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.
According to Don Casey, this table shows anchor holding power as related to scope:
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.
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.
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.
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.
2012 Anchoring (ideas)
Here's the baseline standard for anchoring a boat which you don't want to move. You simply use a bow and stern anchor. It's a very common practice among boaters.
The houseboats come with 14lb anchor (check). With 100+' feet of line they may hold. It would help dramatically if chain was brought and added to the line.
But ideally, according to Danforth's tables each boat should have a 25lb bow anchor with a smaller stern anchor. Bringing a 25lb danforth anchor with 100 feet of line and 20 feet of chain, plus another 10 feet of chain to add to the 14lb danforth as a stern anchor would bring these boats up to typical nautical standards.
1) To anchor:
a) Drop the bow anchor. b) Float backward until a sufficient amount of line has been release. c) Ideally with the help of a dinghy, the stern anchor is dropped well behind the boat.
2) To leave anchor:
a) Tie the bow anchor to a float, or give it the line to someone in a dinghy and release the line. b) Float backward, hauling in the stern line until you're directly above the anchor. c) Pull it in. d) Now _carefully_ to avoid any floating lines, motor up to fetch the bow line and pull in the bow anchor.
3) To add or remove boats
a) There is no issue because each boat has it's own anchors.
By lashing 5x 40'x15' houseboats together we effectively create (for the purposes of anchoring) a 75'x40' boat.
According to Danforth's table, we would need something larger than a 100lb anchor to moor these boats together in a windstorm. We're not going to have any hundred pound anchors because anchors get _much_ more expensive as they get bigger, and also because the cleats on the houseboats are almost surely not capable of handling such force.
Instead, one idea is to use larger anchors (40+lbs) with chain as the bow and 25lb anchors with chain as stern anchors. This would not get us up to nautical standards, but would almost surely be much better than what we had last year.
1) To anchor:
a) First boat drops the bow anchor. b) First boat backward until a sufficient amount of line has been release. c) Each successive boat slides up to the previous boat and lashes off to it. d) Ideally with the help of a dinghy, the stern anchor is dropped well behind the final boat.
2) To leave anchor:
a) All of the middle boats depart. b) The leeward (downwind) boat floats down hauling in the stern anchor line, then pulls up the anchor. c) The windward boat, motors up, hauling in the bow anchor line, then pulling up the anchor.
3) To add boats
a) To add them to either end, they should approach slowly. The boat should be partially lashed off. Then the anchor line should be transferred to the new end boat. Then the boat should be fully lashed off. b) To add them to the middle. Unlash the boats where you want to add a new boat. Using a 25 foot line allow half of the mini-flotilla to drift downwind away, then stop it. Use the cleats! When there is enough space the new boat drives _slowly_ in and then lashed off on both sides.
4) To remove boats.
a) To remove an end boat. Just transfer the anchor line and let them depart. b) To remove a center boat. Use a 25' line to connect boats on either side of what will be the gap. Unlash the boat and then let it depart. Use the 25' line to bring the boats back together, but let the currents work for you, rather than working against them.
Tying off to a tree
It is apparently relatively common practice in the Delta to create a secure anchorage by tying off to a tree. A stern anchor would of course also be needed to keep the boat from being pushed into that tree.
More advanced ideas?
I hope we can try to build upon these ideas...