Multihull Structure Thoughts

In 2025 Astus, the French small trimaran company, plans to release its largest production sailing cruising trimaran. The Astus 26.5 has been designed by VPLP and a prototype will be in the water late 2024. The Astus 26.5 preliminary numbers are 25.8 x 18 foot with a weight of 1900 lbs (estimate). The 32.5 foot aluminium or carbon fibre mast will carry about 415 square foot main and jib with a 400 square foot Code 0 (estimate). The draft over the daggerboard will be 6.5 foot. The outboard engine will be 5 to 10 HP. The tri will be trailable with sliding aluminium beams to reduce the tri to trailering width.

Because of Astus DNA they like there tris to be comfortable and fast, so using VPLP will help ensure that the performance will be very good for what is claimed to be a 4 berth tri. The structure of the tri should also be well done due to Astus build experience and VPLP design experience. The structure will be resin infused foam glass for the hulls and deck with aluminium crossbeams.

The accommodation is described as: “The Astus 26.5 will enable cruising with 4 berths, and even the possibility of standing at the cabin entrance. The forward cabin can be separated with its own toilet area. In the saloon, there's a galley and an icebox”. The cockpit will seat 4 comfortably.

The tri will come in 3 versions: “A leisure version for quiet family cruising. A cruise version, to see further afield. A sporty version to satisfy racers.”

The initial jpegs will give you the idea of an interesting development.

 

To give people an understanding of Astus boats that are already produced I will do an update on the Astus 22.5 trimaran is a trailable sports tri that has limited accommodation. The tri is built to CE certification class (C) and can go along the coastline, on rivers and canals. The designers are VPLP design. This is an extension of the Astus trimaran family. The tri is 22.9 x 16.1 foot which can be folded to 8.2 foot. The weight is 1,460 lbs. The aluminium mast is 29.5 foot carries a 225 square foot mainsail, a 97 square foot jib and a 345 square foot gennaker. The draft ranges from 1 foot to 4.6 foot over the main hull centreboard and rudder. There is optional float based C foils available for those who want higher performance. Another performance option is a carbon rig with textile shrouds. The outboard power is 5 HP.

The accommodation is simple. The main cabin is lit by the side window of the roof and the deck hatch in front of the mast foot. The 4 berths are 6.5 foot long. The 2 quarter berths at the rear are 1.5 foot wide, while the forward is a 6.6 foot long double berth which is 4.6 foot wide at the head and 2.7 foot at the foot. A chemical toilet is located under the front of the double bunk with a small hatch overhead. The kitchen furniture is modular and removable. There is sitting headroom of 5.3 foot.

The performance is good. A test found “The boat accelerates quickly to reach 5 knots upwind, despite a light wind. With the wind stabilizing around 10 knots, without exceeding 12/13 knots, the Astus 22.5 maintains a speed upwind between 6.5 and 7 knots. As the wind remains light, we not reach double digit speeds and will not make long surfs. However, the meager 12-knot reefs allowed us to reach 9 knots with a crew of 3 adults.” Astus says the tri is capable of speeds up to 20 knots. A figure I could believe with C foils and an upgraded rig. Another test found; “In the fresher spells we maintained closer to 12 knots for good periods and peaked at 14.8. All the time the boat felt perfectly comfortable. With 1,150 litres of buoyancy in each float, together with all that beam and a relatively modest sail plan – even given the square-top mainsail and tri-radial sails on our test boat – we never felt the need to de-power.”

To give a feel for stability the suggested reefing is Reef 1 in at 15-20 knots TWS, Reef 2 in at 20-25 knots TWS and do Reef 3 in at 25-30 knots TWS. To quote an expert about downwind sailing: “The real risk to multihulls comes from waves, rather than wind strength, though clearly the two are linked. Because the hulls are narrow, and speeds are high, burying your bows in the back of a waves is the thing most likely to cause a capsize, as the boat will slow suddenly and you’ll be subject to the full force of the true wind.”

The build is listed as “polyester resin infused fiberglass” with stiffening members inside the hull. The floats are claimed to have a single solid skin due to the curved surface. The main hull is probably has foam glass areas. The beams are aluminium and slide in fiberglass channels. The Astus 22.5 takes about an hour to assemble or disassemble for trailing.

