DIRECTIONS IN DESIGN FOR FOLDING TRIMARANS

Article last updated 18th August 2018

Where are we headed with the design of trailerable trimarans, and in particular those that employ the Farrier Folding System?

We explore five different aspects of folding trimaran design to see where improvements might be able to be made.

1. The beam rebates. Do we really need them?

2. Styling and a better cabin for cruising

3. A more stable platform. The X beam

4. A more elegant folding systems. 

5. A better way of attaching the floats to the beams for assembly/disassembly


For family cruising, for racing, for kicking back on the water, it's hard to beat the convenience and versatility of a trailerable trimaran with the Farrier folding system.

 

The fast and easy deployment of the folding system provides the option of storing the boat on the hard stand, folding on the water for berthing in a marina pen of limited dimensions, or towing it off down the highway to a distant regatta or cruising ground.

 

The Farrier range of folding trimarans introduced a whole generation of sailors to multihulls

 

But where is the fizz? Where's the sparkle? Where is the effort to improve performance, bring on some more inspired styling and work on some of the engineering issues that are holding these boats back from becoming not just "accepted" but sought after and sailed with vigor, passion and pride.

 

For quite some time innovation and design development for trailerable trimarans with the Farrier folding system has followed a very flat curve.

 

 

We live in world where technology and innovation are changing our lives almost on a daily basis, yet you could fairly argue that trailerable trimaran design has seen very little evolution since the introduction of the F27 in 1985.

 What's holding this development back? What are the design features that need to be looked at to reinvigorate this genre of sailing yachts?

 

We've been taking a look at the currrent state of folding trimaran design from a range of perspectives including performance, accommodation, structure, style, and ease of contruction which of course affects the price.

 

We propose a range of concepts that could inject some fresh excitement into the world of trailerable trimarans in a series of design focus articles to explore these ideas. The concept model shown in these drawings and renderings is 30'/9.2m long and for now we're referring to it as the X-30.

 

 


DESIGN FOCUS 1. LET'S DITCH THE BEAM REBATES

Why do we have to have these rebates and how to get rid of them?

The beam rebates are ridiculously complex to design, to engineer and to manufacture. And they are not beautiful by any standard.  I've seen many attempts to minimize them or simplify them in various designs but do they really need to be there?

No they don't. We can design and build a perfectly good trailerable trimaran without the need for a complex system of rebates within rebates and mechanical fasteners in areas that are either impossible or very difficult to access. This can be done without making big compromises in the interior living space.


 

 

 

Right; This is the preliminary modeling we did for the beam rebates in the Airplay trimarans in 2012. These rebates have to accommodate two folding struts in two static positions - when the boat is sailing and when it is folded, and also through the range of rotation angles dictated by the folding geometry.

 

The shape of these rebates is further complicated by the intersection of the trajectory of the folding arc as the boat is folded. 



Models for the fore and aft beam rebates in the Airplay 30 looking from inside the boat. The lower strut is highly loaded while sailing and the engineering of the rebate is critical.

However what is equally important is that the laminate specification is clearly documented (not easy for such a complex part), and that the workers are adequately trained and supervised to ensure the integrity of the final structure.


The rebate in the side of the main hull and deck for the RAW30. On this boat the rebate did not extend up into the cabin side because the design was performance focused and we were able to keep a relatively narrow cabin. That's normally not an option for a cruising boat where the upper folding strut requires a slot in the cabin side above the deck.

The complexity of the rebates  is not only in the hull side, it continues up through the deck and into the cabin tooling as well, with the depth of the rebate dependent how wide the cabin is at the location of the forward beam. It is desirable to move the beam aft for performance, but move it too far aft and you're restricting the cabin shape or make the rebate way too big.

So if we take the hull we propose and make it as wide as we can at the gunnels, still allowing for the beams to fit within our nominated width restriction in the folded position, this is what we have.

