Multihull Structure Thoughts

[oldmulti]

The following is not just about the 33 foot power boat but more importantly about its structure and build. HDPE is the high density plastic used in some kitchen cutting boards and can be used in boat building. It is waterproof, scratch resistant, does not need to be painted and can be plastic welded together. There are already 2 multihull designs (a 28 foot and 43 foot catamaran) that have been built using this material. The material is relatively heavier for its “strength” than aluminium, plywood or foam glass but lighter than solid glass. The following is the first more detailed plan of the materials use. PS there are several firms in Holland that produce production power boats for industry using this material.

The 10 meter high speed power boat is 33.5 x 12 foot with a weight of 6700 lbs and a displacement of 22,400 lbs. The draft is 1 foot for the hull and 2 foot with the outboards down. The outboard engines can range from 2 x 150 HP to 2 x 300 HP. The peak speed with 2 x 150 HP is 65 Kph and with 2 x 300 HP is 87 Kph.

The structure is mainly 15 mm HDPE plate and HDPE tubes. The boat is designed for HDPE sheet material (low-pressure polyethylene). With the availability of cutting parts on CNC machines and the manufacturability of welding HDPE material, an amateur self-builder is quite capable of building a 10-meter boat on his own. HDPE welding methods are developed and there is no shortage of HDPE welding machines. Basically, the design of the boat uses sheets of HDPE with a thickness of 15 mm. All structural parts are made of HDPE sheet plastic (designed for sheet widths up to 1200mm), fender bars and keel reinforcement made of HDPE pipe. The assembly of the boat hull is carried out using the standard classic method. Welding of the HDPE boat structure is carried out with a thermal welding machine. Work with it is within the power of non-professionals. The boat can also be built from 12mm FSF plywood or from metal - aluminum or steel.

From what I have investigated so far HDPE boats have been built using 10 to 18 mm sheets depending on the size and supporting frames stringers etc. An interesting topic needing further investigation.

 

[oldmulti]

The second HDPE power boat is an outboard designed for rivers, lakes or coastal areas. The boat could be used for fishing, water taxi, pleasure boat, water skier towboat, work boat, patrol speed boat for border, customs rescue or police service. Again, it’s the structure that is important here.

The planning monohull has an 18 degree V bottom and is 21 x 7.35 foot with a weight of 1,570 lbs and a full displacement of 6,700 lbs. The hull draft is 1.5 foot. The power is a single outboard from 180 to 420 HP. The peak speeds when fully load is with a 180 HP outboard gives 45 km/h, 230 HP gives 60 km/h, 300 HP gives 74 km/h and 420 HP gives 90 km/h.

The boat can be built from HDPE sheet material (low-pressure polyethylene, HDPE), which is gaining great popularity in small shipbuilding. With the availability of cutting parts on CNC machines and the manufacturability of welding HDPE material, an amateur self-builder is quite capable of building a 6.5 meter boat on his own - reliable in operation, unpretentious and maintainable, durable, and not requiring much maintenance. HDPE welding methods are thoroughly developed and there is a large offer of HDPE welding machines. Basically, in the design of the boat, sheet HDPE with a thickness of 10 mm is used. All parts of the structure are made of HDPE sheet plastic (designed for sheet widths up to 1200mm), fender bars are made of HDPE pipe 250mm. After the exhibition of frames, a longitudinal set is cut into the slipway - keel, stringers, fenders, and welded. Then the hulls are sheathed with 10 mm HDPE sheet on the bottom and 8 mm on the sides. Then the hull is turned over and the deck and pressurized compartments are arranged. Welding of the HDPE boat structure is carried out with a thermal welding machine. The boat can also be built from 12mm FSF plywood or 4mm aluminum.

This boat is in the displacement range and speed ranges of below 26 foot multihulls showing the material may be able to be used for EG shell and internal frame work for hulls. Cross beams and load points would require some alternate materials EG Aluminium cross beams bolted on etc. but the majority of the boat could use HDPE. An effective low maintenance cruiser could be developed. Spirt Catamarans did a 28 foot cat using HDPE as the shell building material.

The jpegs will give the idea.

 

[oldmulti]

This is from an item on page 100 of this thread that has been edited and gives an outline of a 28 foot cruising catamaran in HDPE from Spirit Designs Australia.

