Calculating Scope Can Be Very Misleading

[noelex 77]

This is an old thread, but many cruisers (even those experienced at anchoring) underestimate the effect of seabed slope on an anchor’s holding ability and the effective scope.

Steve has produced another excellent video where he has tested the difference. This is well worth watching.

https://www.youtube.com/watch?v=GR-9...feature=shared

[Kettlewell]

Steve's video is excellent, but I have found it is rare to anchor on a significant slope here on the East Coast. Even when anchoring off a beach the short distance between my anchor and the boat usually indicates the same depth or nearly the same at both locations. I really can't recall anchoring on a steep slope almost anywhere. Obviously, there are situations where it is important to keep this in mind, but I have found they are rare. One of the few times I have used this in my favor is when taking one of my Fortresses out in a dinghy to limit swing. I can then sometimes put the anchor in a shallow spot compared to where the boat is anchored. One of these situations has been in the Bahamas when anchored in some of the narrow "creeks" in places like Pipe Creek or Warderick Wells back when you could anchor there. I would put the second anchor up on a sandbank that in some cases was drying at low tide, effectively decreasing the angle of pull on the rode to near horizontal. Rode out some big blows this way.

[noelex 77]

Originally Posted by Kettlewell

I really can't recall anchoring on a steep slope almost anywhere. Obviously, there are situations where it is important to keep this in mind, but I have found they are rare.

This is very dependent on location. In some parts of the world most anchorages only have a gentle sloping seabed, in other areas you are more likely to anchoring on steep slope (Norway is example of the latter).

The slope is easy to estimate just by looking at the spacing of the contour lines on the charts (or using your depth sounder in poorly charted areas) While the slope can be easily estimated, many underestimate the effect of this slope on their anchor security and the required scope they should deploy so it a topic that is worth emphasising.

[chrisr]

the inverse is also true

i always feel very relaxed when we are laying back into shallower water ie up the slope. if we drag, the scope increases

of course usually only happens with an onshore breeze, but still nice

cheers,

[Kettlewell]

the inverse is also true

i always feel very relaxed when we are laying back into shallower water ie up the slope. if we drag, the scope increases

We once dropped our hook into a 50-foot hole in the middle of 20-foot depths in Annapolis, then proceeded to ride out some severe storms that sent lots of boats dragging. Our anchor had to climb 30 feet before it reached level ground! Local lore was the Navy had dug the hole out there for some reason. It was weird reading the depth sounder in the twenties and then having the chain fly out as the anchor sank down some 50 feet.

[team karst]

Originally Posted by Kettlewell

We once dropped our hook into a 50-foot hole in the middle of 20-foot depths in Annapolis, then proceeded to ride out some severe storms that sent lots of boats dragging. Our anchor had to climb 30 feet before it reached level ground! Local lore was the Navy had dug the hole out there for some reason. It was weird reading the depth sounder in the twenties and then having the chain fly out as the anchor sank down some 50 feet.

Just spent our 5th nite anchored inside a blue hole.
Had a 40 kt rainstorm hit with around a 2:1 scope [emoji106] and zero drag.