Merlin Rocket Design Guide
Part 4 - Glossary of Technical TermsRewritten in 2007 by Chris Martin
From class rules: “One bilge keel or chafing piece, of which the cross section shall contain a rectangle not less than 20mm x 10mm for a length of 1200mm, shall be fitted over a land on each side and so placed that the weight of the boat will bear on the main keel and one bilge keel only when the boat is on a level surface.”
As we can see from the above paragraph, there are no major restrictions on the positioning, size or shape of the bilge keels, as long as the boat can rest on them. This has lead to a number of different shapes and positions, in recent years mainly involving Winder-built hulls.
Originally required due to launching trolleys not being commonplace, the bilge keels were seen as sacrificial rather than an aid to performance. Until about 1992 nearly all boats had rather ‘stubby’ bilge keels, fitted in parallel line with the centreboard slot on the join between the first and second plank.
Recent trends are towards extremely rounded and streamlined bilge keels, fitted slightly further back than the norm of about 5 years ago. Other variations tried by boat builders include very flat, squat and rectangular bilge keels in an attempt to improve the planing surface (mostly boats built by Ron Hall and Chipstow Boatyards) and the rounded variety continued right to the transom for the same reason (a few Winder hulls).
All varieties have had their admirers and the differing shapes offered are alleged to influence performance. It however must be said that this is not really borne out by the results over the years.
If the rocker line suddenly curves up at the aft end, the boat is said to have a bustle. This was generally used by designers and builders to help heavier helms get the flat aft sections out of the water in light weather.
The bottom two planks.
The join between two planks where they overlap.
Rise of Floor
From class rules: “(Beam) At mid-length, measured 210mm above the points on the outside surface of the skin, 51mm from the fore aft centreline shall not be less than 1170mm. The measurement shall be taken to the straight line bridging adjacent lands on the outside of the skin along the side of the keel at 2135mm forward from the intersection of the gar plank and the aft face of the transom.”
This rather complicated paragraph is a means of policing the amount of ‘V’ in the hull shape at a given point, and is intended to prevent a hull that is too narrow on the waterline to be seaworthy.
Experience has shown that boats built to the limit of this rule (maximum permissible rise of floor) are the fastest. Some are also the tippiest, though this is also influenced by the overall hull shape, especially the transom.
In the search for maximum rise of floor, a number of boats have initially failed this measurement requiring remedial action. In the late 60’s a number of boats had their certificates revoked due to a misinterpretation of the then rule, giving rise to the rather complicated but more precise wording quoted above.
The amount of curve in the hull when viewed from the side, from fore to aft. The amount of rocker in a hull, and it’s position, influences weight carrying, tacking speed and planing speed.
In the early years of the class, hulls generally had an enormous amount of rocker as they were sailed mostly on rivers where tacking time and light airs speed were crucial. Over the years with more powerful sails, larger spinnakers and the need to plane sooner, the hull rocker has reduced to what we have today. However if you are sailing on a river the highly rockered old boats are still extremely competitive, to the extent that a boat was built in 1999 very close to Jack Holt’s Passing Cloud design from the late 50’s.
Part 1 - Introduction
Part 2 - Modern Merlins
Part 3 - Older Designs