Multihull Structure Thoughts

Guzzis3. Aluminum is possible below 30 foot. Page 48 this thread has a 25 foot trailable aluminum tri that weighs 1600 lbs by Langevin (now retired). Page 22 has a 2o x 20 tri built of 1.54 mm aluminum skins that displaces 2000 lbs. On page 33 the C329 32 foot a full cruising trailable aluminum tri that displaces 10,000 lbs. So it is possible. The weight of 9 mm ply is about 1.5 lbs /square foot. The weight of 3 mm aluminum is 1.28 lbs/square foot. Allowing for framing required in a EG 28 foot cat the weight of a plywood timber cat hull would not be to different to a 3 mm aluminum extra framed aluminum hull. Based on what I know 3 mm plating on an appropriate framing and stringers would be strong enough to handle the loads. Attached are the 25 foot tri jpegs.

 

@guzzis3 we have similar width with no signage across the whole country. If I want to go across state lines with a wider load I would need permits for each state so that limits the easy cross country travel to ~2.6 M wide. Wider is possible, but requires planning for permitting.
In this state a permit to move up to 3.35 M wide is $120 per year, or $20 for a month. No signage needed, just go.
That really opens up the useful hull width here. Length is not restricted in any meaningful way, only in the practical sense
Now it is down to how much you want to handle in terms of weights and forces to assemble things, step the mast etc.
This fits into the "Transportable" category that Woods talks about. I think his trailerable designs are brilliant in the sense that he thought of all of that, and what one person could reasonably do.
I would be tempted to convert as you are suggesting, to a longer boat design. Probably the ideal would be a large open deck design that I could make a custom light cuddy for.

 

The following s a guide to the future and a compliment to the past. McConaghy Boats has released the new Vortex pod racer, a one-person sailing rocket on foils with a relatively easy-to-use pod seating and control centre. The Vortex is 17.4 x 19 foot weighing 330 lbs. The 32 foot mast carries a 180 square foot mainsail and a 36 square foot jib. The Foils are T sections and the draft of the pod racer varies according to the boat flying etc. In full flying mode the boat can be 3.3 foot above the water as it does 30 plus knots. One advertising claim said it was possible to go 40 knots. That is yet to be proved. The take off speed is 12 knots.

And the compliment to history is the Hobie Trifoil designed and sold in the 90’s. Very similar concept with dual rigs.

The collapsible cross beam allows the Vortex to be stored on a cart with the platform raised, keeping installation times to a minimum, and for long-term storage or transport the cross beam and platform can be removed. The Vortex pod racer can be launched directly from the beach or via a ramp. Once in the water, the tiller and front T-fins are easily deployed and retracted from the cockpit controls.

The Vortex Pod Racer is a single hander with a seat available in the pod. Now the fun. The boat is foot steered to allow the skipper to play with all the control ropes that adjust the foils and rig. There are 12 control lines and the builder offers an electronics package to assist you in the foilers control.

The boat is built at the McConaghy factory in China. The Vortex Pod Racer has a carbon fibre hull and foils, constructed using pre-preg foam core. The wings are pre-preg carbon torsional beams with wing fairings to take the torsional loads of the foils and rig. The Vortex Pod Racer is hand finished and sprayed with 2K polyurethanes in a temperature controlled, dust-free environment.

This is another attempt to create the “Laser” of foiling but at $US40,000 it may be a bit of a struggle. The price is similar to A class cats. I hope the boat can live up to the advertising as it will be a very interesting design. The jpegs give the idea. The final jpeg is the Hobie Trifoiler.

 

There is a difference between what's possible and what's sensible. Anything below 3mm aluminium skins is going to be a pig to shape and join but more importantly will have no durability. You can build your 8m catamaran in aluminium but it will require very good fabrication skills and as soon as you start using it it will accumulate dings with every sail. And then there are all the other problems, insulation, electrolysis, grain transitions...

Anyway do as you wish, but building a small aluminium catamaran is a hard path.

 

This is an update on the HH 44 spoken about earlier. I suggested the structure was carbon fibre PVC foam and have found that the structure is more refined, unlike their larger brethren, HH44s will not be built with full carbon hull skins as this does not offer a weight reduction at this size, but resin-infused E-glass and M Foam composite epoxy laminates with a carbon skeleton comprising of all bulkhead, stringers, beams and reinforcing for loaded areas. There is little to justify the material cost of using 100 per cent carbon in a boat this size. Nonetheless, carbon is used throughout the boat where its strength and light weight is suited to the engineering integrity of the structures such as the monocoque longeron, stringers and truss structures that tie together the loads from the hull bows to the spar through the headstay. The curved daggerboards are carbon.

