Multihull Structure Thoughts

Rob I not going into the for and against Harry Proa’s as they are basically another version of a multihull. A well thought out version I might add.

My problem is basically when I sail a 33 foot tri the jib can be changed by one person if there is a problem. Going upwind in a 25 knot wind and having to change a headsail because the sail is ripped, on the foredeck of a 45 foot cat is not a simple job. (roller furling sails can still rip). The size and weight of the sail etc makes life hard as your thrown around by the boats motion, wind in the sail on “deck” with water weighing it down etc. On the 55 foot tri I occasionally sailed on every sail change move needed to be planned to ensure the safety of people and boat. Sailing in good conditions with perfectly operating gear is easy on any size of boat. When thing get tough due to bad weather or there is gear failure size starts to matter. The configuration of the multi is not the issue here, it’s the size of equipment, sails and boat windage that can cause problems.

Finally, not relevant to most Harry Proa’s, I can set a 2000 square foot spinnaker single handed in under 10 knot winds but it takes 3 people to bring it down reliably without doing damage to the spinnaker. Size, wind and wave conditions etc do have an influence.

 

The following cat is one persons version of a cheap simple to build boat to “Bug Out”. The cat is 32.5 x 18.9 foot weighing 2000 lbs and displacing 3800 lbs. The split junk rig has a free standing mast with 405 square foot of sail. Please look at https://proafile.com/multihull-boats/article/the-bug-out-boat

I will leave the designer to describe his construction option for the boat. Simplicity is the theme, cruising is the game. “BUG-OUT is designed for 4x8 sheets of plywood. The 8’ long bows and sterns are identical, the center section is 16’, for a total length of 32’, plus rudders. Each hull is built from 8 sheets of plywood, plus bulkheads.

The hulls (really the pontoons) are built 2’x2’ square in section - rotated 45° to create a diamond. The shape is as rudimentary as it gets: there is no rocker or graceful sheer - just a brutal 16 feet of ‘extruded’ plywood. I am tempted to call the style “Polynesian Neo-Brutalism”. Be that as it may, the simple shape hides a number of virtues besides the primary one: it can be constructed with little more than a T-square.

There are no interior accommodations within the hulls, so they are simple and forgiving to build. The V’d diamond section provides a gentle and well-moderated ride in a seaway. One nice thing (for a designer) is you are not tempted to make the WL beam “just a bit more” in order to fit in more stuff. BUG OUTs hulls have a 13:1 finesse ratio, and will easily exceed hull speed. The hulls also have a high prismatic coefficient, which means they make a good sail carrying platform - not a lot of hobby-horsing in a seaway. They are also reasonably wave-piercing at speed, which is why the the crossbeams are elevated above on plywood struts.

BUG-OUT sports an unconventional rig for a multihull: a Chinese lug, or junk. Specifically, this version is called a split junk rig, or SJR for short. Developed and documented by Slieve McGalliard on the Junk Rig Association site, the clever refinements shows a great deal of promise in overcoming the Achilles heel of the junk, namely, poor windward performance, or L/D ratio.”

Looks like a fun quick build design for simple cruising. The web site has more details.

 

That's the coolest new design I've seen in awhile. The hulls remind me of the original "Nugget" amas. Not crazy about the rig.

 

Child of the Seas (Tama Moana) is a Wharram ethnic catamaran of 37.75 x 15 foot weighing 3500 lbs and displacement 6900 lbs. The sail area 395 square foot in a 2 mast Crab Claw rig.

Glenn Tieman built and sailed a Pahi 26 but wanted something more ethnic and longer. Wharram/Hennke Boon produced what Glenn wanted a longer cat more in sync with ethnic values but with not much more accommodation as he was happy with his Pahi 26 accommodation. The Crab Claw rig and the hull shapes came from Polynesian history especially from a 9 meter canoe of the islands of Tikopia and Anuta found at a NZ museum. First jpeg shows the hull shape found at the Museum.

The Tama Moana (Child of the Sea) has the traditional hull shape. She is built in strip planking, glassed inside and out, over plywood backbone and bulkheads. The cross beams are solid timber poles that are tied to hull with very low stretch fabric bindings. The short masts and Crab Claw spars are again solid timber. She is steered with steering paddles/side rudders. This is a relatively low cost boat for its size but expensive for its accommodation.

Ethnic Designs as Canoe Craft have a basic design principle of maximum boat for minimum cost. Building and sailing one you can be a research participant in a major attempt to recover and preserve the practical, design, handling aspects of Man's first offshore sailing vessels.

Glenn and several others have built the Tama Moana design and have sailed thousands of miles in the Pacific and other area’s. 2 were built, sailed 4000 miles then donated to the people on the islands of Tikopia. The boats can go upwind tacking through about 110 degrees in flat water. The cat can sail well on a reach and running. The rig takes a bit of work but is relatively small and divided into two sails which makes it easier to handle. An interesting design.

