Multihull Structure Thoughts

Martyn Smith also did all the stress analysis on the Freebird 50, the Firebird's big sister.
He was the chief stress engineer at British Aerospace and was involved in the design of Concorde.
I have occasionally had people asking me if I am him (as I am Martin Smyth), but I can only dream about having such brilliance.
Cruising World https://books.google.com/books?id=xiWI0JXgtdIC&pg=RA1-PA57&lpg=RA1-PA57&dq=martyn+smith+British+Aerospace&source=bl&ots=KMLhW2-mcv&sig=ACfU3U2JlRPAiRT4q6xdL4KnptV7Wr3m6g&hl=en&sa=X&ved=2ahUKEwiOz9Xr8dPlAhVnzlkKHStTByUQ6AEwAnoECAkQAQ#v=onepage&q=martyn%20smith%20British%20Aerospace&f=false

 

The My-cat looks like a simple little racer cruiser cat. It has a shorter waterline than many boats for its length and a simple rig. The cat is 26 x 16.4 foot weighs 1300 lbs and displaces 2500 lbs. It carries 474 square foot of sail in a main and a genoa. The boat can be disassembled to 7.2 foot to fit onto a trailer. The boats have a 12:1 length to beam ratio. The boat has 4 bunks. Its built from polyester foam e-glass with a composite main beam and aluminium fore and aft tubes. A nice simple boat good for day sailing etc. Then you read its IOMR rating of 0.998 with 3 crew. This is a seriously fast boat. It was the boat to beat in micro multihull racing (multihulls up to 26.25 foot with “some” accommodation) winning from 1985 onwards. It could perform across the wind range and go upwind and downwind with the best of the specialist cats and tris in the class. The formula 28 class is the modern equivalent of the micro multihull class. If you can find a second hand one in good condition you will be a very happy person.

 

This is about a new material for composite layups. A Swiss company Bcomp has developed the flax base fibre material for use in a resin matrix. The final product is like a unidirectional or biaxial cloth that can be used with polyester or epoxy resin. The strength can an be higher than polyester e glass but it will be light for a given stiffness. This is very interesting. The product was developed because its manufacture only produces about 3% of the carbon emission of the production of carbon fibre fabrics. I will let the PDF’s explain more but 2 EG’s will give the idea.

Bidirectional fabric with fibers oriented at 0° and 90°, suitable for manufacturing fiber reinforced composite products with a high performance and a low environmental impact. ampliTex® 5040 has a very good drapability and is ideal for complex shapes. High laminate stiffness is obtained due to the low crimp twill 2/2 weave.

Performance advantage. Considering that glass fibers have a density of 2.6 kg/dm3 and a tensile modulus of 70 GPa, the flax ampliTex® 0°/90° 300 gsm can replace a 495 gsm glass fiber 0°/90° fabric to have the same stiffness in tension. In compression, the performance of flax is a bit lower, thus the flax ampliTex® 0°/90° 300 gsm can replace a 410 gsm glass fiber 0°/90° fabric to have the same stiffness. This fabric is ideal to be combined with the powerRibs fabrics 5019 and 5020, replacing a 600gsm carbon fiber layer with same performances in bending.

Amplitex 5057. Considering that glass fibres have a density of 2600 kg/m3 and a tensile modulus of 70 GPa, the flax ampliTex UD 150 gsm can replace a 250 gsm glass fibre UD fabric to have the same stiffness in tension. In compression, the performance of flax is lower, so that the flax ampliTex UD 150 gsm can replace a 220 gsm glass fibre UD fabric to have the same stiffness.

General characteristics. Great compatibility with epoxy and polyester

Compatible with infusion based processes (vacuum infusion, RTM), wet layup, bladder inflation moulding (BIM) and compression moulding

Flax fibres always contain some humidity at ambient conditions. Some resins (especially polyesters) are sensitive to moisture and may badly polymerize or create bubbles. In that case, dry the fabrics before use (110°C for 15 minutes)

Fibre weight fraction of 50% can be reached with process pressure > 5 bars. However, the fibres absorb a lot of resin when hand-laminating the fabric and it tends to look “dry” (unless too much resin is used) before pressure is applied. We recommend controlling the amount of adhesive used for laminating and impregnating it with 50 to 60% resin in weight. Excess resin comes out while pressing the fabric. The web site contains all the fabric options and further information.