The HH 44 is 44.3 x 24.5 foot with a weight of 20,740 lbs and a displacement of 27,500 lbs. The LOA with the bow spirit is 49.7 foot. The 56 foot carbon fibre rotating mast carries an 799 square foot mainsail, 550 square foot solent jib, 287 square foot self tacking staysail, 1,181 square foot screecher and a 1,603 square foot gennaker. The length to beam on the hulls is 9 to 1. The draft varies between 4.6 foot over its rudders and 9.8 foot with its carbon fibre C curved daggerboards down. The underwing clearance is 2.5 foot.

Let’s compare it to a Grainger designed Raku 44. The Raku 44 is 44.3 x 23.5 foot with a weight of 14,000 lbs and a displacement of 23,700 lbs. The optional 62 foot rotating aluminium or carbon mast carries a 745 square foot mainsail, a 327 square foot solent, a 575 square foot code 0 and a 993 square foot gennaker. The hull length to beam is 13 to 1. The draft varies between 3.5 and 7 foot with straight boards.

The build of the Raku 44 is e-glass biaxial/ triaxial cloths and PVC foam with vinylester resin or epoxy. Carbon fibre is used as reinforcement on the flanges of the cross beams, unidirectional e-glass is used in other areas of reinforcement. There is a lot of flat panel building with curved hull bottoms added later. The boards are either cedar glass or foam e-glass.

The performance modelling of the cats depends on the sails they are carrying. The first rig configuration is the mainsail and self tacking jib. Under this configuration the Raku 44 will be about 3% faster than the HH 44. Under a rig of mainsail and gennaker the HH 44 in theory should be 10 % faster than the Raku 44, but the performance model I used does not have a correction factor for the length to beam ratio of the hulls. The HH 44 has fatter 9 to 1 hulls than the Raku 44 13 to 1 hulls. The length to beam counts at higher speed. Translation, in the real world either of these boats will deliver 240 to 300 mile days if sailed hard. The sail area under main and self tacking jib is nearly the same. But under mainsail and gennaker the HH 44 needs 40% more sail area to gain 10% more performance than the Raku 44. Translation, it will be easier to achieve a good speed with less sail area with a well setup Raku 44.

What do you end up with? Both cats have about the same payload carrying capacity and a similar accommodation. A HH 44 is a good boat at a high price that will require electric winches to handle 1600 square foot gennakers. The Raku 44 will be a “less exotic” but cheaper build both in the structure and in the rig due to less sail area.

Interesting cats and I would love to see a race between these to performance cruisers. The jpegs are a guide.

 

Today is about a simple home designed tri done by a guy who has a reasonable understanding of what he is trying to achieve. The mainhull shape is chine to allow flat panel foam glass construction. The tri is 23.4 x 7.2 foot mainhull with an anticipated beam of 18 foot over the floats. The tri can fold to 8.2 foot. The weight is aimed at 1,100 lbs with a loaded displacement of 2,200 lbs. The sail area is not specified but a large Hobie rig would be a starter with about 270 square foot and could go to EG 370 square foot. The length to beam of the mainhull is 6.7 to 1. Hull draft will be about 1 foot. The rudder is a transom hung kickup. An offset kickup centre board acts as the front of cabin seat.

The interior has a “double berth” forward and 2 single berth/seats in the main cab area. A camping galley is indicated. The cockpit will seat 4.

The structure is a “copy” of similar sized foam glass trimarans. The hulls are 400 gsm e-glass biax 12 mm klegecell or similar foam and 400 gsm e-glass biax inside with polyester or vinylester resins. The builder “confirmed” the specifications by contacting an F-22 builder. He thought that the inside of his hull was covered with 400gsm glass, and the outside with 600 gsm glass.

The build of boats like this can be interesting. You can build a cheap ply wooden female mold, wax it very well and resin infuse under a vacuum bag the external skin and foam then resin infuse the internal skin. This approach means there is minimal taping and fairing later. Second approach is build flat foam glass panels and cut them to shape then tape them together. Real problem here is 2 fold. Unless you put 1 mm indents in the foam glass panels 50 mm around the edge of the panels when you tape them together you will need to fair your joint. The 1 mm indents allow the tapes to join panels do 2 layers of 400 gsm biax tape then a small fairing job that does not impact our flat panel finish. The 50 mm x 1 mm deep foam “fairing channels” take some effort to do well.

