“On the trot” is an English phrase that means roughly “one after the other” – e.g. “We went to three pubs on the trot”. By analogy, trot moorings have several boats sitting in a line, each having their bow and stern connected to something robust. The boats always point in the same direction regardless of wind or tide, unlike a swinging mooring. The boats can only move as far as the slack in their chains allows, and so will sit a little to the left or right at low tide depending on wind/current.
At Fisherrow Harbour (SundaySail’s local) there are six huge ground chains running north/south along the harbour and secured with anchors at each end (mushroom at the north and fisherman’s at the south). They’ve been there since the 1970s and the chain links are immense, something like 8 inches. They’re hard to find because they’re buried deep in the mud. This is desirable because it stops them moving which reduces abrasion and keeps them away from damaging oxygen.
Each boat owner is then responsible for installing their own riser chains. These chains are somewhat smaller – perhaps 10mm galvanised short/medium link chain for a smaller yacht. At the bottom end, the riser chain is shackled securely to the ground chain. At the top end the riser chain is shackled (or spliced) to a length of rope then attaches the boat itself.
(In a drying harbour with deep mud at the bottom, getting out to the mooring at low tide to inspect or change the riser chains is an adventure in itself, which we’ll explore in a separate article)
Both the bow and stern riser chain need to be long enough for the highest high tide. They should also up at a slope rather than vertically to stop the boat wandering too far. You can do a bit of high school trigonometry to see this. Let’s imagine the highest high tide is 6m. If the riser chain ran vertically, then we’d just need 6m of chain. But when the tide falls to 3m, the slack allows the boat to wander away by about 5m (3²+5²≈6²). In contrast, if we run the chain at 45 degrees we’ll need more chain, about 8.5m, to cope with the highest tide and we’d have to connect it to the ground chain 6m behind the boat. But now when the tide falls to 3m, the boat can only end up 8m from the connection point (3²+8²≈8.5²) which is only 2m extra. So the sloping chain does a better job of keeping the boat in place.
The riser chain and ropes are intended to sink to the bottom of the harbour when the boat is away so that other boats don’t get tangled up on them. This means that the rope needs to be made from a non-floating material – nylon or polyester. Of course, you need to be able to find it again when you return and so a pickup buoy is attached via a thin line to the mooring ropes so you can haul them back up from the muddy depths.
In a drying harbour, the riser chains get dragged up and down twice a day on the tide. The chain links rub against each other and oxygen, which is most plentiful near the surface tries to react with the metal. The riser chains are usually galvanised steel and so have a protective zinc coating, but it eventually gets damaged by the constant movement. The underlying steel reacts with oxygen in the water to form weak iron oxide (rust) and the movement of the chain abrades the rust away exposing fresh metal underneath and continuing the cycle of corrosion. After a few years, a 10mm chain can easily be reduced to 1mm or less. Consequently, the riser chains need to be inspected regularly and replaced every few years. The top end of the chain tends to suffer the most, because it spends longest in the oxygen-rich water near the surface.
The advantage of trot mooring is that you can fit a lot of boats into a small area. That’s also the disadvantage – there’s lots of other boats to manoeuvre around when you arrive or depart!