Natural fibre fabrics http://www.bcomp.ch/en/products/amplitex

 

The Trailertri 18 was Ian Farrier first public folding tri design. The tri was 18.5 x 13.6 foot capable of folding to 7.6 foot. The boats weight is 800 lbs and displaces 1700 lbs with a 23 foot mast carrying 215 square foot in main and genoa. The length to beam on the main hull is 5.2:1. The construction is plywood and timber. The hull is 6 mm on the bottom and 4.8 mm on hull sides. The stringers are 19 x 19 mm the chines are taped glass. Decks are 6 mm ply with 25 x 18 mm closely set deck stringers. This boat was lightly built but structurally sound. I have sailed an 18 and it is good in light to moderate airs. As with all small boats it can sail faster than the numbers would suggest if the boat is kept light. In heavy airs if the boat is really driven really hard, the main hull starts to “plane” and the boat can go fast for its size. But be very alert, as these floats were submersible and you could be going along well then slow very rapidly when a float goes under and the cross beams plough into the waves with you needing to release sails fast. This boat was the start of a line of plywood tris which formed the design basis of the F series of foam glass boats. This boat was way ahead of any other really viable folding trimaran which is the reason there was literally 1000’s of trailtri’s and F boats have been home and production built globally.

 

In posts 132 and 140 I mentioned a 50 foot Myers catamaran Manta. It was mentioned because of its I beam aluminium cross beams. I have found the documentation about the boat which is still in charter in the Western Caribbean. Manta was a development from Myers 44 foot Seabird racing cat (Transpac record holder at one point) and 58 foot charter cat. It was deemed to be ‘convertible” for both functions. The cat was 50 x 25 foot with an estimated light displacement of 10,000 lbs. The boat had a 60 foot mast carrying 1400 square foot in the main and genoa. All standing rigging is 9.5 mm 1x19 stainless steel wire except for the upper shrouds and backstays which are 8 mm. The hulls were 15.5:1 length to beam. The hulls were cold moulded plywood with a fiberglass covering. My guess is a triple moulded 3 mm WRC layup with stringers inside. The boat had a few interest aspects such as the hull were splayed slightly to increase stability, there were instrument clusters on each hull to allow helmsmen to optimise speed, the wing deck was over 5 foot from the water etc. These things were all about getting speed. The Ibeam cross beams are shown in the attached jpegs. I have no idea of the performance of the boat but the numbers indicate a very fast boat in the right hands. PS according to design work done by several designers once a hulls length to beam is over 16:1 fine transoms work nearly as well as full transoms under most sailing conditions. Modern theory says this is still valid IF the buoyancy distribution in the hull is correct. In short don’t make the hull a pot belly with no buoyancy at the ends.

 

John Hitch designed the simple ply Hitchiker 33 and 40 (post 308) then moved onto the Hitchhiker 50 “Wired” The boat was 50 x 36 foot that had a designed weight of 11000 lbs but under later ownership actually displaced 22000 lbs. The sail area is in a sloop rig with 2 forestays with 1 forestay going to each bow. This boat is a very big example of evolutionary design, building and redesign by various owners. The concept of the design is 2 long hulls with an aluminium triangulation structure to join the hulls. Boat was designed to be built with strip plank and glass but after the first part first hull was built it was found to be to labour intensive and John choose to build in standard plywood construction. The idea was to build a cheap light boat that was diagonally very stiff. Hitch hiker hulls and decks are 9mm marine ply, with 50 x 25 mm stringers at 225 mm centrelines on 12 mm plywood bulkheads. The hulls and decks are epoxy saturated, sheathed externally with 6oz above the waterline and 12 oz below. The hull bottom is 37 mm thick. The 4 main “cross beam” connecting tubes are 250 mm joining under the mast and inserted into hulls at the inner gunnel. The base of the aluminium triangle structure had 25 mm stainless steel wires connecting the hulls together. Each side of the cat has a 200 mm aluminium tubes go from the mast joining base to the outer hull gunnel. In the original cat a central centreboard was located on a frame structure between the hulls. Now we start on the modifications. The central centreboard did not work as the board wobbled around too much. Fixed aluminium keels were eventually added by a later owner and found to be more effective. “Wired” was partially destroyed in a fire in 2003. The remaining starboard hull, mast, rigging and sails were rebuilt over the next 16 months and relaunched in 2005 with a small deck cabin that was ”hung” from the aluminium tubes. The overall weight of the boat increased with “cruising gear and the new deck structure” which probably was the reason for the bent tube.

