RETURN TO: Home Page - Performance Yacht Design 101

This page we'll consider # work in progress'. We were recently asked by a client's project manager to explain using common words some of the every day terms we use as naval architects when describing a yacht. At some point I'm sure we'll get around to adding some pictures/graphics, but for the moment this seems enough to be going on with. If anyone reading this still has difficulties then email us at racing@owenclarkedesign.com and we'll know which graphics to prioritise.

1 STABILITY

A yacht’s stability, is its ability to withstand the pressure of the wind in the sails or the action of waves in trying to make it heel and is provided by three components: The yacht’s beam, hull form and the yacht’s displacement (measured in terms of both the mass and the position of this mass in relation to the yacht’s centreline and waterline). For a given wave or pressure of wind, a yacht with a high stability will heel less than one with a low stability.

In the case of hull form stability the general rule for yachts of a similar ‘footprint’ (waterplane area), the wider the maximum waterline beam of the boat, the higher the stability. In general this also leads to higher drag.

A yacht with a greater mass, a lower center of gravity (measured from the waterline) or a center of gravity offset furthest from the centreline (by means of crew hiking, water ballast or canting keel) will have a higher corresponding stability. In the case of higher displacement or the addition of water ballast this will also mean higher drag. But NOT in the case of the crew hiking or keel being canted.

2 RIGHTING MOMENT

Righting moment is the value of the stability that can be measured and compared between different yachts. The higher the righting moment the more ‘powerful’ the yacht because it is able to carry any one sail-plan/sail area to higher wind speeds.

Thus, where higher stability generally means more drag, it also comes with the advantage of being able to ‘apply’ more power. The challenge for the naval architect is to make sure that the drag produced is more than compensated by the extra power. It is a challenge to create a yacht that is fast both when fully powered up and in light conditions when it is not using all its stability. The positioning of the cross-over and the wind speeds/angle that this occurs at is crucial in positioning the design of the yacht when optimising for regattas.

3 ROCKER

Rocker is the shape of the underwater line of the hull when seen in profile view and measured from midships to either end of the yacht. A yacht with a more ‘banana shaped’ profile is said to have more rocker than one with a straighter hull bottom.

Two yachts of the same displacements but different rocker profiles will have distinctly different mid-ship section shapes as well as differences in the bow and stern shapes.

For the same displacement a yacht with greater rocker will be quicker in light air at speeds below the maximum upwind speed. A yacht with little rocker would be likely quicker at higher speeds, and would plane earlier and longer. If insufficient rocker however at high speed and/or in waves such a yacht may experience bow burying, shipping a great deal of water on deck and exhibit characteristics that make her difficult to control and so require early shortening of sail.

The fitting of an interceptor allows a yacht to sail in both modes, effectively altering its rocker aft depending on the sailing speed at the time and so optimise the yacht for the existing conditions.

4 HEELING ANGLE

This is the angle to which a yacht rotates about the axis of its centreline under the action of waves, wind, movement of ballast/mass away from the centreline, or a combination of two or all three of the above.

In general heel angle results in a reduction of the efficiency of the sail-plan and underwater appendages (except with positively angled daggerboards on canting keel designs), but an increase in stability and power at normal sailing heel angles.

5 PLANING

A heavy displacement yacht is limited in speed in flat water by its waterline length. The transom effectively drops into the trough of the wave created by the bow and the yacht is unable to climb up and over its own bow wave. If sufficient power from the sail area is available and the yacht is light enough it can climb over that bow wave and rather than pushing though the water it has the appearance of riding on the wave.

A hull shape that works for planning is almost the opposite of one that is optimised for displacement sailing. To extend the upper speed range of a displacement hull the keel is moved aft, the underwater volume in the stern is increased, but extra wetted surface is kept to a minimum by avoiding flat areas. This added volume distorts the hulls waterlines and buttocks, the shape of the boat aft becomes more distorted and this in turn as well as the yacht’s displacement inhibits planning.

In high performance designs the low-speed wetted surface area configuration is compromised in favour of inducing the yacht to exit displacement sailing mode and plane earlier as well as longer. The goal for this project is to take an up to date modern hull form and optimise it for the Cento class displacement and the prevailing weather data for the  target regattas and then balance these two requirements.

6 BEAM to LENGTH RATIO

The beam to length ratio of a yacht is a dimensionless number and is a simple means of comparing the stability of yacht’s that have comparable hull shape/mid section areas. It is a useful number and indicative but needs to be used with some caution since it is part of the race boat designer’s craft to develop new hull shapes that play on the relationship between a yacht’s maximum beam, waterline beam, displacement and drag.

7 BOAT SPEED vs IRC RATING

IRC is a rating rule to handicap different designs of allowing them to race together. Each boat’s rating (her ‘handicap’) is calculated using measurements of the boat; her length, displacement, draft, sail area, etc. The resulting time corrector, the boat’s ‘TCC’, is her handicap. After a race, each boat’s elapsed time (the time she has taken to complete the course) is multiplied by her TCC to calculate her corrected time (her race time making allowance for the characteristics of the boat). The boat with the shortest corrected time is the winner of the race.

IRC is a permissive rule. It permits features such as asymmetric spinnakers, bowsprits, water ballast, canting keels, ‘code zero’ headsails and all mono-hull hull formats. The goal of the performance racing boat designer is to develop a yacht whose boat speed and ‘efficiency’ is such that although faster, more responsive and exciting than slower displacement designs, the penalty (a higher TCC number) received is such that the yacht is able to win both on elapsed and corrected time.