Multihull Structure Thoughts

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

  1. oldmulti
    Joined: May 2019
    Posts: 1,145
    Likes: 569, Points: 113
    Location: australia

    oldmulti Senior Member

    Today we start Russia week with a cat called “Myth” launched in 1984. The cat is 34 x 17.7 foot with 4 foot wide hulls. The cat weighs 4400 lbs and carries 2 Pyramid rigs on 2 masts. The masts are 37 foot steel water/gas pipes with a diameter of 130 mm and a wall thickness of 4.5 mm. The foremast is secured by two forestays and two shrouds, the mainmast - by two backstays and two shrouds. The masts are interconnected by a cable between them. All standing rigging, made of 8mm steel cable, is attached to the tops of the masts. Rotating "carousels" (spreaders for base of sails) for the pyramid rigs are mounted on each mast, which are metal structures, consisting of four pipes-spreaders with a diameter of 40 mm, four top ties from a steel 10-mm bar the length of each tie is adjustable. Wooden 12 foot booms with a diameter of 100 mm are attached to the sail foot and tangs welded to the ends of the spreaders which can rotate. The sails are isosceles triangles with the length of the front and back leech 32.3 foot each, the ail foot is 11.5 foot. Each sail is 190 square foot (4 sails = 760 square foot in total). A well designed Pyramid rig has very low sheet loads as it is a “balanced rig” with some sail area in front of the mast.

    The hull draft is 2 foot with rudders on the sterns of the hulls. The hull shape of “Myth” is experimental at best. It takes simple asymmetry to the extreme. This is a high wetted surface design with minimal foils for lateral resistance. The length to beam ratio is about 16 to 1.

    Watertight bulkheads divide each hull into three compartments. In the port hull there are three cabins with four berths, in the starboard hull there are two cabins with three berths, a storage room and a loo. Each cabin has a porthole and 6 foot headroom. The is also a temporary collapsible deck cabin if required.

    Now we get to the structure. As with all Russian home build designs, they build with what is available and what they can afford. Most of the designs have reasonable structures but they may not be the best way to build a design. The “Myth” is built from ply and timber. The hulls ply is five-layer 6 mm plywood. Frames are made of 40 X 60 mm pine, stringers are 30 X 60 mm pine. Outside, the hulls are covered with epoxy resin fiberglass. Eight transverse pine cross beams with a cross section of 100 X 60 mm support the bridge deck which is made of 10 mm plywood was laid on the beams. At the bottom, steel strips with a cross section of 3 X 30 mm are fixed to beams crosswise. The masts are supported by pine beams with a cross section of 160 X 80 mm which have 20 mm steel rod dolphin strikers below.

    The performance of Myth is considered good. In local racing it points about 45 to 50 degrees to the wind which is poorer than most of the local monohull sloop rigs. Once on a fine reach to running it performs better than most sloop rigs due to the ability to adjust the rig angle to the wind direction. The speeds in local racing in 20 knots of wind is about 12 knots. The cat tracks very well and can tack without problems. A British magazine tested 2 Prout Quest 31’s, one with a sloop rig, the other with a single pyramid rig and found similar up wind downwind results. Pyrmid rigs work well in light to moderate conditions but become more difficult in higher wind conditions due to difficulty in reefing and the “reefed” sails unbalancing the rig.

    An interesting concept which shows Russian creativity. The jpegs give the idea.
     

    Attached Files:

  2. oldmulti
    Joined: May 2019
    Posts: 1,145
    Likes: 569, Points: 113
    Location: australia

    oldmulti Senior Member

    The cruising catamaran "Yanka Maur" was home designed and built in 1965. The catamaran is 19.7 x 9.9 foot that displaces 2250 lbs. It was designed to carry a 28.5 foot fixed aluminum mast with a 202 square foot fully battened mainsail and a 83 square foot jib. The initial trial mast was 20 foot high carrying a reduced 200 square foot of sail. The length to beam on the hulls is 7.7 to 1. The single V bottom shape is reasonable but is not as modern as the vertical stem slightly flatter rocker of modern hull shapes. The lateral resistance is provided by hull shape only and the kick up stern mounted rudders. The designer rejected centre boards to allow more internal space. The underwing clearance is 1.5 foot.

