Upgrade chain size or use anchor angel?

[RupertW]

Not having a windlass. I used to keep the kellet nearer the boat, so when it came to weighing anchor, you don't have to lift chain at the same time as the kellet.

That's why the site linked to in the earlier pages is so fascinating. Neither the weight of a chain or of a kellet/angel is much use once it's more than about a quarter of the way from the anchor to the boat. At half way it's pointless given his worked example, hence the clear preference for mixed chain and rope, with a 10m rope length and hook for when there is chain still left in the locker for the scope you want.

It's made me rethink some of my current tactics and will look a bit further at the calcs and other views on the same topic.

 

[John Barry]

It's made me rethink some of my current tactics and will look a bit further at the calcs and other views on the same topic.

If you're interested in the calculations the main formulae for catenary curves are:
a = weight per unit length / horizontal force
y = height (sea bed to bow roller)
x = swinging circle (anchor to bow
)y = (cosh(ax)-1)/a
length of rode (s) = sinh(xa)/a

For horizontal force you can use Alain Fraysse's formula of F=0.0089L^1.66V^2 where L = boat length in m and V = windspeed in Kn

These can all be easily modelled in a spreadsheet - have fun.

 

[RupertW]

If you're interested in the calculations the main formulae for catenary curves are:
a = weight per unit length / horizontal force
y = height (sea bed to bow roller)
x = swinging circle (anchor to bow
)y = (cosh(ax)-1)/a
length of rode (s) = sinh(xa)/a

For horizontal force you can use Alain Fraysse's formula of F=0.0089L^1.66V^2 where L = boat length in m and V = windspeed in Kn

These can all be easily modelled in a spreadsheet - have fun.

That much I understood, I think, but interesting new bit to me was where the weight per unit length is not uniform. In other words with a chain for only part of the rode, or to simplify the calculation a "weightless" rode with a kellet at different places on the rode. The calculation above for a uniform weight per unit length is like having the kellet 50 percent of the way along, but moving the weight down towards the anchor decreases the anchor angulation according to AF and therefore increases holding power and catenary.

That's the bit I'm struggling with as that is different to my understanding of catenary forces. If that really is true then it would imply that the best place for extra weight is on the first link if chain by the anchor and that just doesn't make sense to me or anchor designers would put a hefty great weight there.

Great site and I need to do it justice before I even think of changing tactics from my current simple all chain.

 

[John Barry]

There are two ways to model a rode with mixed chain warp

1: You can model the chain and treat the warp as a weightless extension. This means that you need to decide the angle at the warp chain join, for which you'd probably use the natural angle at the end of the chain. This means that although you're modeling a horizontal force you have an angular force, but you're modeling warp with no weight, whereas of course it does have a weight. Generally these errors cancel each other out.

2: You can split the model into discrete elements (I think that Alain Fraysee's spreadsheet uses around 500 elements) and you calculate the weight of each element separately. This means that you can model any warp / chain / kellet combination in discrete sections. Each section is modeled as a catenary with a given specific weight.

Going with option 2 gives more accuracy but it's not practical (with a spreadsheet) to do more than model one specific arrangement (chain weight and length, warp weight and length, kellet weight and position, depth, horizontal force on boat). Using option 1 allows you to model multiple variables more easily (in a spreadsheet) and produce tables using the fixed elements (chain size and length, boat size) and the variables (windspeed and depth).

 

[lw395]

That's why the site linked to in the earlier pages is so fascinating. Neither the weight of a chain or of a kellet/angel is much use once it's more than about a quarter of the way from the anchor to the boat. At half way it's pointless given his worked example, hence the clear preference for mixed chain and rope, with a 10m rope length and hook for when there is chain still left in the locker for the scope you want.

It's made me rethink some of my current tactics and will look a bit further at the calcs and other views on the same topic.

I disagree.
Nearer the anchor it is more effective in getting the rode horizontal at the anchor, but closer to the middle it gives more 'spring' to the rode, i.e., the bow of the yacht can be lifted more by a wave with less increase in tension on the rode.
It also does more in moderate weather to damp the yacht's motion.
It depends what the problem is and what you are seeking to achieve.
I was concerned that my boat wandered around a lot on a mostly rope rode when anchored in shifty conditions, and reducing the chances of dragging in lightish conditions when you can't use enough scope because you don't have the anchorage to yourself.

Given that matey's spreadsheet is talking 11:1 scopes and the RYA talks about 3:1, there would appear to 'scope' for looking at the problem from different perspectives.
I never seek to anchor through a gale, but I think given my boat with two undersized (maybe?) CQR's, I'd use them in tandem, the second one can be viewed worst case as a kellet close to the first.
11:1 is rather a lot of string at HW down west or in Newtown.

