By Richard , in category: Articles
Block Configurations - Ash Blocks (stainless bindings)
Confusion quite often reigns, so a few words of explanation might help here.
With blocks, the "number" ie single, double, treble and so on refers to the number of sheaves (pulley wheels, if you must!).
If the binding - in these cases the metal bit, extends past the shell of the block, it forms a "becket" to which you can attach the end of the rope.
And that gives you the basic variations eg double, single & becket etc.
By Richard , in category: Articles
Winch Selection Guide
|Mainsail Area (ft2/ m2)||120/11||150/14||180/17||210/20||250/23||325/30||750/70|
|Genoa Area (ft2/ m2)||200/19||300/28||350/33||470/44||575/53||825/77||1500/139|
|Boat Length (ft/ m)||23/7||27/8||30/9||34/10||38/12||45/14||52/16|
|Spi. Sheets etc||-||1||1||1/4||4/5||5||5/6|
|Spi. Sheets etc||8||10||10||25/26||31/40||40/41/51||51/70/71|
Please note that the above figures are provided as an initial guide only
By Richard , in category: Articles
Rudder straps - bronze
The first dimension for the rudder straps gives the distance between the straps, the second the length of the straps. We supply rudder straps without fastening holes, since some need to match existing fasteners, others through-bolt, yet more use screws etc. etc.
The boat lengths are only a very rough guide.
Approx max boat length
Depth of strap
Offset of pin
Transom fittings - bronze
Series 1 - craft up to about 4m length
|3 - Hole||23/4”||13/4”||5/8”||3/8”|
Series 2 - craft up to about 5.5m length
|4 - Hole||21/4”||13/4”||3/4”||3/8”|
|3 - Hole||31/8”||21/8”||3/4”||3/8”|
Series 3 - craft up to about 6.5m length
|4 - Hole||31/4”||2"||1”||1/2”|
Series 4 - craft up to about 11m length
|4 - Hole||51/8”||25/8”||1”||3/4”|
By Richard , in category: Articles
Metals in a Marine Environment
To understand the use of metals - or indeed any other material - it is helpful
to know a little about three properties; stress, strain and stiffness.
Stress is the amount of force acting over a particular area. If a 1 ton boat is
suspended by a wire of a cross-sectional area of 1 inch, the stress in that wire is 1
ton per square inch. It doesn’t matter what the wire is made of, the stress will be 1
ton per square inch. By using this concept, the strengths of different materials can
be related. You can have a large piece of wood as strong as a smaller piece of steel
but, because of their different cross-sectional areas, the stresses at which they break
would be very different.
For metals, there are two useful values of stress to consider: the Ultimate
Tensile Stress (UTS) which represents the breaking stress, and the Proof Stress (or
for steels, Yield Stress) which represents the maximum useful static stress. The
table shows sample stresses for a range of materials. These are guidelines only, and
need to be modified to allow for certain further considerations:
- metal is subject to fatigue if put under repeated or cyclic loads.
- stress concentrates locally around holes, defects, sudden changes in crosssection
and so on.
- the fact that even if you can accurately predict the strength of a fitting, it is
rare to be quite so confident of the service loads it will be required to take. Ample
factors of safety are needed.
The subject is further confused by the fact that different alloys, for instance
the brasses or the steels, can have very different strengths. It tends to be that the
stronger an alloy is, the more brittle it becomes.
Strengths Yield/ProofStress (N/mm2)UTS (N/mm2)
Mild Steel 250 400
High Tensile Steel 1000 1500
Stainless Steel 316 325 575
Aluminium - cast - LM4 85 150
Aluminium - plate/bar 80-250 140-400
Copper - plate/wire - C101 200 300
Brass - cast - SCB4 90 280
Brass - plate/rod - common brass CZ108 340 470
Brass - plate/rod - naval brass CZ112 300 440
Brass - plate/rod - high tensile brass CZ114 285 510
Bronze - cast - gunmetal LG2 115 240
Bronze - cast - Phosphor bronze PB1 145 250
Bronze - cast - aluminium bronze AB2 275 680
Bronze - plate/rod - Phosphor bronze PB102 345 485
Bronze - plate/rod - Aluminium bronze CA104 385 720
All figures are for guidance only, and will vary widely depending on
the heat treatment and/or work which the material has undergone
Strain is defined as the extension per unit length, usually expressed as a
percentage. So if our wire was initially 100m long, and extended to 102m under load, the
strain is said to be 2%. The amount of strain at fracture can give us a feel (but no more) of
the brittleness of a material. Pottery breaks at about 0.5% strain, piano wire (steel) at 5%,
mild steel at 30%, rubber at 2-300% and so on. Typical working stresses for metals
induce strains of about 0.2 - 0.3%.
