Having been asked the question over and over it seems a lot of boaters do not have a good grasp of anchoring scope or what relationship it has to your anchor on the sea bed.
For this article I wanted to put scope into pictures. I used a short piece of aluminum to represent an anchor shank, some 12 GA finely stranded wire, my work bench and an upright jig marked off in inches to represent the water depth or bow height of the boat and black tape to represent every foot of rode. The depth was set to 12" or 12' by scale.
I used a scale of 1" = 1', or 1 inch equals 1 foot, because this was simple. The aluminum anchor shank, at this scale from pivot to rode connection point, matched my 35 pound CQR and represents about 32" long.
The wire was drawn tight to represent the rode straightening under a full load on a boat. Of course with 300+ feet of rode approaching infinite scope is nearly impossible but this is relative and illustrative only.
Most boaters only measure scope along the rode length so I therefore did not include the anchor shank length. This is why the 1:1 scope you'll see is not vertical.
To measure the angle I used my angle cube that I use for woodworking. This is a highly accurate device though very, very sensitive. I was not and am not concerned about the small fractions .05 etc. it may be off but more about the general angle to the whole number. I tried very hard to be gentle when setting the cube on the shank so as not to disturb the angle of the rode.
The shank was designed to sit 100% flush with the bench but an anchor would be buried and depending upon the design the shank might too be buried. The bottom line is the angle the rode puts on the shank at a given ratio.
Please note that this is a ratio so the scale does not matter and the angle of the rode at 7:1 would be the same if we were talking 300 feet of rode at 7:1 or 8 feet of rode at 7:1.
The Test Jig
The depth jig is marked off in inches to 12" tall. The jig simply slides along the bench to set the scope ratio. Once the desired scope was set the wire was drawn tight.
The Depth Jig
This is my angle cube designed for use on a table saw or other woodworking tools to determine the angle of a blade or fence. The cube was zeroed to the bench before each measurement was taken to ensure it was as close to accurate as can be possible.
Zero The Angle Cube
Here we go. This is what a 1:1 scope looks like if you did not include the length of the anchor shank. As mentioned earlier this aluminum shank represents the length of my 35 pound CQR to scale. A true 1:1 would be vertical but the extra length allows some angle to the scope.
Scope is figured as 1 times the depth from your bow chock to the ocean floor plus accounting for tide changes and what the actual depth will be at high tide.. So for a 5:1 scope with 10 feet to your bow from the bottom you'd need 50' of rode or 5 X 10' = 50'. 7:1 would be 7 X 10' = 70' of rode. If your depth transducer is 1' below water and not calibrated for offset don't forget to add this into the calculation. Forgetting to add in bow height is most common and can make a fairly big difference if you leave it out. In deep water shorter scopes can be used and often are.
Depth Jig
Again, I have not included the anchor shank as part of our rode length so the angle applied to the anchor in this scenario is 52 degrees at a 1:1.
1:1 Scope Angle = 52 Degrees
Here's what 2:1 looks like and the angle on the anchor shank has dropped tremendously by going from 1 X depth to 2 X depth.
2:1 Scope
As you can see by going from 1:1 to 2:1 scope we have nearly cut our shank angle in half to just about 26 degrees. Still not a scope I'd sleep well on with any anchor.
2:1 Scope Angle = 26 Degrees
Here's 3:1 scope..
3:1 Scope
At 3:1 we certainly don't cut the shank angle in half, like 1:1 to 2:1, but we do lose about 9 degrees of shank angle by going from 2:1 to 3:1
3:1 Scope Angle = 17 Degrees
Here's the visual of 4:1 scope.
4:1 Scope
4:1 scope equals roughly 13 degrees of anchor shank angle to the ocean floor.
4:1 Scope Angle = 13 Degrees
Here's the visual on 5:1 scope.
5:1 Scope
At a 5:1 ratio we are now down to approximately a 10 degree anchor shank angle pull. Many boaters rely on 4:1 or 5:1 in tight anchorages in benign conditions but some have ridden out severe weather on nothing more than 5:1 with the right anchor.
5:1 Scope Angle = 10 Degrees
Here's the 6:1 scope visual..
6:1 Scope
Going from 5:1 to 6:1 you lose roughly another 2 degrees of anchor shank angle.
6:1 Scope = 8 Degree Angle
Here's the 7:1 scope visual..
7:1 Scope
As you can see as the scope gets longer the law of diminishing returns applies to the angle of attack. Going from 5:1 to 6:1 we lost two degrees but from 6:1 to 7:1 only 1 degree. At this point the rode is pulling fairly close to the ocean floor and the angle is what most any well designed anchor can and should be able to handle.
7:1 Scope = 7 Degree Angle
Here's the visual for 8:1 scope..
8:1 Scope
We lost roughly another degree going from 7:1 to 8:1. Keep in mind this is a lot of rode for very little return in shank angle and you are increasing your swing radius dramatically.
There is nothing wrong with more scope but just be sure you have the swing room to do it. As you continue on out in rode length the changes in angle become smaller and smaller and this is why 7:1 is the generally advised scope for anchoring when you need good holding power. In tighter places a 5:1 or 4:1 can work but the angle on the anchor shank clearly becomes more upright the shorter you go with scope.
