Multihull Structure Thoughts

Back in 2017 I was pretty serious about purchasing a Stiletto. In fact, I still have the brochures for the original 3 models. I am attaching them for your files. Oldmulti, Thank you so much for sharing your archive of knowledge!

 

Kevin Baxley has owned a F-27 and F-31, he has also owned two Chris White Atlantic 42s at various times. Kevin Baxley believes a trimaran gives him the best feeling for sailing and ride on the sea. He did not need a large cat for his style of sailing in his 60’s and so started looking for his next design. He could not find what he wanted in 2011 so he went to Walworth Designs to create his trimaran. The tri was the PT 11 sport cruiser. The PT 11 is 36 x 28.1 foot that weighs 6,000 lbs and carries a 50 foot rotating pre-preg Hall carbon wing mast section with a boom that is fitted with a platform and lazy jacks for ease of sail handling. The mainsail cover is cleverly incorporated into the boom platform (an ACC idea -- works great) without any of the bulk that is typical of most sail covers that are permanently attached. A code 0 upwind sail and an asymmetrical spinnaker are flown from the bowsprit. The mainsail is a square head fully battened mainsail, and the jib is mounted on a Harken furling assembly on the forestay. The mainsail and self-tacking jib are 842 square foot, the main and screecher is 1,107 square foot and the downwind sail area is 1,868 square foot. The shrouds are 11mm Maffioli Ultra Wire with Dyneema lashings. Sails are by Quantum/ Thurston of Bristol, R.I.

The main hull based daggerboard draws 4 foot and the underslung spade rudder is on the main hull. The main hull length to beam is about 10 to 1. The floats have 200% buoyancy. A weight of 6,000 lbs and 842 square foot of upwind sail equals performance.

To reduce weight, the PT 11 was equipped with a minimalist yet comfortable interior. The accommodation is a double berth forward, toilet, dinette that can be converted into berths and galley. A 12-volt freezer and additional storage is located under the settee. Auxiliary power is a 21-hp Nanni diesel with ZF saildrive and a Slipstream two-bladed folding prop. The estimated motoring range of 150 miles, which seems low, but given the boat’s ability to move in light air, is OK. A lightweight 320-amp, 12-volt lithium-ion house battery bank provides ancillary power for the freezer and electronics.

The PT 11 lightweight, 6,000-lb hulls, are constructed using uni-directional S-glass with Nomex honeycomb and SP Corecell A500 foam core. All structural and interior partitions are carbon and Airex C70 foam cored panels. The dagger boards are a combination of carbon and E-glass. All laminates are vacuum consolidated and post cured. The resin is mainly epoxy. These materials and techniques were required to limit weight while maintaining high strength. The cross beams are carbon fibre epoxy which are permentaly attached to the floats with stubs ends on the cross beams that slot into the main hull. The float cross arms are connected to the main hull using removable pins. This arrangement allows transportation of the tri for local and overseas travel but this is not a trailer sailor. The design and build engineering required for this form of crossbeam attachment is significant. I have seen the result of a 30 foot tri, with the same cross beam arrangement, that had a major rebuild of main hull bulkheads where the beams slotted in. The torque on 350 mm cross beam stubs is substantial and the main hull internal structure needs to be very good to handle the loads.

The designer calculated the PT 11 will sail at wind speed up to true wind speeds of 8 to 15 knots. Above that wind speed, the boat speed will flatten out due to an increasing sea state. On a reach, the PT 11 should be able to hit the low 20's regularly depending upon the wind and sea state. An actual test said “Sailing performance is excellent. She has proven to be well balanced and a joy to sail. Going to windward with the deep daggerboard and powerful sail plan and high buoyancy amas is a dream. Off the air she picks up her skirts and flies. In light air she will ghost along on the lightest zephyr, easily matching the wind speed. With a little bit more velocity she readily gets into the teens and has hit a top speed of 24 knots.”

