The GB Picos, Bahias and Vagos (and their counterparts at Topper) have a different rationale.
The groundbreaking part is the polyethylene sandwich construction...it is really maintenance free, contrary to GRP which needs some maintenance , specially the early boats with some wooden parts in them.
Furthermore , the Polyethylène construction is cheaper than the GRP...but only once the huge initial cost of the industrial tool has been balanced by the mass sales.
The big advantage of polyethylène sandwich is its resistance to little dings and shocks and it has a big appeal to sailing centres as the instructors team tend to go professional instead of volunteers and cost more, furthermore most sailing instructors dont like maintenance tasks very much and when they have their word in boat choice , they go for the soft plastic sort of boats.
It is obvious that a maintenance free boat reduces the wages expenses in big sailing sclool fleets but those boats are not without disadvantages.
Polyethylene boats are both softer and heavier than their GRP counterparts ...they tend to loose their shape if improperly stored ....which has led the designers to use the old trick of Junkers 52 aircraft and Citoen 2CV and H van fame...the corrugated panel construction and the swages in the thin skin.
Picos and the like all sport the swages / rubbing strakes under their hulls to limit hull deformation but this is not exactly good as far as hydrodynamics priciples are concerned.
The modern equivalent of the 420 is the Laser Vago (in the dacron sail version) but its hull is 116 kgs instead of 80 (imagine sailing and carrying a 420 with a sack of concrete in it) and therefore needs more canvas to move).
Granted it has a double bottomed self bailing cockpit (the soft plastic construction wouldn't allow for a single bottom hull anyway) , it is a good boat with a quality rig but it is not a better boat in quite a number of domains .
The polyethylène also has issues with sunrays and UV's , after some time outside under the sun it becomes brittle and cracks, unless carefully wrapped in covers, (the kind of things never done in a sailing school), specially on stress points like the transom on polyethylène cheap powerboats like the Jeanneau Newmatic (granted , today's polyethylène is marginally better in this respect).
In Multihull oriented France the polyethylene construction has been tried and tested for sailing school catamarans and in many respects soft plastic construction goes better with catamarans than with mono hulls, even with single sheet polyethylène (Like the french made New cats) in lieu of plyethylene trilam sandwich.
The kind of curvature radius involved in a catamran hull is more compatible with soft plastic construction...except for one point: the stress and bend in the rear crossbeam attachment point on the hull.
A catamaran tends to distort when hard pressed (specially the overcanvassed Hobie Cat 16, a kind of US sports car with huge engine and poor chassis, and the bed like style of platform on four posts dosen't help) and the first (New cats) and second generation (Dart 16, Hobie Twixxy, Topaz) of training polythylène catamarans all had issues with the hull cracking in the critical zone of the rear crossbeam attachment.
Both New cat and topper are launching new training catamarans with design tricks to try cure this problem which is not a small one: a puncture in a polyethylène hull (the boat hitting a pointy obstacle) is easily repaired with some polyethylène and a Leister heat gun but a fatigue crack is terminal and calls for an entire new hull (Both Dart and New cat have flirted wth recievership and bankrupcy because of warranty issues).
The ideal material for both dinghies would be possibly some sort of polyethylène or other soft plastic but fully glass reinforced .
There has been trials in this field in France : Boutemy / 2 win of La Rochelle produced very good cats in this kind of material which is both shock resistant, stress resistant, light and industrially manufactured with low manpower costs...but another snag appeared : after some time and minimum wear the fiberglass fibres ends started to show at the surface and induce a nasty skin rash on the thigs of young summer sailors wearing only shorts and T shirts, it is a warranty issue with legal action involved between the boatbuilder and its subcontractor and
it does nothing to help Boutemy out of the current economic crisis.
Another very recent try, this time in dinghy field has been made by Jean Marie Finot , of vendée globe fame...he teamed with an industrial firm manufacturing glass reinforced soft plastic body panels for the car industry .
The material seems fantastic : Lightweight, rigid and shiny lile gelcoat coated GRP, industrially molded at low manpower costs like polyethylène and shock resistant (think of car "shield" like bumpers)...but this time it is the design that is troublesome :Finot is a very good designer of fast keel boats (from Vendée globe 60 ft ocean racers to Open 5,60 dropkeel sportboat that makes a splash both sides of the Atlantic) ...but he is not a good dinghy designer and furthermore he is somewhat of the stubborn kind of guy.
His design (called the Albatros) is a kind of crossover between a dinghy and a keelboat and unfortunately it has many of the design quirks of Finot'searlier design, the infamous Wizz.
The Leaded -30 Kgs-60 pound centreboard (supposedly of the gybing sort) is shaped in the old windsurf style and protrudes under the hull even when retracted which makes trolleying problematic .
The transom is very wide and the boat is certainly stable and can even go reasonably fast fast in a breeze but the wetted surface is BIG!
It is a pity because Finot invested much of his own money in a mammoth hi-tech set of molds (hull , deck, CB , case inner stringer) and refuses to see that his boat is somewhat flawed.
He did test a fibreglass prototype and made a few improvements ( the carbon untapered mast gained a track for hoisting the sail instead of the Laser/wizz sleeven mainsail which is oK on a laser but just plain stupid on a mast with shrouds) but the hull desgn and CB/Keel design remained the same as he had unwittedly ordered the mold construction to start before the trials were completed (it is a huge aluminium old with an intricated set of heating pipes in it and the cost is staggering).
It is a pity because i think if he had merely copied a good but not too extreme hull from a international 14 (before the pressure of racing made them unsuitable for beginners) and bought "off the shelf" components like the aluminium extrusion foils you find on Marttia italian ctamarans or Ovington 29ers and asked for more outside advice in the testing period we would have a perfect boat ...on a limited budget
http://www.finot.com/newsletter/envoi_n ... pt212.html