Thoughts on Stability

Updated: Sep 7

You’re in a steep following sea or crossing a bar, wind against tide. The steering and speed is insufficient, it all goes wrong and you broach. Then what? Have you ever been in a kayak and ventured out beyond a few yards of the beach? If the answer is yes then I am guessing you also have a view on the meaning of stability. In the ultimate I’d say it’s a none linear process. You can wobble a bit, lean a lot then when that become too much, you flip. And the beautiful world goes to rats in a handbag.


Big Boats

Oceangoing vessels are stable on account of both design (and load schedules) and rotational inertia.   It takes energy to get them going over and when they do they tend to roll back in slow motion.  Calculate the load schedule incorrectly and the motion of rolling can become very uncomfortable, on occasions rather than coming back they adopt a list that does not self-correct and you’re on the local news.    I once sailed with a cargo of rolled steel , Pohang (Korea) to Botleck (Netherlands).  The cargo was dense and low in the holds’.  Wave derived rolling was slow on the way over and quick on the way back. Quick enough to make us stumble and even filling the wings tanks with ballast could not completely eradicate the vomit inducing motion.  So the corollary of this? Stability in important but so is a comfortable motion. 


Smaller Boats

The next thought on this subject is that yachts are regularly knocked down.  Dinghies on a daily or hourly basis then lessening with size.  This may cause irreparable rigging damage but very few will not bring themselves upright at some point (unless the keel falls off but that’s a rather extreme case).  In more technical terms they have no “angle of vanishing stability”.  The motorboat equivalent would be exemplified by a rescue lifeboat, Pilot Boat or Coast Guard cutter. Knock them over and they will roll back.  The exact opposite is a catamaran, very stable until it goes and at that point it will never come back. There are rules around this angle depending on where the vessel is certified to operate but let’s leave it at this for now. 


Self Righting

Have a look at the enclosed graph and illustration. This is a GZ curve otherwise a graph of the horizontal distance between Centre of Gravity and Centre of Buoyancy.  As Vanguard heels, the centre of buoyancy shifts creating a righting lever.  The graph shows that, for this yacht at any angle between 01 and 179 degrees, the righting lever remains positive. It’s effectively zero at 0 and 180 degrees and a maximum at 90 degrees (where trawler yachts typically wallow if flooded). We may be battered and bruised, if not scared whit less, but our own “rubber duck” will always come back if we prepare correctly.

All boats will have an angle of vanishing stability. It is a derived but variable figure that may/will change in reality.  Put the yacht over and then a majority of the accommodation becomes subject to hydrostatic pressure.  Additionally, HVAC vents, engine room vents and tank breathers become subject to down flooding and wo-betide anyone with an unsecured port or hatch.  Bouyancy will reduced by flooding as will the righting force to bring you back.   This is the reason a modern motorboat tends to wallow on it side when knocked down, (probably doomed). So in the design you should consider the:

  1. forces necessary to make the vessel heel over

  2. forces resulting from that heel that will bring her upright again

  3. potential motion when rolling in a seaway

  4. potential of flooding to ruin your plans

Summary

In my early years at sea I learned respect for my colleagues and I learned also the limit of our own capabilities.  When leaving port, we prepared for Standby and then prepared for Sea.  When at sea we prepared for bad weather.  When the weather got really bad we prepared for the unknown as best we could but never to meet our Maker.  Preparation will save you and it starts with the design you pick and its suitability for the use you envisage.  


Vanguard and the XPM78 Explorer Yachts are built tough to survive in a tough world. I am not a Naval Architect and leave the mathematics to those better qualified. I do though have direct experience influencing my opinions and this is a platform to express those thoughts. I hope you enjoy the thread.

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