 

We had a talk about boat size. I copied this from a longer Facebook post from Mark O Hara which was posted on the Wharram cats Facebook page.

In fact, if anything this lock-down has inspired me even more and I’m considering building a second boat, maybe a Tiki 21. Where the Pahi 63 is home, base and center of operations the Tiki 21 will be more like the family car. Audrie's home province is 9 hours by road and 2 hours by sea, go figure! But the Pahi 63 is too much sail to handle for one person over such a short distance, something like a Tiki 21 would be ideal for small weekend excursions and shopping trips to the capital with transport and accommodation with a small deck tent sorted out.

The Goldilocks zone is hard to find - not too big, not too small.

 

Thank you, thank you, thank you for posting THIS one.
This has me returning to an old idea I discarded because I thought it was too simple.

 

Carbon does not bend, so you do not have a jammed rudder shaft under the boat preventing you from steering or removing it to seal the leak from the almost certainly damaged lower bearing.
It is also probable that the rudder blade has damaged, maybe pierced the hull when the shaft bent.
A bent shaft is a bear to remove; on the majority of boats it will require slipping and some heavy machinery.
Carbon does not corrode, rely on unseen welds in an oxygen poor environment (cause of crevice corrosion) or have the bonding dissimilar material problems that ss shafts and fibreglass do, resulting in leaky blades and the problems this causes.
A multihull rudder that does not kick up in a collision is extremely poor (I would say negligent) boat design. Not only eliminating the bending/breaking problems but allowing a lighter rudder and drying out.

Carbon is cheap ($20/kg), and easy to fabricate, including changes in section and laminate, allowing more width and wall thickness at the hull, reducing to nothing at the tip and a tiller attachment that does not require machining. Either do it yourself (easy with a vacuum pump or a compressor), or pay a boat builder to do it. If it is not built to a racing spec and does not have a clear finish, it will probably be cheaper than the ss blade and welding. It will certainly last longer

 

Moderator, could you please load the following index of page 46 to 65 on the first page. Thank you for your efforts.

 

Updated now.

 

The following jpeg was found on a sales site. It was a set of “plans” for sale. The interest is who designed it. Miller and Whitworth. Who? The guy changed his name to Ben Lexcen in 1974. He designed Australia 11 with a wing keel that won the 12 meter Americas Cup in 1983. So, what is the about 1970 jpeg showing?

The cat is 60 x 24.5 foot weight is guess 18000 lbs and sail area is unknown. The hulls are asymmetric with a length to beam of 21:1 on the 54.5 foot waterline length. The asymmetry and fine ends make me think the design was heavily influenced by CSK 60 x 23 foot Seasmoke which was launched in 1967. Seasmoke was the fastest ocean racing cat in the world at that stage. The underwing clearance is low at 2.3 foot but the main saloon is large with 6.1 foot headroom. There are 6 single berths and loo in the hulls. The main saloon has a large dinette and galley.

No construction is specified but main mast cross beam appears to be a plywood box beam with timber strips on each corner. Bob Miller (Ben Lexcen) was designing cold moulded monohulls with timber ply internal structures around this time. He was an expert in light weight ply timber structures by the early 70's.

I do not know if the cat was built, but it is an interesting design concept from a guy who often had very original monohull designs and had an obsession with minimal weight in all boats. EG Alan Bond who paid for the AC boats and many of Lexcen’s other designs wanted 4 brass deck vents on a 58 footer called Apollo. When the boat was launched, Lexcen walked over to the vents in front of Bond pulled off the brass vents and threw them overboard.

The first jpeg is the Miller and Whitworth design. The second jpeg is of CSK Seasmoke.

 

Wow what a find OM! IT seems reasonable to suggest that Ben Lexcen/Bob Miller (he was sometimes called BenBob) would have drawn a cat.
I was a racer in my youth (still am deep down). In the winter evenings there used to be a talkfest called Sailfast (this was back in the early 80s) and fast sailors would give a lecture on how they made their boats sail fast. I don't remember much about them but I do remember Ben Lexcen giving a talk. It was before 1983, so the general public didn't about him. Back then I was only interested in racing dinghies and monos, multis were just a side show.
But I do remember Lexcen talking about sailing in Hawaii. I think it was in response to a question but anyway he talked about sailing off Hawaii and the boat was under kite and wanting to gybe or roll and the helmsman is struggling and the crew are wrestling and the boat is straining under kite and everyone thinks they are flying and then from behind comes this apparition. He says that this cat comes sailing past them with one guy at the wheel and lots of women all over him and he just waves, passes the full pressed maxi and sails on. Lexcen seemed impressed. He was a funny guy. His design certainly looks like Seasmoke and there were lots of CSK cats in Hawaii going awfully fast for their time (and they would still go well today)
Okay - my claim to fame. I was Ben Lexcen's paper boy. He would always get his Manly Daily delivered to the door in rainy weather. I used to see him a bit when he got his Eureka 30 built. He would be on his own, usually motoring up and down Middle Harbour when I was out Laser training. We saw each other a lot and got to nodding acquaintance. Then one day I was coming back from after a race and I was flying (in a Laser). I was pretty good then and was gybing flat and fast in the strong southerly, broad reaching down the harbour - Lexcen in front of me. I was going to do a few more reaches, execute a few more perfect gybes and round up and ask for a sail with him. He was looking right at me as I planed towards Clontarf beach for the final gybe, the last of about 6 great gybes in a honking blow. So I gybe, the lower rudder gudgeon breaks (it is plastic!) and I end up in the water the boat upside down and Lexcen looks away. I never did ask to sail with him.

