Challenge: Anchoring on Slanted Bottom

[JPA Cate]

G2L:

Your PM box is full, so here is my response, instead.

Thanks for the fill, G2L. Sorry, we don't have any connections in your area.

Assuming you'll be staying in that bay with the sharply sloping bottom. For retrieving, you'll be able to use a line on the anchor chain to a primary winch. Some people use one port, one stbd. You can use a chain hook, or a rolling hitch. It is tedious, because you can only retrieve one deck length at a time. Using this method, you save your back. Remember you can easily get 3 or 4 meters of storm surge from a cyclonic storm, and that your chain scope is your friend. My point being that the scope which is adequate today may not be, tomorrow. [Side question: is there any hope of getting a mooring down before the season starts?]

This is really a hard problem. You don't want to be able to thump the bottom, when the rope rode stretches, and you're limited chain-wise, swinging room-wise, and depth-wise, and this particular spot seems to be the only even partially suitable one in your area. If you can set your 44 lb. Bruce in the 7m. water, you're already short on chain scope......

Maybe you can do the "V" deal, if you can get another anchor. That can limit your swing, but isn't as suited to a 180 deg. wind shift as a Bahamian moor, which will really keep you in the middle, and was described earlier in the thread.

The bow structure of your tri did not look really strong in the pic you posted. Have you considered adding backing plates at the bow cleats? I'm sure you know to strip the boat. Find somewhere to store the sails if they won't fit below.

Are there mangroves around the edges of the bay that you could spider-web to? Then, assuming the Bruce did not drag uphill, the lines might hold against the wind shift.

Keep on asking, maybe someone can come up with an idea for you to keep your boat safe. Offer the details of what you have available to work with. And, it might help if you re-frame the problem, making sure everyone understands the limitations you're trying to work with.

Good luck.

Ann

[Gone2long]

Originally Posted by Ann T. Cate

G2L:

Your PM box is full, so here is my response, instead.

Thanks for the fill, G2L. Sorry, we don't have any connections in your area.

Assuming you'll be staying in that bay with the sharply sloping bottom. For retrieving, you'll be able to use a line on the anchor chain to a primary winch. Some people use one port, one stbd. You can use a chain hook, or a rolling hitch. It is tedious, because you can only retrieve one deck length at a time. Using this method, you save your back. Remember you can easily get 3 or 4 meters of storm surge from a cyclonic storm, and that your chain scope is your friend. My point being that the scope which is adequate today may not be, tomorrow. [Side question: is there any hope of getting a mooring down before the season starts?]

This is really a hard problem. You don't want to be able to thump the bottom, when the rope rode stretches, and you're limited chain-wise, swinging room-wise, and depth-wise, and this particular spot seems to be the only even partially suitable one in your area. If you can set your 44 lb. Bruce in the 7m. water, you're already short on chain scope......

Maybe you can do the "V" deal, if you can get another anchor. That can limit your swing, but isn't as suited to a 180 deg. wind shift as a Bahamian moor, which will really keep you in the middle, and was described earlier in the thread.

The bow structure of your tri did not look really strong in the pic you posted. Have you considered adding backing plates at the bow cleats? I'm sure you know to strip the boat. Find somewhere to store the sails if they won't fit below.

Are there mangroves around the edges of the bay that you could spider-web to? Then, assuming the Bruce did not drag uphill, the lines might hold against the wind shift.

Keep on asking, maybe someone can come up with an idea for you to keep your boat safe. Offer the details of what you have available to work with. And, it might help if you re-frame the problem, making sure everyone understands the limitations you're trying to work with.

Good luck.

Ann

On the inbox - will fix that. thanx.

On the bow structure. Yes, it could be stronger, but it is a very narrow boat up forward, and the cleats are secured well below with bolts and extra wood. The boat is wood over epoxy.

Also, the captain for the last owner survived a major cyclone by tying to a mooring and securing to the mast. Lots of boats broke from their mooring lines, but ours did not. So that's a trick I will keep in my back pocket as well.

We will be somewhere else by mid April, so a mooring is not the ultimate answer for our "down the line" challenges. Got some help aboard recently, so pulling anchor shouldn't be that traumatic. We have a manual windlass, which could serve the same purpose as that winch trick you mentioned, but it has the same drawback - pretty slow going.

On the mangroves: They are too far. We actually do have adequate swing room, but not a lot of room were we to drag toward the shore. Seems like we are hooked well enough for the most likely conditions, but we need to prep for the unlikely.

