Multihull Structure Thoughts

Discussion in 'Multihulls' started by oldmulti, May 27, 2019.

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Coastal OgreJunior Member

Continuing excellent discourse with this thread!

Is there an equation to translate hull cycles to nautical miles sailed (correlation, approximation or industry norm)?

Let's say you have a large cat (WRC with epoxy) and she has sailed ~90k miles over the course of her life (predominantly in the Pacific - in case that is important to the equation): how many cycles would this be considered?

Thanks!

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oldmultiSenior Member

Coastal Ogre. The answer to this question is very hard. What is a cycle depends on where you sail, how hard you sail and whether you like sailing upwind etc. To simplify it each wave you go over induces a cycle, but it depends on the size of the wave. A 1 foot wave hardly causes any stress, a 10 foot wave causes a serious stress cycle. If you are going downwind or reaching across a 10 foot waves minimal stress. If you have a heavily loaded rig where the mast is being pitched fore and aft in rough conditions its another cycle. If the boat is angling across large wave patterns where the stern of is in 1 wave and the opposing bow is getting into another wave another torsional cycle etc. I do not know of any general rules of thumb. The attached PDF is heavy on mathematics but tries to define wave cycles to boats.

The following web page gives some detail of waves which depends on wind speed, fetch, depth of water etc. then make an assumption a cycle is each wave you meet irrespective of your sailing direction etc. Wind wave - Wikipedia https://en.wikipedia.org/wiki/Wind_wave

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catsketcherSenior Member

I don't know if I have attached this before Coastal ogre but this is something on fatigue. You really won't be able to tell the fatigue limit of a structure unless you can work out the loading the structure is under. A typical cruising cat does not seem to have any issues with fatigue in hulls and crossbeams anymore. Read this Considerations for Seaworthiness http://www.shuttleworthdesign.com/NESTalk.html

There were issues with fatigue in the Spindrift cats in the eighties. Two, Pennant and Broken Arrow (IIRC) developed cracks in the armpits of the main beam. In Pennant's case the cracks were found early enough and the area beefed up and she lasted another couple of decades. In the second case, a Spindrift was off the coast of New Zealand when she developed a crack in the main beam that grew catastrophically and the boat broke up.

Unlike more modern cats, Spindrifts have a underwing joint that many engineers would now baulk at. It was a hard right angle with no chamfer panel or rounding. On top of this the boats were built without uni directional cloth at the join. This led to a hard corner for crossbeam loads into the hull structure. Also the thinnner beam and tall, thin hulls excerbated the stress on the corner. Increasing corner radius is simple engineering to decrease stress and now good designs use radii or steps and fatter hulls to decrease the stress and reduce the chance of fatigue failure.

Some trimarans also lost floats due to catastrophic failure. Two Nicol tris - Triventure and Tryste, lost floats when cracks developed in the aft wing and ripped forward. A Piver Lodestar called Om also lost a float, as did a Lodestar in Pittwater in a gale on the mooring.

So as to your cat developing fatigue issues, if she has rounded corners where the load path changes, has wide hulls at beam connections, uses uni or stitched fabrics at high load areas and is solid around the dock (has a heavy enough laminate) she will probably be fine. The nice thing is that you will usually be lucky and find any cracks before failure if you look carefully. A cat I know of had cracks in her aft beam where the beam transitioned into the hulls. They were small but showed that the area was under stress. So the builder reinforced the area with unidirectional glass and the boat has developed no more cracks after a decade and a half. Cracks are your friend as long as they are short and you get right onto them.

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oldmultiSenior Member

Reinforcing of a hull structure is very much a designer choice. You can theoretically design a boat that has no internal reinforcing structure at all. EG a Laser, but the boats structure would be heavier than it needs to be. By making a thinner hull structure with selective reinforcing structures you can lighten the entire structure. I will take some extreme examples. In the Philippines 6 mm ply is relatively cheap so the local fishing boat builders use it on 60 foot open ocean power boats. The stringers are 200 mm apart and frames are every 500 mm. Why? This reduces the panel area to the point where the structure reacts like a 12 mm ply hull shell with 400 mm apart stringers and frames every 900 mm. In Queensland when Lex Nicol was developing his very light 36 foot foam glass racing trimarans (3800 lbs. Main hull 200 gsm glass cloth 12 mm airex foam 200 gsm glass cloth) Lex pressed on the side of the hull, if there was to much flexing in an area, a vertical foam glass rib was put on the inside. In both cases it was being done by practical “feel”. Other designers like Thomas Firth Jones ply boats use rules of thumb like stringers every 300 mm frames every 900 mm on his 20 to 30 tri and cats. Also there are designers that use full FEA analysis to optimize the internal and shell structure of a multihull (only big manufactures or rich customers here).