The relaunched “wired” on its first cruise it bent one of its 250 mm aluminium tubes (I suspect this was not a designer problem but an increased weight and modification problem) which was repaired with a carbon fibre sleeve followed by a new box ply crossbeam that supported the deck cabin onto the hulls. After another 5 years of cruising the owner then removed the aluminium cross beam structure and reduced the beam to 25 foot and put on a larger bridge deck cabin. His reason was it had more useful room and at 36 foot beam the boat was difficult to park at marina’s etc.

‘Wired” in its original form could cruise at 10 knots and peak at 20 knots. The rig allowed the boat to go upwind well. The hull shaped pitched a bit which resulted in horizontal plates being attached to the rudders to reduce pitching. The concept was interesting but all subsequent Hitch 50’s had solid crossbeams.

 

One of Lock Crowther’s racing catamarans that was not promoted at all was a 45 foot version of D Flawless Super Shockwave 37 cat. The pure racing cat was designed and built for the Round Britain and Island race in 1985 for Simon Frost. The cat was 45 x 29 foot displaced on average 6500 lbs and carried on a 55 foot wing mast 1005 square foot of sail in the main and fore triangle. The cat hulls were 300 gsm s glass style 5800, 160 gsm Kevlar type 2800 20 mm airex foam 160 gsm Kevlar type 2800 inside all in epoxy. The decks 300 gsm s glass style 5800, 160 gsm Kevlar type 2800 16 mm Klegecell foam 160 gsm Kevlar type 2800 inside all in epoxy. There were many bulkheads in the hulls The hull weight was about 0.73 lbs/square foot. The cross beams were aluminium tubes fore and aft with the main crossbeam a fabricated aluminium box structure. The Wing mast was constructed from layers of 2 mm veneers of douglas fir or spruce into an airfoil shape with timber insets in the nose main spar and aft for a track to be attached. Each hull was set up to be a survival capsule if there was a capsize.

The cat called Glucometer 11 entered the race running 18 th on the first leg then climbed to 11 th on the second last leg. On the last leg of the race Glucometer II was run down by a Dutch coaster of Beachy Head. Fortunately her crew Simon Frost and Lock Crowther were rescued. The winner of the 1985 RB&I race was the 60 ft Apricot tri skippered by Tony Bullimore who did the race in 17 days 7 hours. 11th over the line was the 60 foot mono Thursday Child which did the course in 19 days 7 hrs. The last boat in took 33 days. This was a seriously fast cat with a Bruce number of 2.2. Sorry I can not find any pictures but the attached jpegs give a lot of information.

 

Gary Lidgard is an Australian designer who has been spoken about before. I will focus in on his 39 foot design that ended up being the basis of the Fusion 40 kit catamaran. Several of these cats has been built by home builders and many Fusion 40 kits have been sold. The 39 cat is 39.3 x 24.5 foot displacing 16900 lbs. The sail area is 928 square foot in the main and jib on a 55 foot mast. The hull length to beam is 10.6:1. The Bruce number is 1.2. This is a reasonable cruiser that can average 9 knots and top out at 20 knots in the right conditions. This is a foam glass design but I cannot remember or wrote down the exact details when I saw a boat being built, but it was something like 18 mm foam with 850 or 1100 gsm triax outside and 850 biax inside. All pretty conventional. The only difference was the mast step beam being forward of the main beam on a secondary beam that then flowed back into the main beam. Fusion 40’s sail well as the 39 was supposed to sail as well. The jpegs are a combination of the 39, 40 and Fusion designs. The hull frame jpeg are the frames required for the mould used to build the hulls in foam glass before stringers are on it. The problem of a home built foam glass is that you in effect "build" a hull 3 times. Once the mould frame, second lay the foam and fair it, third lay the glass and fair the glass. Flat panels or cheap female moulds end up being faster to build for most home builders. Rob Denny has developed his Intelligent Infusion technique of a cheap female moulds to a point where you can build a 40 foot proa in literally half the time of a convential hand laid foam glass boat.