And finally, stiffness. Return to our 1 ton boat on the 1 inch wire. If the wire were
steel, it would barely stretch at all. If it were wood it would stretch more, nylon more still,
and rubber might simply keep extending to leave the boat on the ground! Each material
under the same stress shows different strains. Divide stress by strain and you have a
measure of stiffness - steel has a high value, rubber a low one. Stiffness - referred to as
“Modulus of Elasticity” or “E” - stays fairly constant for a given base metal irrespective of
the alloy. So wrought iron, mild steel, cast iron, stainless steel and high tensile steel all
have virtually the same E. The table lists values for the usual metals found in boats.
Stiffnesses E (kN/mm2)
Wood - along the grain 9-12
Aluminium & alloys 70
Copper, Brass, Bronze 120
Steel & Iron 210
Carbon fibre/Kevlar/ Boron fibre 300-800
Does any of this relate to the real world? To answer the question, let us look at one
of the more “engineered” aspects of a boat: the rig. If you subscribe to the view that it is
useful to stay the mast such that it remains roughly in the same place, you need to use a
material that is both strong (to avoid large section areas and so reduce windage) and stiff
(to maintain rig tension). Look at the strength and stiffness values and you can see that
steel is as strong as most things, but considerably stiffer. Hence steel in various forms, is
used for standing rigging - any other metal would be a nonsense. Only if you have a
downwind rig such as square rig where the stresses are lower and the windage of less
importance, is the use of traditional rope a feasible option. It is probably fair to say that
the development of windward ability in boats owes as much to the availability of steel
wire as it does to developments in rigs and sails.
Stiffness in rig adjusters is not so vital since, even at high strains, the length of the
adjuster will be so small by comparison with the length of the stay that the overall
extension is acceptable. So bronze rigging screws or deadeyes are feasible even though
they are less stiff than steel screws - and from the point of corrosion rather better.
In practice, a large proportion of a boat’s fittings and, indeed, most things we use
in everyday life are designed for stiffness rather than strength, and in broad terms that
outcome results from designing “by eye” or experience. For example, the pen I’m using
now needs to be stiff, but the stresses in its casing must be trivial. Chairs and tables
shouldn’t wobble too much, but it is easy to work out that service stresses are very low.
Deck fittings tend to pull out - often with a bit of deck or cabin top attached - rather than
All of which means that the selection of materials for general boat fittings tends
to be based on considerations of usage, appearance, weight and workability rather than
pure strength. For example a cleat will need to take so many turns of a particular size of
rope, and that will pretty much determine its size. As long as the supporting structure and
fasteners are adequate, the cleat could be made of almost anything. So in the areas where
stiffness is more important to the function of an item than strength, what looks right usually is.
“If all else fails, use bloody great nails”
With fasteners, additional factors come into play. They need, of course, to be
strong and, it’s a good idea if they are cheap, since you will use hundreds in even the
smallest craft. But perhaps most important is that, once installed, they should neither
corrode, nor be corroded by, the items being fastened - at least for a reasonable period
of time. In this context there are two main types of corrosion to worry about.
The first is galvanic corrosion, where two different metals are connected in the
prescence of an electrolyte. Sea-water is, unfortunately, an excellent electrolyte and
gets everywhere on a boat. The galvanic chart shown below shows the various
electropotentials of commonly used metals in boats. I’ll not to get bogged down in too
much chemistry, but here are three points which help to make the table useful:
- Where two metals are linked, the one to the left will corrode.
- An electropotential difference of 0.1 volt is usually safe and 0.2 volts usually acceptable, subject to the next proviso.
