Part 2 Glued Wooden Boats
From about the 1950s glues were used in boatbuilding – probably before too. Early examples included plywood and glued multi skin hull constructions. In England, Uffa Fox made multi skinned sailing dinghy hulls built upside down using thin veneers laid diagonally onto moulds. In New Zealand, Des Townson and Mosquito craft built multi skin moulded dinghies over moulds.
Considering New Zealand’s background in multi skin yachts like those made by Logan and Bailey, and with the developments made in dinghies it wasn’t much of a stretch to larger multi skin glued planked boats, keelers, launches and commercial boats.
Hulls were typically built upside down, onto bulkheads, frames and stringers, or on a stringered mould. The stringered method echoed the way aircraft were built. The same care was taken in timber selection as for traditional boats except that many components were commonly laminated (glued together) from layers to build up to the finished sizes, and to enable curved shapes to be formed by gluing them around jigs. For example, laminated hull frames.
Framing
Hull frameworks typically consisted of timber internal keel back bone, wooden stem and stern and timber floors through the bottom and timber engine bearers. Plywood bulkheads were glued alongside frames. Laminated timber frames were often used on yachts especially with multiple layers of timber glued and clamped for round bilge boats.
Frames were made on the floor from the lofted sections and braced with cross spawls and legs before standing up, then stringers were fitted. These boats were mechanically fastened by screws, bolts and rivets as well as glued but reliance on fastenings was lessened with the glue spreading the loads all over the component faying surfaces rather than pulling on a series of pins(nails/screws/bolts) like traditional vessels.
Transverse frames and bulkheads were spaced sufficiently to provide good support for longitudinal stringers spaced at around 100mm apart.
Planking
For launches plywood was often used in the aft sections and diagonally laid fwd. Solid timber diagonals were mainly used on round bilge vessels but also on chine vessels of about 6-10mm thickness.
Diagonal (cold moulded) planking was typically double diagonal (2 skin) on boats up to a maximum of about 12m and triple (3 skin) diagonal on larger craft. It isn’t possible to tell definitively without taking a core sample if a hull is 2 or 3 skin. 3 skin was generally agreed to be significantly stronger than 2 skin for the same overall thickness. Some used 4 or more skins for extra strength, thickness or for bigger boats.
Fastening and glue combinations
Fastenings were required to back up the glue in some areas to prevent cracking alongside glue lines and to hold the glue join until it cured. Planking was typically nailed into stringers with copper nails mainly to hold the planking against the stringers until the glue cured. High and even clamping pressure was required with the resorcinol glue that was used, and rows of nails along stringers can usually be seen on cold mornings or where there has been water ingress or on dark coloured hulls.
For commercial glued wooden boats the recommended fastening nails and spacing were specified in the Requirements for the Construction of Wooden Inshore Fishing Boats ,1972 the effort to publish the guidelines including plywood indicates the uptake of glued timber by 1972 was significant.
Resorcinol glue was used until epoxy acceptance (initially with Epiglass’ “Epiglue” began to take from around the mid 1970s). Resorcinol is an excellent glue for wood-to-wood gluing and laminating – in my opinion still superior to epoxy for laminated frames and for slightly damp timber and possibly for treated timber. And it is still used for marine plywoods. It is far stronger than the timber it glues but must be applied to both surfaces and clamped evenly and tightly to form a good bond. It is also not a gap filling glue so all joins had to be a perfect fit.
Glued vessels scantling sizes and skin thicknesses can be reduced compared to traditional timber vessels because of the improved glued connection of the components such as multi layers glued together preventing splitting and cupping that would occur with single layer timber.
Glued construction provided greater structural stability and hull/deck structures were no longer subject to the leaks or movement that was normal on timber vessels. However older non glassed vessels sometimes move around nails along stringers that tends to allow moisture ingress and hulls becoming wet.
Hulls were increasingly sheathed with an epoxy saturated reinforcement skin. And most hulls were at least sealed with a thinned epoxy like Epiglass’ “Everdure”before painting. The approach was to keep all water out of the structure – although this was not always the reality.
