In 2004, Mark posted a question about 5O5 boat longevity on the international list, Matt Hansen from Fremantle, Australia, heavily involved in boat construction replied with this very comprehensive response.

In reply to Mark's question about longevity of hulls I will make some comments. First of all I am speaking as one of those involved with developing the Fremantle 505s and as a 505 sailor of 20 years standing who has owned and sailed 3 Kyrwoods, 2 Van Munsters and now a Prepreg Fremantle 505. This is a long reply but I hope it helps to sort out some misconceptions and clarifies a few points.
I believe the factors affecting the longevity of the hulls are as follows (in order of importance)

1. How the boat has been looked after,
   particularly how it has been stored, transported and lifted in and out of the water. Regardless of construction type a boat will age prematurely if it has been neglected, left outdoors without covers subjected to extreme heat and cold or roughly handled during transport or launching . Proper storage, good covers, a good trailer and dolly plus preventative maintenance are all vital. Problems tend to amplify with time so early repair is important.
   
2. Engineering.
   The strength and stiffness of modern boats is primarily due to better engineering.
   Sandwich construction
   Most builders produce fully cored hulls and decks . The core material separates the two skins in a similar manner to an "I" beam conferring stiffness in proportion to the distance separating the skins and the resistance to shear of the core material. The integrity of this matrix is very important so good bonding between the layers is essential.
   Structural support
   The cored surfaces of hull and deck are supported transversely by bulkheads and thwarts as stipulated by the class rules (Class rules allow now boats built without thwarts). In some boats these are also sound fully cored strong stiff structures but in others the builders have sought to save time or weight by using uncored glass or unsupported marine ply. The thwarts are there to resist the loads placed on the case by the centerplate and Mainsheet, they can only be effective if their outer end is locked to a rigid surface. In many boats the thwart is attached laterally to the inner side of the side tank which is a vertical flat surface with little resistance to compression loads placed on it at 90 degrees. A more structurally sound arrangement is to triangulate the thwart to the junction of the sidetank and floor where the forces will be in shear to 2 surfaces instead of one. Adding additional bulkheads will add even more structural support and can even be employed funtionally as in the chute arrangement on the Rondars and Fremantle 505s that creates 2 longitudinal bulkheads under the foredeck. We also employ additional longitudinal bulkheads between the hull and seat tanks to strengthen this area even more.
   Joins
   All joins are points of potential weakness so the fewer the better. Modern bonding materials have improved the situation somewhat but the fact remains that if a boat could be built with no joins at all it would be better than any current boat. We have reduced the number of joins in the fremantle boats by making the boats out of just 3 major moulded parts.
   
3. Materials
   This is the bit where everyone gets most excited but which probably makes the least difference. There are 4 aspects to consider; Resins, Mat, core material and bonding.
   
   1. RESINS
      Epoxy resins are generally accepted as being the best but they are a diverse group and range greatly in strength, stiffness adhesiveness and durability so the precise type of epoxy resin used needs to be known. ie saying something is epoxy resin is about as specific as saying an animal is a dog..it could be a poodle or an Alsatian.. big difference. Epoxy resin is the resin employed in most pre-impregnated mat (prepreg). Good epoxy resins have great bonding capacity and durability even if flexed, which is to say that they tend not to fatigue as much as other resins if subject to repeated bending and flexing. The key point here is that a well engineered fully cored boat should NOT be flexing in the first place so this property is much less critical. The downside with epoxy is that it is much more expensive and it is damaged by sunlight.
      Vinyl ester is closely related to epoxy but has easier handling characteristics and slightly less resistance to fatigue. Again, if the material is not being fatigued by repeated bending and flexing then it will last just as long as epoxy. Its advantage is that it bonds much better to gelcoat and is much cheaper and generally has slightly better resistance to sunlight.
      Polyester is still a good material but is probably the most prone to fatigue which was a significant problem in the uncored sides of older hulls which become quite soft. Having said that the balsa cored floors of many of the 20+ year old Kyrwoods are still very stiff and the boats still competitive. Polyester is the cheapest of the resins and has pretty good sunlight resistance compared to the other resins and bonds well to gelcoat (which is polyester).
      
