Anchoring techniques

LSP · Post by **LSP** » Wed Jul 22, 2009 10:27 am

Indeed Cpt. Ross ... great visuals

ltbrett · Post by **ltbrett** » Wed Jul 22, 2009 12:15 pm

Great visuals, but they need a little fine tuning. Scope of rode is measured from where the rode enters the water, not from the bow of the boat. In the second picture, all but the far right (10:1) anchors are incorrectly set. This is most commonly caused by trying to set the anchor with insufficient scope. The shank of the anchor lifts off the bottom, pulling the flukes up and not allowing them to dig deep. That's not to say that you need 10:1 scope to get a good set. All you care about is having the anchor line laying flat on the bottom near the anchor during the hardest pull you expect while anchored. The anchor's holding power is a constant regardless of scope, provided you pull the anchor parallel to the bottom. But pull up at all on the shank, the flukes start pulling out and holding power vanishes. That's why you need to let out more rode in high winds/currents/depth. The harder you pull on your anchor line, the more of it pulls up off the bottom. Pull hard enough to get all the line off the bottom, your anchor will start to break free. Chain is heavier than line, so you can get away with less socpe if you have chain. Plus the weight of the chain itself helps hold you in place.

Brett

rossjo · Post by **rossjo** » Wed Jul 22, 2009 12:35 pm

Brett - these are Fortress' images - and they say that scope starts at the bow, not at the water.

Fortress Anchor's Safe Anchoring Guide ...
http://www.fortressanchors.com/safe_anchoring.html

So, if you have a large boat, and the bow is 5' above water that is 10' deep (for example), you "depth" would be 15'. Using 5:1 scope you would need 75' of line. If you simply use the water depth, you'll only end up with 50'. If use your depth finder (at the bottom of your 3' draft),you would base it on 7', and only 35' of line = 1/2 of what they recommend ...

So - let the debates begin ... who's got the Chapman's close by?

ltbrett · Post by **ltbrett** » Wed Jul 22, 2009 1:47 pm

Chapman's defines scope as a multiple of the distance from the bow to the bottom. On pleasure boats, this adds an additional margin of safety. On larger vessels, scope is always defined as a multiple of the depth of water, then add the height of the hawse pipe to get your final scope. In your example above, you'd need a minimum of 55' (5x the 10' depth plus 5' to the bow).

But don't get caught up in the technical definition. The principal behind the definition is to keep the anchor line from lifting the shank off the bottom. Use a consistent yardstick so you can adjust your scope for varying wind/current. Knowing when to go from 5x scope to 7x is much more important than which yardstick you use. Plus you would be just as big a menace in an anchorage swinging around with a 12:1 scope if everyone else is at 5:1 as you would be at 2:1 and dragging.

Brett

rossjo · Post by **rossjo** » Wed Jul 22, 2009 3:47 pm

One last note, for those of us who boat in areas with high tide fluctuations ...

"Depth is the depth of water at high tide, plus the height from water line to the bow roller. "

The last Regatta of the year in Charleston is at Rockville, appropriately called the "Rockville Regatta". This event is a huge party (bands, etc. on boats) with little focus on the sailboats at all. Its always fun at some point in the day when the tide turns, and several rafted groups of 15-20 boat tied to one sport fish with a single anchor breaks loose and starts running into people (who have probably partaken of too many "big boy drinks"). You want to be ready and get clear - but its a hoot to watch.