Now we come to a reality for all designers of home built boats. Do you design a hull structure that is structurally sound alone or do you design an integrated hull structure including furniture as part of the structure. Home builders have a really bad habit of wanting to redesign boat interiors for “their needs” not realising that a bench front or bunk panel may be an important part of the boats structure. A good example is Wharram classic design cats which did not depend on their internal furniture for strength. His later Tiki designs depend on bunk tops/seats for there core structural strength.

In the real world the distance between stiffeners is dictated by the required panel stiffness for a design state which is a function of wave height, vessel speed and vessel displacement but according to Richard Woods some of the biggest loads are usually not from waves but from fenders when side-tied with wind and waves pushing the boat onto a wharf. The stiffeners can take a form of stringers, frames, bulkheads etc whatever stiffens the hull panel to keep deflections within acceptable limits. Designers accomplish this is different ways. For catamarans the interior furniture and arrangement means that all sorts of things can be utilised in order to stiffen the hull panels as everything is basically adjacent the hull panel with a central walkway, so we often see cupboards and shelves acting as stiffeners, doorways and room partitions etc. So with the interior arrangement in mind, the designer can see at what spacing the stiffeners will be located, and design the hull panel accordingly, which is mainly driven by hull panel thickness. Another consideration is the ease of building, especially for home built designs. It's easy to cut and setup closely spaced frames and bulkheads on a strongback then apply the hull sheeting over it to get a fair hull, so it's about the build method not just the design.

Also in multihulls localised high load points are distributed by major bulkheads or cross beams. Bulkheads can also act as transverse frames that prevents torsional stress or twisting of the hull. Unified with a stringer system, they form a structural web and a truss. Removing bulkheads is rather like removing the trusses from a bridge. The overall strength can be reduced to the point of structural failure.

The jpegs are of the internal framework of a Lagoon 77. This boat has a specialist designer and build team just for these bulkheads to optimise the build, minimise the weight and ensure attachment to the hull structure. The internal framework of a multihull is part of an overall design process and each designer uses there own method to achieve an overall design result. There are very few “golden rules”. EG Lock Crowthers later designs had very thick foam (eg 30 mm corecell) thin glass skin hulls to reduce the need for any internal ribs or support stringers between the major strength bulkheads. His earlier designs had thinner foam (18 mm) and some ribs were cheaper in materials but took longer to build. Same intended boat outcome but different internal structures to suit specific needs.

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oldmultiSenior Member

For those who want a really simple way of getting a small trimaran I give you Tricote. This one uses a monohull center hull coming from a Lanaverre 590 (20ft x 6ft8). You cut out the keel and built a structure for the cross arms and add 2 hulls from a Hobie Cat 16, a wing mast, mainsail and jib. The big work is fabricating the folding system. The jpegs give more detail of the galvanised steel tubing cross arms and folding arrangements. Tricote can average 10 knots for periods. New trampolines and fins in the hobie hulls were added after initial sailing. It’s 20ft in length, weighs about 600 Kg, and cost less than 6000 euros (about \$7000 US\$).The boat has good performance and it takes just 1 hour to prepare it from the road to the water. It has 4 bunks. PS I not a big fan of the folding connection bolt arrangement.

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DolfimanSenior Member

From another source here below, the tri is said from late 1986 (the Formula 40 class ended in 1990 anyway). The minimum weight in the class rule was 1,8 t, so ~ 4000 lbs, I suppose that the real weight of these racers, before turning cruising boats, were not a lot higher than that.
Spirit https://trimaranspirit.weebly.com/spirit.html

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Coastal OgreJunior Member

Ok, thanks.

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oldmultiSenior Member

Just a short one on the use of beach cat hulls as floats on smaller trimarans. EG building a Newick Tremilino or a Wood Strike main hull and then putting EG Hobie or Quattro hulls as float hulls. Many designers are suggesting this is a fast way to get a boat up and sailing as not only do you use the hulls but also you can use the rig etc. Good in theory and as long as the tri is not driven to hard when sailing.

