Multihull Structure Thoughts

A tacking outrigger sailing canoe by Dave Gentry who does a lot of small simple skin on frame monohull sailing and rowing designs that can be very light in weight. He also does stand up paddleboards. He decided to do a simple tacking proa for fun day sailing in bays or rivers.

The proa is 13.5 x 8.5 foot with a weight carrying capacity of 250 lbs. The rigs can be an Optimist lug rig of 40 square foot or a crab claw rig with a 9 foot mast and 16 foot yards with a sail area of about 60 square foot. There is a small leeboard on the main hull of unknown draft. Paddle power only. The tacking proa can be disassembled for cartopping if needed.

The accommodation is a maximum of 2 seats and not much else. You need to shift your weight around to help stability.

The construction is a timber framed canoe shape 13.5 or 15 foot long 1.5 foot wide which is skinned with high strength polyethene skins painted. The float is literally a 100 x 50 mm plank of EG Western Red Cedar that is lightly shaped. The cross beams are 50 x 50 mm in oregan or mahogany. There are X shaped plywood connections between the float and cross beam. This is a boat that can be built from a local hardware store materials.

Sailing performance is unknown beyond a video of Splinter moving well in flat water. I suggest you will have fun for very little money.

The limited jpegs give the idea.

 

Yes, it's hard to tell whether they slowed for comfort or not from the information available.

As someone who owns five wingmasts and a whole lot of wide luff pocket sails, and has sailed on wingmasted big multis at various times for about 9gulp) 40 years, I find the claims for the "wing" rig's performance to be over the top. Squaretops and luff pockets simply don't add much speed; for example moving to a high-performance squaretop in a racing cat normally adds about 1% to their overall speed. Not many people are going to notice such a small improvement when cruising.

 

Well they also were using their wind vane, not steering for the waves etc... so it sounds like a reef down hunkerdown cruise.

 

This is for those who aspire to have a multihull but only have a canoe or a slim monohull skiff. Mike Storer sells plans for a couple of “add on outrigger” kits for these boats. The plans can be had in paper or PDF and are fully detailed.

The first plans are the “Mini Drop In Outriggers”, that are designed to stabilise a sailing canoe or other slender boat, to reduce the risk of capsize. They are positioned above the waterline, to allow sailing the canoe as normal, with both floats out of the water till the boat begins to heel excessively.

Each float is 3.95 x 0.66 foot with a buoyancy of 50 lbs. The hull weight is 5 lbs. The crossbeam length can handle a 5 foot wide main hull.

The 2 hulls are made from one sheet of 3 mm or 4 mm okoume (gaboon) plywood. The crossbeams can be made from fir, pine or other medium-density timber with straight grain and no knots. There is no lamination required to build the crossbeam: a simple 6:1 scarf joint is used to create the dihedral angle. The plans contain suggestions for mounting the crossbeam to the main hull, but this will need to be adapted to suit the chosen boat. The beam and floats will stow inside many canoes for storage.

The next set of plans are for the “Drop in Outrigger”. These outrigger (float) plans are for larger monohull boats and can be used with rigs of EG 80 square foot on a EG 15 foot slim monohull of 4 foot beam or under. The concept is to make a simple trimaran (or tacking proa if you prefer) that can be sailed in light to moderate conditions.

The drop in outrigger plan is for floats are 7.7 x 0.95 foot and a freeboard height of 0.86 foot. The buoyancy of about 110 lbs per float. The cross beams can be built to your desired need but think about 12 foot maximum.

The floats are built with Gaboon ply of 3 to 4.8 mm plywood and are simple stitch and glue. The crossbeams are simple laminations with a top and bottom flange with timber block separators. The hulls attach to the crossbeams with butterfly nuts, then the crossbeams are lashed into the main hull. All components under 4.5kg (10lbs) and can be done with 2 sheets ply and minimal timber.

The first 2 jpegs are of the mini drop in outriggers, The rest are of the drop in outriggers on a bigger mono. There is a proa jpeg and trimaran jpegs of the outrigger.

A simple cheap way to go sailing on a “multihull”.

 

The following trimaran design is done by Eric Henseval (from France) for a build in Poland. It is a plywood build and is listed as “easy to build” for the professional or home build design. This is a fast cruising trimaran that has many sail options on a racing type rig.

The “DIVINITE 8.50m” plywood trimaran is 27.9 x 18.4 foot with a weight of 3,020 lbs and a displacement of 4,480 lbs. The 35.8 foot mast carries 510 square foot upwind with a 316 square foot, a 164 square foot fore triangle, a 193 square foot solent, a 102 square foot staysail and a 538 square foot asymmetric spinnaker. The main hull length to beam is about 8.5 to 1. The length to beam on the floats is 16 to 1. The draft ranges from 1.3 foot to 5 foot over the central hull draft. There is 440 lbs of fresh water ballast per float. Transferable from one side to another with a smart plumbing system. The engine can be a 10 HP outboard. The cross beams are demountable by bolts so the whole tri can be transported in a single 40 foot container.

