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Crossbeam folding descriptions
Folding mechanisms for multihulls with a single outrigger

Option 1

This is my preferred version. Crossbeams are mounted directly onto the hull rather than on a sponson. Advantages are that the lower spar directs its forces into the seat position of the main hull, at this level and below a full width bulkhead is located. This gives a better transmission of forces. Addtionally the greater vertical distance between spars results in lower stresses in each of the two spars.

Below please find a photo of a trimaran with a similar mechanism, the spars seem quite modest but are probably adequate for the conditions that the boat finds itself in

Options 2a and 2b

Inspiration for this mechanism comes from Dudley Dix and his 3 fold 6 trimaran. Here the spars are mounted on a sponson that clears them out of any wave action. There is of course quite reasonable loads in the sponson, however this can be built strongly enough to withstand these forces

Now for structural reasons it would be nice if the lower edge of the sponson metup with horizontal shelf which is at the sitting level. This would mean that the clearance between the sponson bottom and the waterline is only 18cm when fully laden. This seems just OK, as this is not going to be a speed machine. Additionally the further away from the hull the sponson is the greater the clearance. Looking at the photo below shows this idea with a larger boat and very low clearance.

Lastly if we take this lower clearance concept and increase trailering beam by a few inches, and reduce distance overall beam by 2 or 3 inches then it becomes possible for the folder outrigger to sit ontop of the sponson, retaining a low center of gravity. This layout is described in the sketch below

Calculations for lower spar - mechanism 2a

Now assume that maximum load of float is it's displacement (say 700kg) and all that load is absorbed by the front crossbeam, calculations are as follows

Tensile yield strength of marine grade aluminium = 240MPa, source wikipedia
Now a 60mm diameter 3mm thick aluminium tube has a cross-sectional area of
= pi x (30 x 30) - pi(27x27)
= pi x 171
= approx 537 square mm
= 0.5376 x 10 ^(-3) sqr meters

Now maximum load that can be withstood (Newtons)
= tensile strength x area
= 240x10^(6) x .537x10^(-3) = 240 x .537 x 10^3N = 129 x 10^3N
divide by 9.8 to give kg = 13150kg

Now load from outrigger may be displacement = 700kg
Now given the 5:1 ratio of given slope of crossbeams, gives a load of 3500kg on each strut in

now 13150/3500 gives a factor of safety of 3.75
This safety factor would hopefully allow of dynamic loads

Some more things to consider are this, when the outrigger works as a float the bottom spar will be in tension and the top spar in compression. In this situation the tension in the lower spar can not exceed 13 tonnes. This seems a high figure and probably more than the structure of the hull could withstand

When the outrigger works as a counterweight, the lower spar will be in compression. This is potentially more of concern because the small diameter of the lower spar is prone to buckling. Happily when working as a counterweight the load will be less as the weight is solely the weight of the unladen outrigger plus any ballast. This combined will be less than 200kg. Thus loads in compression are 3.5 times less than any likely to be subjected to in tension.

To ensure against any buckling a small amount of framing as per tradional truss design can be added at the most minimum of weight penalty and this seems a prudent measure.

Option 3

This is a sliding beam setup. One of the main issues here is trailering beam. If trailering or retracted beam is deemed to be greater then the amount of beam bury can be increased. My present tacking outrigger is just under 6ft from center of the main hull to center of the outrigger. The design above shows an 8ft separation. Looking at photos of traditional tacking outiggers and also catamarans and of trimarans, a distance of 8ft between centerlines seems appropiate. Total beam excluding safety ama when outrigger is deployed works out at a bit over 11ft.

This diagram shows a retracted beam of only 7.5ft. If more beam was deemed acceptable then more options would be created, such as having a wider sponson and greater beam bury. Note on this diagram the retractable safety ama. This is made retractable in order to reduce retracted beam. This is not really as complicated as it seems as the safety ama is subjected to quite low loads and can be very lightly built. One downside of a retractable safety ama as opposed to an intergrated safety ama is that it cannot be used to store light bulky objects.

