How to calculate scope

[Woodland Hills]

When calculating anchor scope do you add in the distance from the anchor roller to the water? My roller is 9 feet above the water so I have been including it in my calculations. Last night we anchored in 11 feet plus 9 feet up to the roller for a total of 20 feet times three equals 60 feet of chain for a three to one scope. Is this correct? Or do I actually have over 5 to one out?

 

[rslifkin]

Yes, you include height straight up from water to roller in the calculation. So your case, scope would be be based on 20 feet, not the 11 feet of water depth.

 

[DavidM]

As noted above, scope includes the height of the bow roller above the water. In your case it is 9' above the water which either significantly reduces available scope in crowded anchorages or requires you to put out double or more rode to achieve a reasonable scope.


BTW, I consider 5:1 to be the minimum scope I would routinely use. The anchor tests done here on TF and others indicate significantly reduced holding at 3:1.


So consider adding a fitting near the water line on your bow to decrease the height above water line to a foot or so. This should be well reinforced with proper backing inside the fiberglass. It takes a bit of rigging to make it work. You can use a short painter that feeds through the bow fitting with a block on it for the main rode so that you can raise or lower the attachment point to raise your anchor. A bit of a PITA for sure, but I would really consider it if I had a bow 9' above the water.


David

 

[O C Diver]

It depends.
I start by using water depth to calculate scope. Then the next issue is your snubber and where it attaches, and if it's a bridle. When I used a bridle, it was fed through 2 hawsuer pipes probably 8' back from the bow roller. So if the chain straightened out in a blow with 7:1 scope, the point where the chain entered the water was maybe 10' ahead of the bow roller. The other option if using a snubber is too spool excess chain out, maybe until it almost touches the bottom. This keeps the snubber hook pulled down and reduces the amount of scope required on normal days.

Ted

 

[jimdavi]

Don’t forget tide depth change as well.

 

[DDW]

Yes add the roller height. 5:1 is reasonable in 9 feet of water, not so reasonable in 100 ft. There is a good argument that (at least with a chain rode) the deeper the water, the less scope is needed.

 

[tiltrider1]

Is your depth gage reading from the bottom of the boat or is it corrected for actual depth? I can see 11’ +3’ + 9’ = 23’ On a 3:1. That would be 69 feet vs 33 feet if you only took your depth reading.

 

[jclays]

When calculating anchor scope do you add in the distance from the anchor roller to the water? My roller is 9 feet above the water so I have been including it in my calculations. Last night we anchored in 11 feet plus 9 feet up to the roller for a total of 20 feet times three equals 60 feet of chain for a three to one scope. Is this correct? Or do I actually have over 5 to one out?

Yes your calculations are correct. Three to one scope is good if you are just hanging out having dinner or watching a concert.
Five to one is more appropriate if you are spending the night and sleeping.

 

[caltexflanc]

It depends.
I start by using water depth to calculate scope. Then the next issue is your snubber and where it attaches, and if it's a bridle. When I used a bridle, it was fed through 2 hawsuer pipes probably 8' back from the bow roller. So if the chain straightened out in a blow with 7:1 scope, the point where the chain entered the water was maybe 10' ahead of the bow roller. The other option if using a snubber is too spool excess chain out, maybe until it almost touches the bottom. This keeps the snubber hook pulled down and reduces the amount of scope required on normal days.

Ted

The snubber is just part of the rode. So I just let out the appropriate amount of chain as if there was no snubber, and just count the length of the snubber as bonus rode. Another 10 feet never hurt any one. But if you need to be pedantic, and want a very precise amount of rode out, then let out the chain in an maonunt less than the length of the snubber. The only kind of snubber that affects rode calculation are those that attach to an eye at the water line.

I too use your method of letting out a big lazy loop of shain behind the chain hook/plate.

 

[Woodland Hills]

I am using a 55kg Rocna Vulcan with 250’ of chain. Last night I had 75’ out in 35mph gusts with no movement. If I used 5 to 1 it would be a problem in tight or crowded anchorages given the difference in rode lengths. We were the only boat in the anchorage so I could have used any scope I wanted, but nearly 4 to 1 worked just fine. I only use my bridle if there is going to be a big blow as the boat sails around less if I have a single attach point. Of course I lock the chain and relieve tension on the windlass.

I like your idea of loading up the bridle with a giant loop of chain to keep it down and will be trying that next time I use it.

 

[dhays]

Is your depth gage reading from the bottom of the boat or is it corrected for actual depth? I can see 11’ +3’ + 9’ = 23’ On a 3:1. That would be 69 feet vs 33 feet if you only took your depth reading.

