Multihull Structure Thoughts

You’re a boat builder, a guy comes through the door wanting a 45 foot Wharram cat. You say OK. A short discussion takes place should it be ply but the builder says foam glass would be longer lasting and building a male hull mould would make it faster. Owner says OK. Unfortunately, this cat was built a decade ago on a relatively low budget so used a slower build approach.

Step 1; speak to the designers about a foam glass version. They agree and slightly redraw the hull shape to suit the build method. Step 2; build a model to confirm the shape and familiarise the build process for everyone. Step 3; build the strongback and the male mould.

The male mould has plywood bulkheads with 52 x 18 mm strips which were edge nailed on with an airgun. Next graffiti the outside of the male mould to make fairing easier for the final shape. Long board sanding is done several times to get the final shape. This is the painful time consuming part. Fine sand the surface (320 grit) and clean for final waxing for the start of glassing for the first hull from the male mould.

The first hull first layers are e-glass biaxial, CSM and polyester resin, this is vacuum bagged on. There was a flange put on gunnel of the hull to stiffen the hull. After the inner skin is finalised it is sanded and faired. 15 mm Coosa board is the glued to the hull at any hard points (EG where crossbeam will rest on hull edges). Then a layer of 15 mm foam is vacuum bagged onto the hull. Around the stem, stern and keel line 9 mm foam was vacuum bagged on. The thinner foam allowed heavier glass layups to be done in these areas for additional hull strength.

The foam is now given a light fairing and sanding to prepare the hull for an external skin glassing. Now the final minimum 3 layers of biaxial glass plus an external layer of CSM is laid on again in polyester resin and vacuum bagged. Now the fun begins. More long board sanding and fairing prior to painting. Now you repeat the process for the shell of the second hull. This is long way of build 2 foam glass hull as you are fairing the hull mould, the inner skin, the foam and then the external skin.

The modern way is to build a male mould then spend more time building a proper female mould. The external hull skin and often the foam core can be vacuumed bagged in one shot and have very little fairing. If you resin infuse a hull you can do the inner skin foam and inner skin all in one shot. You may spend more time up front but by the time you have produced 2 hulls it will be a lot faster.

The female moulds being used now range from Rob Denny’s “Intelligent Infusion” cheap MDF (chipboard) box like moulds for his Harryproa’s. The other end of the scale is hull moulds for EG one off racing monohulls that can be a carbon fibre epoxy mould the holds its shape a lot better during a hull moulding process. BUT please do not make a non rigid mould as I have seen a moulded hull that required a choice of either throwing it out or a lot of fairing to take out the flats, dips and rises left by a “flexible” mould.

The jpegs give an idea.

 

Wow, that doesn't look fun. I'd build a ply one.

 

This is about a man who knows what he wants and has the experience to design it. Paul Bury is a Naval Architect (of Bury Design) who spent time at Wally Yachts, as well as being chief Naval Architect for 5 years at Bill Tripp's office, and knows a thing or two about aeronautical engineering. He also designed the Cure 55 all carbon cruising catamaran. The following project is for the ideal client, himself with his family and friends. It will go into survey to do inshore and coastal charter work, but its main mission is to be able to go places efficiently and in a lot of atypical or prevailing weather conditions, which are often not the most amenable on the planet.

The design concept is a power trimaran that will travel at about 10 knots in cruise mode. Why? Try 1 litre per nautical mile (l/nm) at 9.4 knots and just 1.6 l/nm at 10.5 knots on its single engine. To quote Paul Bury "This is to be a primarily an owner operated vessel capable of long ocean passages, but also suited to coastal cruising. So it is mostly for private use, but is also suitable for day charter (50 Persons On Board in sheltered waters), and then overnight charters (6 POB plus 2 crew) in open waters."

Interesting so far now we get to the unusual parts. The main hull is the modified hull mould from a Volvo Ocean 70 build. The best hull produced from that mould did 596 miles in 24 hours (average of 24.8 knots) under sail. So, you know where the Volvo 70 hull mould is and you do a design that can incorporate that mould with modifications. Next you say you want a long range boat that doesn’t require active powered stability control so you choose a trimaran design. After that you say I only want a practical long term cruiser for a few people, not a crowd. Finally, you are Australian and have to pay $2.5 to $4 per litre for diesel ($16/gallon or about $US10/gallon) resulting in the need to have low fuel consumption. Even in full economy mode an around Australia trip would cost over $25,000 in fuel alone. A conventional 70 footer with twin motors could pay $60,000 in fuel for the same trip.

