Anchor Scope - From Waterline or Bow Roller

[UFO]

I was having a conversation with an old salt and he thought I was crazy for including the distance up to my bow roller for my scope equation.


His point was that my bow roller is 1.6m above the waterline, so if i am anchoring in shallow water - Say 1.5m I will be more than doubling my scope.


His method 1.5m x 5 = 7.5m whereas the way I do it would be 15.5m - which to him was a scope of over 10:1 - He went as far to say that I was being selfish in letting out all that chain!


Also my bridle is 7M long, so I would have to put it 1/2m from the anchor, rather than 8.5M of my method.



What does everyone else do?

[Reefmagnet]

Bow roller + depth at max expected high tide height

[UFO]

Just another question...


Do you let out the required scope and then add the bridle - so letting even more out or is the bridle length included with the initial scope equation - So if total was 10M and Bridle is 5M you would attach the bridle after letting out 5M - Which is what I do.

[noelex 77]

You are calculating the scope correctly.

The rode attachment point above the water should be included and added to water depth. Generally this is the bow roller height, but some boats have a lower attachment point. The water height should be the water depth where the anchor is located rather than the depth of water where the boat ends up after the rode is deployed. You should divide the total length of rode by this number to determine the scope. To calculate the scope at different states of the tide the change in water depth needs to be allowed for.

The total length of the rode should include the length of the snubber extending past the bow roller. This should be added to chain length up to where the snubber is attached. Basically you looking for the maximum distance between the anchor and the first contact point with the boat.

Finally, it is important to realise that while scope is a useful rough measurement, what is really important in affecting the anchor’s holding ability is the angle between the rode and seabed at the anchor. This is governed by several factors. In particular the slope of the seabed can have a very significant impact on the “effective scope”. See this thread:

https://www.cruisersforum.com/forums...ng-137516.html

[tkeithlu]

Quote: "Do you let out the required scope."

I have the feeling that you may have over-calculated this. "Required scope" may only exist in books and conversations. Scope needed to keep your boat from dragging your anchor starts with the nature of the bottom, then the size and type of anchor, then how much chain leader, then the type of rode, then (yes) attachment to boat, then how much current, then how much wind, then your boat's windage, and then how your boat is ballasted. It's a judgement call. Develop judgement for your set up and your conditions. Start with your knowledge of the bases of the problem and a book recommendation, and work from there. Drag it a few times and your estimate plus margin for safety will adjust itself.

Some delightful folks I know, and know now to be cruisers of accomplishment, on their first night aboard their first boat put out 35 feet of rode because they were in 35 feet of water.....

[Reefmagnet]

Regardless of conditions, you need to know how much length represents the unity value for deployment. From there you can calculate how much rode needs to be laid out to suit the situation.

[UFO]

Thanks All. I get it is a bit of an art form which can really only be learnt from lots of practice in different conditions - But it will be fun in the learning

[Pegu Club]

From discussions with other boaters, and from what I have read, it does appear the majority of us include the freeboard/bow roller to the waters surface distance as part of the “scope” to be deployed. I have not heard of anyone recommending doing it otherwise until what the op was told by the “old guy”’.


Fair winds,

[thomm225]

Whatever happened to simply eyeballing the angle of the rode and if in doubt let out a few more feet of "rope?"

[chris mac]

Scope doesnt care about depth of water or height of freeboard.
Distance from rode attachment point to where the anchor sits is what's of value.
For us that's the bow cleats for the bridle. and yes I include the bridle length in my scope measurement as it is part of total length.
Personally I always use at least 5-1 unless that is impossible because of hazards close. And I avoid those spots as much as possible

[DockDoc]

I sailed with an "old salt" olympian, had been on boats for most of his 70 years, sailing many boats.

I too use the roller to water (static number) plus water depth times 7. He did not agree. But as they say, "my boat my rules" so I use the roller to water in my calculations. My bridle is 7 meters so I subtract that from the total length, out the bridle on and back down on it. If I think I'm going to swing and potentially hit someone, I move. I have no idea how much they put out.

