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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 29-03-2013, 18:13   #136
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Re: What is your Bigness Factor?

Quote:
Originally Posted by minaret View Post
Yea, but can it shoot an anchor?
I've never tried an anchor, but if the shank will fit the bore it will shoot it !! maybe a fortress ?? what ya think ?
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Old 29-03-2013, 18:17   #137
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Re: What is your Bigness Factor?

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I've never tried an anchor, but if the shank will fit the bore it will shoot it !! maybe a fortress ?? what ya think ?


Oh yea! Just cast the shank of a small fortress into a beer can of cement, and voila!, anchor cannon!
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Old 29-03-2013, 18:28   #138
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Re: What is your Bigness Factor?

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Originally Posted by Kettlewell View Post
I basically agree, but I don't think it is "completely irrelevant," though it is interesting how little it is discussed in most anchoring treatises. Where displacement comes into play is when the wave action gets big enough to start surging the boat back against the anchor rode. This is one reason really long scope is needed in storms--allows the bow of the boat to rise freely because of the lack of downward pull from the anchor rode.
Not sure the physics work out that way. The force of a wave is the force of a wave. If one boat is heavier than another, it will move less from that force than another lighter boat, would it not? The heavier boat doesn't add to the force, it just reacts more slowly when it is acted upon by that force.

And because a heavier boat moves less when acted upon by the wave, it places less herky jerky strain on the anchor, hence my comment that it might actually be of benefit in anchoring.
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Old 29-03-2013, 18:28   #139
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Re: What is your Bigness Factor?

Quote:
Originally Posted by Kettlewell View Post
I basically agree, but I don't think it is "completely irrelevant," though it is interesting how little it is discussed in most anchoring treatises. Where displacement comes into play is when the wave action gets big enough to start surging the boat back against the anchor rode. This is one reason really long scope is needed in storms--allows the bow of the boat to rise freely because of the lack of downward pull from the anchor rode.
Relative to our length 58, we have low freeboard and short LWL 45. But, when our mass 36T gets swinging and hits the anchor, we impact the heck out of it, flat water or lumpy. In the end, you have to know your boat. The standard modern light-weight, high sided, flat bottom, fin keel, square bow and broad transom and our vessel ar poor comparisons. If you have no experience with a tank like ours you have little basis for understanding that it is a very poor comparison. Relative to these lightweights, we have relatively way more below water than above. GO BLUE
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Old 29-03-2013, 18:29   #140
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Re: What is your Bigness Factor?

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Oh yea! Just cast the shank of a small fortress into a beer can of cement, and voila!, anchor cannon!
I'm going to need one of those big Australian beer cans to fit my Claw.
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Old 29-03-2013, 18:34   #141
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Re: What is your Bigness Factor?

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Relative to our length 58, we have low freeboard and short LWL 45. But, when our mass 36T gets swinging and hits the anchor, we impact the heck out of it, flat water or lumpy. In the end, you have to know your boat. The standard modern light-weight, high sided, flat bottom, fin keel, square bow and broad transom and our vessel ar poor comparisons. If you have no experience with a tank like ours you have little basis for understanding that it is a very poor comparison. Relative to these lightweights, we have relatively way more below water than above. GO BLUE
The only experience I have is with a 65 ton boat 3 feet shorter than yours with 7 1/2' in the water. Moves less than my 12 ton 36 boat at anchor, or so I recall, hence my observation that the weight of the water displaced by the vessel has little to do with how big the anchor needs to be. A 1000 ton barge with 18 inches of freeboard will put less strain on an anchor in a blow than a 1000 ton slab sided ferry.
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Old 29-03-2013, 18:38   #142
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Re: What is your Bigness Factor?

Delfin and Kettlewell,

I hesitate to comment for fear of invoking the wrath of Evan, Jim et al. As they possibly have an erudite post that I have missed.

But

If we have a modern deck saloon, plastic fantastic and a steel Roberts design of the same windage. So say one weighs half that of the other.

So same length and windage but 2 times weight.