Kevin Baxley has a lot of sailing experience in multihulls and had a clear vision of what he wanted. Walworth designs delivered on that vision. A very good tri, well-constructed, that has a very high performance. An excellent example that a practical smaller multi can achieve very good results. The jpegs show parts of the story.

 

François Jaubert is an architect in building construction, he has also trained as a naval architecture. As part of his graduation project, he designed this 23 foot trimaran. I am sorry but this story is going to be short on numbers. The tri is 23 foot long and will weigh 1,350 lbs all up. The wood for the structure weighed 560 lbs. As the tri is not launched and a graduation project, I hope he hits his numbers but theory does not always translate to reality. As Fallguy said, he got 53 lbs of excess weigh out of a 16 foot dingy. The main hull waterline beam looks narrow. The floats look like they are high buoyancy. If it has a Hobie 20 or 21 rig on the tri, it will perform well in my estimation. The indication is Francois wants a bigger rig than a Hobie.

The tri will have Farrier type folding beams constructed from plywood and fabricated folding components. Because Francois says the folding components are complex he has suggested he will not sell plans to the tri. This is a pity. The aim of the boat is a performance cruiser with simple accommodation. There is a berth, some seating and a galley storage area. The headroom is about 4.5 foot.

The construction is strip plank cedar covered with 200 gsm cloth on either side for the hulls main hull deck and some internal furniture. The cedar would be 9 mm or 12 mm thick. The hull bulkheads are plywood as are the float decks. The plywood would be 6 mm or 4 mm thick to meet the weight requirements. All in epoxy.

It took François a year of work to build the 3 hulls and decks of his boat. For this he just used a screwdriver, a jigsaw and a sander. He is not able to afford a rig for the tri as yet, so we will have to wait for a performance report.

The jpegs tell the story better than words. Hope we will see more of this tri.

 

Oldmulti, this has been a great thread. Much to be learned here. Thank you.

What would the performance benefits be for a 45 ft catamaran with a carbon mast (compared to aluminum) ? To be more specific, mast height 0f 60 foot, weight of boat 18,000 lbs. , and a carbon mast lowering the CG by 3 foot. Would it noticeably reduce the pitching moment (hobbyhorse) ?

 

Waypoint. The carbon mast would improve performance if it is lighter and lowers the CG 3 foot. BUT if you are still using 1x19 SS rigging, heavy dacron sails and the mast does not rotate you are only gaining half the benefit. Yes, pitching will be reduced slightly, but if EG anchors were removed from the bow, excess junk thrown out of aft lockers and EG water tanks centralized you would also reduce pitching. A carbon mast is only part of a bigger picture and possible solution to minimizing pitching.

 

The Tricat 6.90 trimaran is a new design that is geared towards family cruising. The designer is Jack MICHAL has already assisted in the design of the Tricat 25 tri. The Tricat 6.90 is 22.65 x 15 foot and can be folded with swing arms to 8 foot. The weight is 1600 lbs with a sail area depending on the version. The family version has a 205 square foot mainsail and a jib of 105 square foot. The sports version has a 238 square foot mainsail and a jib of 116 square foot. Both versions can carry a 260 square foot gennaker and a 366 square foot spinnaker. The float dagger boards draw 4 foot when down and there is a kickup rudder on the main hull.

The folding system for trailing is similar to the Dragonfly tris. It is a one-minute swing wing horizontal folding system. The cross arms have water stays for extra strength. The family version offers four berths, a galley area and a potapotti. An important aspect for sailing is all controls are led to the cockpit for the helmsman to access.

The build of the tri is a polyester e glass biaxial PVC foam resin infusion process. Additional layers of glass are laid in the main hull keel area. The bulkheads and bunk fronts are plywood. The float dagger boards are carbon fibre.

Judging by the performance of the Tricat 20 and 25 this tri will sail very well in light to moderate conditions with peak speeds of 15 or so knots. The tri will handle heavier conditions but it basically a bay and coastal cruiser not an ocean crosser.