The jpegs tell the story. A good effective boat.

 

I will start this with a quote from the firms website: “We have created a hybrid concept which is a real alternative between monohull and multihull. And we have chosen construction methods that are more respectful of the environment than those of shipyards producing “polyester” hulls, which are difficult to recycle. Indeed, the choice to build the hull and all the fittings in plywood (CP) and to use a resin that is both high-performance and bio-sourced (Greenpoxy from Sicomin), reduces our ecological impact, with a carbon footprint of ten times less than that of a polyester sailboat of the same size.”

Interesting statement. The Sailscow 37 is designed by Architect: Gildas Plessis and is a French shipyard managed by Jean-Michel Linck. The Sailscow 37 is 35.4 x 12.5 foot with a weight of 8,500 lbs. The carbon mast is 51 foot high and carries a 527 square foot mainsail, a 505 square foot jib and a 613 square foot genoa. There are other sail options as you provide the sails. The keel is either a fixed keel of 6.3 foot or a lifting keel which starts at 3.3 foot draft to 7.9 foot when fully down. The engine is a 20 HP inboard.

Now we get to the hull shape. The architect Gildas Plessis has done scow bowed boats before including a 29 foot aluminium production boat and several custom designs including some racing Mini 6.5 and Class 40 boats. The very wide bow is not really round, but looks more like the bow of an Optimist. "The Sailscow 37 will be light. But not too light, because you also need to keep the weight to go through the waves without stopping. The volumes in the bow do not require building a sailboat that is too wide. The Sail SCOW 37 will be less wide than the competition.”

Now for the elephant in the room, the architect Gildas Plessis does not hide the fact that "it will be difficult to sail well in short choppy seas upwind. But when cruising, we avoid this kind of condition if we can. Otherwise, in all other points of sail the gain will be enormous. The Sailscow 37 will still sail above 10 knots with no problem."

The accommodation options include a four cabin layout, with two doubles forward, both with rectangular beds, while aft there’s a further double, plus a twin cabin with bunk beds. Alternatively, there’s space for a giant owner’s cabin forward, plus one aft double port and a generous technical and stowage area to starboard. The main cabin has the galley and seating for 8 people. The cockpit is large and open with good side decks and a very wide bow area for sail handling etc. This boat has very good ergonomics for people to live on and move around.

The construction is basically plywood, fiberglass cloth in a sandwich with green epoxy The reason for non polyester construction “officially” is the problem of recycling old boats. Polyester foam glass boats are very hard to dispose of when their service life is complete. I suspect a stronger reason is this boat is available as a kit for home builders. There are other options of partially complete boats through to renting a fully made up boat that is looked after by the yard for about $US50 per hour. The French have no problems with production plywood boats that are well saturated in epoxy. They have a proven long lasting track record and the French do maintain their boats. The kit materials to build a Sailscow 37 consists of the following structural elements, digitally cut (CP means plywood):

Hull frame: 19 mm chipboard (bulkhead support)

Partitions and structures: CP of 12 and 15 mm for the hull

Planking: CP of 12, 15 and 18 mm for the hull

Bow: CP of 5, 8, 10, 12, and 15 mm

Bridge: 12mm CP

Roof: CP of 15 and 18 mm

Now we add confusion. Another source said of a production version of the Sailscow 37 structure is and I quote “the Gildas Plessis plan is also intended to be lighter than other cruise-oriented scows thanks to its construction carried out by assembling panels of glass/foam sandwich composite, lined with a thin plywood panel on the interior side” and lists the structure as Material: sand. glass/CP/PVC/epoxy. It also said “The plywood that appears constitutes the interior skin of the Sailscow. It will receive a 28 mm Airex type foam, then an exterior fiberglass skin (epoxy resin)”. What I suspect is happening is the kit boats are basically plywood and the production boats are semi foam glass epoxy resin.

