Multihull Structure Thoughts

SolGato
Hello there,
I just wanted to thank you for your contributions on the HH tramp variants and the Ostac boats.

Based on your and Oldmulti’s contributions my partner and I bought the Ostac boat that was on Gumtree, it is still in Melbourne and I am in Perth so a trans Australian road trip is coming up towards the end of the month.
Thanks for your input it made a difference.
Cheers,
RR

 

The Class 6 is a performance cruising catamaran that has a few interesting design concepts. The Class 6 is 64 x 29.5 foot that comes in 2 versions. The first is a performance version that weighs 33,600 lbs and displaces 43,700 lbs that carries a 77.5 foot mast with a 1170 square foot main, a solent of 602 square foot, a 1130 square foot genoa and a 3000 square foot gennaker. The next version is the racer model (the performance model needed an upgrade) with a weight of 30,250 lbs and a displacement of 40,300 lbs. The mast is 84 foot carbon fibre rotating with a 1,540 square foot mainsail, a solent of 645 square foot, a 1,323 square foot genoa and a 2,850 square foot code 0. The company will install one light Genoa on Kevlar shroud. You will use it up to 15kts wind. The remaining sails are on fixed shrouds except gennakers or spinakers. The length to beam on the hulls is 13.7 to 1. The draft over the hull is 2.6 foot and over the curved asymmetric daggerboards 8 foot. Translation, in either version they are fast cruiser racers.

The engines are 2 x 80 HP Yanmars and there are 2 x 9.5kw bow thrusters. There is 3000 watts of solar panels that feed into 4 x 200Ah 24V Lithium batteries.

The main design feature of the Class 6 is the “spine” which runs the length of bridge deck and then extends to act as the prodder supporting the forestay. This is a significant structural item that carries the main rigging forces and allows a base for a rigid set of forestays to handle the fraction sloop/cutter rig depending on the situation. If the spine concept is done well, it can also house anchoring arrangements and engine mountings etc. The design team claim the spine can provide wave piercing in heavy seas for comfort and security, extra buoyancy to lift “nose up” in heavy sea and easy sailing with reduced main sail and large front sails on furlers.

As the Class 6 is a semi production boat, the accommodation layout can be tailored to suit from a 3 double berth cabin arrangement to 6 double cabins with attached toilets. The saloon area is large with galley, navigation and dining. There are large doors between the main cabin and cockpit for an open plan living arrangement. The cockpit can be setup for short handed sailing or racing with electric winches to control this sail plan.

The performance version is built from a Gelcoat, vinylester resin infused PVC foam glass, Kevlar and carbon structure. The spine has up to 24 layers of carbon fibre in it to carry the loads. The racing version is built from a painted (much lighter than gelcoat) epoxy resin infused PVC Foam sandwich, mainly carbon fiber and Kevlar structure. There are less loads on the forward hull structure with the spine concept. The forestay loads go onto the spine not a crossbeam then down the forward hulls. Result is you can lighten the forward structure.

The performance claims cannot be verified as I have not read supporting tests but the statement that the cat “can easily match wind speed with Main and XL Genoa (104m2) in light air (3 to 10kts). If you to fly a Code 0 or Gennaker you could go faster than the wind!” It is a very believable claim. This is a 12 to 15 knot average sort of boat in favorable conditions and would have peaks of 25 knots plus.

An interesting design for those with large wallets and a need for speed. The jpegs give the idea.

 

The following trimaran is a good choice for those who want to get very high performance on a cruising boat. The Pulsar 36 is designed by EriK Lerouge whose multihull designs are legendary. This design appears to be a expanded fast cruising version of Dick Newick’s 23 foot Tremolino.

The pulsar 36 is 36 x 29.2 foot with a weight of 6,200 lbs with a displacement of 9,700 lbs. The 53 foot semi wing rotating mast of 80 square foot carries a 640 square foot mainsail, a 330 square foot jib and a 1,300 square foot spinnaker. The draft is from 2.1 foot to 8,2 foot over the main hull based daggerboard. The outboard is 10 HP.

The tri can be disassembled with the main hull being 9.7 foot wide. This design benefits from the latest racing developments with long buoyant floats safety and power in strong winds. The rudders are on the floats for optimum control and easy drying out. The deep daggerboard insures good windward ability and shoal draft.

The flared main hull offers nice cruising accommodation with two large double cabins, a convenient dinette, standing headroom at the galley, chart table and toilet.

