Hull Shapes
See the Design Guide for a detailed analysis of the different hull designs
that have evolved over the years. The key hull measurement for a boat to conform to the rules,
apart from the obvious beam, length and depth dimensions, is the rise of floor measurement.
This rule (4bii) is designed to prevent hulls becoming overly vee shaped in the underwater sections,
and all the builders take the rule to the absolute limit to reduce wetted area. To envisage how the
rule works, imagine the hull sitting in a rectangular frame (like a cradle) positioned at mid length.
This frame is 1170mm wide, and the outer legs are 210mm high. When sitting in the frame, if the hull
touches the top of the legs of the frame, but does not rest on its keel, it is very flat and is a
Merlin Rocket. If however it rests on its keel, but does not touch the tops of the legs,
it is very vee'd and is not a Merlin Rocket. This is not how it is measured, but establishes the
principle of the rule.
Rocker (the amount of longitudinal curve in the keel) is not policed, but the trend has been for the
amount of rocker to reduce – it is most noticeable if you compare an older style narrow hull,
where the forefoot will invariably be clear of the water with a modern hull where the forefoot will
nearly always be in the water. The flatter profile planes better, and is very often linked to a
'flat run aft' where the longitudinal curve aft of the centreboard case is nil, and the lateral shape
of the hull at the transom is flat for about 400mm or so across the centreline.
Hull materials
Over the years, the hulls have evolved from using plywood nailed and roved to frames,
to plywood glued with Cascamite, to plywood glued with Polyester, then Epoxy, resins,
and are now mostly moulded using two skins of epoxy resins in a glass fibre matrix bonded either side
of a foam core (Foam Sandwich). The rules forbid the use of carbon or aramid fibres in the construction
of the hull shell, which includes the transom and the centreboard case. These materials are allowed in
any decking that there may be, and in any internal fit out of the hull shell.
Masts
The trend in mast construction has been to reduce the weight of the spar as far as possible, and to
increase the consistency of the production of the spar as much as possible. Early wooden masts were
very heavy, very variable in bend characteristics, and prone to rot and fall over! The transition
to metal masts was made under the inspiration of Ian Proctor, and the D section made by his company
was for very many years the class standard. Competition from Superspars in the form of their M3 section
opened up the market for a while, until the big shift back to deck stepped masts was made in the late 1980's.
This change was made to enable lighter weight crews to sail the boat across a wider wind range,
and lead to the need for a lighter more flexible mast section, for which the Superspar M1 became the
standard until the advent of carbon masts.
It would be a rare new boat that was launched now without a carbon mast.
The most popular choices are the Superspar Carbon, which is excellent for competing at club and
Open Meeting level; those aspiring to Silver Tiller or Championship success are invariably buying
their masts from Chipstow Boat Yards.
Mast stepping
The first Merlins built had timber deck stepped rotating masts which had at least one set of diamonds,
and were effective, but very heavy. At some stage, this evolved into the hog stepped aluminium mast
with mast gate that characterised the boat through the sixties, seventies and early eighties.
The rig that went with this was a fairly stiff rig, often called the 'Barn Door' rig,
since the technique for coping with being overpowered was to load up the kicker and sit out harder,
while easing the main sheet, which, like a barn door, just swung in and out!
This was when the boat required crew weights of 25 stone to be competitive across the wind range.
During the late 1980's, from boat number 3413 (Canterbury Tales) the shift back to deck stepped rigs began,
and the technique of depowering by introducing mast rake became the norm.
Deck stepping the mast means that the spreaders remain effective across a wider rake range,
and enables crew weights to reduce to the 22 stone mark, while remaining fully competitive.
Mast sections became smaller, and with the introduction of carbon, much lighter,
while also permitting higher rig tensions to hold the jib luff straighter for longer improving pointing.
The lighter masts are controlled by the main shrouds, and a pair of 'Lowers',
which in effect are short shrouds connected to the mast at gooseneck height to control bend induced by
kicking strap tension. For down wind work, the 'puller' has also been added which runs from gooseneck
to a position some 450mm along the foredeck to prevent the mast inverting under the influence of the
load from the spinnaker pole.
While a number of the later hog-stepped boats have been converted to take the newer style rig,
it is rarely completely successful because of the need to run additional controls,
which makes everything very complicated and untidy. Older boats, before about sail number 3200 should not
be converted anyway because the glues used in their construction cannot cope with the additional loads
the rigs impose.
See the library, which has several articles on the subject.
Wing/Rotating Masts
It is the need for maintaining the rig tension that has limited the development of wing masts in recent years.
