Boatbuilding: Cold-moulded and Strip-Planked Wood

307 Pages·2016·8.16 MB·English

by  Ian Nicolson

More titles by Ian Nicolson available from Amberley Publishing: Alfred Mylne:
The Leading Yacht Designer: 1896–1920
Yacht Designer’s Sketch Book
The Boat Improvement Guide
The Ian Nicolson Trilogy
Build a Simple Dinghy
Yacht Designer’s Notebook
First published 2016
Amberley Publishing
The Hill, Stroud
Gloucestershire, GL5 4EP
www.amberley-books.com
Copyright © Ian Nicolson 2016
The right of Ian Nicolson to be identified as the Author of this work has been
asserted in accordance with the Copyrights, Designs and Patents Act 1988.
All rights reserved. No part of this book may be reprinted or reproduced or
utilised in any form or by any electronic, mechanical or other means, now
known or hereafter invented, including photocopying and recording, or in any
information storage or retrieval system, without the permission in writing from
the Publishers.
British Library Cataloguing in Publication Data.
A catalogue record for this book is available from the British Library.
ISBN 9781445651668 (PRINT)
ISBN 9781445651675 (eBOOK)
Typeset in 9pt on 11pt Celeste OT.
Typesetting and Origination by Amberley Publishing.
Printed in the UK.
Contents
Part 1: Approaching the Project
Part 2: Preparation and Materials
Part 3: Laminating and Fastening
Part 4: Building the Backbone and Hull Shell
Part 5: Frames, Floors and Beams
Part 6: From Deck to Keel
Part 7: Reinforcement and Repairs
Part 8: Strip-Planking
Part 9: Reference Section – Boatbuilding Woods
Further Reading
PART 1
Approaching the Project
Why Wood?
Wood has advantages such as a valuable strength-to-weight ratio, resistance to
fatigue and shock loads, ability to stand up to different types of damage, also
repairability, adaptability, and ease of handling and working. Cold-moulding
retains these virtues, and is particularly good in terms of strength/weight and
resilience. It is consequently an attractive option for racing craft of all kinds –
sailing dinghies, powerboats or large offshore racers – as well as small tenders,
fast launches and rescue boats. Strip-planking is also light in weight and easy to
build, but less good in terms of strength and resistance to damage, ease of repair
and longevity. For commercial craft, however, the conditions of use tend to
favour steel in most cases.
As economic pressures increase there is a tendency to build boats for the
widest possible market. Design has gone towards short bow and stern overhangs,
ample beam and high topsides because these proportions suit land storage and
marinas, (where charges are based on length), and allow maximum
accommodation space. When a yacht is wanted that is not short and fat the
available choice in fibreglass becomes very small, and this is where cold-
moulding becomes especially attractive as it suits such hull shapes and is viable
for small production runs. Long, narrow, fast and light launches of the type used
on lakes and rivers, where they combine high speed with minimal wave-making
wash, are another example, and like rowing shells they have been made in this
way for decades.
Both methods particularly suit one-off or ‘custom’ building, as for racing
yachts and dinghies, or short production runs. A set of moulds may be reused for
sister ships, are easy to store in between, and compared to a fibreglass mould are
much quicker to make and perhaps a quarter the cost. The shape of the moulds
can also be altered quite a lot without total rebuilding. This is a unique and
valuable asset because racing boats tend to change gradually. When a designer
has a successful shape one year, next year’s boats are likely to be similar and the
old moulds can often be modified in a few days.
Wood itself is not a particularly expensive material. Just how its cost should
be realistically compared to other boatbuilding materials is much debated.
Comparison by weight alone ignores building costs, storage, the density of the
material, the cost of buying, handling and working, and so on. Cutting away the
forest of prejudices, most experienced people would agree that steel is the
cheapest material, with wood and ferro-cement about equal second, then
fibreglass, and aluminium the most costly. However these relationships change
according to the size of craft involved and the amount of series production. One
advantage of cold-moulding is that the cheaper pieces of suitable timber can be
used, since long lengths and expensive wide boards are seldom needed. Good
quality timber is still necessary, though, and always costs more than rubbish.
Another aspect which makes both cold-moulding and strip-planking
attractive to amateurs is that they can be done by people who are short of
woodworking skills, and by a small team such as a club or even one person
working alone. By careful planning most of the jobs can be handled by semi-
skilled labour, and this is also an attraction for professional builders who want to
stay in business. Second year apprentices can do much if not all the work, and
material stocks do not have to be large.
Anyone worried about his competence should start with a dinghy. Unless
the boat is clearly a disaster it will be useful or can be sold, and it will give
experience as well as confidence. If building is started but cannot be completed,
the materials are not difficult to sell and the experiment will have been cheap.
One disadvantage of all wooden hulls is that they are not impervious to
marine borers unless sheathed under water. However the sheathing need not be
expensive copper. Cascover, which is a tough nylon cloth embedded in epoxy
resin, keeps out worm and is stronger and lighter as well as cheaper. So for the
tropics cold-moulding and strip-planking are still attractive, especially as the
wooden hull, deck and cabin top make good insulation.