A very good performance cruiser for a small family. The jpegs give the idea.

 

The following is about a man who wanted a cheap boat for fun and a possible Texas 200 racer. He choose to do a pacific proa with a lug rig initially. The first version of the was 21 x 10 foot with a 14 foot long float. The sail area was 90 square foot and this version was steered by a sweep oar. This version was quickly modified to a 21 x 11 foot version with a home made Gibbons rig and eventually a kickup rudder attached to the side of the hull. There was another version that went racing in the Texas 200 was 21 x 8.25 foot with a schooner rig. The schooner rig has 2 windsurfer masts with timber inserts in the bottom carrying two 65 square foot mainsails and a jib that is moved from bow to the other bow on the main hull.

Now the only consistency in the build over the versions was the main hull. The main hull was built from 3 mm doorskin plywood, 4 litres of epoxy some cloth and tape. There are timber stringers, bulkheads and deck beams. The hull is 21 x 1.5 foot wide. The first float was a 14 foot deep V plywood build of 3 mm plywood. The next version of the float was solid blue foam shaped and then covered with fiberglass and faired.

The crossbeams are a learning curve. The first cross beams were 2 old Laser masts cut to 11 foot. These beams worked to a degree, but bent when crew sat out on them. There were a few timber variations of the cross beams that started out as I-beams of 6 mm ply. They were weak in torsion. He boxed them in, added 3 mm plywood sides to make them 76 x 88 mm box beams, and wrapped them in 10 oz biax cloth. Structurally they worked well but were heavy. After some sailing, he found setting up the proa from the trailer to water took about 2 hours and about an 1.5 hours to put back on the trailer. So, he decided to reduce the beam to 8.25 foot to allow it to remain a solid proa and easy to trail. He also found no practical difference in sailing stability as he changed to a schooner rig at the same time. The 8.25 foot cross beams were cedar 52 x 104 mm beams with cedar 25 x 104 mm glued top and bottom and a bit of uni glass on the top and bottom are both lighter and easier to make than the overbuilt box beams that they replaced. The final upgrades were the need for a bigger leeboard and bigger rudders and fix the rudder flex, mast flex and the rudder housing flex.

Next, we talk about the rigs. The initial lug rig only lasted a couple of trips. The next rig was a Gibbons rig, which is a short mast with a top yard of 2 windsurfer masts joined at the bases and a triangular sail that’s clew points downward. You pull one end of the yard down near the deck that is the logical bow and you have a good windward rig. BUT this rig takes a bit of refinement to handle well and the sail can generate a lot of power that if not controlled well can be a bit of a beast to handle. After 2 versions of the Gibbons sail and various refinements he got reasonable speed and control in 10 knot winds but at higher wind speeds the rig became hard to handle including a pitch pole. After 25 trips with the Gibbons rig and prior to the Texas 200 he wanted his proa to sail across all wind ranges and modified the rigging to a schooner rig with a headsail. This rig worked in light to heavy wind ranges and could be easily reefed. The rig required special timber booms, yards and mast head attachments which all were home made mainly of wood.

The performance of the pacific proa wass about the same as a Hobie 16 with Hobie 18 performance at its best (actual side by side racing in the Texas 200). This is not to bad for a home built evolved pacific proa. The owner commented “The boat tacks through 100 to 110 degrees, not very good. But it's enjoyable and safe to sail, shunting is no issue, I hit 10 knots on a reach under perfect control in 13 knots of wind a few weekends ago with no worries.”

The jpegs try and tell parts of the story. A cheap fun proa that turned into a reasonable raid type racer with a lot of development. We all learn a lot from a man who started to build in about 2004 and raced in Texas 200 in 2008.

