Tidbit: Anchor Rode Scope Calculator [Updated Mar-2024]

This is one of a series of brief [you hope], no nonsense posts that we call aTidbit: 

noun; small and [possibly] particularly interesting item of gossip or information...

The purpose of these Tidbits is to share succinct posts about lessons learned, or things we use or do that work [or don't...] that are common to most of us boaters.

The goal is to garner feedback from those of you having first-hand experience with a different approach/ solution/ product/ or additional useful information to share...  

We never assume what we are sharing is the ideal or only. In this case it is about the attributes the builder of our boat provided for us [and some we added...] 

We share this information in case there are any points of interest for others, and to solicit things you have done that we might be interested in...

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Determining how much anchor rode to veer is easy, right? 

Note: In this discussion, we will ignore considerations for distancing from other vessels in crowded anchorages. This is all about determing the ideal scope for the conditions in an anchorage with adequate space to do so.

As an example, let say we are preparing to anchor for the night using our 99 lb. Spade primary bower on our all chain rode [and we always set a longish bridle...] 

Our depth sounder reads 50 ft of water. [Assuming the bottom is level.... otherwise we use the deepest depth within our anticipated swing circle...]

Beause of the depth, we know from experience a 4:1 scope ratio is very adequate [read we sleep well] for all but extreme conditions. [In which case we would increase scope up to a comfortable 5:1...]

It is worth noting that deeper water allows using less scope; shallow requires more. For example, we are very comfortable with 3:1 [or even 2.5:1] scope in 90 ft. of water with our primary bower.

So, 50 ft x 4:1 = Pay out 200 ft. of rode. 

Done.

Or are we? 

What about tidal change? [Our current playground can experience 24 ft. tidal variations.]

Therefore, we also need to determine the stage of tide when deploying the anchor [and calculating the scope] to determine the highest predicted tide [remembering storms can influence tidal measurements- among other things...] during our planned stay. [And the lowest tide if anchoring in water shallow enough to ground us...]

Most of us have tide tables built-into our chart plotters and/or apps typically displaying a nice curve— making this pretty easy to do:

But what if all we had were printed tide tables? [I can see you paper-only chart salts smiling...]  Do you remember how to use the Rule of Twelfths?

Back to our example of anchoring in 50 ft. Lets say the tide will increase another 11 ft from the time we anchor. [This is the highest tide during our planned stay...]

Easy; 50 + 11 = 61 ft of depth. 

So we should let out 61 x 4:1 = 244 ft of rode; 44 more feet that we initially figured. 

Done again, right?

Wait. Did we remember to include the height of the anchor rode [bridle in our case] fairlead above the water?

Ours is 5.5 ft... [Lets round to 6 ft.]

Okay; 50 + 11 + 6 = 67 ft.

Then lets refigure scope again: 67 x 5 = 268 ft [an increase of 68 ft from our original guestimate...]

Are we done yet?

Not quite... 

Lets also remember our depth sounder is set to read depth below our keel. Denali Rose draws 6 ft. 3 in. [we will round to 6 ft] so we also need to add this value to the depth gauge reading... [How would you know this if we didn't tell you?...]

Once more:

50 [depth reading] + 11 [predicted tidal increase] + 6 [Freeboard] + 6 [depth sounder offset] = 73 ft. effective depth during our stay at this location.

What is our desired rode length now?  

73 x 4 = 292 ft of anchor rode to achieve a 4:1 scope at highest tide during our stay.

Yikes! That is 92 feet more than the 200 we originally calculated in our head... [A 46% increase...]

If we had anchored using just 200 ft of chain, we would have ended up with a scope ratio of 2.7:1 [at the highest predicted tide during our stay...]; quite a bit less than the 4:1 ratio we prefer for this depth... [But likely adequate for calm conditions with our oversized ground tackle.]

So, we can still do this arithmetic in our head, right? 

Yes. In our [ever diminishing] attempts at staying sharp, we calculate how much chain to veer in our heads each time as we prepare to anchor.

To make this easier we simplify; there are two variables: depth and tide; and two constants [which become one]: freeboard + depth offset.

So, the simplified version is: add 12 ft. to the depth meter reading + how much higher the tide is predicted go [during our intended stay period] when we drop anchor, then multiply by our desired scope ratio, thus:

Staying with the current example: depth reading variable [50] + our constant freeboard and depth offset [12] + highest tidal variable during our stay [11] = maximum effective depth during our stay [73] X 4 = 292 ft of chain.

And lets remind ourselves, our chosen rode length is assuming a level bottom... 

What if you are anchoring on an incline? [Even a slight one...]

When figuring the angle of the rode at the anchor when the rode is under high load [i.e., is 'bar tight' or almost straight...] one only needs to measure the angle between the straight line of the rode and the slope of the seabed in the direction the anchor is set, correct? [Trick question.... more below...]

Yes. And the worse case angle of the rode is determined by the straight line between the anchor attachment point, and the fairlead the rode is first lead through on the boat... In your example, the bow eye [with the snubber- now part of the straight line rode...]

Explaination of trick question, above:

When anchoring, our goal is to keep the angle of the rode at the anchor attachment point smaller than the tripping angle of the anchor. [Typically under ~5° with modern anchors] We typically do this by letting out more scope in incliment conditions, but this common approach doesn't allow for a sloped bottom... [That is another discussion (on Cruiser's Forum) that is well worth reading...]