- The rate of corrosion depends, amongst other things, on the surface areas of the exposed metals. If the fastener is less noble than the fitting, it will
corrode very quickly. If more noble you will get a useful working life.
So is this of any practical use?
- It explains why most fasteners are at the noble end of the scale. Aluminium would be a nightmare, so make sure your pop rivets are monel (aluminium
pop rivets are widely used the in the car industry).
- You can deduce which fasteners to use for which fittings (see table on the next page). Galvanised fasteners should of course be avoided on stainless or
aluminium equipment. Less obvious is that brass screws are unsatisfactory, perhaps dangerous, for bronze fittings.
- There are some alloys which can create their own galvanic couple. The most significant is brass where (in the presence of an electrolyte) one ‘phase’ will
corrode rather than the other. This is known as dezincification. A brass component that has been subject to dezincification is terrifying to behold - it
has the appearance of, and not much more strength than, a ‘Crunchie’ bar. It is also possible to induce a similar effect by working steel. For example, the
heads and points of steel nails will corrode in preference to the shanks because they have been worked. This is not often significant in boats, but may help to explain why,
when you are trying to remove old tacks or nails, the heads keep breaking off!
Fasteners for fittings
Fitting made of: Fasteners
Galvanised Galvanised or Stainless Brass or Bronze
Aluminium Stainless Galvanised, Brass or Bronze
Brass Brass or Bronze Stainless
Bronze Bronze or Stainless Brass
Stainless Stainless or Monel Galvanised or Brass
The second type of corrosion to give concern is attack from various chemicals. In
general, metals form oxides to protect thems elves, the crucial distinction being whether the
oxide forms a hard self-repairing film, as in stainless, aluminium and yellow metals, or
flakes off to expose fresh metal, as in steel. The effects can range from cosmetic if bronze
or galvanised deck fittings become ‘weathered’, to dangerous if nail sickness occurs. The
latter is primarily caused by the generation of acids as woods saturate and break down, oak
being the worst offender. It’s well known that steel can suffer - hence corroded keelbolts
and hull fasteners. But is it less widely appreciated that stainless is also susceptible.
Because there are so many misconceptions about stainless steel (a misleading
term in itself, though not as bad as ‘inox’) it’s probably worth momentarily delving into the
technicalities. As well as iron and carbon, stainless steels include a number of alloying
elements. Of these the most important is chromium (Cr.). If there is more than 12% in the
alloy, a complete layer of chromium oxide surrounds the metal. This layer, the ‘passive’
film, is resistant to most things and will self-repair in the presence of oxygen. Chromiumonly
stainless steels tend to be brittle, so about half as much nickel (Ni) is added to create a
more usable material. 304 stainless (or A2) is one of the more commonly available and
includes 18% Cr and 10% Ni. If you have a stainless sink or exhaust pipe it’s likely to be
304 and, as anyone who’s ever tried cleaning a sink or pulpit will know, is somewhat prone
to attack from the organic acids generated by food, fingerprints and other pollutants.
The chemical and food industries alleviate these problems by adding a dash of
Molybdenum (Mo). Thus 316 stainless (or A4) typically comprises 17% Cr, 11% NiI, 2 %
Mo and is widely used to store and transport some very aggressive substances. So, you
might think that this is the perfect stuff to use as a fastener in or through wood, and from
the sole perspective of chemical attack you’d be right. But we need to reconsider the
environment in which the fastener is doing its job. Imagine a bolt, nail or screw fastening a
plank to a frame underwater. The head, at or near the surface, will be oxygenated enough
to maintain its passive film. The shank, buried deep in the structure, is likely to be starved
of oxygen but will be surrounded by various acids and chlorides. In these circumstances,
the passive film may break such that the stainless becomes ‘active’. This has two effects:
firstly, look back at the galvanic series and you’ll see that the difference between active and
passive electropotentials in 304, and to a lesser extent in 316, is enough to cause galvanic
corrosion. Like brass, stainless can form its own galvanic couple. Secondly, without the
oxide layer, the stainless will corrode about as fast as steel. The upshot is that stainless
fasteners below the water-line - irrespective of the grade - may be no better than mild
steel. Above the water-line (more oxygen and less electrolyte) such fastenings are fine, but
unless you value the extra lustre of 316, there’s little point in paying for it.