The following is about Spirited Designs Stow Away 8.5 sailing power catamaran and especially the material build options. The Stow Away 8.5 is 27.9 x 13.1 foot and displaces 7700 lbs with a payload capacity of 2200 lbs. The fractional rigged of a 35.6 foot fixed aluminium mast carries 452 square foot of sail. Being a power sailor there are two 50 HP outboards that drive the cat at 20 knots peak speed. Under sail the cat can cruise at 7 to 8 knots. The hulls are semi-displacement Tennant canoe sterns type which are efficient under power and sail. The underwing has 1.65 foot clearance. This cat can be seriously cruised around any coast or bay. The accommodation in this cat is good with one double berth and 2 singles. The main saloon has and hulls have 6 foot headroom. The main saloon has the dinette and galley with a loo in a hull. 2 single berths are in the other hull.

But the real interest is in the build options. The standard build is either PVC foam e-glass vinylester or Duflex (balsa or foam) e-glass vinylester or epoxy. There are plywood components. There is/was a HDPE option available in the Stow Away 8.5. You can build the hull in HDPE Polyethylene plastic sheet (think kitchen plastic cutting board material).

There are boat building companies that use HDPE in power boats up to about 50 foot. The tensile and compressive strength of HDPE is about 4000 PSI (pounds per square inch), marine plywood tensile and compressive strength is about 4500 PSI. The Flexural Modulus (ability to handle bending before failing) of HDPE is about 20% of marine plywood. The weight of HDPE is 60 lbs/ cubic foot, marine plywood is about 36 lbs/cubic foot. HDPE advantages and disadvantages are:

HDPE ADVANTAGE: Polyethylene is extremely impact-resistant. If the HDPE is UV stabilized it can be obtained in many colours and does not require any painting or other finishing material. HDPE can be plastic welded together not requiring any tape seam strengthening. Translation you can buy a panel cut it to shape place it over a frame and weld the flat panel hull parts together. Work done.

HDPE DISADVANTAGE: Polyethylene isn’t as structurally stiff as fiberglass or aluminum, so HDPE boats are limited in size. There are few over 30 foot and many of the relatively large models require structural hull supports built from different materials. EG US Bass 20 foot power boats that have 25 mm HDPE bottoms with internal framing that if built in plywood would have used 18 mm ply/timber framing and 18 mm plywood for the shell. The HDPE bottom in this example weighs twice a plywood bottom. A HDPE hull skin saves a lot of finishing work but comes at a weight penalty and requires the same internal framing structure as a plywood build. HDPE is not suitable for structural (EG Crossarm) bulkheads etc which should be either plywood or designed foam glass.

There are many variations HDPE from standard to marine with solid, lightweight and foam cored options. The material is available in 8 x 4.5 foot to 13 x 6.5 panels in thicknesses from 3 mm to 25 mm. The cost is about the same as good quality plywood. In the case of the HDPE Stow Away 8.5 the displacement increase required the hulls to be redesigned to handle the extra structural weight. Investigate and understand what is possible with a designer.

The jpegs show a Duflex build Stow Away 8.5.

 

[Eurosnob]

Put a HDPE skin below the waterline = no more anti-foul?

 

[oldmulti]

Eurosnob. Sorry, from my understanding antifoul is still required. If a variety of 2 pot paints and epoxy finishes (which are basically a variation of plastics) can still get barnacles etc we are doomed.

 

[oldmulti]

The following is the final in HDPE boats for a time. The boat is a 12.8 foot punt type power boat designed for inshore or coastal fishing or pleasure day time touring around. Again this boat is built in HDPE and again shows how small you can go using this material. This would make a good practice build if you would like to understand HDPE construction.

The monohull power boat is 12.8 x 5.3 foot with a weight of 360 lbs and a loaded displacement of 1,000 lbs. The draft ranges from 0.3 foot to 0.5 foot depending on load. The engines can range from 6 HP to 25 HP. The 6 HP will give a peak speed of 15 kph, a 15 HP will give 37 kph and a 25 HP will give a peak speed of 44 kph (about 24 knots).

The boat is an open cockpit pleasure boat with seating for 4 adults (maximum effective load) with storage beneath the seats for fuel, ice box and EG safety gear.

The construction is from 8 mm thick HDPE sheet material and assembled from parts cut on CNC machines (can be hand cut). HDPE welding methods are thoroughly developed and there is a large offer of HDPE welding machines. All parts of the structure are made of sheet plastic HDPE (designed for sheet dimensions up to 2x4m), fender bars made of HDPE pipe 110mm. The hull skin is laid from 8 mm sheet HDPE on the bottom and sides. Then a set is mounted in the hull - a keel, stringers, then a fender, reinforcements and flooring are installed. Alternatively, the boat can also be built from 9mm FSF plywood with fiberglass cloth laid externally in epoxy or 2-3mm aluminum.