I've got some diagrams and sums somewhere not on this PC though.

 

[lw395]

see the East Head thread on the other forum....

 

[ex-Gladys]

....Given that matey's spreadsheet is talking 11:1 scopes and the RYA talks about 3:1....

Sorry all chain a minimum of 4:1, chain and warp a minimum of 6:1...

 

[lw395]

Sorry all chain a minimum of 4:1, chain and warp a minimum of 6:1...

Obviously we've had some inflation since I did my theory course
(maybe these modern anchors need more scope?)
But 4:1 is quite a different picture to 11:1

By the logic of weight being ineffective except close to the anchor, it should not matter whether the bit near the boat is rope, so the scope needed shouldn't change much.

My point is, there are a lot of variables.

 

[Conachair]

Well, after spending a while with that spreadsheet ages ago my 20kg angel/kellet/heavy lump of metal went over the side. Logic being that when you really need it then it won't actually do anything of benifit and then you are then in a potentially dodgy and dangerous situation of having to get a big lump back onboard before doing a runner from the anchorage instead of just the chain and anchor.

Not many things in their favour as far as I can see.

 

[Chris_Robb]

Well, after spending a while with that spreadsheet ages ago my 20kg angel/kellet/heavy lump of metal went over the side. Logic being that when you really need it then it won't actually do anything of benifit and then you are then in a potentially dodgy and dangerous situation of having to get a big lump back onboard before doing a runner from the anchorage instead of just the chain and anchor.

Not many things in their favour as far as I can see.

Plus 1! Prefer to take up the shocks with a nylon snubber - just so much easier to deploy - and works - no calculations necessary to prove.....

 

[John Barry]

Sorry all chain a minimum of 4:1, chain and warp a minimum of 6:1...

The point that Alain Fraysee makes on his website is that a bland 3:1, 5:1, 7:1 or 11:1 doesn't account for the actual behaviour of the system. It doesn't allow for how a catenary works, and it doesn't give different results in 5 knots and 50.

The problem is that no one looking to anchor is going to work out the calculations of expected wind, depth, possible snubbing from pitch, yaw and gusting, etc. Catenary calculations are not that difficult - they're about A level maths standard - but they're more involved than can be done approaching an anchorage. And spreadsheets don't really help - turn PC on, enter variables, try out a couple of other scenarios, what about the swinging room, etc. It all takes too long.

The answer is to precalculate rode lengths to deliver particular acceptable angulations as a table, but you're always going to need to allow for swinging room, additional gusts beyond the expected, additional yaw loads, etc. It's where seamanship takes over from maths.

 

[lw395]

Well, after spending a while with that spreadsheet ages ago my 20kg angel/kellet/heavy lump of metal went over the side. Logic being that when you really need it then it won't actually do anything of benifit and then you are then in a potentially dodgy and dangerous situation of having to get a big lump back onboard before doing a runner from the anchorage instead of just the chain and anchor.

Not many things in their favour as far as I can see.

There probably isn't much point if your rode is chain.
20kg of kellet is not much chain.
Presumably, you have a chain rode because you believe it's good for it to be heavy?
When it comes to a rope rode, forget the spreadsheet and observe what happens in reality.

Reality of course varies from day to day, boat to boat etc.

 

[Conachair]

There probably isn't much point if your rode is chain.
20kg of kellet is not much chain.
Presumably, you have a chain rode because you believe it's good for it to be heavy?
When it comes to a rope rode, forget the spreadsheet and observe what happens in reality.

Reality of course varies from day to day, boat to boat etc.

I have chain cos I believe chafe would worry me sick anchoring long term, especially if there's any coral about.

As far as i can tell the physics is pretty basic in predicting catinary behavior. If you know the force anyway, what's doesn't seem so easy is predicting the forces being transfered on a particular boat as it veers and snatches on the anchor rode. But if you know the numbers then stick them in a spreadsheet and you'll know pretty accurately what's going on down below. Not that you'll be doing that on a blustery day.

With mostly chain there seems little or nothing to recommend hanging a lump of weight off it.

 

[CaptainBob]

Not the conclusion I've drawn

I have chain cos I believe chafe would worry me sick anchoring long term, especially if there's any coral about.

