Multihull Structure Thoughts

The following will be a 3 part story of a Croatian guy who has built 3 cats spread over the last decade. He was initially inspired by Wharram and crab claw rigs and built his version of it in his initial small cat. The small cat is a day sailor camp cruiser at best. The cockpits can carry a few people or stores.

I do not know any dimensions but would guess between 18 to 20 foot with an A frame mast and crab claw sail. The tack of the crab claw sail can be moved to either side to help when going upwind. There appears to be no lateral resistance depending only on the deep V hull shape. There are stern based rudders.

I am certain this is a home based design and build. The hulls appear to be tortured plywood over bulkheads. The skin ply would be thin to create the ripples in the hull. The beams are solid timber. The A frame is of an unknown material. The sail appears to be sail cloth with PVC tubing for the yard and boom.

I have no idea of performance.

The cat was sailed for a few years then the builder was inspired to move onto a larger Wharram based cat.

The jpegs give an idea.

 

Part 2 of the Haka story. The guy from Croatia decided he wanted a larger cat, he choose a Tiki like catamaran hulls about 31 foot long. The reason I say Tiki like is the freeboard is higher in the Croation build and there are variations in the cabin shape and the rig. I do not know if he built the hulls but the cabin, interior, cross beams and rig are all his work.

If this cat closely followed the Tiki 31 it would be about 31 x 17.5 foot with a weight of 3000 lbs and a displacement of 5200 lbs. Thee sail area of the original design was 386 square foot but the rig that was set up on version 2 of Haka was a crab claw ketch rig on A frame masts. The draft is 1.75 foot.

The build is plywood timber. The ply is about 9 mm and the stringers are 50 x 25 mm on the flat. The reason I think this is a “copy” of the Tiki 31 is the keel lines on the hulls are flat not rounded. The cross beams are shaped like the Tiki series but initially there was a foam glass discussed for webs etc. The rig is A frame with foam glass A frames, booms and yards. This took several years to build.

The boat was completed after 5 years of building and launched. There was some limited sailing which resulted in some rig damage, then the owner builder changed direction and wanted a smaller trailer sailor cruising catamaran. The builder then put Haka 2 up for sale which took a long time to sell. The buyer was described as a “crazy Russian”, which allow the Croatian guy to build Haka number 3 which is a 28 foot trailable cruising catamaran.

The jpegs will give the idea.

 

Part 3 of the Haka story. After selling Haka 2 the Tiki 31 look alike, our Croatian guy now could focus on his trailable catamaran. “Kahuna” (name of new cat) will be a 8.5 m trailerable catamaran. This time it will be built inside a plastic shed to allow faster catamaran. The build still took 4 years. This man has stamina and a lot of patience.

Little is known about the 28 foot cat. Pure guess, 28 x 16 foot with a Wharram type wing sail rig of with a 28 foot mast and 190 square foot mainsail and a 150 square foot jib. No indication of lateral resistance. There are daggerboard rudders on the stern.

The accommodation is spread over the 2 hulls and central detachable pod. The hulls contain 2 single berths and in one hull a galley. The central pod has seating and table.

The construction is mainly plywood, timber and XPS foam (think blue Styrofoam). The hull sides, bulkheads, decks and central pod are 9 mm. There are 20 x 40 mm stringers. The hull bottoms are 4 mm plywood with about 30 mm of XPS foam over the bottom then fiberglassed over with epoxy resin. The cross beams are ply box sections with XPS externally for shaping then fiberglassing. Warning, XPS foam is not a structural foam and can break under loads. Please use marine grade foams (eg PVC, PET etc).

There are no performance figures available or launch jpegs.

The jpegs give the idea.

 

Haka Kahuna jpegs extra.

 

I realize this is an old post.. but I believe that green-hull CC-26, originally built in Old Saybrook, CT - is now in my possession as of this past Sunday

 

John Marples the multihull designer wrote an interesting article about fibre rigging. I will quote parts of the article and add some additional information.