The 1960s saw rapid development in glued construction, using plywood and diagonal planking with many vessels made by DIY builders. Legendary designers such as Hartley, Jim Young, Pelin, Augustin, Spencer, Alan Wright, and others produced multiple designs built by amateur and professional builders. There was a tendency for many builders to put their own mark on designs, and although most vessels were effectively custom built, there was a move towards efficiency with kitsets and templates sometimes available for popular designs. With the kiwi spirit alive and well this didn’t stop sisterships sometimes turning out to be quite different when built by different builders from the same plans.
| About plywood Marine plywood is often misunderstood in boatbuilding. Its introduction revolutionised boatbuilding with one of its most ardent champions John Spencer known as “the Plywood King” due to his evangelical enthusiasm of the material. It quickly became and remains in many applications, the go-to material for areas where strong relatively durable sheet material is required including bulkheads for almost all boats, decks of almost all glued timber boats (some with teak overlay), hulls of hard chine dinghies and yachts and launches and even for production GRP (fibreglass vessels) and deservedly so. In the 1970s marine plywood may have been made in New Zealand, usually comprised of durable species for all layers, in some cases believed to be from New Zealand Kauri. This was glued with resorcinol glue as it is now. The early plywood definitely seems to have stood the test of time well compared to more recent BS1088 marine plywood made from tropical timbers like Meranti or Okume Gaboon. Despite the “marine” description the durability of BS1088 plywood made from tropical timbers can be variable. The timbers used are non treated and the BS1088 standard does not refer to durability, it is related to allowable voids and gluing standards. The glue is usually resorcinol, and the bonds are better than epoxy, boil proof and not generally liable to failure, even when wet. However, the plywood’s BS1088 standard may not have been independently quality assured for consistency. Unfortunately, decay is not uncommon. It usually occurs where freshwater finds entry into the plywood such as via deck penetrations like screws or bolts, around windows, cockpit lockers etc. As with all timbers, if the timber, plywood or not, is not durable (such as sapwood, non durable species), and if the moisture content is allowed to reach a certain level – over 20%, and if there is no ventilation but there is oxygen, decay is likely (saltwater is less of a problem in planking as it tends to pickle timber). One of the main disadvantages of fully sealed, non treated timber is that if water gets in, it will not evaporate or dry out easily. Furthermore, inferior “marine bonded” plywood was often sold alongside BS1088 without the BS1088 stamp, without quality assurance. Of course this was typically about 25% cheaper. Same plywood sheet outer faces, same bonding. What could go wrong? Wooden boatbuilders exposed to the new BS1088 plywood through repair work, especially the tropical timbers, were aware of the limitations. To guard against decay it was/is common to use treated pine made for the construction industry for key structural at risk panels. For example under teak decks and for paneled bulkheads. Treated pine construction plywood, while having rougher face grain and surface, has proven durable over time. |
Early epoxies and sheathing
By the 1970s fibre sheathing of timber was being used such as “dynel” an abrasive resistant fabric that soaked up a lot of resin, and glass fibre. The epoxies in the 1970s were mainly Epiglass products such as: a white resin called something like Rapid 90 laminating resin for fibreglass, Epiglue epoxy glue paste, Epifill epoxy filler paste (filler that was really dense and very hard to sand), and Everdure sealer (that contained toxic preservatives that are no longer included).
The epoxy glass sheathed laminated timber hulls made from these products were a far cry from their timber ancestors in terms of finish, likely longevity, dryness, weight and strength.
New paint systems
The glued and sheathed vessels were very stable and not subject to movement like that experienced on traditional planked vessels. Hull and deck surface stability was compatible with the new hard “two pack” marine paints which became available around the late 1970s, such as the high tech product of the time known as Epiglass Reaction Lacquer.
Two pack marine paints are more than a tough, hard protective coating. A good system applied correctly by a skilled applicator will not only last but retain a fine near perfect finish with requiring only an occasional polish. This is a preferable coating than the more porous gelcoat used on GRP vessels due to superior waterproofing and lack of porosity.
Movement showing in finish
One of the main features and advantages of multi skin glued constructions, especially kauri, is structural stability.
Any hull movement is usually very slight only able to be seen in certain lights or early in the morning where condensation often forms on the nail rows of stringers or vessels painted dark colours that will move slightly more due to the hotter temperatures of their structures.
Blemish/defects can occur on glued timber hulls and decks such as:
- glass sheathing fibre “print through”,
- Unusual/inconsistent movement that can be felt by fingers and nail depression into the timber clearly visible.
- Exposure to sun continuously on one side of the vessel (such as in a marina).
- It is possible that movement can originate from the build stage where a partially finished vessel may have had water ponding in the bilge or gluing and painting may have been carried out in very moist conditions, or the boat may have taken many years to build so moisture can be locked in and cause movement as it tries to dry out when exposed to the elements.
