Multihull Structure Thoughts

Final pages of Harris paper.

 

Multihulls around 25 foot range in weight from 380 lbs C class cats to 6000lbs plus cruising catamarans. They can built in many materials but for this item I will stay with thin skin foam glass cats, tris and proa’s.

C class cats are 25 x 14 foot weigh from 380 lbs to 1000 lbs depending on their age. The standard rig is 300 square feet of mainly a full wing mast. The minimum that was used (cat named Invictus) was 200 gsm biax carbon fibre 5 mm H80 pvc and 200 gsm biax carbon fibre. An additional layer of unidirectional carbon fibre was put on the flanks of the hulls from the cross beam to the bow to reduce flexing. This was all done in epoxy with resin infusion vacuum bagging.

Next came 9 mm nomex cores (cat named Cogito) with the same 200 gsm CF skins but without the unidirectionals on the flanks as the increased core thickness provided the stiffness required. These hulls weigh about 40 lbs each. But the manufacture of the hulls have changed to autoclave for heating and pressure curing requiring prepreg CF cloths. Also the pressure control in the autoclave is very important as too much pressure deforms nomex cores especially around the edges of sheets. As foiling became more relevant additional strength was introduced into the hulls around the dagger foil cases connecting to the cross beams and at the transom to support the foil rudders.

Some “low cost” C class cats (cat Rafale weighed 400 lbs all up) have been built using CF cloth H 80 core and a “heat blanket” that has electric strips running down it on top of the CF resin matrix and peel ply etc but under the vacuum bag to provide a light hull half but at a controlled temperature for several hours instead of the expense of an oven.

Next is an Arbor concept tri he was designing intending to build. The tri was 30 x 29 foot and was to displace with crew 1500 lbs. Its intended skins either side was 153 gsm unidirectional s glass at 90 degrees to the 6 mm western red cedar core. I don’t know if Shawn built the boat. To give an idea of how extreme this is a Gold Medal winning Tornado had a 200 gsm s glass unidirectional either side of a 3 mm WRC core. Boyer A class cats had 163 gsm kevlar cloth either side of 5 mm foam with 195 gsm cloth in the hull ends. 8 mm foam is used around the daggerboard case and main cross beam.

Finally, early versions of the Elementary 25 foot proa, if you wanted a high performance structure was 300 gsm CF cloth in epoxy either side of 5 mm pvc foam. I am sure Rob has developed the structure further with female molds and intelligent infusion. The basic message is you can build many types of multi’s very light if you want a good bay racer/ mini cruiser. Just don’t hit anything at speed unless you have a good repair kit.

 

I will risk all and talk about “unusual” wing rigs. In 1999 an IMS 50 foot mono had a fixed unstayed wing mast. Krazy K performed really well upwind to the point that other IMS owners refused to race against it at the Admirals Cup until its rating was “corrected” so the boat would loose. The owner and designer had developed the concept over 3 years so the boat was competitive. The concept worked to well in flat water and 10-15 knots of wind, especially upwind. But the boat did not sail under the rig in racing conditions after it left the Admirals Cup when its rating was adjusted to make it noncompetitive. The rules forced the owner to change to a normal stayed sloop rig. The boat won races with a normal rig. Now something interesting. Junk rigs which have fairly flat sails and a mast permanently on one side of the sail have been tested as to performance on both tacks. As long as the mast is less than 15% from the leading edge of the sail the boat performs about the same on both tacks. The though is the leading edge of the sail creates a turbulent "bubble" which reattaches to the sail after about 15% of the sail cord so the airflow does not “notice the mast”. Maybe the same may apply to a fixed wing mast when reaching. Only a thought.

Next is a J/90 racing mono called Blackwing where Eric Hall (famous spar maker) tested a free standing rotating carbon wing mast. "We certainly knew quite a bit about how to engineer a conventional stayed carbon mast through our experience with numerous offshore and one-design classes, but this presented a unique challenge," said Eric. A balance had to be struck between the size and shape of the spar, how it would enhance the effective airflow into the mainsail, and what stiffness and weight to achieve — all the while bearing in mind that this ultimately had to be a concept capable and practical enough for series production. For mounting Blackwing into a J/90, the region of the cabin house around the partners was reinforced with radiating gussets that taper away from the bearing assembly mounted at the partners. At the mast step a simple socket lined with self-lubricating plastic was devised, where the trailer ball mounted on the butt end of the mast would sit when stepped.

