1 The Obvious
Ensure there is sufficient depth of water at all tides in an environment that is without waves or swell. In fact if there is wave action in excess of 300mm the location is most unsuitable for the permanent berth.
2 The Limit
To ensure the vessel is well secured a berth such as a fixed or floating structure should be at least 80% of the length of the vessel. If this is not possible then the distance can be augmented by a mooring pile that supports the vessel in association with the berth.
3 The Wind
The primary thought is always the safety of the vessel and to this end the owner will always wonder what forces can impact this safe harbour. In all of the calculations of current or berthing impacts it is the power of the wind upon the sail area of the vessel that must be resisted by the lines and cleats.
4 The Spring Line
This is the diagonal one running longitudinally bow to stern. This should be as long as possible. It will prevent the vessel moving forward and aft along the dock face.
Lines should be used to their maximum length wherever possible as this allows more stretch to be applied initially as well as less chance of breaking under strain. In poor weather double lines is done before a storm, and even direct to piles if in doubt about the cleat strength.