Multihull Structure Thoughts

MG did a 12' version of lil nip which uses 1 1/2 sheets for length. The more interesting detail though is he spec'd the boat with thinner ply and stringers. He mentions the smaller boat can be built with the same scantlings. I don't know how much if any weight is saved but the plan sets together give the builder plenty of options.

I have not found a builder who has posted a finished weight for any of these 3 designs, 8',10',12'. I would be very interested as they lend themselves to cartopping if light enough...

 

In a similar (sort of) fashion to Fatcat, I sketched out a design 20 odd years ago for a small fishing power cat (15'5' x 8') that I wanted to build at the time - however other things got in the way, and I never did build it. I am still hoping to build it one day though, and if the concept works, use the same semi-displacement hull form for a larger cat - say 24', or even up to 32'.
I called her a Moses cat, as here in Barbados any small (under 22' or so) open fishing boat is called a 'Moses'.
Attached are copies of my rough sketches.
Re the 'CAD' drawings, this was a very basic and simple program I had then called Plyboats - it was in DOS and worked very well. The hulls of the 49' powercat in my avatar were designed with this, along with a couple of other power cats, and I have designed and built a few dinghies with it.
She is not intended to be a 'fast' planing boat - rather, to potter along fairly sedately with minimal power and hopefully good fuel economy.
Since sketching these thoughts I would change some things - a wave breaker extending aft into a pod for the outboard engine would be good, in similar fashion to Fatcat.
Hull extensions (2' - 3' long) for swimming / boarding would be useful, as shown.
And I think that I would probably just have a shallow cockpit at bridgedeck level with a small console and a saddle type seat, similar to what one might find on large RHIBs.

 

“Halo” was designed by LOMOcean design for a Kiwi couple who wanted a light weight structure, large sail area, rotating carbon fibre wing mast and hydrodynamically optimised hull form and dagger boards. Halo is 46 x 26.3 foot. Further details are unknown. All system weight has been concentrated as close to amidships as possible to make for a sea kindly motion. Wingdeck clearance is large in line the vessel's offshore operations. Power is inboard diesel engines with shaft drive coupled to large diameter folding propellers.

Galley, dining navigation, communications and relaxing are up, with cabins and heads down. The port cabin is set up as the owner's suite with a double cabin aft, a small office / workshop / living area mid ships and a dedicated head and shower compartment forward. The starboard cabin incorporates the guest accommodation and day head. With a double cabin aft, head and shower compartment midships accessed from either forward or aft end and a forward cabin with sleeping space for four adults in twin singles up on the wing deck and a twin bunk up forward. All cabins feature large floor areas, heaps of storage space and full standing headroom in excess of 2m. The extreme ends of the vessel are not used for accommodation or storage to keep weight centralised.

Construction is from medium tech composite using PVC foam core sandwich, epoxy resin and stitched / knitted e-glass reinforcements augmented by carbon and aramid fibres in specific locations. A bow sprit attached to the monocoque forward cross beam is used to stiffen the forestay base. The interior will be kept light with minimal use of wooden trim and highlights.

For a cruiser the helming positions are a little exposed but if self steering is mainly intended it could be OK. The cockpit area is large and has steps to the water. The dingy davit is a single T section. Good for looks but will have to be well engineered to handle the uneven loads of a dingy with or without engine and maybe some water as it is pulled up. Minor issues compared to the whole design.

An interesting design. If anyone knows more please advise.

 

Alexandre Fortabat is a naval architect who produces some interesting designs. His latest is the Eco Cat which is 42 x 25 foot. The weight is 16,000 lbs depending on the material used, displacement is 22,000 lbs loaded for travel. The 55 foot fixed or rotating aluminium or carbon mast carries 1100 square foot of sail. The fixed keels draw 4.2 foot. The wingdeck has a high clearance. The power is Oceanvolt electric (from 2 x 10 kW), the battery pack is well centered for best positioning.