If they are both enjoying the same wind speed does not the heavier model move at half the speed of the other thus the momentum, which becomes the load on the anchor, is the same. Equally in waves we have the same energy from the waves, does not the heavier boat be more stable, by a factor of 2, but at the end of the day the impact on the anchor the same.

Is there not a way to minimise skittishness (the wind effect)?

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Old 29-03-2013, 18:52   #143
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Re: What is your Bigness Factor?

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Delfin and Kettlewell,

I hesitate to comment for fear of invoking the wrath of Evan, Jim et al. As they possibly have an erudite post that I have missed.

But

If we have a modern deck saloon, plastic fantastic and a steel Roberts design of the same windage. So say one weighs half that of the other.

So same length and windage but 2 times weight.

If they are both enjoying the same wind speed does not the heavier model move at half the speed of the other thus the momentum, which becomes the load on the anchor, is the same. Equally in waves we have the same energy from the waves, does not the heavier boat be more stable, by a factor of 2, but at the end of the day the impact on the anchor the same.

Is there not a way to minimise skittishness (the wind effect)?

Jonathan
I think that is correct. The only practical aspect that applies to anchoring is that while the total force is the same for both boats, it seems to me that the heavier boat will move less putting less strain on the anchor. A slow steady pull seems less likely to pull an anchor free than applying the same force in jerks. Or by jerks, for that matter.
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Old 29-03-2013, 19:13   #144
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Re: What is your Bigness Factor?

Quote:
Originally Posted by JonJo View Post
The idea of the poll has sparked much response, in terms of poll numbers. There seems to be much support.

If you think the poll too simple come up with a system that invokes less criticism and offer a new poll.
Jonathan
I agree!
This is an interesting thread but I still believe the poll calculation is incomplete in considering all the key factors:..

My post # 56 links how it is professionally calculated by international organizations.

From that...Quite clear what is missing in this poll is ‘Displacement and Windage Area’. (this would also give better value to power boat owners)

Perhaps someone with better Poll Making experience than mine can create a user friendly formula where we plug in our measurement factors. (Keep in Metric for decimal clarity) and then we can analyze the new poll results in terms of new design factors like CQR vs. Manson… etc.

Simplified IMO Calculation Factors
“Equipment Number” calculated as follows:
EN = ∆2/3 + 2,0 hB + A/10
Where:
∆ = moulded displacements, in tonnes
B = moulded breadth, in metres
h = effective height, in metres, to the top of the uppermost house; for the lowest tier “h” is to be measured at centerline

Where:
a = distance, in metres, from the Summer Load Waterline amidships to the upper deck
hi = height, in metres, on the centerline of each tier of houses having a breadth greater than B/4
A = area, in square metres, in profile view of the hull and house are within the Equipment length of the vessel and also have a breadth greater than B/4.

NOTES
1. When calculating h, sheer and trim are to be ignored, i.e. h is the sum of freeboard amidships plus
the height (at centerline) of each tier of houses having a breadth greater than B/4.
2. If a house having a breadth greater than B/4 is above a house with a breadth of B/4 or less then the
wide house is to be included but the narrow house ignored.
3. Bulwarks 1,5 m or more in height are to be regarded as parts of houses
4. The equipment length of the vessels is the length between perpendiculars (On yachts we can have long bowspirits and davits so measure from perpendiculars as shown)
5. The total length of chain given in Table 1 - col. 4- is to be divided in approximately equal parts
between the two bow anchors.
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Old 29-03-2013, 19:19   #145
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Re: What is your Bigness Factor?

[QUOTE=Delfin; Or by jerks, for that matter.[/QUOTE]

Very droll
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Old 29-03-2013, 19:23   #146
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Re: What is your Bigness Factor?

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Originally Posted by Delfin View Post
I may be wrong, but it seems to me that displacement is completely irrelevant. The force applied to a 65 ton boat with zero wind is the same as a 10 tonner with equivalent windage. Same with 5 knots or 50 knots because the force acts against the surface presented to the wind, not the weight of the water displaced. Heavy displacement might actually be beneficial in the sense that when another force - waves - act against the hull, the vessel with greater displacement will be more resistant to movement resulting from that force than a lighter vessel.
Yes you are mistaken. Displacement is everything followed by windage area, where length is just on component of Areas
Anchors and Ground tackle are meant to stop vessels that are already moving, be it from currents or wind
Imagine you are on a dock trying to stop a 1,000lb yacht by hand going at 1knt,
Now imagine if it is a 10,000 lb yacht going same speed…
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Old 29-03-2013, 19:27   #147
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Re: What is your Bigness Factor?

