Multihull Structure Thoughts

This is about an electric foiling power boat. It is interesting on 2 fronts. One it is electric powered and it foils at up to 30 knots with its cruise speed of 22 to 25 knots for a 50 miles range. The second part is a 25.3 foot hull that weighs 670 lbs. So lets look at the Candella Seven. It’s a monohull sport power boat of 25.3 x 7.9 foot with a weight of 2,900 lbs including all foiling fittings, equipment, battery, engine and all accessories in the total weight. The draft varies from 1.4 foot to 4.3 foot over the foils and 5 foot over the propeller. The power is Torqeedo Deep Blue 50i 55 kW (75 hp) engine and a BMW i3 40kWh Li-ion battery.

Note that when the battery approaches its 0 level, the Candela Seven switches to an economy mode ensuring that it can travel eight miles at a speed of three knots. Charging is done from a standard power outlet. Charging time from 0 to 100% is approximately 12 hours. Power Consumption at 25 knots is 1.3 kWh CE. The efficiency of the Seven is difficult to beat — it is 4-5 times more energy-efficient than a comparable gas-powered planning boat and converts 90% of its chemical energy to mechanical energy. The cost of ownership of the Seven, according to Candela, is 95% less than a gas-powered planning boat.

The foils have a highly dynamic sensor/control system paired with a highly responsive foil. “The foil is straight, but we wanted to be able to twist it in the water, depending on speed, roll angle, pitch angle and yaw angle when running.” The boat needs to be travelling at 17 knots before it lifts onto its foils.

The accommodation has six seats, distributed in such a way as to optimize the balance of the boat. On the other hand, the storage spaces are few, and the equipment almost non-existent. To reduce weight there is no fresh water system or refrigerator on board the boat. To get such a light large boat with heavy batteries etc means being ruthless about any unnecessary weights.

Now we get to the structure of boat. Remember this is a vessel that can hit waves at 30 knots (not preferred but its designed to handle it.) The hull is single skin carbon fibre with an internal structure of carbon fibre panels. The hull is comparable in design to an aircraft fuselage — a skin surrounding a lattice of stringers and ribs (see the jpeg of the hull structure). The hull is infused, using the same UD carbon fiber and SR1710 epoxy, fabricated on composite tooling also made with SR1710 epoxy. Hull thickness below the waterline is 3 millimeters; hull thickness above the waterline is 2 millimeters. To build the stringers and ribs, Candela starts with 3-millimeter-thick solid laminate carbon fiber/epoxy infused panels and then CNC machines them to shape, depending on where in the hull the laminate is being placed. To build the hull structure — stringers and ribs — these cut shapes are then assembled and attached to each other and the hull with adhesive bonding and mechanical fasteners. A hull takes about 40 hours to make. The foils are UD carbon fibre built the same way.

All of the above is built by Candella Boats in Sweden. The jpegs give the idea of a very innovative boat but the structure is the thing that interests me A single skin is nice if the supporting framing and ribs are relatively simple to do, It is the stiffness of the panel that is mainly important in multihulls. You either get stiffness by panel thickness, material stiffness and or supporting ribs frames reducing the area of an unsupported panel. Light, fast to build but expensive. The jpegs give the idea.

 

I ran across this Rob Denny Harryproa design whilst looking for something else. I knew Rob had played with Kite rigs before but did not realize one of his clients, Trevor Jack had built and seriously tried a Kite powered Harryproa. The boats name is “Kitetik” and is 50 x 49 foot with a claimed weight of 1120 lbs (a number I doubt, Rob can you please verify). The accommodation float hull is 21 foot long. The sail power is various sizes of kites from 119 square foot to 282 square foot. The steering rudder/boards are on the long hull ends and can be kicked up.

The accommodation is minimal with a couple of single berths and a few basics. The build is foam glass an I suspect it was done using the Intelligent Infusion technique.

The performance of this kite powerd proa is based on written words and 2 short video’s. Quotes from here “We measured (apparent) wind of 2.8 knots at one stage. I think the highest apparent wind speed we measured was 12 knots and it was typically 8-10. So perhaps 6-8 knots of true. When we looked, we were often doing somewhere close to 5 knots (the log unfortunately was set to record data only every 5 minutes but from that small sample there were several points around 5 knots)” This was with the bigger kite.

