Challenge: Anchoring on Slanted Bottom

[Gone2long]

Originally Posted by boatman61

Regarding anchoring I ALWAYS circle the anchorage area to verify the charted depths concur with actual depths.. storm erosion, shifting banks and bars etc.. then I will pick my spot knowing what my surrounds are and possible get outa jail free routes.
Regarding your chain.. if your concerned about your splice at the shackle (bad idea 2 fail points in one) I would definitely run another line and make a bridle.. can be managed solo by hauling in first one half then tie off and take in the slack on the other till the shackles on board.. disconnect the bridle then deal with the chain.. IF you feed into an open locker.
One thing I would definitely do ASAP however is get rid of that shackle and do a proper tapering splice direct to the chain.

Roger the above. Thanks.

G2L

[Gone2long]

Originally Posted by cruisersfarm

If you don't have the right gear (only 45' of chain?? no rope rode? nothing?)

Anchor somewhere else.

Then, dock the boat, go ashore and get a job for a while.

Proceed to the local chandler and buy the following:

1) An anchor appropriate for a 40' cat (windage)
2) At least 100' of chain
3) Another 200' of 3/4" or so nylon rode (splice that to the bitter end of chain)
4) A bridle setup if desired

Basically, the question is similar to, "how do I drive a nail into solid oak with this stick of butter?"

Thanks for your note, but your reply seems to indicate that you have not read the entire thread. Please read the entire thread to better understand my situation. Also, the boat is a bit different than most might expect. It only weighs 4 tons.

G2L

[barnakiel]

Over slanted bottoms, you drop the heavy one in the deep then the light one in the shallows. Alternatively, you can drop the heavy one as above then forego the light one and tie to a rock, a tree, etc.

Level: beginner.

b.

[cruisersfarm]

Originally Posted by Gone2long

Thanks for your note, but your reply seems to indicate that you have not read the entire thread. Please read the entire thread to better understand my situation. Also, the boat is a bit different than most might expect. It only weighs 4 tons.

G2L

Displacement isn't the factor. A 40' cat has windage. 25 knots is nothing. You're going to drag when you get a storm through. Especially at this 5:1 scope.

Reread carefully. Noticed that you dripped additional info in with each post, changing the situation.

You need to grab a book or website link on on marlinespike seamanship as well, while at that chandlery.

You can't increase scope because you can't splice your rope rode into the chain?


There is nothing at all safe about this anchoring setup. A wreck waiting to happen.

[wrwakefield]

Originally Posted by Gone2long

OK,

So everyone has probably heard the various formulas: 5 to 1 rode, more in bad conditions, chain as long or longer than your boat, etc. And, we have heard the various disagreements about some of the formulas.

My challenge is as follows: Lets say, you are going all chain. Its a 40 foot multihull with only 3' draft, and you are in 10 foot of water, with a 35 pound, old style Bruce anchor.

You have 45 foot of chain out and you can't let out more because 45 feet in one direction, the water is only about 6 foot deep, and you are seeing 3-4 foot tides and some rocks on the bottom here and there. In the other direction, the water drops off to about 22 feet, only 45 feet away. You have swung 180 in both directions and continue to swing with radically changing, winds which vary from about 10 -25 knots.

Is your 45 foot of chain enough to hold you when you swing into the deeper water and get hit with 25 knot gusts? If not, what is the solution?

Got some ideas, but hoping to hear from more experienced folks.

Thanks,

G2L

G2L,

You have received lots of very good advice already, and perhaps even already advised about what I'm going to mention...

I have had good success is similar anchoring situation using a Bahamian moor.

i.e., set a stern anchor/kedge [in leu of a shore fast] and bring its rode to the bow- attaching to the main anchor rode. Center the boat between the 2 anchors and stay in one place [relatively speaking.]

I run the stern/kedge anchor warp through a thimble attached to a line secured to the main anchor chain, then over the bow roller to a cleat. That way the stern kedge is attached to the main anchor rode, but still adjustable. Or you can always just tie the stern/kedge anchor warp directly to the main rode with an icicle hitch.

This will help protect you from dragging downhill with your main anchor when you get those 180° wind shifts, and still allows you to point into the wind.

In case this is useful.

Cheers!