Quote:
Originally Posted by Delfin View Post
I think that is correct. The only practical aspect that applies to anchoring is that while the total force is the same for both boats, it seems to me that the heavier boat will move less putting less strain on the anchor. A slow steady pull seems less likely to pull an anchor free than applying the same force in jerks. Or by jerks, for that matter.
That's all well and good, assuming you have a slow steady pull. If you don't, it will take a lot bigger anchor to overcome the inertia of a 40,000 pound boat than that of a 20,000 pound boat. This could happen from the vessels sailing at anchor, as well from wave action.

If you're standing on a pier and two 30' boats come drifting by and each skipper throws you a line, which one are you going to succeed in stopping, the light displacement boat or the heavy displacement boat?
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Old 29-03-2013, 19:37   #148
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Re: What is your Bigness Factor?

One last post to put perspective on sizing anchors.
  • We don’t choose our ground tackle based on normal conditions but something that will hold us in bad conditions
  • Not just wind but tidal current surges.
  • That implies that your yacht is already being tossed around by those forces
  • Maximum Moment Force is then a factor of your displacement.
  • Break out Force on your anchor is also severely affected by anchor chain weight and deployed chain length plus anchor depth to minimize the BoF

In terms of safety…Heavier Displacement Yachts have a greater “Potential’ to create more ‘Kinetic Force’ and that is what the ground tackle must be sized for.

http://www.school-for-champions.com/..._potential.htm
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Old 29-03-2013, 19:39   #149
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Re: What is your Bigness Factor?

Yes, FSMike, but if the energy causing the boat to come 'drifting by' is intermittent (as in the energy sources causing yawing and ranging) and esentially finite,
then the amount of energy 'coupled' to the boat could easily be independent of mass, given that it has to couple via some sort of drag (which acts on area, not mass)

The more massive boat will not be accelerated to the same speed by the same drag forces acting on a lighter boat.

I'm inclined to agree with those who think the mass probably crudely cancels out, and the applicability of SnowPetrel's rule of thumb across a huge range of masses seems to bear that out.

Unless ..... the two boats encounter the sort of waves you hope never to have entering an anchorage. (NB: a proper fully developed wavetrain, not just a windchop)

In this case, I can well agree that mass is effectively travelling downhill, in which case energy IS coupled via a mass channel rather than an area channel.

(Due to the force of gravity, Potential energy acquired at the top of a wave is traded into Kinetic energy as it travels down towards the trough)


I think in such circumstances, the massive boat is in trouble if the entire anchor system is not upgraded relative to the lighter one with the same wind and water drag.
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Old 29-03-2013, 20:26   #150
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Re: What is your Bigness Factor?

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Yes you are mistaken. Displacement is everything followed by windage area, where length is just on component of Areas
Anchors and Ground tackle are meant to stop vessels that are already moving, be it from currents or wind
Imagine you are on a dock trying to stop a 1,000lb yacht by hand going at 1knt,
Now imagine if it is a 10,000 lb yacht going same speed…
If you say so.

However, it does seem odd that anchor manufacturers don't make more of displacement if it is 'everything'. For example, on the Rocna site they do recommend going up one size if your vessel displaces twice what another vessel of the same length weighs, while they recommend going up 4 or 5 sizes if your vessel is twice the length of another. Then again, they make the same recommendation to go up one size for a multi-hull, whose principal characteristic would be lower displacement but greater windage.

On your example of stopping a a 1,000 pound yacht or a 10,000 pound yacht, if they both are moved along a flat place in reaction to a 100 psi force, it will take 100 psi force to stop either one of them. If you incorporate gravity, the force increases, but I'm not clear how much of a factor gravity would be in anchoring.
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