Another video of the smaller 118 square foot kite showed the kite had a lot of power and the proa was moving well until something broke in the kites attachment to the proa. In both videos the person controlling the kite was constantly attending to the kites needs. Kites produce a lot of power for a small area, but control and handling issues have not been well resolved. EG to launch the kite, a dingy had to be sent out from the proa to set up the kite for flying.

An interesting approach, I do not know if there has been any further development. Can anyone tell us about serious attempts at a kite powered multihulls?

The jpegs give part of the idea. The Harryproa and Youtube will show short video’s (under 60 seconds) of Kitetik Harryproa flying the kites.

 

I think this may qualify as a “serious attempt” at a kite powered multihull. See video link.



Regarding the construction process of the Candela, agreed the results are impressive, especially the short build time.

I saw some of the young solar boat challenge teams using a similar process to build their boats using pre-fab CF sheet CNC cut to create ribs that they bonded in as a support structure.

And as I mentioned before, I think foils are the key to making this style boat viable as far as efficiency and range goes.

One challenge that still remains and is mentioned is weight distribution.

Getting a boat full of passengers to sit still can be a tough task.

I assume the foiling system has/will have load and balance sensing capability and will be able to trim accordingly (I have not done a deep dive on the tech), otherwise I have to wonder what happens when someone’s hat flies off and they react without thinking by standing up and lunging for it?

just how important is weight distribution and shift when underway?

I really have no idea how sensitive balance is with these new boats, but when I think about light boats like a Hobie 14 powered up at speed or just a regular foiling surfboard, minor shifts in weight have a huge affect in handling, steering, etc.. I guess what I'm getting at is with the boat hull being light, narrow, and foiling, I wonder how mindful passengers have to be about weight shifting either at speed or when transitioning to foiling plane.

I don’t have any personal foiling experience, only what I have seen quickly develop locally over the years in surfing,

And on that note, here in Hawaii we are already starting to see the challenges of regulating the technology with regard to personal craft.

By law, anything motorized and capable of a certain speed must be registered and follow the “rules of the road”, and yet it’s not uncommon to see operators who are so tired they can barely stand up, let alone maintain balance at 30MPH, ripping around through our mooring field as if it were an obstacle course during the summer. The few times I have watched this the riders reminded me of a drunk trying to ride a bike. I once watched one dump as he was heading for a moored Tri, when he got back on and set off he started pulling the Tri with him as the Tri’s mooring line was caught on the foil.

Anyway, enough of my griping. The reason I bring some of this up is because it relates to the operator and how skill plays into safety.

We’ve all seen the videos of foiling boats wildly crashing and the behavior of “unqualified” captains operating boats. Designing safety precautions into the technology (like modern electric vehicles) is what will likely set some brands apart from others because everyone will want to have a go at piloting it, and boats as we all know are already dangerous in the hands of some, let alone a foiling boat full of people.

I have to think that second to design and construction of an efficient hull, “flight” systems will be an important part of developing electric foiling boat technology for the masses.

Now assist foiling (I think that’s the term) is an interesting concept that might be a good fit for semi-planing cruising type Eboats.

Some power boat manufacturers are using it to smooth out the ride and improve efficiency, and I wonder if the same technology could be used to improve efficiency and the top speed of low power cruising boats by reducing wetted surface area without the complication of full foiling.

Any improvement in range/top speed could be exponential in that you could also reduce motor and battery size and still achieve the same overall performance.

Aquila boats for example is one of the manufactures incorporating this tech into their ICE power Cats.

And BTW, since OldMulti recently did another feature on it, and rumor has it that production has ceased, a Farrier F-22 kit just popped up for sale on Ebay if anyone is interested.

 

Kitetik was the first professionally built infused Harryproa. A falling out between the owner and builder (underquoted despite having been deeply involved in a near identical test boat I built next to his shed) meant Trevor got it cheap. I haven’t weighed it since it was launched, but based on the components and other boats (the 24m cargo proa weighs less than 3,ooo kegs/6,600 lbs) we have built, I am confident the quoted weight is close.
Trev uses it to test ideas on how to auto fly, launch and retrieve kites, it has yet to be seriously powered up. The biggest Kate so far is 24 sq m/250 sq'. Launching that from the deck was an interesting challenge. The days of dinghy launch are long gone.

Kitetik is the next step from his kite powered foiling nacra cat which has easily exceeded 20 knots and my test bed Harryproa fitted with an inclined foil under the short hull and the kite attached to the lee hull. The foil has 300mm/12” chord is a sec and is lifting a bit under 200 kegs even when fully ventilated.

