Here's a quick run down on anchoring technique. Even with extra crew on board I anchor & moor solo about 85-90% of the time.
Scope: This is perhaps the single most important and over looked aspect of setting your ground tackle. Scope is the angle of attack, if you will, of the rode or anchor line in relation to the bottom. The longer the scope the more parallel to the bottom the rode will be and the less likely to yank the anchor out from a more vertical pull. A short or steep scope angle will most certainly yank the anchor out of the bottom and will not hold well when the wind picks up.
How do I know what my scope should be? Scope is easily calculated, but often calculated incorrectly. Scope is simply the max water depth, plus the distance of your bow chock to the water, plus any off set for your depth transducer. Huh? Ok, you pull into an anchorage at low tide and it has a current water depth of 10 feet. The area you are in has a ten-foot tidal range (Maine). So your max water depth will be 20 feet. You know your bow chock is 4 feet off the water and your depth transducer is 1 foot bellow the surface and not calibrated as such. So you simply add 20 feet of water depth, to 4 feet of bow height, to 1 foot of transducer depth for a total of 25 feet of scope basis.
To set your anchor you should be using a minimum of 5:1 scope but the preferred setting scope remains 7:1. So the 10 feet of water you read on your depth sounder was actually 11 because your transducer is a foot bellow the waters surface and when the tide is added to the bow height your 10 feet of water depth turned into 25.
So let’s pretend you think you set your anchor at a 5:1 scope, based on the 10 feet of water depth you saw on your depth gauge. Don't feel bad as many sailors and boaters do this. A 5:1 scope for 10 feet is simple it’s 5 X 10 = 50 feet of scope. Oh, oh the tide comes in and you have mis-calculated your scope! For the example from above you ACTUALLY have 25 feet from the bottom of the ocean to your bow not the ten feet you mistakenly calculated.
For this same 5:1 scope you would need 125 feet of rode not 50 feet. 50 feet of rode for a 25 foot scope basis is a VERY dangerous 2:1 scope or almost vertical at high tide. You are nowhere near a 5:1. Again, this is a VERY common mistake. Please calculate scope carefully and always add the bow height and max tide.
Rode: This is the second most overlooked aspect of anchoring. At a minimum you'll want to be using 1.5 times the boat length of chain then a suitably sized, & highly elastic in nature, nylon rode. An absolute bare minimum chain length is one times the boat length despite what some anchor manufacturers recommend. Chain also prevents abrasion of the rode on underwater coral or rocks. An all chain rode is always better but you will need to use a very elastic snubber to prevent shock loading of the chain and on a 22 footer all chain is not necessary.
Why is the chain important? The chain serves a few purposes: 1) It serves as a weight to help prevent the anchor line from snapping tight. It keeps a curve or caternary in it during mild to moderate winds helping to keep the angle of attack on the anchor correct. In high winds a sentinel or kellet may be needed to maintain caternary but even kellets stop working as the wind rises. 2) It prevents the nylon anchor line or rode from chafing on coral or rocks on the bottom. 3) It aids the anchor in proper setting by keeping the shank down so the flukes/fluke can penetrate when backing down.
Anchors: All anchors are not created equal and there is far too much to be written on this here. Some anchors do not re-set well on a wind and tide shift and thus should not be used when a wind or tide shift is expected. Some anchors perform better than others do for certain bottom types and it should be up to the boat owner to thoroughly research which anchor will perform best for his or her environment.
In a general summary Danforth types which include the Fortress do not like to re-set on wind and tide reliably, Bruce or Claw styles are generally good setters and re-setters but offer low holding and should be up sized at least one or two sizes beyond the recommended size. World cruisers, and authors, Beth Leonard and Evans Starzinger use a claw that is a full five sizes bigger than the manufacturers recomendation. CQR’s or plow styles can give false sets and must always be properly set and checked for set. The new generation anchors such as Spade, Rocna & Manson Supreme & Bullwega are generally excellent performers, and practically set them selves, but they are not the be all end all's, there is no perfect anchor, and should also always be set and checked for set...
When I mention partially set CQR's this photo is exactly what I'm referring to. This pictured CQR is NOT SET.
Photo From Sail Magazine Anchor Test Report:
Technique:
#1) Examine the anchorage: Make careful observations & based on weather predictions chose a spot that will be better protected from the prevailing winds. Also take note of how others are anchored and envision a 7:1 scope to mentally picture where their anchor might be on the bottom. DO NOT drop on top of someones anchor.