Now the down side. Most beach cat hulls have insufficient buoyancy in the bows to handle the tri under full power unless the floats are relatively forward compared to the main hull. The internal structure of a beach cat hull is not significantly strong enough to carry the full weight of the tri being pitch forward down a wave putting a lot of pressure on the leeward float bow. Beach cat overall hull structures are designed to carry eg a 400 lbs boat plus 400 lbs crew, a total of 800 lbs. These small tris (eg Strike 18) displace 1400 lbs. This is a significant weight increase which would more than double loads on a beach cat hull if the tri is driven hard.

The internal structure of a lot of beach cat hulls depend on a 50 mm thick horizontal Styrofoam shelf without glass in the forward half of the hull. Styrofoam breaks down over time with repeated cycles. Most floats designed specifically for trimarans, have solid plywood or foam glass shelves in the forward sections of the float hull to take the bending loads. In the case of Newick he later designed specific floats for the Tremilino to get additional buoyancy in the float for faster sailing.

Finally, most beach cat hulls have the dagger boards or hull shape incorrect for lateral resistance which may require modification.

One qualification. The early Hobie 14 and 16 hulls were strong and could take all sorts of abuse but they were not the best hull shape and lacked buoyancy.

Overall, building floats that are designed for a tri is probably a better path and will only take a little longer to do. The performance of the tri will also be very likely to be better.

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oldmultiSenior Member

Running Cloud is a beautiful trimaran whose design was started in 1968 and the tri was finally finished in 1973 after being built by Richard Flint in 4 years as a part time project at his boatyard. The tri is 52 x 31.75 foot displacing 14,000 lbs and carrying 1325 square foot of sail with main and genoa. The main hull length to beam is 12:1. The internal accommodation is generous with a rear cabin double berth, a double and a wide single in the main cabin and a good galley and dinette with a proper bathroom forward. This boat sails very well and was still winning races in 2007 in Asia. In 1986 it crossed the Atlantic in under 14 days in a “cruiser rally”. Why is it so interesting? Richard was 25 when he designed it and 30 when he launched the boat. He spent \$20,000 (in 1973 \$) building the boat. He made nearly everything himself including the mast, sails, blocks, steering gear etc. Richard extended the tri 3 foot to 55 foot after initial sailing to improve reaching and off wind speed. He tried a wind vane for the single handed Transpac in 78 but the boat accelerated so quickly that he had to buy a Tillermaster steering unit and attach a small steering tab to the main rudder. The boat has 2 daggerboards one in the fore peak in front of the forward crossbeam and one in the cockpit. This was to help balance the boat and provide more space in the main cabin. What was the tri made of? Foam glass with limited plywood for bulkheads etc. Very few multihull people in 1969 were attempting foam glass in 52 foot home designed trimarans. The boat can average 10 knots in cruising mode and has topped 20 knots regularly. A very impressive boat and builder.

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oldmultiSenior Member

The G Force 44 was designed by Glenn Henderson and built in 1992 by Hans Geissler of G-force (G-Cat cats). The G-Force High Tec 44' trimaran is 44 x 27 foot but will fold to 15.5 foot. The tri weighs 8,600 lbs and displaces 11,000 lbs. The 49 foot mast can be raised and lowered manually for clearing overhead bridges and carries 695 square foot sail. The main hull length to beam is 6.7 : 1. This is a cruiser that will be OK in lighter winds but will need to be pushed in heavier winds especially upwind. The boat heavily loaded has averaged 11 knots in a 18 knot winds in small seas.

The folding wing structures are basically very heavy aluminium tubes with water stays under. The video shows many details. The main hull is strip planked with 18 mm duracore and the floats are 12 mm duracore with 600 gsm 45/45 in epoxy on either side. The glass layup is doubled below the waterline externally. The boat has a low aspect ratio keel on the main hull. This is not the best looking boat externally or internally but it does its job well. The jpegs gives more clues.

Video launching of the g-force 44 ' trimaran zuleika https://www.facebook.com/GCatOwnersAndEnthusiasts/videos/launching-of-the-g-force-44-trimaran-zuleika/741517945992323/

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oldmultiSenior Member

MTC 40 trimaran designed by Nick Bailey. The tri is 40 x 35 foot displacing 9400 lbs and carrying 1000 square foot of sail area. Several versions were built starting in 1993. One in Brazil was built in 1995and is still sailing. Another was built in 1998 extended to 43 foot and sailed by Ross Hobson in the 2006 Route de Rum and capsized (the 2 people were saved). The boat hulls are strip plank cedar with composite cross beams. The cross beams have foam glass webs with unidirectional carbon fibre in the top and bottom flanges. The reason for showing this design is that there is some dispute over the designer of Spirit tri mentioned in a previous post (490) as Nic Bailey had some involvement in the cross beams of Spirit. The concepts look similar in the main hull, floats and rig but the beams are different. I suspect the strip plank hull structure would be the same. That is 12 mm strip plank western red cedar with 2 x 250 gsm UD E glass in epoxy either side. The jpeg gives more detail.

