Hi Unicorn here 50,000# 49LOA 180ft all chain 60# CQR - no issues CQR looooves mud - drop and sets 30ft - resets loose 10ft around 180s no issue.
Have not seen thread owner lately - miss a little bit the current flow force vector in this discussion - craziest situations had when wind vector and current flow force vector come from different directions.
Wind force vector is easy to calculate or approximate and any anchor/ground tackle system has a theoretical limit - I placed mine around 1200lbs max holding power in best ground conditions -
wind pushing boat nearly perpendicular to current 2,5kts dont know 900# holding a boat my size in plaze?? - thats when I start praying for wind direction change - drag likely.
a calc for my boat rough guessing windage size and coefficient if hit from bow - I would feel ok till 45kts with my CQR (in east coast mud - sand I dont know honestly) - anything hitting me above I would daisy chain secondary Danforth - scope is king.
windspeed(kt) force(lbs) (warm southern temp)
10 30
15 68
20 121
25 189
30 275
35 371
40 485
45 613
50 757
55 916
60 1090
65 1280
70 1484
75 1703
80 1938
85 2188
90 2453
95 2733
Ya'll have a good one!
For anyone wanting to make the calculation for their own boat herewith some additional figures.
Windspeed it usually calculated at 10 mtr above the surface, but our boats are usually between 2.5 and 5 mtrs off the sea level. Therefore you can deduct a bit to get to real wind speed on your boat. At 2.5 mtr that would be 70 % of the wind speed, for 5 mtr that would be 80 %.
However, if you want to calculate with the complete wind speed that is also fine.
At 4 Bft: 70 % of 14 kts = 9.8 kts
At 5 Bft: 70 % of 19 kts = 13.3 kts
At 6 Bft: 70 % of 25 kts = 17.5 kts
At 7 Bft: 70 % of 31 kts = 21.7 kts
At 8 Bft: 70 % of 37 kts = 25.9 kts
At 9 Bft: 70 % of 45 kts = 31.5 kts
At 10 Bft: 70 % of 52 kts = 36.4 kts
In order to calculate the forces on your hull (the part that is full in the wind) you can use the following figures:
Then calculate the total frontal surface of your boat and for my boat that is about 19 sqm.
Bft N/m2 kp/m2 N Kp
4 15 1.5 285 28.5
5 26 2.6 494 49.4
6 49 4.9 931 93.1
7 79 7.5 1501 150.1
8 106 10.6 2014 201.4
9 157 15.7 2983 298.3
10 204 20.4 3876 387.6
In other words, you can calculate the total force that the wind can put on your boat. As long as the holding power of your anchor plus chain is more than this wind force, you will not drag.
However, dragging is also a result of the angle of the shank with the sea bed. The maximum holding power of the anchor is at 0 degrees between shank and sea bed.
But at only 5 degrees angle you will no longer have 100 % holding power, you will only have 85 %.
At 10 degrees it is already down to 70 %
At 15 degrees it is 60 %
At 20 degrees it is 50 %
At 25 degrees anchor has lost all holding power.
So even if your maximum holding power is e.g. 2000 N and the total wind force is only 285 N, when the angle of the shank gets up to 25 degrees you won't have any holding power at all and you will drag, no matter what. Which means it is imperative to keep that angle at 0 degrees at all time, which means the chain needs to provide the holding power at the given wind speed.
As I have stated in another topic on anchoring, I let my chain to 90 % of the work, which means if I need a holding power of 1500 N I will drop at least that amount in anchor chain (and most of the time much more). It is the approach the large cargo ships use as well, they rely basically on the chain and not so much on the anchor itself.
Since I have taken that approach I have never dragged an anchor and before I applied this approach we have dragged a few times. In my opinion that was mostly because I did not understand the mathematics behind anchoring. Since I read this book I do understand the mathematics and it makes perfectly sense.
Now, where we are there is no current, so I don't have to calculate for that. But if you have to deal with current and wind it becomes a different story. If anyone is interested I can post those calculations as well and if anyone is interested in the book, it is called: Das ist Motorboot fahren, written by Joachim Schult. It is in German, but nowadays Google translate can solve a lot of language problems for those who don't speak or read German.