The hulls and decks can be made of strip plank cedar with glass and epoxy or foam glass with vinylester. A new full carbon version is on the design board. The ideal to save weight aloft will be a rotating carbon mast. A homebuilder can build a cheaper wood/carbon/epoxy wing mast in 300 hours. Beams are laminated on the same plug and may be either bolted or glassed on the hulls and decks.

I have no questions about the claim of a high performance cruiser. A 53 foot mast, 970 square foot upwind on an all up displacement of 9,700 lbs equals speed. 10 knot averages possible peaks of 20 knots possible.

The limited jpegs give the idea.

 

This custom 23 foot racing trimaran is not quite what it seems. Yes, it is a custom but is from many modified production boat parts. A classic “I have these bits and will put it together to form close to what I want.” Mark Zollitsch and Dave Peterson did most of the design and assembly/build of this tri that has won the NW Multi-hull Championships on Vancouver Island and numerous Puget Sound area races. It is faster boat for boat, than most F-boats including F-31 in 10+ knots, and Multi-23, similar speed to Diam-24.

The ”Osprey” tri is 23 x 16.5 foot with an all up sailing (including motor and rig) weight of 1000 lbs. The 31.4 foot carbon mast (from a Nacra 6 with an extra set of diamonds) carries a 228 square foot mainsail and 100 square foot jib. There is also a code 0. There is a daggerboard in the main hull.

This tri is a pure racer with limited storage. Let’s get to the build.

The main hull of Osprey started life as a Mike Leneman (So.Cal multihull dealer and builder) L-7 trimaran ama. After doing some float tests and loading a thousand pounds on it, it was decided to split the hull down the middle from 10 feet back, pry it apart and add almost 9 inches of width for added buoyancy in the stern. Then the free board was raised about 6 inches, splayed it open (no stern in yet) built internal structure, bulkheads, floor, ama sockets, and finally added decking. The floats started life as Nacra 6.0 hulls and were extended 3 feet. The crossbeams came from a broken Mumm 30 carbon mast, which had sockets made for hull connects and waterstays added for extra stiffness. The carbon Daggerboard is from a Hobie Tiger and the rudder is an N-20 daggerboard. Some aluminium components were manufactured such as the rotating raising lowering mast step, kick-up/raise-lower rudder cassette and steering system.

Result is a fast tri as indicated above. EG a high of 12-14 knots near the end of the race that was mostly in the 6-12 knot wind range.

An interesting high performance tri built in about 2010 and was sold about a month ago. The jegs give an idea. There was no formal plan just a lot of napkins drawn on during meals.

 

Pretty sure those cross arms came from me. I used to buy truckloads of reject carbon masts from Omohundro and I sold Mark enough Mumm 30 sections for a trimaran. That was a very long time ago though, so I could be wrong.

 

You go along to the Bishop Museum in Hawaii and find information on a 15 foot Fijian outrigger sailing canoe or “Thamakau”. You want to know more and decide to build one and learn how to sail it. What could go wrong. First a few modifications as dug out canoes from trees are not that common now a days, so the adventure begins. First build a 250 mm model, then just build a bigger version.

The “Thamakau” main hull ended up being 14.5 foot long and 1 foot wide at the waterline (14.5 to 1 length to beam). The beam is about 6 foot. Displacement unknown but about 270 lbs. 10 foot high rig when full crab claw sail up. Crab claw sail about 30 square foot. Draft unknown. Steering appears to be a moveable oar.

This is a pure day boat for sheltered waters for 2 reasons. There was a lot of learning going on and the guy spent about $80 US on the proa and EG For a sail, he used a canvas tarp from Home Depot, and hand sewed the edges.

To the build. The main hull was a skin on frame construction. The frame was made from recycled baidarka gunwales, ash ribs, and pine stem boards. The fore and aft decks are marine plywood, and the turreted deck in the middle is of cedar boards, leftover from a fence project. The mast is a tree found on a beach on Whidbey Island. The skin on the main hull is polyester, sealed with a two-part urethane coating. The yards were made from scrap timber laying around. All the components are lashed together with rope as can be seen in one of the jpegs. The float and crossarms are timber.

So, you start with the basic proa then you start to learn how to sail it. The first day you find it will move under sail. Next you try some control manoeuvres and promptly capsized. After a few more capsizes he got a feel for the purpose of the outrigger and his body weight to counter act a capsize. End of day one.

To quote the owner “A few days later, I took the canoe out again. An entirely different experience this time: Light winds gently increasing during my two hours out. Where I hadn't shunted a once during the launching, this day I shunted 8 times or so. Everything worked perfectly-- no tipping over at all, and I only dropped the boom in the water once while shunting. One improvement (of several) I made after the initial launch was to rig a rudder attachment. I used a single-blade paddle-- a replica of a Koniagmiut qayaq paddle. Upon sailing the second day, I discovered I had absolutely no need for it at all as a steering method-- the canoe is extremely responsive to weight shifts, and I found I could sail a full circle just by repositioning my body: Turn right? Just lean forward!”

