Multihull Structure Thoughts

A 50 foot Hawaiian beach charter catamaran is being built by James Bett enterprises. The client started with a 27 foot plywood cat then upgraded to a 44 foot foam glass cat. The client needed a new cat as sailing into the beach 5 or 6 times per day to load and unload 30 customers is hard on the cats structure. So the client spoke to Bett and said he wanted a tough boat. The client choose Titanium.

What is Titanium? It is neither a precious metal nor rare, yet among industrial metals it has the reputation for being very expensive. It's the fourth most abundant metallic element and the ninth most abundant of all the elements in the earth's crust. It is well-named, as the strength of this metal is amazing. It’s 45% lighter than steel, yet it’s just as strong. It’s twice as strong as aluminium, but it’s only 60% heavier. It resists corrosion in sea water. It is resistant to high temperatures, as its melting point is 3,034.4 degrees Fahrenheit (1,668 degrees Celsius). It even resists abrasion, cavitation, and erosion even at high-flow velocities. Price of Titanium is about 2.5 times aluminium, although it in 2006 it was as high as 9 times. It is a very hard metal to recycle (melting point 3000 degrees) so can be found relatively cheaply as scarp if you live near an aircraft factory or an old nuclear reactor (yes, some cheap scrap came from Russia that was radioactive).

Back to the 50 foot cat. The jpegs show parts of the build. Every part of the boat you see in that photo is 3 mm (.125") Titanium as well as the hull plating. Keelson may be 6,5 mm (1/4" instead of 3/8") as Jim Bett didn't like the big difference between the thickness of the two plates for welding. There are lots of pitfalls welding with Ti. In the 50 foot charter cat it would have used 4.8 mm 6061 aluminium weighing 2.65 lbs/square foot verses 3mm Titanium CP2 weighing 2.77 lbs/square foot. The displacement between the aluminium and titanium versions would be very much the same and the much higher strength to weight of the Ti boat should have no problem getting signed off by the Coast Guard.

The 50 foot hulls are asymmetrical with careful compounding of the tighter inner plate at the turn of the bilge at the stern. One of the capabilities that brought the build to Betts is an old Dutch powered feed English Wheel. It takes a lot of pre-forming of a plate to closely match its final shape on the frames and stringers as the Ti is tough and resilient stuff and the Wheel is a necessity. As thin as the plate/sheet is you can't just throw a flat piece on the stringer/frames and draw it down to the frames like you can with aluminium. Look at the substantial strongback and bracing is to keep the hull shape true.

Titanium is an interesting material that is very tough if you want a very good build material. Just do a lot of research on how to weld it and engineer it. Sorry I only have a few jpegs.

 

I've worked with titanium. Absolute pig of a material. Aircraft people are used to it and have the tools and skills to work it but there are all sorts of pitfalls with it.

Bit like solar cells, or photovoltaics. Mostly silicone, doped with bromine. Cheap as chips but growing it into the right sort of crystals takes unbelievable amounts of energy so they are dear as poison. I know about the paper that claims "modern" methods mean they put out more energy over time than it takes to make them. I meant to chase his method to see if it's flawed (I suspect it is) but my strokes happened before I got a chance.

Anyway I'm drifting. Point is unless you have a really good reason titanium is a mad material to make a boat out of.

 

I am confused with this catamaran. It is not an outright racer nor is it an outright cruiser. The ESC 40 is a “40-ft catamaran designed to be two to three times faster than the typical cruising cat – using carbon fibre to reduce weight and provide increased hull strength” according to the advertising. OK. The ESC 40 is 39.5 x 23 foot that displaces 12,000 lbs (ready for sailing with crew I hope). Assuming its well designed and built, that means it should weight about 8,000 to 9,000 lbs. The 55 foot mast carries a 635 square foot mainsail, a 280 square foot self tacking jib and a 1300 square foot gennaker. The hull draft is 2 foot. The rudders are retractable transom hung and with curved retractable daggerboards providing better upwind handling and performance at speed. The length to beam at the gunnels of the hull is 10.3 to 1 which makes the hulls 3.85 foot wide. I am guessing that the length to beam at the waterline is about 11.3 to 1 judging by Hal Whitacre other cat designs.

Hal Whitacre of Whitacre Yacht Design did the design. Oh, Hal also owns and operates another business, Bruce Roberts USA and in that capacity, he has designed over 100 steel and aluminium boat kits in addition to many fiberglass and wood sail and power boats. Most recently, Hal was the chief Naval Architect for Gunboat Catamarans when they were built in the USA. I am now getting more confused. I fully accept the design talents of Mr Whitacre and his capability of designing in carbon fibre foam. But Open Waters yachts said this cat can sail at 2 to 3 times faster than your typical cruising cat. If you’re comparing to a Lagoon 40 that can hit 12 knots maybe 2 times as fast but if you are comparing to an Outremer 45 that can hit 20 plus knots, I question the statement.