The accommodation comprises of a main saloon with 2 seats/berths with vision 180° when sitting looking forward, convertible in double berth. The main saloon has 6 foot headroom and also has a galley, chart table, ice box location and a porta potty. There is a double berth forward. There is storage in the floats.

Construction is a plywood epoxy glass with aluminium tube/mast section cross beams that are bolted vertically onto the hulls. “Easy to build” is Eric’s claim. Eric has designed many smaller multihulls before and really knows how to design simple to build effective cats and tri’s. I will accept his words.

No performance data but the rig, light weight and hull shape indicates high average speeds will be possible EG peaks near 20 knots and easy 8 to 10 knot averages in short bursts.

The limited jpegs will give the idea.

 

Sorry, no entry today life is getting in the way.

 

That Henseval design is very nice indeed. He's done remarkably well to fit that much space into a boat that looks so nice.

 

Frank Bethwaite with his sun Julian were prolific designer and builders of performance daysailor racing dinghies and other design concepts, they wrote books on the subject and designed mono’s like the B 14 and B 18 and were instrumental in the development of the 29 and 49 dinghies. One of the more unusual designs was the HSP (High Speed Project) that went into production in various versions. According to his son “40-50 of them sold, around the world, big up-side was heaviest part was 15kgs, so launching and retrieving single handed was a breeze. Probably did +20 knots, but remember this was late 1970's.”

There were many development versions and at least 2 production versions of the HSP. The first was for a single crew and was 16.5 x 16.5 foot long weighed 180 lbs and had a sail area of 175 square foot. The 23.8 foot mast was aluminium with a fiberglass tip which allowed the mast head to blow off in stronger winds. The length to beam on the main hull is about 9 to 1. The draft over the central hull daggerboard was 2.3 foot. The single person 16.5 foot is a 40 year old design and somewhat primitive in construction, monolithic E glass Polyester and alloy tubes. The design started with a clean bow but after nose diving a “duck bill” bow flange was added to limited nose diving. The duckbill on bow didn't really cure nosediving and it had a few moments with main hull buried up to rear post while still doing 15+. This model of HSP still reached 20 knots plus and could still sail faster than windspeed under good conditions. One owner reported: “I would routinely hit 12-13 upwind tacking through 90 degrees.”

The second version was for 2 crew and was 19.7 x 17 foot weight about 275 lbs in foam glass. The waterline beam on the main hull is 1.66 foot giving a length to beam of about 11.5 to 1. The rig size is unknown but sufficient to push the boat to a measured 22/23 knots. This version was built in foam glass and according to Frank/Julian was over weight. Later builds were lighter, the lowest weight I have read is 250 lbs. Owners reported the 2 person HSP could be pushed to faster than wind speed but required cautious handling in rough waters. These were very wide boats that depended on crew weight positioning as much as float buoyancy for stability, also the rig depended on the mainsail to stand up and initially they had a light boom that if incorrectly treated would bend causing rig instability. Tricky stuff but if it all worked well in the right conditions very few boats could keep up.

These 2 models inspired others in to develop similar boats in the early 2020’s EG the last 2 jpegs of Scissors. The first jpegs are of the single crew model then the 2 crew model and then Scissors.

 

The following trimarans are advertised on AliExpress as the “U-Boat” models UBP-K7 (single seat) and UBP-K3 (dual seat) but look familiar (yes Hobie tris). I do not know if these tris are the source manufacturers or a close copy. These tris have all the same characteristics of the Hobies including the peddle drive system. So, we talk about both models.

The first UBP-K7 is a 16 foot x 2.45 foot main hull with 12.45 x 0.72 foot outriggers with a weight of 197 lbs and a load capacity of 400 lbs. The carbon fibre mast carries a 64.5 square foot battened mainsail. The draft over the pedal drive is 2 foot. The outboard rudder is kickup.

There are no performance figures, but this bay/river sailor should be fun. Cost delivered before Christmas is $A6,500 or about $US4,225. Pity the shipping cost $A2,500.

Next is the larger 2 seat model has an 18.05 x 2.66 foot main hull with a 13.25 x 0.66 foot float, the overall beam is unknown. The weight is 235 lbs and a load capacity of 600 lbs. The carbon fibre mast carries a 92 square foot battened mainsail. The draft over the main hull centreboard and double pedal drive sail is 2.3 foot. There is an option of an electric outboard.

The basic hull structure can have such additions as a choice of colour, but additional accessories such as a life jacket, electric motor, bimini top, kayak trolley, and fishing chair can be added at extra cost to “enhance your sailing experience”.