High strength fiber rope has around for some time now, but until it was produced in a new form, it was not suitable for standing rigging. Although stronger than steel cable, the basic rope suffers from having high stretch. Most of the stretch is called “constructional stretch”, which is caused by the fibers settling into a tight braid as load is applied. Once the settling is complete, the rope only exhibits “material stretch”, a characteristic of the type of material from which it is made. Dynex Dux and STS have been “heat set” by compaction into a more condensed braid under heat while under tensile load. This process removes the constructional stretch, leaving only the material stretch. Dynex Dux is not made in less than 5mm, so SK-75 Dyneema rope can be used.

Another stretch characteristic of continual elongation over time when under load is called creep. Creep is virtually non-existent in steel cables, but is measurable with synthetic fiber ropes. For both Dynex Dux and STS, the amount of creep in normal usage will not be a factor. Rigs with high standing preloads may require an occasional tightening. For most small craft with floppy rigs, or trailer sailers, it is insignificant.

Not only is the fiber rope a direct replacement for steel cable, but much of the attachment hardware can be changed to lashings to eliminate other metallic components. Avoiding the use of expensive turnbuckles and toggles and replacing them with simple lashings will reduce costs significantly. Off-the-shelf components are available to allow the boat builder to fabricate all shrouds and stays for the boat. Many of these fittings are designed to connect to existing tangs and pins for direct replacement of metal cables. Others allow the builder to optimize the spliced-loop end cables for original construction.

For small boats, the stainless steel rigging cable can be replaced with Dyneema SK75, Dynex Dux or STS fiber rope of the same size. The fiber rope will be 50% stronger. Dynex Dux minimum diameter is 5mm, so for anything smaller, use SK 75 Dyneema line from your local marine hardware supplier. For thimble terminals, use heavy duty “Sailmakers”, complete ring thimbles about 50% larger than specified for the rope diameter. If larger than 3/16”, consider using commercial terminator fittings.

You should replace the rigging more often as fiber will degrade slowly in UV exposure. If you don’t know how to make a brummel splice, learn how to do it and you won’t ever need a rigger again for your tri, except for the forestay. Use stainless cable for the forestay as a furler has to transmit torque all the way to the top and dyneema won’t do that without additional bulk which would spoil the fine entry for the jib.

Standing rigging products include: LIROS D Pro Static article 01504 – 12 strand Hollowbraid from specially engineered Dyneema DM20 fibre delivering near zero creep, LIROS Heat Stretch System, LIROS Coating System

Marlow M Rig Max – 12 strand plaited construction from super pre-stretched Dyneema DM20, near zero creep, Armourcoated

Hampidjan DynIce Dux – super pre-stretched Dyneema SK75 12-strand construction – although the initial stretch of SK75 is not comparable with other fibres, it is more responsive to heat stretching, which results in near zero creep and constructional bedding in Hampidjan’s super-enhanced DUX version.

Jimmy Green Break Load Comparison Guide for 12 strand Dyneema Fibre Rigging for: Hampidjan DynIce Dux, LIROS D Pro Static, LIROS D Pro XTR, Marlow M Rig MAX and Marlow D12 MAX78 is in Fibre rig 3 jpeg below.

DynIce Dux Weight and Break Load Information Chart including a comparison guide with standard 1×19 stainless steel wire by equivalent creep/stretch DynIce Dux (DD) is in Fibre rig 2 jpeg below.

Attached below is a jpeg of the next step if you want the ultimate in light weight fibre rigging, Carbon Fibre.

In all cases there is larger sizes of fibre rigging but i limited the charts down to rigging suitable for EG a max of a 40 foot cat or tri.

 

...all fine & good, but there is this little thought in the back of my mind, that some drunken vandal can cut my standing rigging with the swipe of a pocket knife...

