Anchor in Shallow water

[Shaneesprit]

I see lots of Anchor questions, but never seen anyone discuss how much rode they use when Anchored in shallow waters?
I am thinking of less than 2 meters deep. Possible 1 or 2 meters of tide.
Obviously in the multihull section, as the fin keels will of fallen over by this point or is that what they say about us? 😂

[Mike OReilly]

Not sure if I'm allowed to comment since I'm once of those monohull folks, but I prefer anchoring in shallow waters. I like it because it means I'm close to shore.

I'm happy to swing in waters with just a few cms between my keel and the bottom. I just came out of an anchorage where my six-foot keel was scraping the bottom at low tide.

[GRIT]

You've always got an open mind Mike, I think you're allowed to comment anywhere!

We also anchor in shallow waters, down to 0.6m at low, unless we want to go aground, which we often do.

In shallow water, we still put out a fair amount of scope, I'll put out 15m in 1m of water. Our anchor roller is 1.7m above the waterline, so we're still basically at 5:1.

We love really shallow waters, the waves are so small, there's usually much less movement, so bouncing off the bottom has never happened. yet.

Cheers.
Paul.

[Don C L]

2 meters deep and 2 meters of tide. Does that mean you are on dry land at low tide? In any case the same rules for scope apply: 5:1 good. But what is the bottom like? I am guessing mud so be sure you have an anchor that does well in mud. Shallow water may have old tires and shopping carts, so I’d use chain. Is this shallow water near an open bay? Will waves be breaking in there if the wind pipes up?

[fxykty]

Take a look at the anchor rode calculator that is posted on another thread here - because there is almost no catenary the amount of rode required to reduce anchor load on the boat is well beyond 5:1 in shallow water. So if you’re expecting more than 20 knots throw out lots, as in 20:1 or 40m in 2m of water!

Or you can follow the 2 boat lengths plus water depth rule; that yields a reasonable result and is easy to calculate.

A stretchy snubber/bridle is especially useful in very shallow water to reduce anchor loads.

[Fore and Aft]

Shaneesprit, we normally let out a far bit of chain. Usually, the anchor is in the deep and then we swing into the shallows. We have a swing keel mono hull and touching bottom never worries us. We love sitting on the bottom in a sheltered spot.
If it's a regular thing you might want to set your toilet sink up with a Y valve so you can divert your sink water into the toilet. We do that so we can keep flushing the toilet into the holding tank, even though we are high and dry.
Cheers

[flightlead404]

Originally Posted by GRIT

You've always got an open mind Mike, I think you're allowed to comment anywhere!

We also anchor in shallow waters, down to 0.6m at low, unless we want to go aground, which we often do.

In shallow water, we still put out a fair amount of scope, I'll put out 15m in 1m of water. Our anchor roller is 1.7m above the waterline, so we're still basically at 5:1.

We love really shallow waters, the waves are so small, there's usually much less movement, so bouncing off the bottom has never happened. yet.

Cheers.
Paul.

Indeed, but as I recall you have kick up rudders?

We've shared two anchorages with a cat named GRIT over the last year, and as I recall it was fairly in the mud in St. Auggies in April.

[Mike OReilly]

Originally Posted by fxykty

Take a look at the anchor rode calculator that is posted on another thread here - because there is almost no catenary the amount of rode required to reduce anchor load on the boat is well beyond 5:1 in shallow water. So if you’re expecting more than 20 knots throw out lots, as in 20:1 or 40m in 2m of water!

Or you can follow the 2 boat lengths plus water depth rule; that yields a reasonable result and is easy to calculate.

A stretchy snubber/bridle is especially useful in very shallow water to reduce anchor loads.

This is true. There is almost no catenary in shallow water. But one thing which appears to be missing in those other anchor discussions is the effect of friction in shallow waters. A heavy chain laying on the bottom produces a lot of drag, shock-dampening, and sheer holding power.

I tend to lay at least 5:1, but rarely over 8:1. The vast majority of the time, I'm only swinging on my chain while in shallow anchorages (under 15' of water).

I agree with you on the need for a good snubber. We always run dual snubbers in a bridle layout off the bow of our monohull. It's excellent for dampening, but it also tends to helps to limit our sailing at anchor (not that my boat does a lot of that anyway).

[MicHughV]

Ha ha, spend any time in the B'mas, and 2-3M (6-9') is a pretty typical anchoring depth. Bottom is usually sand over coral.

I've anchored there a gazillion times and use the same setup pretty much everywhere I go. I use 75' of chain with maybe 10' or rode out, to act as a snubber and so the roller doesn't make a lot of noise. I don't think about it much when I'm anchoring, as I use this same setup pretty regardless of where I'm at.

Anchor selection is important though. For me, nothing beats a Bruce in these conditions.

I've dove on my anchor in about 35 knots of wind and was surprised to see most of the chain laying flat on the bottom, with maybe 10' arcing up towards the boat.

Obviously everybody will have their own anchor and rode selection, but that is what I use.

