Anchoring Technique ....!
I would be amiss to post a horror story about anchoring: If you don't know how to anchor... DON'T !!!!!! without also giving a reminder of anchoring technique.
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. A short or steep scope angle will most certainly yank the anchor out of the bottom and will NOT hold when the wind picks up!
How do I know what my scope should be? Scope is easily calculated but often calculated incorrectly. Scope is 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 remains7:1. 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, as MANY sailors and boaters do. 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!!!!! 50 feet of rode for a 25 foot scope basis is a VERY dangerous 2:1 scope or almost vertical!! 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 should 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!! An all chain rode is always better but you will need to use a very elastic snubber to prevent shock loading of the chain.
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 and 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) 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 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 size beyond the recommended size. CQR’s or plow styles can give false sets and MUST be properly set! The new generation anchors such as Spade, Rocna & Manson Supreme are generally excellent performers and practically set them selves..
When I mention partially set CQR's this photo is exactly what I'm referring to. This pictured CQR is NOT SET!!!!
#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 someone else’s 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 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 is RUDE and inconsiderate! Boats on permanent moorings will swing around their bows, but will move very little compared to a boat on an anchor so be careful 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!
#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 and gently and methodically. As you begin to get to about a 4:1 (your rode should ALWAYS 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: As the boat approaches a 7:1 put it in neutral and 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! If the anchor moves or drags you’ll need to start over! No AUX sailboat engine should be able to budge a properly sized and set anchor! If it does you need new ground tackle! The average sailboat AUX engine can barely replicate 25 or 30 knots of wind with the engine and most can only replicate about 12-17 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 great deal of time in my life diving on anchors and I can assure you a solid 85% 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 set.
#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 want more scope or at least a 7:1. Try and pick areas that will allow you to use the max allowable scope in case of a micro-burst or wind. If you leave your self only enough room for 4:1 you’ll likely get exactly 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…
Quite the dissertation. I'd have to argue with the 80% power going backwards. That seems excessive to me unless your boat is way under powered. Some boats could drag their anchors around all day with 80% astern power.
You have also neglected to tell folks how they can tell if the anchor is set or dragging. It may not be apparent to some......evidently.
It's interesting to note what the West Marine/Sail Mag test boat used as an engine and prop to try and drag anchors intended for 35-40 foot boats!!!!!! They did not drag them around the bay with a sailboat aux engine!!
"We were able to apply and accurately record how the anchors reacted to up to 5,000 pounds of load, thanks to the 52-foot, 92,000-pound research vessel Shana Rae, which we chartered to do the pulling. Equipped with a 375-horsepower diesel engine and a 40-inch-diameter propeller, Shana Rae allowed us to apply more force on each anchor than it would ever experience in “normal conditions.”
Converting HP to thrust is a complicated science, with all the variables, and far to complicated to go into here. Suffice it to say that depending upon prop and other variables thrust is anywhere from 20lbs to 35lbs. per hp.
So let's take a look at a Universal M-25, a very common sailboat aux engine. At 3200 RPM it develops 21 hp however max RPM is rated at 3000 RPM. At 80% of rated RPM it will develop about 18hp. Now if you figure you have a two blade sailor prop with a 45% efficiency, as many do, and a reduction gear that is half the speed in reverse as forward the equation would look something like this.
18 hp X 25lbs thrust = 450lbs Now subtract your prop efficiency loss and reduction gear X% of forward for a given transmission and you're likely down to a thrust of bellow 200lbs.. If you can drag an anchor around on less than 200lbs of thrust than as I said you have some serious issues..!;)
BTW my old Catalina 36 had an M-25 in it. In 42 knots of wind that boat will be pulling between 1800 and 2100 lbs... Even in 30 knots it's pulling between 900 & 1100 lbs...
When powering back on your anchor, watch your SOG (speed over ground) display on the GPS -- with the rode tight and the anchor set it should read 0.0 knots. If it doesn't, you are dragging or swinging, or something else is amiss -- the COG (course over ground) display can be informative here.
Once your anchor is set, leave your GPS on, zoomed in to it's closest scale. With cookie-crumb tracking on you can see how your boat wanders around at the end of your rode, and when you wake up in the middle of the night can instantly tell whether you've moved from where you started the night. (I sometimes even take a hand-held GPS to bed with me so I don't have to crawl out of my bunk to glance at it...)
Many GPS's can set an "Anchor Alarm," but I find the cookie-crumb tracking to be the most useful.
If you're really clever and coordinated you can punch the "Mark" button at the right time and store the exact coordinates where you dropped your anchor. Then your GPS will tell you bearing and range to that "waypoint" -- if the range is greater than the amount of rode you have out, then you know something is amiss.
Lots of other techniques to monitor for dragging, too, like taking bearings to fixed objects on shore, having a feel for the motion of the boat (rolling side to side vs. pitching fore-and-aft), feeling and/or listening for vibrations transmitted up the anchor rode, staying attuned to other sounds (surf, dogs barking on shore, etc...), and smells (exposed tidal flat mud, etc...), setting the shallow and deep alarms on your depth sounder, etc....
Ok ive got 2 anchors out on my boat. Both are on 200' of line and 30' of 3/8''s chain with 30# bruces on each one. Ive got a max depth of about 35'. Both of my anchors are out, one up river and one down river. Both only have about 15-20' of line left on the boat. They hold great, untill the wind blows across the river, then im screwed.
My boat is a Pearson 36 weighing in at 13tons. All extra windage has been removed save for the boom and bimini. Any ideas to get it to stay put without an extra anchor, other than getting around to putting my mooring down.
Halekai...nice write up but I also disagree with the 80% power rule and the length of chain rule.
I have found that some anchors in SOME bottoms need to just work themselves into the bottom slowly rather than being dragged. I have found that the CQR in particular and Delta's to a lesser extent benefit from some time with light loads on them to bury and set properly particularly in mud or muddy sand bottoms. For years I followed the back down hard advice with my CQR until I learned that all it was doing was giving me a lot of unnecessary exercise. Setting the anchor for me means making sure to snub it at about 1/3 scope to insure it grabs...then letting out full scope to allow it to set naturally. The only time I use the engine is when it is dead calm and I need to put on a light load to get the initial set. I have no objection to a test "haul" in reverse, but I think it should be delayed unless conditions are deteriorating and you need to be sure now. I do keep an anchor watch for at least 1 hour after setting.
While I carry a full chain rode on my primary, I would note that Fortress explicitly states that only 6 ft. of chain is advised with their anchors in 25 ft. of water or less. Danforth recommends 8ft. with their 100lb. anchor. So...fluke type anchors may not benefit from excessive chain. For other types I think at least a boat length is needed.
So...there's a bit of difference of opinion but by and large if everyone followed your instructions, we'd be way better off than with what one can see in almost any anchorage daily!
Great post, very informative! Thanks for sharing!
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