After more sails the owner found the proa went down wind and broad reaching well but upwind the proa was slow. The main hull is symmetrical and has no foils to resist leeway. Also this is a small boat with little sail which would limit performance.

A fun cheap proa which taught the owner a lot. The jpegs give the idea.

 

The Triune is a unique 9m trimaran from Musters Marine in Poole, which was way ahead of its time. Muster Marine started to build Piver trimarans but Robin Chaworth Musters did the design work on the Triune 30 improving the basic Piver concept with EG a fiberglass round bilge main hull, asymmetric floats etc. This boat was built in 1966 and renovated in 2020’s.

Triune is 30 x 20 foot with a weight of 6,300 lbs. The 40 foot speaderless fixed aluminum mast (119 x 225 mm) carries a 195 square foot mainsail, a 107 square foot jib, a 182 square foot genoa and a 50 square foot storm jib. The rigging is 1 x 19 stainless steel of 8 and 10 mm. The draft ranges from 2 foot to 5.75 foot over the pivoting centerboard. The engine is a Yamaha 8 HP outboard that can drive the tri to 8 knots. Maximum speed under sail is about 12 knots. The asymmetric float improves upwind performance along with the centerboard compared to 30 foot Piver Nimble tris of the time.

The accommodation is forward a “double bunk”, a toilet shower then the main saloon. The main saloon has 2 wing berths, two seats with slide in tables and aft the galley area. The cockpit is in the stern. There is a lot of storage space in the floats.

The main hull construction was solid polyester glass with plywood decks, floats and plywood box cross arms. All the external plywood structures were covered with glass and epoxy. The 2020 rebuild included replacing a float bow.

This was a tri ahead of its time in 1966 but was superseded in the late 1970’s by other designers. Good work for its time. The jpegs give an idea.

 

Pivers make me nostalgic.
One of the first multis I sailed on was a 40' Piver. (Victress?) It was a nice day, I was impressed by the comfortable pilothouse, the level sailing, the huge space below.
I did notice that the motor had to be started every time we tacked.
Later that same year I managed to get invited for a daysail on a 46' Wharram Oro. That tacked without the engine, but the girls all kept their tops on....

 

We featured the standard Havkat (Havcat) 27 before but this Havcat 27 has been modified to become more of a cruiser. The standard Havcat 27 has an internal galley, toilet and bunks but an open air seating/table in the wingdeck cockpit area. This owner modified the cat to have a cabin area over the seating table. The changes require more explanation later. Lars Oudrup of Denmark was an excellent designer of cats. He designed many very good smaller fast practical cats that could be home built or done in production. The Havkat 27 is a light weight cruising / racing catamaran of which 25 were built in series production in Denmark starting in the late 70’s.

The standard Havkat 27 is 27.23 x 14.86 foot with a weight of 4,300 lbs and a displacement of 5,700 lbs. The 35 foot fixed aluminium mast carries a mainsail of 195 square foot, a genoa of 248 square foot, a jib of 162 square foot and a staysail of 108 square foot. The length to beam on the hulls is about 8 to 1. The draft varies from 1.3 foot to 4.9 foot over the lifting rudders and daggerboards. The power is a 10 HP outboard.

Now we go to the modified version shown here. Warning, speak to a designer before you do changes like this. The owner wanted more interior space so he moved the main mast bulkhead 600 mm forward then moved the external seat table area forward and then did a cabin roof over that area to form a bridge deck cabin area. Reason for the designer involvement is you are moving the weight of the mast and main bulkhead about 10% forward, you are adding the weight of the cabin roof, unless you change the sails you are moving the centre of effort forward which may mean you have to alter the daggerboard position and the weight shift forward may cause a nose down sailing attitude unless you compensate with weight aft. This is not a simple change.

Result the sails are on the now mast head sloop are a mainsail of 215 square foot with 7 battens, a jib of 215 square foot, a spinnaker of 430 square foot and a 615 square foot gennaker. Also, the payload capacity has decreased so I suspect the cat is “overloaded” most of the time.

The accommodation has improved with a galley in one hull, an aft single berth, a double berth forward across the wing deck and a navigation area, toilet and shower in the other hull. The seating table area on the bridge deck is good. The hulls have 6 foot headroom the bridgedeck has 4.5 to 5 foot headroom.