Finally, we get to performance, in light airs a big rig is required to get over the large wetted surface of a wide flat hull but when the Sailscow 37 gets some wind and heels it is the equivalent of sailing on one hull of a catamaran and the design develops a lot of righting moment and has in effect a narrow “hull” to sail on. This is good reaching and running. Upwind things are little more difficult. In light airs with some seaway it is a problem, but once the wind fills in you can either power through or drop off a few degrees and foot a lot faster to get to the same point. There are no serious sailing reports I have found yet but the successful performance of the monohull Class40 scows in ocean racing show these boats can sail very well on most points of sail.

Tomorrow we will see some other models from this company but these are interesting “hybrids”. The jpegs give part of the idea.

 

Yesterday we spoke of the Sailscow 37 which was the first build by Sailscow. There was immediate request for other Sailscows. Next came the Sailscow 40 which is more cruising oriented with more internal space and features like a covered cockpit etc. The Scowsail 40 is 40 x 13.6 foot with a weight of 12,540 lbs and a loaded displacement of 15,700 lbs. The 54 foot carbon fibre mast carries a mainsail of 475 square foot, a 225 square foot staysail and a 452 square foot genoa. The draft ranges from 4 foot and 8 foot over the daggerboard keel. The motor can be 30 to 50 HP inboard.

The 40 footer is wider, heavier and carries a larger rig. The interior is similar in layout to the Sailscow 37 but has more elbow room in all areas. The intention of this boat is to be a true cruiser more than a racer. As per usual the next client wanted a slightly larger version of the Sailscow 40.

Result The Sailscow 42 was born. This is a serious long term fast cruiser with space to spare. Again, a similar layout to the 37 and 40 but with a lot more space in each area. Each double bunk cabin has its own ensuite in the 42 footer. The main saloon on the Sailscow 42 has seating for 8 to 12 people and a large galley and navigation area. The forward part of the cockpit is covered for heavy weather or very sunny sailing.

The Sailscow 42 is 41.3 x 14.6 foot with a weight of 15,700 lbs. The 60 foot carbon mast carries 1,000 square foot of sail upwind and up to 2,420 square foot of sail downwind. The draft ranges from 4 foot and 8 foot over the daggerboard keel. The motor can be 30 to 50 HP inboard. The construction of the Sailscow 42 glass/foam/epoxy sandwich, no mention of plywood. As this is a new design we may find the boat in its final format may have EG plywood bulkheads etc.

As with most of these scow designs they probably will have low ballast ratio’s as the box like hull design have more form stability requiring less keel weight for overall stability. The design balance of form stability to keel weight requires experience especially in a larger cruiser model. You will notice that the Sailscow 37 has a 2 foot narrower beam, is half the weight and carries a 10 foot shorter mast than the Sailscow 42. For boats of 5 foot in length apart this is a significant difference. The Sailscow 42 will be a more comfortable boat in a seaway, have more internal space but I do not think it will be significantly faster than the Sailscow 37.

Now we get to a very interesting option. Further clients came saying they wanted the concept but in a less expensive, smaller package. Architect Gildas Plessis who has designed 29 foot boats (and all the other Sailscows) of a similar types again designed a 28 foot Sailscow 28 for the company. This is more a fast cruiser racer with good internal accommodation. The Sailscow 28 is 28 x 10.3 foot with a weight of 6,700 lbs and a displacement of 9,000 lbs. The 41 foot carbon mast carries 785 square foot upwind and a maximum of 1,450 square foot downwind. The draft varies according to the keel option chosen. The long fixed keel draws 7.9 foot, the short fixed keel draws 6.2 foot or you can have a lifting daggerboard keel. The engine can be a 10 to 20 HP inboard or outboard. The build can be a plywood kit with fiberglass and epoxy over to a full production build.

The 28 footer internal accommodation has again 3 double berths, a single separate toilet shower and a main saloon with good seating, a smaller galley and a navigation area. As with all the scow designs there is plenty of deck space available for sail handling forward and cockpit areas.

The jpegs give some diagrams of these boats. All of the models have the capability of sailing well but will surprise many on their reaching and running capabilities. The scow mini 6.5 can do 10 knot averages over a 1,000 miles. The Class 40 monohull scows can average above 10 knots on long distance legs therefore I see no problems with 8 knot cruising averages for the larger models. The 28 footer will very much depend on the talents of the crew but if they can drive hard it will see mid teen peak speeds and up to 10 knot averages over shorter distances. A group of fun boats that could be a half way version of a multihull.

