Problems with ropes chafing is one of the chief counter-benefits of chain, and is one of the many reasons behind the traditional value of chain. As soon as rope is used with a mooring system, friction between it and the contact points it must traverse result in wear and heat which, once the forces in the line are adequately high, are very serious problems.
The obvious environment where abrasion becomes an immediate danger is bad weather and rough water. However, chafe is also accumulative; UV damage and microscopic wear on a rope's fibers can add up over time. Together, the various scenarios comprise one of the most prevalent causes of mooring system failure, and damage or loss of property.
Chafe can be greatly reduced or even eliminated completely with various techniques.
Flexible PVC tube or a similar material can be employed to endure the bulk of abrasion, in a role sacrificial to the rope itself. The pipe should be tied in position with simple string, to avoid it slipping along the line and out of place. If the line is likely to stretch much, the guard should be tied in position relative to the fairlead or chock (i.e. to the boat, not the line).
These however can literally burn or melt through when under serious stress. A better solution is fabric reinforced rubber hoses, such as exhaust hose. Even wire reinforced variations work well.
Chafe guards do not solve the problem, they simply make it manageable. They themselves will eventually wear through, must be replaced when necessary. Hoses should be watched for wear on their underside – they need periodic inspection and adjustment.
Holding the line clear
|“||Forces in rode can be substantial. I've used these line and block variations often. Bad gusts in Porto Santo, Madeira... good quality 1" polyester burned through (chafed) in about three minutes and shrinking (stretching) from 1" to 1/2" with the surge loads. Only thing that would work was a snatch block. Plastic tube burned straight through.||”|
— Peter Smith, Rocna designer
If it is practical, a small secondary line can be employed to hold the main rope mostly clear of the point which it much traverse. If the angle is not great (such as a rode deployed to an anchor at high scope), this can relieve the rope from the bulk of abrasive contact. This small line must naturally then hold a share of the force in the entire rode, this given by elementary trigonometry, but if the angle is shallow this share will be small, and no component of the system is any longer actively moving and rubbing longitudinally against any hard surface. Moreover, the failure of this line is not immediately dangerous.
The secondary line can either be tied directly to the main rope, or around its chafe guard (see above). Or, to a snatch-block which the rode is passed through. Using either the pulley or the chafe guard stops the line crushing the rode, removes the pressure concentration, and softens the change of angle.
If you are using rope as a shock absorbing snubber, and the chain has not left the foredeck, then the chain should be safely secured in a chain stop or alternative, even though there is no tension on it. Should the rope fail, the chain will then not suddenly shock load the windlass as it takes up.
Using a chain section
Another excellent idea is to splice a short length of chain into the rode at the wear point. This can still be covered with tube as above, to protect the boat, but it is guaranteed to never fail on account of chafe. This reasoning can be applied to all critical rode elements prone to chafe. Mooring lines are an exception, as metal and chain should not be permanently kept at the water's surface. If the chain is kept above the water however, it is ideal.
A last resort
In an emergency Vaseline, silicone grease, or lastly generic grease rubbed into the rope will greatly help mitigate chafe.
Different ropes handle chafe better than others. The rope section has more on this. However, all ropes require chafe protection.
Chafe when on moorings
Although the Rocna Knowledge Base is principally about anchoring, moorings are a closely related topic, and a new problem is often presented when the anchor remains unused on the bow of a boat! The section of the rode from the mooring to the vessel must be safely clear of the anchor. It may be necessary to remove the anchor from the bow entirely.