    The catamaran was designed as a cruiser for sailing in the Caspian Sea (frequent wind changes, shallow depths, lack of shelter) and along rivers. Day sailing capacity is four to six people. The vessel has the maximum comfort for its size. The cabins headroom is 4.5 to 5.2 foot and the cat has been cruised for a month with 2 people. There is a single berth (also acts as seat) in each hull with a small galley and table in the starboard hull. A "loo" (read bucket) is in the port hull.

    The bridge, deckhouses, deck, hatches, etc. were made from construction plywood 6 and 5 mm thick. The 10 frames and stringers are from coniferous wood. Casein glue was used. The fore beam is 120 x 50 mm, the mast cross beam and aft cross beam are box sections with solid timber on one web face and 6 mm plywood for the other 3 faces.

    The designer wanted waterproof ply and better timber but the change of materials caused changes in the structural drawings and EG beams had increased cross-sections and fastened with galvanized screws instead of stainless steel. The construction plywood was impregnated with linseed oil which seals well especially if the joints and edges of the sheets are protected. Again, Russians do not always have access to good materials and make do with what is available.

    With an initial trial 20 foot mast and reduced sail of 200 square foot, the cat easily climbed waves and decks were always dry, even in strong winds. There were no major wave impacts on the bridge deck with only a few splashes into the grating. The waves from the stern do not hit the transom, but go under the cat. With the reduced sail area of 200 square foot the average speed was 4-4.5 knots in moderate and light winds. The maximum speed exceeded 7 knots and the cat moves well in the slightest breeze.

    This is a practical small cruising catamaran that is similar to the Jarcat 20 but has a wider beam. A nice concept. Jpegs give the idea. The table below is the table of offsets for the hull.
     

    Attached Files:

    Last edited: Jan 28, 2021
  3. peterAustralia
    Joined: Mar 2006
    Posts: 369
    Likes: 30, Points: 28, Legacy Rep: 233
    Location: Melbourne Australia

    peterAustralia Senior Member

    Nice, not sure it would go to windward well with flattish bottom and no boards. There might be space for an external leeboard-daggerboard on the outside of the hull, flush with the hull side at midships
     
  4. oldmulti
    Joined: May 2019
    Posts: 1,145
    Likes: 569, Points: 113
    Location: australia

    oldmulti Senior Member

    peterAustralia. As with many of the Russian designs the ideas and structure may be a little old but are basically good. It would be easy to add either fixed keels or EG a dagger board or lee board as you suggest in one hull of this cat which would improve its windward performance a lot. Tomorrows entry will be a little mundane, but in a few days there are some really interesting Russian multi coming that could be built today. Thanks for the comments.
     
  5. oldmulti
    Joined: May 2019
    Posts: 1,145
    Likes: 569, Points: 113
    Location: australia

    oldmulti Senior Member

    Triton is a Russian “copy” of a Cross 24 tri designed in 1968. The tri is 23.5 x 13.8 foot and displaces 2,200 lbs. The wooden or aluminum mast is 28.7 foot high and is a sloop rig with a 183 square foot mainsail, a 79 square foot jib and a 160 square foot genoa. The main hull length to beam 7.3 to 1 when light reducing to 7 to 1 when loaded. There is no additional lateral resistance beyond the hull shapes. The hull shape is classic Cross (without the keel) of that time which means they will sail well in light to moderate airs but upwind and heavy airs these types of tris whip from float to float in the wrong sea states. V floats can work well if designed correctly, but early designs had short low prismatic (fine ends) floats provided little pitching resistance and picked up wetted surface quickly.

    The accommodation has 3 berths in the main cabin with a small galley, seat and table. A portable loo can be fitted. The headroom is limited to 4.3 foot.

    The structure is ply timber with 6 mm ply on hull sides and 8 mm ply on main hull bottom. The frames are 50 x 30 mm, the stringers are 50 x 25 mm or 60 x 35 mm depending on location. The keel is laminated from 3 layers of 35 x 25 mm timber. The deck and cabin are 6 mm ply. The 2 main full width cross beams are 6 mm ply on one face with 40X60 mm top and bottom flanges and a 4 mm plywood face web. The structural design is reasonable and appears from the article to be engineered (although parts of the plan could have been copied from Cross). There is one interesting warning though “It is advisable to make all metal items of equipment from an aluminium-magnesium alloy, by the way, they will not corrode either. However, you cannot install copper fasteners, but you need to use steel, galvanized.” As I have said Russian’s work with what they have.