The last 4 jpegs are of the Sailscow 37 to give a feel for the shape and build.

 

Well you know what they say.
“Gentlemen don’t sail to windward”

 

Good god no ! You might spill the gin & tonic !
Lord knows what would become of the cucumber sandwiches ?!

 

Here is an even more radical design than modern scows: something between a scow and a cat.

Viper 550 by Wojtek Wierzbowski

krzys

 

krzys. Have you any more detail on the Viper 550. It looks like a concept I saw drawn by an Australian guy who did not build a boat. I be interested to know how the Viper 550 sailed. Did the dual bows make any difference going upwind?

 

The following is about a performance catamaran that has just done a Transatlantic Race where the co skipper said: “Broad reaching here we were doing from a slow 18 knots to a top speed of 28 knots” During another race he said “the boat weighs over 30 tons but we still keep sailing at over 30 knots with a peak of 32 knots” The skipper also noticed, sitting in the main cabin, the bows were being pushed down near the water, he went out to see what was going on and the crew on deck were quite calm as the cat was sailing normally above 20 knots. OK what is this performance cruising catamaran?

“Allegra” is a APC78 built by Green Marine in 2014, the exterior design and structure was done by Nigel Irens with the interior design done Nauta Design. It has 4 double bunk cabins and occasionally is used for charter work during the year and its peak speed reached under sail is 35.5 knots. The Swiss owner is happy with its performance as Allegro finds 100 foot racing monohulls like “Rambler” good boat for boat to boat competition around a course and Mod 70 tris good competition in some wind strengths.

Before we get into Allegra dimensions a little history. Gunboat between 2012 and 2016 developed and built several Gunboat 78’s. Allegra and a Gunboat 78 are very similar. A Gunboat 78 is 78 x 33.25 foot with a weight of 56,000 lbs and has a displacement of 65,000 lbs The carbon mast is 102 foot high and carries a 2,570 square foot mainsail, a 408 square foot staysail, a 807 square foot jib, a 1,400 square foot genoa and a 3,464 foot asymmetric spinnaker. The draft ranges from 2.5 foot over the hulls to 11.4 foot over the daggerboards down. The underwing clearance is 3.9 foot. The 2 x Yanmar model 6BY3 engine are 160.00 HP each.

The build of the Gunboat 78 initially was composite construction of vacuum bagged epoxy foam sandwich and Aramid honeycomb Nomex with carbon fibre inner skins and Kevlar outer skins for impact resistance.

Now we get to Allegra. Allegra is 78 x 31.25 foot (listed as 84 foot long including the prodder) and has a sailing displacement of 76,000 lbs with crew. The rotating carbon mast (102 foot) and boom are from Future Fibres with carbon fibre rod rigging initially carried a 2,570 square foot mainsail, a 408 square foot self tacking staysail, a 807 square foot jib, a 1,400 square foot genoa and a 3,464 foot asymmetric spinnaker. The draft ranges from 2.5 foot over the hulls, 6 foot over the rudders to 14.76 foot over the daggerboards down. The underwing clearance is 3.9 foot. The 2 x Yanmar model 6BY3 engine are 160.00 HP each. Allegra has had an upgrade to its sail plan (do not know the details) and its foils to increase its speed. Allegra also has ballast water tanks in the sterns to improve performance when racing down wind.

The build of Allegra is composite construction of vacuum bagged epoxy foam sandwich and Aramid honeycomb Nomex with carbon fibre inner and outer skins very with limited Kevlar. Jpegs of Allegra and the Gunboat 78 show minimal differences in appearance and detail. The only line of importance is the term “custom beam” in Allegra which is 2 foot narrower than the Gunboat 78.

The jpegs give the accommodation detail of the cats but Allegra has comfortable cruising accommodation for 8 guests and several crew. Allegra is not a stripped out racer but has genuine cruising accommodation. There is a forward cockpit for sail handling at the mast and in the aft cockpit 2 steering positions near the outer gunnels for helming and some protection. But racing big boats like Allegra does have some interesting down sides.

