Why Am I Dragging ?

[GordMay]

Paul posted an applicable opinion, back in 2006.
➥ http://www.cruisersforum.com/forums/...hill-2949.html

[lorenzo b]

First of all, thanks for all the help on this.
When I dragged off 79th st at NYC I was in very soft mud in 20 ft of water on a short rode. Ended up going to Flushing, Queens, and pigging out on Asian food for a week.
This time I'm in good sand and I don't think my anchor chain wrapped around the exposed fluke, though that has happened in the past. This morning we added 100 ft more of chain which we had stored so now we are in 10 ft of water, 6 ft to the roller, about 210 ft of 1/2 chain weighing 700 lbs and a 350 lb Northill anchor.
If we drag now I'm going to beach this puppy.
In a severe storm I have an addiitional 400 ft of 1 1/2 1nch nylon rope I can add and hook it all to the padeye at water level. If that doesn't hold I'm just going to have to go with the flow because I'll never be able to pull all that up and stow it in a severe storm.

[MarkJ]

Originally Posted by lorenzo b

now we are in 10 ft of water, 6 ft to the roller, about 210 ft of 1/2 chain weighing 700 lbs and a 350 lb Northill anchor.
.

That should fix it!

Maths is still interesting: 1050 pounds holding a boat of 140,000 pounds... (427 kgs holding 63,500kgs)

Thats 1 unit weight holding 140 units..... on one solitory fluke.....

I'd be asking Manson Suprime for a quote...

[zenigma]

On my 40' sailing vessel I use 80' of G4, 5/16 chain and have 300' feet of rode and apply a ratio of 7:1 scope, including the height from bow to water surface. It kept me off the hard in a 30 knot blow but when it gets iffy, I set a second anchor in a v-shape off the bow. It reduced my swing and gave the slipping anchor a chance to re-set itself. Mind you, it took me 2 hours to get under way again the next day. Good thing is, as sailors we don't have appointments to keep, so it did not matter.

Zenigma

[klem]

The vessel in question is starting to become of a size that it will not act like a cruising yacht at anchor. The large displacement means that the loads on the anchor and rode are very different. The motion tends to be much slower and waves have a very different effect on the hull. Comparisons to cruising sailboats are really not valid in this case.

The most obvious way to see whether you had enough scope out is to look at the rode and see whether it still has a lot of catenary and pulling in the horizontal or whether it is pulling up on the anchor. As long as it is laying on the bottom for a little bit before the anchor, there is enough scope for the conditions. It does not take much force at all to take the catenary out of 1/2" chain but the conditions described are really not that extreme.

I suspect that one of the problems was related to the holding bottom. A northill has a very small surface area and does not hold well in soft mud or sand. In addition, smooth ledge is a very bad situation for any anchor since they rely on mass alone to provide holding power. These anchors do well in bottoms that have a lot of structure like really thick mud.

Another problem may well be that you fouled the anchor with the chain. This type of anchor is very prone to fouling with tide or wind changes. It is also prone to fouling when it is dropped, if it flops the wrong way and then need to turn around.

Another issue that people occasionally encounter in places with lots of current is that the water can scour out the soil around the anchor.

I don't see anything inherently wrong with your anchoring setup except that the total rode sounds very short and won't give you that many options. It will limit the bottom types where you can count on the anchor holding well. I have spent around 1000 nights at anchor on a 125 ton vessel with a 500lb fisherman anchor and 3/4" chain and it has been the most reliable anchor setup I have ever used. Only in winds over 30 knots did we ever use more than 3:1 scope, most nights were spent with something like 80' of chain out in 30' of water with the hawsepipe 10' off the water. The reason that it worked so well was due in large part to only anchoring on good thick mud and rocky bottoms. If you are going to spend a lot of nights on soft bottoms, it may make sense to look at either a much bigger anchor or a different type of anchor.

[Dockhead]

Originally Posted by klem

The vessel in question is starting to become of a size that it will not act like a cruising yacht at anchor. The large displacement means that the loads on the anchor and rode are very different. The motion tends to be much slower and waves have a very different effect on the hull. Comparisons to cruising sailboats are really not valid in this case.