The construction is solid GRP layup below the waterline, foam sandwich GRP above the waterline, underwing and decks. The interior jpegs show a ply timber main crossbeam structure with plywood interior cabinets etc.

The original Havecat 27 could sail very well on all points of sail and this version I suspect would match its performance with increased sail are. The majority of the Havkat 27’s are used as fun fast cruisers. Think a 7 knot average, 15 knot peak type of boat. If raced hard they live up to their handicaps.

The first jpeg is the stand Havcat 27 layout, the following jpegs (bar the stern jpeg) are of the modified Havcat 27.

 

I have a leckerli for you. Cleaned up some old hard drives and found this:

Apparently there were some variations on the theme. Besides the 47ft version in PDF-2, the attached PDF-1 shows a 37-foot version, and @TrimaranMan attached a 40ft version in #4352. Note the differences - while the pictures show round bilges, the drawings show flat bottoms with rounded chines ... Has anyone seen one of these models sailing?

 

In 2003, when Corsair Marine had already built over 1250 of her Ian Farrier designed F-27, F24 and F31 trimarans, they wanted to extend their fleet with a trailerable 36' trimaran that combing seagoing cruising capabilities and the performance of a racer. The use of the most modern materials and building methods helped to restrict the empty weight to well below 3000kg. Designed from Ian Farriers work by Corsair Marine and built by Corsair Marine the 36 (later 37) started production in early 2003 Under the then Paul Koch owned company. After Corsair was sold Paul now has a major interest in the Rapido brand of trimarans.

The 36 is 36 x 25.6 foot (folds to 9.9 foot) weight 5600 lbs and displaces 8,500 lbs OMR laden (everything except crew). Originally it had a 46 foot (later extended to 47.5 foot) rotating aluminium mast that carried a 354 square foot mainsail and had a 317 square foot fore triangle. The headsail options extended to a 1,083 square foot screecher and optional spinnakers. The length to beam on the main hull at waterline is 10 to 1. The draft ranged from 1.9 to 6 foot over daggerboard. The engine is a 15 to 20 hp 4 stroke outboard that gave around 6 knots.

The accommodation is a double berth forward, a toilet shower area beside the daggerboard case, a single bunk/seat opposite in the main saloon a galley and opposite a dinette. The aft cockpit has a double berth under. These boats had EG compressor fridges, a reasonable stove sink area etc and could seriously be cruised for a couple for an extended period.

Now we come to performance, yes, these tris are fast for a cruiser. One test reviewer said: “Under main alone we broad-reached across a windy Vineyard Sound doing a relaxed 12 knots. Later, on a beam reach in about 12 knots on Narragansett Bay, with my 12-year-old daughter steering (one hand on the tiller extension, one hand holding a book), our speed jumped from 11 knots to 15.3 knots as I trimmed in the main and the sprit-mounted genoa (known as a screacher). And when I crewed in a local race with Multihull Source dealer Bob Gleason, we saw 17 knots on a tight reach.” Another test report said: “With Steve Marsh of the Finish Line, at the helm, we sailed close-hauled in 8-12 knots of wind with boat speed consistently registering 8-10 knots. She seems to pivot on her daggerboard, and tacked through 95-100°. When the wind angle moved deeper than 35°, we hoisted a “screacher,” essentially a high-clewed, 180% drifter, and speed increased to 12-15 knots. She heeled 5-10°, carving through a modest chop on the surface.” Translation the 36 can sail at windspeed and reports have stated peaks of over 20 knots. Expect 10 knot averages under good conditions.

Construction is fun. There are 30 different moldings, including 10 in the deck and hull, and 11 in the areas belowdecks, as well as two amas, four akas (crossbeams), and all the precision-engineered parts that allow the whole thing to be folded up. The 36 uses vinylester resins, carbon fiber, double-bias fabrics with Kevlar. The build used vacuum-bagging and curing process that relies on precise blends of fiberglass, resin, and catalysts. The lamination schedule consists of a layer of NPG gelcoat, a skin layer impregnated with vinylester resin to prevent blistering, and multiple layers of uni- and bi-directional fiberglass. Kevlar is laid in high-stress areas on the bottom, daggerboard trunk, and at bulkheads. Akas are constructed of layers of fiberglass, carbon fiber, and foam, which increases stiffness. This is not a cheap or easy build.

A good fast cruiser if you can find one second hand. The jpegs give the idea.

 

A short story of 81 year old Mic Dales, who completed his Drifter 16 trimaran from free plans from Duckworks. Mic had some previous home boat building experience and wanted a fun machine for his son and grandkids etc. in Florida. The real story is the build from start to finish took 108 days.