Second problem with thin glass layups is you need to do them well. That means no pin holes in the layup and the surface sealing has eg an good epoxy paint over the top. You will be surprised as to how porous an average thin glass layup is, especially if a cheap paint system (eg exterior house paint) is done over the top.

The final approach is to do a male mold lay the foam over then glass the outside fair paint and take off the hull and do the interior glassing. This is the hardest to do well because you in effect build the same hull 3 times to produce 1 hull. Believe me when I say a cheap female mold and resin infusion looks the hardest but can end up being the easiest in the long run especially in smaller chine boats.

An interesting design attempt. Hope it all works for him. Jpegs give the idea.

 

Thick foam and thin glass is the expensive option. I guess it saves a few kg but $$$

8mm foam wold be enough with slightly heavier skins maybe as much as 600gsm. Bit heavier but much cheaper....

Here is a thought. Make the planks with a light glass either side, join and fair them, then "tape" with wider strips of glass so they but up against each other. If you were careful you might be able to create a smooth surface which would need less filling and fairing. 2c

 

I feel a little bit sad to be taken as a bad example....anyway I want to thank again the people who get the time to answer to my questions.
Of course I will not advise anybody to build boats with wrong materials as I did, especially because it is a time consuming process and to get to the end is expensive anyway !!!
I have no experience with cat and cat building, but I have been sailing and working in the yacht industry for 20 years and I was in brazil when I visited the shipyard of batavento sailmakers and the owner told me about this free plan.
Never thought to build until life circustances bring me to live in dominican republic. Before to build I showed the design to a friend who already builded few cat....off course bigger than this one.
Back to the construction I have lunched the boat a few days ago with a prindle 18 mast, mainsail and jib. Used also the rudders and the beam that I made wider to go with the design. The daggerboards were not ready so I sailed without.
Boat looks balanced on the rudders, just difficult to tack. I was so happy that could not sleep for 3 days !! Total weight should be around 350 kg.
May be the boat is not going to last long....and you can be sure that I will write this down too. Yes, I got your comment as A personal attack, but my answer is just to say that I have maximum respect for boat designer and boat builders....and off course keep reading your and others people post and trying to understand as much as I can...ciao.
Luca Campana

 

Here is a link to Luca's thread re the build of the sailing cat mentioned in his post above.
Rival 21 from batavento sailmakers brazil https://www.boatdesign.net/threads/rival-21-from-batavento-sailmakers-brazil.62169/

Luca, I don't think that anybody was 'attacking' you in the thread above - rather, they were / are worried about you throwing money at a boat being built with (some) inferior materials.

 

Lucianbarba. My apologies, if you thought you were being "attacked", I support your design choice but using MDF (chipboard) as hull material is not good. You need to seal this material very well for it to survive even a short time in a marine environment. The rest of the build, rig etc appears to be well done. I hope you enjoy the boat and when you can afford to, upgrade the hull skins.

To todays design. The Scarab 8 meter trimaran is an older design by Ray Kendrick of Team Scarab. The tri is a performance cruiser built mainly in plywood. The Scarab 8 is 26.4 x 21 foot that can be trailed with an 8.2 foot with a farrier type folding system. The empty weight is 2350 lbs weight. The displacement would be about 3300 lbs. The 36 foot mast carries 430 square foot of sail on a fractional rig. The draft with the centre boards in the floats. The rudder is a kickup on the main hull.

Please be careful about the PDF jpeg which shows a 10 to 1 length to beam on the main hull, as a quick calculation shows the drawing would be for and unloaded boat, a loaded boat would be at least 100 mm lower in the water. Rays other tris range from 7.5 to 1 to 8.5 to 1 length to beam on the main hull. If the main hull is 10 to 1 length to beam please ensure you build this tri as lightly as possible. Choose your plywood carefully.

Accommodation consists of a small double berth and three singles, space for a small galley and a porta-potti.

The boat was designed to be built using multichine stitch and glue plywood epoxy resin and fibreglass with a pivoting transom hung rudder and fibreglass/epoxy beams built using a top hat section. The ring frame supporting the folding system is structural grade aluminium with plywood faces. Ray has on his later designs the ring frames supporting the folding system are foam glass with a lot of solid glass at attachment points, lighter simpler to construct and less maintenance.