The jpegs give the idea.

 

[oldmulti]

Nimbus 9 is a single seat mini trimaran from the drawing board of Sailrocket designer Malcolm Barnsley. She is aimed at people with reduced mobility, but also learners and anyone seeking the thrill of a fast ride with greater comfort safety than the standard racing dinghies and beach cats. The de-assembled boat can be easily be accommodated on a standard size car roof rack, the heaviest item being the main hull at 18kg, and the longest being the assembled platform (7 foot) is within the legal width limit (8 foot) and hence can be trailed. Allow 45 minutes for first time, 20 minutes once familiar. Only 2 tools needed 13mm spanner and mallet or light hammer.

The Nimbus 9 is 9 x 6.5 foot with a boat weight of 96 lbs and a displacement of about 290 lbs depending on crew weight. The rig is from an Optimist with a freestanding 8.5 foot mast above deck and a 38 square foot sail. The waterline beam of the main hull is 1.25 foot giving a length to beam of 6.8 to 1. The hull draft is 0.75 foot with a daggerboard draft of 2.5 foot. The floats in the study print are not the same as the floats in the jpegs. The floats in the real tri are 9 x 1 foot.

The rudder control is by foot pedals when you are sitting in the fixed forward facing central seat and there is a single sheet for sail control. Speeds over 7 knots are achievable in windspeeds of 15 knots and above. With typical adult pilot mass of 75kg operation in windspeeds up to 25 knots (force 6) is safe in sheltered waters, where speeds of 6 knots can be achieved reaching and 4 knots upwind. Significantly higher speeds are likely with lighter pilots.

Nimbus 9 is built from Gaboon marine plywood by the stitch and glue method. The hull shells use 4mm ply with a full external sheathing of 250gsm woven glass in epoxy. Stringers and deck beams are from Meranti. The bottom of the main hull is protected from abrasive damage by a bonded-on Formica sheet which is sacrificial and can be replaced. The crossbeam tubes are 75 mm diameter x 2.5 to 3 mm walls.

The final 2 jpegs are of a foiling version that was to done for Speed Week. The foil calculations were done and in theory a detailed spreadsheet suggested 14 -15knots of wind should allow the tri to fly if the foil areas and angles are right. Reassuringly Chalmers University managed to get an Optimist foil borne (Nimbus 9 uses an Optimist rig).

This would be a very simple boat to construct and sail but is a sheltered water tri for 1. Great fun trainer.

 

[oldmulti]

The 10 foot sail racing multihull class had popularity in about 2010 to 2018 with many designs done by professional designers for a fun class. The speeds in theory were meant to be from about 4 knots to about 9 knots but some better designs could hit 11 to 12 knots in good conditions. The following design was done by Eric Henseval and marketed by a couple of Italian guys.

The trimaran is 10 x 8.8 foot with a weight of 155 lbs and a loaded displacement of 325 lbs (yes, you as crew need to weigh under 170 lbs to get the best boat speed). The 20 foot fixed mast (about 52 mm diameter) carries a 86 square foot square head batten mainsail. The main hull length to beam is 7 to 1 at waterline level and the float is 10 x 1 foot at deck level. The draft ranges from 0.7 foot over the main hull to 2.45 foot over the daggerboard. The tri can be disassembled for transport.

This a one person boat intended for racing and therefore has a seat for one and not much else.

The build is a 4 mm plywood shell covered with 180 gsm e-glass in epoxy. Any taping of plywood panels are done with 300 gsm E-glass in epoxy. The cross beam, mast bulkhead and transom are all done in 8 mm plywood. The gunnels in the main hull are 12 x 30 mm solid timber and 22 x 22 mm in the floats. The cross beams are solid timber with a maximum cross section of 95 x 75 mm tapering at either end to 30 x 70 mm. The floats can be bolted or lashed onto the hulls.

The boats are strong enough but are fragile so do not jump on a deck or run onto rocks etc. If they are well built, they will serve you well.

For fun, this tri is a good solution. Many PDF plans are available at the following web site:

Growing the 10 Foot Trimaran Class https://smalltrimaran.co.uk/growing-the-10-foot-trimaran-class/

The jpegs give the idea of this tri.

 

[HelmutSheina]

The last one is a bit like the Gumprecht Drifter 12.