Image from Cruiser's Forum discussion

Are we having fun yet?

Back to where we were with our original example: 

We have determined we need to veer ~292 feet of rode. [Lets round-up to 300 ft.]

A chain counter would make this easy— especially since we gravity drop our anchor and rode. [i.e., FAST!]— but we are the chain counter... 

While the chain is whizing by, we need to keep track of the plastic zip ties screaming past  as well so we know how much we have veered... [Our rode is well marked with colored zip ties every 30 ft— or 5 fathoms... or even shots (15 fathoms of chain, or 90 ft) if that is how you think... ] 

Our marker pattern is one red ziptie @ 30'; 2 reds at 60', 3 reds @ 90'; 1 white at 120', etc. , through Blue, then Yellow... So which color, and how many are we shooting for to reach 300 ft? 

We are also experimenting with a 3 ft. length of yellow pollypropelene line woven into the chain links at each set of markers to aid visibility— especially during our typical high speed gravity chain deployments.  

So far so good... 

A cheat sheet can be handy for determining which set of rode markers we want. This is because our markers designate either foot [30] or fathom [5] increments— probably making the marking scheme more complicated than it need be... [I'm constantly rethinking this, but old habits... Keep reading to see why a change isn't a priority...]

Our chosen rode marking system [that doesn't require keeping a mental count...] helps offset my self-awareness that I can [and do...] make mental mistakes even when I'm rested and alert— let alone when I've got too much to think about when I should be concentrating on anchoring maneuvers.

Enter my dynamic cheat sheet. [Another way of looking at it is I'm basically very lazy, so it is easier to make a calculator; trust but verify— even one's self...]

 
The following screen shot of our prototype scope calculator is displaying the example values from above :

 

After selecting the 4 variables at the top left [Units, Depth sounder reading, etc.] the Scope ratios are calculated and color coded based upon value. [See Scope Color Key in middle right, above.]

Find the desired ratio in the Scope column and read the required length of rode.

In the same row, find the Rode Marker [colored zip ties in our case] you are looking for [Note we have two anchors, but typically use the primary bower- 99lb Spade.]

In our example, looking at the Scope Color Key for our desired ratio range, then choosing a scope from that range from the Scope column. [Notice the hot pink Preferred Scope Range column on the right is highlighting our preferred range taken from the Scope Color Key for the calculated Effective Depth. (In this example 3-3.9:1. Lets round up to 4:1 just because we can...)]  

Next we slide left to the Primary Bower Rode Marker column to find we are looking for one yellow ziptie on the chain portion of the rode to achieve 4.1:1 scope for the effective depth at the highest tide during our stay; 300 feet of all chain rode. 

If we desire more or less rode for any reason, we can choose accordingly. 

The next screen shot is a close-up of the calculator itself showing a new set of variables. 

How much scope [and which primary bower chain marker] would you choose? [Assuming no other obstacles to consider in the anchorage...]

Close up of business portion of calculator showing more typical depths in our cruising areas. 

Note the color coding automatically shifts in the Rode Length and Scope columns to help guide weary eyes.

Your turn: Pretend like you are just finishing a great day of boating and [if like us...] are a bit tired and hungry [an perhaps a little dehydrated- and maybe a bit cranky...] and you are approaching your anchorage for the evening... 

Oh, and you are expecting some weather in the next couple of days, so don't screw it up...

How adept are you at calculating everything in your head now? 

Test yourself: 

Quickly calculate the rode length at 4:1-ish scope in your head for a depth sounder reading of 65 ft., consulting your tide resource for the highest tide during your stay to determine how much deeper the water well get. [Lets say 7 ft. for this example.]

Which chain marker [set of colored zip ties] are you shooting for? 

Quickly now- since you are already gravity dropping [fast releasing] the chain in anticipation of sundowners— and you just saw a pair of white zipties go by...

And your answer is?...

Here is what the calculator came up with if you want to double-check yourself:

How did you do?

Did you remember to stop short of your chosen final rode length to allow for the length of your snubber or bridle? [Our bridle adds 20-35 ft./leg depending upon how much we let out- which depends upon anticipated conditions...]

At this point I suspect most of you are wondering about my mental state [me too...]   

Do I really need to use a calculator to anchor?

Absolutely not! 

The arithmetic isn't difficult. [But building the calculator was fun- even if unnecessary...]

Typically I use the calculator in between the first soft anchor set [short scope] and the final hard set [which is performed after the total desired rode length is veered, and the bridle is deployed.]

Trust [thyself] but verify...

What are your methods of simplifying the hidden complexities of determining desired scope and anchor rode length?

Below is a link to our live calculator. Have at it... [Note: If several individuals use it at once, you may see changes being applied by others... more insanity...]

This prototype was developed using Google Sheets. [Yes, our verson also works when not connected to the internet...]

Here is the online version of our live scope calculator. You are welcome to use it online [and/or copy or download a version that runs in Google Sheets— or convert to the spreadsheet application of your choice...] 

Or print it out for your dart board...

Update 28-Mar-2024: For a calculator that takes into account all of the variables discussed above and more, it is hard to beat the Anchor Chain Calculator [app available.] 

Since its release [after we created our less capable version, above] it has been our go-to choice. Highly recommended.

It is also worth noting that a link to the Anchor Chain Calculator mentioned here was added to the list of Additional Resources shown on our calculator when it was first released [July 2022?]

Please comment if you discover any errors or omissions, and of course share your enhancement ideas.