Multihull Structure Thoughts

[Skip Johnson]

More re wingsails. The original trials with a wingsail on the Bionic Broomstick, which originally had a cambered panel staysail, were oriented towards having adequate performance in very light conditions. The reasoning being if you could get a wingsail to work in the lightest of breezes adequate performance in stronger wind was pretty much a given. A supposition based on on the arcane workings of aerodynamic physics centered around Reynold numbers.

Here's a short you tube clip of the Bionic Broomstick on a very small lake or large pond surrounded by trees in those light conditions.

 

[oldmulti]

Skip Johnson’s (sorry about the spelling) next Proa after Nomad was a “simple proa” that could sail as a Pacific or Atlantic proa. “Trival Obsession” which is about 15.5 x 7.5 foot overall excluding rudders. The mast is 17 foot 52 mm tube mast carries a 51 square foot battened stay supported mainsail.

Now things get a little interesting. The blue first jpeg is a plan of Trivial Obsession which shows a hard chine hull doubled ended hull with a “foil board” float design. This is an open cockpit proa. The steering system took 3 attempts to get right (and Skip had done several proa’s before). Each proa is almost like a new development. The rig was another development. The wingsails were rejected due to handling issues and a simpler rig was tried. The mast was a simple aluminum tube with a stay on which the mainsail was attached that stay was on a small floating yard which allow the mainsail to be slightly forward of center on each tack. When the main is sheeted in the stay becomes rigid providing a good sail foil shape and the main acts like a “genoa”. Good solution.

The build again was an improved cheap and dirty. Plywood, timber and aluminum crossbeams and mast. The power is an oar. The steering system is an improvement on previous proas after development.

To quote Skip: “Sailing out of a marina in fairly close quarters and I have several gps tracks showing short shunts thru 90 degrees getting out thru the large tire breakwater. Only worked with wind from SE or SW a large hill made other directions somewhat variable. I don't think the current set up is quite as weatherly yet but I'm confident in the long run I'll be able to shunt thru 90 degrees or better in most conditions. Not bad for a craft that'll do it in less than 2' of water. To add to the confusion with a couple of reefs in TO can also tack as well as shunt or operate in either Pacific (preferred) or Atlantic mode.”

Trivial Obsession worked OK but Skip had other plans and salvaged all hardware and burnt the hulls. The jpegs give the idea.

 

[oldmulti]

“Nomad” was created for 2011 WoodenBoat’s Design Challenge but Skip Johnson did not enter. Chuck (of Duckworks) said it seemed a shame to let all the design work go to waste and it was published in Duckworks ezine. Here is the preliminary design of Nomad. Take with a grain of salt, as the real build done years later had a different rig and there was a multitude of bits that were changed.

Nomad is about 23 x 9.5 foot with an unknown displacement. The designed mast is 22 foot high with a full sail area is 138 square foot and a 38 square foot staysail. Nomad has a rig that has low inertia, powerful, low mainsheet loads, easy to shunt, easily reefed in about twenty discrete steps with no change in lateral CE and balanced running downwind. The length to beam on the main hull is 12 to 1. The draft over the rudders is under 2 foot.

When Skip’s trailerable proa P52 hit the water in 2007 several things were immediately obvious:

The long lean dory style hull with a fairly high prismatic coefficient and flared sides made a superb open water boat.

Telescoping beams work better than swing arms. Solid slats work better than a trampoline. General ergonomics are good. The boat needed a foot rest for sitting in the lower pod position.

It really is difficult to make a crab claw sail go downwind.

As much as I like steering oars, simple and versatile, they are not the best solution on a boat this size. This one came hard for me, I really liked the oars.

The extra weight (10 to 20 lbs) of the ¾" T&G ply bottom is worth every ounce. This insight was reinforced one night as I listened to the bottom grind on the oyster shells underneath as the tide went out in a howling norther; too exhausted to stir and move the boat.

The pod/cabin structure would be just as happy lowered a bit to reduce windage and weight. The berth flats would be better served with some small cross beams rather than a longitudinal girder support.