 

The following simple tri is being built by Tom Raidna based on a professional design. The tri is 16 x 13 foot (an be folded to 5.7 foot for trailering) and weighs 225 lbs. The designed displacement is 650 lbs. The mainsail sail area is 100 square foot with a 93 square foot gennaker. The main hull length to beam is 11 to 1. The daggerboard draws 3.5 foot when down. The rudder is a stern mounted kickup.

This tri has the same characteristics as others, very simple easy to build hull shapes that are actually quite refined to achieve the balance between performance, ease of build and appearance. Notice the gunnel angle flats on the floats which add to stiffness as well as improve looks. The chine step in the hull sides add to the hull stiffness as well as add space in the main hull (max 2.5 foot width at mid hull chine level in the main hull) etc. The result is a reasonable hull shape that can also be used as a mini cruiser with a tent top option. The design is intended for 1 or 2 people.

The build is plywood, timber and aluminum crossbeams. The hulls are mainly 4 mm plywood with 6 mm plywood main hull bottom and bulkheads. The joints are mainly taped seams and the hulls are 200 gsm fiberglass cloth covered in epoxy. The aluminum beams are solidly attached to the floats. When you want to launch, you lift the floats into position then insert the tubes into the main hull receiving tubes and push into the sailing configuration. Again with a bit of thought you could develop a simple folding system with EG hinges that may make launching faster.

An interesting simple design for day sailing or as a amp cruiser. The jpegs give the idea.

 

One very interesting article I saw come up recently mentioned that due to the cost and difficulty of obtaining insurance the Ocean50 trimarans are now carrying a Yves Parlier designed kite to act as an emergency jury rig. During the last Route du Rhum the fleet was shadowed by a power trimaran which a number of teams had signed a contract with that could right the boats if they capsized and they could sail independently to the nearest port under kite jury rig. The power trimaran would then continue on shadowing the fleet. One Ocean 50 capsized and they righted it at sea which has all sorts of positive benefits as far as easy repair of the boat if it happens on a timely fashion.

 

Corley. That is a very interesting approach. The kite will probably help if the boat is upright but has a rig issue but the capsized boat recovery by a "rescue" boat is an interesting approach. Some of the Farrier tri guys are having real problems getting insurance cover especially if they are home built. Big production cats done by Pajot, Lagoon etc have less problem especially if they are in charter fleets. Multihull insurance globally is a real problem especially if you cannot show a reputable designer and builders name associated with the boat.

 

I found this advertisement of a racing cat which is very performance orientated. I would call this a pure racer and a small amount camp cruiser, accommodation is an afterthought at best. The real problem about this tube cat is who did the design and build, all I can find about the cat is it named “RITALIN” a ADHD 28 with no indication of who built it. Folks, link the name to the “boat brand” and you find it a “solution” to some medical issues. But back to the cat, RITALIN is 28 x about 22 foot with an OMR measured weight of 1,300 lbs ready to race. The 45 foot carbon fibre wing mast carries a 416 square foot mainsail, a 185 square foot headsail and a 390 square foot screecher. The hull length to beam is about 16 to 1. The cat has deep dagger boards and kickup stern rudders. The outboard is 5 HP.

What does this all add up to, an OMR rating of about 1.2 depending on the sails carried. That implies it can sail at about 1.2 times wind speed in theory. RITALIN has raced and taken line honours against larger boats but its racing history is hard to find. Anyone who has more detail in Queensland, can they fill in the gaps please.

Now we get to the interesting hull shape. We are talking a high prismatic hull that has very full flat bottom sterns with a near chine like shape. The bows are reversed. The decks and bottoms look as though they came from a similar mould. I would like to know if the shape is an enlarged beach cat or because the weight of the crew is a large proportion of the boats weight and the sterns need buoyancy to support them.

The construction is listed as carbon fibre with what looks like 2 carbon fibre cross beam tubes and a prodder for the forestay and screecher attachment points.

As I said this is a performance cat that has an interesting hull shape. Hopefully people know more information. The jpegs give the idea. Tomorrow an interesting precursor to this cat that may or may not be related to this design.

 

Today we will talk about Box Boat 8.5, a 2013 design by Matt Johns and built by Dave Bigger, to be a one design class. The Box Boat 8.5 is a tube racing cat with no thought to accommodation. The Box Boat 8.5 is 28 x about 20 foot with an OMR weight of 1450 lbs ready to race (hulls and cross beams came in at 780 lbs). The 42 foot carbon fibre rotating wing mast (2 parts for container or trailering) carries a 377 square foot mainsail, 247 square foot headsail and a 485 square foot spinnaker. There are deep daggerboards and kickup rudders on the stern. The cat is designed to disassemble and can be fitted into a 40 foot container. “Box Office”, a 8.5 in QLD has a slightly larger rig with an OMR rating of 1.145.