The build of this cat is full carbon foam epoxy. The core is a structural-grade foam with a high shear strength characteristic: if the part is of a complex shape, we cut the foam into sections, thermoforming it to the required shape. They use five different sizes and densities of foam on the boat, optimized for local strength requirements. Each cat takes 35 rolls of stitched carbon fibre to complete and the cat parts are epoxy resin infused under vacuum. Final finishing such as taping and sanding is hand done before painting.

The ESC 40 can be had as a kit boat to a fully finished cat. This allows for EG 2 hulls to be put in a container shipped to a location then a final assembly done or have a transportable component cat for transport to new sailing locations.

The accommodation is unusual. A hull has 3.7 foot wide “double” berth aft, either a galley and or a berth centrally and a toilet forward. The wing deck cabin can have either a double berth or a dinette. Plenty of deck space though.

This is an interesting design that has the capability of going fast. But I compare ESC 40 to a Schoinning Arrow 1200 Sport or a Stealth 12 S cats that have better accommodation, about the same sail area and displacement without a full carbon fibre build. I need to know more of the claimed advantages of the ESC 40. Sorry about the limited jpegs. Details can be found at: Open Waters Yachts https://openwatersyachts.com/

 

The WYD 47 is a performance cruising catamaran designed by Whitacre Yacht Design and is being built by Bett Boats in Washington state. The cat is 47 x 25 foot with a displacement of 39,600 lbs. The 61 foot fixed aluminium mast carries a 880 square foot mainsail, 400 square foot self tacking Solent and a 1500 square foot asymmetric spinnaker. The draft over the rudders is 4 foot and the draft over daggerboards is 7.4 foot. The length to beam of the hulls is unknown but I guess about 10 to 1.

The accommodation has 2 double berths on the wing deck forward of the mast. The main cabin contains a dinette, galley and steering position. I would imagine the hulls also contain toilets and extra berths. The aft cockpit is covered and integrated with the main bridge deck cabin. I am seeing approach in several designs of late and if the cat can be controlled from a rooftop cockpit or have control ropes lead into the internal steering position, I see no problems with this arrangement in larger cats.

The structure is foam core with e-glass on either face in vinylester. There is carbon fibre for extra strength where needed EG flanges on bulkheads.

The numbers indicate the performance of the WYD 47 will be good but not ultra high performance. As a cruising cat, it does not need to have very high performance but good cruising speed across the range. If your cat can maintain 8 to 10 knot averages and peaks of 18 knots with a good sea motion you will have a good cruiser. I suspect this cat will be able to do this.

The jpegs give the idea.

 

A short one on how a male mould can be built for a multihull. This approach uses cheap material for a “one off” mould so a production fiberglass female mould can be made. The same approach can be done to produce a “one off” female mould with the framing of the structure on the “outside”. The cat is a power catamaran from Argus boats. We will talk about The Argus 35 cat tomorrow. The cat is 35 x 15.4 foot.

The male mould build is built from MDF. The MDF in this case is CNC cut (could be manually done) into the frame shapes. Either have a concrete floor or good strong back arrangements to hold the framing accurately. The framing is then covered by MDF. This design has a lot of flat panels which makes the covering of the frames relatively easy. Any curved area’s in the hull mould can have the MDF sawed through to a depth of 50% which allows greater curving on the MDF. In the case of the Argus 35 the rounded hull bottoms were built in a separate mould then attached to the basic MDF male mould. You can then do fillets etc to provide the final shape prior to painting and waxing. Once prepared the male mould can be used for a female mould.

Rob Denny Intelligent Infusion does the reverse approach using a cheap MDF mould to provide a female mould for hulls. Check the Harryproa web site.

The jpegs give the idea.

 

The Argus 35 power cat is a displacement cruising cat, designed by Roger Hill of New Zealand, that has good fuel economy. The cat is 35 x 15.4 foot and displaces 9400 lbs without fuel, stores and crew. The hull length to beam ratio is 13.5 to 1 with its round bilge full displacement hull. The underwing clearance is 1.75 foot. The cat draws 2 foot with its outboard motors. The twin 90hp Suzuki outboards on the demonstrator consume 28 litre/hour at 16 knots, equating to 1.75 litre/nautical mile. Top speed of 20.5kts results in 65 litre/hour. An Argus E35 cruising cat did a 6500 nautical mile circuit of Australia using less than $10,000 worth of fuel in 2015.

The accommodation has 2 double berth cabins, featuring queen-sized beds with European-style slat bedding, forward of midship, a galley in one hull, toilet in the other with a dinette and seating in the bridge deck cabin. The steering position can be forward in the bridge deck cabin and/or on a flying bridge. The cockpit is large and can be integrated with the main cabin to provide a large space.

