Multihull Structure Thoughts

Outremer

 

A little fun. The following 2 documents are ratings for multihulls. In both cases they are calculated handicaps based on some design criteria with adjusted variations based on race results. They are indicative of the boats characteristics with a slight adjustment for the skippers/crew skills. The word document comes from he Queensland Multihull yacht club OMR ratings as of 2016 (updated to current). It contains current racing boats and older boats no longer racing. The higher the number on the right the faster the boat is. It contains a few surprises. EG a Mod 70 ft racing tri rates at 1.426, a ORMA 60 tri rates at 1.476. A foiling capable DNA TF 10 (10 mtr trimaran) rates at 1.051. A Weta 4.4 mtr day sailing tri rates 0.807. For comparison a Seacart 30 ft rates at 1.050. A Prout Snowgoose 37 0.628. Please study the documents as they give a realistic assessment of relative performances of many boat designs EG the farrier tris range from OK performance to some models like the 28 R that can really perform. Have fun. The original excel rating sheet for QMYC is available on their web site. It contains many basic dimensions such as weight beam sails etc. 12 hour later edit. The dutch 1991 rating sheet ratings are on the right hand column in this case its the higher rating is the slower boat. The fastest boat has the low number EG Duo rates 98 fast. Apache rates 148 slow. The QMYC ratings are fast is the high number slow is the low number.

 

Great piece overall catsketcher.

Alternatively you can just build the structure strong enough ( WITH IN REASON ) to handle the loads.

We build hulls strong enough to cater for the loads exerted by unstayed mast on our decks, like wise sampson posts.
We don't expect ether of these to tear holes in our boats .
There has been a trend in recent times to ignore what engineering tells us and build underspeced rudder shafts.
The debate should be over. Enough first hand evidence exists.

A well known, highly respected multihull designer said he would have speced 2 1/2 inch solid SS bar for the stocks, after the BE GOOD TOO incident.
How on earth did someone thinks 1 1/2 " was going to cut the mustard.

 

Sadly the rule does not take hull shape into account. So other than confirming light with a cloud of sail equates to fast (and dangerous) it tells us very little about how the boat, most of us here would own, will perform loaded with water, food and toys (even if there was enough room to stow it) for a 3 month short handed cruise.

 

Expeditionary is a Harryproa designed in 2011 for European raids. The 35 foot x 20 foot proa is being discussed due to its empty weight of 800 lbs. With an additional crew rig etc weight of 400 lbs the total displacement is 1200 lbs. The rig is a 2 mast schooner carrying 514 square foot of sail. This gives a Bruce number of 2.1 (very fast). Lee hull 10.5m/35′ long x 450mm/18″ waterline beam (23:1 L:B) and a prismatic coefficient of 0.75. This is very high, but proas don’t have to tack, so their hulls can be optimised for speed rather than for turning ability. Rob Denneys words from here.

“Empty weight: 360 kgs/794 lbs. This may seem light, but Elementarry (25 foot) weighs 120 kgs/265 lbs, the 15m/50′ short handed racer we are currently building is on target for 500 kgs/1,100 lbs All these weights were accurately predicted, so I am confident 360kgs is achievable. CONSTRUCTION

The entire boat, (masts, booms, beams, rudders, hulls) is built using flat panels, bent into shape with tailored male/female joins on the edges. Where required, additional laminate is added to the bent shape. For minimum weight, the panels would be vacuum infused foam and glass (plus carbon for the mast and beams) on a full length table, but could also be done on a smaller table. The hulls could also be plywood. Either way, there are no building frames required for the hulls or beams (hence no table of offsets) as they are self aligning around the bulkheads. We have built 2 x 15m/50′ infused flat panel proas in the last 12 months and the system works well. The panels include all necessary strengthening/stiffening, bulkhead landings, mast and gin pole steps, deck bearings, beam hinges, hatches and hatch rebates, reinforcing for the beams and masts and simple, self aligning male/female joints for the panels. With infusion, it is possible to do all this on the panel, with millimetre (0.04″) accuracy. There is no secondary laminating and all glued components are male/female joints, so it is very simple to build. The panels are all fair and the joins all rebated, so there is no fairing, apart from the 200mm/8″ foam/glass pieces on each bow. Peel ply on all areas to be glued means no sanding is required. The base laminate is 400 gsm double bias either side of 12mm H80 foam (12 ounce either side of 0.5″ H80 foam), with 100mm/4″ wide strips of unidirectional glass where required and an extra layer of db between the beams. The localised strengthening of the beam sockets and mast bearings is heavy (70k) carbon tow, included in the infusion.

