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Old 29-04-2017, 07:42   #1
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Anchor Calculations,

I would like to know where the calculations for anchor sizes came from,
Is there a formula specific for anchor size to boat,
How do they work out what size is needed for a boat,
Windage by weight by size by area facing the wind, ??????
Its so variable,

I used to install concrete panels, a 20 knot wind and a 20 ton panel is near impossible to control,
Over a certain wind strength, the crane wont lift, Its too dangerous, It can blow the crane over,
Its not a very high wind strength either,
Its about 35 knots and no crane will lift any thing,

My boat has a front of some 14 feet x 10 feet, Approx, Plus mast area,
Thats a lot of flat area to be hit with a wind, Any wind,

So how is it calculated to say in a 50 knot wind what size anchor is required to hold the boat steady in one spot and not drag,

Being in a Marina or protected bay in high winds is not an answer,

Other than what manufacturers recommend for their anchors,
I have not seen any where how they calculate that out,
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Old 29-04-2017, 07:57   #2
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Re: Anchor Calculations,

There are no calcs for anchor sizing. The
They are sized empirically by the manufacturer.
Rule of thumb for cruisers is go up one size if your boat is at the bottom to middle of an anchors size range and 2 sizes if near the top.

The loads on the rode however can be calculated approximately. The are a number of methods. The methods explained in "The Complete Book of Anchoring and Mooring" by earl Hinz is the one I am most familiar with.

Generally I am of the opinion that the ABYC tables are the easiest to use for monohulls and are conservative to begin with. I have interpolated boat lengths and beams to get length to the nearest 1' and beam to the nearest 0.2'. For cruising I would use the 42kt windspeed column unless I was going somewhere special like high latitudes.
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Old 29-04-2017, 08:30   #3
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Re: Anchor Calculations,

What Adelie said. Though it is possible to calculate wind loads on surfaces, given wind speeds, the amount of area, & coefficient of drag. Harken has the formula for flat surfaces (sails).
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Old 29-04-2017, 08:33   #4
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Re: Anchor Calculations,

A number of investigators, including me, have, have tested loads under varying conditions with different rodes. Some of my testing was in Practical Sailor Magazine.

  1. If on all-chain in shallow water, with no (or short) snubber, the ABYC tables are correct. It is that bad once the wind comes up.
  2. If on rope, chain with a long snubber, or chain in deep (over 20 feet) water, the load is 3-5 times lower. (The wind-only load is about 5 times lower, but gusts and waves can add). This is NOT to say the ABYC numbers are wrong for specing hardware. You want it to last and to safely tolerate anything.
  3. It is practically impossible to say what an anchor will hold. I've done a lot of testing, and a given anchor's holding capacity can vary from 500-5000 pounds (35- to 45-pound scoop anchor in soft mud vs fine sand), or even worse on hard pan. There is a learning curve there, far beyond what I'm going type here.
  4. How much any anchor holds, regardless of claims, goes down as the scope (at the bottom) is reduced. How much depends on scope, wind speed, and water depth, because these all affect actual scope at the bottom. Also anchor design (some are more vulnerable than others).
Number 3 is why anchor debates go on and on and on, and why vendors don't agree. It's like herding cats.
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