The reason for the varying numbers above is the design can be home built or professionally built with a common frame but varying shell materials from plywood to balsa glass flat panels. Alexandre wants the most eco-responsible construction solution possible. Alexandre Fortabat proposes a cat with developable shapes, built from flat panels. This cat could be in plywood or with sandwich panels made on marble. Thus the marble as well as the silicone which encloses the sandwich are reusable. This sandwich is composed of a balsa wood core (recyclable) and flax_fibre skins with biosourced epoxy resin. The construction is then as clean as possible. Alexandre says “For this construction, the working time only for the platform is estimated at about 3500 hours for an enlightened amateur.”

The design is computer generated ensuring the shapes are developable and able to be built as a “kit” in flat panels first. Then just assemble the kit near the water for launch.

The cat is an open bridgedeck with 3 double berth cabins, toilets in hulls. There is also a galley in one hull and seating in the other hull. The accommodation spaces can be adjusted to a 4 cabin layout. The bridgedeck has a large seating area with a Bimini cover. Access to all sail controls etc is excellent as is the helming positions.

This cat is a smaller version of his 45 and 49 foot versions that have been designed previously for less eco friendly builds using EG foam glass and vinylester resins. An interesting cat worthy of investigation. The jpegs give the idea.

 

Moderator. Could you please post the following updated indexes on page 1. They will replace the existing indexes on Page 1. Thankyou.

 

I can see a book deal on the horizon.

 

Today we discuss a fine trimaran that is designed to cross oceans with its initial target being a trans-Atlantic record attempt. “Samson” is 44 x 9.5 foot with a loaded displacement of about 4000 lbs. The length to beam is 13.5 to 1 on the main hull. The draft is 200 mm excluding appendages. The motive power is 2 humans, yes it’s a row boat.

Lomo NZ designed “Samson” which is an innovative wave piercing rowing boat optimised for ocean crossings. The vessel is designed for maximum efficiency – a crucial feature for a vessel powered by muscle alone. Refined geometry gives this vessel the ability to self-right, while a forward canard assists with course keeping. Departing from New York City, the pair will set out across the Atlantic Ocean, rowing almost 4000 miles to their destination in Paris. When completed, their journey will be the first crossing in the West to East direction. The attempt is expected to take between 60-80 days depending on weather.

Why is being done? To help high light issues about protecting and preserving our environment and giving our future generations the best opportunities in life.

The construction is foam carbon fiber in epoxy. This is a serious vessel designed and tested to achieve a task powered by 2 very fit people. I wish them well. The jpegs give the idea.

 

One man had a dream of building his own catamaran with a more traditional rig. He decided to design and build his own crab claw rig cat. The cat is 14 x 6.8 foot and weighs 168 lbs ready to sail. The sail are is 107 square foot. He decided to simplify the construction by doing a skeg on the hull or a “boardless” hull shape. Kickup rudders are on the sterns of the hulls. The aim is a day sailor that will give an understanding of the crab claw rig.

He decided on 14 foot hulls because the boat had to be easy to handle on land and this was his first boat building project so he wanted to keep it small. Next time he would do 16 foot or 2 full ply sheets. The crab claw rig was chosen because it has smaller lighter spars with a rig that has a lower centre of effort. It may have a weakness upwind but in theory the crabclaw goes downwind and reaching better.

He said the best method of construction was tortured plywood construction because it does not require a mold, is relatively inexpensive and results in a very light hull. Based on the density of plywood and the hull design, I estimated that I could create a 14' long plywood hull weighing less than 50 lb with a total boat weight of under 200 lbs,

The fun begins. He cut out 8 hull bulkheads per hull from 9 mm plywood. The bulkheads are setup with a 9 mm ply “skeg keel” is inserted to lock in the hull shape. Gunnels were inserted. The hulls were inverted and 3 mm ply attached to the hull sides by screws until the epoxy glue set onto the bulkheads and gunnels The hull bottoms that had the most curves was covered with shorter pieces of ply that could be partially shaped to form a skin. Call this a partially tortured hull.

Since the crab claw requires simple round spars for the mast, boom and yard I could make them myself from carbon fiber and fiberglass. The masts are approximately 12 foot long and 65 mm in diameter with a 3 mm wall thickness. The straight yards (spars) attached to the sail had to be made flexible enough to bend to fit the curvature of the sail and were made from 25 mm internal diameter with 2 mm walls fiberglass. They were not stiff enough and had a 25 mm copper tube push in them to stiffen them up .

