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View Poll Results: What is your Bigness Factor?
0.5 - 0.9 = Light 18 13.85%
1.0 - 1.2 = Normal 33 25.38%
1.2 - 1.4 = Conservative 37 28.46%
1.5 + = BIB 42 32.31%
Voters: 130. You may not vote on this poll

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Old 30-03-2013, 10:27   #166
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Re: What is your Bigness Factor?

I'll revise my comments--the biggest factor to consider is how many Pain Killers the crew has had, and than it's windage!
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Old 30-03-2013, 10:48   #167
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Re: What is your Bigness Factor?

The results of this poll proves that the majority of respondents do not know how to anchor and have made the faulty decision to compensate their flawed behavior with a heavier anchor. The heavier anchor will compensate up to a point, but when the conditions worsen, it will let go if it is not set with correct scope.

I have witnessed this anchor over-weight phenomenon and ultimate breaking loose too many times.

If I see a large anchor, I avoid anchoring in harms way...problem is, they are usually down when we arrive to anchor.

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Old 30-03-2013, 12:51   #168
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Re: What is your Bigness Factor?

For what it's worth, when in cruising mode we carry a 66# steel Spade on 300 ft of 5/16 HT chain. VALIS is 44 ft LOA. The chain may be the weak link in the system. We also carry a medium aluminum Fortress on the stern, with a rope/chain ride.

I have a 44# Delta with rope/chain that's my racing anchor, and I keep this aboard as a cruising spare. I have a 44# Danforth-style (West Marine) anchor, but that was my old stern anchor, and is now in the garage at home.

So, our number is 66/44 = 1.5

Don't know about the number, but I like our Spade.
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Old 30-03-2013, 13:04   #169
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Re: What is your Bigness Factor?

Quote:
The majority of respondents do not know how to anchor and have made the faulty decision to compensate their flawed behavior with a heavier anchor.... If I see a large anchor, I avoid anchoring in harms way....
Sounds good. Anchor as far away as you like.
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Old 30-03-2013, 13:07   #170
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Re: What is your Bigness Factor?

Quote:
Originally Posted by Snowpetrel View Post
The key here to my mind is that they are scaling the displacement to the power of 2/3. Reducing it to the same as LOA^2 as in my formula. Basically the Equipment number and my formula (loa^2/anchor weight) are both scaling based on windage not displacement. This shows that windage is the dominant factor in anchor loads not displacement.

...
oh heck, SnowP -- do you have to keep making sense?

It's an anchor thread, g*d*m*t!

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Old 30-03-2013, 14:55   #171
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Re: What is your Bigness Factor?

I'm curious after watching the credit card sailors video--what anchors are major charter companies equipping their boats with in the Virgin Islands? Does anyone know?
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Old 30-03-2013, 15:07   #172
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Re: What is your Bigness Factor?

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Originally Posted by cfarrar View Post
Sounds good. Anchor as far away as you like.
+1. Yes, please do.
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Old 01-04-2013, 03:23   #173
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Re: What is your Bigness Factor?

Quote:
Originally Posted by conachair View Post


F=MA, that's the biggie.

Plus this must be one of the best analysis available to the likes of us trying to get a handle on the forces and behavior of a boat at anchor. At least the catenary behavior & increase in boat speed in a gust or acceleration of the boat back to zero velocity by the anchoring system are fairly straightforward, since Newton's time anyway. How accurate you can calculate those forces in a very dynamic system like a boat at anchor in some fierce gusty wind seems a bit more difficult.
Worth having a play around with the spreadsheets, intuition can be very far off the mark. (Unless you are on an internet forum, where UFO's are real and Elvis lives )


Tuning an Anchor Rode

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Thanks conachair ! ... those links were very interesting
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Old 01-04-2013, 04:51   #174
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Re: What is your Bigness Factor?

Quote:
Originally Posted by Delfin View Post
Which is why displacement is largely, if not completely irrelevant to picking the right anchor for a vessel. The inertia of the more massive vessel will resist acceleration response to a given force, like a wave, resulting in less acceleration than the less massive vessel. But it still has acceleration because it still has movement, however if they have the same basic shape in the water the force required to cancel the acceleration is the same regardless of the mass of the vessels being compared - F=ma. If both vessels are drifting, then you have a different scenario since the same inertia that resists movement against wave action also resists stopping moving once started. Dealing with the dragging anchor scenario is where displacement does matter and is probably why the recommendation to increase one anchor size for heavier vessels is made by anchor manufacturers. Once stopped, displacement means nothing when comparing vessels.