The power tri is 70 x 24 foot and displaces 45,000 lbs. The length to beam on the main hull is 7 to 1. The float length to beam is about 22 to 1. The draft is 4.8 foot over the rudder and propeller. A single main diesel of 350hp will drive a massive, single, 1300mm controllable pitch propeller (CPP) for very high propulsive efficiency. Electric drives are in each float hull for in port propulsion and manoeuvring via digital controllers. The big outboard rudders are for control of her whilst under way. 5000 litres of fuel give a comfortable 3000nm range at 10 knots, with suitable reserve. 350hp delivers 15.5knots peak speed. Translation a relatively slim, light hull with trimaran like stability being driven at about 1.25 of the square root of the waterline in cruise mode with a single engine and high efficiency propeller equals cheaper cruising than majority of 70 foot power boats.

The accommodation is spacious and practical with 4 double berth cabins with private ensuite toilet shower area are well separated from one another in the hulls. There is a very spacious full headroom engine room in the mid section of the main hull. The saloon is large with multiple seating and table areas, a very large galley, a helming and navigation station forward and a cockpit accessible toilet for day guests. The deck space is large with 2 cockpit seating table areas aft.

The construction varies. The mould from the Volvo 70 is 8 mm solid carbon fibre and is modified to narrow it down to provide the centre hull. The outer hulls, internal structures, decks and cabin are to be PVC foam, with E-Glass/Vinylester skins. All the internal domestic gear will be from freely available commercial sources with minimal specialist gear to be used. This boat will be going places where there are few specialist facilities available and repairs need to be done by local people. The tender was shown a few days ago and again is built with the go far and have fun adventure mission in mind and is stored on the fore deck lifted on and off by a crane.

This design is worth a lot of study as it is very well thought out and is a very practical long distance cruiser that will do 250 miles a day for a week at a time. This design is intended to sail where Paul lives (Eastern Victoria Australia) that backs onto Bass Strait where comfort as sea, economy of operation, and the ability to punch through the inevitable Southeast Australian coastal chop were seen as 'critical'. Also, his real-world appreciation for use at sea, the vessel's construction, stability and damaged stability are to Australian Commercial NSCV standards. Very good. The jpegs give the idea.

 

Bury Design has some interesting designs to meet the needs of prospective clients. The following 2 designs started with Bruce Jones CEO of Triton submarines requesting designs that could be used to carry passengers and launch and recover a Triton Submarines 3300/3 submarine. There has been an increasing intertest in chartering the submarines in a many unique locations in the world and an increasing interest in private owners of a submarine to travel to selected locations.

Two concepts were explored. The first the brief was simple: a robust, efficient vessel to carry a crew and a Triton 3300/2 submarine around the world. Influenced by extensive experience in sailing boat design this design evolved into a long, lean, streamlined vessel, heavily orientated towards efficiency and sea-keeping ability. The narrow hull form coupled with a long water line length gives exceptional performance figures with extremely low power requirements and ocean spanning range. The hull geometry and the gyroscopic stabilisation system combine to provide an incomparable ride quality and safety in even extreme conditions.

The second concept was designed specifically to support a Triton Submarines 3300/3 submarine, this catamaran features an innovative design built around the submarine launch and recovery system. Featuring a dedicated enclosed hangar for the submarine, crew quarters and extensive open areas for guest entertainment this vessel provides an unrivalled compact support platform. The wave piercing hull configuration and aluminum construction delivers a high performance and lightweight vessel with good performance and sea-handling characteristics.

I do not know the dimensions beyond the monohull length (95 foot). Both vessels would be built in aluminum. Yes, if you can afford a Triton Submarine you can likely afford the required support vessel. The charter guest would get an interesting view of the above and below sea level.

The jpegs are of the cat, 95 foot mono and triton submarine.