The computer system on Allegra plots the optimum sailing angles and speeds to reach a given mark, you are rated by the system as to what percentage you are achieving the optimum VMG. If you are at 98% good, if you are at 70% you are changed for another helmsman. It doesn’t matter if you are sailing fast, if you are not reaching the optimum VMG numbers you are in trouble. Next there are in effect 2 crews when racing. The sailing crew doing the sail setups and helming and the engineers below decks. The engineers have to keep all the electrics, hydraulics, computers, routing information feeds, sensors, battery condition information, generators, ensuring power winches are functioning etc. The engineers in transatlantic races are working as hard as the sailing crew as there are always water sensors, engine alarms, rigging load sensors going off when Allegra is being pushed hard in a big seaway. Also, there is a small side issue, if you are going to race seriously you need a few family and a super star crew who don’t come cheap.

The result of all of this Allegra has won the around Gotland Runt race. It has also won the Morca multihull division of the Fastnet in 2021 and 2023 plus many others. This cat is not only seriously fast but can win under handicap as well.

And the final word from Paul Larsen co skipper: "Allegra is constantly improving and being upgraded. She has a new set of sails which are an improvement. The boat is sailed very well and cleanly and we are getting to the point where we're only making small gains." EG, Allegra's centreboards now can be raised flush with the hull and they have removed weight by only using day tanks. Those believing Allegra may be a stripped-out cruising boat, will need to reappraise. Responding to this Larsen usually invites people to inspect the fully stocked 'sauce drawer' in Allegra's well-appointed galley. "While you are surfing down waves at 30 knots, you are literally sitting on a couch looking at it through a window - a pretty edgy couch admittedly! It is like watching a tornado about to envelope your house!" He was talking about this as he was sailing in 39 knots true wind speed with a staysail and one reef in the main. We are talking 50 knots plus apparent wind with one reef in the main. The stability of this cat and the strength of the rig is amazing but the result is thinking 30 knots boat speed is normal.

The jpegs give the idea of Allegra and there are a few of the Gunboat 78. Also have a look at this youtube which shows the run accross the Atlantic: .

 

Yann Quenet is a prolific designer of small simple boats ranging from dinghies to small monohull cruisers (16 to about 28 foot) and a few multihulls. He does them mainly in plywood and in simple chine shapes. His 16 foot CatKedac coastal cruising cat is a good example of simplicity, cruise ability and an easy affordable build. The next multihull design he did was a camp cruising 18 foot tacking outrigger that comes in 2 versions of rig. One rig is a classic mainsail and mizzen which could be more expensive to make or a standing lug rig which may be cheaper and easier to make. The interesting thing in the lug rig is the main mast is on the accommodation hull but the mizzen is on the float hull. The offset lug rig may provide a better cockpit space but the mizzen will be in disturbed air on some points of sail.

The Bit and Kontel is 18 x 12.5 foot and displaces about 1200 lbs. The conventional rig has a 25 foot mast with a mainsail (foresail) is about 120 square foot, the 16 foot mizzen mast carries about 40 square foot. The lug rigged model would have about the same sail areas but shorter masts. All masts are freestanding and I would suggest either wooden or some used wind surfer masts for mizzens or yards. The length to beam on accommodation hull is 7 to 1. The draft over the float based daggerboard is 2.7 foot. This boat is small enough to be rowed or use a 5 HP outboard for motive power. The proa is transportable as all components are relative small allowing disassembly or reassembly.

The accommodation is 2 bunks in the accommodation hull and storage in the float hull. This is a camp cruiser at best, but can provide some bunks that are well protected for the occasional rainy night.

The construction is not specified in detail but I would suggest 6 mm plywood for the hull skins with timber framing and likely to have taped seams at the chines. Cross beams are either timber or aluminium beams depending on material availability. The cross arms and hulls are connected by rope tie down as in Wharram cats.

I do like the kayak storage which also acts as a “safety ama” if capsize is likely. The kayak may not provide much buoyancy if it doesn’t have enclosed ends but it will provide a few seconds of stability to allow you to loosen sheets etc to prevent a capsize.

An interesting design. The jpegs give the idea of the concept.

 

Colin’s 33 ft Sunyacht Fremantle seems to have a low bridge deck clearance, any thoughts.