Bill

[Nicholson58]

We have done this in fresh water, no tide, no big seas but savage wind. We set one anchor in 50 feet and a stern anchor on the beach. - Then used a winch to stretch us tight between them. Other times, set the primary shallow and a second anchor deep on the slope to keep us off the beach. If you can't make it work - move.

[wrwakefield]

Originally Posted by Nicholson58

We have done this in fresh water, no tide, no big seas but savage wind. We set one anchor in 50 feet and a stern anchor on the beach. - Then used a winch to stretch us tight between them. Other times, set the primary shallow and a second anchor deep on the slope to keep us off the beach. If you can't make it work - move.

Nicholson,

We also prefer a beach kedge to a shore fast using a tree or rock...

The only time we prefer a shore fast over a beach kedge is if we can loop it around a tree and back to the boat- eliminating the return trip to shore before shoving-off... [Remember, I'm lazy...]

Up here, the shore fast to tree [or boulders...] is more usual since we have few beaches that lend themselves to kedge anchors... [rocky instead of sandy...]

Thankfully, we don't often require either; preferring to anchor in deeper water [40-90 ft] when possible.

Cheers!

Bill

[barnakiel]

Originally Posted by Nicholson58

We have done this in fresh water, no tide, no big seas but savage wind. We set one anchor in 50 feet and a stern anchor on the beach. - Then used a winch to stretch us tight between them. Other times, set the primary shallow and a second anchor deep on the slope to keep us off the beach. If you can't make it work - move.

+1

Yep.

Where this can't be had, you want a crew on the boat when you go ashore - boats can easily drag off shelves and slopes if anchored with one anchor only, and go sailing without their crews!

We have 'rescued' a very fine Ovni in this situation in St Pierre, Martinique. She was about half a mile offshore when we found her drifting towards Panama with her chain hanging vertically from the bow and her anchor swinging free some 100' below the boat ... ;-)

b.

[Stu Jackson]

Originally Posted by cruisersfarm

<SNIP>

Reread carefully. Noticed that you dripped additional info in with each post, changing the situation.

You need to grab a book or website link on on marlinespike seamanship as well, while at that chandlery.

You can't increase scope because you can't splice your rope rode into the chain?

There is nothing at all safe about this anchoring setup. A wreck waiting to happen.

If you can't make a splice, then have you considered tying an anchor hitch for the time you're there?

Anchor Hitch | How to tie the Anchor Hitch (Fisherman's Hitch) | Boating Knots

There are many knots that you should know, especially if you can't splice.

Good luck.

[deblen]

Deeper water is "calmer" water. You may get an uncomfortable swell in deep water,but you won't have to suffer the backwash of confused waves created by the sea running abruptly up a sloping bottom & piling back on itself as it tries to seek it's own level. It's like being in a tidal current whirlpool-& then add wind-nasty! Think bathtub slosh.
Of course,we sometimes have to work with whats provided,but avoid shallows if possible if there is going to be any weather.

You could ballast one side of boat o match the slope of bottom,but you would have to lay parallel to the slope at all times.

Cheers/ Len

[Rapanui]

The problem of determining enough scope on sloping ground would seem to be most easily visualised as geometry. If you desire the same holding power on a sloping bottom as that that you would get on a level bottom, then you need the same angle between the line of the anchor stock and the seabed. In the worst case of strong wind or tide, the catenary of the rode dissapears and the rode is more or less straight, and therefore the problem reduces to straight line geometry. As an example, it is quite easy to show that if you require 5 x depth scope on a flat bottom, on a sloping bottom of 1:10 pulling down hill, you will need ~ twice the scope i.e.10 x depth. The other way round you would need half the scope i.e ~2.5 x depth.
Most anchoring is done close to the shore, and so a sloping sea bed is commonly met. It is difficult or at least time consuming, to work out the slope of the seabed in practice so we compensate by increasing the scope in all cases by a personal factor determined by negative experience i.e. dragging.
The good news is, when that cozy anchorage under the lee of the land turns in to a weather shore, your anchor should hold better - provided it can cope with a 180 degree windshift and reset itself!

[facciatosta]

Originally Posted by noelex 77

Another alternative would be to stern tie to shore. Here the anchor will be kept on an upslope. This would significantly improving the effective scope.