 

The following is about a power catamaran that can be adapted to many rolls. It has the same basic platform from the gunnel down and has different cabin structures to suit the need. In these examples, it can be an oil hose vessel or a dive support boat. The cat is 34.4 x 13.1 foot with a light displacement of 15,300 lbs and a fully loaded displacement of 18,700 lbs. The length to beam on the hulls is 7.9 to 1. The draft is 2 foot over the hull and jet drives. The power is 2 x 300 HP Cummins diesels driving through Castdli 340C jet drives.

Power cats that can do reasonable speeds with planning hulls, do not need to have wide spacing between hulls as the hulls are planning. Displacement power catamarans require finer hulls and wider spacing to prevent pulling waves that interact between the hulls and make the cat slower (or less efficient). This cat’s projected speeds are up to 26 knots and with 600 HP of diesel power driving it I could accept this cat would be capable of those peak speeds. PS 600 litres of fuel will limit your range to about 100 nautical miles at peak speed with some reserve. At slower speeds (9 knots max) you could probably go 500 miles.

As indicated a variety of mainly commercial functions could be done by this platform ranging from offshore support vessels to commercial fishing ventures or charter work for diving groups etc. ALUCRAFT boat builders and designers (West Australian company) that does the designs for these cats also sells aluminium pre cut kits to build them if required in different countries. ALUCRAFT designs and builds are mainly aluminium and their commercial boats meet all required marine standards construction standards. Result you get tough boats.

Sorry about the limited jpegs but they give some detail.

 

Eclipse Hull Mold for Sale - Woods Designs (tapatalk.com)

45 foot Wharram makes Eclipes ...

Can not wose

 

Today is not a criticism of any specific production boat but a statement about things to look for and understand. The F-27 was built by Corsair Marine in the United States between 1986 and February 1997, with 450 examples completed. Recently a F-27 came up for sale and had a few issues. As this tri could have been a minimum of 26 years old and up to 37 years old, it depends on the maintenance schedule as to how good it is.

In this case there were a few issues. There are cracks on the beam attachment to the floats. There are cracks in the beams. There has been a sloppy repair around the base of the daggercase. There is piping that does not have a known function. In the first jpeg the deck appears to be coming away from the hull. There are failing gel coat and or paint in several areas.

So, what is a real issue versus cosmetic here. The beam cracks and delamination are known issues with earlier Farrier tris and Ian Farrier and Corsair wrote documents about how to upgrade or repair beam delamination’s. This is a serious issue and should have been addressed when it first appears. Leaving beam issues unaddressed allows water ingress and further cracking/fracturing. The beam "repair manuals" are quite specific about what is required for the repairs and the work is not to hard but needs to be done well.

The daggercase repair appears to be badly done. Beside the lack of trimming of the glass there are air bubbles in the layup. There are other samples of bad workmanship done as “reinforcement” of the structure that appear to done as “strengthening” by the repair person. You would need to understand what caused the need for the repairs and if the daggerboard had been damaged at any point which means a full inspection. if there is a crack in the case more problems may appear.

The three bows need to be inspected inside for major impact damage. The cracking near the U-bolt in front of the beam, and the fact someone decided to add a corner support to another beam, seem like clues the boat hit something hard and wracked out of true. Hitting something could be done sailing or trailering. Even a small hit may stress some of the folding joints or beams, so there may be hidden damage to the primary beams-to hulls structures

The deck joint issue in the first jpeg is a serious issue and requires a full inspection of the extent of the joint failure. Repair may appear to be simple but ensuring full rigidity in the joint is mandatory. The torque twisting on multihulls is large an any weakness will only cause another failure.

Another characteristic of some F-27’s is that they had some Balsa cores but according to F-27 Class Rules adopted in 7-22-03 and revised in 4-28-08 says “Balsa core may be replaced by PVC foam or epoxy core.” Interesting. If any of the cracks and splits allow access to a balsa cores the problems could be very significant.

This tri has not had a good life and has not been maintained properly with very sloppy repair work done already. To bring this tri back to full sailing capability will require a major refurbishment by a skilled person. I have seen well done repair jobs on many other boats but sometimes the effort involved is more than a boat is worth.

F-27’s if well maintained are very good second hand boats that serve their owners well but like most older boats if they are not maintained they suffer.

