Looking for enlightenment as I'm sure I wouldn't be the first to go through this exercise. I normally put out anchor chain equivalent to about 5 times the depth (at high tide). I also run my motor in reverse when at anchor to top up the batteries when needed. If I'm in about 4m of water and have about 20m of 10mm chain out I will drag. If I'm in 10m of water and have 50m of water out I never drag while charging the batteries. My motor is only 30hp and I am only running it at about 1000rpm so I' don' believe I exerting a lot of extra force on the anchor. Well not more that it should be able to handle using the 5:1 formular for anchoring.
So it got me thinking (quite painful as some of you others are aware
) that if I was in shallow water like the pioneer bay at Airlie Beach (3-4m depth) and I was under the illusion that my safe anchoring practices of 5:1 would keep the second love of my life safe, then I'd probably be shortly ringing the insurance company if a strong N/E set in. So I did some calculations (with regard to wind force increase and required rode to obtain the same catenary sag in my 10mm chain) and came up with this. If my anchor rode, when set at 5:1, is the minimum requirement for 10 knts of breeze, then in 20 knts, I would need double the rode out, in 30 knts, I would need about 3 times as much and in 40 knts, I would need about 4 times the original rode calculation of 5:1.
Now I never did lots of calculations for different depths and I have anchored in over 30 knts and never dragged probably using about a ratio of 7:1 so I would say that my existing anchor set up at 5:1 has a higher wind rating then 10knts. But doing the calculations sure has he helped me get my head around the fact that 5:1 in a shallow anchorage means I'm more likely to drag than when using 5:1 in a deep anchorage.
Maybe this is why my mates in their cats who creep as close to the beach as they can and then deploy at 5:1, sometimes have problems with dragging.
Some facts. (If you start with an anchor rode length/setup that is only capable of holding the boat at 10 knts and not allowing for current or wave surge)
The extra wind force on the hull at 20 knts from 10 knts is 3.7 time more, but only requires twice the anchor rode to be let out
The extra force on the hull at 30 knts from 10 knts is 8.38 times more, but you only need 2.8 times the anchor rode let out
The extra wind force on the hull at 40 knts from 10 knts is 14.8 times more, but you only need about 3.8 times the anchor rode let out.
One thing is apparent is that the correlation between force applied to the anchor rode and the required rode length is not linear. The ratio reduces as the force increase and applying the 5:1 ratio works fine in deeper water but not necessarily accurate for shallow anchorages. Maybe each boat and it's associated anchoring equipment has a minimum rode length. I know I can get away with 20m when conditions are ideal and I'm not running the motor, but I wouldn't use the old 5:1 for shallow anchorages in $hit conditions.
It would be good if some one has done all this before or knows where this type of information can be found. For all I know, I could have it all wrong (again).
Things I think about when anchoring...
-Water depth
-Vessel displacement.
-Wind resistance of vessel.
-Enviromentals; Tidal flow, wind speed and seabed (rocky, sandy, grassy)
-Weight and type of anchor, certain anchors are better in certain conditions and some of the new types are great all rounders.
-Chain weight & length, both apply different forces (short link weighs more than long link per meter, but long link with the same weight will add length to your rode).
-Length of rode in total (5:1 in normal conditions and ~7:1 in windy conditions and potentially allow for 10:1. Limiting factors may be proximity to other obticles as the boat can pivot 360degrees plus possible drag during the anchor resetting itself).
-Multiple anchors (can prevent boat pivoting and add holding power).
www.boatus.com//boattech/casey/anchors-and-rodes.asp
^That link has a little chart of holding powers.
Another good link...
anchormarine.com.au/index.php/page/allaboutanchors
I enjoyed watching the 'bag of bricks' slid down the rode that the guys in the 'hold fast' documentary used.
Hello SirJman, thanks for the information and links. I have read a lot of this stuff before and put a lot of it into practice. I suppose what I'm looking for is information that goes along the line of
"The required ratio of rode to depth for 10mm chain to get the same holding capacity under a certain set of conditions (wind, currrent etc) for the following depths
# 3m is 10:1
# 4m is 9:1
# 5m is 8:1
# 6m is 7:1
# 7m is 6:1
# 8m is 5:1
It's the shallow anchorages that seem to blow the 5:1 ratio to bits so maybe I've got to work it out for myself by doing some real life tests
Cheers
I don't think there is any basis for changing the ratio at different depths, except that small errors (in either your depth estimation or your chain measurement) have a proportionally bigger effect. For this reason I will often let out an extra 5-10m to err on the safe side, if in shallow water.
The depth that you use in your calculations should be maximum water depth (at the highest tide you anticipate while anchored) plus the height of your bow above the water. Ignoring this principle would give worse holding at shallow depths, for the same calculated ratio. Does your depth gauge measure depth below the keel, or water depth, and do you take this into account?