 

Musings on boat size.
I posted above a post on boat size. I wondered why it was that I didn't want a large cat. It seems funny but here in middle class Lake Macquarie my 38ft cat is reasonably big. But if I go to Sydney I see a division. The new boats are larger and the older boats are, usually, smaller. I have talked with a few people with larger boats and they always seem to be richer than me - and I am a teacher on just over $100 000 a year, which is way over the median income. I have a good salary and yet my very simple 38 footer, on which I do all the work, is about the easy limit for my budget.
So when I see a cruising world ad come out spruiking the merits of some 50-60ft boat I wonder, who are they selling it to? I can see that secondhand boats crowd the "smaller" boat market and so I postulate that marketers are targeting the only niche they can, the larger cruising boat.
Still, watching the yachting press is a little like watching Kevin McCloud and Grand Designs. I find the largesse off putting and the money required almost obscene.
I watched 4 of of my friends the other day working on their own boats in the mooring group. We all have boats between 34 and 40ft. We all work on them ourselves and know their systems intimately. We all seem to like doing so and we all use our boats a lot. No one is uber rich although we are all comfortable. None of us are talking about getting bigger boats, none of us need the increase in cost or time. Our boats are big enough to live on and yet small and cheap enough to keep, for years at a time, between cruises and be used for daysails, for sitting on watching lovely days pass by and to slip on local boatyards. There is so much more to boat owning than long distance cruising, even for a long distance cruiser.

 

Ian Aitken was intrigued by the article in Multihulls magazine in 2002 featuring Harrigami, he evolved and built a Harryproa type design on a farm in New Zealand finally launching the boat in 2017. The rig hull is 36 foot long and can be folded to 32 foot (to allow for NZ trailing laws), the accommodation hull is 23 foot long. The overall beam is 15.5 foot but the whole boat can be folded to 8.2 foot for trailing. Hull shape underwater is basically semicircular but with a 150mm flat section in the middle, (to make it more comfortable to walk/stand on the inside of the hull bottoms). There is a small amount of rocker in both hulls, about 100mm in the LW hull and the ends of that hull are flatish to give some dynamic lift to the front and squat to the rear. This is to help counteract the normally nose down attitude of these boats. It still sails with a slightly depressed bow. Weight is about 2700 lbs, with a displacement of 3500 lbs with all cruising/camping gear aboard but not including any crew. The length of mast is 38 foot with a 4 foot bury and 34 foot above deck. The mast took 2 years research and another 2 years of part time to build. Mainsail 215 square foot, jib 105 square foot, carbon fibre wing mast 50 square foot.

The boat is foam glass and Ian learnt how to vacuum infuse by making the rudders and beams first. Then made a rectangular spar which is the guts of the mast. The main rig hull has both ends of 3 foot of ‘glass covered foam and the accommodation ends are 1.5 foot of closed cell polystyrene with one layer of 200gsm glass cloth skin.

Rudders are NACA 64a 010 400 mm chord 1.7 m in the water. They rotate 180 degrees when shunting. Have an adjustable latch that allows them to kick back. This latch can be triggered with a short boathook from the deck and the rudders then pop up to the surface (because they are much lighter than water). They can be wound down using a brace/crankhandle from the deck also.

9.9 hp Yamaha 4 stroke outboard motor. Long shaft electric start, high thrust prop. Gives top speed of 8 knots. The outboard is raised 800mm on a sliding mount so that it clears the LW hull that folds under the brigedeck. An electric trolling outboard can be mounted near the middle of the bridgedeck. This can turn full circle and is used to move the boat in any direction.

This design evolved from a 3.25 foot long model that he made about 10 years ago. It was fitted with RC gear and after trying 4 masts 4 boom designs 3 different rudder sizes and locations 2 different LW hull shapes and 3 different folding geometries I decided to go ahead and do the calcs for a maxi size trailerable one. A very creative man who had the dedication to carry through with a project over 10 years. Rob Denny’s initial Harry proa’s designs provided the inspiration. There are video’s on line showing how the boat unfolds and sails. The jpegs give an idea.

 

Such a huge amount of time and energy. I would love to see a video of her sailing and folding.

 

Catsketcher look at this site it has 3 Jacknife video's. http://harryproa.com/?p=1987