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Knowledge Base

Rode Length / Scope

You can have the ultimate anchor but if you don't give it enough chain length- It will be USELESS.

Scope

 

The scope is the ratio of the length of the rode vs the distance between the seabed and bow roller.  When you calculate scope you need to add the depth of water to that distance between the waterline and bow roller.

 

Much has been written about the scope and there are internet forum threads that have devoted posts over months discussing the issues.  We are not going to go into detail, for more information use the search engine on your favorite sailing forum.

 

The older designs of anchors benefitted from high scope ratios, 5:1 - 10:1, as the higher the scope the greater the chance that the tension angle on the anchor was near horizontal.  Today we are much luckier than earlier generations as most modern anchors are much more forgiving and will perform adequately at ratios of 3:1 - 5:1.

 

We at Viking Anchors take advantage of this more forgiving nature of most modern anchors and we happily deploy, to engage and set our anchor at 3:1. We always deploy the anchor with our yacht moving slowly astern. This ensures we do not drop all the chain on top of the anchor and the chain is deployed in a straight line ready for the tension to be increased and the anchor fluke point is aligned ready to engage.   If the anchor does not engage almost immediately we apply tension then there is something unusual about the seabed, or more likely, the anchor has been fouled by a ‘foreign’ object.   Popular anchorages can have old beer cans or towels blown off lifelines ( we once caught an old gas cylinder and many have caught old crab pots, supermarket trolleys, or discarded fishing nets).  In more isolated locations seaweed stalks or waterlogged trees can be a problem.   We can then immediately retrieve the anchor, check and start again or move location slightly.  Because we have not deployed much chain as the scope ratio is only 3:1 it does not take long nor use much power to retrieve.  We know some derive great satisfaction from sailing to (and from) anchor and it is a skill every skipper should have.  However sailing is meant to be a pleasure we happily anchor with the engine running, one reason being we will not deplete the batteries and we will power set at about 75% of cruising revs.   Once we have ensured we have engaged the anchor and started to set we will then deploy more rode.  Normally we would deploy to a 5:1 scope but if the weather is questionable - we might deploy more.  However, deploying more rode might not be possible in tight or busy anchorages.  We would always power set to ensure the anchor is set and not moving.  We have measured the tension in a 5:1 rode at various wind speeds and measured the tension in the rode at different revs and 75% revs are about the same tension you would experience with a 30-knot wind impacting on the yacht (measured at the masthead).

 

When we decide where we are going to anchor we set the location at the point of deployment as this we can later be used to set our anchor alarm.   Our anchor alarm is then centered on the anchor, not some indeterminate offset.

 

Generally, we would only deploy a 5:1 scope and would use a long snubber, at least boat length, to give us elasticity in addition to catenary but we would only attach the snubber when we have completed our power set.  One way to check if the anchor is securely set - simply touch the rode with 2 fingers laid and touching a couple of chain links just forward of the bow roller - if the chain jerks and jumps - the anchor is not set.  If it is steady - the anchor is engaged and not moving.

 

To ensure you know how much rode you have deployed the chain needs to be marked with a simple code.  We write the code into the underside of the chain locker hatch - so that anyone who deploys the chain knows exactly what the marks are.  We mark every 10m with cotton ribbons or hemp/sisal cord.  The ribbon or cord passes through the gypsy and unlike cable ties, paint and plastic inserts do not contaminate the seabed.

 

Once the anchor is set and the snubber attached we would then attach another snubber or secure a chain lock.  This practice ensures you are not relying on the windlass as the ultimate fall back.  The additional snubber can be a short length of Dyneema with a chain hook (or soft shackle if you prefer) and attached to a strong point independent of the windlass.

 

The penultimate task is to set the anchor alarm incorporating the length of rode and snubber deployed + a few more meters (2-4).  Finally, with all the formal tasks its time to break out the chardonnay (if you are in a warmer climate) or a decent malt whisky if further north and enjoy the atmosphere and scenery. 

 

We do find that many anchor alarms on modern chart plotters are very unobtrusive, especially if the plotter is at the helm and the owner's berth is in the bow.  There is usually an option of wiring into the plotter a piezoelectric alarm - the use of which denies any sleep once it is activated!

 

Setting an anchor should be an easy task and really should involve no anguish and can be completed by one person - leaving the rest of the crew to prepare those welcoming drinks and set to and prepare that gourmet dinner!

 

Here is a table showing the importance of the scope and how it affects your anchor holding.

 
Scope % Maximum anchor Holding Power
2;1 10 %
3:1 40%
4:1 55%
6:1 70%
8:1 80%
10:1 85%
20:1 89%

 

Try to use a long chain if possible. No need for more than 10:1 scope. A long chain with a good anchor will do the job.

 

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