Shallow Water Anchoring

[nautical62]

In the Bahamas, I have often anchored with less than a foot under the keel at low tide. I find doing the math and anchoring shallow often is the key to enjoying places that would otherwise be inaccessible or to spending the night in a protected, calm, shallow bay instead of spending the night rolling around out in the slop.

[sailorboy1]

Originally Posted by Bash

Not if I can avoid it. Silt scours the bottom paint. Those of us who don't haul our boats out every winter usually try to make a paint job last 2-3 years. That won't happen if you keep anchoring shallow.

Agreed. On the plus side that would have taken care of the mussel farm I grew this past season on the bottom of ny wing keel.

[s/v Jedi]

There's 2 problems to recognize with shallow anchoring:

1. when you become "stuck" in mud or sand at low tide you can get in trouble when a squall with wind shift comes around or the tide (current) turns.

2. the scope on anchor rode should not follow a 4:1 or 6:1 ratio like for deeper water. You need some minimum length of chain (rode) out and we use 100-120' when room allows it but never less than 60' chain. We have seen too many examples or boats with 25' rode in 8' water that get swept away with the first mild squall that comes along.. endangering others and inflicting damage all around.

ciao!
Nick.

[markpierce]

Originally Posted by s/v Jedi

2. the scope on anchor rode should not follow a 4:1 or 6:1 ratio like for deeper water. ...

The vertical difference between the water and the point the rode leaves the boat needs to be considered. That distance is constant regardless of water depth. For example, if the water is four feet deep and it is three feet from water to point where rode leaves the boat, one needs forty-two feet of rode for a 6:1 ratio, not twenty-four feet.

[s/v Jedi]

Originally Posted by markpierce

The vertical difference between the water and the point the rode leaves the boat needs to be considered. That distance is constant regardless of water depth. For example, if the water is four feet deep and it is three feet from water to point where rode leaves the boat, one needs forty-two feet of rode for a 6:1 ratio, not twenty-four feet.

yes but that is always the case, regardless of water depth.. it just becomes a bigger factor when it gets shallow water.

But this is not what I mean. Let's take a 4:1 ratio as is normal to use with a new tech decent sized anchor. Say we have 4' water and 3' freeboard, so 7' total. this would calculate to 28' rode to get the same angle as one is used to when following the 4:1 rule. However, 28' of rode is just too short .. the boat is too close to the anchor and there's very little weight of the chain out. I have never seen a boat anchored like that stay put in 30+ know squalls with wind shifts. Also, a nice wave on the stern will take the boat over the anchor on a rode that short.

So, the point that I wanted to make is that you always want some minimum rode length, like 60' or 100'. I have anchored with much less rode out than that but never at night and never left the boat in those cases.

ciao!
Nick.

[Mark Johnson]

In thousands of successful nights spent on the hook, it is surprising how many times I have been caught in the middle of the night by a 60+ knot short thunderstorm, (or long duration 40 knot winter fronts). I anchor our 34' tri accordingly.

I always use my 30' bridle, so the boat doesn't saw around, and always take the height from the water to the bow roller into the math when calculating the 7 to 1 scope. With my 1/4' HT chain acting more like rope, and usually anchoring in shallow water, this 7 to 1 scope is very important! (It is the big heavy monohulls anchoring in DEEP water with 3/8" chain that get away with 4 to 1 scope.) Smaller, lighter boats can't get away with it!

If an anchorage is too crowded, I don't do the standard... "squeeze in there anyway, OR use TOO short a scope". I anchor somewhere else! All over the Caribbean I was regularly anchored WAY away from the crowd, because other than true emergencies, the above rule is etched in stone. Yes, I have a planing dinghy...

I set my 35' DELTA firmly at full reverse RPM! In firm sand or mud, I can do this right away, and I'm good to well over 60 knots of wind. In really soft Chesapeake jello mud, I can "usually" achieve this by starting S L O W. I reverse at 1,500 RPM for long enough to casually drink a beer, then 2,500 for a few more minutes, very slowly working up to 3,400 RPM. I have a folding prop, so it has less reverse thrust than a fixed one, but anyone, over time, can figure out what amount of reverse RPM = what amount of wind... "more or less".

If I can't get proper holding like this and continue to drag, I try a couple of more spots.
If this doesn't work, It is dark by now, so I put my huge emergency Fortress on the forward wing, ready to deploy. (Since it got airborn once, I now lash it down!)

I never trust hard or broken up "gravelly" bottoms, or thick grass, and if clear and < 30' deep, I freedive on the anchor. Several times my anchor looked great from the "viewing bucket", but I found it was hooked on a marl ledge with 6" of sand over it. This is when I poke around "down wind" and find a GOOD all sand spot or hole in the marl. I leave my lobster stick there as a marker. Then with a BIG breath, dive down, pick up the anchor, QUICKLY walk backwards across the bottom, and drop it in my marked spot. So far, this has been much quicker than the speed that the boat falls back, and my wife is at the wheel... "in case".

Improper anchoring is the most likely way to loose ones boat! Getting it right is an acquired skill, but one well worth mastering, for your boat.

Hope this is of use... Mark