Good point. I have my depth sounder set to give me water under the keel. From my anchor roller to the bottom of the keel is conveniently ~10'. So I just add 10 feet to whatever the depth sounder tells me and use that to calculate scope. Figuring exactly where I'm at in the tidal cycle is more of a trick in some places.

 

[Spinner]

I have 300 feet of all- chain rode. This last summer, I anchored twice in 100 feet of water....put out most of the rode and slept like a baby! Also note that my Vesper AIS gives me an anchor alarm on its cell phone app. It’s not feasible or needed to carry 700 feet of chain!

 

[Bigsfish]

Ted and others

I also use a lazy loop on my chain when using a snubber. But your remark that that loop reduces the amount of scope needed on normal days, doesn’t that loop become straight in a blow so there is no advantage? At least that’s the way my feeble brain works.

 

[caltexflanc]

Ted and others

I also use a lazy loop on my chain when using a snubber. But your remark that that loop reduces the amount of scope needed on normal days, doesn’t that loop become straight in a blow so there is no advantage? At least that’s the way my feeble brain works.

You are right, the loop is not part of scope. What it does is add extra weight to the system for the boat to move against, and, in most cases, aid the security of the hook to the chain. I suppose you could argue that it reduces the need for additional scope, but I have always seen it as icing on the cake, so to speak.

 

[koliver]

When calculating anchor scope do you add in the distance from the anchor roller to the water? My roller is 9 feet above the water so I have been including it in my calculations. Last night we anchored in 11 feet plus 9 feet up to the roller for a total of 20 feet times three equals 60 feet of chain for a three to one scope. Is this correct? Or do I actually have over 5 to one out?

Here is a good example of the east v west difference.
Here, we would never dream of anchoring in 11 ft of water. From the look of your boat, that would leave only 5' or perhaps a lot less, of water beneath your keel.
Scope calculations should take conditions into account. In shallow water, any wind waves will cause proportionately more movement than in deep water, so more scope should be required than in deeper water.

Here in SW BC, average anchoring is in 50' of water, and few use more than 3 to one.

 

[FF]

There should be at least 2 choices of snubber diameter , perhaps 3/8 for fine days and 1/2 for modest wave action.

It takes about 15% of the lines break force to start to get some stretch , to smooth the ride , so thin line is required on nice days.

The stretchy snubber also helps the anchor stay set on nervous boats that dodge from side to side.

 

[psneeld]

And when I anchor in the Indian River in Florida often there is less than 2 feet under my keel.

Different situations, different boats and different captains.... and no particular method is right or wrong.

 

[Codger2]

BTW, I consider 5:1 to be the minimum scope I would routinely use. The anchor tests done here on TF and others indicate significantly reduced holding at 3:1.

I completely agree! Too much info is complicating a relatively simple task.

 

[Maerin]

Here is a good example of the east v west difference.
Here, we would never dream of anchoring in 11 ft of water. From the look of your boat, that would leave only 5' or perhaps a lot less, of water beneath your keel.

You'd be apoplectic in the Bahamas, where we routinely anchor with 2ft under the keel, travel across the bank with 1-1/2 ft under the keel. We had west coast owners join the group at our 2011 Selene rendezvous, they were freaking out at the water depths. :lol: It's all in what you're accustomed to!

 

[rslifkin]

You'd be apoplectic in the Bahamas, where we routinely anchor with 2ft under the keel, travel across the bank with 1-1/2 ft under the keel. We had west coast owners join the group at our 2011 Selene rendezvous, they were freaking out at the water depths. :lol: It's all in what you're accustomed to!

Yeah, I would not be happy with that! I get nervous with 3 feet under the props and by 2 feet I'm at idle and sweating.

As far as scope goes, I generally treat 5:1 as default unless conditions require something different. In good weather, deep-ish water and a tight space, I'm ok with a bit less. In questionable weather, I'll tend towards 7:1 if at all possible.

 

[Bigsfish]

Boating in the Keys and Bahamas I often anchor with one foot under the keel at low tide (protected anchorage). I have on rare occasion bumped the keep on the bottom (no big deal) and Maerin is correct many times I’m moving at slow speeds with only a foot or two under the bottom, you need to read the water. It’s also common to bump over sandbars to get to a particular place. I try to use 7:1 scope. Once your use to it it’s no big deal.

 

[DDW]

Yeah, I would not be happy with that! I get nervous with 3 feet under the props and by 2 feet I'm at idle and sweating.

As far as scope goes, I generally treat 5:1 as default unless conditions require something different. In good weather, deep-ish water and a tight space, I'm ok with a bit less. In questionable weather, I'll tend towards 7:1 if at all possible.