The construction of both boats is rotomoulded from the robust exxonmobil lldpe ll8446.21 hull material. The cross arms are aluminium and the mast is carbon fibre.

Again, there are no performance figures, but this bay/river sailor should be fun. Cost delivered before Christmas is $A 7,500 or about $US4,875. Shipping to Australia only costs $A 5,500. Hmm.

Sorry about some of the jpegs, the single seat first, dual seat second.

 

So you cannot afford to go cruising because the boat is going io cost so much. Let’s talk about Stephen Wolf and his lady friend Margo who did a 10 year 40,000 miles circumnavigation and had a baby half way around. They visited 32 locations globally (nearly as many countries) stopping from a few weeks to months. A very good way to get to know the world. Reason for the interest. They did the trip in a trailable 24 foot Piver Nugget built by Cox Marine in Britian. The tri was named “No Name” and returned from its circumnavigation much modified, EG daggerboard, rudder, parts of the rig and improved interior.

The Piver Nugget (Mk 1) is 24 x 14 foot, had folding wings to reduce the beam to 8 foot and weighed about 1,700 lbs. The claim of Stephen was a weight of 1,000 lbs and a carrying capacity of 1,000 lbs. The 26 foot 150 mm x 50 mm solid spruce mast was held up by 4 mm rigging and carried a 250 square foot basic sail plan but had a 300 square foot drifter and a 25 square foot jib. The length to beam on the main hull was about 7 to 1. The draft over the hull was 0.7 foot and over the daggerboard was about 1.5 foot. The motor was initially a 5 Hp outboard but Stephen disposed of it early in the voyage and just used a range of sails for progress and paddling if required.

The accommodation on the original Nuggets was more of a day sailor with a “shelter”. The shelter was converted into a minimalist cab that a double berth and small galley was installed. Sitting headroom only. This was the basic accommodation for 10 years bar short shore stays. Youth is a wonderful thing.

The construction of the Piver Nugget trimaran is is largely plywood 6 mm for hulls on 18 x 25 mm stringers and frames with 9 mm for decking with 18 x 40 mm deck stringers and 18 x 50 mm deck beams. The keel was a 40 x 100 mm. Strong for a 24 foot boat. The cross beams were plywood boxes and were heavy as about a third of the timber fibres were running in the wrong direction. The entire boat is covered with one layer of light 156 gsm fiberglass. Plan options show both trailable and deep-sea models. No cross-arm hinges on deep-sea model. Also shown in plans is optional head; possible widening of settee-berth to form double--two wing bunks.

The performance of the Nugget was considered by the owner as being good for its time (70/80’s) but it lacked windward performance EG The tri turned back from beating into a 5 foot head seas as it was not getting very far, and 80 to 100 miles a day was considered a good days sail. A lot of small cruising monos could match this but on the rare really good days off the wind “No Name” could get days above a 100 miles which small cruising mono’s could not match.

A couple who found a cheap way to see the world and had a lot of adventures along the way. Read about part of the journey on the following web page(s): April 2023 - Latitude38 https://www.latitude38.com/issues/april-2023/#76

The limited jpegs give an idea.

 

Part 2 of the 24 foot Piver Nugget story. The following set of plans came out of a very old “How to build 20 Boats” magazine. This is the first version of the Piver Nugget and was designed in the early 1960’s. Hence the box floats on edge. The plans detail the build. The dimensions of the boat may be slightly different to later versions of the Nugget design, but the materials and structure are very similar. Do not be inspired by the cross beams as there are better ways to do them. Jim Brown built and sailed one of these tris very early in his multihull journey. To expand the size of the jpegs double click on the jpeg then click on the top right corner angled arrow (beside the X). That will take you out to a lager page then if you see a plus size on screen press and it will expand the page again. Use your mouse key to move the expanded page around.

 

Art Piver's books are some of the more entertaining reads out there, quite funny really.
And if well built those boats are capable of serious voyages.
Nobby Clark, an ex Spitfire pilot, was the sales arm behind the Brit Piver commercial builds. They turned out quite a few.

 

I think I have done this tri before but I must update the index. I will talk about the SYAS 12 mtr performance (read racer) tri. Syas Performance does design of sailing, power boats and industrial design. They are a shy group with only an email contact point. The design looks viable but please check the companies background.

The Syas 12 is 40 x 35.9 foot with a displacement of 7670 lbs. The carbon fibre wing mast of 57 foot carries a basic sail area of 1,162 square foot has many options depending on your budget. The main hull length to beam is about 11 to 1. The float length to beam is 16 to 1. The draft is 8.5 foot over the central hull daggerboard. The Bruce number is 1.52 (good). The inboard engine appears to be a sail drive.

The accommodation is limited to 2 single berths in the main hull. The main cabin is focussed on a navigation station and a small galley and not much else. The cockpit is set up for real sailing with seating at either side half way to the floats.