 

I think it is remarable, there is nearly no Rocker and no swept up stern to be found at the hulls and the Hitchhiker 33 seems to
sail well without them.
Thank you for your excellent work, oldmulti!

 

This piece of research was done in 1980 but still has some relevance today. A French guy paid for some serious tank testing to understand the effect of stern shape, trim and displacement on a 27 foot catamaran hull for hull speeds up to 14 knots. The base cat hull was semi circular sections from midship to the bow. The 5 shapes in the after sections were 1. Narrow transom with aft overhang like Myers Seabird. 2. Medium and slightly immersed semi circular transom. 3. Immersed transom with the same section as the midship section. 4. Flatter transom with elliptical section. 5. Flat transom with hard chines in the aft third of the hull. Results:

Up to 3 knots hull speed. 1 hull shape was slightly faster than 2. Very near are hulls 3,4,5. Transom drag at that speed is purely psychological as all the drag is friction.

3 to 6 knots hull speed. All shapes about the same.

Above 6 knots hull speed. Shape 5 has the edge with the hard chine and fuller transom creating a flatter wake with smaller drag.

Change of trim experiments concluded the was little advantage in altering the trim. If you pushed the bow down you added wetted surface, if you depressed the stern you added drag.

Change of displacement effects the drag. If you can fly a hull you can reduce a cats drag.

Translation. Semicircular sections forward and a flat transom with hard chines aft is the best compromise for a cat in this 1980 test. Over the last 35 years the hull shapes have evolved toward this shape especially with greater understanding of buoyancy distribution and the effects of hull pitching.

The only problem is the shape compromises required for simplified building or accommodation requirements.

 

Manfred. The 33 hull shape is used on the 40 and 50. It literally is the same hull shape just stretched out. The lack of rocker and upsweep at the stern is due to the hull being basically a canoe shape under water. The low aspect ratio keels provided a pivot point for tacking. Of the owners I have spoken to of the 33 and 40’s they all say they sail reasonably and track well. Upwind they need some persuasion to tack unless the rig is really well set up. Nearly every 33 or 40 I have seen has horizontal plates on the rudders or near the sterns to minimise pitching. The owner of the 33 I spoke to in Queensland, who had sailed around the world, said she would do it again in a 33 but with bulbs on the bows and a simple low deck cabin to hide behind in rough or very hot weather. A good basic boat but I suspect you could build a faster cheaper cat in modern materials now.

 

The Maine Cat 30 is a cruising catamaran of 30 x 18 foot of 6000 lbs displacement with a 2000 lbs payload capability. It has a 366 square foot of sail in the main and 134 square foot furling jib. The cat sails well in light airs doing about 50% of wind speed and can do up to 15 knots. It has a 2.1 foot underwing clearance.

Her hulls have buoyant ends with deep forefoot and flaring topsides, which build up buoyancy rapidly when immersed. The deadrise aft is gradual and flattening in the underwater sections. The Maine Cat 30 hull shape has a beam to length ratio of 1:10 which yields a maximum waterline beam of 2 feet 11 inches when displacing 6,000 pounds. The underwater foil on the Maine Cat 30 is a daggerboard in the starboard hull with uphaul and downhaul lines located on deck. The fixed, balanced rudders are protected by 9 inch bumper keels located at the deepest section of the hull. The "bumper-keels" which are not part of the main hull structural lay-up The Maine Cat 30 is being built in female molds utilizing vinylester and polyester resins, with 450 gram CSM, 2 plies of 1605 (520gsm 45/45 with 150 gram CSM) bi-directional stitched fabrics and a 300 gram CSM either side of a 12 mm Core-Cell foam. All assemblies are finished with peel-ply interior surfaces and gelcoated with 944 Cook Composites to eliminate the need for expensive and heavy hull and deck liners. Bulkheads and fabric headliners have been eliminated to save weight. All interior cabinetry is vacuum bagged foam core construction bonded in place. Additional strength and safety have been added to the hulls below the waterline with additional fiberglass reinforcement. A small amount of wood trim is used for handholds and drawers. The hull-deck joint is entirely glassed over with tri-directional fabric. In the bow of each ama is a collision bulkhead. These form two flotation chambers that, combined with a third under the master berth and the buoyancy of the core, theoretically would float 12,000 lbs.