I find that 3:1 is adequate in most conditions, ie up to about 15kt, unless very shallow as noted above. It's very rare that I would use much more than 4:1.
5:1 gives you about a 80m diameter swinging circle if anchoring in 8m - pretty hard to find that amount of room sometimes unless you get in before lunchtime!
Bit pointless discussing anchor rodes with out first mentioning what sort of anchor. With a Spade or Rocna you can get away with a shorter rode if you need to.
I use a SS Spade/Ultra [replica].
The idea was not to discuss different anchors, it was really about the length of rode in shallow anchorages. I've talked with guys who use less than 5:1 in shallow anchorages but they also don't drive their anchors in by reversing the boat after first laying down the ground tackle. I'm not saying this is necessary because in places where you have 2 directions of tidal flow, the anchor needs to be able to reset it self anyway. But by driving your anchor in by reversing the boat, you get a more accurate indication of just what your holding is going to be especially when things turn to $hit. I use a Delta but have a Manson Boss (not much fun to store inside the boat) on hand if I want to try something else (plus the CQR I'm not going to mention)
It's open to discussion what sort of anchor....what sort od bottom.
Professional marine books distinguishing anchor rodes and wind strength.
Hi all
A old bloke down here tells a story about another bloke who went for a boat licence that goes like this
Examiner: suppose you are anchored and it starts to blow 30 knots, what would you do
Pupil : i would let some more chain out
Examiner: suppose it blows 40 knots what would you do
Pupil : i would let some more chain out
Examiner : what if it starts to blow 50 knots what would you do
Pupil: I would let some more chain out
Examiner: Now it is blowing 60 knots, what would you do
Pupil: I would let some more chain out
Examiner: and just where are you getting all this chain from
Pupil: The same place you are getting your bl#*!% wind from
Regards Don
I can understand what you are saying about the shallow depths. With the idea, (and I'm no engineer) the sag in the chain and the weight of the chain play a big part in keeping the anchor dug in, and the angle of effort low enough to prevent the anchor lifting.
Therefore, say you have a 1.8m draft, and you anchor in for example, 2.4m at low, and 3.4m at high, 17m of chain and rode is not enough, sweet F.A. actually, to gain the benefit of the flex, and sag to absorb the shock loads that would be induced by a 3,4,5,6 etc tonne boat surging backwards in a gust or strong tide/current.......
Say somewhere about 25m or 30m minimum, subject to wind, boat, chain, anchor style etc, of chain and rode would be required before the advantage of the weight of chain would be advantageous.
Just my thoughts.
Possibly another way to express what I mean, is imagine a 50ft boat, 20 tonnes or so, with a 1.8-1.9m draft,.........anchored in the same depth, 17m-20m of 12mm chain and a good modern anchor is still inadequate........
Anchoring is a highly debatable topic. One needs to compare cheese and cheese, not chalk and cheese.
The most important thing of course is choice of anchor.
Here is a very good article published by Yachting Monthly on various anchors. It is on the Rocna web site so obviously Rocna believe it has proved their anchors to be the best.
That may be the case but what the article does show is that the Manson Supreme is up there with the best of them and is probably the best value for money.
I have used a Manson Supreme once and when we reversed to dig it in, when the chain straightened out we stopped with a jerk. I would trust my life and yacht on a Manson Supreme.http://www.rocna.com/sites/default/files/press/press_0612_wm_ym_testing.pdf
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I reckon anchor type and weight has A LOT to do with it as Ramona mentioned. Take it to the extreme and imagine you had 2000kg anchor on your 30 footer. Won't need much out...
Also vessel weight doesn't really effect the load much either. It's more about windage. None of the guides give weight brackets The anchor is only trying to stop the wind blowing your boat away. Imagine an anchor off the stern and you measure the load on the anchor with some heavy duty scales. Now put a up the headsail . Same boat, same weight, more windage, more pull obviously.The biggest error that can be made in shallow water is not to include the height if the bow above the water. The 50 footer mentioned above prob has a bow roller 2m off the water so in (3.5 m depth plus 2m) x 5:1 comes out close to 28m which is prob about right.
I'm also careful about putting out more than one anchor for the hell of it due to the possibility of them fouling each other. I reckon a really good handful of reverse when setting an appropriate anchor is the equivalent of close to doubling the windspeed up to say 15-20 kts.
I'm not sure what draft has to do with anchoring rode length apart from not wanting to sit on the bottom and then anchoring is optional! :)
Can never have too much chain either. I've got to put out 50m of it before I get to the rope. In 20 kts of sea breeze and 5:1, the chain still snakes on the bottom from where the boat has been wandering around during the day so no load on the anchor at all really.