The construction E-glass and foam hulls with carbon fibre in high load areas. Carbon fibre beams. Carbon fibre wing mast with synthetic rigging.

The jpegs give the idea of what should be a very high performance trimaran.

 

During the early 1970s David Keiper sailed 20,000 miles in his 31 foot hydrofoil trimaran, Williwaw, throughout the South Pacific. David wrote a book, “Hydrofoil Voyager” (still available on Amazon) about his experiences and there is a video that was made from rare vintage clips taken before the famous open ocean cruises. Dave sailed from San Fransico to Hawaii back to San Fransico before sailing the Pacific to New Zealand via Cook Islands etc. This is a true hydrofoiling cruiser that was home built and could be sailed single handed for extended periods. This trimaran truly was radical. A few words from David first.

“The tri is now named WILLIWAW and is 31 ft long overall, and I' m expecting a total displacement of 3000 lbs, including two persons and their supplies. In addition to designing a foil system that should give a lift/drag ratio of 14 or 15 at take-off, I had to work out a rigid but lightweight method of construction, and an improved sail rig. Calculations indicate that a 13 knot wind will be required to become fully foil borne. Lacking that wind, the boat can be operated as an efficient trimaran by retracting the foils. The abbreviated pontoons are located forward of a mid-ships and serve for initial stability and for structural fastening points for shrouds, bow foil and lateral stabilizing foils. The aluminium foils have a 150 mm chord length, and will be set with minimum dihedral of 30°. The bow foil will span the width of the boat and will thus have a very high aspect ratio. It will be set for a fairly high lift coefficient at the take-off speed of 12 knots.”

“The hull construction is curved frame members (every 500 mm) and planking are of 6 mm plywood. Angle blocks spaced every 150 mm along the frames fasten frames and planking together (initially it had no stringers just triangular connecting blocks. Stringers came later). Bottom and transom will be of 12 mm in plywood. There will be a thin fibreglass skin over the whole boat. I have less surface area to plank, fibreglass, and paint than a standard trimaran, and also no built-up cabin structure (5 ft headroom). The hull weighs only I,300-1,400 lbs, but is extremely rigid because of its proportions and its doubly-curved plywood. Practically all of its weight contributed structurally, including inside shelves and benches. The living quarters appear spacious, since they run the full length and width.”

I have sailed the craft once so far, in a light air, without hydrofoils. She balances and manoeuvers well. However, it is so easy to get confused about wind direction, because it generates its own wind going upwind, and kills its wind downwind. When a Force 3 wind is generated close-hauled, with 380 sq ft of sail, the craft heels about 15°, and the underbelly adjacent to the leeward pontoon begins planning." (Normally, a leeward hydrofoil would prevent such a heel.) Hydrofoils will be fabricated for WILLWAW next month (Jan. '67). The hydrofoils will add 400 lbs to the craft, but this isn't much more than the weight saved by not having large pontoons.”

Williwaw could average 160 miles plus/day whilst cruising with 1 or 2 people, that is 1000 miles per week. This is for a 1970 31 foot home built boat. The reason for this item is an additional PDF and video has been found which gives more detail of Williwaw. The PDF can be found at: International hydrofoil society article: https://www.foils.org/wp-content/uploads/2018/01/AIAA1971Nov20-AeroSail.pdf

The jpegs will give an idea of Williwaw. The video of Williwaw sailing in San Francisco Bay is at:

 

The follow analysis of a 24 metre (78.7 foot) power trimaran is technical but gives an interesting design/structural insite of an aluminium potential build. The paper is technical but even a glide over the top will give an understanding of the forces involved and where to focus on in the design and build.

The power tri concept is shown in the jpegs and is 78.7 x 25.95 foot with a displacement of 70,790 lbs. The length to beam on the main hull is 9.25 to 1, the float hull beam is 13.2 to 1. The draft is 4 foot. The power is 800 KW.

The hull structure is 8 mm, the underwing is 8 mm, the deck and bulkheads are 6 mm, the forward collision bulkhead is 8 mm. These are just the aluminium plate thickness. The underlying framing and stringer structure is where the real analysis is done.

The analysis was done by FEA (computer Finite Element Analysis). The power tri was put through a series of FEA tests EG slamming into head seas, extreme hogging loads, structural load pressure to break a float off etc.

The conclusion was: “The structural analysis of the initial model showed that performing the scantling with the classification house, the maximum obtained stress is below the admissible stress for each classification house between 5% and 32%, depending on the house used, which allows for a relative safety margin in the material.

The finite elements method allows not only to verify not to exceed admissible stresses, but can also identify high stress efforts, which may be diminished with the adequate structural optimization processes, modifying the structural design geometry for example, and that not necessarily can increase scantling in general.”

The technical paper is at: Untitled Document https://shipjournal.co/index.php/sst/article/download/123/373?inline=1