Thanks for your input.

G2L

[Muckle Flugga]

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

Lay a second anchor, mate. Lay a much deeper and longer chain and your bower in the deeper water, and lay a kedge off the stern into the shallower. Or else lay the bower as suggested and tie a line or two ashore, if close enough.

[Muckle Flugga]

Originally Posted by boatman61

First.. I hate swivels.. the rest.. I'd rather not comment on from a picture.. to much at stake.
Get rid of the swivel and use the largest bow shackle that will fit the chain..

Interesting. What makes you hate swivels?

[Muckle Flugga]

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.

Forgive me, but this just seems like you hadn't set the anchor properly, and is not any reason whatever not to take a line ashore. If you had not set the anchor properly on the flat, the same thing would have occurred, regardless of your shore line. Anchoring with a good long bower up into a slope with a line to shore is not only standard, but also very secure indeed. There is nothing wrong with this style of anchoring. It is the general standard in the med. Just set your anchor correctly next time.

[boatman61]

Originally Posted by Muckle Flugga

Interesting. What makes you hate swivels?

Three years of assisting servicing 80% of the private/club moorings in Poole Harbour.. and salvaging boats that broke their moorings.
We would replace the swivels on the mooring chains/blocks every winter due to erosion from the sand raised by tide/current movement.. shackles however were a different story.. no moving parts.
I know an anchor swivel does not 'normally) get that constant wear to grind it down but it sure as hell opened my eyes to how easily they can fail.
But.. that's just me.. what others choose to do is not my responsibility.

[Gone2long]

Originally Posted by Ann T. Cate

G2L,

Just a few more thoughts....

Part of what you're discovering is that 40 ft. of chain is so short that it's hard to work with safely, for the boat, and I'm assuming you do not have another anchor with some chain on it, due to storage and weight considerations. If you have only what you have what you have writte about here, the idea of anchoring back out in the 22 ft. would be a good idea, or even further out, so you could swing. The problem with the line ashore or to a stern anchor in the shallows using rope rode, is how to place them so that no coral chafe occurs and leaving is easy if it must be done. If possible, dive the setup once it's all secure, and make sure the rope section of the rode cannot chafe on the coral around there. [Yeah, chain chafing on coral is noisy! But rope is silent. ]

wrwakefield suggested 5/8" triple strand nylon for the rode. I would have thought that 1/2" would probably do for a 4 ton boat. However, I have a prejudice against the plastic thimbles. S/s thimble.

I'm wondering if you need the swivel. Some people like them; others, not so much. Jim took ours out of the system. Yours is ready for re-galvanizing, but the chain looks really good. Have you put seizing wire on the shackles?

Hope you can get your o/b sorted fast. It must feel rather fraught, sort of trapped in a not-the-best-situation, and the boat often at risk.

If you have a second anchor aboard, you might try placing it further out towards the deeper water, and lie between the two with the second anchor limiting your swing towards the rocks. The danger with this is that the two rodes might foul each other; however, for short periods (like one wind or tide cycle) we have done it with success and no fouling.

Good luck with finding a creative solution.

Ann

Thanks again for your input. The swivel came with the anchor and rode, and I just assumed it was useful, in order to keep the line from twisting.

Apparently, many folks feel that it is either not necessary or a bad idea.
Why might it be a bad idea? Will ask boatman the same question.

Thanks,

G2L

[Gone2long]

Originally Posted by noelex 77

Two anchors can be helpful to reduce the swing radius (as in the OP's example), but it is important to realise that you will still need a large, or very large scope on the anchor that is holding on the downhill slope. Even this may not be enough. I will explain why.

Note: Without an image of the terrain around the anchor, all we can work on in this example is the average drop in depth from the anchor to deeper water.

In the OP's example the depth drops 12’ in 45’, suggesting the downhill slope near the anchor may be about 15°. Thus if an huge amount of rode could be used and the wind was strong enough to straighten the chain near horizontal, the anchor shank would be pulled 15° to the substrate simply to get the shank horizontal, equivalent to the angle of pull on the shank with a 3.75 :1 scope on a flat substrate.

This suggests that if a strong wind is blowing the boat "downhill", a large anchor with a large length of rode is needed. In addition despite the large mathematical scope, you will need an anchor that performs well on a shortish scope ie 3.75 :1.

It is also worth noting that in situations like this two anchors does not add to redundancy. In fact it creates the problem that if either anchor fails the yacht will drag.

