Multihull Structure Thoughts

Wow that is a really informative post. I'm guessing that the pvc tube [with sump]set in the timber cross beam is for emptying the bilge in the float. It is tempting to move water out to the flying hull with a system of valves to distribute water/weight[with baffles] via the bilge pump, but it's an extra risk of overloading the beam and float and not quick to remove if things get ugly I'm guessing. Still, having the system in place could have safety factors, along with bilge water level gauges for each float visible from the helm[as long as they don't leak..], especially if a float is being pushed hard/submerging for hours at a time. I suppose those beams would have to be bigger/stronger again if the floats/amas were larger. Were Twiggy's made with aluminium tube also? I'd imagined them of larger diameter, anyway, thanks for the great posts Oldmulti, it's giving me some real insight and ideas.

 

This is probably the best thread I have ever read on an internet forum. So much real world information! Thanks Oldmulti and keep these great posts coming!

 

No Twiggy I know has been built with an aluminum cross beams. The PVC pipe in this boat was for a bilge pump. Twiggy Mk1 had smaller floats than Twiggy Mk4 very full floats but no changes were made to the original timber cross beams. The glass beams were being provided as an option. Water ballast in multi is occasionally built as an option but many dont use it after initial trials. EG Russell Brown built it into his Proa JZerro and virtually did not use it. But Russell Brown also rebuilt a Guedgon 32 (32 x 8.5 foot cat) and used water ballast in his single handed win in the race to Alaska. PS Russell is an EXCEPTIONAL sailor and a nice guy.

 

Additional thoughts on trimaran crossbeams can be found on Grainger Multihulls News https://www.graingerdesigns.net/custom-and-racing-designs/mauritius-20-trimaran/news-and-race-results-01/ This page covers many topic but the rethinking cross beam design for larger trimarans is interesting. It separates the 2 main functions of a cross beam that applies to all multi hulls. Function 1. The vertical support of any hull weights and rig loads and function 2 to act as a torque box to resist any fore and aft or rotational moments. Hulls on a cat or a trimaran often try and move independently as they travel over waves etc, if the entire multihull platform is not rigid enough rigging goes slack, the boat twists and can break at joints etc. Now there are 2 types of cross beam structures, the first is a raw beam connecting 2 or 3 hulls without any wing deck. These beams have to handle both the up and down and torsional forces. And the full bridgedeck EG catamaran where the main beam has to handle the vertical forces and the rest of the wing deck structure (if correctly designed) can handle the torsional forces. The raw beam structure need to be able to support the vertical loads and be a minimum of about 33% as strong fore and aft to take the torsional loads. The wingdeck model need to have some form of forward wing structure (often the D section leading edge of the wing deck) to handle the torsional loads. If a designer says a whole wing structure should be built a certain way just do it. There are many unseen forces which can catch out designers. EG Nigel Irens famous racing trimaran designer a few years ago had one of his racing trimarans fracture part of a crossbeam close to the main hull. Reason why? The foam in the foam glass beam fairing was to rigid and fractured the glass foam matrix as the boat raced through waves. More flexible foam was inserted, problem solved.
PS Wharram states that his designs are "flexible" to allow for these forces. His latest designs EG Mana 24 have rigid box beams which indicate he appears to be moving toward a more integrated approach.

 

One of my favorite designs is a boat called Sardine Run a 18 x 14.5 foot trimaran designed by Eric Hensalvel. The boat weighs 600 lbs and has a maximum displacement of 1100 lbs with a sail area of 220 square foot. Sardine Run Plans https://www.duckworks.com/product-p/eh-sardinerun.htm The boat is an excellent design for home building as it fully utilizes the build materials without adding any unnecessary bits. The hulls are basically 8 mm ply taped together with 450 gsm biaxe cloth and epoxy with minimal framing and BH's. The thick plywood single skin hull shape give the boat its structural strength. The external of the boat is covered with 300 gsm cloth. The cross beams are simple 100 x 3 mm aluminum tubes with wire water stays to give additional structural support. Where the cross beams cross the main hull an additional 2 layers of 8 mm ply is attached to the gunnels with a partial BH to provide additional support. The cross beams are bolted to hulls with 10 mm bolts. This design is all about the dimensions of each panel to provide stiffness and supporting angle lines to substitute for timber stringers etc. Look at the inward sloping bow deck panels that provide additional stiffening in the forward topside and add to the looks of the boat. From the sailing videos I have seen this is a fast fun boat that should be easy to build.

 

Sardine twin 5.5 is the catamaran equivalent of Sardine Run. Sardine Twin 5.5 Plans https://www.duckworks.com/product-p/eh-sardinetwin.htm It is 18 x 11.5 foot tube catamaran weighting 800 lbs and displacing about 1300 lbs carrying 205 square foot of sail that can go to 270 square foot. The hull construction is the same as the trimarans but it uses 150 x 3 mm aluminum cross beams (at least 6061 t6 or 6083) with a dolphin striker on the main beam. The differences between the trimaran and catamaran are interesting. Although Sardine Run Trimaran main hull is same up to about 75% from the bow the stern on the catamaran has been modified slightly by adding more buoyancy aft by lowering and flattening the run aft. The increased stern buoyancy is probably need to support the weight of the crew and provide more asymmetry to minimize pitching. Tris have one big advantage over cats, the center of buoyancy in the floats can be ahead of the center of buoyancy in the main hull when the float is depressed. This if done correctly can reduce pitching quite a bit upwind. The crossbeams have been enlarged due to having to carry greater loads over a longer unsupported span and to have to carry a slightly higher displacement. The righting moments of both boats are similar so they can carry the same rigs but the cat will be slower in light airs due to more wetted surface due to being a cat. Both good designs if you want a camp cruiser for bays or short coastal hops.