The loads on a staysail type system are stout, significant compression in the mast, tension in the stays to match even with a relatively wide staying base. Hence, there is no substitute for having the lowest stretch staying available. The float could use a little more buoyancy.

The construction of 3 full lengths of 2 foot wide strips of 6 mm plywood stitched and glued to a carefully shaped 19 mm T&G bottom, makes the hull. End bulkheads are moved out a bit and the cabin structure is lower and shorter than Nomads predecessor. The berth spaces have the head out in the open cockpit. Center cabin area is longer now and it was prudent to add a laminated frame to fix the relation between hull and cabin on the leeward side. A flat now extends across the hull at center of boat to brace hull near the waterline and provides a space to slide the requisite port-a-potti. Berth flats are an integral part of the hull framing are supported by multiple small cross beams. Telescoping arms slide through plastic bushings at outside of pod and at cabin side, hold open pins are accessible from outside the boat. Slats between pod and float are held in place with bungie cord.

The accommodation is a camp cruiser style with 2 single berths, an area for a porta potti and storage for food and camp stove etc. A minimalist cruiser for a week or so with the usual limitations of water and EG ice.

Float - A stretched version of Tom Speers P30012 bidirectional foil (P30007.75?), the float has the required buoyancy in a narrower section than the previous double ogive section.

Rudders - Real P30012 sections, worked very well on predecessor after steering oars were abandoned, these are vertical rather than the previous slanted blades, loads were light enough for two finger steering. The only time rudders started to popup in a non bottom contact mode was at over 10-12 mph while dragging large loads of seagrass.

Launching - Typically to rig and launch Nomad's predecessor took 30-45 minutes, working fairly steadily but not rushed.

This 2011 design formed the basis of “Nomad” with wingsails built later then a foam glass version which we will discuss tomorrow. PS Skip is still having fun as an 80 year old still thinking of his next boat. I feel so inadequate. The jpegs are of the 2011 design in Duckworks ezine.

 

[Skip Johnson]

IMO anyone who has posted almost three thousand items re multihulls and received over two thousand likes by a small multitude of like minded people has little reason to feel inadequate. I for one greatly appreciate your effort , experience and expertise . I 've enjoyed and learned a lot since discovering this site.

 

[peterbike]

IMO anyone who has posted almost three thousand items re multihulls and received over two thousand likes by a small multitude of like minded people has little reason to feel inadequate. I for one greatly appreciate your effort , experience and expertise . I 've enjoyed and learned a lot since discovering this site.

Ditto. thank you.

 

[oldmulti]

The final (for now) Skip Johnson proa. The “Questing Beast” more commonly known as the QB. This was the proa that Skip used in the Texas 200 in 2024. Basically a down sized “Nomad” but buiolt in foam glass with a schooner rig of cambered panel staysails. We have featured this design previously so I will do a quick update with a few jpegs. Also the lats jpeg item is a PDF of the initial drwawings of the 16 foot “Bionic Broomstick” Skip put in the video above.

QB is 21 foot Pacific (can be sailed as an Atlantic) proa. The designed weight of the proa was to be 320 lbs. The rig is a chambered panel staysail schooner of unknown sail area. The accommodation again was 2 single berths with the heads of the berth being in the cockpit. The rudders are kickup which means minimal draft.

QB is constructed from a foam glass sandwich. Foam is 12 mm 5 lbs/cubic foot Gpet foam. Fiberglass is primarily 9 oz tooling cloth with 12 oz biaxial taping and some 22 oz unidirectional tape spot reinforcement. The 9 oz tooling cloth was a mistake according to Skip; it should should have been 12 oz biaxial throughout. The fiberglass was mainly vacuum bagged and all fiberglass work was topped off with a layer of peel ply. The main hull bottom foam panel was 12mm Corecell M80. Sliding cross beams are 3” x 1/8” wall 6061 aluminum tube sliding in HDPE bushings. Masts are sectional step tapered aluminum flagpoles 3” diameter base, 2” diameter cap. Sails are 4 oz Dacron with a leading edge luff of seat belt webbing. Battens are 3/8” c.f. tubing.