 

The KD 1000 is a Bernard Kohler performance cruising cat design. The cat is 32.4 x 20.4 foot that weighs 4,500 lbs and displaces 9,400 lbs. The fixed mast is 41 foot high and is a glass fiber flag pole as its base. The boom is a double sided sprit (wishbone) boom. It carries a 322 square foot mainsail that is on a roller furler, a fractional roller furler jib is 145 square foot and a genoa of 430 square foot on a prodder to the masthead. The mast has swept forward spreaders to counter the forward thrust of the staysail boom. The length to beam is 10 to 1 and the daggerboards draw 5.5 foot and stern based kickup rudders.

The accommodation has a master berth on the wing deck centrally for most comfort room. The galley is in a hull to lower the centre of gravity and it allows a feeling of space in the saloon for sitting and navigation. The bathroom is large for this size of boat. There is full headroom throughout. The clear split from living area, kitchen and bathroom ensures a pleasant stay on the cat also for long term cruising.

The KD 1000 is a plywood/glass/Epoxy composite construction. The hulls have a multi chine cross section with rounded side panels. Construction time about 400 hours per hull. The hull have 9 or 10 mm plywood on timber stringers with framed plywood bulkheads. The main cross beam bulkheads are plywood panels with structural timber framing. The wing frames are ply with timber deck/underwing stringers. The underwing is a ply timber framing ply sandwich. The coach roof is a modified geodetic construction where panels are bent and glued over templates. The roof has 3 layers of 4 mm plywood. Easy to bent and immense strong after the epoxy is cured. The roof is finished with glass clothes. Some inside stringers are added at the corners of the plywood panels intersects. Decks are plywood on deck beams.

The designer the cat will be fast as speed predictions show EG a speed of 10 knots in wind force 4. The boat will reach above 20 knots under good conditions. Bernard’s speed predictions are accurate if the cat is driven by a good crew.

This is good design by a very good designer. Again this design is light easy to build and will be relatively cheap if the rig is built to plan. The jpegs tell the story.

 

The all carbon F101 is a foiling trimaran, aimed specifically at sailors who thought foiling was too difficult. The F101 concept originates from Rob Andrews and Alan Hillman experience of teaching people to foil in the international Moth class and developing a way of making foiling easier. The F101 is 17 x 8.4 foot and weighs 180 lbs ready to sail. The preferred crew weight is between 155 lbs and 270 lbs. The foiler can carry up to 330 lbs in light weather but is restricted to 220 lbs in stronger wind. The sail area is a 97 square foot and carries a 60 square foot mast head gennaker. The Gennaker is not required in winds above 12 knots. The mast is a 2 part all carbon structure.

Alan Hillman found a lack of initial stability in monohull moth designs which limited sailors to develop foiling skills so “The trimaran configuration gives you righting moment direct from the foil in the middle hull,” he says, “and gives a measure of safety. It’s hard to capsize the boat.” In short, the tri configuration allows people to sail the F101 as a “tri” so they can focus on learning the foiling aspects.

The F101 will take of in take off in wind speeds of 8 knots and has a target boat speed range of 10 to 25 knots. Tests of the F101 support the initial speed estimates and relative ease of sailing.

The main foil under the main hull is has a flap on the back edge which is controlled by a wand under the main hull. As you’re sailing, the foil wand hanging behind the trailing edge effectively feels where the boat is riding relative to the water and actuates the main flap. In light winds, it gives you more lift, and the boat pops up on the foil. Get too high, and the wand drops down even further, forcing negative lift on the flap, which brings you back down to your desired height and prevents the foils from breaking the surface.

The technology for the wand and flap interaction has been developed in Moth sailing. It took years and some bright people to optimize the arrangement and is now used in many other designs from 10 foot to 50 foot multihulls.

Construction is all carbon epoxy with some PVC foam construction for the main hull, foils, floats, beams and 2 piece mast. An all up weight of 180 lbs for a 17 foot tri is impressive.

The jpegs give the idea. The PDF is the Owners manuals which gives many insites as to the F101 and how to sail it.