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oldmultiSenior Member

Formula TAG is a very fast cat designed by Nigel Irens. This pure racing cat in its original build is 80 x 41.5 foot that displaced 19500 lbs with a 90 foot mast that carried 2400 square foot of main and jib. Formule TAG was an innovative catamaran project, begun in 1982 as the brainchild of Canadian skipper Mike Birch with support from Techniques d’Avant Garde (TAG). The catamaran had pre preg carbon fiber/Kevlar Nomex hulls and main beams are box beams of carbon fibre Kevlar nomex webs and flanges. The boats components was post cured at 120 degrees centigrade. It was built by Nigel Irens and his team in Canadair facilities in Montreal using aerospace and Mclaren F1 carbon fibre composite monocoque technology.

Formule TAG was the first boat to beat the 500-mile-a-day barrier, clocking 517 miles in 24 hours during the 1984 transatlantic Quebec-Saint-Malo race. After this the boat has had many lives and a huge number of races. I will give a short history of the major changes. In 1993 after a first failed attempt at the around the world race under the name of ENZA the cat was repaired in the McMullen & Wing yard in Auckland, and reshaped for the race, the “big cat” now measured 28 meters. Its hull was reinforced under its waterline, its rig was resized, its structure lightened. The skippers were Peter Blake and Robin Knox-Johnston. ENZA New Zealand crossed the finish line of the Jules Verne Trophy on 1 April 1994, recording a time of 74 days, 22 hours, 17 minutes and 22 seconds for its circumnavigation. Beating the record by almost 5 days. The distance was 26395 miles at an average of 14.7 knots. The boat at this stage was 92 x 41.5 foot displacing 21800 lbs with a 105 foot mast and 3800 square foot of sail (with genoa) upwind. Down wind 6700 square foot of sail.

Tracy Edwards brought the boat and named it Team Legato to sail in The Race in 2001, then competed in Edwards' Oryx Quest round the world event as Team Daedelus in 2005. In 2007, Tony Bullimore tried to sail the cat around the world single handed but gave up at Tasmania. The cat was extended to 108 foot to get “extra speed”. Under Tony’s ownership in 2010 the cat capsized in the Bay of Biscay “Conditions at the time were slight seas, good visibility, and winds 11 knots. The delivery skipper said that they were hit by a sudden gust of wind. 'The catamaran accelerated from 15 to 30 knots and we were not able to slow her down.'” The boat was towed to France, righted and found to be badly damaged. It was stripped of all its gear. In 2012 they were still deciding what to do with the boat.

This boat shows that a well designed, well built boat can with stand a lot of modification and abuse. This boat has circle the globe on at least 5 occasions mostly in all out racing conditions. Any boat that can average 15 knots over 26000 miles requires investigation of its structure and design. Superb design by Nigel Irens.

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DolfimanSenior Member

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oldmultiSenior Member

The firebird is a very fast 26 foot cat designed by Martyn Smith. The cat is a racer cruiser 26.25 x 17.25 displacing 1200 lbs in racing trim. The cat carries 330 square foot of sail in its main and jib. The boat is foam uni directional glass throughout with carbon fibre in selected parts like the cross beams. The main mast beam on this boat weighed 17 lbs. The rudder kicks up with the end of the hull. Pete Goss successfully raced one in the 1988 single handed Trans Atlantic race finishing second in class in 23 days 3 hours. One was used as a trailer sailer across Europe. Several others won micro multihull championships. In Britain the 50 mile around the island race had a Firebird named “Orion” sailed it in 4 hrs 27 minutes with the time and speed of the legs breaking down as follows: Needles 13.3nm - 1 hour 35 mins - 8.4kn vmg upwind, St Catherine's 12.8nm - 55 mins - 14kn vmg two sail deep reach, Bembridge 12.4nm - 1 hour 15 mins - 10kn vmg spinnaker down wind (carefully) and Finish 11.6nm - 42 mins - 16.5kn vmg. The overall result was 50.1nm- 4 hours 27 mins - 11.25kn vmg. That is velocity made good (vmg) the average speed through the water would need to be higher especially upwind. “Orion” is also famous for doing a very spectacular capsize when racing very hard. A very fast 26 foot boat with limited accommodation and bunks.

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