Because wood is an insulator, and above all because it is so attractive, cold-
moulded hulls do not need lining provided the interior is well finished, and glue
spillages have been minimized, also wiped away before they harden. Furniture is
quickly and easily joined to the hull, and given plenty of manpower for
laminating the shell a cold-moulded boat can be afloat in a very short time after
the design is complete. A fibreglass boat from an existing mould is built quicker,
but if the pattern and mould have to be built then the wooden one will be afloat
much sooner. So where speed of building matters, cold-moulding is sometimes
the best choice.
Finally, a well built cold-moulded boat should last as long as a carvel-
planked one, say fifty years.
Cold-moulded Hulls
The term ‘cold-moulding’ covers a number of related building techniques, but
essentially consists of gluing layers of thin wood skins or planks together like a
multi-layer sandwich. The result can be considered a composite material, and is
very different from traditional wood boatbuilding in which separate pieces are
fixed together with metal fastenings (sometimes assisted by glue) so that in use
each part can move very slightly in relation to its neighbours. A cold-moulded
hull is rather like the shell of a nut; the waterproof outer shell is also the strong
structure which makes the nut so tough. If a nut’s shell has a weakness it is at the
join of the two halves, and in the same way cold-moulded boats (like fibreglass)
unless properly designed and built, tend to have less strength at the deck/hull
join. The analogy does not hold true, however, in the advanced form of
construction seen on some racing boats, where the hull shell is largely to keep
out the water and contributes much less to the rigidity and strength of the whole.
An internal space-frame takes over much of the structural role as well as linking
the stresses from the rig, keel, etc. that would in the conventional version be
taken partly by floors, framing, stringers and so on as well as the hull shell.
However, a hull built in conjunction with a space-frame structure is still
laminated, though more lightly.
Cold-moulding is the best way of using wood because it accentuates its
good properties and gets round most of the problems. Many woods are subject to
faults such as cracks as a result of uneven drying-out or thunder shakes; a sound
tree has curly grain near knots and other variations.
But by using thin narrow planks one can be virtually certain of eliminating
all faulty wood; if there is a hidden defect it must be small since each piece is
small in section. The tendency to warp or shrink, which varies according to the
species of wood and how it is cut and dried, has always been a nuisance.
Laminating veneers or thicker layers together contributes to dimensional
stability, and modern glues used with well dried timber result in a long-lived and
stable shape. Naturally paint and varnish have to be kept up, but cold-moulding
does not suffer from the drying-out cracking and open seams sometimes found
on carvel hulls, solid timber and laid decks after exposure to hot sun.
The strength of wood is of a special kind. Tough stringy fibres are bound
together by a gluey substance so in a collision it deals with the distorting forces
in two ways. Part of the energy of the blow is absorbed by crushing, as the wood
is progressively compressed; this is an excellent way to react to damage because
the force is decelerated. At the same time the fibres bend, steadily building up
resistance which further helps to absorb the force. This springiness, which
remains in cold-moulding and strip-planking, is remarkably effective in
minimizing damage. It also absorbs vibration, so slamming against steep seas, or
the vibration of a mast in a gale, are neutralized by a wooden hull.
Chafe damage, such as a boat suffers when grinding against a quay or
pontoon, affects a cold-moulded hull more severely than carvel or clinker, much
more than steel, but less than fibreglass. Such damage to wood is easily repaired
if it is slight, and is not really difficult (though it is time-consuming) if it is deep.
Fibreglass is very vulnerable to quite mild abrasion and even if this is repaired
just by filling and painting, so many coats are needed and the paint is not the
cheapest, that repairs are relatively expensive. Steel of course is the ideal
medium where conditions are rugged; it stands up to a lot of abuse without any
harm except to the surface coating. However it is heavy, massively so when
compared with a carefully designed cold-moulded hull. Aluminium is
comparable in weight and toughness, but less so in cost until the larger sizes are
reached.
A heavy duty cruiser designed by A. Mylne & Co. for owners who wanted a yacht capable of taking the
ground at low tide on the rocky shores of the Scottish east coast. As a result she is unusually strong, with no
cabin top to break the continuity of the deck. Her overall length is 38ft and she was originally designed
with a ketch rig. The two men who built her had no previous experience in boatbuilding, so the construction
was worked out to avoid difficulties like reverse turns near the garboard.
There is nothing so dangerous as a claim that one particular boatbuilding
method is lighter than another. Each material and technique has the backing of
designers, suppliers and owners who constantly promote their favourite. Big
companies have invested millions in fibreglass, so they have no intention of
letting the material fall behind in popularity for want of advertising, research,
press trials, development and so on.
Sticking my neck out with the certain knowledge that howls of protest will
arise from many quarters of the boatbuilding world, I quote from the Henry and
Miller paper before the 1978 meeting of the U.S. Society of Naval Architects
and Marine Engineers (see table on page 9).