The Blackwing spar rises over 3 foot taller than the conventional J/90 spar, has an 1.6 foot chord length, and is several kilos heavier, despite not having spreaders or any standing rigging, save for a Vectran headstay to support the jib luff, and a set of Vectran running backstays. These adjust on a simple 2:1 system to help reduce headstay sag and support the spar downwind when the large masthead asymmetrical spinnaker is set.

The promise of enhanced performance due to greatly reduced windage has translated well from theory to reality, as the prototype has outperformed many other boats of its size in races around Rhode Island.”

A later response from Eric Hall had some very relevant comments:

1. First to say is in my comments below, the “una rig” was a fully rotating airfoil shaped mast. Everything should be understood in that context.

2. It was inferior going to windward.

a) A lower frontal area airfoil section would have improved it (but would have had sideways stiffness problems).
b) Any rotation past in-line with the apparent wind slowed down the boat where added drag trumped added power (as flaps do on a plane)
c) There is no substitute for a jib to improve upwind performance
d) The jib seems to add lift while improving the flow around the main

3. Beginning with a close reach the Blackwing rig was superior.

a) With the mast able to rotate, the wind “sees” a much better mast shape with less turbulence that a conventional non-rotating mast.
b) As soon as a jib is eased, its positive effect on the mainsail reduces quickly
c) Rotating past apparent wind adds substantial power this time trumping added drag
d) Adding a standard J90 A-sail made this system a real weapon!

Blackwing mast is CF skins with Nomex core. The words of Eric Hall about performance of wing masts with sail cloth mains should be understood. Yes I know rotating wing masts are easier on multis. This is looking at the a different approach without wires.

 

With all due respect oldmulti I am pretty sure this does not represent the rig or what happened.
The rig was a rule beater/rule cheater.
From memory the rule took the athwartships staying base into account. Krazy K had a forestay, a backstay and a really thick section wing mast buried in the boat, that didn't rotate but was designed to twist by its very clever designer, having no side stays, the effectively staying base measurement for the rule was zero.
This allowed them to carry extra sail that was not penalised.
What the designer/ french team admitted later was they had not read the bit in the rule that allowed unforeseen rule cheater/beaters to be handicapped so as not to render the rest of the fleet as mooring minders.

This..... But Kandler also admitted that the French “had not read the rule” and Rule 101 clearly states that the Offshore Racing Council can issue a rating which it considers appropriate in the case of innovation in design and construction.

So the performance of this rig tells us little about unstayed rig performance.

 

Bruce. Thank you for the correction. Rules have never been big in my life. But the concept of wing rig twist is interesting. C Class et al solid wings have multiple parts that try and emulate some twist in the upper parts of the wing so designing it into a solid "slightly flexible" wing is a good idea. Juan Kouyoumdijan was the designer and this was one of his first break though boats. He has continued to be an innovator in mono racing since and has designed global race winners etc which were controversial due to there swing keel configurations etc. He pushes the edges now, not breaks the rules.

 

I will ask the Moderator to put this list of items from page 16 to 30 on the first page. May take a day. When I get time I will do the next 15 pages.

 

I came across this diagram of 3 cats used or intended to be used in The Race. Club Med won the racearound the world in 62 days. The hull lines of the 3 boats are in the first jpeg. Notice the differences for what is intended to be the same task. In each case they have very slender hulls but each has its own variations of bow ,stern and mid section.

Team Phillips (TP) top left is the original wave piercer with a thin bow that is intended to drive through waves. TP stern is relatively small and the mid section is basically a semi circle. When you view video’s of this cat it could drive through a seaway very well with minimal pitching. TP is 120 x 69 foot and was meant to weigh about 69,000 lbs. The 135 foot high free standing wing masts that weighed 11000 lbs per mast. TP had no difficulty averaging 23 to 25 knots in its short life but her masts and demise will be the subject of another discussion.