The basic build is in 9 mm Gaboon ply in the main hull with 6 mm decks. Ray normally has e-glass unidirectionals on the inside of the hull and biax or triax on the outside of the hulls in epoxy. The floats on the jpegs below were built in 12 mm foam and glass which is not shown on the material list. The ring frames are made from welded 6082 T6 aluminium as are the control arms for the folding system. The cross arms are a top hat section fiberglass moulding that has worked very effectively on Rays range of tris. The windows are 8 mm Acrylic or polycarbonate.

This tri could probably be built with a full foam e-glass shell skin which would reduce the weight by 200 to 300 lbs. The shell would be EG 10 mm PVC foam with 600 gsm biax on either side. The bulkheads etc would still be ply etc.

The jpegs below are mainly of a German kit provider at Trimaran Bootsbausatz - Trimanufaktur Bootsbausätze • Erste Baubilder http://www.trimanufaktur.de/Fotos

The pdf contains the study print drawings of the Scarab 8 meter. An interesting design.

 

Part 2 Scarab 8 meter.

 

So, I have a question to interrupt the thread. All this talk of carbon fiber..

Isn't cf a mismatch in a layup of epoxy and M foam? Elongation at break is like 1% versus the others at 5-8%. Is this overlooked?

On the other hand, the hull doesn't 'care' what the bulkheads are made of. Nor would it 'care' what tabbing reinforcements were made of. My boat, for example, used 600/225 tapes for most of the tabbing. The stuff offers incredible stiffness; the weight penalty is severe. A 3 layer tabbing is 150 oz/yd with resin by hand. If one were to use cf, perhaps a comparable layup would be what 1/3rd the weight? My transom was hand laid with 8 layers of 1700 or 134 oz. Elongation hardly seems relevant.

Of course, the hulls of my boat are another story.

But when does the mismatch really matter? At impact, of course. So does the mean only hullsides subject to dock bumps ought to consider the mismatch? Thanks and apologies if the question is amateurish.

I did a calculation that my boat is 300 pounds heavier due to the mat component alone in the 600/225 tapes. So, it sounds like carbon tapes would have been really wise. Also, easier to fair thinner. Could I be say 600 pounds lighter cf tabbing? Or does the elongation still offer to keep my boat from shattering like glass in some bad seas?

 

Fallguy, a simple question but it needs some generalisations for some answers. The question is “Isn't cf a mismatch in a layup of epoxy and M foam? Elongation at break is like 1% versus the others at 5-8%. Is this overlooked?”

If you are doing a sandwich hull shell then you can look at it in 2 ways. The core material can add EG tensile strength to the structure or the core is just a separator to the real strength which is in the skins. Simple example. A western red cedar core adds real tensile and flexural strength therefore you can add lighter skins to get a total “strength” solution. For Western red cedar cores carbon fibre skins are a better match due to similar elongation characteristics (about 1%). E-glass elongation is in the 3% range. For hull shells with foam cores e-glass elongation is a better match to foam. I will not get into the variations of polyester, vinyester and epoxy characteristics beyond saying please use vinylester or epoxy with higher “strength” unidirectional or fabric materials.

Lets turn to masts to highlight another issue. Aluminium masts work well as they are cheap relatively light can handle the compressive and bending loads well. A wooden mast matches an aluminium mast and if it’s a wing mast cane be lighter. Now let’s look at fiberglass and carbon fibre masts. Pure fiberglass masts made of e-glass can be heavier if they need to be as stiff as an aluminium mast as the elongation of e-glass allows the mast to flex more. To counter this flexing stiffer walls are required which is either additional thickness on the walls or higher tensile strength materials. Carbon fibre masts used in masts are a good substitute for aluminium and in theory could be built much lighter with thin walls. Reality the walls have to be about as thick as aluminium masts as the walls if they go out of column buckle and bang, back to thicker than required walls to minimise the chance of buckling. A carbon fibre mast will be lighter than an aluminium mast and if it is very well designed and controlled, much lighter. Home built carbon masts are possible but be conservative.

Now we get to Fallguy’s question. If you are going to do a cross beam top and bottom strength flange DO NOT mix e-glass with carbon fibre. Use one or the other. E-glass and carbon fibre elongation differs. Translation the carbon fibre will take the initial load and break before the e-glass takes up any substantial load. But if you have a rib or an underwing stringer the walls can be e-glass foam and the caps can be carbon fibre. The carbon fibre caps need to be designed to take the full load, the underlying rib is a notional separator from the hull or underwing.

