How to calculate scope

[psneeld]

Something I have observed.....


Unlimited scope...not much required of the anchor.


Short scope, more required of the anchor.


Anchor set.....unless you dive on it and have knowledge of the substrate from surface to several feet down...it's only a guess/hunch/experience...but very little on fact.


Enjoy discussing it as a hobby...but much of what is discussed seems pretty consistent with experienced cruisers who anchor say 100X a year or more in all sorts of bottoms and situations.

 

[Bigsfish]

Scott

When I was younger I dove on my anchor whenever I was spending the night and would check to see the anchor was buried. As I get older I find that a difficult task so I use the engines to really test the anchor to see how well it is set. I guess I’m lucky because once I have set the anchor I’ve never had it break out.

Since I boat in the south do you guys From the north dive on your anchors in winter? My rule is only go in the water when it 80 degrees or more.

 

[psneeld]

Heck, I don't.

But then I have realistic views of anchoring...not the anchoring fantasy world that some think they have down pat.

 

[Bigsfish]

LOL. I agree anchoring is not an exact science like understanding women, you do the best that your comfortable with and hopefully you get lucky and your boat is in the same place when you awake.

 

[Nomad Willy]

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

Fish,
Read my post #28.
More than 8-1 will probably be “better” w some sets of variables but one could easily arrange a set of variables whereas more than 8-1 would not be better. But it would be guesses.

“assume” that more than 8-1 “might” be better .. of course re your choice of words.

You can’t assume what I think you’re saying. Too many variables that would open that door but there would also be too many variables that would likely enable it.

I think your statement assumes a more is better scenario. Not so

 

[Bigsfish]

So in your world how does one know how much scope would be best for his boat? Since there are many variables such as Botton composition plus the substrate.

Your correct, I usually try to anchor with approximately 8X scope for over night and if the weather people tell me to expect a big blow (over 30 knots) I will go to 10X. I guess I’ve been doing it wrong but I don’t know to determine the more correct (better) way.

Thanks for your input.

 

[OldDan1943]

LOL. I agree anchoring is not an exact science like understanding women, you do the best that your comfortable with and hopefully you get lucky and your boat is in the same place when you awake.

If at all possible, try to determine the bottom material. That may assist you in determining your scope.
Once you believe you have your anchor set, back down on it a little, to make sure.

 

[psneeld]

I would bet most people who try to point out precision in anchoring also have an active inerest in area 51.

 

[Woodland Hills]

So in your world how does one know how much scope would be best for his boat? Since there are many variables such as Botton composition plus the substrate.

Your correct, I usually try to anchor with approximately 8X scope for over night and if the weather people tell me to expect a big blow (over 30 knots) I will go to 10X. I guess I’ve been doing it wrong but I don’t know to determine the more correct (better) way.

Thanks for your input.

Are you including the 8 or more feet from your anchor roller to the waterline? If so you must have a lot of anchor rode! Or never anchor in water deeper than 22 feet as at 10:1 you’ll have 300 feet out........

 

[psneeld]

Except in tropical cyclone weather why would you ever have 10 to 1 scope?

 

[OldDan1943]

Except in tropical cyclone weather why would you ever have 10 to 1 scope?

There is such a thing as Bahama anchoring. It involves 2 anchors, set one, row the other out, slack the first and set the other then, tension both as you normally would. Both off the bow. This is usually done in a tide effect river. The object is, when the flow is reversed, the other anchor will assume the major load.
As I recall, the anchors are set 45 degrees, maybe more. Yea, a pain in the butt to retrieve them but, far less than unintentionally beaching the boat and getting towed off at high tide.

 

[Bigsfish]

Old Dan. For overnight I back down pretty hard. First one engine at 1000 rpms then the other, let things settle for a minute or two then both at 1000 moving up to 1500. Not sure that is correct but most things I do are wrong.

WH. Yes we are 10 feet above the water plus 5 feet to the transducer so I add 15 to the transducer depth then calculate the XXX scope.

Scott. Didn’t I see you in Arizona two weeks ago sneaking onto government property at night?

 

[Bigsfish]

Old Dan. That is a Bahamian Mooring, I’ve done it any number of times in the Bahamas if anchored in a channel with a tide flow. Anchors are 180 degrees apart, set the first into the current drop back and set it then drop back further and drop the second anchor. Motor into the current to set the second anchor and adjust the scope until you in the middle. It’s a PIA as you can swing and get the rode fouled plus get one wrapped on the props. I avoid those places as much as possible.

 

[psneeld]

Thanks Irv...nice when people in the know chime in.