 

[tane]

We have done an item on this cat before but an interesting PDF study plan has appeared. Toru (or Little Spruce Goose) is a C/S/K (Rudy Choy) design built by Tom Sauter in about 1962 to 1964. It is a cruiser racer sailing catamaran that was very lightly and well built.

Toru is 36 x 15.5 foot of unknown displacement. The 46 foot fixed aluminum mast carries a 350 square foot mainsail, a 440 square foot genoa, a 220 square foot jib and a 820 square foot spinnaker. The length to beam at the waterline is 14 when not healed, the heeled waterline is 15 to 1. The draft is 1.45 foot. The engine is a 33 HP outboard. The hulls are classic CSK asymmetric which were good in rough water but were prone to pitching and often had daggerboards added to help in upwind work.

The study plan jpeg gives the accommodation layout. The deck cabin only has about 5 foot headroom and was to be modified to give more headroom. A practical cruiser layout.

The construction is light. The hulls were moulded in 3 layers of 3 mm spruce. The crossbeams and decks are plywood backed by timber framing and stringers. A well built and maintained 60 year old plywood timber build can be a good buy.

The jpegs and PDF will give the idea of an interesting design.

...when catamarans were beautiful...

 

[oldmulti]

This is an update on the Bernd Kohler KD650. The KD 650 is an affordable trailer catamaran for amateur construction. The boat is designed for fast coastal sailing. One of his main features is the aerodynamically efficient Gunter rig. Every effort was made to end up with a practical, cost efficient and fun sail boat.

The KD 650 is 21.3 x 11.5 foot overall and can be disassembled to 6.5 foot for trailering. The weight is 760 lbs and the displacement is 1,700 lbs. Each hull weighs 198 lbs. The rig is a Gunter rig is to have a cheap and powerful rig. The mast is a 14.1 foot aluminium tube, diameter 90 mm OD 84 mm ID (3 mm walls and weighs about 22 lbs). The gaff is made from shaped spruce with some UD carbon fibre. The sail area is 200 square foot. The hull length to beam is 16.6 to 1 with a prismatic coefficient of 0.57 (tells you how full the ends are). Because of the high length to beam ratio the ends do not have to be very full, if the length to beam was EG 10 to 1 the PC could be in the 0.6 or above range. The hulls have an asymmetrical hull shape with a 1/3, 2/3 asymmetry and do not need boards. The draft hull of the hulls is 1 foot and the rudders 1.6 foot. The power is a 5 HP outboard.

The accommodation is limited to 2 bunks aft under cockpit seats with a seat and small galley in 1 hull and a toilet area in the other hull. Functional for a weekender or short coastal trips.

The construction is mainly plywood over a minimal timber frame done with epoxy and aluminium cross beams bolted on. The hull sides are 6 mm ply, the hull bottoms 8 mm plywood, stringers are 19 x 25 mm. There are 5 bulkheads, 6 or 9 mm backed by a 19 x 30 mm timber frame, in the design and internal furniture, EG 9 mm bunk boards, form part of the structural strength in the design. The cabin roof is 2 layers of 4 mm plywood, and the remaining decks are 6 mm plywood. The rudder blades are made from 4mm plywood some carbon fiber and spruce. The are built in a jig to make sure to end with a profile which resembles the NACA 0012 section. The rudder will kick up when an obstacle is hit. The aluminium cross beams are 140 mm OD tubes with 4 mm walls for the mast beam and rear beam. Bernd approved a builder to use 130 mm OD with 5 mm walls for the mast beam and rear beams.

Bernds comments on performance: "The boat will have a speed of 17 knots at a wind speed of 21 knots (this is the wind speed a hull starts to lift of the water surface) and a speed of 12 knots with a wind speed of 12 knots. Not bad for such a small boat.”

The jpegs give an idea of the catamaran. The black jpeg was used as a basis of an accurate 10% model of the cat which the builder found some errors in the full size design.

 

[oldmulti]

David Kagan. He has a small Hobie Bravo cat then added outriggers to it. This perhaps is a quadmaran, since the Bravo although narrow has a catamaran hull shape. He did it for a few reasons but mainly the Bravo cat is a narrow boat and doesn’t necessarily fit two adults in total comfort. He thought adding floats would increase the stability and provide more room for 2 people.

The Bravo cat is 12 x 4.4 foot with a weight 195 lbs. The 19 foot carbon fibre mast carries 86 square foot of sail area. The draft is 0.5 foot. The floats are 8 x 1.25 foot and expand the beam to 10.5 foot at full extension and can be compressed by sliding beams to 7.5 foot for trailering.