Thanks for the detailed explanation. The math of it all is kind of baffling at this point. Gotta do more research of my own.

Appreciate all the info supplied on the various threads.

G2L

[Gone2long]

Originally Posted by noelex 77

Yes, that is correct. This is the major reason for using a large scope in strong wind. It ensures the angle of pull on the anchor is as close to the seabed as possible.

So on flat ground assuming there is zero catenary, the chain is pulled up at 12° for a scope of 5:1. In preparation for bad weather if you let out 10:1 this decreases to 6°. The anchor's holding ability is greater with a shallow angle of pull closer to the seabed. So the holding power increases as more scope is let out.




No that is wrong. The angle is decreased with more scope but the angle between the chain and the seabed can never be less than 15° (assuming the chain is taught with no catanery)

If we assume an infinite scope (obviously impossible but this is a best case scenario) the rode would be parallel to water surface (ignoring the curvature of the earth ). In diagram form it will look like this. You can see the chain is pulled up at 15° relative to the seabed. On a flat seabed the angle would be zero.



In practice, if the wind is light to moderate, the weight will ensure the chain will sit on the bottom and the pull will be parallel to the seabed, but in strong wind the catenary becomes minimal and the downslope will have a significant effect.

Thanks, the graphic definitely helped me better understand the "math" and "geometry" of the situation.

Regards,

G2L

[boatman61]

Originally Posted by Gone2long

Thanks again for your input. The swivel came with the anchor and rode, and I just assumed it was useful, in order to keep the line from twisting.

Apparently, many folks feel that it is either not necessary or a bad idea.
Why might it be a bad idea? Will ask boatman the same question.

Thanks,

G2L

Check how much play you have in it.. if there's any slop dump it.. if hardly any you should be fine but be aware.. there's no visual sign of weakness.

[Gone2long]

Originally Posted by boatman61

Check how much play you have in it.. if there's any slop dump it.. if hardly any you should be fine but be aware.. there's no visual sign of weakness.

Thanks boatman,

That's exactly the kind of stuff I need to know. Read your response to Flugga. Very interesting.

G2L

[Don C L]

Originally Posted by boatman61

Three years of assisting servicing 80% of the private/club moorings in Poole Harbour.. and salvaging boats that broke their moorings.
We would replace the swivels on the mooring chains/blocks every winter due to erosion from the sand raised by tide/current movement.. shackles however were a different story.. no moving parts.
I know an anchor swivel does not 'normally) get that constant wear to grind it down but it sure as hell opened my eyes to how easily they can fail.
But.. that's just me.. what others choose to do is not my responsibility.

Yes, and somewhere way back in the recesses of my mind I remember reading an article about the strengths of swivels and being shocked how weak they can be compared to the chain and shackles, even if they have the same working load limits, so I haven't considered using them for years. I believe the most common ones usually say something about not being designed to swivel under a load. And are they really necessary? So what if the line and chain spin around a little?

[Muckle Flugga]

Originally Posted by Don C L

Yes, and somewhere way back in the recesses of my mind I remember reading an article about the strengths of swivels and being shocked how weak they can be compared to the chain and shackles, even if they have the same working load limits, so I haven't considered using them for years. I believe the most common ones usually say something about not being designed to swivel under a load. And are they really necessary? So what if the line and chain spin around a little?

Well, the swivel itself only takes full load if all the catenary is lost. I rate Kongs but they are expensive. TBH failing them I would tend to agree with Boatie. However… they do in fact do a good service, which is to stop the chain roll from tripping the anchor, as can happen if it is heavily bunched up and exerting an axial load on the anchor shank. It also renders the likelihood of bunching somewhat lower.

[boatman61]

Originally Posted by Gone2long

Thanks boatman,

That's exactly the kind of stuff I need to know. Read your response to Flugga. Very interesting.

G2L

G2L.. please do not take all my comments as gospel.. in some area's my knowledge may be more limited than my mouth..

[Stu Jackson]

Originally Posted by Gone2long

I appreciate your comments and links, and tell me why you think it might be a bad idea to have the swivel and/or the shackle.

Thank you for your detailed responses.

G2L

Swivels may well be unnecessary.

Ground Tackle & Anchor System Sizing TABLES & Swivels

Usually in boating, more connections and pieces tend to lead to a situation where if there are more "things" more things can go wrong. If it isn't required, it's unnecessary.