The spars were made by the following process which requires a long pole set on a rotating motor to act as the mold for the spar. The outer diameter of the pole becomes the inner diameter of the spar.

Choose a pole (copper tubing or similar material) of the appropriate diameter and length for the tube you want to make. It must be stiff enough no to flex under it's own weight or else you will get wrinkles in the final part.

Support the pole at each end in a way that allows it to roll easily (two wheels mounted next to each other work well). One end must be connected to a motor so that the pole can be rotated at about 1 revolution per second.

Using small pieces of scotch tape, tape a layer of paper to the pole (this will make is possible to slide the spar off the pole when it's cured).

Now tape a layer of plastic (polyethylene film from McMaster works well) over that to keep the epoxy from soaking into the paper and sticking to the pole.

Cut the cloth (fiberglass of carbon fiber) to the desired length and width (the width of the material and the pole diameter will determine the final tube thickness).

Turn on the rotation and paint a thin layer of epoxy over the plastic sheet.

Turn off the rotation and lay one edge of the cloth along the wetted pole (wrap it around enough so that it doesn't fall off - the smaller the pole the harder this is).

Begin painting on epoxy over the cloth to wet it out.

Manually rotate the pole so that more cloth is wrapped around it and wet out the newly wrapped cloth with more epoxy.

Continue to repeat this until all of the cloth is wrapped around the pole.

Now turn on the rotation and use a squeegee to remove excess epoxy and tighten the wrap of the cloth around the pole.

Leave the pole rotating until the epoxy is cured.

When cured, you should be able to slide your new spar off the pole.

The cat sailed as expected. It can sail upwind but is good reaching and running. It took time to solve the sail yard issues and control issues but he enjoyed his time. A 10 to 12 knot sort of cat at peak speeds. The jpegs give the story.

 

A short but informative item. An "Arafura Cadet" is an 11 x 5.1 foot plywood cat that weighs 90 lbs for the hull and beam structure. It carries a 17 foot mast with a 20 square foot jib and 55 square foot mainsail. It has a central dagger board. There was over 150 of them built in Australia and there is still some class racing happening.

The cat was designed in the late 60's (Australia converted to decimal currency in 1966. Notice the 40 cent or 4 shilling price) and the design is still an effective cheap home built trainer for people between 6 and 70 years of age. The plans tell you about the plywood and timber version. Modern versions have aluminum beams and trampoline decks. So please read the plans .

 

The Paper Tiger is a very successful 14' Catamaran designed in 1969 by Roy Given in New Zeland, weighting 160 lbs with 100 ft² sail and daggerboards. More than 3200 were and are still build. She is a very active class in New Zeland. Plans are available at the association
Paper Tiger Catamaran https://www.papertigercatamaran.org/ptcia/

I built my plywood one in France in 2015 and sailed her with great pleasure since, aside my A class cat and a 20' Strike 20 Trimaran
This cat is worth to develop around the world !

 

At the age of twelve (1971) having saved up all my morning paper round money, I built an arafura cadet with my father looking over my shoulder. Rolly Tasker's factory had just burnt down, so I got some smoke stained sails for half price.
In my local club races, I chased around an Arrow (arafura big bro 14') & a paper tiger.
The paper tiger was the fastest in light airs, but he never turned up if the breeze was over 8kts.
So I never found out who would win in more breeze ?

 

Peterbike mentioned an Arrow which is the bigger brother of an Arafura day catamaran. The ARROW is 14.1 x 6.1 foot with a minimum weight (hulls and fixed fittings) of 110 lbs The ideal one person crew weight is 160 to 185 lbs. The fractional sloop has a rotating 20.2 foot by 89 x 65 mm (maximum) aluminium mast. The mainsail is 82 square foot and the jib is 29 square foot for a total 110.00 square foot. The Arrow has a central kickup daggerboard. The kickup rudders are transom hung.

The design was done by Neil Fowler and Roy Martin (Roy designed Miss Nylex the first solid wing Little Americas Cup cat) in the early 60’s. Neil designed the Arrow to fill a gap between the little Arafura and the 18 foot Arafura. He designed the tortured ply Mosquito in 1966 then designed the 20 foot Wirraway, which was a micro cruiser with a small cabin, commercially produced as the Red Baron.