 

[Nomad Willy]

Fish what anchor are you using?
Nice that you have all that anchoring room to swing about but it’s not needed. But you will say it’s free so why not take it. But unless you’ve got one of my experimental anchors that failed half as much scope will work just as well 97% of the time.

I anchored once at 10-1. Calder Bay on the north end of Prince of Wales Is. it’s big, just about 20’ deep all over beautiful and say’in “I’m all yours” even a pack of otters to entertain me. That was a clue of things to come as in the morning I pulled up the stinkiest mass of sticky black mud I’d ever seen. Probably took those Otters 3000yrs to make that mud w all the shells. Took almost 1/2hr to scrub off. All the time the little Otters were playing and laughing at me. Never anchored at 10-1 again and life has been better.

 

[dhays]

Scott

When I was younger I dove on my anchor whenever I was spending the night and would check to see the anchor was buried. As I get older I find that a difficult task so I use the engines to really test the anchor to see how well it is set. I guess I’m lucky because once I have set the anchor I’ve never had it break out.

Since I boat in the south do you guys From the north dive on your anchors in winter? My rule is only go in the water when it 80 degrees or more.

Never dove on an anchor in my life. Water is too cold, too deep, and too murky.

 

[Bigsfish]

Scott. I’m sure I’m not in the know, Watfa tells me all the time I don’t know crap. Her philosophy is if you have a penis your wrong.


Willy. I had a boss and loved it but the new boat is 10 feet longer and a Boss would be too big for the bow. I thought about a Rockna and many of my friends on the forum have them and swear by them (and I respect these guys) but the hoop bothered me so I bought a Spade 130 lb anchor. So far it’s been perfect but it’s only two years. Did I do OK?

 

[markpierce]

Calculating scope is easy if one considers the height above water of the anchor roller and water depth, as well as changes in sea height due to tidal influences in the calculation.

Have 200 feet of chain for the rode. Sufficient in most areas of the San Francisco Estuary (its bays, sloughs, delta are mostly thick mud and shallow). Knock-off Bruces have caught and held unfailing for me in the SFE as well as the prehistoric Danforths, despite 3-knot reversals in tidal currents.

 

[Nomad Willy]

Hi Mark,
Still worried about those prehistoric Dans gett’in your finger eh?
Well they will ... so be careful.

Fish,
The Rocna is almost bullet proof. Unless you get into slime type mud or ultra short scope.
The Spade is bullet proof as far as I know. Haven’t used either one but read extensively on both.

Because the Spade has the ballast pound for pound the Spade will have a bit less fluke area. Probably a wash. But setting performance is more important anyway. There the weighted tip may be an advantage. But from all I’ve read and heard you’ll not be disappointed in the Spade. You may be interested in knowing that the Frenchman that designed the Spade went to some trouble testing fluke shape and concluded that concave shape was harder to break out. You’ll also notice the Rocna Vulcan is almost a mirror image of the Spade. They were impressed too.
Yes you did good.

 

[Bigsfish]

NW. Thanks for the info, never knew the Spade was a French invention (I guess I shouldn’t hold that against the anchor). Since you blessed me with “you did good” if it drags and I wreck my boat can I send you the repair bill.

So far no problems with the Spade once it is set. Once I had a hard time getting it to penetrate to set so I move to a different area where the bottom wasn’t limestone.

Thanks.

 

[Nomad Willy]

Fish,
At what scope did you experiment the set failures?

 

[Bigsfish]

Seenor Eric

I really can’t remember exactly but the place I went to is one that I haven’t found an anchor that will hold there so I would guess 6:1. Just wanted to see if the Spade might be a miracle anchor. Nope.

Thanks

 

[Nomad Willy]

Yup ..... no miracle anchor.
The bottom (seafloor) is the biggest variable in anchoring.
When my other anchors fail to set I get out the old 13# Danforth. The one that came w the boat years ago. Don’t recall it failing but my memory ..

I’ve overcome many “fail to set 1st try” by laying out the rode again. Starting by lowering the anchor to the bottom and picking it up 6” to a foot. Then insuring that I’m moving aft very slowly (less than walking) I lower the anchor till I feel it contact the seafloor and then lower it more quickly as rode goes out.

Then we get to the subject of this thread.
Being accurate about my scope has never been a priority for me. I have a leader of 50’ (newer line) joined w two shackles that serves as my 50’ mark. Then I mark w black electrician’s tape at the 100’, 150’, 200 ... 250 ..... I observe the marks and make a small effort to differentiate between 150’ and 175’. When I pull up the slack the scope can be seen by the angle of the line in the water. When it’s shallow I pay a little more attention to the numerical scope.