There are engine options with diesels available but the preferred, cheaper and lighter option is 4 stroke outboards which are easier to maintain as well. The fuel tanks are 600 litres which allow 340 mile range at 16 knots. If the cat is slowed to 8 to 10 knots it can travel 600 miles on a tank. There are 400 litres of water and a 50 litre holding tank.

The Argus 35 has a solid e-glass fibreglass bottom, with vinylester resin below the chine, and composite foam e-glass vinylester resin construction above the waterline and on the decks. There are lateral and fore aft watertight bulkheads just in case, and structural foam-cored fibreglass keels that let you rest the boat on the dry without damaging running gear.

One test report said: “Designed with narrow hulls and a deep forefoot, the E35 rises slowly in choppy conditions so the hull slices through waves rather than bouncing over the top providing a comfortable ride combined with outstanding fuel economy.”

An interesting design that appears to be a very good coastal cruiser. The jpegs give the idea.

 

The Albertza 40 catamaran was designed by Dutoit Yacht Design of South Africa for charter work for an owner operator. The cat is 40 x 23.8 foot with a weight of 15,400 lbs. As a charter cat I could imagine a 6,600 lbs payload so an all up displacement of maybe 22,000 lbs. The 54 foot fixed aluminium mast carries 845 square foot fractional rig. The headsails are not self tacking due to the forward cockpit. All headsail sheets go to the coach roof. The cat hulls have a length to beam of about with 8.5 to 1. The draft of the low aspect ratio keels is 3.25 foot when empty and underwing clearance of 2.46 foot.

Dutoit Yacht Design of South Africa may not be well known but think about the Balance catamaran and Knysna catamarans as other examples of this design houses work. The accommodation layout of this cat is spacious and comfortable to suit the charter needs. In each hull there is a queen size aft double cabin with a toilet shower room and a double berth cabin forward. The bridge deck cabin has a dinette, galley, saloon (?) and chart area/bench space available. There is a forward and aft cockpit. The aft cockpit has all sail controls lead to it. This cat is seriously comfortable.

The construction of the boat is semi-custom using the moulds of an existing hull and adding a customised deck, superstructure and interior. Material used is GRP and balsa core for the hull, deck and bulkheads.

If anyone knows if this vessel was completed and have any idea of its performance I would be interested. This could be the basis of a serious cruiser with reasonable performance. The jpegs give the idea.

 

The following power semi foiler cat was designed by Bieker Boats for a Florida client who wanted to build the boat himself for recreational boating. The cat is 29.5 x 9.1 foot weighing 2,950 lbs and displacing 4,600 lbs. The hull length to beam ratio is 10.3 to 1. The draft is 0.9 foot. The underwing clearance is 1.25 foot. The power is two 90 HP Suzuki 4 stroke outboards. The cat can run in displacement mode and the foils can be retracted. With the flip-up main foils down combined with the “moustache” foils mounted to the motor drive legs theoretically reduce displacement 40% when down giving faster cruising speed and better stability in a seaway. The main foils can be manually adjustable for trim over ±2 degrees.

Bieker started with his commuter cat hulls as a template since they had performed well then added features suitable to warm weather boating in Florida, like a larger deck, minimal cuddy cabin, forward seating and a permanent hard top for shade and paddle board stowage. The cabin contains a double berth, a toilet in a hull and storage in the other hull. The wing deck headroom is 4.5 foot with the hull headroom of 6 foot over a short floor length. The cockpit has good seating and an effective sun shade hardtop. The forward area has some forward seating and netting.

The construction is basically plywood hulls and bulkheads with PVC foam carbon fibre for underwing and cabin roof. The hulls have 6 mm plywood sides and the hull bottoms are 9 mm plywood. The hulls have 200 gsm and 300 gsm e-glass cloth in West System Epoxy inside and outside. The 18 x 10 foot wet deck and hard top has 20 mm PVC cores with 3 layers of carbon vacuum bagged on both faces. Structural reinforcing details are carbon composite. For tapes and reinforcing 400 gsm double bias carbon fibre and 300 gsm unidirectional carbon fibre in epoxy is used. The forward crossbeam tube is carbon fibre (ex boom of a 6.5 mini mono). The hull build jpegs give the idea.

The foils have a main spar of 167 layers of pre-preg 190 gsm unidirectional to form solid foil plank. 144 layers are at 0 degrees with the remaining 23 layers dispersed though out the layup at 45/45 degrees. The foil is cooked at 120 degrees C in 5 or 6 separate runs as the entire 167 layers of prepreg material could not be laid up in one run. The jpeg gives the idea.

The performance is good. At a non foiling cruising speed of 20 knots @ 4,200 rpm, the fuel consumption is 12 Litres/hour (3.1 gal/hr). The cruising range is 320 nautical miles.

This is an interesting design that should provide a lot of fun. Part of the way through the build the builder decided he would use the boat as a prototype for a production cat. Jpegs give an idea.