As far as i can tell the physics is pretty basic in predicting catinary behavior. If you know the force anyway, what's doesn't seem so easy is predicting the forces being transfered on a particular boat as it veers and snatches on the anchor rode. But if you know the numbers then stick them in a spreadsheet and you'll know pretty accurately what's going on down below. Not that you'll be doing that on a blustery day.

With mostly chain there seems little or nothing to recommend hanging a lump of weight off it.

Not the conclusion I've drawn. Seems a combination is best. Heavier grade chain can add 40% or more holding with just a small increase. Adding some elastic action can reduce anchor loading by 50%. And adding a weight to the rode can again increase holding by a considerable amount. Combine all three with an anchor that sets reliably and you've increased your holding by more than 100%

I emailed the anchor man about his spreadsheets and hes looking into getting then workingminnods format after receivingnsimilarcomments from others also.

 

[lw395]

I have chain cos I believe chafe would worry me sick anchoring long term, especially if there's any coral about.

As far as i can tell the physics is pretty basic in predicting catinary behavior. If you know the force anyway, what's doesn't seem so easy is predicting the forces being transfered on a particular boat as it veers and snatches on the anchor rode. But if you know the numbers then stick them in a spreadsheet and you'll know pretty accurately what's going on down below. Not that you'll be doing that on a blustery day.

With mostly chain there seems little or nothing to recommend hanging a lump of weight off it.

Yes, chain is the answer to chafe.
The physics is quite basic, if you know the conditions you are putting in. But they are very much up for debate. Steady strong wind is easy to calculate, gusts and waves much harder IMHO.

 

[vyv_cox]

There probably isn't much point if your rode is chain.
20kg of kellet is not much chain.
Presumably, you have a chain rode because you believe it's good for it to be heavy?
When it comes to a rope rode, forget the spreadsheet and observe what happens in reality.

Reality of course varies from day to day, boat to boat etc.

The biggest problem with rope rodes occurs not when the wind is strong, when it may well be the better material, but when it is light and shifty. I have seen many collisions between boats on rope and those on chain, due to the differing drift patterns of the two. A kellet in these circumstances is very beneficial for rope rodes and will also help to prevent wraps around keels and props.

I find the weight of a chain rode to be highly beneficial for stern- to berthing in crosswinds, where it holds the bow quite effectively. Chain is also more resistant to being wrapped around another boat's prop than is rope, a constant scenario throughout the Med.

 

[Conachair]

Not the conclusion I've drawn. Seems a combination is best. Heavier grade chain can add 40% or more holding with just a small increase. Adding some elastic action can reduce anchor loading by 50%. And adding a weight to the rode can again increase holding by a considerable amount. Combine all three with an anchor that sets reliably and you've increased your holding by more than 100%

I emailed the anchor man about his spreadsheets and hes looking into getting then workingminnods format after receivingnsimilarcomments from others also.

I do same as most, chain with a snubber. Problem with a weight in there is when you need it most it does hardly anything. You haven't increased your anchor holding by much at all. Then you have a big weight to get back onboard in nasty conditions. Really can't see the benifit.

 

[bbg]

The biggest problem with rope rodes occurs not when the wind is strong, when it may well be the better material, but when it is light and shifty. I have seen many collisions between boats on rope and those on chain, due to the differing drift patterns of the two. A kellet in these circumstances is very beneficial for rope rodes and will also help to prevent wraps around keels and props.

I find the weight of a chain rode to be highly beneficial for stern- to berthing in crosswinds, where it holds the bow quite effectively. Chain is also more resistant to being wrapped around another boat's prop than is rope, a constant scenario throughout the Med.

FWIW- having used a kellet on a mostly rope rode once, I wouldn't do it again. In gusty conditions the boat would creep forward during the lulls as the kellet went to the bottom. In the gusts the boat would get blown back for several boatlengths, accelerating all the time until it was brought up with a jerk. My impression was that the kellet resulted in greatly increased shock loads because it increased the fore-aft range of the boat between lulls and gusts, allowing the boat to pick up more momentum as it was blown back.

 

[lw395]

FWIW- having used a kellet on a mostly rope rode once, I wouldn't do it again. In gusty conditions the boat would creep forward during the lulls as the kellet went to the bottom. In the gusts the boat would get blown back for several boatlengths, accelerating all the time until it was brought up with a jerk. My impression was that the kellet resulted in greatly increased shock loads because it increased the fore-aft range of the boat between lulls and gusts, allowing the boat to pick up more momentum as it was blown back.

I've never had that happen. Not sure why. Possibly because I keep the kellet near the boat. I noticed some jerking if the kellet was on the bottom part time.
If it's very gusty, all sorts of things can happen with most types of rode.