Setting the anchor is key. The water is so clear there, you can count the blades on single plant. So I like to find a patch of sand with no growth to drop the anchor.

Never had a problem doing this. Occasionally had to use two anchors, but those times were rare.

[GRIT]

Originally Posted by flightlead404

Indeed, but as I recall you have kick up rudders?

We've shared two anchorages with a cat named GRIT over the last year, and as I recall it was fairly in the mud in St. Auggies in April.

Yes, Kick up rudders.

I know of no other Cat named GRIT, so I'll bet it was us. We often anchor in Salt Pond, but usually have a little water under us at low tide. We've also been known to anchor just off the fort, which is also pretty shallow.

If you see us again, and feel like a chat, come on over and introduce yourself. I often dinghy over to other boats to chat; it's part of what I love about cruising.

Cheers.
Paul.

[chris mac]

Shallow anchorages need more scope as mentioned above. An easy way to look at it is,
Put 60 feet of chain on the ground as a minimum. Then add the vertical to your bow roller.
If strong winds or waves are expected, more doesn't hurt.
All through the Bahamas the minimum I put out is 75 feet. It sure makes sleeping easier.

[Tin Tin]

One of the issues for catamarans anchoring in shallow water is that the bridle will often drag on the bottom at low tide. A standard chain hook easily comes adrift when this happens, so a robust hook mechanism that locks to the chain and can not be easily dislodged, but still easily disconnected if in a hurry, is needed. There are many debates about what is best for this purpose. As far as scope goes in shallow water we use 5:1 or 7:1 if weather looks grim. Even so we once had 100ft of 1/2" chain out in 6ft of water and still manage to drag an 88lb Rocna though a sea grass bottom when a 25knot wind reversal hit us. (Lubbers Landing, Bahamas, not the greatest bottom for holding)

[GRIT]

Hi Chris mac and fxykty. As regards "more scope" could you explain "why" for me?

I never really understood the more scope in shallow water thing. I pretty much always anchor with 5:1, deep water or shallow water. I figure the angle to the anchor is the same at 5:1, no matter the depth of the water. I'm happy with 5:1 up to 30 knots or so. If it's forecast to blow more, I'll likely put more out. One thing I should mention is that we're fond of what some would call oversized anchors. Presently, our primary (Ultra) has a mass of 60kg, and our secondary (Spade) 55kg. But regardless of the anchor size, I'm at a loss to understand why more scope in shallow water.

We anchored with less scope on our 39 foot Corbin (3:1), using a 30kg Genuine Bruce anchor. And the same before that with our SS 35, with a 15kg Genuine Bruce.

We cruised full time on each of these boats, from Canada to Trinidad, to the UK and the Atlantic coast of the EU. We always held great, and never drug, once set.

I suppose someone should define "shallow" too. Just so we're all on the same page.

I'm not being argumentative here, and I don't want to start a whole debate, but if someone could either explain to me why they use more scope in less water, or post the link to the thread someone mentioned earlier, I'd appreciate it.

Thank you.
Paul.

PS. To me, 5:1 means water depth plus anchor roller height, then multiplied by five. It offers a pull of, roughly, 11 degrees off horizontal.

[Sailmonkey]

Originally Posted by GRIT

Hi Chris mac and fxykty. As regards "more scope" could you explain "why" for me?

I never really understood the more scope in shallow water thing. I pretty much always anchor with 5:1, deep water or shallow water. I figure the angle to the anchor is the same at 5:1, no matter the depth of the water. I'm happy with 5:1 up to 30 knots or so. If it's forecast to blow more, I'll likely put more out. One thing I should mention is that we're fond of what some would call oversized anchors. Presently, our primary (Ultra) has a mass of 60kg, and our secondary (Spade) 55kg. But regardless of the anchor size, I'm at a loss to understand why more scope in shallow water.

We anchored with less scope on our 39 foot Corbin (3:1), using a 30kg Genuine Bruce anchor. And the same before that with our SS 35, with a 15kg Genuine Bruce.

We cruised full time on each of these boats, from Canada to Trinidad, to the UK and the Atlantic coast of the EU. We always held great, and never drug, once set.

I suppose someone should define "shallow" too. Just so we're all on the same page.

I'm not being argumentative here, and I don't want to start a whole debate, but if someone could either explain to me why they use more scope in less water, or post the link to the thread someone mentioned earlier, I'd appreciate it.

Thank you.
Paul.

PS. To me, 5:1 means water depth plus anchor roller height, then multiplied by five. It offers a pull of, roughly, 11 degrees off horizontal.

I’m curious as well. I’ve had a few instances in the Bahamas, in a shallow anchorage, where I had 45’ish out and a wind reversal put a boat on top of me. The other boat had something like 80-90’ of rhode in the water depth.

[rslifkin]

There was a long thread on here where the effects of catenary, etc. were assesed. The end result was a few things. Chain catenary is far more effective in deeper water. And in deeper water the height change from waves is less significant as well, having a smaller impact on rode angle. So shallow water typically needs more scope to make up for those issues.