If everyone is bow and stern anchored you need to do the same or there will be “swinging” issues! If everyone is bow anchored only please, please, please do not bow and stern anchor. All boats must swing naturally, and in unison. If one boat is bow and stern anchored it will not swing with the crowd and there will be fiberglass on fiberglass contact. Anchoring contradictory to the crowd already there is always rude and inconsiderate. Boats on permanent moorings are generally on a 2:1 scope and will swing around their bows, but will move very little compared to a boat on a 7:1 anchor scope so be careful and stay far enough away from anchoring near permanently moored boats.
In light air, boats with an all chain rode will not swing as far, or as fast, as those using a nylon/chain rode so take note of who has all chain to the deck. Choose your spot and visualize your boat swinging in unison with the others in a 360 pattern. If your spot has you hitting other boats during this 360 visualization exercise find a new one..
#2) Prepare & set: Once you’ve determined your “spot” calculate your scope as described above. For the best results use 7:1 for setting. 5:1 is an absolute bare minimum for setting and should be avoided if you want consistent results. As you approach your “spot” shorten the dinghy painter so it will not foul the prop when backing down. Slide the gear shifter into neutral and gently glide past, and over, where you actually want the anchor to set. Once beyond your “spot” slip it into reverse and get the boat going in a straight line backwards but SLOWLY at perhaps .3 to .5 knots. Lock the wheel or tiller to keep her as straight as possible and walk carefully & slowly to the bow.
#3 Play out the rode: As you begin to move backwards begin playing out the rode. Do not just drop a pile of chain or rode to the bottom, it will tangle the flukes. The rode must be played out while moving backwards, gently and methodically. As you begin to get to about a 4:1 (your rode should be marked in feet or meters) gently snub the anchor for a test bite. This will orient the anchor to a proper setting angle if it has not already happened. If you begin to feel resistance let off your snub and continue playing out line until you hit 7:1+ gently snubbing along the way every now and then. The greater the scope used in setting the better the result and better the odds of a first try set will be.
#4 Setting the Anchor: With the boat at 7:1, with a good test bite on the hulls backwards momentum, let the weight of the boat and the remaining momentum partially set the anchor and come to a stop. Once the boat has finished stopping, and is back to a taught line, not jerked forward from nylon rode stretch, run the engine up to full cruise RPM, usually 80% of max rated throttle, and finish setting or burying the anchor! With small outboards you'll want to use full reverse as they tend to have lower reverse thrust when compared to inboard engines. If the anchor moves or drags you’ll need to start over. No small AUX sailboat engine should be able to budge a properly sized and set anchor for the given boat. If it does you need new ground tackle or need to re-set and try again.
For example 30 knots on a 36 foot sloop is about 900 pounds of force on the anchor. My 36 footer has a 44 h.p. diesel spinning a 16" prop and can only develop just over 500 pounds of reverse thrust at 80% of max throttle or nearly 50% less applied force to the anchor than 30 knots..
This last step, 80% of max throttle, is very important and is one many overlook. Bottoms are often made of “layers” and the top silt layer is easily penetrable and will hold fine in light conditions but not moderate or high winds. You want to dig the anchor into the next layer, the one that is much harder, and will hold even in high winds to be properly set.
I have spent a good deal of time diving on anchors and I can assure you a solid 80% of the anchors out there are not properly set. With CQR’s this is usually represented by a partial sideways set meaning it is laying on its side with the tip partially buried. There was a perfect picture of a CQR doing this in the Sail Magazine anchor-testing article from last year. If you are not back-winding the sails or using 80% of your engines capacity your anchor is not really properly set checked.
#5 Shortening scope: Now that you set the anchor it is somewhat safe, depending on your choice of anchor, and chain/rode configuration, to shorten to a safer swinging scope for the anchorage you’re in. 4:1 is the generally accepted minimum for calm conditions or winds bellow 10 knots. 5:1 can usually be safe to around 14-15 and any wind speeds over that you will generally want more scope. Try and pick areas that will allow you to use the max allowable scope in case of a micro-burst or suden storms or high winds. If you leave your self only enough room for 4:1 you’ll likely get precisley what you ordered, the “disaster plate special with a side order of heartburn and severe anxiety”.
Hope this helps and that I did not forget anything…
