What was the deepest water you could practically anchor in?

[Daniel]

So, how deep could the water be and the ship still be able to anchor? And what's the limiting factor?

I think having a 100-fathom anchor cable doesn't let you anchor in 100 fathoms of water. I think that for the anchor to bite properly, the anchor cable needs to be at an acute angle to the sea floor, and pictures I've seen of ships at anchor show the ship several hundred yards away from being directly above their anchors. You would only go directly above your anchor when you were preparing to haul it up.

But is the length of your anchor cable the limiting factor? Beyond some length, does the combined weight of the anchor and cable become too much for the crew to lift up, or for the capstan to withstand?

Edited October 13, 2013 by Daniel

[Red John]

Daniel,

Ummm . . . "Aye" to all . . . basic rule is you should only anchor in a depth equal to 1/3 of your anchor line's length.

Sooo, a 100 "fathom" anchor line (1 fathom = 6 feet, thus a 600 foot line) would allow you to anchor in a roughly 33 fathom depth, or approx. 200 feet straight down. All of this being the general rule, your basic starting point. Wind, weather, current, bottom conditions, how loaded your vessel may be (weight), etc., all play a role when you are actually going on the hook. Plus, are you putting out only one anchor, or a pair, or more? All in all, "more art than science" is probably best way to describe it.

Yes, needs to be an acute angle, and yes, the weight of the whole thing and storage space are all limitations. Not to mention the number of crew, space for them on your capstan, capacity of your capstan, all of that.

yours, aye-

[Daniel]

Thanks!

I found a book on line by Jan Glete called Swedish Naval Administration, p. 455, which says that large Swedish warships generally had anchor cables 100 to 140 fathoms long, with smaller warships having proportionately shorter cables. This matches well with an example from the Ship Model Laboratory, which mentions a 120-fathom cable on Nelson's Victory. So it sounds like a typical large warship would have difficulty anchoring in more than about 40 fathoms of water, or maybe up to 47 for an exceptionally long cable.

There's another ship-modeling post that says that, until the 19th century, most anchors had straight arms, not the gentle curve we're familiar with today!

[flagman1776]

It is quite correct that one can not anchor in water, the length of your rope. Yes it needs the angle mentioned above. An anchor with a heavy stock helps immensely but in addition a length of chain, weight dependent on the load on the line, acts as a shock absorber, requiring the line pull to lift it off the bottom before applying force to the anchor. Likewise the weight of the chain, and longer chain with more weight, changes the attack angle of the line to horizontal on the sea bed, greatly increasing holding power. Seldom seen these days is additional weights can be slid down the rode (anchor line) on a tether stopping short of the anchor, and like wise acting as a shock absorber. Modern nylon has additional give under load, again reducing shock loading.

I recall a scope ratio of 7:1 but few anchorages allow such swing room. I recall a very bumpy night spent in an exposed anchorage in Buzzards Bay when we could not find adequate clearance in the harbor. Our heavy Folding Yachtsman anchor held beautifully, with it's chain & oversized 5/8 rode in a 30' Pearson Coaster. We kept an anchor watch & every hour eased the line a few inches as she snubbed short as the storm passed... to limit the chafing on the chocks in any one spot. I don't think anyone got much sleep, all wedged together in the dinette amidships. The yachtsman anchor is similar to the Admiralty Pattern folding stock but with diamond flukes to avoid fouling.

http://www.anchorbuddy.co.nz/anchoring.html

http://www.anchoring.com/article_info.php?articles_id=9

[Coastie04]

Oooh, here we go... There are a large variety of factors that will account for how deep you can anchor, and I'll try to touch on some of the major points here, though they have literally written books on the subject.

First off, the 3:1 ratio (aka scope) mentioned above is generally considered the minimum today. It's good for normal conditions (light wind/current, etc.) with good holding ground (mud is usually considered the best, with rocky/shale or weed bottoms being much less favorable). I have heard of some modern sail and power boats occasionally setting a less than 3:1 scope, but it's generally considered an 'anchor hook', meaning that it's good conditions and that they're going to be anchored for a very short period of time (and generally everyone's up and can react fast if they start to drag). For moderate wind/current or less than ideal holding ground, a 5:1 scope is usually recommended. For storms, or if there's room in the anchorage due to other vessels or shoal water, a 7:1 or greater scope is recommended. Generally, the more line you have out, the less likely you are to drag anchor.

