 gsm carbon fibre inside or 12 mm Nomex all done in epoxy. The bulkheads and rear beam are done in 12 mm Nomex with 627 gsm carbon fibre on either side. The racks are 75 mm Nomex with carbon fibre top and bottom and weight 10 lbs a piece. All components were built in prepreg carbon fibre and after vacuum bagging were cooked in an oven for 18 hours at 180 degrees.

    The molds for the cat hull and other components were made from simple chip board and timber materials with cabin structures and bulkheads done as a flat panels. The jpegs give the idea.

    Now how fast is this cat? To quote the crew. “Wind was 10-15 and boat speed was 12 – 15 knots. We hit a max speed of 19 knots on a screaming reach out of Port Townsend and finally when surfing down seas we were topping 20 knots plus.” This is a faster than wind speed boat and if handled well will embarrass many larger multi’s.

    The jpegs give the idea.
     

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    Last edited: Dec 26, 2020

  15. oldmulti
    Joined: May 2019
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    Location: australia

    oldmulti Senior Member

    Further 24 foot Turning Point catamaran. Nomex carbon fibre build.
     

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