The  International Twopointfourmetre Class

Metre rule explained

 

 

Here’s an attempt to explain the meter rule for those who are not familiar with it.

Regard it as an ongoing project. Maybe the best help I can get is that you who read it ask me whatever

questions that arise as you read and try to get into the text.

 

Later a description of the measurement procedure will follow.

 

If you are not familiar with the rule it’s good if you tell me what could be explained better.

If you are familiar with the rule you probably have something to add too.

 

The metre rule in its relative simplicity typeforms the boats built to the rule into a certain design space

by penalizing what it views as non acceptable excessive shapes and measures/control the speed enhancing

factors for a heavy displacement type of yacht.

 

For a displacement boat like the 2,4mR, speed is mainly a function of waterline length.

Speed to Length Ratio = velocity in knots/Sq root of waterline length = v/sq root of lwl.

This is the speed that a pair of waves can move through the water. Above this speed the boats begins

to try to climb its bow wave.

 

 

 

The maximum speed is ordinarily considered to be 1.34 times the square root of the LWL in feet

(2,34x the sq root of LWL in metres).

 

The length measurement L in the rule measures the assumed length while under speed and the distribution

of the volume of the under water body and thus measures maximum attainable speed.

 

For the 2,4mR class the L is the length measured 36mm above the waterline the boat floats at in measurement

trim plus a measurement of the volume of the ends of the boat, the girth measurements.

 

 

The speed potential of the hull is then balanced against the size of the sails (power available).

The longer the boat (the higher the potential max speed) the less sails you can have.

 

The principal variations you can play with within the rule are the length that rules maximum attainable speed

and sail area that rules maximum accessible power.

This is the centrepiece of the metre rule.

 

Most of the other limitations are there to type form the boats into a specific design space.

 

The major limitations are the minimum allowed displacement that is related to the waterline in the

flotation plane (LWL), the value given to the forward girth measurement and the aft girth measurement

and the girth measurement amidships that puts a heavy penalty on boats with less depth of the canoe body

than 300mm.

 

 

Other type forming limitations are the value for the freeboard allowed for in the formula (292mm),

the prohibition of concavities above the waterline and the maximum allowed upwards slope of the after

body of the boat.

 

The third major factor determining the speed upwind is the righting moment (RM).

The RM is controlled by the scantlings (the building specifications and the restrictions imposed on them).

The specified minimum weight for the hull and deck mouldings (3,6kg/sqm) and the minimum weight for

the mast and rig, determines how much of the boats weight can be lead ballast located in the keel.