 

He would have to have a really sharp pocket knife
I used 5 mm Amsteel Blue for the cable railing around our back deck some years ago. The only issue is the negative temperature coefficient; cables get fairly loose in winter.
Along with the cable supported light fixtures inside there were 118 brummel splices involved. There are 12 on my proa schooner ;-)
Oops, I forgot the 3mm lashing tails, House 177 splices, Proa 16 splices

 

In yesterday’s fabric rigging item there was a mention of a Brummel eye splice to attach the fabric lines to a termination point. This is done better by pictures than description. The strength of a Brummel eye splice is very high. The basic principle is the Brummel eye splice is commonly used for Dyneema rope. Passing both ends through the rope at the base of the eye locks the slippery braided line, and then a long tapered tail is buried inside. The simple Brummel splice works as a first splice because both ends can be threaded through. The second splice, on the other end of the line, requires a different technique because the eye can’t be passed through the braid.

The jpegs give the idea.

 

Part 2 of Brummel eye splice.

 

Nice how-to on the Brummel splice. Definitely one of the most useful terminations for high-test single braid line. I'm not quite sure of the meaning of the last sentence. Are you saying you can't effect a Brummel splice in the other end of the rope as well? I've put Brummel splices on both ends of line for sailboat andr igging and traction kite bridles several times. - with and without thimbles.

I picked up a set of Samson tubular fids years ago. Then I started making my own 'needle' fids with music wire and rarely ever bother with tubular fids on single braid. The example in the thumbnail is used on single braid from ~ 1/8" - 3/16" (3-5mm), and is made from 19 or 20 ga music wire. I've made them shorter, with smaller gauge wire, to splice single braid dyneema as thin as 0.5mm (200 lb test fishing line) for kite bridles. My largest one, using 17 or 18 ga wire, works on 1/4" - 3/8" (6-10mm) line.

The elongated 'diamond' tip makes it easier to insert through the braid. A tapered hot-wire cut, or even just an angled scissors cut, at the bitter end helps with threading through the eye. The twisted wire shank is epoxied into a deep hole into a wooden dowel handle, with a bit of plastic tube as a bushing at the exit to help relieve bending stresses. You can pull really hard if needed, and the bitter end stays wedged into the tip of the eye without sliding out. There is no messing around with masking tape on the bitter end, or stuffing line into a tubular fid only to have it pop out.

 

This is reality versus dreams. Designer and boatbuilder Tim Clissold, whose talents have mostly been put to work on sail-powered multihulls, has spent several years designing a 20 foot electric powered tri (Elotri) for the average weekend boater. This power tri will not consume 60 litres of fuel for 100 km but will go 50 km on a single charge, stay for hours of fishing or overnight etc whilst the solar panels assist recharging the battery for the trip home.

The Elotri is 20 x 7.7 foot with a weight of 1,700 lb and a displacement of 2,700 lbs. The draft is 1.5 foot. The 4 x 12 volt batteries Lithium Phosphate LiPO4 11kW/hr are wired in series to provide 48V DC. The solar array is 720-watt flexi-panels. The energy stored is feed to a 3kW 48V electric motor which provides a 30 mile range (50 km) at a speed of 4 to 5 knots. The engine draws about 500 watts at cruise speed and the solar array provides 100 watts/hour input in moderate conditions.

Tim Clissold did test models of the trimaran hull form to minimise drag whilst carrying the weight required. The main hull has a narrow entry with chines developed into floats on either side, for minimum drag and maximum efficiency. One of the longest and most complex development processes was around the propeller – critical for translating the energy provided by the electric battery system into forward motion. Clissold used a 3D printer to produce a series of experimental prototypes. “If you’re doing something completely different, you’ve got to think differently,” he says. “I can afford to be experimental – it only costs $40 a prop to print them. I would love to do tank tests but you can tell from the wake and speed how effective they are.” A very nice experimental solution before paying for a metal propeller.

The logic of this power tri is most people don’t travel far from their launch site in harbours lakes etc that have speed limits etc. Why pay $100 for fuel to go 100km for a weekend when you can travel for free with solar panels. The Elotri has accommodation for 2 in its forward cabin with a toilet and storage for a portable fried and stove etc. The cockpit can be used for another 2 bunks if required. The layout is useful for a camp cruiser for a weekend.

