Multihull Structure Thoughts

Today is about an open wing deck catamaran designed by Uniworks Boats in Finland. The cat is 40.1 x 32.5 foot with a weight of 7600 lbs and a displacement of 12500 lbs. The 52.5 foot mast carries a 534 square foot mainsail, a self tacking jib of 355 square foot and a 730 square foot genoa. The length to beam on the hulls looks about 13 to 1. The draft is from 1.6 foot to 8 foot over the daggerboards. The engines are 2 diesels of 33 h.p. with sail drives.

The intended function of this cat is fast day charter catamaran. There is a large cockpit, which is designed for 10 passengers and two crew members and two cabins for a relaxing break, two toilets, shower and galley. The hulls are flared above the waterline which allows a reasonable amount of internal space that could be converted into a very good fast cruiser. There could be a couple of large double berth cabins aft, the galley and a seating area could be from mid to forward in one hull with seating, navigation and large toilet in the other hull.

As a fast cruiser concept this cat would be faster and more comfortable than EG a Wharram Tiki 38. Deep daggerboards, wide beam, light weight and powerful sail plans really make a difference. We are talking about 300 mile average days versus 200-mile days. The Uniworks 12 would also take about the same time and effort to build as the 38.

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

This is an excellent concept that for the racer cruisers amongst us and would be a lot of fun. As a charter catamaran your clients would be enjoying a few thrills on the days with a bit of wind. Sorry about the limited jpegs.

 

Here is a very small folding trimaran (collapsible amas), designed and built by a small tri lover in Italy. The designer wanted a tri to race in the 10 foot class with:
1) The design and build of a trimaran that can compete in speed and manoeuvrability with other boats of the Class but that can be built and assembled by anyone.
2) Not to use epoxy resin, nor to force the builder to nerve-wracking hours of sanding and painting.
3) To build a boat with an average weight of existing boats and which can be loaded on the trunk of a car.
4) That it takes up very little space so that it can also be stored in a corner of an apartment.
5) To allow the transport of two trimarans on the same car.

This is an interesting set of requirements. He came up with a tri that is 10 x 7 foot with a weight of 130 lbs and a displacement of 280 lbs. The 17 foot mast carries a 60 square foot sail that has a pocket around the mast. The draft over the daggerboard is 3 foot.

The tri is designed for one person and should only be used in sheltered waters. The tri is deemed to be collapsible. More accurately the tri can be broken down into parts. The most interesting part is the floats. The floats can be carried as flat panels. When you want to sail you pump up a yellow inner tube inside the hulls for shape and rigidity. The pumped inner tube in the hull also provides buoyancy for safety.

Now we get to the construction. Its different. The material is a product sold by Alcan, DIBOND, registered by Alcan Composities. It is a sandwich composed of a 1.4 mm polyethylene core and 0.30mm aluminium skins. Dibond slabs are suitable for resisting the most aggressive atmospheric agents of our cities and are used to cover the facades of buildings and in advertising billboards. A panel of Dibond is 150cm x 305cm by varying thicknesses depending on the stiffness required.

The panels are cut to shape, then the shapes are glued "dry", with 3M double-sided tape, keeping them in register by means of screws inserted into holes made before applying the tape. The screws are removed and pop rivets are inserted in some locations to add strength. The crossbeams are a simple ply web with top and bottom timber flanges that can be removed. The mast step is a plywood panel between the 2 hull sides and the mast is supported by a 3 stayed wire rig. Dibond is also used to build a rudder box with a built-in hinge. The total weight of the boat is competitive with a plywood boat with fiberglass externally and EG timber framing and ribs, which is not required by the Dibond version. The outside of a Dibond hull can be painted if wanted.

This is a fun experimental tri for one to test ideas. No performance numbers, but if the weight and hull shapes are similar it should be competitive and a lot of fun. The jpegs give an idea.

 

Mark Gumprecht designed and built this trimaran in 2008 to explore the possibilities of a using simple dory shaped hulls and stitch and glue construction. The trimaran is 12 x 10.5 foot with a weight of 200 lbs. The 18 foot wooden wing mast carries a fully battened main and a roller jib. The mast is held up by 2 side stays and a forestay made from 3.2 mm wires. There is a lee board on the side of the main hull. Please study the very smart kickup rudder arrangement as seen in the jpegs. This is a light effective easily built rudder system. The tri is easily trailable on a small flat bed trailer.

The dory hulls are built from 6.4 mm Luan ply from Home Depot. It has hollow box beams with two layers of ply on the top and bottom, and one on the sides. There are stainless steel removable pin hinges on the top and bottom of the beams. You just remove the bottom pins to fold the amas. The exterior is fiberglassed with System Three epoxy, and varnished. There are solid ply seats on either side of the main hull between the crossbeams. He added spray deflectors on the bow which makes the boat much dryer. Mark said all small tris need something like that, because water will splash right up the bow, hit the beams, and end up in your lap.

Mark says the tri goes to weather and tacks well, and will hit 10 knots or more on a reach, but doesn’t require the gymnastics of sailing small cats, and you have a comfortable place to sit. He mostly sailed the tri in the local bays and rivers, and in the ocean on moderate days.