    An actual sailing “Triton” peaked at 10-12 knots with a calculated maximum speed of 14 knots. The main resistance speed is 8 knots, which is at the peak of wave drag. Once the tri gets over the 8 knots resistance point the tri can sail at 10 to 12 knots with favorable winds.

    This design gives a hint as to how a Cross 24 was built and this tri could make a reasonable small cruising tri. BUT do not expect modern performance, this design will give adequate performance. If you want a small ply tri, a Buccaneer 24 (full plans on this thread) will give a lot more performance. The jpegs give the idea with the last jpeg a Cross 24 for comparison.
     

    Attached Files:

  6. oldmulti
    Joined: May 2019
    Posts: 1,145
    Likes: 569, Points: 113
    Location: australia

    oldmulti Senior Member

    The trimaran “Nun” took 5 years to build by 2 people. The hull shell took 2.5 years to build, the fit out, rigging etc took 2.5 years to finalise and “Nun” was launched in 1986. The tri is 40 x 27 foot with a weight of 5,900 lbs (actual with stores but without liquids/crew), the displacement is 7200 lbs. The 44 foot high, 500 x 100 mm plywood and timber wing mast (jpeg. 4 mm ply shell with 20 x 100 mm main spar, 60 x 40 mm leading edge, 20 x 100 mm bulkheads every 700 mm) carries 860 square foot of sail in a fractional rig. The mast area is 65 square foot. The boom is 13 foot long. The main hull length to beam of 11 to 1. The center board draws 4 foot when down and there is a kick up stern rudder. The mast base is 55% back from the bow which is very modern thinking.

    The performance of this tri is described as “good”. With the numbers indicated above of the weight, wing mast and fine hulls, I have no doubts about its capability to get near 20 knots. The accommodation is good with 5 berths over 3 cabins, good galley and sitting area, also a separate loo.

    The structure of the main hull is timber frames 35 x 35 mm backed by 6 mm plywood placed 1000 mm apart with 35 x 45 mm stringers with 6 mm plywood used on the shell. The keel is 45 x 60 mm either side of the centre board case combining to a 45 x 120 mm fore and aft. An additional layer of 6 mm ply is added to the inside of stringers in the bow region, underwing areas and at chainplate attachment areas. The radius of the main hull bottom ply is 550 mm with triangular 6 mm ply panels bent around and “sewn” together with 6 mm ply backing strips on the inside and 3 layers of fiberglass on the outside of the joints. Floats have 35 x 35 mm 6 mm ply backing frames 1000 mm apart with 35 x 35 mm stringers and 6 mm plywood shell the keel is 35 x 100 mm (tapered at ends). Decks are 6 mm ply. The structure of the hulls are near the edge but reasonable if the displacement is kept to 7000 lbs. Now we get to the cross beams, these would need to be reviewed by a designer, as I know smaller lighter tris that have stronger beam structures. The beam specified are 220 x 220 mm boxes with 35 x 35 mm timbers in each corner and 6 mm plywood faces. There are water stays to help support the beams. Windows are 12 mm Perspex. The tri was 200 lbs heavier than anticipated due to “a significant amount of epoxy paint (the layer thickness was about 0.5 mm), which is heavy”.

    This is a reasonable design with an OK structure. The cross beam dimension and structure would need to be checked by a designer. The jpegs give the idea. The last jpeg is a section of the mast.
     

    Attached Files:

  7. oldmulti
    Joined: May 2019
    Posts: 1,145
    Likes: 569, Points: 113
    Location: australia

    oldmulti Senior Member

    The following was inspired by the British 30 foot "Iroquois" catamaran. The Russian designer could not find any useful structural information about the fiberglass version so he did a timber version in 1970. The "Gardarika" cat is 29.5 x 13.1 foot that weighs 4400 lbs. The 36 foot aluminium mast is a fractional rig carries a 250 square foot main, a 130 square foot jib and a 210 square foot genoa. The length to beam on the hulls is 7.8 to 1 with an under wing clearance of 1.35 foot. The designer probably had access to the lines that were published in Yachting World in the late 60’s as the hull shapes etc are fairly close to the “Iroquois”.

