Re: Anchor Questions
Back to your original question about chain need. For the flukes of navy style and Danfort anchors to grip properly, and for mushrooms and grapnels to grip properly in a different fashion, the shank of the anchor, the part the line (rope) ties to needs to be parallel to the bottom......not talking about a buoy mooring mushroom here which winds up vertical.
If the boat is in any depth of water, it is above the anchor such that the shank is not parallel, but tilted up at some angle. The deeper the water the worse it is. The higher the shank the less holding power.....like when you retrieve the anchor with no slip ring, what do you do? You pull on the anchor rope which lifts the shank, which pulls the flukes out of the bottom material and up comes the anchor. On a Danforth with the slip ring you just drive past the anchor, the ring slides forward where the flukes are and you lift it out backwards.....but that action has nothing to do with the purpose of the chain.
The purpose of the chain is to put weight between the shank of the anchor and the anchor line to force the shank back down. Doesn't take a lot. The pull of the boat (on a wave crest) has to lift the chain off the bottom before it can lift the shank and when the wave passes is releases tension on the anchor line and the chain falls back to the bottom....acts like, is, a shock absorber assisting in keeping the boat shock off the flukes.
For a 12# Danforth and a 18' boat in reasonable water, calling 2-3' swells reasonable and a 20 mph wind, 6' of 3/8 chain is adequate.....was for me and my 18' Starcraft Holiday I/O which had high sides that wind liked. Used a 100' of 1/2" Nylon line and never anchored in water deeper than about 25'. Nylon line is an excellent choice as it resists the elements, abrasion, and it stretches. The stretching absorbs some of the shock from the boat which otherwise would be transmitted to the chain, lifting it then dropping it, and ultimately to the anchor flukes.
Worst line I ever used (another boat) was polyethylene; like ski ropes are made of. It does not do well with chafeing like an anchor rope would get from your anchor rope chocks on you boat and your boat riding up and down with the swells......degrades with sunshine also.
Then there is what's called "scope". Scope is the ratio of the depth of the water to the length of the anchor line. To assist in keeping the shank parallel to the bottom, a minimum of 3:1 scope is recommended and for rough conditions, some go to 10. That means the length of the line (that is actually in use between the boat and the anchor) is 3 to 10 times the depth of the water. If you are in deep water that means a lot of line out or a humoungus anchor which is really not manageable as the boat gets larger.
So there you have it. I learned this as a teen in the Sea Explorer Scouts, BSA and never forget it. Another thing I never forgot was a question on a merit badge I was trying to obtain. Question was: What is an anchor the symbol for? Answer was: Hope. With an anchor you have confidence (hope) that you will not drift out/off to harms way including out to sea, nor onto the rip rap.
Mark