The masts, beams and rudders are uni carbon with glass for the off axis loads. The glass adds a little weight, but saves a lot of money. In ply, they would be 10mm/3/8″ lee hull, 6mm/1/4″ windward hull with a couple more frames, stringers along the panel edges and 200 gsm glass cloth on the outside. The hulls would have smaller deck and chine radii The beams, rig and rudders would still be carbon composite.”

 

The T 29 tri is 29 x 21 foot and carries 570 square foot in main and genoa in a conventional fractional rig. Weight is 2760 lbs. The reason for the interest in this boat is the second T 29 had a reefable soft wing mast and sail placed on it. It was created by the team of wing designer from France (Hugues de Turckheim), wing camber builder from Switzerland (MB Composite), platform designer from Sweden (Stefan Törnblom), wing envelope sail maker from Sweden (Mats Johansson, Gransegel), and builder from Hungary (Pauger Composite), this quick 1200 kg all carbon is the T29 trimaran is the first production multihull to be equipped with a hoist-able and reef-able wing. Maximal sheeting load on the main is greatly reduced compared to a conventional wing mast rig, from around 1600 to 80 kg, thanks to the balanced wing and the boat tacks through 70 degrees with unusual ease for a multihull. More about the wing from the technical consultant on the T 29, T 30 (same boat with 8 inches more beam) project.

“ Here is some information. T29 was the prototype, non folding version, of which two have been built. One of these has a conventional rotating carbon wing mast rig and sails in Brittany, France. The other, which is mine serves as a platform for development of a soft wing sail, now in its 7th generation, presented in Seahorse magazine august edition? This sail is still not ready to hit a larger market. The 7th generation this year comprises a number of simplifications, such as getting rid of the boom, removing parts of the elements in the camber system and taking the control lines to the outside of the wing to create more power than what was previously possible. We can now produce about 400 mm of displacement between the windward and leeward parts of the wing envelope which is about the maximum of what is required to produce sufficient power at open wind angles. Also, the nose of the wing can easily be pointed upwind. Close winded the wing tacks through 68-70 degrees at 12 knots true wind speed and at 8 knots of boat speed. Obviously, there is no conventional jib that can be used at such angles. VMG is approximately the same as tacking through 90 degrees with a jib at the same wind speed. The wing rotates around a 15 m high modulus spreaderless carbon mast weighing 52 kg, that can be canted 12 degrees to either side. Canting improves performance by 10-12%.

Stefan Törnblom and I are discussing offering a new type of Aerorig on the T30 using hydraulic downhauls fore and aft. With modern high modulus textile and carbon materials this would certainly be a very interesting alternative to the rotating wingmast in particular. As a comparison, we have measured a maximal main sheeting load around 1600 Nm on the T30 with wing mast compared to 90 Nm maximal on the wing sail version when sailing close winded at 15 knots true wind speed and no reef in either sail.”

 

A small discussion which stems from another thread. The question was asked about float shapes asking why floats are big forward and fine aft. John Shutleworth in 1982 in the following provides a good answer to this:

Brittany Ferries Article http://www.shuttleworthdesign.com/BritFertalk.html

But time has moved on. The general principle still applies but floats are now much fuller and in large racing tri’s that act more like a catamaran with 2 hulls hanging in the air the floats have fuller sterns. For a cruising tri full forward floats with finer sterns are good but modern designers design for higher performance and have been using fuller floats. The examples are an original Val tri, a much later version of the Val hull lines, Rogue Wave 60 foot tri and an ORMA racing tri float lines with associated photo of the tri. Finally trimaran float design and positioning takes a lot of experience to get right over a full sailing range. Each designer has there own criteria and trade offs to get the performance range they require. Cats are simpler to design for all round performance.

 

A question on multihull bows. Nowadays it seems almost all cats/tris have a vertical or near vertical bow stems (some raked forward, a few raked aft). Years ago quite a few had rounded stems, and even overhangs, but no longer? Is the vertical bow, or even raked aft stem, all about getting more waterline length for a given size of boat? I know if there are rounded bows and they are too close to the bridge deck, the bow does not come up quickly enough and the bridge deck can pound. Is there a disadvantage to overhangs. I have read that if the boat has a fine bow and fine stern it can hobbyhorse, with such vessels overhangs can be used to reduce excess pitching. Russel Brown built three large proas with fine ends an without overhangs, as also did/does John Harris with the CLC Madness proa. Do these vessels avoid hobby horsing because they have a very long waterline length, do they hobbyhorse? And also,,, that the darn is hobby horsing anyway (excess rythmic pitching?)