The Drifter is 15.5 x 11.25 foot day tri that weighs 130 lbs. They are light open boats. The rigs on all models are freestanding windsurfer masts with simple sails that if straight cut with minimal battens, roll up around the mast. Most people use windsurfer sails. The sail area varies between 60 to 80 square foot.

The Drifter 16 is built with 3 mm plywood and timber. The timber framing and stringers is not overly complex but is required to built as per plan as the hull skins need the support structure. The hulls are covered by 130 gsm e glass and epoxy. The cross beams flex but are strong enough to handle the bay and river sailing that the boat is designed for. A simple leeboard is used for leeway resistance. The full PDF plans are below.

The materials list for the boat is:

12 sheets 1/8" mahogany plywood
8 1/2" x 1/2" x 13' stringers (spruce or fir)
4 5/8" x 1 1/2" x 16' stringers
2 5/8" x 1" x 16' stringers
3/4" spruce or fir for rudder and leeboard (can be glued together out of strips)
3 2" x 5" x 11' 6" medium density spruce or fir for beams
3 1/2 gallons of epoxy
20 yds 50" 4 oz. fiberglass
A 16' or 17' windsurfing mast


These are simple fun boats that sail well. The jpegs give the idea. The first jpegs is the original, the blue tri is Mic’s build.

 

Anything will float. This is very short. I found this jpeg in a thread about the worst design ever. Not quite the worst but it would come close.

The “cat” is about 4 x 4 foot and would support about 260 lbs. The foam blocks are about 4 x 1.25 x 1 foot. The garden deck chair was purchased on price not structural integrity. The 12 volt battery under the seat is wired directly to the electric trolling motor. The timber cross beams look like a collection scrap timber lashed onto the foam blocks.

You would need a light weight skipper and work in no wave water in a lake/river. I suspect a 2 or 3 knots will be the top speed. This is the type of vessel where your ability to swim to shore is the most important safety feature. The lone jpeg gives the idea.

 

Skip Johnstone occasionally visits this thread. He is a prolific home boat builder designer who has built about 35 water craft of all types. There is an article in Small Craft Advisor describing them, the one that raised my interest the most is “Nomad” a 25.5 foot proa that has a Schooner rigged with two wingsails and end mounted rudders on the wings.

Skip built Nomad as an upgrade from his good P52 proa. The P52 was built cheaply out of $10 utility plywood. The P52 proa could cruise at 10 knots and peak at near 20 knots with minimal sail area. Nomad was 25.5 foot proa built from Okume and Meranti plywood. A higher quality of build.

The radical part was the installation of a Schooner rig with wing sails. The wing sails were “solid” (fabic and thin ply over frames on an internal mast) The wings then fitted over a stub masts attached permanently to the main hull. Good so far.

I will quote Skip here: “The stub masts on Nomad were way up in the air. After some effort John and I got to where we could mount and dismount the wings but it still seemed a little iffy. An initial splash at Lake Somerville was unremarkable, there was no wind but the boat moved nicely with its AD scull and the rudders were serviceable but difficult to raise and lower. Subsequently Susie and I took the boat to the Port Aransas Boat Festival but John had a previous commitment. I bowed to some pressure and tried to mount the wings with some local help but the wind caught the first wing and sent it flying along with the gin pole and associated stuff. No one was hurt but I was seriously upset, someone could have been hurt.”

After this event Skip moved to Oklahoma and had other full-time commitments limiting any sailing and he sold Nomad. Before the new owner could pick up the boat an EF2 tornado destroyed the boat along with other boats made by Skip, his shop and did extensive damage to his new home.

It’s a pity that Skip did not have enough time and support to solve the issues with the wing sail rig as we could have all learned something. The concept of the flap has been tried on some larger tris like the Walker wingsail but did not appear to be a major commercial success.

Skip, if you read this please add to or correct any details. The lone jpeg gives the idea.

 

Thanks Oldmulti, the only corrections I've got is last name is Johnson ;-) and the wings were hot wired foam with light fiberglass skins and uni carbon spar strips. The tail surfaces are controlled by a circular cam setup and act like an automatic transmission (some of my real sailor friends were horrified at the idea of not having to constantly tweak things).

Re wingsails I believe there's a great deal to be learned and if I'm still able I suspect there will be another small Bionic Broomstick solo boat once I'm finished (ha) with QB.
Peter Worsley's circular cam control is an essential part of the deal and works surprisingly well. Mass balancing around the pivot axis is also necessary; the land speed guys don't need it but their machines aren't rocked around by waves.