We’ve previously outlined the advantages of big anchors (shorter scope, less swinging room, security, quicker setting). But we neglected one huge factor, reducing scope reduces the risk of fouling.
Here is an example. Assume you are anchored in forty feet (12m) of water including an allowance for the height of the bow roller off the water.
In light airs, with five to one scope, you would veer 200 feet (60m) of chain. Most of that, say 150 feet (45m) is on the bottom. The area swept by the chain on the bottom is equal to the radius squared times 3.1416, or 17,671 square feet (1673 square meters).
Now lets switch to our big anchor and reduce scope to 2.5 to one. The chain lying on the bottom is reduced to 75 feet (22m) and the area swept drops to 4418 square feet (418 square meters). The huge reduction in swept area is a function of the circle radius being squared in the area formula.
The chain on the bigger anchor and shorter scope sweeps 1/4 the area of the conventional set up. So, whatever the risks are from boulders, coral heads, wrecks, etc. they are a lot less with the bigger hook.
On Wind Horse, our normal practice is to set on a longer scope and then shorten down, even when there is plenty of swinging room, to reduce the risk of fouling.
Posted by Steve Dashew (January 13, 2010)
January 17th, 2010 at 1:23 pm
This is a very valid point, Steve, and one that I hadn’t really considered before now.
For cruisers concerned about their impact on the marine environment, your line of reasoning leads to another interesting conclusion: The small anchor / long scope / heavy rode setup, in your example, will do at least four times more damage to the vegetation and fish habitat on the seabed as will the heavy anchor / short scope setup. Not only does the heavier anchor have less chain sweeping the bottom in light winds, it is also less likely to drag and tear up seabed vegetation in heavy winds.
February 10th, 2010 at 11:28 pm
Steve,
I do fully agree with oversized anchors and reducing scope. Having been working at as Captain on the Great Barrier Reef in Australia for several years, I have seen what damage anchor chains do to the reef. BUT…the anchor chain is not just there to connect an oversized anchor in the shortest way to a vessel (hopefully in a permanent manner). One aspect we are forgetting with changing the scope is the secondary function of a heavy anchor chain is the spring effect of the chain. Anybody who has been diving on the anchor chain in strong wind conditions, has seen that even then at least 50% of the chain stays on the bottom, so no jerking can break out the anchor. My principle is that I could short change the scope on day anchoring but I sleep much better at night with the recommended scope AND a big anchor.
I also must admit that I have been caught shortening up my scope and just before dark someone anchors too close up unaware of my choice and the fact that I may increase scope on short notice to deal with weather conditions.
Happy Sailing!
We do very much enjoy your website.
February 11th, 2010 at 10:05 am
The scope and spring action required is a function of load, as you have pointed out. Our experience is that if wind or sea state requires added scope, then we lengthen the chain. For spring and noise reduction we use a pair of three strand snubbers. Ten feet (3m) is the normal length, used 99% of the time. We also carry a heavier 50 foot snubber (15m) which has been used once in four years.