How to estimate the length of a single line Mooring Strop

Single Line Mooring Strops ~ for Swinging Moorings

  1. Measure or estimate the freeboard at the bow i.e. the vertical distance between the bow roller and the surface of the water
  2. Decide the optimum angle from the mooring buoy attachment to the stemhead – this will be the load bearing angle of the mooring strop
  3. Calculate the hypotenuse with the help of the appropriate geometric formula
  4. Subtract the height of the mooring buoy if the attachment point is a ring on top or add on the depth if the attachment point is beneath the surface.
  5. Add on the distance from the bow roller to the strongpoint on deck to which the strop will be secured
  6. Check your theoretical estimate against a working example before ordering a strop to made up to a specific length
  7. Check with your local harbour/river mooring authority - they will almost certainly have advice, guidelines or even rulings on the mooring strop scope (length) and the mooring strop diameter - in addition to rulings on the scope and size of the riser and ground chain

Mooring Strop Illustration

On the River Percuil in St Mawes for example, the local moorings authority give an advisory on all aspects of your mooring including the maximum scope of the Boat Strop.

This is in the interests of keeping swinging circles and consequently, overlapping contact between moored boats to a minimum e.g. the mooring strop should be restricted to a maximum length calculated as follows:
The height of the bow fitting above the water, plus the distance from bow fitting to mooring cleat, plus between 0.3 metres (1ft) and 0.45 metres (1ft 6ins) or 0.75 metres (2ft 6ins) in deep water moorings. If the riser extends above the surface of the water, the extent should be subtracted from the total length.

Theoretical examples of how to estimate the length of a strop for a single swinging mooring

Consider carefully what angle is best for your individual mooring

2 metre Freeboard Example at 45°

Mooring Strop Illustration 45 45 90 angle

2 metre Freeboard means a 2 metre distance from stemhead to the surface of the water

The hypotenuse can be calculated by geometrical formula = 2.83 metres

Distance from Bow to Buoy (no allowance for rake in the stem) = 2 metres

2 metre Freeboard Example at 60°

Mooring Strop Illustration 30 60 90 degree angle

2 metre Freeboard means a 2 metre distance from stemhead to the surface of the water

The hypotenuse can be calculated by geometrical formula = 2.31 metres

Distance from Bow to Buoy (no allowance for rake in the stem) = 1.16 metres

2 metre Freeboard Example at 30° - illustrates how the length of strop becomes too long at a shallower angle

Mooring Strop Length 30 60 90 Triangle

MOORING INFORMATION

2 metre Freeboard means a 2 metre distance from stemhead to the surface of the water

The hypotenuse can be calculated by geometrical formula = 4 metres

Distance from Bow to Buoy (no allowance for rake in the stem) = 3.46 metres

Hypotenuse Calculator