This may be a cheap boat but as one person said to me “That cheap boat will be the most expensive boat you will buy”. The jpegs give the idea.

 

Guzzis3, please google for "Ouabain", latin "strophanthus gratus" or "strophanthus kombe". An old well known med to avoid strokes.

 

Manfred.pech. Thank you for the medical suggestion, my partner did some research and found it interesting. I hope it can provide some assistance to Guzzis3.

 

This is about one mans dream and shear perseverance to achieve his goal. Trevor Payne had an idea about a power catamaran 20 years ago then did some research to find out if the idea had been done by anyone else. He eventually received patents in Australia, USA, Europe and China for his power cats and spent 15 years (without much income) building and refining 2 prototypes of the cat to prove the concept. So, what is the concept? SkiSea is a new hull concept out of Australia that uses special skis to provide hydrofoil-like lift. SkiSea creator Trevor Payne says this approach allows for greater fuel economy, stability in rough waters, a shallow draught, and higher speeds while generating minimal wash or bow waves.

The SkiSea is a planning power catamaran that hull dimensions are 23 x 10 foot (overall including extended bows and motors etc 27 x 11.5 foot). The dry weight is 6,950 lbs. The length to beam is different here, under displacement mode the length to beam is a barge hull of 2 to 1. The length to beam of the demi hulls is 60 to 1. The length to beam of the “skis” when planning is about 12 to 1. The power is either twin 300hp outboards or a diesel inboard as an option.

So basically, we have a catamaran structure that has 2 “skis” attached to the bottom, then you add enough power to get the cat to plan on the skis. As the skis and associated very narrow demi hulls are thin they can punch through any small waves etc. The only issue is how high the underwing rides above the sea to minimise pounding. If the underwing is close to the sea, you have a partial cushion of air to ride on, if the underwing is high, you may have less initial pounding but you lose the cushion of air effect. Interesting issue.

When powered by twin 300-hp (224-kW) outboard engines that give the craft cruising speeds in the region of 65 to 75 km/h (40 to 47 mph/35 to 40 kts) and top speeds in excess of 93 km/h (58 mph/50 kts), even in choppy conditions that would slow most other hull types. A report said: “The design was adept at traversing bad choppy conditions at very quick speeds, whereas traditional hulls find this a very hard and uncomfortable operation. With the high cruising speeds attainable, there was also a noticeable fuel advantage coming through for the prototype” and “Some other fortuitous advantages also manifested themselves in our trials of both the 2-tonne (2.2-ton) prototype and 3.1-tonne (3.4-ton) production model. Perhaps the most important of all of these could be the almost zero boat wake from the SeaSki while traveling at cruise speeds. The bow waves generated by the passage of the boat are no more troublesome to other boaters or the source of bank erosion/damage than surface conditions caused by a mild 20 km/h (12 mph/11 knot) wind”. Fuel consumption figures was not available.

The build is aluminum with the “ski partial demi hulls” an integrated aluminum extrusion. The skis are a minimum of 5 mm 6082 aluminum. The remainder of the build is plate aluminum of a minimum of 3 mm. The demi hull are sealed and also act as a 150 litre fuel tank.

The following web address will give access to further details and video’s: High Speed Boat | Offshore Power Boat https://www.seaski.com.au/

The jpegs give the idea of an interesting boat.

 

The following catamaran is used as a floating school, education tool about the marine environment. Sea Explorer plays a fundamental role in the Association Terre Marine Ecole de la Mer, which Franck and Bourguignon set up in 2014 to contribute to sustainability in the Mediterranean. They aim to engage the public toward greater understanding and enthusiasm for marine life and more responsible behaviour. In winter, the yacht is used for scientific research and educational film projects; in summer, it welcomes the public.

Michel Frank conceived this design with naval architect Nicolas Fauroux and structural designer Rivoyre Ingénierie. Micheal had the cat shell built by professionals in France in the Lerin Islands and then he and volunteers put in 38,000 hours to complete the cat from as many second hand and or recycled items that he could find. Many companies donated previous model equipment and left over building materials to help the build be completed. The cat was completed in 2017.