This boat is about performance to quote Matt Johns “The cat achieves speeds of 12 to 13 knots upwind. Downwind, 20 knots is attainable in breezes more than 12 knots. Matt reckons it is really enjoyable to sail in 15 knots of breeze and it comes with a single reef to make it more manageable in stronger breezes, particularly during training and less competitive sailing”. After the 2013 Hamilton Island race week where the Box Boat 8.5 lead several races against much larger multihulls observers thought the 8.5 sweet spot is 12-14 knots running downwind and it performs best in up to 18 knots of breeze. Any cat that can run 60% faster than wind speed and can peak at 25 knots plus I would consider fast.

The construction is of interest. The Box 8.5 is unique as it has solid core hulls (as in surfboards) made from sections of polystyrene, precision CNC machined, which are segregated in the hulls by factory made bulk heads for each load point; forestay, beam sockets, winch base and side stays. The sections are butted and glued together before the yacht is taped with carbon fibre patching then completely wrapped in the cloth and laminated with an epoxy system. The simplicity of the build method means that homebuilders can assemble the yacht themselves with little or no help from qualified boat builders further reducing overall cost. The separation of skins by the solid core results in hull stiffness and allows us to keep the yacht light, the first hulls and crossbeams weighing in just over 780 lbs. The crossbeams are carbon fibre as is the prodder.

The building platform kit includes everything you need to put your Box 8.5 together. Clip together CNC routed build jigs to make it even easier to assemble, female moulded foils, and beams are also in the package as is carbon tillers, tiller linkage, bow pole and dagger board lifting posts, basically everything you need to complete your platform.

Result a home made very high performance cat with e-glass epoxy hulls with some carbon fibre and carbon fibre foils, beams and rig.

The jpegs give the idea. As I said yesterday, I suspect this cat inspired yesterday’s 28 foot design Ritalin.

 

Interesting when comparing those last two Cat designs the difference between beam designs, and I’m surprised to see that the one with round tubes uses a similar thru bolted connection method like you would see with aluminum, and without the need for any kind of strap or clamp on top.

Both boats look like a lot of fun, and their lightweight certainly makes assembling/disassembling more feasible with a simple trailer support system.

I wonder if any modern day improvements could be made to the center hinged folding design the Viva 27 originally used.

A few people I talked to bad mouthed the system, but I didn’t really get a good explanation as to why.

If you could have a modern folding fast Cat like the boats above that could remain assembled and partially rigged so they could be launched folded, finish rigged on water, and was narrow enough on trailer to fit in a container, I think it would be pretty cool and more usable for day use and easy to store and transport.

Basically a large light fast Cat that takes as long as a Farrier/Corsair to go from trailer to sail that could be easily rigged, launched and sailed single hand for day use like a small beach cat.

 

In 1994 I visited Lymington, U.K. and spoke with the designer of a folding 8.5m catamaran. This boat received an award for design and could be unfolded on its trailer. The Aluminium beams were hinged at 3 parts and the 'operator' stood in the cockpit of one hull and, using a sheet winch on the boat the boat unfolded while simultaneously raising the mast and tensioning the trampoline. I can't remember the name of the boat or the designers name (James Labatouche?) but he took me out to this boat moored nearby. It felt stiff and had good accomodation in each hull with standing headroom amidships in each hull.
SolGato, this boat took 15 - 20 minutes from arrival at a slipway to raising sail (very favourable compared to Farrier tris).
I was offered the manufacturing rights to this design but never took up the offer. I believe that the boat actually took part in a Round Britain event but have no information as to how it performed.

 

Phlames. I have seen one photo and an article on this cat many years ago and cannot relocate them. Can you remember if the the beams and joints had a slight angle built into them to help with alignment in the fold and unfold? Any further information would help. PS Catsketcher's Breeze 7 design is a different folding cat but works on similar principles. When I found enough stuff we will talk about Takeaway, Brine Shrimp, Viva 27, Breeze 7, Stilleto, International 23, Evergreen and several other genuinely trailable catamarans that are real and have actually sailed and trailed on many occasions. These boats exist but each has its issues. The Farrier or Dragonfly system for tri's is just easier at launch time.

