Question details:

Why should a different gauge used for draught restriction than for length restriction (relevant to Marblehead, RG65 and Nano class)?

Answer:

Use of the length restriction gauge adapted to also restrict the draught of the Marblehead class was proposed when the draught limit was introduced in that class (late 1990s).

The conclusion at the time was that use of a combined gauge would have major disadvantages –

  • it would not restrict the draught of a boat as accurately/effectively as a transverse draught restriction gauge does
  • it would be easy to get more draught by lowering the bow and stern of the boat into the water
  • it would tend to encourage boats that were not manoeuvrable

The following notes expand on this. In the time leading up to the Marblehead rule change the major section, relative to the true waterplane, of all available M designs were placed on a single piece of paper#. There was a spot on the 'hulls' about 50 mm off the centreline that all the sections passed through with a + or - 3 mm variation. This + or – 3 mm range is a very small variation from the average value being + or - 0.5% of the overall draught figure.

The bow and stern profiles of the same designs were all plotted, relative to the true waterplane, in the same way and there was a large vertical range at the 'measurement points' created by the ledges in the length restriction gauge. A + or - 10 mm range, or more, was found.

This test indicates that, in a mature class, the transverse draught restriction gauge is approximately three times better/more effective for controlling draught than the length restriction gauge.

It was also considered how the different restriction methods could be exploited to gain additional draught.

Using the transverse draught restriction gauge

The section shape of a hull near midships could be altered to increase draught marginally. However the change that is necessary would have the following effects:

  • increased wetted surface area
  • increased form drag (due to distorted section shape)
  • decreased prismatic coefficient (or beam, or waterline length) for the same displacement

These effects were all counterproductive and would tend to discourage such changes or negate them if tried.

Using the length restriction gauge

The bow and stern profile of a hull could be altered to increase draught markedly. The change would have the following effects:

  • longer waterline length (if not already maximised)
  • increased prismatic coefficient (or decreased beam or decreased hull depth)
  • marginally increased wetted surface area for same displacement
  • decreased manoeuvrability

The first and second might be viewed as positively useful. The third of these effects was the only one that directly adversely affected straight line speed but was marginal in size. The fourth, which may not be considered as a problem, was probably the most serious negative factor.

Based on the data and brief analysis above the conclusion at the time was that the transverse draught restriction gauge would be the better choice for the following reasons:

  • It controlled draught several times better than a combined gauge
  • With a transverse gauge any exploitation of section shape could give marginally increased draught but created three disincentives and no clear benefit
  • With a combined gauge any exploitation of profile shape could give a marked increase in draught, had two potential benefits and only one negative factor which might not be recognised as such.
  • It would be unwise to use a draught restriction system that might encourage designs of boats that were not manoeuvrable.
  • Dedicated length and draught restriction gauges are smaller, easier to handle and transport than a combined gauge

For the same reasons the Marblehead system is proposed for the RG65 and NANO class rules.

End

Question details:

What is the point of the NANO class?

Answer:

The advent of 3D printing fittings and other parts for rc boats raises the possibility of a class where the hulls and other parts are largely constructed by 3D printing and, probably, by the end user. Although this may well work in the existing RG65 and FOOTY classes, those class rules permit conventional moulded carbon fibre for the hull and deck structures and this confers a large weight/stiffness advantage to boats built using ‘hi tech’ methods

A new class rule devised specifically to cater for those who want to create their own boat, primarily by printing it themselves, would seem to be a logical step for the sport.

Such a class is an ideal one in which students of all ages can test their design and build skills. Perfect for craft, design and technology projects. The possibility of designing the boat on Friday and going racing on Saturday has become a reality.

Importantly the class raises the possibility of readily available low cost boats within an open class that is ideal for those starting the sport of rc sailing. You do not have to choose a one design with no freedom to experiment. 

Objectives of NANO class

  • Sailors are quite likely to be able to design and build the majority of their own boat
  • Hull and associated structural parts may only be made by 3D printing
  • No hull/boat certification process
  • No sail area measurement
  • Event measurement only
  • Draught/length ratio not so large that fin technology rules – 0.45 length maximum
  • Tightly controlled rig size and number

Provisional class rules have been launched in May 2017. It is anticipated that feedback from users may assist with refinement of the class rules, if required, to help achieve the desired objectives.