The Arrow is a popular catamaran class local to Australia. Originally designed for amateur builders in ply and timber, it started to have a few professional builders do fiberglass skin versions but the weight and shape rules were maintained. Result the about 1800 boats all were competitive in the right hands with the best sails. The peak speed claimed was “Will plane on a reach where speeds up to 20 knots have been achieved.” The cat is relatively fast for a 14 footer but I think 20 knots would have been a one off.

The Arrow construction in ply and timber as originally specified in 1964 is 4 mm Gaboon plywood hulls with 7 bulkheads with some timber reinforcement and 18 x 18 mm chine, one stringer per side and gunnels. The crossbeam structure as shown in the jpegs is 50 x 38 mm box section attached to hulls by stainless steel bolts. The foils were originally timber then were made in glass. Either work well. The hulls are covered by 138 gsm glass doubled over the bottom. Please read the PDF’s as they contain details of the rig and the control measurements of the hulls with the minimum building requirements.

The jpegs give the idea of a very popular cat that still is raced seriously in many areas of Australia.

 

Peterbike , a link to the Australian Paper Tiger association :
Log into Facebook https://www.facebook.com/groups/6251883876/
The designer's name is Ron Given !
The original design technology was the same as Arafura, but the hulls shape are more evolved (not a dory shape).
They are built out a simple male mold. Ply was first used and still to day, but also fiberglass and carbon for the hulls and appendices.

 

A short one, Australian Will Richards managed to get his Arrow Cat foiling at the first attempt in time for his national’s invitation race in 2013. The Arrows 50 year old design surprisingly suits the foiling mode of sailing well, due to its single centre board and very low sailing weight. An Arrow is 14.1 x 6.1 foot with a minimum weight (hulls and fixed fittings) of 110 lbs and a rig of 110 square foot.

The Arrow is a standard Arrow with a large T foil where the daggerboard would be. There are no secondary foils, as the cat supposedly carries standard rudders. This means Will is very athletic and talented as he has to balance the cat foiling by his weigh shift fore and aft in variable winds and sail settings. This is not easy especially when you are responding to wind strength and shift variations. I know Kite board foilers can do this but the kite helps maintain stability. To quote one observer “Its not completely stable yet but it also doesn’t have any wings on the rudders either.”

No details are available about the foil but it looks like a timber foil glassed over. The rest of the arrow looks pretty standard. To achieve the sailing results on Wills first try is pretty impressive. There are Naval architects who don’t get foilers right on their first try.

The limited jpegs give the idea.

 

The Seawind 24 was one of the first production cats in Australia. It sold very well and eventually was sold in USA any several other countries. The Seawind 24 is 24 x 16 foot with a weight of 2000 lbs. The sail area is 345 square foot on a fractional rig aluminium mast. The length to beam is 11 to 1 with maximum waterline hull beam forward of centre. The draft with daggerboards down is 3.2 foot.

The Seawind 24 was designed as a bay and coastal cruiser racer. It can sail well especially in higher winds as its has a powerful rig and was wide for its length. The Seawinds other great feature was it was strongly built of solid glass with a very rounded shape which minimises the need for a lot of stiffeners or foam cores. The crossbeams are large aluminium sections.

Now we get to the point of this post. Because they are relatively cheap on the second hand market many are purchased and converted to what people think is a “sensible” cruiser. Some do a good job by adding EG a removable tent on deck to increase space, others add on a Binimi for sun shelter. Then there are some who “convert” the 24 foot Seawind into a full bridge deck full headroom cruiser with galley and seating in the main saloon and a large forward cockpit.

The highly modified Seawind is no longer a coastal cruiser racer but at best a bay cruiser in moderate conditions with downwind performance assured. Upwind performance will require the assistance of the large outboard to help it. The underwing clearance is reduced and the original freeboard is lower. The interior space may be increased but the sailing capability would be significantly reduced and in any serious weather could cause be a significant problem. I doubt there would have been a full structural analysis of the modifications so I would need to be convinced it has not affected the total strength of the cat.

Don’t be to inspired by this modification. The jpegs are of the modified Seawind 24 and the original cat then tent versions.