The cat is 79 x 37 foot with a “displacement” of 60,500 lbs. The 90 foot A frame aluminum masts carry up to 5 genoa’s of an unknow sail area. Michel has vast sailing experience and wanted an A Frame mast so that the mast connected directly to the crucial strong points of the boat, the hulls. This would allow the boat to remain light and strong. It also allowed a proper ladder and crows nest lookout points for spotting dolphins and whales.

The draft is 4.3 foot over the hull rudder and propellers. The hulls have a flat bottom for beaching and have no keels or boards for lateral resistance. The hulls were designed to be long and thin to provide the “lateral resistance’ like a Hobie 14/16. The engine power has 2 John Deere PowerTech™ 4045TFM85 engines with direct drives for port maneuvering and emergencies. For additional motoring the cat carries lithium batteries (54 kWh used EV car battery), battery management systems, 4 x 10kW electric motors and solar charge controllers with 25kW of solar panels. The intention is to power the cat by wind and solar with minimal fossil fuels.

The accommodation is vast but the main cabin is used for education and a “lecture” hall. The catamaran is equipped with microscopes, underwater camera and aquariums covering 250 square meters (2700 square foot) spread over three decks. The hulls have the sleeping spaces, toilets and galley equipment.

The structure is basically aluminum. The multichine hull has a flat bottom and is a simple shape to build. The remaining wing deck and cabin structure is flat panels so its basically cut and weld. Simple fast shape. BUT like all big boats addition things like engines, winches, deck gear, plumbing, electrical lines etc take the same amount of time as any 79 foot cat. The reason the cat took 38,000 hours (18 years of 40 hour weeks) to build this cat. Also the comment about the A Frame mast reducing the loads and weight of the cat is only partial valid. At this size of cat to add some additional weight to strengthen the main mast carrying for a single mast is a minor addition.

The performance of the cat is moderate for 2 reasons. The owners have no reason to push the cat as it is an education tool and why sail hard with children on board who may get scared or seasick. Result the statement “typical sailing speeds of 5-7 knots”.

An interesting total concept and catamaran. The jpegs give the idea.

 

The ski's idea is interesting. I wonder what marine architects/engineers think about applying this to a yacht ?

Also the "A" frame mast idea is interesting; although I think I prefer Bernd Kohler's version.
It appears that his website is down ?
He has a 45' cat with an A frame mast made from fibreglass flag poles

 

Peterbike. The following link still appears to work for Bernd Kohler site. https://www.ikarus342000.com/

The A frame mast has been tried many times, the smaller the lighter the boat the better it works structurally but it is not the best mast option for fast boats as it is no lighter and interrupts airflow to the sails.

The ski idea has been tried many years ago on an older style trimaran floats. It only partially worked at higher speeds and was a real drag at lower speeds. I suspect modern foils provide a better option on trimaran floats

 

However the links for the individual boats do not work.

There is a good assortment of Bernd's designs available on Duckworks -
Plans & Kits - Plans by designer - Bernd Kohler - Duckworks Boat Builders Supply https://duckworks.com/bernd-kohler/?limit=40

 

A-Frame Mast Rigs

Do a little search on this forum and you will find a number of A-frame rigged vessels.
....to name a few.....
Catbird Suite, WishBone Sailing Rig https://www.boatdesign.net/threads/wishbone-sailing-rig.1999/page-11#post-484935
WishBone Sailing Rig https://www.boatdesign.net/threads/wishbone-sailing-rig.1999/page-12#post-485560
WishBone Sailing Rig https://www.boatdesign.net/threads/wishbone-sailing-rig.1999/page-16#post-499688
Rainbow Warrior, WishBone Sailing Rig https://www.boatdesign.net/threads/wishbone-sailing-rig.1999/page-14#post-492050
SMG 50,
Procyon, PROCYON project........a Bold experiment https://www.boatdesign.net/threads/procyon-project-a-bold-experiment.10771/

One might have a look thru this rather long subject thread
WishBone Sailing Rig https://www.boatdesign.net/threads/wishbone-sailing-rig.1999/

I played with the idea as well,.
WishBone Sailing Rig https://www.boatdesign.net/threads/wishbone-sailing-rig.1999/page-3#post-109336