Yacht Stability
Factors affecting the tendency to capsize
Factors which may cause a yacht to capsize:
Flooding.
- A vessel with a large amount of water below can become very unstable.
Free-surface effect.
- The free-surface effect, is large amounts of water rushing unimpeded across the boat, this will make a boat extremely unstable. It is what sank the car ferry, Herald of Free Enterprise.
Large breaking waves.
- A breaking wave has a rotational component that could roll a yacht over. Particularly if the wave height is the same or greater than the vessel's beam when broadside to the waves.
Anything which moves the COG upwards.
Resonant rolling and broaching.
- Resonant rolling occurs when heading downwind, especially with a spinnaker set. It may lead to a broach or an uncontrollable turn up in to the wind.
Factors that improve a vessel's stability:
Pump out the bilge water regularly.
Increased length.
Heavy displacement for the size of vessel.
A high ballast ratio or a heavy keel in relation to the boat's overall weight.
High angle of vanishing stability.
High re-righting factor.
A good RM at 90° of heel.
Modest beam to length ratio.
Good but not excessive freeboard.
Minimal topside flare.
Absence of large nearly flat areas of deck (increases inverted stability).
Large angle of heel before flooding can occur (look out for hatches not set on the centre line of the vessel, keep hatches closed).
Increased keel profile area.
Absence of bulwarks (allows water to run off the deck quickly).
Rough weather.
A yacht will only capsize in extreme conditions, if you understand where not to be and what not to do you will probably never experience a capsize.
When in a small craft in rough weather DO:
- Be aware of category and capability of your boat (some boats are only designed for use in estuaries).
- Close hatches, wash boards etc (keep the water out).
- Pump out bilge regularly.
- Be alert for strong gusts and down-draughts (near cliffs).
- Avoid being caught beam on to a breaking waves.
When in a small craft in rough weather DO NOT:
- Sail through areas of overfalls, tide races or areas where the bottom shelves rapidly.
- Sail in conditions that are outside your level of experience or the category for which your yacht was designed.
EU Recreational Craft Directive Design Categories (RCD)
The EU has included in its design directive a classification based on the stability of the yacht and the area, for which it is designed to be used.
|
Category |
Type |
Wind force |
Wave height |
|
A |
Ocean |
>F9 |
>4m |
|
B |
Offshore |
<F8 |
<4m |
|
C |
Inshore |
<F6 |
<2m |
|
D |
Sheltered waters |
<F4 |
<0.5m |
How to find out about a yacht's stability rating
For a secondhand boat, look in the yachting press, when a magazine tries out a boat they often look at its stability. Most magazines offer a back issues service where you can get a copy of an old report.
Alternatively, many classes of boat have an association; members are often very helpful to anyone thinking of buying a vessel in their class.
Information from the RYA Stability List
Attitude to yacht stability.
The idea of this section is to promote awareness of the issues on yacht stability, not to deter anyone from going to sea. It is very unlikely you will ever be on a vessel that heels more than 90°, which leaves a big margin for error.
If you do ever get into a big storm remember the points discussed here!
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