Skip completed QB in time for the Texas 200 which is a 153 mile race. He only completed 68 miles. Several “operator errors” limited QB’s progress and Skip withdrew. Skips summary was “General impression is that QB is definitely fulfilling its design requirements as a comfortable quick camp cruiser. It’s certainly comfortable, a silicone gel seat pad is a significant element in overall long term comfort for an 80+ year old butt.”

One issue was with QB’s rudder system which had push rods. When weed got caught on the kickup rudder the load on the push rod weakened then broke one push rod. Result was, Skip will probably switch to a cable system for rudder control rather than pushrod.

QB is an interesting evolution over at least 5 proa’s and a total of 35 boats to a very good trailable camp cruise solution. These proa’s are worthy of close study.

The jpegs are of QB and the PDF is a line drawing of “Bionic Broomstick” built from blocks of foam and covered with fiberglass.

 

[peterbike]

That is a very interesting rig on QB.
Questions for Skip.
Is it your brainchild ?
Do you think it would upscale ? to 35/40 ft ?
thanks.

 

[oldmulti]

A short one about traditional proa’s built today using available materials from the islands. This is about John Scull (a Canadian) who purchased a flying proa whilst at a beach bar for he and his wife to have fun sailing. The result is the fly proa of Kapingamarangi.

The sailing canoe itself was about 22 feet long. Its sharply-pointed, fine hull narrowed at both the top and bottom. It was carved from a single breadfruit log and planks for gunwales were then sewn on with sennet (coconut fiber) cord. The seams were sealed with breadfruit pitch. The canoe hull had a sharply vertical entry at both ends. I was told that when they first brought their canoes from sandy Kapingamarangi to volcanic Pohnpei, they had experimentally rounded these points so the canoes could be easily dragged ashore on this rocky island. It was found, though, that the modified canoes performed very poorly to windward and were only suitable for paddling or motoring.

The canoe needed work. No nails, screws, glue, or fasteners were used on the canoe and outrigger was held together with sennet lashings. Somewhat like Manila rope, sennet has a coarse surface so that lashings and slip knots hold fast through friction. When tying down shrouds or the sheet, a turn of the line around an outrigger strut serves as well as an expensive cam cleat.

Preparation of the sail and the canoe continued fitfully for over five months. When everything was ready, John and crew went on many day sailing trips both inside and outside Pohnpei's barrier reef. The Northeast trades usually blew at Force 5, raising whitecaps on the lagoon. Occasionally there were brief rain squalls accompanied by storm force winds.

In Kapingamarangi tradition both women and men sail. The women’s job is to stand on the outrigger, shifting their weight to keep the float just skimming the water. The men’s job was to hunker on the narrow main hull handling the crab claw sails and shifting weight forward or back to maintain trim for upwind or downwind sailing. Speed and safety depend mostly on the skill and alertness of the crew. For maximum speed the outrigger float is kept skimming just above the waves.

When sailing downwind the canoe is steered by dragging a paddle in the water. When sailing on a reach or when beating, however, steering is accomplished by shifting the crew around or controlling the sail, rather like a sailboard. Move the weight forward and the canoe turns down wind; move the crew aft and it heads up. The center of effort of an Oceanic lateen sail is well forward; sheet in the sail and the boat falls off the wind; ease the sail and the boat points up. Point up a bit too far and the whole rig comes crashing down.

It seems to be essential to the proper performance of the canoe for the outrigger to be completely rigid except for extreme flexibility with respect to twist in the vertical dimension. This flexibility, along with an alert crew, reduces the chance that the outrigger float will submarine when it strikes a wave. Should this happen it brings the canoe sharply up into the wind with a surprising and usually unpleasant result.

When a violent tropical squall is seen approaching, the rig is quickly taken down and laid on the outrigger frame. The canoe will now naturally lie to with the outrigger to windward. As long as the crew keeps its center of gravity low the canoe can ride out any weather in this way. Voyaging canoes caught in hurricanes make very little leeway and are in now danger as long as the canoe does not break up.