Anchoring is one of those things you just gotta try on your own boat I reckon. There are just too many variables to calculate it absolutely accurately. If you drag you have to change whatever you can so you don't. My 2 cents but I'm only new at this and willing to learn.
I found anchoring (especially if you're not on board to watch it) is all about peace of mind. If you have the swing room, there's no such thing as too much chain. I used to anchor my 37ft cat in three metres at Redland Bay. I'd let out about 50m of chain (then the bridle), full reverse for two or three minutes swinging the rudders both ways to auger her in to the mud. She never dragged. Year before last we had nearly 60knot winds, the two monos anchored near her were in the mangroves, mine never moved. Serously, it's ALL about peace of mind.
My two cents.
Peter
Select to expand quoteAquanMarine said...
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...I'm not sure what draft has to do with anchoring rode length apart from not wanting to sit on the bottom and then anchoring is optional! :)
My point about depth below the keel is this. To calculate your rode length, are you using what your depth sounder says and then just multiplying by (say) 5? If that's the case, you're wrong if the depth sounder reads depth below the keel, and the error will be more pronounced in shallow water. You'll be less wrong if your sounder reads depth from the surface, but to be correct, you still need to add the height of your bowsprit above the water.
For example, if you are in 3m of water, your bowsprit is 2m above the surface, and you want 5:1, you should use 25m of rode. But if your draft is 2m and your sounder is therefore reading 1m, you'd calculate 5m of rode. Big difference.
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Thanks guys for all the feed back but what Supersonic27 said May help get the thread back on track. Every anchor has a critical angle of pull when it will start to break free. So it doesn't,t matter what anchor you have, if you start to pull the anchor above its critical angle then it will break free. This is where the length of rode becomes important. And from my original calculations I determined that there must be minimum length of rode for every anchor (no matter what type) for the depth of water (plus the height of the anchor roller) you are anchoring in. And this minimum length can be influenced by wind conditions and doing things like running the motor in reverse. I know I can anchor my boat in shallow water with 20-30m of chain out, but there is no way I would get off the boat for a week and leave it to the elements. If I was to put out 100m (all chain) I would be a lot more confident. But there must be a figure in between somewhere that is the minimum for certain conditions.
From what I have been told, a lot of the boats that washed up on the shores of Airlie Beach were actually on moorings. This sort of reminds me of what having a heavy anchor on a short rode would be like.
The displacement of the boat may not affect the anchor in an anchorage with no swell but when a large swell is pumping through, a 20 tonne boat is more of a handful than a 4 tonne boat.
I am a fan of keeping the anchor chain held down as low as possible.
By that, I mean we do not have an electric winch on deck and cannot use massive amounts of chain. The solution for us is to slide down the rode a heavy weight, such as a great big D-shackle attached to a rope and keeping it bobbing just off the bottom. With the angle of the chain now down low,this helps prevent the snubbing which can dislodge the anchor.
There was a mob many years ago selling something called an 'Anchor Buddy'.
This method is probably only good for yachts up to 5t displacement .I upgraded the original 27lb unbranded plough to a 35lb Manson plough and there is a difference! The back up anchor is a Bruce. Have yet to find decent holding in fine sand though.
The slipway here had a few examples of unbranded anchors where the flukes had simply broke off. Not what you want in a gale near a lee shore!
So what we need is the appropriate size anchor with a pivoting shank that will allow the anchor to set, ie allow the arms, wings, flutes to go over centre and dig in but then pivot to say 60 degrees up off the bottom before releasing them again. Then you set it normally at 5:1 but if it's a tight anchorage be able to bring some in to say 3:1 quite safely. Is there such a anchor? Can see myself getting the welder out these holidays!
CQR anchors have a pivoting shank, with a plow shape head including a weighted tip. However CQR anchors arn't good in all beddings. Danforth anchors have no problem bedding, but aren't generally well weighted and don't hinge.
I have a bruce anchor and am very happy with it's value, weight to bedding capabilities.
I've tried my true Bruce, not a crappy badly made copy, tied to my mates 400hp Malibu. Won't let go one bit thankfully.
Select to expand quoteAquanMarine said...
So what we need is the appropriate size anchor with a pivoting shank that will allow the anchor to set, ie allow the arms, wings, flutes to go over centre and dig in but then pivot to say 60 degrees up off the bottom before releasing them again. Then you set it normally at 5:1 but if it's a tight anchorage be able to bring some in to say 3:1 quite safely. Is there such a anchor? Can see myself getting the welder out these holidays!
You need to build a Spade. Chase up the line diagrams or templates and make a plywood copy of the appropriate size and see if it will fit on your bow or anchor locker. If it wont and its probable, do the same with an Ultra. Otherwise work your way down the list of good anchors, Manson Supreme, Rocna etc.
The last thing you need is a pivoting shank. The anchor needs to be able to reset itself easy with out reversing etc. You may not even be onboard when it resets.
