Why should a different gauge used for draught restriction than for length restriction ?

Question Details

Why should a different gauge used for draught restriction than for length restriction ?


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 65 and NANO class rules.