John clocked his proa canoe at 11 knots over a one- mile stretch in the lagoon and he knows he often went much faster. Voyaging canoes in the Caroline Islands have reportedly made long passages at average speeds of as much as 11 knots and larger Marshallese canoes have been clocked at 16 knots.

John and his wife had fun whilst in the islands. The jpeg give idea of why even old proas could sail well using basic materials. They do require good crew to get peak performance but proa’s were developed over 100’s of years. You don’t need exotic materials to go fast.

 

[Skip Johnson]

That is a very interesting rig on QB.
Questions for Skip.
Is it your brainchild ?
Do you think it would upscale ? to 35/40 ft ?
thanks.

The cambered panel staysails on QB are the result of a long journey to find a sail rig uniquely suited to a proa. The balanced club sail on P52 was really close, capable of driving a 6.99 meter (22'-9") craft easily at 10-12 mph with 48 square feet of polytarp. But there were two downsides, it wasn't that easy to reef and it required a lot of tension to work well particularly upwind.

I had fiddled with a number of approaches but hadn't come up with anything that showed much promise until I came across a short article by Robert Biegler in an AYRS journal where he proposed that a sail with numerous rigid battens would not require the same tension to maintain shape as required in a jib. The Bionic Broomstick was originally built to trial the idea with a three panel sail of polytarp and wood lath battens and showed great promise before I wandered off onto wingsail land.

After my experience with Nomad I went back to the cambered panel staysail idea with Trivial Obsession. The cambered panel sail had five panels with parallel luff and leach for the bottom three panels. Sailcloth was a cheap 4 oz dacron cut with shape on the sides attached to the battens which were 1/2" pultruded fiberglass rods, a bit heavy but one dollar each on ebay. The boom was a piece of aluminum tube arranged to roll up for reefing with a 3d printed ratchet assembly at the tack. The boom was held in place with a bridle from the tack and clew to an adjustable point about 25% back from the tack. There are a jack lines, 3mm dyneema, from tack and clew to the head of the mast to keep boom in place with the sail rolled up. the bridle attachment goes to a multipart tackle downhaul. The process is to to have the rig tightened down with the sail rolled up and to slack the downhaul with the sail raised to where the halyard slacks the jacklines a bit and then tension the downhaul. From Trivial Obsession I learned; flat cut panels in dacron don't work as well as they do in polytarp. The bridle leg from the tack has to be rigid for the roller reefing to work.

All this has carried on into QB and I plan on a lot of testing this spring when it warms up, there's a lot to learn about proper geometry of the bridle and I've an ongoing issue with raising (unrolling) the sails(s). It's possible to raise the sail with out it running wild and out of control but it almost needs three hands and I've only got two. I'm still working on incorporating a runaway escapement in the ratchet mechanism but that's getting a bit off the beaten path.

Yes I think it would upscale but at this point it would require some experimentation.

 

[peterbike]

Yes I think it would upscale but at this point it would require some experimentation.

Thanks for the explanation. I would be happy to assist, but from australia I would need faster than light travel -
which we haven't covered on the forum yet ?
maybe coming up soon ??

 

[oldmulti]

Just a short comment on the Sail GP in Australia. The new T foil cats are very responsive to driver, wing and foiler controller input. Result is the GP 50 cats can sail at 2 to 3 times wind speed (wind speeds in Australia were 11 to about 16 knots). Peak speeds were about 47 to 48 knots. The interesting part was the winning cats were on foils from 95 to 100% of the 10 to 15 minute races, that included tacks etc.

The real revelation was the importance of strategist and those on board the cats reading the what was at times, variable wind conditions, around the course. At times a 7 th place boat could have a good leg and go to third place. There was a 300 meter race to the finish line where a cat did not trim its boat well and the cat was defeated by a well trimmed following boat that was sailing 10 knots faster.

