What Are Engine Belts (V and Poly-V)?
Rubber belt transferring drive from the crank pulley to the alternator and pumps - the part that takes charging, and on many engines cooling, with it the moment it snaps.
The belt is the rubber band that transfers the crankshaft pulley’s rotation to the alternator, the circulation pump and, on some engines, the raw water pump. Look how many systems stop when one belt snaps: charging stops, and if the circulation pump stops the engine can overheat within minutes.
In short, the belt is the cheapest but most far-reaching weak link in engine failure scenarios. The table below sets out the symptoms.
Belt symptoms
Symptom | Cause |
|---|---|
Black dust, glazed surface | Slip: incorrect tension or pulley misalignment |
Screeching noise (especially on start-up) | If it’s slipping, there’s a cause. |
Cracked, worn backing | Once it’s past due, waiting for it to snap is just picking your breakdown day at random. |
Rule of thumb for tension: about 1 cm of give with thumb pressure between two pulleys. There should be a spare for every belt on board, plus the spanners needed to change it.
At least once, carry out a trial change in port; nobody plans to learn how to change a belt for the first time in a rolling seaway.