Next is Playstation top right. Playstation is like an enormous Tornado. The hull lines have a high fine bow, rounded semi circle mid section and full sterns. The cat was lengthened to 125 foot x 60 foot beam. It carries over 6000 square foot of sail in its basic rig. This boat suffered pitching in many conditions due to its rig size, power and fine bows and had been known to have its forward beam hit waves which limited its average speed. In flatter water this boat could power up to very high speeds but in a seaway it had to be treated carefully.

Club Med bottom left (and her 2 sister ships) were fast in many conditions. Club Med was designed by Giles Ollier and was 116 x 54 foot. The cat was deliberately had a narrower beam to keep it lighter and more responsive to sail variations. The hull shape is more balanced in the ends with a slightly deeper mid section and had flair above the waterline to pickup buoyancy when going through waves and “heeling”. Ollier had designed several 70 foot racing cats prior to this and had some good experience from them. The bows may be fine at the waterline but there is a lot of buoyancy above and the transoms are slightly above DWL in light airs. The hull shape was the result of 35 virtual models and two 12% tank test models comparing it to 15 sail plans that were analysed and 43 appendages tested. In short the initial design testing and experience of real previous designs by Ollier helped him produce a potential winner. Club Med could sail through a seaway much better than Playstation which resulted in higher average speeds and resulted in a win for Club Med.

Finally, it was the sailors sailed the boats that made or broke the boats. Each one of these boats had structural issues which we will discuss later.

 

I would like to thank the Moderator for posting the "next" version of the index pages 16 to 30 on the front page. The Moderator goes unnoticed but is very helpful.

 

No problem at all - thank you for a very interesting thread!

 

Team Phillips, Playstation and Club Med all had their structural problems. Team Phillips (TP) had the greatest failure eventually leading to the loss of the cat so we will start there. When team Phillips was conceived it was very advanced in all aspects. Adrian Thompson the designer tried to achieve many things in one design. Long thin light wave piercing hulls, wide overall beam, freestanding wingmast rigs and advanced construction. Each had their issues ultimately effecting the cat. TP was designed to be semi flexible with an anticipated movement of up to 5 foot in the bows and the 135 foot masts allowed to move up to 16 foot at the mast head.

The long thin hulls had a length to beam ratio of 24:1. According to Bill Roberts the designer of RC 27 and RC 30 cats, any length to beam above about 16:1 requires additional strengthening on the hull sides as there is so little effective width compared to length to prevent bending of the hull. Secondly, TP had 48 foot of unsupported bow before the first cross beam. This unsupported length added to the stress on the bows. The hulls were carbon fibre either side of a nomex core then two 6 mm thick by 500 mm wide unidirectional stringers ran the entire length of each side of the hull. During the heat curing process of the hull tiny amounts of moisture could not escape from the nomex core causing a ”weakness” in the cure of the carbon stringers to the rest of the hull. A 40 foot section of bow broke off while sailing in moderate winds due to a suspected failure of the carbon stringer attaching to the hull composite. The hull was repaired and all hull structures were strengthened.

It then suffered from problems with the pioneering bearings that supported the massive 135-foot-tall masts which required further repairs. The masts weighed 11000 lbs each and were basically carbon fibre. The base of the mast is about 1.5 foot in diameter of which 1 foot is solid carbon. See the first jpeg. The masts were reputably 2000 lbs overweight. This put additional loads on the bearings and through the rest of the structure.

TP after repairs was test sailed in heavy conditions to prove the cat, but was abandoned during a freak storm in the mid-Atlantic in December 2000. 70-knot (130 km/h) winds and 10-metre (33 ft) waves started to produce cracks in the crew's central safety pod and forced Pete Goss to send out a mayday signal. He abandoned ship with the rest of his crew, and the vessel broke up several days later. TP was a wide flexible structure that in difficult conditions flexed more than expected and was not up to the wave impacts that can result from a heavy storm.