The hulls have a double berth in the aft quarter of each hull. There is a single berth forward in the port hull. The starboard hull forward has an enclosed head with a shower.

 

I don't know why I get a little concerned about the practically of foiling boats for cruising. The GC 32 in a race. Its a long way to fall.

 

The Raider 30 cat is a Tony Grainger design. The cat is 30 x 19.6 foot weighing 2800 lbs and for racing displaces 3600 lbs. An all carbon foam version weighs 3300 lbs for racing. The boat carries a 390 square foot main, 175 square foot jib and a 311 square foot screacher.This boat already has an impressive track record with several offshore racing wins including a 330-mile event in which they beat, boat for boat, 22 multihulls and 50 monohulls. The only boats that beat the prototype Raider were an Open 60, a 56-foot carbon racing cat and a 60-foot racing cat. In a recent 15.5-mile race in Florida in light air the Raider beat a F-31 by 10 minutes, 56 seconds. The high performance wing mast versions can keep pace with Seacart 30’s in heavy airs offshore.

The Raider is not exotic in its build. The only carbon fiber aboard is found in some reinforcements in the rudder and daggerboards; otherwise, the hull is vacuum-bagged glass over a Divinycell foam core. Hull is 858 gsm quadaxial outside in Vinylester resin 12 mm divinycell and 607 gsm quadaxial cloth polyester resin inside. The spars are all aluminium, on both the standard rig (with a 42-foot, 6-inch fixed mast) and the racing rig (with a 46-foot wing mast). The boat can also be derigged for trailering.

With just 8 to 10 knots of wind the boat runs at 11 to 15 knots. It starts to fly a hull when the wind hits 12 and has plenty of volume forward to keep the lee bow from stuffing into waves. In just 15 knots of breeze the boat will purportedly move at over 20 knots, with a maximum speed somewhere in the mid-20s.

The reason for showing this boat is to compare it to the Maine cat 30. The main cat has a 10 : 1 length to beam ratio. The Raider has a 11: 1 length to beam ratio. The Maine cat 30 has a displacement length of 107. The Raider has a displacement length of 60. The Maine cat has a sail area of 500 square feet in the main and jib. The Raider has 565 square feet in main and jib. But the killer difference is the displacement. The Maine cat is 6000 lbs, the Raider is 3600 lbs. The difference in performance is dramatic. The Raider will top out at 25 knots and exceed wind speed across the range, the Maine cat 30 will run to 15 knots and only match wind speed when reaching in a narrow range. This is not to denigrate the Maine cat 30, it is an excellent cruiser that due to its hull strength will take you far and wide. The Raider will get you there faster with less accommodation and need a lot more vigilance along the way if you are sailing fast.

 

I based the folding concept I use on my little cats on Wired and Robyn Chamberlin's cat Excess.

In some ways the use of a simple beam structure for the main beam of a cat is not the only way to do things. In many other areas of engineering simply supported beams are used only up to a certain size - in bridges for instance, simply supported beams are used in short roadways only. For larger bridges the tensile and compressive loads are taken by individual members. For example large bridges are usually suspension bridges (with the tension in the wires and compression in the towers) or arch (with the tension in the bedrock and the compression in the arch). You can do more for less weight if you go this way.

Wireds tubes take the compression loads from the rig and distribute them to the hulls in a much less stressful method than a normal simply supported beam. If you want to break a stick, you will break it by bending, not by compressing it, bending breaks things easier than compressing things. That is why we don't like our masts bending too much.

Wired could be really wide and much lighter than the other 50 I know of -Alia. She had massive normal beams and was not particularly quick, whereas Wired could get up and go. Same hulls, different weight and rig.

What is interesting for me is that the central radiating point under the mast allows the hulls to independently rotate. I found out when I tried to use something similar on a mock version of my folder that there needed to be other structures to resist torque (one hull bow up and one hull bow down). I used a back beam as did Robin Chamberlin on Excess. But what John did is marry his twin headstay rig with the pyramid structure. You can tighten the rig a lot (because the pyramid can take lots of load) and because the mast is supported by four wires leading to each bow and stern lifting one bow would move the mast, which would move the whole structure, causing the cat to be very stiff. The mast needs to be an integral part of Wired, without it she will flop around. That is probably why she is pictured being launched with her rig on.