    The first cat was built by a carpenter and ten 14 to 16 year old kids using basic tools in under a year as part of an education project. The vessel was launched with everyone going on a few day very cramped cruise. The cat has 1 double, 3 single berths and convertible dinette double berth. The galley and loo are in the hulls. The vessel sails well having achieved 16 knots in local racing. The kickup center boards and rudders help the performance of the cat.

    The structure is again dictated by available materials but is reasonable. The plywood is claimed to be 5 mm on the hulls but in the structure inset jpeg the underwing, the cabin top and other areas are 6 mm ply, so I am assuming the hulls are covered in 6 mm ply. The hulls have 20 x 160 mm bottom frames, 90 x 20 mm side frames with 30 x 50 mm stringers, 50 x 80 mm gunnels and 50 x 100 mm keels. The roof frames are 70 x 20 mm. The underwing cross members are 50 x 70 mm with at the mast bulkhead two 50 x 100 mm diagonal struts to support the mast thrust. The diagonal struts go to each hull inner hull gunnel of 50 x 160 mm. This is a wooden interpretation of the steel structure that that supports the “Iroqouis” mast.

    This is a “nice” copy of an “Iroqouis” cat but please remember the narrow 13.1 foot beam. Later design cats of this length have beams up to 50% wider. The jpegs give the idea. Tomorrow we go into the unusual again.
     

    Attached Files:

  8. oldmulti
    Joined: May 2019
    Posts: 1,145
    Likes: 569, Points: 113
    Location: australia

    oldmulti Senior Member

    "Blagovest" is an original steel catamaran that is 46 x 23 foot and weighs 18,000 lbs (a number I doubt, more likely 25,000 lbs plus). The schooner rig has two 47 foot aluminum masts that carry 1290 square foot of sail. The length to beam on the hulls are 14 to 1. The stern mounted kick up rudders draw 2.5 foot when down.

    The structure is steel. The hull shell thickness is 5 mm steel and due to the curved surfaces, the strength and rigidity of the shell is high enough to only require minimal frames every 3.25 foot. There is only one longitudinal stringer, at a height of 3.25 foot (from the flat keel line) which also forms the support for the double berths.

    Simplified contours with a narrow (500 mm) horizontal bottom is used. All bent surfaces unfold into a cone or cylinder. The constant section cylindrical components are about 70% of the hull length, which greatly simplifies construction. Darts are then cut into the bow and stern sections which are pulled together and welded to form the bow and stern “hull shape”. The crossbeam trusses are made of steel welded. The upper and lower pipes, as well as the strut under the mast step are thick-walled steel tube with a diameter of 120 mm. Four diagonal braces are cut from rods with a diameter of 25 mm. The lower tube is sealed in a 120 x 450 foam "profile", covered with fiberglass, for streamlining.

    One of the unusual design solutions is the crossbeam attachment structure that concentrates loads only in articulated joints. The designer claims “it is known that the rigid fastening of the bridge is a problem in multihulls: it is here that the greatest number of breakdowns and cracks occurs”. Modern designs have less problems in crossbeam attachment and the only advantage I see in this arrangement is the ability to assemble and disassemble the cat for building or repair.

    A Queensland based 50 foot aluminum racing trimaran (Mooloolaba Fire Truck. Page 1 of this thread.) is built on a similar idea a “tube” of metal and tapered ends but it uses 5 mm aluminum for its shell. This tri weighs 8000 lbs.

    In “Blagovest” there are 6 double cabins for crew and guests in the hulls and are separated from one another by watertight transverse bulkheads. Each cabin is 13 foot long and 5 foot wide and has a separate access. The bridge deck pod is used for seating, galley, engine, steering and sail control.

    Tests on Ladoga, the Baltic and the White Sea showed that “Blagovest” can continue to sail in 30 knot winds and can tack through 110 degrees. The cat can average 8 knots under the engine and can average 10 knots under sail in favorable conditions with average daily sails of 120 miles during daylight hours.

