Multihull Structure Thoughts

Further Arrow 40.

 

The Raku 40 is a relatively new design from Tony Grainger. It is described as a performance cruising catamaran and is 39.3 x 23.6 foot weighing 9000 to 10000 lbs at launch with engines and rig. Displacement can be a maximum of 16800 lbs. The wetted surface of the hulls 339 square foot. The mast can be a 55 foot aluminium section or a 61 foot carbon fibre wing mast depending on your performance needs. The standard rig has a 620 square foot mainsail, a 245 square foot jib, a 440 square foot code ) and a 1205 square foot asymmetric.

To quote the designer “The length to beam of the hulls varies between 12.5 : 1 to 11.8 : 1 depending on the payload being carried. As the figures show the WL beam is finer for the lighter boat. For this style of boat we avoid having the waterline any finer than about 1:13 because the higher sinkage rate limits payload and reduces wing clearance. It also increases wetted area which is a hindrance to performance in lighter conditions.” The standard design comes with daggerboards and cases with fixed keels available as an optional design feature. For kit construction the price of the materials is the same or both options.

The Raku 40 is built in composite with glass skins, PVC foam cores and epoxy resins. The hull shoes are moulded by strip planking foam glass. Everything else is built from flat foam glass panels except the forward curve in the wing deck which is kerfed, and some kerfing in parts of the cabin top. Carbon fibre reinforcing is deployed for chain plates, daggers, rudders, the longeron, forebeam and strategic reinforcements in the structure.

You can either get a kit from some manufacturers or purchase plans (including the cutting files for the panels) and manufacture or commission a supplier to manufacture you own panels. Note that the panels should be vacuum laminated or infused and you need a CNC cutter to cut the panels to shape, If you don’t have facilities to CNC the laminated panels you will need to make templates from the cutting files.

Details are available at Raku 40 performance cruising cat with inspired style and performance potential. https://www.graingerdesigns.net/cats/raku-40/

The Grainger Raku 40 and Schoinning Arrow 40 are very similar boats that will give, in cruising mode, very similar performance which will be high in both cases. These are 250 mile per day boats in the right conditions. The build time of both these boats will be similar if built from kits (may I suggest, from my experience, about the maximum size for home building). Take your choice. These cats can provide real performance and cruising capability which can be sailed short handed without the need for expensive powered winches etc.

 

Sardine Boats produces several designs. On page 3 of this thread we spoke of Sardine Twin 18 foot catamaran. But we will focus on the trimarans produced by Sardine Boats and the designer Allel Behidj. The first tri is the original 18 foot Sardine Run. It is 18 x 14 foot that can be disassembled to 8 foot wide. The weight is 600 lbs and displacement is 1100 lbs. The sail area is 220 square foot. This boat has many youtubes and sails very well. The basic boat has a dory mainhull that has a flat bottom. The hulls are built from 8 mm plywood with tapped seams. The shape of the hull and thickness of plywood provides most of the longitudinal stiffness. There are 3 bulkheads or partial bulkheads to provide shape to the main hull. This is a very fast boat to build and can just provide a single berth cabin that is 2.6 foot wide. The cross beams are 100 x 3 mm aluminium tube with waterstays. The first 2 jpegs are of the 18 foot tri and the third jpeg is of the main hull shape.

The next design was the Sardine Run 19 foot with foldable sliding beams. It is 19 x 14 foot that can be folded to 8 foot wide. The weight is 670 lbs and the displacement is 1450 lbs. The mainsail area is 183 square foot, jib is 65 square foot and gennaker is 175 square foot. This tri had a dory bottom aft and V bottom forward in the main hull. Again its built with 8 mm plywood but it is taped together with Basalt fiber (similar to e glass but slightly cheaper). The hulls exterior are also covered by basalt cloth and epoxy. The fabrics are made by Basaltex . A satin of 300gr / m² (exterior of the hull and floats) and a double bias of 450 gsm (reinforcement of seams) are used. The main partitions are made by combining basalt and carbon fibers. The resin used is Gurit SP 106 epoxy resin. Other parts will be produced in vacuum stratification (rudder frame, boom, rudder, daggerboard etc). Again the hulls are shaped by a minimum of bulkheads or partial bulkheads. The cross arms are aluminium that slide inside Basalt tubes on the main hull. The single berth cabin is 3 foot wide. The next 6 jpegs are of the Sardine Run 19.

