Multihull Structure Thoughts

The Wavelength 780 trimaran was designed by Bob Forster in the early 2000’s. Bob did full time building work with OSTAC (Ocean Sailing Trimarans & Catamarans, Australia) helping build the F-31 moulds and boats. The ownership of this company was later to take a controlling interest in Corsair Marine in America. When Bob left OSTAC he wanted a tri to replace his boat and act as a test bed for the practical and theoretical ideas accumulated over a long period of involvement in multihulls. I was hopeful that the design would prove attractive to potential ‘home builders’ and at the same time provide me with a business project in semi retirement. He thought a tri in the 7.5 to 8.5 metre range provided the best compromise. A cruise oriented boat with high beam clearance was considered a must to avoid cold showers commonplace on many of the trailerable tris. Higher beam clearance also provides the option for more volume in the main hull and a longer term cruise layout with dedicated bunks, storage lockers, galley, sinks etc.

Now we run into a problem. The dimensions of this boat are only partially known. Length is 25.6 foot with a rated weight of 2910 lbs (actual boat weight with engine, safety gear and rig). The 32 foot aluminum wing mast with a 258 square foot mainsail, a 116 square foot roller furling jib, a 145 square foot genoa and a 580 square foot spinnaker. The swing centreboard is in the main hull. There is a kickup rudder on the main hull stern. A 10 HP outboard is sufficient. The tri is fully trailable with a Farrer type beam folding system that permits towing at 8.2 foot.

The accommodation is a berth forward, 2 wing berths and a seat in the saloon with a small galley and either chart spot or more galley space. There is a portapotti forward and storage under the cockpit. A good weekender for 3 or a short term cruiser for a couple.

The main interest is the construction method. Bob had a lot of experience with foam glass construction but he was still attracted to ply as a simple and economical building material without the need for higher tech processes (vacuum bagging) or materials (foam, carbon fibre). The problem was he wanted to build a complex curvature, no compromise shape, normally outside the province of sheet plywood, and not fully achievable, even when ‘Constant Camber’ or tortured ply techniques are used. Bob had worked alongside a talented pattern maker in earlier boat building days, where flexible ply was used extensively to build complex mould shapes, he was well aware of the potential of light thin strips of flexible ply. So he developed an interesting compromise technique.

Thin 3.6 – 4mm flexible ply strips are laid up in a female mould as for vertical foam strip construction. The narrow thin ply strips (170 -180mm wide) are easily trimmed with a jack plane to follow the mould curvature and are laid at approximately right angles to the longitudinal axis of the mould. Tech screws at the top and bottom of each strip hold them in place temporarily on the batten mould. After all the ply strips are fitted dry in the mould, light weight, bull nosed cedar stringers are then glued side on, rather than the traditional edge on, along the laid up strips at predetermined intervals. This is the first element in tying the structure back together lengthwise. A lightweight unidirectional glass is then laid up over this structure in one operation, effectively butt strapping the exposed joins between the ply strips, to create the inside skin of the hull. Any necessary bulkheads or other frames are then fitted before the half hull is removed from the mould and turned over. The 2 halves are then joined and a flat ply bottom added on the main hull to complete with the main hull. Minimal fairing of the outside faces of the ply strips is required before fitting the outside unidirectional glass layer and any additional reinforcement required.

Empirical testing showed that the method, in its basic form, was significantly lighter than an equivalent cedar strip plank layup and not too different from foam sandwich. The system is flexible, in areas requiring more strength, for example under the waterline and on the deck, the ply layers can be double, where the second layer butt straps the first and where extra light weight unidirectional glass can be added selectively to stress areas. For hulls this is an interesting technique. Crossbeam construction is unknown but a Farrier type folding system is used.

Performance is good and in bay racing “New Horizon” the third build matches F-24’s. Light upwind work is very good with 7 knot plus averages possible. Peaks of 18 knots have been seen and 10 knot averages possible.

The jpegs give part of the idea. A very good design but unfortunately Bob has been hard to contact for a copy of the plans, build instructions and build CD.

 

After yesterday’s Wavelength 780 plywood build technique we will look at Kurt Hughes plywood Cylinder moulding technique. Kurt famously said “if you need bent plywood make bent plywood”. Cylinder moulding followed which allowed round bilge plywood hulls of any size to be made from flat plywood sheets. This method of building is fast and is structurally strong. Also, it allows (not recommended) lower quality plywood to be used in a hull shell construction.

Logic of this is hull global forces require about 70% of the strength fore and aft and 30% longitudinally (this is a big generalisation but roughly correct for most multihulls). In cylinder moulding you put several layers of thin sheets of plywood over a simple mould and glue them together to form the hull shape. Thin plywood (3 and 4 mm) often has 3 equal layers so you end up with 66% of grain in one direction and 34% of the grain at 90 degrees. Good for global shell strength.

The plywood panels that are made then are cut to shape, joined at the keel line and bow and then slightly tortured into a hull shape. The jpegs and Kurt Hughes PDF description give a better overview of this. The hulls can have more shape and thicker skins than single sheet tortured ply hulls.