So in importance, it came down to good position and speed at the starting line, sailing fast in all directions and having a good strategist who can read the wind variations. All crew members count in these races. Boat reliability is important but all cats are very similar with a lot of common spare parts like rigs etc. The problems occurred on top boats like NZ and middle of the pack boats like Brazil.

Finally the performance variations of the boats surprised me. EG Australia was fast on day 1 races but did not go well in day 2 races. Nz was slow in some races but very fast in others etc. Some performance differences were due to tactics but other times the cats just choose the wrong side of the course and finally the foil trimmers did not adjust the foils for the optimum speed. In variable winds, a gust can create a burst of speed and the cat rises on the foils unbalancing the cat which can lead to a nose dive killing speed etc. These cats are all about team work. This is the reason this style of foilers are not for cruising boats yet.

A few jpegs of GP 50 foilers.

 

[oldmulti]

Goldcoast Yachts in the Caribbean does many interesting sailing and power multihulls. The main market now appears to be power cats and the occasional tri. To give an example of the work I will compare a 2014 power catamaran of 70 foot with a 2025 power trimaran of 70 foot.

The 2014 wave piercing power catamaran is designed to take 70 passengers on an offshore route. The cat is 70 x 25 foot and is powered by 2 x Cummings QSL9 405 hp diesel engines. It has 2 x 400 gallon fuel tanks. The top speed is 28 knots. The displacement is about 45,000 lbs.

The cat was built with some ply timber and some foam glass. Goldcoats yachts liked building in timber ply a lot until supply lines became expensive and troublesome. They started transitioning to foam glass to satisfy client requests.

The 70.8 x 30.5 foot power trimaran carries 30 passenger over similar offshore routes and is powered by 2 x 240 HP Yanmar engines. It has a single 300 gallon fuel tank. The cruise speed is 23 knots with na top speed of 27 knots. The displacement is 30,000 lbs fully loaded. The main hull length to beam is 16.6 to 1. The draft is 3.5 foot.

The hulls, decks, cross beams, wing decks, bulkheads, roof, cabin sides, windshield and heads walls built with Corecell foam and fiberglass using Proset epoxy resin via resin infusion and vacuum-bag processes. Miscellaneous other components built using West System construction methods.

Customer route requirements in ferry work often dictate the statement of requirements for design work but the 2 vessels above give an interesting comparison. Generalizing. The tri is a lighter but more complex build. It takes less fuel to move a passenger on the tri than it does to move a passenger on the cat at full load. The overall initial cost, maintenance and running cost of the mechanical equipment in the tri would be less. There are no comments about the seakeeping qualities of these specific vessels but owners and passengers of other Goldcoast wave piercers are complementary of their performance in seaways.

The jpegs give an idea of each boat. The PDF give line drawings of the 70 foot power tri.

 

[oldmulti]

“Salone” was designed and built as charter catamaran that can take 185 passengers in its 220 square meter saloon. When at maximum passenger load the cat bis basically an inshore vessel but can go further offshore with a reduced passenger load. The naval architecture is H. Loizou and the designer is A. Demetriou. The cat was built in Cyprus under Rules of Cyprus Shipping Department and in accordance with Lloyd’s Rules starting in 2012 and launched in 2015.

The “Salone” is 102 x 43 foot with a draft of 7.9 foot. The displacement is about 250,000 lbs (about 111 tons). The length to beam on the hulls is about 12 to 1. The power is 2 x VOLVO PENTA D9 500 hp diesel engines. The cruising speeds is 11 knots with a peak speed of 12.6 knots. There is also a 5KW generator. There is also hydraulic tank with separate Volvo engine 200HP that powers 2 side bow thrusters SH 420/386 TC and Muir Anchor Lift.

The build is composite foam sandwich.

The accommodation is focussed a big 220 square meter luxury saloon, big bar, TV, music surround system, Wi-Fi, dining area, kitchen, BBQ area, 4 WCs included a disable WC. All WCs are flat with decks with no steps for easy use. There is 2 swimming ladders and shower facilities at the back of the vessel. Upper deck is open with sun beds at the front and a big shaded area including bar with seats and two WCs. Two VIP cabins will be enclosed in hulls.