The increased power that should be available from the wing mast used in conjunction with the existing rig
makes it a desirable grail, and there have been a couple of notable attempts were made to make an effective
wing mast. One involved a large aluminium section, which worked ok in terms of power generation,
but was so fantastically heavy that the net result was poor. A further series of trials using a carbon mast
were undertaken, but the conflicting requirements of a light section, and significant rig tension rendered
the trials a failure.
The principal problem was how to attach the rigging while still allowing rotation –
the solution has not been found to date. As late as 2002 a boat was developed specifically to carry a
carbon rotating wing mast; so great was the anticipation of the power that the rig would generate,
that it was built without a spinnaker. The reality was that the expectations never materialised,
and in the light weathers at Salcombe, the lack of spinnaker was sorely missed.
What future developments may be, remains to be seen.
Booms
The story here is similar to the mast tale; apart from a flirtation with very full sails and bendy booms
in the 1970's, the holy grail is to get the lightest, stiffest beam available to enable tension up the
leech of the mainsail to be achieved and maintained while using a centre main sheet system.
Carbon fibre is very popular – apart from being light, it doesn't hurt as much if the boom hits you!
Older rigs commonly had a bolt rope along the foot of the sail which slid into a track along the boom,
however, most modern mainsails are cut with a 'loose foot' that is only attached to the boom at the tack
and the clew, dispensing with the need to have a track along the top of the boom, not only saving weight,
but making it easier to let the sail take up a full shape down wind when the outhaul is eased off.
Sails
Mainsails are available in either Dacron or a variety of 'Laminate' materials.
Dacron is the usual club level material for the sail; it is economic to buy and if looked after will last
a good length of time. It suits older boats very well. Merlin Sailors with higher ambitions will often
look towards a lighter weight material that has lower stretch for it's weight that Dacron, and will hold
it's shape across the wind range. The materials that are used for this are Mylar and Kevlar;
these materials are more expensive, and although pretty bullet proof, do shrink and so rarely last more
than a season at top level.
It is probably unwise to look to buy a modern mylar sail to use on a older style 'barndoor' type rig
[see Why Deckstep the Mast];
the characteristics of the modern mast means they are more bendy and the sails have more luff round to
cope with this, and will therefore not fit a Proctor D properly without a sailmaker's attention.
Jibs are invariably made from Dacron, because it copes with the flapping that jibs suffer better than the
laminate sails, and is a lot cheaper. Once again, though, it would be a mistake to buy a modern jib to
use with an older style mainsail. The sheeting angle (the distance from the centreline of the boat to the
jib fairlead) rapidly narrowed in the late 1980's, requiring jibs to be shaped substantially differently to
match their mainsail. An older job will not work with narrow sheeting and vice versa.
Spinnakers have undergone two major transitions in the last thirty years, from the very small, very high
cut sail around up to the 1970's, to the larger sail that went with the longer pole in the 80's and 90's,
to the current rules sail. The latest difference was to make the measurement rule an Area rule, meaning
that, as long as it is symmetrical, the owner can dictate what shape he wants the spinnaker to be,
tall and narrow, or short and wide, or any stop in between. The newer sails are more powerful,
and are unlikely to suit older narrower boats as well as the 1980's sails, due to their reduced righting
moment.
Spinnaker Pole Systems
Twin poles or single poles is the main choice here. Twin poles have achieved great popularity in the last
twenty years. They are trickier to set up correctly, but are both easier for less experienced crews to
operate, and quicker for the more experienced. When things go wrong, though, boy do they go wrong!
The main advantage of single poles is their simplicity. The spinnaker pole length has progressively
increased over the life of the class to the current maximum of 2.3m. For many years the length was fixed
at 6 feet, and there are still some poles of this length in circulation. Old narrow boats are usually
better served by using this size pole rather than the newest longer one.
Centreboards
The rules of the class limit the maximum weight of the centreboard, the amount by which it may protrude
below the keel, and dictate that it must retract fully above the keel.
Other than that, it is pretty much unrestricted in shape or size. See Guy Winder's article on the subject
for more information about optimum areas and chord positions and depths.
Rudders
Rudders for Merlin Rockets are unrestricted in shape or size, and much experimentation has taken place
over the years. Early designs had swept back blades, and used a traditional rudder stock allowing the
blade to be lifted out of the water without removing the assembly from the transom.
Modern rudders have vertical leading edges, and are often 'fixed' that is to say they do not have a stock
and the blade can only be lifted out of the water by removing the whole item from the transom.
Fixed rudders are lighter, more rigid and have less 'play' ensuring more accurate steering,
for this reason they are the first choice for most of the leaders in the fleet. Once again, a visit to
Guy Winder's article will enlighten the interested.