    “Blagovest” is interesting in its build and its design options. This is not a performance machine but it does its intended function of a reliable cruiser that could handle grounding better than most cats. Jpegs give the idea.
     

    Attached Files:

  9. oldmulti
    Joined: May 2019
    Posts: 1,145
    Likes: 569, Points: 113
    Location: australia

    oldmulti Senior Member

    The design and construction of “Odyssey” took 7 years and was launched about 1970. The design was inspired by Rudy Choy’s “Lani Kai” catamaran. The cat is 36 x 16 foot with a weight of 5,850 lbs and a displacement of 8,000 lbs. The 42 foot aluminium mast (4 mm walls) carries a 388 square foot main, a 194 square foot jib and a 322 square foot genoa. The asymmetric hulls length to beam is 13.9 to 1. The asymmetric hull provides lateral resistance. The rudders are kickup attached to the sterns. The underwing clearance is 2 foot.

    The design was conceived as a fast cruiser. The design was calculated to have a maximum speed of 15 to 17 knots under good conditions. The catamaran has shown good seaworthiness and speed. In 35 knots of wind and a wave height of 4 foot, under the staysail alone, it’s speed was 8 knots upwind. In 25 knots of wind with full sail up, its speed was 10 knots upwind. The only problem is that the asymmetric hulls do not provide sufficient lateral resistance and there is a lot of leeway. It can point reasonably well but the VMG is not good. There is no substitute for deep dagger boards and a decent rig to get too windward.

    The hulls have 3 single berths over 2 hulls and a galley in one hull. The bridge deck cabin has 2 seat/bunks with 5.3 foot headroom and some storage space.

    The structure is timber plywood with a safety factor of 6 designed in. The seaward sides of the hulls have 10 mm plywood with 40 x 60 mm framing, the tunnel side of the hulls is 7 mm plywood. The hulls have 2 stringers per side with bunks acting as stiffeners. Keels are 70 x 120 mm. The hull bulkheads are 7 mm plywood. The hull decks and underwing are 7 mm ply on 40 x 40 mm framing. The cabin top is 5 mm plywood on 30 x 40 mm framing. There are two 400 mm deep main timber/plywood cross beams with a 10 mm thick plywood faced box forward beam.

    The designer wanted to increase the mast length and sail area to improve the cat’s performance. This is an old design that had its limitations but still worked OK. Asymmetric hulls are rare now and almost all designs have either boards or low aspect ratio keels which improve windward performance and overall boat control.

    The jpegs give the idea
     

    Attached Files:

  10. oldmulti
    Joined: May 2019
    Posts: 1,145
    Likes: 569, Points: 113
    Location: australia

    oldmulti Senior Member

    The final Russian cat for a while is a 13.75 x 7.5 foot biplane rigged, foam glass, bridge deck cruising catamaran with a double berth and sitting space that was launched in 1985. The cat weighs 670 lbs and displaces 1,100 lbs. The cat’s biplane rig has a 14 foot 32 mm steel pipe mast on each hull with a 95 square foot junk sail on each mast. The masts are interconnected at the top and have limited rigging fore and a short distance aft for support. The hulls have a length to beam of 9.2 to 1. There appears to be no centre boards etc but the cat has kick up stern rudders.

    Now we get to the foam glass hulls, expanded polystyrene and foamed epoxy resin were used. The sides above the design waterline is 20 mm thick polystyrene foam with 2 layers of fiberglass inside and 3 layers of fiberglass in epoxy and polyester resins on the outside. The gaps between the foam sheets are filled with foamed epoxy. Large Styrofoam blocks are shaped to form the bow, stern and below the waterline for buoyancy and full hull shapes. Please be warned. Polyester resin and polystyrene do not mix well as polyester softens polystyrene, epoxy is OK. Next low density polystyrene breakdown easily. High density polystyrenes are OK in small inshore boats but are not advisable in EG above 20 foot boats. The bow and bridge deck are made from two layers of plywood with foam in between. The cabin is built from plywood. The cross beam structure is plywood with timber strips. With the masts stepped in the hulls there are reduced bridge deck cross beam forces.