The down side of this hull build technique is that it is not as predictable in the final shape as a foam glass build. Reason is various grades of plywood bend differently as does differing keel joining glue and glass lines can slightly effect the shape. Finally, when you have a 48 x 8 foot by 9 mm thick panels to be moved around and tortured you need more than a few friends or some good lifting equipment.

The upside of this technique is, it is FAST to build hull shells once you understand how to use this build technique. Do a 16 footer first to learn how do it, then try a larger build.

The jpegs are of a 48 foot cat hull build and other items from Kurt Hughes writings. The PDF gives the best overall description is at: CYLINDER MOLD MULTIHULL CONSTRUCTION https://www.multihulldesigns.com/pdf/cm/CYLINDER%20MOLD%20MULTIHULL%20CONSTRUCTION.htm

 

I think those photos are of both cylinder molded panels and of tortured plywood construction. Cylinder molded construction is problematic because the panels are very stiff lengthways and need to be forced into a curve. Constant Camber is a similar method, but has the longitudinal curve built into the mold. Kurt Hughes very publicly badmouthed Constant Camber, but then began introducing longitudinal curve into his molds to avoid problems with his method, but Constant Camber was patented and he was threatened with a lawsuit. If it sounds like I have little respect for Kurt, sorry. I've other reasons too.
He make nice, flashy drawings though.

 

Archipelago Expedition Yachts has produced a new aluminium 47 foot long range power catamaran. The Archipelago 47 is designed by Chartwell Marine naval architects who normally design offshore windfarm catamarans. The Archipelago 47 is 47.6 x 21.3 foot semi displacement catamaran of about 35,000 lbs displacement. The hulls have tunnels for the propellers to reduce the draft and increase propeller efficiency. The hull length to beam is about 10 to 1. The cat is powered by twin 420hp Iveco diesels that provide a top speed of 30 knots. The two 3000 litre (total 6000) fuel tanks can provide a 3000 mile range at cruising speed. A variety of propulsion options available are (including hybrid and electric) depending on requirements.

This cat comes from a working boat heritage and as a result is designed and intended to be used offshore. Its capable of maintaining EG 20 knots in 6 foot seas due to the high wingdeck clearance. At slower speeds it has the range to cross the Atlantic. This is a serious cruising cat. EG in this cat you can safely navigate from the helm under all conditions as the Archipelago 47 features a unique saloon divide which allows the helm to safely navigate the boat from a dark, well-equipped wheelhouse, whilst family and friends can enjoy a warm lounge environment with underfloor heating, AV and galley, to keep them entertained and cosy on the coldest winter nights.

The accommodation has an owner’s double berth cabin and toilet in one hull. In the other hull there are 2 double berth cabins with a toilet. The bridge deck has a large galley and seating area with a helming and navigation position forward. All of this area has sufficient space to lounge around in as required. The main cabin roof is set up to carry solar panels to minimise fuel use on electricity production.

The build of this cat is mainly aluminium for strength and light weight. There are web frames about every 2 foot (estimate) with deep longitudinal stringers throughout. The majority of the plating is slightly curved flat plates. Some areas have curved plates but this would be a relatively easy shape to construct. The jpegs will give you more detail about the structure. This cat appears to be designed and built by very competent people.

The jpegs give the idea of a very good long range power cruiser.

 

I guess we should be thankful he never designed a Proa then.

 

Yeah.... I remember the early cylinder mold boats all floated oddly, transoms down, bows up. Lots of rough finishes, it put me off Hughes boats. I haven't seen a cylinder mold boat built in years so no idea how the current crops sit on their lines. Various comments Kirk has made over the years kept me from being interested as well. Everyone tries to get around plank shaping but it really isn't hard to learn and opens up so many better hull shapes no matter how you do it. Double diagonal, glued lap, strip etc...

 

Small catamarans are interesting due to there trialability and or lower costs. The following power catamaran is a coastal capable cruising and or fishing catamaran that has a variety of wheel house options. The designers and builders are Auto & Marine Composites Ltd in Britain. The company builds kit cars, racing yachts and catamaran workboats from 20 foot to 40 foot. The AMC 690 W is 22.6 x 9.8 foot (optional 10 foot) with a draft of 2 foot. Some are powered by twin 40hp Suzuki 4 stroke outboards. There is an option for twin inboard engines. The hulls are semi displacement multi chine hulls.

The initial AMC 690 is used for taking disabled people on short sea adventures due to the large cockpit space available with the short wheel house. The longer wheel house has an enclosed toilet space. The full headroom wheel house has the helming position and can have a small galley and additional seating in the longer version.

The builds are mainly solid fiberglass hulls with limited bulkheads in the hulls. The underwing has a core as does the wheel house and decks. These cats are build as genuine fishing workboats so they should be long lived. They also are built to SFIA Standards.

This is not a high speed cat, it is intended to be an economical reasonable range coastal catamaran.

The jpegs give the idea to an interesting small cat.

 

I briefly considered the 26 tri stretched to fit the 8.5 rule here. Then I watched the cylinder mold video - not for this boy. I read somewhere that the 40 footer in the video was one of the ones that ended up with less than ideal bouyancy aft.