 

Wojtek began to think about her in late 1990s, built in early 2000s. He chose the size of 5.5m to be easily compared with popular Micro class boats. However, as far as I know, Viper naver took part in any Micro race (by the way, formally Micro class rules do not ban multihulls, but nobody ever designed a Micro cat). Wojtek seemed to be glad of the Viper's performance, especially on engine. On the other hand, I never heard about any performance measurement under sails nor comparison with other boats. She was for sale in 2011, that's the last information about her. Wojtek passed away in 2017...

krzys

 

More Designs of Yann: plan de catamaran https://www.boat-et-koad.com/plan-de-mulitcoques.php Does not look like a trimaran -- it is more like this:

 

A small item about the hull shape developments in the F18 class. Up to 2021 the F18 class had fairly conventional hulls with a length to beam of about 19 to 1, round bilge hulls with a vertical bow, fine bow and wide stern. In 2021 a new F18 developed by Cirrus was created, the Cirrus 901 that meet the class rules but had a much flatter hull shape which showed some interesting results. But first we will describe the F18 class rules.

An F18 is a day racing catamaran that prohibits foiling. The cat is 18 x 8.5 foot with a boat weight of a minimum of 403 lbs. The minimum crew weight of 2 people is 257 lbs with weight correctors of half the weight between the crew weight to the assumed crew weight of 336 lbs. The minimum corrected crew weight is 297 lbs for an all up including crew minimum displacement of 700 lbs. The 29.8 foot mast must have a maximum circumference 385 mm and a minimum of 4 mm stainless steel 1 x19 wire. The mainsail is 183 square foot, the jib 46 square foot and a 226 square foot gennaker. All sails are to be from soft cloth with battens allowed but no solid sails or full wing masts. The length to beam is about 19 to 1. The maximum draft is 4.6 foot of draft from the 1 rudder and 1 daggerboard per hull.

The hull shells may be built from epoxy, polyester or vinylester resin, wood, injected plastic, glass fibre, glue, gel coat, paint and/or metal fastenings. A core of PVC or balsa or felt may be used. The beams shall be made of extruded aluminium profiles of constant section.

Now back to the Cirrus 901. Cirrus produced “conventional” hull shapes up to 2020, but during Covid the designer Matthieu Marfaing had an opportunity to rethink an F18 hull and its total drag. He choose to go down the flat stern and flat mid section with a T-bow (triangle bow) flat on the bottom and cut away at the deck. According to the designer the total drag of the hull across the performance range was lower. In EG light conditions there may be more drag but that was offset by less drag in higher wind conditions were the Cirrus 901 was partially “planning”.

The designer’s comments: “Is clear a flatter bottom can generate additional lift at semi planning conditions. The T-Bow (triangle hull) is a new design. Its shape is original for its passage through waves and its lateral grip capacity. The very shallow draft gives it a lot of mobility (tacking ability) and the ability to start quickly. Some Work on flat hulls has been done in the past, but are very different from our hull design. The resistance to pitch pole is extreme! Absolutely no comparison with a classic F18. This is probably the safer f18 on the market regarding the risk of pitch pole.” And also, about the development of the design: “3 prototypes and 2 tools were produced to validate the performance of the new 901. We also sailed more than 50 hours with each prototype in variable conditions from 5 to 35 knots to validate on the one hand the performance but also the validation of the construction choices.”

The great thing about the F18 is you can theorise and design, but it is on the race course that you get the proof of any design improvements. From what I can find the conventional hulls are still winning but a Cirrus 901 came 4th in a major series in early 2o24 with some guy called Mark Melvin as crew (don’t know if associated with Pete Melvin but the cat was sponsored by MM Melvin and Morrelli). So, the shape has some potential.

The jpegs give the idea. The first jpeg is of the previous model the Cirrus R which was Cirrus “conventional” hull shape. The Cirrus 901 are the following jpegs. No performance numbers as this is boat for boat racing but they are fast in all directions.

 

I checked out a Colin Ayres Imp in Perth many years ago.
Not so much low clearance, more like NO clearance.

A cosy houseboat on flat water, bound to be a slow slamming horror at sea.