- unventilated lockers that border the inner hull with external hull movement only in the region of the lockers
- ingress around chainplates travelling down into the planking and decking
- ingress via decks into top of planking
- window, hatches or belting penetrations
- Rogue sections of non-durable timber like sapwood – accidentally used during the build, repairs or upgrades
- trapped/ponding internal moisture causing external movement or softness such as in sealed voids, inside plywood radar archways, in flying bridge coamings and cockpit box coamings, in leaking lockers with water ponding
Although it is not really a surveyor’s job to guess at the story of what might have happened when defects are found in survey/inspections, these types of issues should be noted as observations. And it is good for the surveyor to understand the potential causes as they can potentially lead to discovery of related issues.
Evolutions of 3 skin diagonal timber hulls
Occasionally glued timber hulls were built using a single skin of glued strip planking with or without glass sheathing externally. This provided a smooth inner surface for water to drain freely to the bilge without becoming trapped by stringers. But there is a higher potential for single skin strip planked glued hulls the same thickness as diagonal hulls to split. A heavier external glass skin instead of diagonals is a viable option to provide extra backing to the longitudinal grain.
However, another approach from around the early to mid 1980s was to use an inner strip planked skin followed by two or more diagonals. This provided a smooth inside planking without a planking split risk and relatively wide frame spacings could be used instead of stringers.
Other new developments also emerged in the early 1980s that were more significant,
- the widespread use of West System Epoxy (and later in the 1980s competitor products such as East system, and Epiglass HT9000 and others
- strip planked cedar core boats.
- Engineered boat structures and laminates
The West System epoxy was designed for use with timber and resin fibre reinforcements. The base resin and hardener could be easily dispensed and mixed aided by proprietary mixing pumps and was used for laminating glass fibres and brushing resin. Additives known as “extenders” enabled resin to be thickened into filler or adhesive or both by the addition of the appropriate “extender” powder/s (such as silica or microbaloons or talc and many others). This was a big step and a whole new system of epoxy boatbuilding.
The West System mantra was all about keeping water out of the timber and encapsulating the boat structures with epoxy. Generally, hulls and decks were glassed over with a sheathing cloth of about 200g box weave (0/90 deg) using epoxy resin. Then the glassing was coated with extra resin or a slurry of thickened resin to fill in the weave, followed by fairing to bring the surface to be ready for an epoxy primer or a marine paint system.
If the structure was properly encapsulated, then water could not enter and if water could not enter the wood could not get wet and if the wood could not get wet it did not need to be durable. This was good in theory but not quite always 100%. Holes were to be plugged with filler then re drilled and/or various other techniques were advertised to maintain the integrity of the system. This approach was a step further than applying Everdure to keep the wood dry.
While West System and wood epoxy in my opinion really did prove to be a revelation, there are still many places and situations where water has passed the epoxy barrier. For example when, in the rush to complete a vessel, holes are drilled for deck fittings and sealant is relied on to seal fittings. When a fitting moves the seal is broken and water can seep into the timber (think cleats). Durable materials are still required.
Strip planked cedar
Widespread take up of strip planked cedar planking proved to be a big step forward from about the mid 1980s. Using the new epoxy methods strip planked cedar – a fraction of the weight of kauri – was used as a hull planking material and was far faster to build.
But cedar was too soft to be used without a thicker than normal sheathing. Fibres such as Kevlar and directional woven glass fibres were used to provide a hard structural supplement to the cedar’s properties.
Initially, (around the early 1980s) strip planked cedar boats tended to be built with slight trepidation, on a backbone like on a multi skin boat but with a thin barely structural internal skin and a thick external skin strengthened with Kevlar below the waterline.
This approach changed with the introduction of more scientific approaches to structural design.
The science
Following the introduction of advanced products by Epiglass and West System; Around the 1980s High Modulus (now Gurit) began supplying advanced fibres and core materials and composite engineering advice. Run by mechanical engineering graduates of the University of Auckland’s Engineering School, High Modulus’ engineers worked with boat designers – most who were boatbuilders rather than Naval Architects, to provide engineered laminate specifications for their designs. High Modulus also supplied the reinforcements that they specified to the builders.
Another company, Adhesive Technologies, run by chemists represented West System Epoxy and suppled resins and advise on specialist epoxies and vinylester resins to use in conjunction with the newer engineered fibres. Mostly, the wood epoxy boats used the West System.