[Shrew]

Originally Posted by UFO

His point was that my bow roller is 1.6m above the waterline, so if i am anchoring in shallow water - Say 1.5m I will be more than doubling my scope.

He's looking at it wrong. You're not doubling the appropriate scope. He is half-ing the appropriate scope.

Scope should be calculated based on the depth at high tide plus height of bow pulpit.

Actual Depth should include the offset for the depth sounder (sounder depth below waterline) to account for the difference between the depth sounder reading and actual depth.

So if:

Depth Sounder Reading: 10 feet
sounder offset = 1.5 feet (below surface) (+1.5)
Bow pulpit = 6 feet (+6)
Tide Range = 8 feet
Current Tide = Mid tide (+4)

(((10 + 1.5) +6) + 4) x 5 = 107.5 ft.

I suspect many would think this is a lot of scope. There are two kinds of people, those who have dragged and those who will.

[AiniA]

Originally Posted by UFO

Just another question...


Do you let out the required scope and then add the bridle - so letting even more out or is the bridle length included with the initial scope equation - So if total was 10M and Bridle is 5M you would attach the bridle after letting out 5M - Which is what I do.

I think you are overthinking things a bit. Anchoring is not that scientific and yes, experience matters

[TrentePieds]

UFO:

As others have suggested: You are permitting spurious accuracy to intrude on your notion of how to anchor. It ain't all that complicated :-). "More (scope) is better" is always warring with "that's enuff!" and which of them must win will always be determined by the particular circumstances obtaining at the time you anchor. And on how circumstances are expected to change while you are there.

Obviously you can always change your scope to accommodate changing circumstances, so the "7-5-3 rule" is always just an approximation that gives you a place to start.

The fact that you are asking the question suggests that you are fairly new to the game, and it's right that you should seek to learn from others. So FWIW here is my take: Say you have a five foot draft. You should obviously never anchor where you boat (as it swings to the anchor) will have less than, say, 3 feet of water under the keel at the lowest state of tide during your stay. So don't anchor where the water within the boats "circle of swing" will be less than 8 feet deep during your stay. If it's gonna get choppy, then add a coupla extra feet for bouncing up and down in the waves.

If your total rode is, say, 300 feet, then at a scope of 7 you can't anchor in water more than 40 ft deep at the HIGHEST state of tide you expect during your stay since 40 x 7 = 280. So there you have the limits (as defined by the contours on your chart) of the places you can anchor. No less than 8 feet at low water, no more than 40 feet at high water. That's really easy to remember. No math required :-)!

Once you are within those limits, set your hook on the longest scope you can. I don't bother with measuring in feet. Fathoms (the reach between your hands when you stretch our your arms) is close enuff. If I want, say, 180 feet of scope, I just haul 30 fathoms of rode out on deck, lower the hook till it's "up'n'down", and pay out hand over hand as we back down. Paying out hand over hand means that I can feel what the hook's doing on the bottom. When I feel it "bite", I pay out more rapidly so the rode goes slack and permits the hook to lie still till all the rode is out. Then I belay the rode and back down hard to ensure the hook has indeed set.

I really don't care what length of rode is out at that point, because it's so easy to adjust it as circumstances change. How d'ye know that circumstances are changing? Well, that's what you keep an anchor watch for :-)

Now, there is more stuff to learn about, but that will get you off to a reasonable start :-)

TrentePieds

[Squanderbucks]

The “right number” is not exact. No matter which rule of thumb is used - it’s an approximation for any given condition. Unless you are a supertanker anchoring in shallow water it doesn’t matter. What’s the difference - maybe 10 ft at max.
Let’s say 30 ft of water and scope of normal X7 the difference would probably 30-40 feet of rhode.

If the conditions are such that you are really worried then let out more. Remember the 5x or 7x are rules of thumb just to get you something to start with.