Anchoring
Contents
Anchor Basics
Here's a typical danforth type anchor like the one's we've been using:
They work by lying flat on the bottom, and having their flukes (blades) dig into the sand or mud. "Scope" refers to the ratio between the line length and the depth. The more scope, the flatter the anchor lies and the better it holds.
Scope Table
According to Don Casey, this table shows anchor holding power as related to scope:
10:1 100%
7:1 91%
6:1 85%
5:1 77%
4:1 67%
3:1 53%
2:1 35%
[[1]]
Note this is approximate and it assumes a flat seabed. If the floor is sloped, then the calculations get more complex.
Example 1: If someone tries to use 50' of line in 25' of water, then the scope is 2:1 and the holding power of anchor will only be about 35% of it's max.
Example 2: If the same line is used in 10' of water, the scope will be 5:1 and the holding power should be more than twice as strong as it was in the first example.
Chain
On all boats except the very smallest, chain is attached to the anchor line. The chain adds weight to the end of the line and helps the anchor stay set on the bottom. I don't have any numbers for how much chain helps, but it's important.
Addendum: a crucial benefit of the chains is that they reduce the initial vertical force you exert on the anchor when you tauten the line, helping set the anchor into the riverbed rather than pulling it up or just along the bottom.
Danforth Holding Power
Weight : Holding Power : Boat size (20knot winds)
14lb : 920 : 31'
16lb : 1300 : 36'
25lb : 1600 : 40'
43lb : 2000 : 45'
70lb : 3000 : 55'
100lb : 3500 : 60'
These numbers assume a minimum of 4-8' of chain.
[[2]]
Removing stuck anchors
After 4 days of anchoring in last year, some of our anchors were stuck in the mud. Pulling up failed to free them, so we tied off the anchor line to a cleat on the houseboat and motored forward at full speed to break them loose.
This year, with a longer event and bigger anchors we are likely to have anchors which are really stuck in the mud. Trying to yank them out with a cleat and the force of a motor is likely to damage the boats.
Instead, here's what one reference recommended and I'm paraphrasing: Pull the line tight, and then let the motion of the waves to gradually work the anchor loose.
On the delta, we probably won't have the waves to help us work the anchors out. But a group of people pulling, with patience, should be able to slowly work an anchor out of the mud.
2011 Anchoring
In 2011, almost all of the anchors used were 14 pound anchors (TODO - verify that!) that came with the houseboats. The came attached to 50' of line (TODO - verify that!).
As we can see from the info above a 14 pound anchor is only strong enough to hold a 31' boat in a stiff breeze while they were being used to hold 40' houseboats.
In addition with 50' of line, assuming 20' depth, the anchors only had about 40% of their holding power (2.5:1 scope). As such, they didn't hold at all. After we tied two ropes together, we had closer to 5:1 scope and the anchors mostly held.
The picture below shows the approx. strategy used for anchoring:
Even with all of the anchors, the city broke free once and floated downstream.
It took a very long time to assemble the city and to take it apart. Once boats were in place very few of them moved until the end of the event.
Sean's Basic Anchoring Guide
BEFORE you toss it in the water…
1) CHECK YOUR GEAR. Captains, make sure to acquire the proper chain, anchor line, and connectors before you leave the house! Before you take your boat from the pier, look at connections between chain and anchor, chain and line, and make sure the line is in good shape. 2) MEASURE WATER DEPTH AND LENGTH OF ANCHOR LINE. For Ephemerisle use a minimum 5:1 ratio of anchor line to depth. You can assume this means a minimum length of 125 feet of anchor line. 3) TIE THE LINE TO YOUR BOAT. Cleat it at 125 feet (or longer), and tie it again to an even stronger part of your houseboat.
WHEN you toss it in the water….
1) CAST ANCHOR. Do this in pairs, one person throwing the anchor, the other person throwing the chain. 2) FEED OUT THE ANCHOR LINE. As you drive SLOWLY away from the cast anchor, feed the line out by hand, giving the occasional tug. 3) TUG AND HOLD. Tugging on the line, you should feel the anchor “set” into the mud. The line will go taught, you will let go of it, and the boat should stop moving (be sure to put it in neutral at this point!). If the anchor doesn’t hold, pull in the line and start over.
AFTER you tossed it in the water…..
1) Check the tie off. Make sure the line is attached to the boat, and isn’t tangled on anything. 2) Check your position. Look in a direction perpendicular to the line made by your anchor line and boat. If you are drifting, your anchor hasn’t set properly. Start over! 3) Check your position AGAIN. And AGAIN. And AGAIN. Lots of things can cause you to come un-stuck from the bottom. Be watchful!
Removing stuck anchors
After 4 days of anchoring in last year, some of our anchors were stuck in the mud. Pulling up failed to free them, so we tied off the anchor line to a cleat on the houseboat and motored forward at full speed to break them loose.
This year, with a longer event and bigger anchors we are likely to have anchors which are really stuck in the mud. Trying to yank them out with a cleat and the force of a motor is likely to damage the boats.
Instead, here's what one reference recommended and I'm paraphrasing: Pull the line tight, and then let the motion of the waves to gradually work the anchor loose.
On the delta, we probably won't have the waves to help us work the anchors out. But a group of people pulling, with patience, should be able to slowly work an anchor out of the mud.
Archive
See 2011 anchoring and 2012 anchoring for reference.