Rather than simply cut out a big rectangular box at each beam position we carve out the minimum shape required, and hence the complexity.

The hull shown here is 30'/9.2m long and the folding struts are from the Raw30/Airplay 30 design.

 

This arrangement requires that the upper and lower struts be aligned in the athwartships axis. We have to use some design gymnastics to prevent the upper folding strut from colliding with the upper folding pin for the lower strut as this would otherwise impede the completion of the folding. And so the design is further complicated.

 

In the image at right you can see that the upper and lower struts would be intersecting at the top in the folded position if the pivot pin was to go through the upper strut without a break.



The rebates used in existing designs are not only in the side of the main hull and cabin top, they are also in the underside of the cross beam, lending more complexity. Access to nuts and bolts for the metalwork is difficult in some designs, impossible in others. Note that in the photo on the left the nets have been partially detached to allow the beams to fold. With the X-30 configuration the nets can stay permanently attached.

Looking for Solutions

How we did away with the beam rebates

In 2012 we reconfigured the geometry of the Farrier folding system so we could raise the beams and use high buoyancy floats.

For the 30-X concept design we reconfigured it once again and also cut away the sheer line starting at the forward beam and running aft.

So now the inboard beam ends simply sit on the ledge created by the cut away and the upper folding strut at the aft beam simply sits on the coaming at the aft deck.

 All of the other struts mount on the hull side with no rebates required in the main hull or underside of the beams. The cabin is free to extend right out to the 2.5m trailerable width limit between the beams.

For this version the trailerable width including the floats and beams is 2.9m.


30'/9.2m Trimaran in Folded State


DESIGN FOCUS 2. INSPIRED STYLING AND A CABIN FOR CRUISING

Studies in how car forms can express emotions of luxury, privacy and elegance. From a study by Andhika Dimas.

One of the big issues I've grappled with in designing trailerable trimarans is what I refer to as the "tunnel effect".

High performance trimarans by definition have long skinny hulls and the feeling of spaciousness below decks is not something you would want to brag about. So how to minimise this effect?

The deck height is determined by the by the height of the beams and this in turn is determined by the need for wave clearance. The nets are attached to the beams on the fore and aft edges. You dont want the cabin side windows under the nets and accordingly the bottom edge of the cabin side windows has been determined.

 

We're left with two options in determining the cabin profile.

1. Keep it long and low and suffer a low window profile which contributes to the tunnel effect by restricting light in the cabin. 

2. Have the cabin profile stick up relatively high above the deck.

 

The second option is not in keeping with our aesthetic preferences and not in keeping with a style that is suggestive of "modern" or  "sporty". Our eyes generally prefer the look of cars and boats that have a cabin structure that is modestly proportioned in relation to the main body, especially with more length and less height. If we can lower the height where the nets attach to the hull without changing the beam height we have more surface area to play with in the cabin side and more options in finding styling solutions.


Comparison of midships section in the saloon for the concept X-30 (double blue line) and the Farrier F32A (Dashed black line). The cross section area for both boats is very similar but the X-30 carries more width down at seat level. The beam to length ratio for the X-30 main hull is 1:9

The X-30 carries the deck height right to the transom allowing for the option of an aft cabin with a lot more volume than the F32-A


The spar and side deck panel that connect the beams just outboard of the cabin side are integral with the complete beam structure. They serve four purposes.

 

1. They contribute significant additional rigidity to the platform.

 

2. The side deck (shown with a wood texture) provides a solid footing for access to the mast base and foredeck.

 

3. The spar (the fore and aft beam) serves to attach the nets so they don't have to be partially detached for folding as is the case on existing folding designs.

 

4. With this configuration we are able to drop the side deck down lower  than where the nets would be normally attached to the side of the main hull, so effectively the cabin side is higher, allowing for a bigger window and more light in the cabin.