This is a fun day sailor that can now handle 2 adults with a bit of comfort. This is a cruiser not a racer. The float hull shape and additional weight does not help its performance but the additional stability will help in stronger winds (up to the limits of the mast strength).

After the Hobie Bravo cat hull was modified slightly to have an aluminium tube place on the hull at the cross beam positions the floats were created. The floats were designed on Powerpoint (yes, the Microsoft product) to get a shape. A cardboard model was made and when it looked right the plywood panels were cut out. Next interesting point the plywood panels were internally taped together using mainly PL construction adhesive and drywall fiberglass tape to make the interior seams. For exterior seams PL construction glue filled any gaps and all was allowed to dry for a week to get the PL construction glue to full strength. The exterior of the plywood hulls was skimmed with PL construction glue and exterior house paint applied. This approach may be good for a small boat but may not work as well for EG a 12 foot or larger boat.

To quote the builder: “PL needs 4 to 5 days of curing to reach impressive strength. It will not be impressive if you test it after only 24 hours. For example, in the early stitching stage, I placed small blobs in the stems to close the hull up at the bow. After 24 hours, the PL did not develop sufficient tensile strength to hold the hull closed if stiches were removed. But, wait one more day, 48 hours, and the hold is very strong. A couple days later, it’s much, much stronger. So, I was careful not to attach the deck for 7 days after all the glue work inside the hulls was finished to give the glue several days to cure.”

The next part again is interesting; he used square 42 x 42 mm steel fence post (wall about 1.6 mm) as the sliding component of the crossbeams to slide inside the aluminium main hull tubes. The float to the cross beam attachment is done with unitstrut fence attachment components. Cheap and simple. When fully extended a pin is placed in the aluminium beam that passes through the square steel tube to hold it together.

There was no performance figure give but the jpegs give an idea of the craft.

 

[oldmulti]

The Strike 20 is a high performance trimaran designed by Richard Woods for home builders. This is an extension of Woods Stike 15 and 18 designs. The Strike 20 is aimed at fast day sailing/racing, the smaller Strikes are more aimed at fast cruising.

The Strike 20 is 20 x 14.4 foot that can fold to 7.5 to 8 foot. The empty weight is 650 lbs and the displacement is 1340 lbs. The mast can be a carbon fibre or aluminium mast from an EG Hobie 21 of 31.5 foot with EG 150 x 80 mm section. The mainsail is 210square foot and a 86 square foot jib. Other rigs can be used from EG F18’s etc. The length to beam on the main hull is 9.4 to 1. The float length to beam is EG 13.2 to 1 at deck line depending on which day sailing cat is the float provider. Draft depends on the float hulls, if they are a 18 foot cat with their own daggerboards and rudders the draft may be 4 foot with the boards down. Electric outboard will work if motive power is required. The hull shape of the main hull is flat bottomed with a slight V introduced in the forward 25% of the hull.

This boat is a pure racer with seating for 3 and some storage if required.

The build is mainly plywood and timber with aluminium tubes for cross beams. The main hull bottoms are 6 mm with 6 or 9 mm bulkheads with timber edging. The gunnels are 50 x 25 mm, stringers are 36 x 25 mm and some of the permanent cross beam structures are 100 x 25 mm. Hull sides can be 4 mm if its good quality or 6 mm if its questionable. The fold up cross beams are 100 x 3 mm aluminium tubes which need waterstays to strengthen the structure. The floats can be from a donner catamaran or be built from EG Woods Pixie timber cat plans. The exterior is covered by 200 gsm cloth covered in epoxy and paint. The simple hull shape and the use of cat hulls with simple aluminium beams means this is a relatively fast build.

The speed of this tri is very good. To quote some owners; A builder after his fourth sail "yesterday we achieved 9 /10 kt to windward, 16 kt broad reaching under gennaker (14.8knots over 500 m and 14.3 over a nautical mile)." And "I have now built a Richard Woods Strike 20 with Nacra 5.8 outriggers, which is much faster than my former Dick Newick Tremolino".

The jpegs are many spread over 2 items. A good simple, well designed, easier to build trimaran for fast fun.

 

[oldmulti]

Strike 20 part 2 jpegs

 

[revintage]

Got this from the Strike 20 builder, to my FB-group a few days ago. Nice boat where the 5.8 hulls are now replaced by 6.0.
https://www.facebook.com/story.php?story_fbid=26090540000623117&id=1192604934176635