 

The boat in the above post re Kurt Hughes seems to have evolved enough in design to be sitting right. Kurt like many designers has evolved himself and his designs over the years. I've got the full build file and it came out ok. The thing with these forums is there seem to be the same people pushing the same barrow as has been pushed for years, while you never see the designers with work on here banging the drum from the soapbox that is the internet. FWIW we are presently putting the finishing touches on a design with Kurt and our first hand report is he has been 100% professional to deal with in every aspect, facts are wonderfull things.

 

I wish I hadn't commented on the cylinder molding. I seem to have a hard time learning that saying anything negative makes me look bad and feel bad. That's too bad because I have a whole lot more to say on that particular subject.

 

FWIW we are presently putting the finishing touches on this. Kurt/JYD collaboration.

 

Well I did mention I haven't seen a recent Kurt cylinder mold, it is nice to hear he has made some progress.
He has paid attention to the paper pushing getting those cats charter certified.

It looks like Kurt has deleted the infamous article from 2012 where he ranted about Dick Newick and other designers while comparing his current designs to boats from the 70s. We had a discussion here on the 4th page of " New 27' Trimaran design by Kurt Hughes" where people posted thoughts and observations. It is also good to hear Kurt has matured since then, has to be good for business.

Cheers

 

Today is about a 72 foot catamaran ferry that could easily be converted into a fast cruiser. Sea Boats Australia design and create aluminum kits for mainly working boats and other pleasure craft. The aluminum plates are marked out, any extrusions required are included and plans provided to allow the build and meet any approving authority requirements. Also additional plans can be provided for EG electrical, plumbing, engine installation etc. This vessel is designed for open water use with up to 125 passengers and crew.

The 22 MTR ferry is 72.1 x 20.7 foot with a weight of 55,400 lbs and a displacement of 66,600 lbs. The draft is 3 foot. The hull length to beam is about 15 to 1. The engines are 2 Scania DI16 072M 662kw @ 2100RPM, Marine Diesel Engines 4 stroke, direct injected with marine transmission of ZF 500 with mounts and gauges and oil cooler. The propulsion is provided by 2 MJP H350 water jet units with hydraulic steering and Carden shafts. The speeds achieved are: Lightship speed (approx): 34 knots Loaded cruise speed (approx): 30.7 knots @ 85% power. Loitering speed (approx): 4 knots.

The accommodation as planned is for an wing deck cabin that can accommodate the 125 passengers and crew. There are also 2 toilet areas. The bridgedeck is a vast open space that could be converted into cabin spaces for a cruiser etc.

The build is mainly aluminum plate 5083 H116 and extrusion 6061 T6. The aluminum thicknesses specified are:

Hull bottom plate: 6 mm plate

Hull side plate: 5 mm plate

Deckhouse plate: 3 and 4 mm plate

Deck plate: 3 and 4mm plate.

There is extensive framing and stringer support infrastructure throughout. Any internal accommodation paneling is foam glass veneered panels.

This is an interesting design that has potential to be a fast cruiser from Sea Boats that has done many aluminum cats of a similar size that have performed well. Sorry about the limited jpegs.

 

On the topic of cylinder molded ply hulls and tortured ply hulls. Can we please have peoples real experience of these types of builds please. I have seen a 45 cylinder moulded hulls being built in Cairns Australia. The guy "invented" a way of interlocking the keel lines prior to the epoxy bog and taping was done that ensured the keel was the correct shape prior to torturing the hull. The hull lines were very fair and very close to being symmetrical when finished. I do not know what it floated like as a complete boat. I have also seen several Tornado cats being built. As long as the plywood was consistent and good quality the hulls generally came out well.

If you have experience of either of the techniques and understand positive and negatives we need to understand what is going on.

Next, please understand that what a designer specifies and what is built is not always the same. EG Malcolm Tennent had a couple of people try and sue him for an incorrect design in his large thin hulled canoe stern power catamarans. The vessels did not trim properly with the bow down in one case. The first thing Tennent asked for was a detailed structural build schedule and then the weight of any items on board and there locations. The lawsuit disappeared when the owner demanded a different layout and got the builder to move the galley forward, shift some water tanks and the builder added some additional structure to "strengthen" he boat which all resulted in the trim down situation. The design was not built as the designer intended.

If there is a trim problem with a boat being built to plan then it is the designers problem but even this can be corrected by moving some items around, EG batteries, spare anchor gear etc. Not good but possible. But we need to understand what is really going on here. No designer abuse please, just structural build and launching facts.

 

Hazarding a quick guess by looking at old Hughes lines it would appear the keel profile which is a gentle rocker doesn't give enough volume aft. This is changed over time. This was likely a product of the "hard to bend" Russell mentioned. The old lines were hand drawn then Kurt went to computers. Were volume calculations done? I don't know.

Being a local PNW the early days seemed to be his arts and crafts movement where getting affordable fast builds afloat was his market and what brought people to his door. The finishes aren't the designers fault but a product of the target audience. Many first time builders don't have the skills or habits developed for pro results or the temperament when fast and cheap is the goal. I played with some tortured ply models but not cylinder mold or constant camber. I imagine finishing out the shell time is similar for all 3.