 

Lots of angles here! One thing I'll mention is overhang conventional bows are much better at sledding over drift if you live in a land of logs like the PNW. Naturally this is contrary to current race tech but for cruising.....

 

Peter many will respond to this each with there own reasons. Give me a little time for the cat tri response and I hope those with real proa experience can give some guidance. Rob Denney of Harryproa in effect said at one point his very high prismatic (full ends) hulls with appropriate buoyancy distribution helps to dampen pitching for his proa's (I hope I have this correct). More to come later.

 

Plumb bows and reverse shear bows tend to have foam impact zones to take the crash but are a bit slow till you get to Port and replace the smushed zone.... Lacking ports with foam ancient proas and canoes tended to have overhangs.

We had a fun thread about this stuff once where I championed replaceable bows for racers or inflatable ones for the pop goes the weasel effect. Then you can get the prismatic where you want it and the flotsam and jetsom have to take their lumps in the pursuit of speed.

 

PeterAustralia. Hobby horsing is as you said “rhythmic pitching” for want of a better term. Some boats start a pitching sequence and if wave/ripple conditions are just right the boat continues to pitch knock wind out of sails and making life uncomfortable. Now the topic of bow shape. Boating is driven by 2 things fashion and practicality. Reason for saying that is a good designer can make any shape bow work in an overall design. EG in 6.5 meter racing monohulls scow shaped (really full blunt bows) are race winning shapes. Nearly every 22 foot production monohull has a sharp bow. Blunt full bows don’t sell well to the public’s idea of a boat shape. Now multihull bow shape.

Originally multihull designer had fine bows and fine stern shapes with masts well forward. The leading lights of ocean going multi design at this stage were Wharram and Choy who both had big forward overhang and fine sterns. Then to limit pitching other designers like McDownie Alpine added fullness to the sterns. Result the bows were a bit overpowered. So more forward overhang and fullness above the waterline was introduced to help compensate. This helped increase speed and under some circumstances reduced pitching. As people built lighter boats with better materials and more powerful rigs hull shapes were pushed harder and fine bows at the waterline with a lot of buoyancy above would try and push over every wave creating a strong pitching moment upwards. This is OK its what you would like to happen. But racing being what it is, some designers wanted a smoother motion going through a seaway to allow the rig to develop more power. 2 things happened at this stage. Small cats started to have vertical bows and push there rigs back closer to the pitch centre of the cat (about 55 – 60% aft from the bow). The small cats were faster. Big boat designers took notice and applied it to larger cats/tris. It worked, the boats went faster. Then small boats said if we drive through some waves not over them then there will be less pitching, but we still need some fullness in the bow. Result, wave piercing bows where the waterline is fuller than the deck line. These bows can to a point drive through wave tops with minimal pitching. Again, big boat designers followed the trend and found their boats were faster. For big heavy cruising cats this is only of minimal use and is more of a fashion statement and as Cavalier has pointed out there are other considerations in this world besides pure speed.

Finally reverse or Axe bows have even gone into large ships with a real cash flow advantage. Aurora ships used to have conventional bows and when going down to the Antarctica from Chile would travel at 12 knots until heavy weather when the old models would slow to 9 knots for passenger comfort. The new Axe models can maintain 11 knots in the same heavy weather because they drive through wave tops not over them. This means schedules are kept and more sailings are possible with greater passenger comfort. Hope this helps. The jpegs are of a smaller beach cat with reverse bows and fine deck lines and of the Aurora ships.

 

Libertist 853 is a trimaran designed by Erik Lerouge that is equipped with permanent minifoils. The tri is 28 x 23.5 foot weighing 3160 lbs displacing 4700 lbs with a rotating carbon mast carrying 440 square foot of main and a 204 square foot jib. The cruise-racer is made from epoxy glass foam sandwich with cross beams of carbon fibre. Its built in female moulds, using the vacuum-bag method and resin infusion.

