Anyone seeing a photo like this for the first time will almost inevitably stumble across this detail: the mast is not exactly vertical on the trimaran, but tilted to windward. This is not a defect or an optical illusion, but a deliberately used trimming tool: the so-called canting rig. The picture was taken during the RORC Caribbean 600The MOD 70 trimaran "Argo", which took the line honours of the multihulls in an extremely close duel against the second MOD 70 "Final Final - Zoulou".
What is a MOD-70 trimaran?
The MOD-70 class (Multi One Design 70) was created in 2011 as the one-design successor to the earlier ORMA-60 trimarans. Over many years, the ORMA boats had become increasingly extreme, complex and expensive machines - fast, but also vulnerable. The idea of the MOD 70 was to retain the fascination and speed, but at the same time limit development and operating costs and make the platform more robust.
What the photo shows
In the picture, the trimaran is approaching the viewer almost head-on. You can recognise the typical load distribution of a trimaran: the lee float is working in the water, the windward float is unloaded in the air. The mast and sail are above it. In these boats, however, the rig is not just a mast with sails, but a wing mast that rotates around its longitudinal axis and forms a profile in the incoming flow. On the MOD 70, this mast can also be canted to windward by around four degrees.
That doesn't sound like much, but it is geometrically enormous: with a mast height of just under 30 metres, these four degrees shift the masthead more than two metres to windward.
This tilts the mast to the windward side
On these trimarans, the rigging, i.e. the shrouds, do not end in shroud tensioners, but in a hydraulic system. Each shroud is connected to a separate hydraulic cylinder in the main hull. On the windward side, the cylinder pulls, while at the same time the leeward side gives way in a controlled manner. The mast tilts to windward at the foot of the mast via a corresponding bearing.
How the power gets into the system
The hydraulics need pressure - and on these boats this often comes from the crew. Two grinders are installed in the cockpit of the MOD 70, which not only operate the sheets and halyards, but also drive a rotary pump for the hydraulics. The hydraulics are not a secondary consumer, but a central system: in the MOD 70 concept, they are also used for the mainsheet via a cylinder under the boom and for the outhaul.
The effect of mast tipping
The canting rig shifts the sail's centre of pressure to windward. This means that the sail plan acts less downwards on the lee bow, which has to absorb all the heeling moments. In addition, C-foils provide lift to leeward to further relieve the float. More modern trimarans often no longer have the canting rig, but more effective foils. For the designers, it is a matter of weighing up the respective advantages and disadvantages. Weight, handling, resistance.
A side effect of leaning to windward is that the weight of the rig acts less as a lever to leeward, thus additionally increasing the righting moment.
Lars Bolle
Chief Editor Digital