 

Seen and sailed the F101, top class design with very very good attention to detail. Sadly the day we sailed it was in less than 4 knots of wind, it would just about get up on its foils before flopping back down. Impressed though that we could still sail it around at will with all the drag of the foils. Seemed really stiff and well thought out and one of those boats you really feel deserves to make it as a design.

Sadly for us we've been in Covid lock downs since and I've not been able to get back and sail it on a better wind day.

 

Another interesting design and build is the Flying Mantis, a British made sailing foiling trimaran which can also sail as a displacement trimaran with a daggerboard in a non foiling mode. The Flying Mantis is 15.25 foot length over all with a 13.75 foot waterline and a 7.9 foot beam. The ready to sail weight is 190 lbs with a maximum crew weight of 270 lbs. The 2 part 20.5 foot long carbon fibre epoxy mast carries a 95 square foot main and a 26 square foot self tacking jib. The trimaran can be disassembled into two 28 lbs float/cross beams and a 60 lbs central hull. The remaining components of foils mast sails ropes etc add the remaining 72 lbs. Basically, this boat is easy to transport and assemble or trail as a full unit when required.

To convert the daggerboard sailing (DBS) version to the foiling version requires the central replaced by a T foil and rudder T-foils. The foiling system is connected to a control wand. The control wand is attached to the end of a bowsprit, well forward of the central foil and close to the water. Being forward enables control of the pitch and ride height, and eliminates flow distortion from the central foil. A twist-grip tiller controls the angle of the rudder T-foil, giving positive lift upwind and negative lift downwind, (less chance of pitch-poling). The Flying Mantis is designed to provide early take-off at lower boat speeds enabling more fun in lighter winds. If the sailing conditions are not great for foiling, it’s easy to convert back to the daggerboard sailing.

The hulls are an epoxy sandwich carbon construction with a painted and lacquered finish that weighs less than the usual gel coat. The foils are a carbon pvc foam epoxy structure. The high performance, lightweight construction of the Flying Mantis means that this boat should not be run up onto or dragged along a beach.

The performance of the Flying Mantis can do up to 25 knots of boat speed in the right conditions. Tests sails have not reached that yet but others are claiming they have achieved it yet. 8 knots of wind appears to be take off speed.

Another very interesting design which is easier to sail than a foiling Moth monohull due to its better initial stability. The jpegs give the idea.

 

The following information over the next 2 days is a partial edited summary of one man’s approach to building a monohull foiling Moth. Moth rules are a maximum of 11 foot with 1.6 foot extensions at either end (eg rudder mounting or wand mounting). Maximum beam is 7.4 foot including wings. The sail area is 86 square foot in a single sail. There are no weight or material limitations which result in 23 lbs hulls and all up weights of 65 lbs. No multihulls, sailboards or trapezes allowed.

The reason for the interest is this guy developed a very good, light carbon fibre boat, from a home build approach. (PS when I say home build I mean in his lounge room). The first part will describe his mould and main hull build technique and the second item will describe the remaining build.

The Male mould. Cut MDF ribs matching measurements from your design drawings and place them on a strong back
Fill the voids between the ribs with solid foam ensuring a tight fit, or hold in place with glue or kebab sewers.
Shave the foam down to the same level as the ribs. Take great care where the hull curves as you do not want to take too much away.
Once you are happy with the shape you can then fill any small voids.
After this, using lightweight fibreglass e.g. 80-100 gsm, cover your mould with two layers - this will give it some protection from dents and scratches, plus a solid skin to work from.
Once set, sand it flat. If there are any imperfections then sand/fill them to get a good mould.
Cover in release tape (I used parcel tape). Make sure there are no creases in it.
Cover in release agent, as specified by the product you are using.