 

Trimaran cross beams and catamaran cross beams both need a deck and torque box structure to with stand many forces as indicated in a previous post. The Twiggy foam glass beam is relatively simple but if you are going to construct it in ply and timber it requires good design and a lot of work. The following PDF will give you an idea of the work that can be involved. The leading edge of the cross arm wing needs strength to resist the day the float hits something at speed. Also the forward under wing ply need to be good quality as it often takes the first wave hit shock loads. I have been on a 37 foot cat that had a one foot square hole punched in its 12 mm thick ply forward wing deck which had a timber support in it. The boat was going upwind in 30 knots. In trimarans the faster you go the closer the outer end of the cross beam gets to the water. Also look at the way ribs and keel structures were done in the foam glass hulls in earlier designs.

 

The Tri 650 is a swedish racing trimaran 23 x 20 foot that displaces 1400 lbs and weighs 900 lbs. Its float and main hull are 400 gsm carbon rovings either side of 8 mm H100 divinycell. There is a 300 mm wide strip of 300 gsm uni carbon fore and aft on the inside and outside of the keel line BH's and internal structures are 400 gsm carbon fibe on either side of 8 mm divinycell. The cross beams are 243 x 140 mm with a 3 mm unidirectional carbon fibre shell (plus some wraps of carbon fibre around the beams). Inside the beams is a 10 mm divinycell vertical web with 2 mm unidirectional glass on either side. The beams go the full width of the boat are are glassed into the shell. Armature built but well done.

 

A start discussion on bridgedeck catamaran cross beams. There are many ways to build a catamaran cross beam from the simple EG Catalac 28 which was a shhet of 19 mm ply glass to the top deck and hull mo;ding to something a lot more complex. The Catalac was built similar to a mono, 2 very strong glass molding that carried most of the forces that only required a bulkhead head to separate the moldings and carry the mast load. The more complex crossbeams are built from a variety of materials from all wood and ply basic boxes with strong strong top and bottom flanges of multiple layers of timber with timber BH's separating the flanges. The web faces fore and aft are plywood. EG 7012 and 7008 below for a think is a 45 foot cat. The more complex approach is shown in 1480 study. which is mainly a foam glass structure with timber inserts to take varying loads. The boat is a 48 ft cat. Shuttleworth and Hughes have designed cats around 50 feet with freestanding masts in the bridgedeck structures. These beams are really complex to design and build and will be the subject of a different post. There is a lot more to come on this subject.

 

John Shuttleworth is a good designer and engineer. Result his designs tend to be more complex to build but are structural sound whilst being lighter than others. As an example his 52 x 33 foot cat displacing 30000 lbs carrying 1100 square foot of sail on a freestanding mast aerorig. The crossbeam design in this cat is interesting and complex. The free standing mast puts up to 76,000 lbs of side force on the bridgedeck roof and 64,000 lbs underwing when sailing. Freestanding masts mainly put lateral (and fore aft) loads on the cross beam structure. Stayed rigs put mainly compression loads on crossbeams, if the rig is a 3 wire rig stayed to outer side of hulls etc. As shown in the attached diagrams the structural box sub beams are designed to go along load paths to resist the forces. The cross beams also have to handle the normal dynamic loads of the hull structure. The next loading the crossbeams have to handle is shock loads and working cycles over a lifetime and the final consideration is the safe working load of the materials used in the build of the beams and a safety factor required for any bad building technique/work. Result designers often design EG cross beams to use safety factors of 10 times what theoretically is required to hold a mast up. Kurt Hughes 55 cat design and Eric Sponbergs 60 foot cat with free standing masts have similar structural considerations with Hughes find that the loads on the cabin roof reduced the further away from the mast as the "roof' structure acts as a full flat support plane. Please read through Shuttleworths Aerorig document article aero design http://www.shuttleworthdesign.com/52AeroDesign.html will give a better insight into one form of crossbeams. Attached are diagrams of Shuttleworths cat and 2 diagrams of Hughes 55 foot cat.

 

Mast section as beams has a lot going for it after reading about the all the detail in these various beams you write about Oldmulti.
Building that at home is amazing, 22ft racing tri, home build.. @Sailing Anarchy

 

Couldn't find it...

 

You can just google , 22ft racing tri, home build.. @Sailing Anarchy ,, it should come up.

 

Woods Strider catamaran is an excellent small tube cat design 24 x 14 (or 17) foot displacing 1800 lbs carrying from 270 to 370 square foot of sail area. It was designed 30 years ago and are still very good boats. The reason its for the discussion is the 3 different versions of main crossbeams available for the same boat. The original cross beams were timber and ply triple box beam of 160 x 125 mm overall dimension with two 6 mm ply fore and aft faces and two internal 6 mm ply webs separated by 35 x 25 mm top and bottom flange timbers between the faces and first web. The centre timbers separating the centre webs 65 x 25 mm flanges top and bottom. Various reinforcing timber are done at hull connection points. A dolphin striker is also attached. Attached word document comes from woods free sample plans. Next beams that were available were 150 x 3 mm aluminum beams with a dolphin striker on the main beam. Finally came a"deep mast beam" that can be fitted to all smaller woods cats to about 26 feet. This beam is 500 mm deep x 118 mm wide with 9 mm ply faces and 100 x 30 mm top, bottom flanges and diagonal trusses. The function of the deep mast beam is it structurally sounder and can carry bigger rig loads with greater margins of safety. Attached is a plan from woods web site of the deep mast beam. Each type of mast beam have worked well over many years. Its your building and material choice.

 

Oldmulti,

Great thread, thank you.

Are you familiar with the Malcolm Tenant Streaker 23?
Comments, thoughts?

Cheers