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Mi6 Proa
Mi6 on the beach

What's a Proa?

Design Details

Commercial Considerations

The Designers


The Mi6 Proa (pronounced 'em-eye-six'), resulted from a 1993 design competition sponsored by System Three and Modern Boating magazine. The Mi6 design was the eventual winner out of 80 entries from Australia and abroad. The following page is a summary of the designers' 1993 design submission for the contest.

Contest Outline:

To design a vessel suitable for use by two adults and two children which can be constructed from Coretech panels and System Three epoxy.

Our Brief:

We decided to adopt a brief to design a vessel that was to be used as a day sailer/picnicker, primarily for exploring inland waterways, lakes and harbours. It should be able carry not only 4 passengers but their picnic lunch and Esky or camping gear. It is also to be inexpensive and trailerable. Within the boundaries of this 'global' design brief we identified the following local criteria.

* The vessel needs to be a place where a family of 4 could quite happily expect to spend at least a couple of hours. To this end a large, roomy cockpit is of prime importance, somewhere where it is possible to lounge rather than just sit.

* The vessel should be straightforward to rig and easy to sail.

* The vessel should be able to offer lively performance in breezy conditions whilst remaining as upright and stable as possible.

* The vessel has to have a classical feel about it while at the same time being otherwise modern, if not radical, in appearance.

* To be trailable it needs to have a "trailer borne" beam of not more than 2.4m

* Hull shape should lend itself to easy construction from Coretech and System Three epoxy by an average handyman.

Test Model

As part of our design submission, we built a 1: 5 scale model/working protype of the Mi6 proa. This was felt necessary in order to test the feasibility of the somewhat unconventional features of the design. It also served as a valuable tool in the styling development of the full size craft, the appearance of such a novel craft being of great importance.

The prototype was constructed primarily from 2.5mm balsa sheet using construction techniques as close as possible to those of the full sized boat. Having said that it should be noted that as this is a working prototype not all of the construction details could be scaled directly. They are however, conceptually the same.

The model was fitted with radio control for the test sail. Despite strong, gusty winds (giving scale wind speeds of up to approximately 60 knots) which were shifting through 180°, the model behaved admirably. The rig and rudders operated as expected, the unique 'reverse tacking' ability of the design being well exhibited on several occasions. The structure of both main and outrigger hulls came through the heavy conditions without a problem. The model appeared quite fast and had good acceleration in gusts.

The Prototype

First prize in the competition was a choice of either $3000 cash or $6000 worth of System Three building materials. The designers opted for the building materials. A full scale protoytpe of the Mi6 proa was built over a period of six months, and launched in April 1994.

What's a Proa?

The proa configuration is in itself a very old concept, probably one evolutionary step from the dugout canoe. It can best be described as a twin hulled vessel in which the load carrying or main hull (the vaka) is held upright by the (usually) smaller outrigger hull (the ama).

Sail driven proas fall into two categories.

i. Pacific - sailed with the ama (outrigger hull) to windward. As such the ama is effectively a ballast weight of small volume (needing only to support it's own self weight and that of the cross beam). The Pacific proa is often sailed flying the outrigger, with the crew able to act as additional ballast.

ii. Atlantic - sailed with the outrigger hull to leeward, using a volumetrically larger hull than that of the Pacific proa, the buoyancy of the ama supporting the main hull as it heels. The Atlantic proa is a modern western variant of the proa concept. Some dispute that the Atlantic proa is actully a proa at all.

Boats of proa arrangement generally enjoy the following. They offer low wetted surface areas, have high righting moments when sailed on the correct tack and have the potential for good performance due to their slender hull forms.

On the other hand, tacking a reversible proa is generally a pretty slow process. Tacking usually involves bearing away onto a beam reach, releasing the sheets and bringing the boat to a halt. The rudder is then removed (or raised in a two rudder system) at one end of the boat then taken to the other end (or lowered). Sometimes the rudders are incorporated into the centreboard in which case the rudder not in use is stored by raising it into the centreboard case. Once the steering apparatus has been rearranged you can then proceed onto the new tack. The mainsail is resheeted at the other end of the vessel, the old jib lowered and the new jib raised. The second problem with proas is that they can run into trouble when caught on the wrong tack. An Atlantic proa will simply fall over if suddenly taken aback in a stiff breeze, whereas a Pacific proa may have it's small volume outrigger forced underwater.

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Design Details




Cross Beams


The Hulls Mi6 Profile

The main hull overall dimensions were derived by working around the ergonomics of the main cockpit. The thwarts and backrests form the flared upper section of the main hull while the hull proper forms the footwell. A strip planked, round bilged hull for minimum wetted surface area was considered but eventually a simple, dory style hull was decided upon. This was selected for ease of construction as well as providing a very useful flat floor to the cockpit. To enhance what could have be an otherwise quite ordinary looking hull, a 100mm wide chine plank was introduced. Made from 16mm thick cedar or oregon this plank is shaped to provide a soft chine.