    The cat cruises Moscow reservoirs and sails OK but is not fast. The cat has never gone faster than 7 knots under sail or power. The reasons for the lack of speed may be heavy weight, fullness of the bows, large hull width and too small a distance between the hulls. But the designer builder does not consider low speed to be a disadvantage for cruising.

    May I suggest for such a small cruising cat with a junk rig and EG 2 people on board, 7 knots is not to bad. A relatively cheap fun cat that could be trailed by any car to inshore lakes or rivers. The jpeg give the idea.
     

    Attached Files:

  11. oldmulti
    Joined: May 2019
    Posts: 1,145
    Likes: 569, Points: 113
    Location: australia

    oldmulti Senior Member

    Outremer 45 is an interesting cat due to one of its owners. Jimmy Cornell is a high profile long term cruiser who has written many books (eg World Cruising Routes 8th edition, Cornells Ocean Atlas: pilot charts for world oceans) and articales about his cruises and monohull boats he has built for cruising. His latest boat is a Outremer 45 which he wants to do serious cruising with minimal environmental emmisions. The Outremer 45 is 45 x 23.4 foot with a weight of 17,400 lbs. The 58 foot carbon Axxon mast and aluminum boom with a fully-battened square-top main of 730 square foot mainsail, 420 square foot jib, 475 square foot genoa and a 785 square foot gennaker. The hulls length to beam is 10 to 1 and the sterns are immersed. The cat has daggerboards that draw 6.7 foot down and spade rudders that draw 3.4 foot.

    Jimmy Cornell did his first world voyage was between 1975 and 1981 when the world was still in a “semi pristine state”. He continued voyaging and tried to retire in 2010 when he was 70 and sold his 43 foot aluminum monohull. He then decided to get another boat, Aventura IV and transited the North West Passage. He quit again but climate change is “getting out of control” and Jimmy decided to sail around the world on a fully electric boat along the route of the first circumnavigation 500 years previously.. His logic is for a catamaran is he wants to be able to do the entire voyage under sail and regeneration of energy is essential. A boat that has sufficient deck area available for solar panels and would be fast under sail whether monohull or multihull. He has launched an Outremer 45 to achieve his goals with the following comment “the most important factor – to accept that we now live in a world and a time when we must be ready to change our ways, from what we eat, how we live, how we travel; and that certainly includes how we sail.”

    Jimmy Cornell described what he considers the problem areas of the voyage. “There are three critical areas along the 30,000-mile route and tackling them in the right way and at the right time is challenging. The most difficult is the 350-mile long Magellan Strait, where contrary westerly winds boosted by the narrow high-flanked gorge will put Outremer’s narrow hulls and daggerboards configuration to a tough test of her windward going capabilities. Potentially even more dangerous are the violent unpredictable williwaws winds that roll off the high-sided cliffs at 40 or more knots and drive the boat relentlessly onto the opposite lee shore. We survived such dire straits on Aventura III in that area and barely managed to keep off the beach with engine screaming at full power.” Jimmy really has been there and done that. His choice of a cat has shocked many monohull cruisers.

    The Outremer 45 hulls and decks are built in vinylester with a divinycell core. The hull layup beneath the waterline is solid glass. Outremer uses carbon fibre in high-load areas to ensure rigidity and durability over time. The hull/deck joint is not just glued on an inward-turning flange, but securely glassed in all around, and the bows include multiple “crash boxes” to prevent water ingress in the event of a collision. The daggerboard trunks are also not only rock-solid, but the boards are designed to break away first. Outremer director of sales Matthieu Rougevin-Baville said “an Outremer 45 grounded for four days on a reef off Tahiti and came away with little more than some scratches below the waterline.”

    A test sail produced. “A close reach in 15 knots of breeze, the Outremer sailed at 9-10 knots. Equally impressive was the boat’s motion in the 4-5ft seas, which was easy and predictable, in stark contrast to all too many cruising cats out there that have a nasty, whippy motion in a seaway is. The Outremer 45 is a boat that takes good care of its crew. Bearing away toward the channel into Port Everglades, and with the wind falling light, we hoisted the A-sail, which kept our speed up in the 10-11 knot range on a broad reach”. This is a boat capable of 200 mile day averages.