DESIGN FOCUS 3. CREATING A STABLE PLATFORM

RETHINKING THE BEAM CONFIGURATION FOR TRAILERABLE TRIMARANS

One of the essential elements required for good performance is a stiff platform. Wracking of the hulls will detract from keeping a tight rig and any flexibility is especially detrimental to upwind performance. This is one of the reasons why a lot of offshore racing trimarans have their beams in a semi "X" configuration.

 

With a Farrier type folding trimaran the boat is even more susceptible to twisting because the beams are not rigidly connected to the main hull. They have x axis pivot points where the lower strut attaches to the beam and the main hull, and the inboard beam end is connected to the main hull with just one or two locking pins.

 

Some models that use the Farrier folding system use diagonal wires under the nets to minimise the twisting and the fore and aft displacement of the beams relative to the main hull.

 

What we have proposed is a much more rigid one piece configuration which incorporates  the semi "X" formations for torsional stability. This constributes to a much more elegant overall design with two other distinct advantages from an engineering point of view. We will explore these in Design Focus points 4 and 5.


Not convinced about platform stiffness?

You can try this at home. I did it myself when I saw trimarans being designed with X beams in the 1980's You just need ten paddle pop sticks and some super glue.

Glue the sticks up in the formation shown here and try to twist the structure. 

Caution; don't stick your finger in your ear or anywhere else if you have glue on it.

If you do this please send photos and comments, we'll post them on our Facebook page.


DESIGN FOCUS 4. A MORE ELEGANT FOLDING SYSTEM

LOSING THE LADDERS

The aluminium struts in the images above are typical of the struts that have been used on existing folding designs including the the Raw30 and Airplay 30 designs we did in 2012. The X beam configuration allows us to go to a more elegant single element lower strut.

With the "X" beam configuration we have a very stable platform that doesn't require additional diagonal bracing to prevent fore and aft movement of the float relative to the main hull.  We can use a single strut instead of a double "ladder style" strut to carry the beam load into the hull and we don't need diagonal wires under the nets as you see on some designs. 

 

The folding struts can be carbon or alloy and they can be foil shaped for minimal windage. Designs that use rebates in the hull sides require the upper and lower struts to be aligned in the athwartships axis. We have done away with the rebates in the hull side and the under side of the beams - so the upper and lower struts no longer have to be aligned.



DESIGN FOCUS 5. BEAM CONNECTIONS FOR ASSEMBLY AND DISASSEMBLY

INTEGRAL BEAM ALLOWS FOR MORE SECURE CONNECTIONS AND EASIER ASSEMBLY/DISASSEMBLY FOR CONTAINER SHIPPING

Most trailerable trimarans have to detach the float from the beam to enable container shipping. Various methods have been used including sleeves in the float and plates at the outboard beam end that bolt onto a deck plate.  Both of these methods can be complicated to build and can be susceptible to cracking. The integral beam structure has two very significant engineering advantages.

 

1. The integral beam minimises torsional movement in the beams that would you get with with two separate beams. This torsional movement is the primary cause of cracking or failure at the beam to float join.

 

2. The integral beam  distributes the beam to hull connection load more widely between the beams, providing the opportunity to create a rebate or a channel for positive location and use bolts for better load distribution easy assembly/disassembly.


Top Left: Drawing for a beam in sleeve arrangement. In this detail the beam is drawn into the socket and located with bolt in line with the beam axis. From an engineering standpoint this the best arrangement for positive location of the beam but it is not easy to build.

 

Top right: The beam might be glued into the socket with or without a flange on the deck. Obviously this eliminates the possibility of disassembling the beam from the float.

 

Lower Image: The integral beam allows for a range of connecting mechanisms. Under sailing loads the highest load concentration will always be at the fore and aft extremities of the beam platform but the integral beam arrangement significantly reduces the load concentrations. We're not showing the exact connecting mechanism here but it will probably be a channel or a rebate along the deck of the float that the beam structure will bolt to.

 

 

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