A default of 5:1 is fine on the east coast. In the PNW, that means you will need 500' of chain for many anchorages. It is also unnecessary:

With chain at least, the catenary allows for the rode to pull parallel to the seabed at the anchor until a certain tension has been exceeded. Once the rode is parallel to the seabed, additional rode will not add to anchor holding. The tension required to lift the chain is dependent on the weight of the chain and depth, as well as scope. 7:1 in 10' is about the same as 5:1 in 60'. With rope rode there is no catenary and this does not apply. If anchored in waves, this also doesn't apply, since surge becomes the dominant factor.

 

[Sangraal]

We use the eye we had made and fitted on the bow to reduce scope as our bow roller is very high on the KK54. It reduces the amount of scope significantly in most situations. 5:1 to 7:1 depending on the situation we are addressing. 55kg Rocna Ancor on high test chain

 

[Bob M]

I use an anchor bridle but hadn't heard of anchor snubber until reading this thread.

An anchor snubber definition: 1/2" nylon line attached to anchor chain and forward cleats that relieves stress on windlass, absorbs energy and helps hold the anchor in place, which is close to my bridle set up except my bridle lines are much thicker.

I have two, what I call snubber lines, that I use when docked. They look like this. Again, the lines are thicker than half inch. Would these lines be appropriate for anchor snubber lines?

 

[ProMaritime]

Yes.

 

[Driftless]

The Navy taught us to use 7 to 9x the water depth to anchor (which I took to mean 8x), measured to the waters edge (meaning to where the chain enters the water, not counting the height of the hosepipe.

More scope never hurt.

After all, what good is it doing you in the chain locker? Better to be in the water and not needed.

Are you afraid of getting it wet?

 

[captaindah]

Snubber

I mounted a "U" bolt at the waterline, attached a heavy duty snap shackle to it, ran a line through it with a Mantus chain hook one one end and a deck cleat on the other end, so I eliminate the need to add the 35' of chain for the 7' of distance between my pulpit and the water's surface.

 

[Nomad Willy]

Driftless wrote;
“More scope never hurt.”


Of course it can. There’s some notion that if some scope is good more’s better. Not at all. Too much scope can easily get you into all kinds of serious trouble .. or grief. Too much scope and your swing’in all over the anchorage. Bang’in into other boats, getting grounded on sandbars, rocks or hazards like old wrecks.

And I don’t hold this to be fact but I suspect there’s a best or optimum angle of rode t anchor for maximum resistance to a breakout. And I don’t think it’s 0 degrees. It’s somewhat less. Perhaps 7 or 8 degrees. If you watch video’s of anchors in action very closely you will see anchors breaking out vertically. I’ve seen that happen w Claws and Danforth types (including Navy and other non-penetrating anchors). Danforth types that do most often have too much throat angle. The Fortress anchor is adjustable in this regard and w an excessively wide throat angle breaking out vertically is most likely. And a hard bottom either won’t allow a set or they will breakout vertically.

That raises a question. If horizontal isn’t the very best rode angle what is? Throat angle varies considerably from anchor to anchor and even considering the same type .. like Claws. The Lewmar Claw has much more throat angle that the Bruce. One could almost say increasing the rode angle would affect the Lewmar less than the Bruce as the Lewmar would have a horizontal fluke when the Bruce w the same rode angle would have a positive angle of attack. Like the Bruce would have a fluke pointed upwards toward the surface. so one would think the Bruce would be more likely to break out .. or being pulled “straight out” with no catenary.

But of course often things aren’t as they seem to be.
A huge element in the above scenario is considering the angle of attack of the fluke and fluke loading and in a real way “stalling” of the fluke surface in a liquid.

So taking all the above into consideration horizontal may not be the optimum rode angle. And on a related plane many anchors may optimally set better at scopes way less than 8-1 and hold better at another scope.

 

[Woodland Hills]

It is easy to get too much scope for a given location if you swing onto shore. I also have to add 70’ to my scope to get to my swim platform. In the Chesapeake where we spent the summer there are many quiet protected anchorages that are relatively narrow. If I anchor in 10’ of water, add 9’ from the waterline to the roller, times 3 plus 70’ gives a distance of 127’ from my anchor to my transom for a swing circle of over 250’. The actual scope is only 57’, at 5 to 1 the scope is 95’ for a total circle of 330’. This can play havoc in a crowded anchorage if everyone else is using 3 to 1 and has a 40’ or under boat.

 

[Bigsfish]

Willy. Might one assume that more than 8:1 might also be better?