    A good cat that being seriously sailed by Jimmy Cornell will influence many monohull cruisers to think about catamarans. The pegs give the idea. The final jpeg is of Cornells new cat.
     

    Attached Files:

    fallguy likes this.
  12. bajansailor
    Joined: Oct 2007
    Posts: 1,608
    Likes: 478, Points: 83, Legacy Rep: 37
    Location: Barbados

    bajansailor Marine Surveyor

  13. redreuben
    Joined: Jan 2009
    Posts: 1,754
    Likes: 106, Points: 63, Legacy Rep: 349
    Location: South Lake Western Australia

    redreuben redreuben

    Anyone care to speculate on what gives the Outremer its comfortable motion ?
    What do the hull sections look like, rounded or more parabolic?
     
  14. Tony.Ellen
    Joined: Nov 2019
    Posts: 7
    Likes: 2, Points: 3
    Location: Sweden

    Tony.Ellen Junior Member

     

    Attached Files:

    fallguy likes this.

  15. oldmulti
    Joined: May 2019
    Posts: 1,145
    Likes: 569, Points: 113
    Location: australia

    oldmulti Senior Member

    R.C. Johnson better known as Skip Johnson designed many great looking tris and cats that performed well in the 70's, 80’s and 90's but was not known much outside the USA. Skip Johnson’s mast aft catamaran was designed in 1976 and is 42.6 x 23.5 foot with a weight is 8,130 lbs and a displacement is 11,700 lbs. The aft mast is 48 foot long although the rig is only 43 foot of the deck. The rig is mainly a genoa and staysail on a mast aft rig. The main genoa is on a roller furler is 690 square foot and the staysail is 283 square foot. Mast aft rigs often require strong furlers and serious winch power to wind in big headsails. The hull length to beam is 11.7 to 1. The hulls have retractable centreboards and spade rudders. The underwing clearance is 2.4 foot aft and 3 foot under the forward engine.

    The cat is designed to be built by amateurs using foam sandwich built on a male mold. The hulls are polyester fiberglass (CSM, woven rovings), Airex foam and polyester fiberglass inside. Balsa is the core in high impact areas like the underwing. The cross beams are plywood with timber top and bottom flanges, the bulkheads etc are plywood.

    Now a discussion about the center of gravity of the entire design. We have a fine stern hull shape typical of the time and the centre of gravity is important. The mast aft is partially offset by the engine forward but there is also a lot of weight in the stronger longer mast required, the additional rigging size and chainplates required to support the mast and the structure supporting the mast base aft. Result is you are putting weight at the ends of a fine ended boat. Result the cat has to drive further into a wave to provide the buoyancy to lift that weight. This means more pitching.

    I sailed on regularly on a Spindrift 37 and Spindrift 45 that had hull shapes similar to this cat. Both the cats had modifications to the bow and stern to provide more fullness in the ends (bulbs at the bow, more U shape ate the stern). After the modifications both these cats were 1 to 2 knots faster through the water especially upwind and they pitched less. They were also more controllable downwind as the bows did not dive as deeply into the backs of waves etc. The down side was they had slightly rougher rides due to the extra fullness in the hulls.

    This partially addresses Redruben question as to why the Outremer 45 hull shape is more “comfortable”. The hull shape, weight distribution, centre of gravity of the rig, the mast position on the cat, efficiency of the underwater foils and the length to beam of the hulls dictate how a cat sails through waves. Full asymmetric ends of 10 to 1 plus length to beam hulls with as much weight as possible centralized and a light efficient rig will generally have a comfortable motion. But be warned, I have thrown up on fine ended boats that others are OK on and they have thrown up on full ended boats I fine OK. Each person has there own version of comfort.

    Back to mast aft rigs. If this 42 foot cat had a Fractional Bermudian rig it would be lighter in mast and rigging dimensions and also would have the weight of the mast/rig more centrally located with a lower rig centre of gravity. Less pitching moment and probably a faster smoother boat.

    The accommodation layout is in the jpegs. The base of this design is good, the rig is not my preferred but could be used. Later designs generally have fuller ends and better fractional rigs but if one of these became available cheap it could be good fun. The jpegs include his Seawings 24 and 36 foot tri’s Skip also designed.
     

    Attached Files:

Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.