Basalt fiber reinforcement has the following characteristics: High breaking load and high Young modulus give good rigidity and resistance (13.7% more resistant and at least 17.5% - up to 34% - more rigid than E glass) with very good wetting characteristics and UV resistance.

Finally, there was a request from a Californian guy to get a bigger version with farrier type folding beams. The Sardine Run 23 was designed. The tris is 23 x 17 foot weighing 1120 lbs and displacing 2000lbs. The mainsail area is 236 square foot, a 107 square foot and 270 square foot gennaker. The Sardine Run 23 is planned for the day or weekends in the San Francisco Bay Area and the Pacific Coast with a minimum of comfort for 3-4 people.

The construction is again plywood, epoxy, Basalt glass / carbon. The structural model used in the smaller tri’s appear to be repeated in this boat with more sophisticated cross arm structures and associated bulkheads. The final 4 jpegs give the design. Again, the main hull has a aft dory shape with a V forward section. The floats have a V bottom.

These boats are simple to build with minimum components but the 18 and 19 are both excellent performers from the video’s. As a minimal cruisers these tris would be great fun.

 

For those who requested a little more about the Sardine Run 18 trimaran.

 

Weren't the original Sardine Run 18 and Sardine Twin designed by Eric Henseval?

 

Clarkey. Yes you are correct. Eric Henseval designed the original Sardine Run 18 and Sardine twin 18 foot cat, he also did a tube 8 meter cat. The problem is that the designer for the Sardine 19 and 23 foot tri is claimed to be Allel Behidj who is associated with the company Sardine Boats. Now, I do not know where the cross over is. Eric may have done the base design for the 19 and 23 with Allel doing the cross beams etc. or refining the design for "production". Eric most certainly did the complete design for the 18 foot tri and cat. Thanks for the correction.

Also for everyone, the "AYRS meeting with Bernard Rhodes" is a 1 hour "Zoom" meeting with a "history" link available if you click into the thread. A bit of a travel tale but also includes some very good insites into sailing small (22 foot) trimarans half way around the world. Details like the need for good plywood, hatches that seal properly, some hints on self steering gear etc.

 

I really love these small tris like Sardine Run but have always been concerned that they occupy an awkward niche. Too large to self rescue in the event of a capsize but small and light enough that going over is a distinct possibility.

 

Clarkey. Yes, I also like small tri's, especially the Sardine Run 18 for its sheer simplicity and performance. It will not match an 18 foot cat (except in light airs), but it takes less effort to sail fast due to the wider beam and greater displacement. The down side is exactly as you said, if it is capsized it will take more effort to right. BUT Bernard Rhodes 22 foot ocean crossing trimaran with 14 foot beam "Kliss" was deliberately capsized and self righted by Bernard. He flooded a float and used a halyard from the mast head to a "Float" (cannot remember I think a blow up plastic canoe) to pull the mast up to the surface. He then applied some weight on a board to right "Kliss" and pump out the float. This was not fast or easy but was possible. It only works in smaller tri's where human weight/strength can counteract the forces.

The reality is for bay sailing you have to learn the limits of the tri and sail accordingly. Make sure you have very effective and easily worked reefing systems and good sail controls so that the boat can rapidly adjust to varying conditions. I have owned a small tri (24 foot) at one point, it was real fun and capable of coastal work BUT it always was hand steered when sailing fast and under rigged when cruising.

 

The next Eric Henseval boats are cats. First is the Sardine Twin which is 18 x 11.6 foot with a weight of 830 lbs and a maximum displacement of about 2000 lbs. This boat is basically 2 of the main hulls of the Sardine Run 18 trimaran with a slight deepening of the aft part of the hull to carry the crew weight further aft. The main is 201square foot, solent jib of 53 square foot and a Gennaker 177 square foot. The hull length to beam at the DWL is 11:1. The cat literally has twice the accommodation of the trimaran. The sardine twin is a stich-and-glue plywood construction, a very simple way to build your home-built craft. The hull and deck is plywood 8mm thick - glass - epoxy resin with no stringers and a few bulkheads supporting the bolted aluminium tubes beams and provide a watertight forward bulkhead. The mast could be a second hand one, if taken from an old Hobie 18 or equivalent. The 2 centerboards are sliding in their trunk, where trunks are integrated in the outside hull shell plating, to gain volume for accommodation. the first 6 jpegs are the Sardine Twin 18.