The sliding mechanism would have a lower center of gravity when trailering so might be a fraction more stable. However with the outrigger in this position the tralier itself may have to be cleverly designed

My feeling is that because of the modest overall beam, and with the sponson aiding in supporting the crossbeam that a stay from the bottom of the main hull to the outrigger is not required. However if a greater beam was desired then such a stay may be prudent

Now looking at some real life examples. Skip Johnson and his P52 proa originally trialled the folding mechanism to get down tralierable width. He then progressed to a sliding mechanism as he noted structural problems with the hinged mechanism. The greater depth of bury with a single sponson multihull means that loads are spread out over a larger distance and thus peak loads are reduced. Retracted beam is 7ft 4" and sailing beam is 10ft 4". Kind thnaks to Skip Johnson for permission to use these images

Option 4

This is a swivelling mecahnism for crossbeam storage. This mechanism is used on the dragonfly series of trimarans. It has potential on a tacking outrigger as this style of craft has a smaller outrigger whcih is usually placed well forward. Thus when the outrigger is retracted it does not increase the length of the craft, an advantage when it is being towed by a car. Like the dragonfly trimarans stays would be required linking the bottom of the main hull up to the top of the outrigger. The sponson would give support for the beams in addition to stays.

This mechanism seems to have potential, however I feel it requires a higher degree of engineering skill than option 1. I have shown it as an example that others may wish to expand on


All mechansims shown seem to be simple and effective options for a stowing a single outrigger multihull. Obviously lines would be needed to control the outrigger whilst it is being rotated up for trailering for options 1 and 2. With optins 1 and 2 there is the issue of the outrigger wishing to go backwards and parallel to the main hull. To avoid this diagonal lines would be required within the rectangle shape between the 2 hulls.

For options 1 and 2 The loads on the struts and pins are high but can be calcualted, The 60mm tubing seems appropiate for a relatively small outrigger. A larger outrigger would of course require stronger struts. The loads on the pins would be high, probably into the several tonnes region at times of extreme stress, such as a collision or buffetting by heavy seas. However the use of pins in catamarans and trimarans is not uncommon and the loads on this mechanism would be lower when compared to most other boats due to the smaller size of vessel it is intended for.

If additional strength is required for the lower strut, then 2 tubes one behind the other could be utilised. This would minimise any drag as frontal area would remain unchanged. Such a mechanism would double strength. The other issue of course is that whilst the crossbeam struts may be strong enough, when these loads are transferred to the hull, will that be strong enough to withstand the loads.

In my opinion Option 1 seems to be the best method. The 'hinge' could even be stainless steel U bolts fixed to the hull, through which 80mm tubing rotates. This would be a simple and effective solution. The bottom spar may produce a little spray but it would be modest. One thing of note, Option 1 would require a drop in bathtub side sponson to allow the crew a little space to get fresh air and sun. This would be a lightly built structure and would not be required to transfer any loads. becasue it is a drop in type it can be made wider for more comfort.

The sliding mechanism also has potential. Loads will be higher than option 1, but the bury is very large for a mulithull so tranferring these loads to the main structure is doable too. Not my favourite but appears workable. Kurt Hughes has made many successful multihulls with a similar setup. He uses much less bury but adds a line to support the loads. The 'line' or stainless steel wire stay idea is not new and is used in many, many boats, such as Dick Newicks's Tremolino trimaran, Crowther's Bucanneer 24 and many many more.

Safety Ama

Last but not least a few notes about the safety ama. For a start the question is why bother?

The answer is that for very little downside, a great deal of capsize protection is created. The safety ama can be very lightly built as it will not be subjected to high loads, it does not need to be highly shaped either, just a little fairing fore and aft will be fine. The ama is not intended to provide the entire buoyancy for the craft, but just sufficient so that if the boat is overpowered by a gust it goes over a fair way, but stops a long way short of vertical. A range of 45 to 55 degrees maximum heel seems realistic here.

It should be noted that all four of Russ Brown's boats have safety amas. The proa Equiibre that did not have a safety ama, tipped over at anchor in the Caribbean (this is my understanding), though thankfully nobody was hurt. Thus in my opinion there a lots of plusses for installing an integrated or external safety ama. Volume for the ama would be in the range of 280L which can be found in a box 10ft x 1ft x 1ft. If a large diameter PVC pipe was readily available then that would be suitable too.


Some notes on hinges. Obviously they need to rotate easily and also be stong enough. Additionally the metal hinge needs to be securely fastened to the beams. One method may be stainless steel U bolts though which tubing rotates. Another may be a large strong hinge. Below is a photo of a very run down Piver Nugget. The hinge can be seen. If I did not know better I would say that the crossbeams look like solid 4" x 4" timber, but that cant be right now can it? By the way I did a little maths and I worked out that 0.5 inch diameter stainless steel bolt has a sheer strength of about 2.3 tonnes. Does this sound about right? Maths is available on request