A clear cockpit length of two metres was decided upon as the minimum which still afforded a reasonably spacious seating area. The main hull was then faired around the cockpit resulting in an overall length of 6m. Highly cambered "end decks" were added at each end of the boat mainly for appearance. These would be constructed from 4mm ply. The rest of the hull is nominally 11m foam cored Coretechpanels for lightness, although balsa cored panels could be used if greater durability is required.

The outrigger is the same length as the main hull for speed and also to provide maximum diagonal stability in Atlantic mode. The construction technique is identical to that of the main hull and also incorporates a rounded chine plank.

The outrigger has enough volume to carry twice the displacement of the boat. It is however much narrower than the main hull and has less rocker or spring in the keel line. The difference in hull shapes is intended to make the boat as efficient as possible through it's entire speed range. In light winds it would be sailed in Pacific mode, flying the outrigger as much as possible to reduce wetted surface. The main hull with much of it's volume concentrated around midships has a lower prismatic coefficient, Cp, providing lower resistance at lower speeds. The outrigger conversely has been designed with a high Cp. The ends are relatively more voluminous than the main hull, giving lower resistance at higher speeds. Therefore, in heavier winds, the proa would be sailed in Atlantic mode, with the outrigger becoming the major load bearing hull, possibly even to the point where the main hull is "flown".

It should be noted that the above idea is not unique to proas. Trimarans work in much the same way without even thinking about it but pay a price in terms of weight, windage and cost of the extra hull. Conventional catamarans do not have the advantage of the ability to select the right hull for the right conditions, and with two identically shaped hulls better suited to higher speeds, are not often noted for their light air performance.

Design Details

The Rudders Mi6 Rudder

The Mi6 proa has two large rudders which provide both lateral resistance and steering. They are mounted 3 metres apart on the inboard side of the main hull. The rudder section is symmetrical fore and aft as it has to operate in both directions. Both leading and trailing edges are fairly sharp. The section shape is therefore less efficient than would otherwise be achieved with a normal, unidirectional foil section. Larger than normal rudders have therefore been used to provide good steerage. We have endeavoured to counter the increased rudder drag to some degree by giving the rudders an efficient, elliptical plan form.

The two rudders are linked so that steering is achieved by moving both rudders simultaneously. In order to keep the rudders balanced, they are mounted on transverse axles attached to the rudder shafts. This allows the rudder to trail slightly aft due to drag, the extent of this trail being limited so that the centre of lateral resistance of the rudder lies slightly aft of the rudder shaft axis.

After trials, we are now redesigning this rudder system to incorporate a dagger type system for lowering and raising the rudders. We also found that it is not necessary to allow thee rudders to trail aft, as the system balanced quite well withthe rudders fixed in the vertical position. See the rudder linkage conceptual drawings for more details.

More Design Details

The Rig Mi6 on Water

We opted for a balanced or 'swing' rig to reduce sheet loads. The rig plugs straight into the mast box of the main hull, in much the same way as a Laser rig does, to allow 360° rotation.

The rig uses a wishbone boom to which is attached both the clew of the mainsail and the tack of the jib. The mainsail is similar in appearance to that of a sailboard except that it runs in a conventional mast track for ease of rigging and reefing.

The jib is an unusual delta shaped 'lop sided hang glider wing'. There is no jib sheet and the slot is trimmed by adjusting the clew position with a simple control line.

The sail plan is of modest area for ease of handling. The mainsail has a large roach to improve it's efficiency.

More Design Details

Cross Beams Mi6 on Trailer

The cross beam structure consists of two 125mm OD x 3.0mm wall aluminium tubes. Stainless steel 'S' simply hook over the tube wall at the outer ends of the cross beams. Both hulls are pulled together against these hooks by the tensioned trampoline.

To dismantle the boat ready for trailering the trampoline is slackened, the beams are unhooked and. The cross beams are then ready to slide out. Special, short 'trailering' cross beams are then inserted to reduce the overall beam of the trailer borne boat to a manageable 2.2m.

The cross beam supports in both hulls are of 16mm plywood.

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Commercial Considerations Mi6 Under Construction

Due to it's inherently straightforward construction technique, the Mi6 proa lends itself to production in kit form. The hull panels are easily developed using CAD and the chine planks can be preshaped to a large degree prior to assembly.

The specialised fittings for the rudder and rig assembly could be manufactured in bulk and sold as part of the kit. As is common practice, the manufacturer could offer the boat in various stages of of completion including a 'sail-away' version with trailer.

The concept of the Mi6 proa also lends itself to a certain degree of customisation through the provision of "optional extras". Such options currently under consideration include:

* padded seats and backrests

* Esky compartment and mini bench

* zippable stowage bags

* customised colour schemes

* "aero - shade" cockpit awningAdded to the above is the inherent versatility of the design, the Mi6 proa being ideally suited for use in general recreation, weekend camping, day hire at tourist resorts and for racing. For the latter, a larger rig than the one shown in this submission may be desirable and an asymetrical spinnaker may be added. The unique manoeuvring capabilities of the Mi6 proa would make for some interesting tactical situations and could require the revision of some existing racing rules.

Quite obviously before anything more than one off production of the Mi6 proa is commenced detailed market research would have to be undertaken, accurate production costs determined and current competition assessed.

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