The most obvious way to see whether you had enough scope out is to look at the rode and see whether it still has a lot of catenary and pulling in the horizontal or whether it is pulling up on the anchor. As long as it is laying on the bottom for a little bit before the anchor, there is enough scope for the conditions. It does not take much force at all to take the catenary out of 1/2" chain but the conditions described are really not that extreme.

I suspect that one of the problems was related to the holding bottom. A northill has a very small surface area and does not hold well in soft mud or sand. In addition, smooth ledge is a very bad situation for any anchor since they rely on mass alone to provide holding power. These anchors do well in bottoms that have a lot of structure like really thick mud.

Another problem may well be that you fouled the anchor with the chain. This type of anchor is very prone to fouling with tide or wind changes. It is also prone to fouling when it is dropped, if it flops the wrong way and then need to turn around.

Another issue that people occasionally encounter in places with lots of current is that the water can scour out the soil around the anchor.

I don't see anything inherently wrong with your anchoring setup except that the total rode sounds very short and won't give you that many options. It will limit the bottom types where you can count on the anchor holding well. I have spent around 1000 nights at anchor on a 125 ton vessel with a 500lb fisherman anchor and 3/4" chain and it has been the most reliable anchor setup I have ever used. Only in winds over 30 knots did we ever use more than 3:1 scope, most nights were spent with something like 80' of chain out in 30' of water with the hawsepipe 10' off the water. The reason that it worked so well was due in large part to only anchoring on good thick mud and rocky bottoms. If you are going to spend a lot of nights on soft bottoms, it may make sense to look at either a much bigger anchor or a different type of anchor.

For what it's worth, I'm going to disagree with almost everyone here and agree with this comment.

I do NOT think you are dragging because of inadequate scope. You've got better than 6:1 which should be plenty for those conditions. We Yanks have a fetish for scope and consider 7:1 and 8:1 normal for even benign conditions. Over here 4:1 is quite common, and is necessitated sometimes by very huge tides. People don't drag more here than they do in the U.S.

I would look at the bottom conditions and look at the anchor. The anchor's huge size ought to make up for any defects in its design, but something is not working.

Maybe the bottom is simply poor. I massively upgraded my ground tackle inspired very much by problems I had in one particular place. Afterwards was amazed to find that even with state of the art and oversized ground tackle I still had trouble setting the anchor. The answer is a difficult bottom.

That being said, of course 100 feet (30 meters) of rode is far too little for such a vessel. Surely you don't have the luxury of alway anchoring in 12 feet of water. I anchor in waters where there are tides of almost 50 feet in some places (Gulf of St. Malo). I have 100 meters (330 feet) of 1/2" chain on board my boat, which is much smaller than yours at about 24 tons. If I were you I might look at the whole system -- anchor, rode, windlass. Remember that it has been conclusively (in my view) proven that there is no disadvantage other than susceptibility to chafe of a rope rode, compared to chain. Maybe you could just add a few hundred feet of rope to your existing chain.

[ActiveCaptain]

Originally Posted by Dockhead

I do NOT think you are dragging because of inadequate scope. You've got better than 6:1 which should be plenty for those conditions.

Perhaps that's why you might not be agreeing with the previous posts. The way I calculate it:

12' of water + 6' of bow height + 5' of tide = 23' of total height. And I'd bet the bow roller is actually a little higher - that boat looks too big to have a bow only 6' off the water. But still, 100' of chain / 23' of total height gives 4.3:1 scope. That's just not enough when the OP complains of dragging in conditions of 2-3 knots of current like he's experiencing now. A trawler also generally has a larger profile to the wind than most narrower sailing vessels accounting for even more force against the anchor. And I know the winds around Beaufort, NC have been in the 20+ knot range because I'm anchored about 45 miles away in the same conditions right now.