   2. MAT
      The choices are of fibreglass- a silicate, carbon fibre and kevlar
      Carbon is the most glamorous material and for good reason Its strength and stiffness to weight ratios are the best of the three, meaning that much less of it needs to be used to get the same benefit.
      Fibreglass is heavier but has good compression and tension characteristcs and is much cheaper.
      Kevlar has good tensile strength but poor compressive strength is the least stiff of the three
      materials and is prone to stretching so it is a poor choice if used alone. Some builders use it in combination with other materials where it may have some benefits but we remain of the view that it is best left out.
      The arrangement of the fibers of the mat also confers different properties. The traditional chopped strand mat is easy to use and has good impact characteristics so is good in areas of the boats that get focused pounding eg by buttocks on seat tank tops and feet on cockpit floors. A thin layer of chopped strand is often used to get a good bond to the gelcoat and prevent 'print-through' of the underlying woven mat. Woven mat comes in various weights and weave types with generally better tensile strength but less impact resistance to highly focused impacts such as kneecaps and trapeze hooks. Unidirectional mat has all the fibres running in one direction as a single sheet. bidirectional has 2 discreet sheets running at a specified angle (ie not woven together). These orientated mat types confer enhanced strength and stiffness along the long axis if the fibres and can be used accordingly.
      
   3. CORE MATERIAL
      The main choices for core material in 505s are foam, balsa and nomex honeycomb.
      As mentioned previously the role of the core material is to function as an "I " beam, keeping the inner and outer skins a constant distance apart and resisting shear movements between the skins. There are many types of foam and balsa with varying characteristics. Generally speaking end grain balsa has better shear resistance than all but the highest density foams but tends to be heavier. We use end grain balsa in the floors of our conventional layup boats because of this advantage and its greater compression resistance. The lighter foam is used elsewhere such as the sides of the hull, foredeck sidetanks and bulkheads. Unless very high density foam is used in the floor the boats are prone to getting small dents in the bottom when being loaded and unloaded from trailers or from kneecaps and trapeze hooks when capsized.
      Honeycomb is probably the best core material of all but it is the least forgiving for the builder. We use it exclusively with our prepreg boats where it can be bonded to the skins under optimal conditions. As usual it is much more expensive than the other core materials.
      Marine ply has far greater structural strength than any of the other core materials but is much heavier. It is used as a core material in some extra high load areas such as the mast step in our boats and used in a stand alone manner in the waterats reinforced with wooden struts to give the I beam effect. Wood can rot so always check this closely in an older boat particularly in the areas where it can stay moist for long periods.
      
   4. BONDING
      This refers to the bonding within the laminate, between the laminates and between the moulded
      components of the hull. Resin and Mat work together by bonding to each other . Optimal resin to mat
      ratio results in the best strength and stiffness within a given layer of the laminate. Pre-impregnated mat has carefully controlled mixture of resin and mat which is closest to the ideal and is inevitably better than even the most skillfully wet laid laminate. Traditional methods of wet layup involve allowing each layer to cure before adding the next so that a discreet bond has to be formed between each layer as the next one is added. The use of vacuum bagging aids this process by compressing the layers together to enhance the contact area for bonding . Once again prepreg layups have an advantage as all layers cure together so the resin forms a continuous linkage rather than having to form a bond between an older 'set' layer and a new 'setting' one. One way to get around this inherent disadvantage of the wet layup is to use the resin infusion technique where all the mat and core material is laid in the mould , a vacuum applied and the wet resin allowed to flow into the matrix under strong negative pressure wetting out the mat as it spreads. Using this technique strength weight and stiffness characteristics approaching the prepreg layup can be achieved. Karl Otto Stromberg has illustrated this nicely in earlier correspondence on the Swedish "Otto 505" which I think is on the Swedish website. Windrush, the builder of the Freo 505 has employed this technique extensively.
      Finally bonding between the component moulded parts has improved over time. Epoxy glues naturally bind best to epoxy resin boats and less well to polyester. Some of the newer glues such as 'plexus' bond extremely well to virtually everything but are very expensive. Glues work best when the two layers to be joined are in close proximity to each other so if you see a lot of glue or filler between the bonded surfaces beware! Glue will generally be the weak link so a poorly engineered boat that flexes and twists will be prone to splitting along its glue lines. It follows that you should closely inspect these glue lines when looking at an older boat.