Add wind however, and the whole scenario changes because you don't have just energy transmitted by waves, but energy transmitted by the wind which increases by the square with a doubling of wind speed. So one vessel will experience 25% of the increase in force that a vessel of the same displacement but double the windage will experience when the wind speed doubles (Wind pressure (Psf), = .00256 x V^2). And that explains why anchor manufacturers make the same recommendation to go up one anchor size for very light displacement hulls like cats or tris. Because they dance around at anchor they present more windage, hence have more force to negate with the anchor.

Hard to figure out how to factor in windage in your calculation, but that doesn't negate its utility.
Originally Posted by Snowpetrel

So I don't know... As a seaman I feel like you that a bigger boat has more inertia so needs bigger anchors. But the maths, Both imperical and theory doesn't seem to support this idea. Do we size an anchor for wind load or inertia? It seems wind load wins in the real world samples I have checked.

Snowpetrel…. I believe your instincts are correct…. I think there are times when you’ve got to forget the maths and just look out the window.

Delfin my difficulty with your conclusion is that you are thinking 2 dimensionally (Windage and Acceleration) and forgetting the dynamics where that building sheer momentum (energy) is going and how some of it is transferred back.

Tried to find an illustration in my old textbooks. Best I could find is one drawing of breaking sheer when advising Master’s which anchor to drop when near a shoal area.

Illustration shows #2-3 as the natural sheer,…… then as winds become strong…. it starts sailing and creeping upwind to #6 where the anchor catenary is stretched maximum and then the head turns and she quickly starts to pick up speed due to energy transferred from stretched out chain falling back to bottom.

Broadside at #7 with a 60 knot wind the vessel is both sailing forward and lifting heavy chain off bottom.

We have all seen or experience position #8 which is when that momentum can cause chain to be wrapped across the bow and we have what is called a ‘breaking sheer’.

Another dimension is ground swell creeping into an anchorage where a heave at the wrong moment of critical breaking sheer is also displacement affected.

The bottom line is we don’t choose an anchor for poor conditions but for very bad conditions where the worst case scenario requires a complete anchor system.

Classification societies use both Broadside Area and Ship Displacement to calculate Size and SWL of ground tackle for very good reasons.
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Old 01-04-2013, 04:59   #175
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Re: What is your Bigness Factor?

In the figure 1.7 illustration …. Which anchor (Port or Starboard ) should be used?... and why?
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Old 01-04-2013, 05:32   #176
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Re: What is your Bigness Factor?

Quote:
Originally Posted by Dockhead View Post
I would think that if you want a simple metric, it would be much better to divide anchor weight by boat displacement.
Agreed.
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Old 01-04-2013, 05:33   #177
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Re: What is your Bigness Factor?

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Originally Posted by Pelagic View Post
In the figure 1.7 illustration …. Which anchor (Port or Starboard ) should be used?... and why?
Takes me back a bit, if memory serves correctly, that illustration is from Danton's Seamanship, one of the bibles for aspiring Mates and Masters.
Last read about 25 years ago.
If I remember, the answer to your question, in this case would be the port anchor.
The anchor is more likely to be broken out as the ship nips the cable across the bow. In the picture, this be at position 8. If the anchor breaks out, the ship is in danger of continuing on towards the shoal.
We were taught that if anchoring near a danger, then use the anchor which is on the side away from the danger.
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Old 01-04-2013, 05:57   #178
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Re: What is your Bigness Factor?

Quote from Alain Fraysse's linked material:

Quote:
Obviously, the Long Chain + Short Nylon Line is the winner, except for small boats that have on-board weight problems. Actually, there is no boundary between the mixed-rode versions: one can choose any Chain/Nylon mix inside a wide range, say, from 40/60 to 80/20, with no significant performance differences (test it yourself with our spreadsheet!). Practically, this involves having 2 rode elements at one's disposal before attempting to anchor.
In other words, use plenty of nylon in your rode in order to greatly reduce the forces on your anchor.
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Old 01-04-2013, 06:18   #179
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Re: What is your Bigness Factor?

Whenever a boat moves behind an anchor, it's energy is multiplied by it's mass. Like a bullet that is heavier at same or even lower speed has higher impact.

Any movement incl. sideways while anchored must be countered by the anchor. This is when the energy from the boat gets transferred into pull on the anchor.
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Old 01-04-2013, 06:38   #180
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Re: What is your Bigness Factor?

Whoa ! agreed :force=mass x acceleration. To argue that displacement (mass) is not a relevant part of any anchoring considerations is disingenuous. I need some help moving out of my 2nd. floor apartment (flat) and will be dropping some items to my cruising friends standing below on the street. Now physics 101 teaches us that all items are accelerated equally by the force of gravity and will arrive to helping hands below at the exact same speed (velocity). Now who here will catch my prize baseball and who wants to catch my piano?
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