[Dockhead]

Originally Posted by ActiveCaptain

Perhaps that's why you might not be agreeing with the previous posts. The way I calculate it:

12' of water + 6' of bow height + 5' of tide = 23' of total height. And I'd bet the bow roller is actually a little higher - that boat looks too big to have a bow only 6' off the water. But still, 100' of chain / 23' of total height gives 4.3:1 scope. That's just not enough when the OP complains of dragging in conditions of 2-3 knots of current like he's experiencing now. A trawler also generally has a larger profile to the wind than most narrower sailing vessels accounting for even more force against the anchor. And I know the winds around Beaufort, NC have been in the 20+ knot range because I'm anchored about 45 miles away in the same conditions right now.

4:1 will hold. It's not ideal but it will hold. And he's got that only at the peak of high tide. His problem is not scope.

[MarkJ]

Originally Posted by Dockhead

4:1 will hold. .

Originally Posted by ActiveCaptain

That's just not enough when the OP complains of dragging in conditions of 2-3 knots .

It will : It won't : It will too....

Well lets have a better look at the 15 knot wind and the 3 knot tide on a 70 ton blob of tin.

Can't some mathmatical genius determain the force that wind and current is gunna have?

It must be immense! And I would have thought the current being 95% of the problem. (and as Klem discussed the reversing tide)

What does a 70 ton full displacement boat draw? 6 feet? (water to bow must be over 6 feet, btw.) in 12 feet of depth all the moving water is pushing against the boat - its got no where else to go.

[mintyspilot]

Originally Posted by MarkJ

It will : It won't : It will too....

Well lets have a better look at the 15 knot wind and the 3 knot tide on a 70 ton blob of tin.

Can't some mathmatical genius determain the force that wind and current is gunna have?

The wind force will be proportional to the cross-sectional area of the boat normal to the wind direction.

The density of air (in modern units - americans please take note) is 1.2kg/m3 and 15 knots = 7.5m/s (approx). A cube of air 1 metre on a side will have energy of 1.2 / 2 * 7.5 *7.5 Joules = 33.5 J.

Since the air is blowing at 7.5 m/s that is 7.5 cubes of air giving up 33.5 J of energy to each square metre of boat cross section = 252N.

If we assume that the boat sits "nose to the wind" and presents a profile of 10 feet high and 12 wide = 120 sq feet = 11 sq m then the force of a 15 knot wind would be 11 * 252 = 2772N which is approx 1/4 of a tonne of force.

If it gusts up to 25kts = 12m/s which changes the force to about 1.1 tonnes

At 35 kts it gets equivalent to around 3.5 tonnes of force.

Note how quickly the force increases with wind speed.


---- Assumptions ----

The numbers given will be too high because of the following assumptions made:

1. I have assumed that the air transfers all its energy to the boat surface
2. I have assumed that the boat is represented by an ideal cross section normal to the wind
3. I have negelected any rotor effects, bernoulli effects or form drag.

We can now apply "Finangle's Formula" and guesstimate tha the actual effect will be (say....) 1/2 the calculated force. Nonetheless, the force will still increase very rapidly as the wind speed goes up.

[lorenzo b]

Update
Adding another 100 ft of chain seems to do the trick.

[sabray]

thats what Im thinking without the math add rhode

[gcaptain]

I agree with my friend Jeff Siegel... the problem is not with your anchor but your rode. You have a few options (pick 2 to be safe):

1) Lengthen the rode. Longer is always going to be better and it does not matter if you lengthen it with chain or nylon line but I'd prefer the latter as it will give you added elasticity and, depending on the bottom, help keep your anchor from jerking loose. Personally I've dragged anchor a few times on large boats with a 5x (5:1), even a 7x (7:1) scope, but never when I've laid out a 10x (10:1) rode.

The problem is this won't work in an anchorage exposed to the current because, when the current shifts 180, you'll start bouncing into the boats around you which are on a shorter scope.

2) Add heavy chain between the anchor and your existing chain. You want to weigh down that end so the angle between the seabed and your rode is as small as possible. This way when the anchor gets pulled it digs deeper rather than pull out.