Tying stern to shore is not good idea.
Happened to us in exact same situation - very slanted bottom . It was catamaran too. We dropped anchor about 100 feet and tied stern to shore( did not loop back because rocks were too far). Drank aperitif( as usual) then lunch, afternoon nap, 3 hours later wind picked up and drove catamaran to the shore dragging anchor. We started engines but could not raise the anchor or move cat away from shore because anchor chain and rope both were in tension. We had to to cut rope in the hurry and motor around the anchor. As mentioned above it is better to loop rope back for quick recovery, still there is only so many seconds to start motor get rope loose etc.

What I would have done differently - I would not anchor in such situations unless it is really necessary and then keep watch all the time and be ready to move.

[belizesailor]

Originally Posted by Gone2long

Thanks, especially for the final comment. That makes sense, and I have seen a lot of boats do that around here. To add some detail, I should note that we have held here for over a month in 25k winds, swinging all around, and one reason I have not put out a stern anchor or a line to shore is that I was worried about what might happen if the boat tries to swing into the wind when tethered to shore, especially in a 2' swell and 25knot wind pushing us around to wind, with a tide coming from the same direction. What is your take on that?

One of the reasons we may have held may be that there is a lot of coral and rocks around, and, when I dove the anchor, it seemed to be holding in sand, just a foot or so on the deep side piece of coral, so it is probably hooked under that coral as well.

Surrounded the anchor are other pieces of coral and rock, so when we swing deep, the anchor still holds. Granted, this is not the best of all possible worlds, so, we are definitely looking at going deeper, where we can let out more rode, and perhaps avoid the rocks, etc.

Thanks for your insights,

G2L

Geez, sounds like a really crappy anchorage...why not just go somewhere else?

[noelex 77]

Originally Posted by facciatosta

What I would have done differently - I would not anchor in such situations unless it is really necessary and then keep watch all the time and be ready to move.

As you point out, there is often little margin for error when tying the stern to shore. Even a short drag can sometimes rapidly put you in trouble, but I would not dismiss the option entirely. With good anchoring gear in the right situation it is quite safe. It helps to have an anchor that does not move backwards very far as it digs in deeper.

There are some factors that make the anchor more secure:

• The anchor is generally pulling uphill
• The direction of pull is constant
• A very long scope can be used (no swinging room is used)
• Tucked in close to shore the wind is usually less

The wind direction is important. Blowing directly from the stern is ideal. A beam wind can cause real problems. Boats have much more windage and anchor forces can become very high.

Your suggestion of doubling lines and leading them back to the boat is good advice for all mooring lines. However, I don't always do this.

I usually use old halyard/sheet lines and I would cut these if I needed to leave in a hurry. I have not needed to do this so far and I think it would be a very rare event. Normally be possible to return and retrieve these later (I would be more worried about the pollution rather than the loss of rope).

If the line is long I have lights /old fenders attached for visibility. Plus I often use chain for chafe protection around rocks or canvas around trees. This loss of these items would hurt more, but they are not expensive (the lights for example are only a few dollars each). A system to retrieve this gear in a hurry from the boat if an emergency occurred could be rigged, but there are some problems, the chain for example would likely jam on the rocks. The retrieval would also take some time which is not ideal in an emergancy.

[Gone2long]

Originally Posted by facciatosta

Tying stern to shore is not good idea.
Happened to us in exact same situation - very slanted bottom . It was catamaran too. We dropped anchor about 100 feet and tied stern to shore( did not loop back because rocks were too far). Drank aperitif( as usual) then lunch, afternoon nap, 3 hours later wind picked up and drove catamaran to the shore dragging anchor. We started engines but could not raise the anchor or move cat away from shore because anchor chain and rope both were in tension. We had to to cut rope in the hurry and motor around the anchor. As mentioned above it is better to loop rope back for quick recovery, still there is only so many seconds to start motor get rope loose etc.

What I would have done differently - I would not anchor in such situations unless it is really necessary and then keep watch all the time and be ready to move.

Thanks. A sobering experience. As some have suggested would be a better solution, I am looking for a new location, but we would need our dingy outboard to be working before we could move.

Regards,

AB