Building A Female Mould: Ensure that the male mould is well coated in release agent.
Cover your mould in 2 layers of lightweight 100g fiberglass and wet through with resin.
Add more heavy weight fibreglass - the more layers you put, on the stiffer and better the mould will be.
Build braces into you mould so that it will hold its shape and sit upright when complete.
Once you feel you have a solid mould, release the male from the female mould. Use small wedges to help pop it out.
Once released clean the female mould, ensuring that you have a clean surface to work with.
Fill and smooth any areas that may need it and then paint with a strong paint.
Compound and polish. The shinier it is, the easer the final hull will come out.
Once happy with the finish cover in release agent.

Building A Hull: Take time in preparation including cutting all bagging materials and all hull materials
Using the mould, lay and tape one layer of 200gsm of biax carbon +/- 45 on the centreline, ensure wetted thoroughly, Peel Ply over the top, and vacuum down until set.
Once set, remove bag and Peel Ply, mix up resin with mirco balloons in it to form a thin paste, then spread over your carbon on mould and foam sheets. Lay foam onto mould, tape or tack in place with small nails or staples, add a layer of Peel Ply, then vacuum again.
Once this has set, remove from bag and Peel Ply, ensure surface is smooth, then wet foam out with resin. Lay one layer of 200gsm carbon fibre at 0 degrees of centreline, Peel Ply and vacuum.
Once this has set, remove from bag and carefully from mould (sometimes easier said than done).
The hull is now ready for internal fitout.

The web address is: How to home-build a Moth - James Sainsbury's step-by-step guide! https://www.sail-world.com/news/229181/How-to-home-build-a-Moth
More tomorrow sorry about the limited jpegs. Visit the web address.

 

Today’s discussion will be on lifting foils used by the Moth class, tomorrows will be on the build of other structural components of a Moth. The reason for this item is there is a lot about how to build the foils but very little on the sections of foils used in successful racing Moths.

The basic of foiling is to have EG a fore and aft T foil that provides lift for the total weight of the boat and crew. The size and shape of the foil matters. Some foils are larger which allows easier take off and control but are often have a slower top speed. Other foils are smaller thinner and have a higher aspect ratio which permits higher top speeds but need more wind to take off. As some Moths reach 25 to 35 knots in optimum conditions, the size, type and placement of the foils become very important for control and personal safety.

The table below, in the jpeg section, is a combination of information gathered by Adam May and Ray Vellinga. Both parties have a strong interest in hydrofoil sailing boats. You will notice a strong similarity of foil sections with variations of foil dimensions. Other sections are used in Moth foiling but about 70% of a fleet uses basically these sections.

The span and chord differences are due to the sailing conditions and user experience with Moths in a foiling mode. Also some Moth sailor prefer the main foil to do most of the lifting, other sailors prefer a balance between the fore and aft foils.

The control wand mechanism to control the ride height of the main foil needs to be well developed to allow it to be adjustable for the conditions as well as doing its intended ride height control. The rudder T foil is mainly controlled by a twist grip on the tiller handle. Learning how to adjust the ride height and pitch control whilst sailing is quite a learning experience. Get it right and you will sail very fast in moderate conditions, get it wrong and you will be swimming.

Most of these foils are constructed of carbon fibre epoxy with some PVC foam for shaping.

This is the VERY simple guide and due to America Cup and many other classes adopting foils the wing sections used have evolved quite rapidly to provide more lift with less drag. In catamaran Americas Cup they had 3 of 4 sets of foils to suit the expected wind conditions on a day. At $400,000 per set it was an expensive game.

The jpegs show some foils and sections. The PDF is a serious tank test study of T section foils. It is a good read for those who want to really understand the characteristic and maths of foils.

 

After Moth Foils I will do the second part of a Moth build. This will be about internal hull, deck parts and wings.