The next design is the 8 meter catamaran which is 26.25 x 15.75 foot tube cat that has an estimated displacement of 3500 lbs. The sail area is unknown. The hulls have a 14:1 length to beam at the DWL. The 2.1 foot draft is achieved with a low aspect ratio keel. The hulls are 3.75 foot wide at gunnel and have 5.9 foot headroom. This boat has real accommodation and could be used as a very effective coastal cruiser with good performance. I do not know if this design was built but it would be a simple build being flat panel plywood. Again, a simple dory (flat) bottom plywood construction with taped seams, using furniture, bunks and bulkheads as stiffeners. The hull shape is the main fore aft stiffener. Glass taped seams and a glass covered exterior shell. The cross beams appear to be aluminium.

Finally Sardine boats have designed their own version of a 8 meter (26 foot) catamaran with a full bridge deck cabin. I do not know any other details but I show in the final 3 jpegs the design proposal.

 

Old Multi I am curious on you thoughts to the following:

Why do cats always gain volume as they get longer (even Schionning and Grainger) add height/ beam to the hulls
Can a 40-45' cat keep the same bridgedeck volume and hull beam, but lengthen the hulls. (similar to the oram 60 Ciao)
How much extra time to build does 2-3 meter extra hull length add.

I am a fan of Kurt Hughes cats like this one Kurt Hughes Multihull Design - Catamarans and Trimarans for Cruising and Charter - 48' Day Charter Catamaran https://multihulldesigns.com/designs_other/48chcat.htm

I would like to stretch the hulls and keep the topsides to a minimum ( 1.95m to stand in hull)

Do the loads go up significantly for just hull length?

 

Slingshot. I have to be careful here. If you are adding say 10% (about maximum) extra length to a cat hull and change nothing else from the original design such as beam or freeboard etc it is a cheap, easy to build addition that MAY slightly improve performance. If you expand everything (eg increase beam, freeboard, hull beam, rig length etc) you need to talk to a designer. Even 10 % additions in all directions will add extra weight, drag, impact on the structure strength etc. If you require a good answer to this, please speak to Kurt Hughes. He is approachable and is asked questions like this occasionally. If the boat is going to be used for chartering, authorities often require sign off for any variations to the original boat plan.

 

On page 46 of this thread I did a mathematical exercise on several smaller cats’ stability. I am going to repeat this on smaller tri’s and cats vs eg Hobie Tiger 18 foot day cat. Unfortunately, this is a mathematical exercise to highlight the theoretical differences and does not take into account the skill of the sailor or the flexibility of the rig. Some sailors can sail a multi that should overturn in 20 knot winds in 25 knots due to their skill to feather the rig at the right time etc. If you have a flexible rig with eg a fat head main, the top of the main can be made to twist off in stronger gusts etc. Also wave action is not included in this calculation. Just be aware according to the things I have read that once the wave height is greater than the beam of the multi, you have a problem, especially if you have under 100 % buoyancy of tri floats.

For demonstration purposes we will compare the Hobie Tiger (18 x 8.5 foot, displace 925 lbs, 235 square foot) with 2 people on trapeze for stability calculations, Sardine Run 18 tri (18 x 14 foot, displace 1100 lbs, 220 square foot), Sardine Twin 18 catamaran (18 x 11.6 foot beam, displace 2000 lbs, 255 square foot), Kliss trimaran (22 x 14 foot, displace 2200 lbs, 340 square foot) and the Eric Henseval 8 meter catamaran (26.3 x 15.75 foot, displace 3500 lbs, 400 square foot sail area). Now all of these boats have different displacements and sail areas so I will add a final feature which is a performance calculation of a Bruce Number and “average speed over 24 hour” calculation based around Richard Bohemers work.