Add 20'ft of something heavy and toss it over. That last 20' will probably be too thick for your windlass so, what I've done is to tie a retrieving line from the anchor up 25' and fastened to the chain (I use a zip tie, but that's me). Heave up the anchor until you can get the retrieving line to your windlass then pull the last 20' up by the soft line and secure the anchor... you can either remove or secure the heavy chain.

3) Add a second anchor to the equation. Personally I try to avoid this as retrieving two tangled anchor lines takes some patience (run it about 45 degrees off the first to help avoid this)... but I always have the second anchor ready to go.

My set-up is a 45lbs anchor with 20' of thick chain and 150' of 1/2" nylon and good chafing gear (as your local fire department to save you a few feet of hose the next time they replace it!). And I know your thinking 1/2" is way to thin but you'd be surprise how strong nylon is once you've removed chaffing from the equation (have you ever heard of an anchor line parting mid-way down?). But your boat is larger than mine so feel free to go with 5/8" but remember... if it's too heavy and cumbersome you'll end up leaving it in the locker next time you drag.

4) Grab the snorkel gear and dive down to take a look. You'll probably figure out quickly what is going wrong and might even be able to seat the anchor by hand.

This one I do whenever possible as it's both fun and worthwhile. What I don't do, but I hear works well, is take the opportunity to tie a second anchor to the first and digg it in 10-20' ahead of the first.

5) If it doesn't feel right, try again. We've all been here: You drop the hook with plenty of scope and backed the engines to dig in the anchor. She holds fine so we call it good and head below... but still it just doesn't feel right.

If you find yourself in this scenario pick-up the hook and try again. That voice in the back of your head is like the shy nerdy kid at school... it never shouts for attention or argue but it usually does have the right answer!

6) Reduce the sail area. Remove the bimini, take your sails off the boom and clear the decks. Bring all the gear below, you'd be amazed at how much wind a bimini can "catch".

If possible stow or swap your dingy too.

--
But the truth is their are no foolproof ways to anchor a boat, even the best sailors drag once in a while... so the next two are not really solutions to your problem but will help keep you out of trouble the next time you do drag.

7) Tie the bow of your inflatable to the bow of your boat and leave it ready. Dragging anchor isn't a problem as long as someone is awake and the engine starts on the first try. Having the dingy ready tow you will keep you out of trouble (remember to tow bow-to-bow with the dingy in reverse!).

8) Let out the 100' feet of chain and clip it to a good bouy, then attach another 50' of line. This won't really solve your problem but is an effective early warning device as the bouy will sink underwater before your boat starts to drag.

Last get out into a clear harbour and drop your anchor down to 2x the water depth then practice your shiphandling skills. 2x is not enough to hold you but is enough to practice maneuvering with a dragging anchor (P.S. you'll have extra force on your rudder while pulling in reverse and, when going forward, your pivot point will move up to the bow).

[MarkJ]

Originally Posted by gcaptain

4) Grab the snorkel gear and dive down to take a look. You'll probably figure out quickly what is going wrong and might even be able to seat the anchor by hand.

Lots of people say this, but in my memory there have been very few anchorages where that is possible. The one in the OP situation sounds impossible (or unsafe) too as he would be swimming with a 3 knot current, going down the chain I feel is risky and will only take up down, not along.

The anchorage here the boat next to me lassoed a rock in 30 feet water and he tried to snorkel it - visibility about 3 feet.
Then he suggested I use his SCUBA gear to go untangle it. I suggested he go have sex with himself. I'M just a 'normal' SCUBA diver, I don't go where I can't see and I don't go play with chains and other things like that. I like SCUBA to go Wow at pretty coral.

Oh for the days of 10 foot crystal clear water....

Is it a cruising trueism that if the water is clear enough to see the anchor its neatly dug in already?

The anchorages with all the space junk are deep dirty murkey and scary!


Mark

[donradcliffe]

If he dragged at 79th st in New York City, I understand--been there, done that with a good anchor and a 8:1 scope. Forget diving on the anchor there--even if you can hang on in the 3 knot current, the visibility is zero, and you're going to have to get shots for every disease known to mankind.