This is a very well conceived, easily built tri for days sailing. The jpegs give the idea. Mark used this design as his base design for his Drifter day trimaran plan range that are now available for free at Plans & Kits - Free Plans - Duckworks Boat Builders Supply https://duckworks.com/free-plans/

 

Crossbeams are a major structural component in a catamaran design. There is no simple answer to crossbeam design as crossbeams can be an aluminium tube to a structured box built as part of total wing deck structure. The following is from Richard Woods to give a basic idea.

"There are two stages in creating a successful catamaran crossbeam solution. First you must DESIGN the structure, only then can you CALCULATE it. The former is usually the more important and certainly the one most people get wrong.

There are several factors to consider when designing crossbeams: First, you need stiff crossbeams, not just strong ones. Fortunately stiff beams are nearly always over-strong. By stiff I mean one without any obvious deflection. Engineers normally consider that to be 1-2% of length. Why a stiff beam? Well imagine crossbeams made out of rubber. They would never break, but would be so flexible you could never keep the two hulls in line and the mast would fall down as the rigging flexed.

How can you make a stiff beam? Well, actually it’s not just the beams that you want stiff, rather it’s the boat as a whole. I’ve found that the best way to do this on an open catamaran is to have two crossbeams plus a separate one to take the mast loads. The actual positioning of the beams is also very important. Although crossbeam size and placement is often complicated by rig and accommodation considerations, the beams must take priority! If they are too near the middle of the boat then the bows can flex up and down and you cannot keep the rig tight. If too close to the ends (especially to the bows) there isn’t enough boat to take the loads and, furthermore the beam cantilever is longer. "

The following web pages give an interesting view of crossbeams design and build: Changing Catamaran Beams from Wood to Composite. https://www.boatdesign.net/threads/changing-catamaran-beams-from-wood-to-composite.64278/

Crossbeam design is where professionals really earn their money, The jpegs show Wharram type beams, a free standing mast lattice beam and a full wing deck beam structure. The loads on a beam are not just vertical but also torsional so beam positioning longitudinally on a cat is important. This is a big subject and each time it is discussed on the web leads to many voices producing a view. Sailing catamaran beams have higher point loadings than power catamaran beams so it depends on what you are designing for. Please make sure the designer really understands what you intend to use the boat for. Also if the designer says build it this way, build it that way. As a builder of a Woods 30 foot Skua found when he tried to “improve” the way a beam was mounted, the beam moved from its mounts.
The PDF is John Perrys version of converting a timber beam to a composite beam. He was a very competent engineer.

 

"There are several factors to consider when designing crossbeams: First, you need stiff crossbeams, not just strong ones. Fortunately stiff beams are nearly always over-strong. By stiff I mean one without any obvious deflection. Engineers normally consider that to be 1-2% of length."

What do catamaran and trimaran designers consider to be acceptable beam deflection?

 

Sorry folks, I have just been told I have to go to hospital for a day or so. Nothing serious but I may not be posting anything for a few days.

 

@oldmulti we all hope that it is just a routine visit to the hospital, and that you will find some wifi there so that you can still keep tabs on what is happening in the Multihulls forum.

 

Pretty lonely and boring without you, Oldmulti.

 

This one was posted up in another forum, and a quick search here didn’t show any mention of it yet in this thread.

Looks like an interesting design and maybe a little less wet of a sail than say an F27 with the good amount of freeboard it has and judging by the one photo of it powered up.

Currently listed for sale on CL in Florida.

 

I missed a $6000 9.2 tri that looked a lot like a fixed beam Farrier, made of a composite including balsa and timber core old multi, , it created quite a stir for an hour and a half on gumtree, pity I didn't screenshot the pictures .The bloke helping to sell it was genuinely decent, either the best buy of my lifetime or well, who knows,...the new owner I suppose. It was so far out of my affordable microcruiser tri ambition I couldn't process it and 45 minutes elapsed, ha. regards and get well soon.

 

Some Multihull design content to keep you entertained while OldMulti is away:

 

Here's a picture of my motorboat.
Anyone know how Oldmulti is doing?

 

Hi Russell, looks nice, are the cat hulls custom built?

 

On the subject of small power boats:
The UNDP has given us money to set up boat building courses teaching Fijians how to build sustainable (no power tools, moulds, temporary frames, ply, timber or metal ) versions of these to supplement the outboard powered pangas/banana boats/fibres. After discussions with the locals, it is clear that while the mini cargo proas will be ideal for fishing and non time critical passengers and freight, there are circumstances (sheltered waters, no wind, windy rivers) where motors are required. Consequently, the course is being extended to cats and (maybe proas) with solid decks and roofs for shade and solar panels. The power generated will be used to run small (the pangas rarely have less than 40 hp) petrol outboards with the powerhead replaced by electric motors.

The hulls we are building are super simple: flat bottoms, square sections, no rocker, double ended. This is for ease of build, maximum load carrying and efficiency regardless of load. As can be seen in the video, they get along pretty well.

I am looking for data points for similar boats (7-10m long, mono or multi) so I can decide whether "super simple" can or needs to be improved on. The data required is any or all of: length, empty weight, max load, cruising and flat out speeds, fuel consumption and build time/cost. Any other information (materials, seakeeping, photos, etc) appreciated.

Thanks.

 

Russell, please tell me you put on a Santa outfit and cruise that thing around during the holidays?

If Santa didn’t have flying rain deer…. or Bond didn’t have a Glastron…

That’s a great shot!