The Hobie Tiger has an 8.5 ft beam 925 lbs displacement and can handle a maximum of 16.35 knots wind speed. Its Bruce number is 1.6 and can average 8.9 knots over 24 hours.

The Sardine Run 18 foot trimaran has a 14 foot beam 1100 lbs displacement and can handle a maximum of 22.5 knots wind speed. Its Bruce number is 1.43 and can average 8.3 knots over 24 hours.

The Sardine Twin 18 foot catamaran has an 11.6 foot beam 2000 lbs displacement and can handle a maximum of 22 knots wind speed. Its Bruce number is 1.26 and can average 7.6 knots over 24 hours.

Kliss 22 foot tri has 14 foot beam has a 2200 displacement can handle a 23.5 knot wind speed. Its Bruce number is 1.4 and could average 8.5 knots over 24 hours. (Its longest 24 hour run was 180 miles plus)

The 8 meter cat has a 15.75 foot beam and 3500 lbs displacement and can handle a maximum of 24.5 knots wind speed. Its Bruce number is 1.32 and can average 9.2 knots over 24 hours.

Summary. The Hobie Tiger is the fastest boat but it will capsize in the lowest wind speed of 16.3 knots.

The Sardine Run 19 tri is the second fastest in day sailing and can sail in winds up to 22.5 knots without reefing. To give perspective about Sardine Run’s capability, Kliss an ocean crossing tri which is heavier but has more sail area is relatively fast in most day sailing but can sail in winds to 23.5 knots without reefing. There is little difference between these 2 boats in real sailing capability. Kliss has more accommodation and is designed to be an ocean crosser but Sardine Run 18 is more modern, easier to build and has better hull shapes.

The Sardine Twins 18 catamaran is slower than the Sardine run 18 tri but due to its increased displacement has about the same stability as Sardine Run 18 and Kliss. The Henseval 8 meter is larger, longer waterline, heavier cat with a larger rig that can handle 24.5 knots wind speed before reefing. It also has the fastest long distance speed of all the boats here. Once you have a boat that can handle 25 knots without reefing you have a boat that can handle the majority of sailing you will do.

So, the bigger the boat in all dimensions the safer (assuming its well built) and with an appropriate amount of sail to the displacement helps to maintain your speed. The lighter the boat for its length the faster it will go if it has reasonable hull shapes. In short, for Clarkey, the Sardine Run 18 tri is a very good boat for its size and is a lot more stable than an 18 foot day cat, but if you want to cross oceans the 22 foot Kliss is very impressive for its age and size.

 

Thanks for your thoughts.

I am in the camp of more length and less volume. I have been on a Hughes 60' and it was massive. I don't need that volume, but I would like the water line. I am not ready to pull the trigger, so I have held off phoning Kurt. What are your thoughts on his flat panel one side infusion table then conform to shape and strip foam the hull bottom. It seems like a good balance between speed and cost.

One more question. Is flat table infusion process like Kurt's smooth enough for a vinyl wrap or is fairing still required?

 

Slingshot. I like your logic more length less volume can produce a fast light boat with an average size rig. The Gudgeon 32 is a smaller example of what can be done but its at the 'edgy end' of the scale. Hughes flat table infusion panels and strip plank foam glass bottom is similar to the grainger approach etc. The method is much faster than the male mould, then foam, then outside glass then fair model. The surface finish on the infused panels is really up to the quality of the flat surface and wax job you have as the initial mold base. If it is done well the surface finish on the panels will be very good and could be vinyl wrapped. If not done well a minimum of finishing should bring the surface up to the required finish.

 

Allel Behidj is actually the builder, founder CEO of Sardine boats. About him, in french :
https://www.ouest-france.fr/pays-de...antier-naval-mais-grandes-innovations-4074580
More photos about the Sardine Run 19 construction :
Sardine Run 19 https://www.sardineboats.com/voiliers/sardine-run-19/
Eric Henseval, naval architect, previously collaborator within VPLP. His multihulls designs :
Multicoques https://hensevalyachtdesign.jimdofree.com/multicoques/