Solgato. The Viva approach was a simple approach that if well engined with a dolphin striker worked well. Only problem is a 27 foot boat and especially its rig is a big thing to tow, set up for launching etc. Even the Farrier boys limit moving there above about 25 foot boats to much.

 

To give you an idea of what is possible Here is a story about a guy who decided to cruise around the UK on his cat Biscuit. Biscuit is a Sprint 15 (also known as a Dart 15) which is 15 x 7 foot and weighs 220 lbs with a 21 foot mast carrying a 132 square foot mainsail. The low aspect ratio keels draw 0.23 foot. This is a beach cat, not a cruiser. Accommodation was either onshore or a tent pitched on a beach. Basic rations were taken but the majority of meals were at local pubs or restaurants.

The owner decided to sail around the UK to see the country. He had attempted this previously but found he was unprepared so he decided to do it properly. He prepared the cat for the task. He strengthened and doubled up the chain plates and shroud plates to back up the existing steel rigging with dyneema string. The boat's original sails had a reefing point inserted, allowing him to reduce the sail area to about half. He already had a Garmin GPS watch which would tell him his speed and heading, as well as tracking the course. He purchased a 24 amp hour battery pack and a solar panel that would unfold to about 1.5 foot long. He bought a new hand-held VHF radio with USB charging which I tested and would last several days between charges. He also purchased a £190 Chinese waterproof mobile phone with a 10 amp hour battery and purchased the latest digital charts for the new phone.

The first day sailing took him from Shanklin on the Isle of Wight to Swanage. Just 42 nautical miles but his body wasn't prepared for six and a half hours of close hauled sailing. After getting off the boat it took ages to get his back straightened. There were many high and low moments during the trip with some strong winds but many hours of good sailing and if things got to rough, he stayed onshore until it passed.

He covered 2,249 nautical miles on the trip. He broke two battens, wore out the main halyard and destroyed the metal plate at the bottom of the mast. An old drysuit, several pairs of gloves and a pair of old boots were worn out. Sun peeled his nose and his hands got very fragile and wrinkled from being constantly wet. Other than that “Biscuit” and he were in pretty good condition and was fitter than when he started.

"Roller coaster of emotions" sums up how he felt doing the trip. He went through the darkest despair and experienced the heights of elation. The worst moments were mostly when he was becalmed with big seas. “A sail flogging from side to side in chop will drive you mad eventually. I was rarely troubled by too much wind while I was on the water.”

The good bits were very good indeed. “The west coast of Scotland was far more beautiful than I could have imagined. I landed on some truly amazing beaches, some of which were basically deserted from one year to the next. I landed on five Islands: Anglesey, Walney, Eigg, Mull and Lindisfarne. I saw porpoises, dolphins, a minke whale a yard from the bows and hundreds of seals. I sailed past countless puffins, sea eagles, razorbills, gannets, guillemots, skuas, terns and dozens of other sea birds species that I cannot name.”

Final comment “Would I do it again? Don't be ridiculous.”

The jpegs are of the Sprint 15 and some shots of the cat and sailor. The fuller story can be found at North Island to Starboard - Around Britain in a 15ft catamaran https://www.yachtsandyachting.com/news/222145/North-Island-to-Starboard

 

Oldmulti, from memory the aluminium beams had three cast aluminium hinges - one in the centre, one each end just as the beams joined the inboard side of each hull. There was a central track under the 'bridge deck' netting that held sliders that would tension the netting as the boat unfolded. There was a seperate system that raised the mast simultaneously with the unfolding. All was controlled by a single person at a sheet winch near a rear cockpit.
A very clever system. There was a patent for the hinge design and patterns existed for the castings that were designed to fit 'standard' aluminium mast extrusions.
As a result of a recent house move I have, unfortunately lost a lot of my information about the boat as well as a lot of other multis information. For this reason I sincerely thank you for your treasure trove of data in your posts.

 

Phlames. Thanks for the additional information. I will search on. Part of the reason for this blog is the 22 GB of boating information, sailing and building experience that could be available to people who have a similar intertest. I also hoped others would use this resource to record there knowledge or information about multihulls structures etc so the information does not get lost over time. EG if I remember correctly you built a 26 foot Tennant cat, you know the build process and I assume any strengths or weakness after sailing the cat. This information would be of value to many.

General. Please do not disclose full plans of a designer who is still making a living from his/her work unless they have given approval.