The tri has a big rig and moderate displacement with a wide beam. The result is a powerful boat capable of good performance that is assisted by its mini foils. Lerouge is reluctant to put full foils on a “cruiser” as full foils only work in limited ranges of conditions in relatively “heavy” boats intended for cruising. So, he put on minifoils in the floats that helped hold the bows up. Notice the fullness of the float bows and sterns combined with the minifoils means this tri can really power up to provide performance like this. It can do 14 knots under spinnaker in 8 to 12 knots of breeze. Its capable of 11 knots upwind in 20 knots of wind. Its also done 22 knots in 30 knots of wind. This boat will be worth further investigation and may give a hint as to the future direction of cruiser racer tri’s. The jpegs are of the Libertist 853 and a future production version of the Libertist 703 tri (23 x 19 foot weighing 1900 lbs displacing 3130 lbs with 310 square foot of sail).

 

28ft. Trailerable Folding Catamaran "Clamcat" was designed by Kerry Thomas. The Clamcat is 28 x 18 foot with 3 foot wide hulls that fold under the central pod to be trailable at 8.2 foot wide. The folding systems is similar to the Woods Sango or Wizard. The length to beam on the hulls is approximately 10:1. The displacement (estimate) 4000 lbs weight about 2200 lbs.

Kerry Thomas is a NZ guy who describes the design as: "The Altair ClamCat 28 was designed for a client who wishes to cruise and occasionally race locally in Florida waters. The design criteria included Trailering, shallow water and light wind capability, excellent unloaded performance, double berth, adequate galley and maximum length 28 ft. This boat is designed for local inshore racing and coastal cruises with optional rigs to optimise for racing or coastal cruising. Retractable foils allow sailing in shallow water. The cruising configuration is ok for offshore minimalist cruising, the boat is not intended for a large payload. Construction of ply with cedar for hull bottom and deck is simple and quick. The folding mechanism is simple and easy to construct. The folding system is unique and developed by myself and the client. It had to fit within 28' and 8'6" max folded width for trailering in the US. As the final design is to be used in Florida it has a bigger rig. The design fits within the rules for cruisers built in NZ for offshore including the design loads, openings etc. As anywhere on the NZ coast is likely to get offshore weather. Not stability as no cat is stable to 110 degrees. These rules are based on offshore racing Catagory 1. I would not recommend either for long offshore passages is the limit on actual load with a 28' catamaran. After you have added essential gear there is not much spare displacement for stores. The added weight of a folding system limits you even more.” Construction plans and instructions are aimed at the home builder. I do not know how many were built or completed.

 

Now a very well known boat, the Corsair Pulse 600 tri. The tri is 19.6 x 14.75 foot (can fold to 8 foot) weighs 990 lbs from the factory and depending on use (racing or day sail) displace up to 2500 lbs. The 31 foot mast carries a 186 square foot main and a 76 square foot jib. This tri is basically a fast day sailor that will perform very well on all points of sail. Why the interest in a production boat? The way its been designed in 2 features. One the ring frame in the main hull supporting the forward cross beam structure and the mast step that is literally separate from the clip on forward cabin cover. Two the shape and volume of the floats.

The construction of the boat is PVC foam carbon fibre and epoxy with a carbon fibre ring frame that takes most of the forward loads. That ring frame then just “attaches” to the hull structure which can be deigned to resist water forces and forestay loads. The aft traveller is also part of the aft beam structure to handle the rear cross beam loads. This is a smart way of putting many of the main hull structural loads and forces in just 2 major components which helps lower build costs. The second major feature are the floats. They are very full in the bow and stern and are high volume overall. This boat is designed to be sailed hard in racing and is in the “I am actually a 3 hulled cat” category. 1 float in the water 2 hulls in the air when being pushed really hard. Result floats that look more like cat hulls than older style tri floats.

According to the advertising information the Pulse 600 is: “Easy folding on and off the water. Open transom with self-draining cockpit. Composite rudder and dagger-boards Large waterproof storage forward of the mast which can be a weatherproof lock-up cabin. Featuring lightweight carbon reinforced construction, this boat will get your pulse racing in even the lightest of winds. The Pulse 600 is a sports boat that is about pure fun. The open cockpit is designed for a crew of up to four but can equally be sailed single or double handed.” Test sailing of the tri have produced in 12-15 knots of breeze in semi-sheltered waters some great runs where the boat went along at around 13-16 knots with the kite up. The rudder balance felt so controlled that taking your hand off the tiller extension and letting the boat go was never an issue. Other reports have said the Pulse 600 can top 20 knots and is a great light wind performer. The jpegs give an idea of the boat.