After scope, there's also the matter of what style of anchor you have. Most anchors today are designed to dig in to the sea bed with their flukes, and thus you want them to be pulled along the sea floor, and not up at all. Yes, chain or weights added to the line do help this (my personal boat has 30' of chain coming off the anchor before switching to a few shots [90 feet] of nylon rope). Older style anchors were much less efficient at digging in. If you go far enough back, the first anchors were just heavy stones tied on to the end of a rope. These were still effective in relatively moderate conditions, provided the rock is heavy enough. This technique is still used today for buoys in many places, and the buoys generally have a much smaller scope in order to limit their swing circle. I won't get in to the factors that determine the length of chain on a buoy right now, as it is only marginally related to the current topic. Compared to most of the anchors today, the anchors used a couple hundred years ago were much heavier. The Lady Washington, for example (iirc-it's been a number of years), has 3 anchors: two 1200 lb anchors and one 800 lb anchor. She also has a chain rode, as compared to the traditional cable. A different passenger vessel (non-sail) that I've worked on more recently is a larger, but similar weight of vessel and has one approximately 50-70 lb modern anchor with a short rode of chain followed up with cable. I've seen some with a large, but relatively lightweight aluminum anchor as well, preferring to rely on the weight of chain and design of the anchor rather than it's weight. The extra weight of older style anchors can allow you to anchor with a minimal scope (maybe 2:1?) in calm conditions when you have a crew ready to pay out more cable if conditions change. Anyone who's hauled up an anchor (especially by hand) knows that at some point, usually when the anchor rode is nearly vertical, you have to break out the anchor from the bottom. This shows that at least if a boat is preparing to get underway, you can haul up to short stay (the point just before breaking out the anchor) for a while without dragging anchor in good conditions.

As for how much the crew/capstain is able to withstand, that's a very hard to define number. Each boat/crew is different. Are you talking all hands in good health with a new boat, or an old rotting boat at the end of a long voyage with a short handed, scurvy struck crew? This is where a lot of the art vs. science comes in to play. Today, many boats have capstains/windlasses that are powered and can only haul up with a certain force. Usually the SOP today is that you use the engines (obviously not available at the time) to drive up to where the anchor is, hauling up line as you go so that it doesn't foul anything under the boat. Then, it's just a matter of breaking out the anchor and hauling it's deadweight (which is lighter while it's still in the water) strait up. Breaking out the anchor, if the windlass/capstain can't do it, can be done sometimes with the engines, though that's usually not recommended.

Additional factors to keep in mind: if you have large tidal changes, the scope will decrease at high tide, and your swing circle will increase at low tide (when there's less water between you and that potential shoal). Luckily, in my area, even though there are large tide changes, there are usually not very many crowded anchorages (certainly nothing like the Caribbean). Although the holding grounds are often less than ideal, and there is often the very real threat of high winds, we've got room to use as much scope as possible. For safety, I try to have at least a 5:1 scope at high tide. Given that most of my line is nylon, my back only complains a little about it when I am hauling up the anchor. I sleep much better for doing it, though!

Recommended reading if you want more detail:

Knight's Modern Seamanship (rev. by Noel)

Naval Shiphandling (Crenshaw)

Merchant Marine Officer's Handbook (Turpin & MacEwen)

[Commodore Swab]

Every night when we go to bed we rely on our anchors to keep us where we are and thus are set well. Our anchors range vary from bruce, danforth, and even the good old fisherman the anchor most closely resembling a traditional wood stocked anchor. As far as bottom conditions the bruce sets best in mud, the danforth in sand, and the fisherman on rocky bottom. That all irrelevant though as we are talking traditional anchors so I will only talk about the fisherman. Once a fisherman anchor is set due to the design the scope can often be shortened up considerably to as little as 2:1 as it has "hooked" on the bottom. Sailors keep an anchor like this on hand for that emergency where they might be getting blown onto the rocks as it very well might save their boat. So in answer to your question depending on conditions a min of 3:1 scope that is 3 times as much anchor line as the depth.

Here in the Florida Keys working on salvaging some of the fleet that sank here in 1733 I can tell you that most of the anchors that were used in attempts to save the ships are sitting in 20-60 feet of water. On anchor recovered was 14 feet from fluke to fluke with a ring large enough swim thru.

[Silver]

as part of our drill we anchor a 40ton vessel in about 20-30 ft. on the james river it is all done by hand (weighing). anchor wt.600lbs our cable is 2inch and and we range out 10x the expected depth. have not seen the windlass used.