First is the bulkheads and transoms which are a 200 gsm carbon fibre 45/45 biax, 5 mm pvc foam and a 0/90 200gsm carbon fibre. After building the front T foil you can build the daggerboard T foil case. This comprises of 10 layers (wraps around the vertical foil with a small spacer separating case from foil) of carbon fibre at both 45/45 and 0/90 in epoxy. Cut out inner skin and foam in hull at the daggerboard T foil location and biax to the hull bottom skin. Build two half bulkheads either side of the case. The hull deck is (guess) 200 gsm carbon fibre biax, 5 mm foam, 200 gsm biax carbon fibre epoxy. The nose cone is two layers of carbon - one biax and one plain over the join to the hull.

In the hull is a tube for the pushrod between the prodder and the pushrod to the flap down the Main T foil vertical shaft. The rudder normal lives on a series of carbon tubes attached to the transom of the hull.

The T foils are made from a shaped PVC foam template covered with unidirectional high strength carbon fibre with some 45/45 carbon fibre biax wrapped around the foils in epoxy. The flap is attached with a kevlar flexible attachment. The prodder is a 5 mm PVC core with unidirectional carbon fibre on the shaft.

Fixed wings are made from blue modelling foam to shape front and rear wing bars to desired shape. Then the blue foam is wrapped in 4 layers of 300 gsm carbon fibre unidirectional, alternated with +/- 45 biax 200 gsm carbon fibre. Pour acetone into the end of your wingbar and let it melt the blue foam away.

Make some joiner inserts made from a carbon fibre sandwich that will join either side together. Try and get them as far up the bars as you can as this will help with strength towards the end of the wing. The wings are tied to the main hull at the point of the bulkhead that supports the mast base. The rear wing attaches near the transom/bulkhead.

To build a Moth you use a lot of carbon fibre, thin PVC foam, peel ply and epoxy with vacuum bagging or infusion your friend. The same techniques could be used to build a smaller light weight cat or tri hulls and crossbeams. The previous foiling tris were built with carbon fibre in production molds. This approach could produce a home built experimental multihull with or without foils.

The jpegs are general, not specific examples.

 

Here is a small fun cat for home building designed by Dudley Dix. Dudley designed a 9 foot cat called the Oppikat which was intended for children but adults liked the cat. There were requests for a bigger version that could carry an adult. The Bigakat is 12.1 x 6.9 foot weight 200 lbs and carries a 20 foot round tube aluminium mast with a 84 square foot battened mainsail and a 27 square foot jib. The rigging is simple with a forestay and single side shrouds, attached to the chainplates with lashings. The jib is on soft hanks or piston hanks over the forestay. The keels are low aspect ratio with fixed rudders attached to the transom. The draft for the keel and rudders is 270 mm. The hulls length to beam 10.8 to 1.

The cat is designed for a novice sailor so the keels are sufficient for some leeway prevention, tacking and running onto a beach without damaging the rudders.

The Bigakat 12 is primarily intended for wood construction by amateurs. Bulkheads, transoms, decks and the upper panels of the hull sides are all made from 4mm plywood. A 19 x 19 mm gunnel and chine step strip is in each side of the hull. The lower sides and bottoms of the hulls are skinned with 6mm strip cedar and sheathed with a light layer of glass cloth (probably 200 or 300 gsm cloth) in epoxy resin. Each hull is built over 4 permanent bulkheads and 5 temporary ones, which are set up upside-down on a board that can be mounted on a large table, workbench or sawhorses. Once skinned and glassed, the hull is removed from the board, the temporary bulkheads removed from inside the hull. Those temporary bulkheads are then set up on the board again with new permanent bulkheads to build the second hull. After waterproofing the insides with two coats of epoxy, the decks are fitted. The cross beams are 85 mm aluminium tubes attached to the hulls with 2 bolts per hull. The mast beam has a stainless-steel dolphin striker attached.

This cat is designed for an adult and child or two teenagers to learn and have fun on and as a result is structurally stronger than a light weight racer would be. A lot of fun in a small package that should have reasonable performance. The limited jpegs give idea with the last 3 jpegs are of the smaller 9 foot Oppikat.