Tabbing in a bulkhead to a e-glass foam hull could be carbon fibre without to much effect but its easier to use eg epoxy with 600 gsm biax tape and no CSM. A transom could be carbon fibre if its is designed to attach to a e-glass foam hull. So why isn’t carbon fibre used all the time?

Beside cost, the down side of carbon fibre is its strength and lack of elongation. When carbon fibre is loaded beyond its limits it shatters, e-glass will flex and fracture a little before it fully breaks. I can assure you a multihull structure flexes and moves continuously as it sails. When a performance cruiser is a “full” carbon fibre structure they have to put additional carbon fibre weight in the EG bottom to with stand sitting on the hard or on a point load like a rock. A well cared for racer can have a much lighter shell structure as it doesn’t bang into wharfs or hit bottom on rocky shores etc.

The use of s-glass or Kevlar has superior characteristics to e-glass and can get near carbon fibre strengths in some characteristics. Kevlar according to John Shuttleworth does not work well with some resin systems which limits it usefulness in boat applications. S-glass can be very useful if you have a racer or need specific strength in eg cross beams. But the real truth is a well designed and built e-glass vinyester structure will be light enough as long as you don’t load it down with airconditioners, fridges, electric winches, several dinghies and full fabric liners in all cabins etc.

Basically use higher strength resins, minimal or no CSM and vacuum bagging or resin infusion to minimise weight. And also trust your designers specifications and don’t ”strengthen” the boat structure because you want to be “safe”. A table to compare material characteristics.

 

The following is a proposal for the Volvo inshore racing circuit during Volvo global race layovers. The design was not selected but it was one of many interesting design proposals. The Volvo Ocean Race Inshore Foiler is 39 x 29.5 foot with a weight of 3,136 lbs and displacing 4030 lbs with the 3 crew and 1 guest. The 55 foot rotating carbon fibre wing mast has an area of 107 square foot with a 688 square foot mainsail which is a deck sweeper. and a 807 square foot code 0 for light wind take offs. The draft varies from 8 foot to 2 foot depending on how high it is foiling off the water. There is a bow steering foil with 2 aft support foils. The crew work in an enclosed cockpit.

The performance is very fast! The foiler will take off in 6 knot wind speeds and start sailing at 10 knots. Upwind in 15 knots true wind speed the foiler will sail at 25 knots. In 15 knots of true wind speed down wind the foiler will peak at 36 knots. These are modelled projections but compared to the 50 foot 46 knot AC boats it appears to be realistic.

The structure is aimed at light weight using environmentally friendly construction materials. Central hull part made in Carbon Prepreg/Nomex sandwich for achieving high stiffness/weight ratio, while removable bow and stern laminated in single skin of Bio-composite made with Bamboo Unidirectional fibers and bio-epoxy for 33% of the total hull weight (97 kg). The central hull is made in Pre preg Carbon/Epoxy. The removable hull bow is made Bamboo/Bio Resin as is the removable hull stern. The ECO-cost of the composite due to the natural fiber low carbon foot print and the low amount of resin required. Beam having D shape will be done in Carbon Prepreg laminated in a female mould. Unidirectional tapes will run at the highest thickness section position. Single skin layout for the U shaped beam nose and sandwich for the vertical. Single skin Bamboo/Bio - resin made by unidirectional thin laminated plies will be used for the trailing edges fairing while natural fibers cloth/Bio - resin will be adopted for the more complicated shapes of beam/hull/nacelle connection fairings.

This is a very interesting concept. The jpegs give part of the story but the full proposal and design details are in the PDF.

 

Much of what you put into post #2068 has been on my mind, namely don't mess with things that not only work, but have been proven on many different builds of a great design. The fixer in me started out wanting to tweak some designs, but I am coming back to the way of thinking, don't mess with success. I might stick on some really small improvements, but nothing that alters the balance or weight or windage, or structure as intended.
I really like the woods micromulti cats, so much so that it is hard to choose one. My only concern is what the daggerboard represents as a collision/grounding hazard, and want to incorporate some sort of safety there to have the board fail before it destroys the hull it is in. Puget Sound has plenty of trees etc barely below the surface. Dad hit one at speed in his 12' homebuilt and sheared the lower unit right off a Wards 25 HP outboard. Luckily he got a quick tow in, but that was a total engine loss.
As fast as his little 25 HP runabout was, one of those fast daggerboard cats would be nearly as fast.