While on the subject , I’d like to tackle the nonsense of shot-blasted s tainless fittings which
seek to ape the appearance of galvanised fittings. The ability of the passive film to selfrepair
is optimised if the surface of the stainless is highly polished. By forming millions of
sharp peaks during shot-blasting, you significantly reduce this ability, which is why such
fittings rust. If you want the appearance of galvanised fittings, try galvanised fittings.
Steel and Galvanising
Unprotected mild steel has no place on a boat because of its propensity to corrode, but
it’s a good material if suitably protected. This is usually achieved by adding a layer of zinc -
“galvanising” - which has two benefits: firstly, zinc has good resistance to chemical corrosion
and, secondly, it will corrode preferentially to the steel in the presence of an electrolyte. There
are different types of galvanising, the key variable being simply the amount of zinc attached to
the steel. For a useful life in a marine environment you need a covering of about 100 microns of
zinc (1 micron is one thousandth of a millimetre). This can be provided by hot dip galvanising
(up to 125 microns), painting (about 40 microns per coat) but not usually by electroplating,
which tends to be limited to about 20 microns. So the BZP (Bright Zinc Plated) fastener
available from your local hardware shop might be fine for the greenhouse, but won’t last for any
useful time on a boat. For marine fasteners you need hot dip (or spun) galvanising.
Unfortunately the cost of galvanised fasteners is increasing. In particular, galvanised
nails are becoming increasingly rare and tend to come in large quantities. Apart from getting
fasteners galvanised yourself - remembering that threaded components need to allow for the
layer of zinc - options are restricted to paint coatings, which are only effective if unchipped, or
the substitution of other materials.
Boat nails and roves widely used in the construction of traditional wooden boats are
some of the few specialist boat fasteners still produced in copper. For these relatively flexible
structures copper nails are perfect: easily worked, corrosion resistant and ductile enough to
allow for movement. With the advent of glued construction methods and, of course, plastic
hulls, it’s quite surprising that copper boat nails are still available. The range, is however,
reducing. For example, 3/16” and 1/4” roves (5mm & 6mm) are no longer made, so canoe
builders will have to clench their nails. Also disappearing are the ‘odd’ sizes so useful on a refastening
job where moving up one size can very effectively re-tighten the hull.
Brass is most commonly available as woodscrews - up to 14 gauge - and as machine
screws/bolts. Remembering the problems of dezincification, brass screws should only be used
in protected environments, for example in interior furniture, or in applications where your life
will not depend on them.
The usual alloy for fasteners is silicon bronze. As well as being used for bolts,
coachbolts and ringshank nails, this is one of the few materials in which very large woodscrews
(up to 30 gauge) can be obtained. It is sufficiently resistant to corrosion to have a very long
working life (perhaps thirty to fifty years) so, in terms of value, bronze fasteners, though expensive, are competitive.
Copper-based Alloys - which is which?
Name Designation Alloy Elements Typical Uses
Common Brass CZ108 Zn 37% Interior Fittings
Naval Brass CZ112 Zn 37% Sn1% Pre-war boat fittings
High Tensile Brass CZ114 Zn 37% Mn 2% Al 1.5% FeSnap shackles, Propellers, Winches
1% Pb 1.5% Sn 0.8%
De-zincification resistant (DZR) Brass CZ132 Zn 36% Pb 2.8% As 0.1% Hull valves and skin fittings
Aluminium Bronze CA104 Al 10% Ni5% Fe5% High strength fittings
Phosphor Bronze PB102 Sn 5% P 0.2% Fabricated/wrought fittings
Silicon Bronze CS101 Si 3% Mn1% Fasteners
Gunmetal LG2 Sn 5% Pb5% Zn5% Cast hardware
Aluminium Bronze - cast AB2 Al 10% Ni5% Fe3% Stanchions, some mast hardware
Al - Aluminium, As - Arsenic, Fe - Iron, Mn - Manganese, Ni - Nickel, P - Phosphorus,
Pb - Lead, Si - Silicon, Sn - Tin, Zn - Zinc