Please do not think TP was a “bad” design. It was a very good sailing cat that had a good hull shape and a powerful effective rig. The issue is the design and build of the structure. This design was pushing the limits of known design and building at that stage. Also, the budget for the boat was limited not allowing for a lot of testing or rebuilds if an item was overweight or did not meet expectations. We all learnt a lot from this design and some of it is only now being applied to larger cats like wave piercing bows. The jpegs give some ideas of the size of this boat and its components. Playstation and Club Med tomorrow.

 

Old Multi, you mentioned the smaller Telstar trimaran - I have a catalogue for her from the mid-70's, which I will scan and post. In the mean time here is a copy of the catalogue for her bigger sister the 35.

 

And here is a copy of an article from Yachting World in England in the mid 70's about a Kelsall designed trimaran 'Azulao'

 

Bajansailor. Thanks for the input. Azulao looks interesting, does anyone know of its performance and weather it used hydrofoils in any racing? The rudder kick up system is my favorite approach. It gives the advantage of an "underslung" spade rudder but will kick up if it hits anything. Good general concept if the hydrofoils worked as required.

 

20 th in class.
8th corrected.
1976 OSTAR
The Royal Western Yacht Club of England | OSTAR 1976 https://rwyc.org/club-history/ostar-history/ostar-1976/#

 

Playstation was to be the fastest transocean multihull possible when Steve Fossett had it designed. Playstation originally was 105 x 60 foot, the estimated weight was 39000 lbs with its 140 foot carbon mast and 7250 square foot of sail upwind and 11650 square foot of sail downwind. The initial design/build had a few weaknesses with the forward freeboard being too low and the bows too fine. This caused the cats forebeam to plough into waves at speed which slowed the boat dramatically and had a few structural effects. The initial thought was the rig was to powerful for the cat. So Playstation was modified.

The boat was lengthened to 125 foot with higher fuller bows which raised the forebeam higher off the water. The boat maintained the same rig and beam but the actual weight of the cat was found to be 60, 000 lbs at this point. The hull length to beam ratio was 22:1 in its longer form. The idea of the additional length was to reduce pitching. It only partially worked. Playstation was fast in its original form running 580 miles in a day in idea conditions. In its modified form it was faster in a seaway but after 16 days in The Race 2000 withdrew with multiple failures of structure and equipment. In 2001 it crossed the Atlantic in 4 days 17 hours. It was built of carbon fibre and nomex.

The root cause of its problems was an overly ambitious design with a very powerful rig. Literally the cat was faster than its bows could handle and the resulting pitching was a problem that stressed the boat. The final difficulty was its 11650 square foot down wind rig that threatened to cause a pitchpole if not handled correctly. Again, we learned a balanced hull and appropriate size rig is better than pushing the edges.

Club Med and her 2 sisterships were 110 x 54 foot weighing 50,000 lbs carrying 130 foot carbon fibre wing masts with 6000 plus square foot of sail. The cat took 60,000 man-hours of work to build. Club Med won The Race in 2000 in 62 days. But near the end of The Race Cub Med broke its forward underwing and had to be nursed over the line.

Club Adventure came second in The Race in 2000 and was Club Meds sister ship, built by the same company at the same time as Club Med. Club Adventure was in a 2001 attempt to break the transatlantic record. In 25 knot winds with full sail during that attempt Club Adventure broke its port bow with the forebeam being the only point of attachment left. They managed to get the boat to safety by motoring at 5 knots and putting additional temporary stays on the mast.

Analysis of the breaks indicated each of the sisterships were built very well but the safety margins were narrow and the characteristics of products like nomex and carbon fibre were not fully understood. The core materials were too stiff in parts and wave impacts when driven hard caused problems. All 3 sisterships are still sailing and with some strengthening have completed other races. Remember, the bigger Orange 2 (built a few years after Club Med et al) 120 x 60 foot has 0.8 mm high strength Carbon Fibre skins either side of Nomex except in the bows and forward underwing which have PVC cores. We all learn from failure.