Dumb Anchoring Question

[Hillbillylad]

I have looked, believe me I have looked.
When anchoring in say ten feet of water, the bibles go on to say you will then drop back and set your anchor at 5 to 1 or whatever. I get that bit.
But, here's my confusion.
You sail up to a beach, and in my case that can be pretty shallow with a draft of 3 foot, and drop anchor in say 5 feet of water. But as you drop back you are pretty quickly in 20 feet of water under the keel. So which depth do you use to work out your cable length? The 5 feet where you dropped the anchor or the 20 feet under the keel?
I know it sounds stupid but it is confusing the buggery out of me.

[Reefmagnet]

It's where you drop your anchor. The scope gives you the angle, say, to the vertical. The greater the angle the better the holding. P.s. don't forget to add the height above the waterline to your bow roller in your depth calcs.

[44'cruisingcat]

In that circumstance it would pay to let out additional scope though, because if you drag a little, your anchor will move into deeper water, reducing the effective scope.

[Pelagic]

Normally you set your anchor towards the danger, ie the shallowest area for your draft, so you are not setting in 5ft…. but deeper and then setting towards the shoreline or reefs to end up sitting in more than 3ft if you get a strong onshore wind.

So you would calculate minimum amount of chain based on the deeper depth, tide and freeboard. (plus weather)

You did not ask a dumb question…… but there are better questions to ask yourself:


1 What type of slope is there on the bottom and if it is steep can I drag into deep enough water where my anchor comes off the bottom or becomes useless because of a small scope?


2 Alternately, if I put out too much scope on this inclined bottom, can the wind blow me ashore or aground?


3 Is there a bottom indent or deeper patch showing on the chart, where I can drop my anchor in a spot where it has to drag uphill in all directions? (safest procedure)


4 If I want to be really close to shore on a steeply inclined bottom, is it better that I drop in deeper water, back towards the beach then use a beach line to tension against the anchor pull and sit nice and secure where I want to be?


My favorite anchor question is: “When you drop your anchor, does it go all the way to the bottom?”…… it was hard not to smile!

[cal40john]

The reason for the amount of rode on modern anchors is to pull at a nearly horizontal angle so the flukes dig in rather than get pulled up and out. Theoretically if the bottom is flat where the anchor is you could use the depth at the anchor. If the bottom is sloping down where the anchor is it could be impossible to have a pull on the anchor that wasn't up on the shank and pulling it out.

The only real world experience I have with sloping bottoms is where you set the anchor uphill towards the shore and tie the stern to the shore so the you're always pulling the anchor uphill and so at flat angle on the shank relative to the bottom.

If you anchor in 5 feet of water and it's 20 feet deep in one direction, aren't you going to be aground when you swing around to the other side when the wind or current changes?

I believe the seamanship books tend to say try to find a fairly flat spot to anchor in if you can.


John

[Oliver L.]

Hilbilly,

I don't quite agree with the previous answers. The sense of the additional chain length is to always make sure that the chain pulls at the anchor laying flat on the ground. There is a formula for that (called the "chain law" in German) which tells you how much chain will withstand how much pull.

Now if you anchor in 3' and you set 15' of chain, that fulfills the rule of thumb on a plane. When the ground drops, you have a chain that is descending, and that is much easier to lift off the ground - because the pulling angle is much bigger. And when you lift the chain of an anchor on a slope even to horizontal, you increase the danger of slippage immensely.

My solution: Pick a spot about 30' downwind from your anchor; take that depth and multiply it by 5 and add it to the 30'. That was, the first 30' should always hug the ground and you should sleep undisturbed...

Oliver

[Pelagic]

Originally Posted by Oliver L.

Hilbilly,

My solution: Pick a spot about 30' downwind from your anchor; take that depth and multiply it by 5 and add it to the 30'. That was, the first 30' should always hug the ground and you should sleep undisturbed...

Oliver

Since we are discussing basic anchoring principals you must also take into account swing room.


3ft depth is usually fairly close to shore and winds do change at night

[afmstm]

Another way of looking at this problem is to remember that the point of "scope" is to ensure that the angle between a taut rode and the surface that the anchor is dug into, is kept below some small angle. Anchors are designed to hold up to some critical angle for a given force. Go above that angle, and the anchor drags.

For a horizontal sea bed, it's obvious that scope is just the tangent of the angle (opposite over adjacent). So, a "5 to 1" scope corresponds to an angle of about 11 degrees.

Now, if the direction of pull is in the direction of the slope of the sea floor (by which I mean you end up in deeper water than the anchor), then clearly the angle is larger than for a horizontal sea bed. If you assume the sea bed is flat but sloped, and do the trig, to a good approximation you should simply take the depth where your boat finishes up as the depth of the water. If you drop in 3 feet, and back out 100 feet of rode, finishing at a depth of 30 feet, then you have a 3 to 1 scope, not a 30 to 1.

Conversely, if you start deep, and finish shallow, then your scope is increased. You'll realize this when you try to stern tie on a steeply falling shore like in British Columbia. Drop in 40 feet of water, back up letting out a total of 60 feet of chain, and end up in 10 feet of water, stern to shore. That's 6 to 1 scope. Tied to shore, you can't swing so you are fine. Just make damn sure the anchor is set!

So the rule of thumb in calculating scope over a "flat" sea bed, whether sloped or not, is to base it on the depth of water you end up in. Of course, as pointed out, shifting winds and currents may be a bigger problem.

[SaucySailoress]

Originally Posted by Pelagic

Since we are discussing basic anchoring principals you must also take into account swing room.


3ft depth is usually fairly close to shore and winds do change at night

See, I wanted to say that, but figured the guy probably has a cat and the beach is sandy... and clearly he's just putting the anchor out because it's too much effort pushing off from the shore in the morning if he runs up the beach...

[sailorboy1]

Originally Posted by Pelagic

My favorite anchor question is: “When you drop your anchor, does it go all the way to the bottom?”…… it was hard not to smile!

And the answer is .........

Would have figured we could have gotten a good debate going on this one.

[alan_za]

If letting out 25ft rode causes a 15ft change in depth, well you are setting your anchor on a steep slope where it is very unlikely to set properly or hold well.

[David_Old_Jersey]

Originally Posted by SaucySailoress

See, I wanted to say that, but figured the guy probably has a cat and the beach is sandy... and clearly he's just putting the anchor out because it's too much effort pushing off from the shore in the morning if he runs up the beach...

Can't be bothered to re-type and tweak my post of earlier today (#59) .......

http://www.cruisersforum.com/forums/...tml#post836472

[Vasco]

I would say that your scope should be in relation to the ten foot depth. However I regularly anchor in ten feet of water and nearly always put out a hundred feet of chain, swinging room permitting. I like to sleep at night.

[Wotname]

Originally Posted by Pelagic

.......
My favorite anchor question is: “When you drop your anchor, does it go all the way to the bottom?”…… it was hard not to smile!

Originally Posted by Don Lucas

And the answer is .........

Would have figured we could have gotten a good debate going on this one.

OK I will bite. Doesn't it depend on what sort of anchor it is?

The mono ones seem to go straight to the bottom 'cause they have a heavy keel and stuff, then they lean over and dig in 'cause they are slow while the multi ones being light and unsinkable, sort of skate along just below the surface 'cause they are fast and stable.

[Waterway Guide]

I'd also try to figure out if there's a ledge of some sort which could be pretty dangerous, but the angle of the bottom certainly needs to be factored in.