The hull is made of foam and glass, with some carbon components, and weighs around 800kg all up, with about 100kg of that in the batteries.

The real virtue of this approach is to provide a simple power tri that can act as a day or weekend cruiser for sheltered waters that can be trailed home and uses its solar panels to recharge its batteries. Minimal expense for some slow boating watching the scenery go by or do some good fishing. A different way of thinking but very realistic in this cost constrained world.

The jpegs give the idea.

 

This is about cheap housing and boating combined. A couple of years ago Brett and Sheena Parkin were casting around for a clever way to achieve affordable housing for themselves in Auckland, New Zealand. They got an idea of buying a house boat as their Auckland resident. After a search they found a suitable craft in the South Island of New Zealand and then they need to sail it to the North island across Cook strait, an occasionally very rough bit of water.

The Cat A Rac is a 42.6 x 23.6 foot catamaran houseboat of heavy displacement (EG It has a steel hull structure and there is 6500 lbs of liquids on board this cat). The engines are 1980 Ford D series truck engines of about 80 HP each. The fuel burn is approximately 8 litres/hr with a cruise speed of 5 knots. Now we get to the real issue. This is a catamaran hull with a 2 story full size cabin structure on top. This is a LOT of windage combined with the weight and hull shape is the reason this cat can only do 5 knots with 160 HP pushing it along.

The cat was originally a “back packer” charter boat with 2 double berth cabins and 25 bunk beds for the back packers. It was surveyed for 30 guests overall. When Brett and Sheena finally set it up as a “house” Cat A Rac had good facilities EG a nice kitchen and dining area etc. Space was not a problem.

The downside of this cat was its ability to move anywhere if there is wind or waves. Cook Strait is open water that has variable weather from calm to 40 plus knots of winds. I will quote Brett on the trip “Often the direction we were travelling (Course Over Ground) and the direction we were facing (Heading) were perpendicular to each other! We were travelling sideways! We were also facing some pretty big swells. While we were able to keep our bow roughly into the swells, we were fairly comfortable. Lots of pretty scary up and down, but not terrible. It was when we were side-on to the swells that things did get… well… not good! I was completely absorbed in keeping the boat facing the right way into the swells and wind, desperately trying to push her on towards a point we’d previously identified as a safe anchorage”. Now it got scary. “To gain a little extra turning power into a particularly big swell, I pushed the throttle down on one engine and watched in horror as the engine died on me. Down to one engine and in the roughest conditions I’d experienced on Cat A Rac.” Cat A Rac eventually crossed the Strait but had modifications done after that journey to improve both the boat control and upgrade the accommodation. After 2 years Brett and Sheena found a house to live in and are converting Cat A Rac back into a charter boat.

Translation, inappropriate weather, too much windage, insufficient engine power and or inefficient propellers resulted in a dangerous situation not helped by a failing engine.

The build is a catamaran hull made from steel and an upper superstructure from plywood and timber.

The jpegs give the idea but it would be better to have a more seaworthy design appropriately powered for travelling in open water.

 

Same vibe. Radically different execution!

 

Looks like the feature of the Elotri was well liked.

I had just seen a video of it and interview with the designer on YouTube prior to your posting.

The concept really lends itself well to rivers and protected waters, and I can appreciate the multi-functionality of its features and the collapsible solar roof, both of which are things I worked hard to incorporate into my boat.

My only critique is that in the video and even in some of the photos posted, the boat appears to be listing quite a bit.

While I understand the desire for a cuddy cabin, I think I’m more of a fan of a true multihull platform for stability and efficiency.

One way to incorporate a “cabin” is to make the deck of a Cat transformable into an enclosed space using the solar hardtop.

I have a shower curtain enclosure that I can drape from my hardtop, but have considered adding rigid solar side panel wings that can open for increased solar, but then be closed to create the cabin.

Recently I came across a nice execution of the concept on the design of the Millikan M10 which you might want to do a feature on if you haven’t already @oldmulti .