Resorcinol use faded away in the early 1980s as epoxy’s far greater versatility, less stringent application advantages and similar price took over. While resorcinol remained an excellent glue for wood to wood gluing and is still used for gluing plywoods, its superior strength was not required as wood was weaker than epoxy as well. In many cases glue strength only needed to be compatible with wood not excessively stronger. Similar density was better and microbaloons enabled easy cleaning off of strip planked cedar – faster and better.
Wood core
Progressively but quickly over the 1980s the internal keels(hogs), stem and backbone structures from strip planked cedar boats disappeared as the inner skin of the cedar strip boats increased in thickness with decreasing internal structure and more strength in the skin – specified by High Modulus or other engineers.
With the reduced structure hulls were built predominately on temporary MDF or particle board frames. Towards the end of the1980s designers began providing computer cut frames, output from the yacht design and CAD software to builders.
Wood, glassed over tended to be retained for mast steps, keel floors and high load areas. It was common for some kauri or similar timber to be used in the top hull planks and keel area hull planking.
Laminated frames gave way to foam/glass engineered ring frames and bulkheads were glass taped in position.
Full transition
These hulls were now, if painted, indistinguishable from a foam or end grain balsa core composite with no mechanical fastenings or indicators of them being a wooden boat at all. The only differences would be that the fibre skin laminate specification could be designed to allow for the strength of the fore/aft grain of the cedar compared to a core that has minimal and equal directional strength like end grain balsa or closed cell PVC foam like Kledgecell, Divinycell or Airex. And for practical purposes the outer skin on a hull with an 80kgm3 core density may need to thicker to prevent denting than cedar with a density of around 320kgm3.
Decks
Sheathing using early epoxy and box weave glass fibre became the norm on hulls and decks that were not overlaid with teak. Teak decks screwed over plywood with black caulking between the seams came into vogue around the late 1970s and has proven to be one of the main issues found on vessels from that era. Teak is soft and wears easily, the overlays were relatively thin. Over time teak decks have often been an issue due to ingress into plywood and glassed painted borders from the fastenings and seams as they degrade.
Decks on glued timber boats were typically made from glued timber in the form of plywood sheet, sometimes from multi layers where a lot of deck camber or other curved shape was required. Some decks were made as ply/balsa/ply of ply/foam/ply glassed which produced a smooth interior surface with no beams.
Cabin coamings could be either plywood, timber or timber with a plywood backing to prevent splitting. Decks and coamings were mostly epoxy and glass sheathed typically using 200gsm cloth. The cloth provides some abrasion resistance if paint wears through and also carries/bulks out the relatively thin layer of epoxy resin.
The epoxy saturated glass sheathing over plywood prevents plywood veneers (peeled off logs rather than being sawn) from “checking” as checking in the plywood provides a pathway for water to get in.
Sometimes unusual hammer tap soundings in certain areas when surveyed can indicate other constructions such as:
- rounded shapes often used cedar or other light wood strip planking or diagonal or foam cores with glass both sides.
- Sometimes thin plywood skins were used either side of foam or balsa with glass fibre skins.
It is worth considering these sometimes-unexpected construction details when encountering softness in decks in strange areas that could be due to the use of foam or similar cores.
Through the 1990s to the present, wooden hulls
Over the 1990s the wood epoxy construction boom slowed down. However, a few professional builders have maintained good businesses building semi-custom vessels. Evolutions of existing vessels which enables the scope and cost to be accurately defined such as the Upfold Elite launches are an example.
There is also a good market for repairs and alterations using wood epoxy – often with glass reinforcements, although the cost of labour and overheads is a barrier and many owners of wooden boats of all types cannot afford to get repairs done.
Occasional one-off vessels are still built but custom vessels in general are at the time of writing rarely built, mainly limited to export vessels for wealthy international owners with very specific needs and deep pockets.
Production built GRP vessels now dominate in new boat sales of all sizes. Most larger non trailerable vessels are imported.
Older glued timber vessels present good value but most being well over 20 years old now are due for refits to bring them to fully serviceable, including paint and often engines. This is not always the case. Continuously maintained vessels are sometimes found that are as good as new.
The hulls of most glued timber vessels that I see are usually pretty good – but there have been significant exceptions. As general types, triple skin kauri and strip planked cedar hulls are both in my opinion as good as each other.
The issues tend to lie in the decks, repainting being due, and a myriad of other deterioration as to be expected for vessels over time. The older and less valuable the vessel, the less owners are inclined to spend.