Then when approximate scope is achieved I Set my anchor. I think scope is important but exactly how much out there .... not so much. More so at short scope. But at short scope the tide is much more important.

 

[MurrayM]

On the question of knowing when & if the anchor has set properly...

Around here it's mostly mud where it's shallow enough to anchor, but twice now we've found harder ground where the anchor wouldn't set.

We use a rope/chain technique told to me by a sailboating friend years before we started boating. It probably stayed with me because it was accompanied by descriptions of hair raising anchor dragging shenanigans he'd seen by other boats.

Head into wind and/or current, come to a stop, lower anchor to bottom, let wind push you back or put into idle reverse while paying out 1:3 scope (pay it out as fast as you are moving), tie off rode to sampson post, keep engine in idle reverse, put hand on rode and lift rode up occasionally to feel tension on rode, when anchor has set (which is obvious when rode becomes very tight) pay out the remaining rode to get the scope you want.

If you have button controls in the pilothouse, you'll have to get out in the weather for this to work

 

[Nomad Willy]

Yes Murray I like to pull up on the anchor line on deck to check it’s tension. If the anchor is bouncing over rocks it can clearly be felt. And if an anchor is chocked w mud one can feel the heavy/steady drag. Ect ect ect ....
And 3-1 is more or less universal for me.

 

[rslifkin]

I usually set mine and check the set as follows: drop the anchor, once it hits the bottom, start slowly backing down while paying out rode. When desired scope is reached, cleat off the rode and let the boat (help with the engines if needed) gently come back on it. As the rode comes tight, there should be a tug that snaps the bow towards the anchor if you're not perfectly in line (if there's wind I often fall back from it at an angle). When doing this, make sure the initial tug when it tightens up is gentle, not hard (so make sure boat speed is low).

After that initial tug, let the boat settle, then gently bring up tension again and hold it using both engines in idle reverse. If all is holding well after a few seconds, raise RPM a little, let it sit for a few more seconds. Then back to idle and then neutral. In mud, that usually gets it set deep enough that it takes some work to break it out and that any wind and other forces on it should only set it deeper, not cause it to break out (unless of course the force is enough to just drag the anchor through the bottom).

 

[AusCan]

I probably anchor out 80% of the time when I'm cruising with the boat, and so far I've dragged anchor once.
It was completely my fault as I was initially planning a short lunch stop and we only let out about 30 feet of rode in 10 feet of water for about a 2.5:1 scope. Plans changed and we stayed the night. My plans overlooked letting out more rode.
I woke up at about 2 a.m. and the wind had picked up to about 20 knots. The boat was travelling at about 3 knots (luckily heading offshore). I let out more rode for a 6:1 scope and it set immediately and held firmly for the night.

btw- One other factor to think about when "calculating" scope would be the angle of the sea floor. On a sloping bottom, more scope is needed to achieve the desired angle when the wind and/or current is pushing the boat towards deeper water. I'm sure someone can work out the number for this, but I just try to generalise the whole situation and add more or less rode to suit.

 

[Nomad Willy]

Auscan wrote;
“One other factor to think about when "calculating" scope would be the angle of the sea floor. On a sloping bottom, more scope is needed to achieve the desired angle when the wind and/or current is pushing the boat towards deeper water. I'm sure someone can work out the number for this, but I just try to generalise the whole situation and add more or less rode to suit.”

I like the “try to generalize” part but I’m wondering about the sloping bottom part. I agree one should add scope but one should only need it when pulling the rode/anchor “downhill”. Uphill even less scope would be required. IMO. And downhill extra chain or chain weight would be beneficial.

Several times I’ve anchored at the head of an inlet w a short beach. And deep water near. Lots of grief could come to pass. If you’re too close to the beach (or will be w a tide change) setting pulling toward the beach would have gremlins attached.

Good point though as anybody going north in the PNW US will need skill + a bit of luck. Every time I’ve been in this situation I’ve had a bit of luck as in no wind.

 

[Shrew]

....we only let out about 30 feet of rode in 10 feet of water for about a 2.5:1 scope.

You weren't even at 2.5:1. If you bow pulpit is 4 feet off of the water, you were really at 2.145:1. I personally wouldn't go lower than 3:1 ever and prefer to sleep at 4:1 or 5:1.

 

[Nomad Willy]

Shrew,
I’ve anchored at 2-1 in benign weather quite a few times. Never in wind over 10 knots though. And 3-1 is my regular scope.
Go through Steve’s Vids and see how well anchors do at 2-1. Many even take quite a load.