The jpegs give the idea of what can be done if you have the money. This could be the ultimate cruiser.

 

[oldmulti]

Bruce Roberts design does a very large range of boat designs. The designs are mainly aimed “cruising boats”. When it comes to multihulls their favourite design mode is power or power sailing catamarans. The following is an example of 11.5 meter (38 foot) power sailing catamaran. The designs are biased about 60% power and 40% sail. All of them can be can be built as a power only vessel or with the added rig designed for the cat.

The 11.6 meter power sailor is 38 x 17 foot with a displacement of about 22,000 lbs depending on the material it is built in. The Aluminum mast is 40 foot and carries a 270 square foot mainsail and a 220 square foot jib. The length to beam on the hulls is about 8 to 1. The power version draws 1.9 foot, the power sail version draws 2.8 foot over the low aspect ratio keels. Now the fun part. Your outboard power can be 2 x 30 HP or up to 2 x 150 HP Honda 4 stroke engines are suggested. The speed difference is big, to quote one owner “with 160 HP twins I now get 30 knots out of her". The fuel “economy” would be questionable.

The accommodation has an aft double berth, shower toilet and a single berth forward in each hull. The bridgedeck saloon has a dinette, large galley and internal helming position. Sail controls and external helming are an afterthought.

Construction can be in solid glass with some foam, foam glass or aluminum. The plan sets are available for each option. The chine hull shape allows a relatively early build with the glass versions using cheap female moulds and flat panels.

As with all Bruce Roberts designs it is aimed at being a simple cruiser to build and own, it is not a fast performance pure sailor nor is it a pure power boat. It is a compromise that suits owners who want reasonable accommodation without to much effort in getting from A to B.

The jpegs give an idea.

 

[oldmulti]

The following is a one off but its performance would indicate others may like it. This is a fast, long-range maxi trailer “trimaran” power boat. It has a stable, efficient hull that performs well in moderate sea conditions that has an excellent layout for stand-up sportfishing. “S.J. Seahawk’s” owner and designer is Phillip Morris and is constructed by Morris Metal Products Limited. Before building S.J. Seahawk, a 3D model of the boat was developed in Solidworks to help the design and fabrication techniques for the boat.

The “S J Seahawk” is 31.8 x 8.2 foot with a weight of about 7000 lbs. The draft is 1.5 foot. The power is 2 x Suzuki DF350 dual prop outboards. The fuel consumption is 74 litres/hour (total) with both engines @ 3,800rpm (cruise speed). Fuel capacity is 630 litres giving an operating range (est.) 255 nautical miles at 30 knots cruise. The peak speed is 54 knots (100 kph).

Now the fun part, this is a trimaran hull shape that has 2 tunnels that help provide lift at speed and the tri also has a aero dynamic cabin shape leading to a real aerofoil (at rear of the cabin) that provides up to 1100 lbs lift at 50 knots. This helps the boat have good seakeeping manners at 30 to 50 knots in EG 3 foot head seas. The second requirement is the power tri is trailable to be able to enter fishing competitions around New Zealand. The specialized tri axle trailer weighs 3,300 lbs which requires a Dodge RAM 2500 tow vehicle to haul the 9,700 lbs total load.

The accommodation is more a sportsfishing open boat shape but has an aero dynamic cabin shape to enclose the helming and crew space due to the above 50 knot speeds the boat was designed for. There is a small galley, toilet and bunk area but the main design feature is a large open fishing cockpit.

The boats performance is described by one test report as: “There’s no climbing onto a bow-wave or powering out of a hole. This thing just goes. Directly into the chop from a standing start, we saw 32 knots in 9.5 seconds and a top speed slightly north of 54 knots (100kph)” Another test report comments that the ride at all speeds is very good with the air lift at speeds helpful.

The build is an aluminum hull up to the gunnel, then a 63 kilogram foam carbon fibre cabin roof and fore deck. The aero dynamic wing is also carbon fibre.

The jpegs give the idea.