8:1 Scope Angle = 6 Degrees
For this article I wanted to put scope into pictures. I used a short piece of aluminum to represent an anchor shank, some 12 GA finely stranded wire, my work bench and an upright jig marked off in inches to represent the water depth or bow height of the boat and black tape to represent every foot of rode. The depth was set to 12" or 12' by scale.
I used a scale of 1" = 1', or 1 inch equals 1 foot, because this was simple. The aluminum anchor shank, at this scale from pivot to rode connection point, matched my 35 pound CQR and represents about 32" long.
The wire was drawn tight to represent the rode straightening under a full load on a boat. Of course with 300+ feet of rode approaching infinite scope is nearly impossible but this is relative and illustrative only.
Most boaters only measure scope along the rode length so I therefore did not include the anchor shank length. This is why the 1:1 scope you'll see is not vertical.
To measure the angle I used my angle cube that I use for woodworking. This is a highly accurate device though very, very sensitive. I was not and am not concerned about the small fractions .05 etc. it may be off but more about the general angle to the whole number. I tried very hard to be gentle when setting the cube on the shank so as not to disturb the angle of the rode.
The shank was designed to sit 100% flush with the bench but an anchor would be buried and depending upon the design the shank might too be buried. The bottom line is the angle the rode puts on the shank at a given ratio.
Please note that this is a ratio so the scale does not matter and the angle of the rode at 7:1 would be the same if we were talking 300 feet of rode at 7:1 or 8 feet of rode at 7:1.
The Test Jig
The depth jig is marked off in inches to 12" tall. The jig simply slides along the bench to set the scope ratio. Once the desired scope was set the wire was drawn tight.
The Depth Jig
This is my angle cube designed for use on a table saw or other woodworking tools to determine the angle of a blade or fence. The cube was zeroed to the bench before each measurement was taken to ensure it was as close to accurate as can be possible.
Zero The Angle Cube
Here we go. This is what a 1:1 scope looks like if you did not include the length of the anchor shank. As mentioned earlier this aluminum shank represents the length of my 35 pound CQR to scale. A true 1:1 would be vertical but the extra length allows some angle to the scope.
Scope is figured as 1 times the depth from your bow chock to the ocean floor plus accounting for tide changes and what the actual depth will be at high tide.. So for a 5:1 scope with 10 feet to your bow from the bottom you'd need 50' of rode or 5 X 10' = 50'. 7:1 would be 7 X 10' = 70' of rode. If your depth transducer is 1' below water and not calibrated for offset don't forget to add this into the calculation. Forgetting to add in bow height is most common and can make a fairly big difference if you leave it out. In deep water shorter scopes can be used and often are.
Depth Jig
Again, I have not included the anchor shank as part of our rode length so the angle applied to the anchor in this scenario is 52 degrees at a 1:1.
1:1 Scope Angle = 52 Degrees
Here's what 2:1 looks like and the angle on the anchor shank has dropped tremendously by going from 1 X depth to 2 X depth.
2:1 Scope
As you can see by going from 1:1 to 2:1 scope we have nearly cut our shank angle in half to just about 26 degrees. Still not a scope I'd sleep well on with any anchor.
2:1 Scope Angle = 26 Degrees
Here's 3:1 scope..
3:1 Scope
At 3:1 we certainly don't cut the shank angle in half, like 1:1 to 2:1, but we do lose about 9 degrees of shank angle by going from 2:1 to 3:1
3:1 Scope Angle = 17 Degrees
Here's the visual of 4:1 scope.
4:1 Scope
4:1 scope equals roughly 13 degrees of anchor shank angle to the ocean floor.
4:1 Scope Angle = 13 Degrees
Here's the visual on 5:1 scope.
5:1 Scope
At a 5:1 ratio we are now down to approximately a 10 degree anchor shank angle pull. Many boaters rely on 4:1 or 5:1 in tight anchorages in benign conditions but some have ridden out severe weather on nothing more than 5:1 with the right anchor.
5:1 Scope Angle = 10 Degrees
Here's the 6:1 scope visual..
6:1 Scope
Going from 5:1 to 6:1 you lose roughly another 2 degrees of anchor shank angle.
6:1 Scope = 8 Degree Angle
Here's the 7:1 scope visual..
7:1 Scope
As you can see as the scope gets longer the law of diminishing returns applies to the angle of attack. Going from 5:1 to 6:1 we lost two degrees but from 6:1 to 7:1 only 1 degree. At this point the rode is pulling fairly close to the ocean floor and the angle is what most any well designed anchor can and should be able to handle.
7:1 Scope = 7 Degree Angle
Here's the visual for 8:1 scope..
8:1 Scope
We lost roughly another degree going from 7:1 to 8:1. Keep in mind this is a lot of rode for very little return in shank angle and you are increasing your swing radius dramatically.
There is nothing wrong with more scope but just be sure you have the swing room to do it. As you continue on out in rode length the changes in angle become smaller and smaller and this is why 7:1 is the generally advised scope for anchoring when you need good holding power. In tighter places a 5:1 or 4:1 can work but the angle on the anchor shank clearly becomes more upright the shorter you go with scope.
8:1 Scope Angle = 6 Degrees
