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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, 20:31   #151
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Re: What is your Bigness Factor?

Quote:
Originally Posted by FSMike View Post
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?
You're describing situations where the vessels are moving and as a result you are now dealing with kinetic energy. Kind of the point of anchoring is that you don't have a lot of kinetic energy to deal with since you hopefully aren't moving. Wave action contains a certain amount of energy. Apply that energy to a 50 ton boat and it will move less than a 20 ton boat. Don't blame me, blame Newton.
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Old 29-03-2013, 20:56   #152
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Re: What is your Bigness Factor?

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Originally Posted by Pelagic View Post
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.

Potential Energy by Ron Kurtus - Succeed in Understanding Physics: School for Champions
Very helpful link. Apparently:

Force = ma

where

F is the applied force in newtons (N)
m is the mass in kilograms (kg)
a is the resulting acceleration in meters/second-squared (m/s2)


So, from this we can conclude that the force of a wave will be equal to the mass of the vessel times the acceleration. So a boat with a mass of 30 tons will have one half the acceleration of a vessel of 15 tons in response to a given force. When both boats come up short against the anchor rode, the acceleration will be zero, waiting for the next wave. In both cases the force to be countered will be the same, and if that force is to be countered comes from the resistance of an anchor, the anchor can also be the same.

Now if you're not anchored but drifting, then the potential energy of successive waves will be translated to kinetic energy that will build up and be greater for the heavier vessel because of its greater mass, and then you really will need a bigger anchor for the heavier boat if you want to toss it out and transfer all that energy to the resistance of the anchor.
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Old 29-03-2013, 21:55   #153
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Re: What is your Bigness Factor?

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Originally Posted by Delfin View Post
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.

Inertia is the factor most considered in anchoring.
Motion and the Law of Inertia by Ron Kurtus - Succeed in Understanding Physics: School for Champions
which is then applied to matter
http://www.school-for-champions.com/...gy_kinetic.htm
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Old 29-03-2013, 22:07   #154
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Re: What is your Bigness Factor?

If one therefore has a skittish boat it does not matter how big the anchor is the yacht is going to be, well, skittish. The anchor will hold (because they went BIB) but the yacht is still going to sail as the wind veers from one side and the other. If the wind is sufficiently strong and the chain length finite then the anchor at one end and the yacht at the other need to take the load of the skittishness (through the snubbers). Conversely the heavier yacht, of the same windage, will not be skittish but moving serenely. Even if both yachts have the same length of chain it is likely the heavier yacht might have a heavier duty chain. Neither anchor will move, I'm assured of that, but one might be less comfortable than the other?

Any justification for somehow taming the skittishness (other than buying a heavier yacht) to better engender a decent night's sleep?
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Old 30-03-2013, 02:28   #155
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Re: What is your Bigness Factor?



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 )


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Old 30-03-2013, 04:21   #156
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Re: What is your Bigness Factor?

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Originally Posted by Kettlewell View Post
This discussion of how to compensate for displacement of the vessel brings up another factor, which is how much stretch there is in your anchor rode. The more stretch the less the force on the anchor and the boat. Nobody would jump off a bridge with anchor chain tied to their legs, but they do it happily tied to big bungie cords that absorb lots of the force. In really extreme conditions you need lots of bounce in your rode to mitigate the forces--either really long snubbers or long lengths of nylon on the end of your chain. I have never done so, but I've wondered about shackling a nylon rode directly to the anchor in addition to the chain for hurricane conditions so that the chain acts more as a backup (and extra drag on the bottom), which you would of course have to attach to the boat with long snubbers anyway.
You definitely need a lot of stretch in your snubber in strong conditions.
Most people use a snubber that is way too short.
However I also believe that you can have too much stretch.
The ideal is even out the force at the end of the veer. Too much stretch will see the sored energy catapult the boat forward, the unstable condition will allow the head off the boat to fall off.
The ideal length is going to vary considerably depending on a multitude of factors, but I think using a snubber all the way to the anchor would prove counterproductive.

I have done some crude experimentation and around 20-30m seems optimum, but I would be interested to hear other experiences.
It is one area where a strain gauge may actually be of practical use (rather than scientific interest)

BTW in bad conditions it worth considering rigging more than one snubber. They do break often (I seem to have more trouble than most in this regard). If you cannot lay out more anchor chain, which is a common situation in bad weather, it is possible to upset the main anchor when you motor forward. Besides it no fun to on the foredeck at 3am in strong wind.
It is easy to tie a second loose snubber that will take over if the first one fails.
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Old 30-03-2013, 04:51   #157
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Re: What is your Bigness Factor?

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Originally Posted by noelex 77 View Post
I have done some crude experimentation and around 20-30m seems optimum, but I would be interested to hear other experiences.
It is one area where a strain gauge may actually be of practical use (rather than scientific interest)
.

This is probably time to start a new thread, but do you really use a 20-30M snobber? I use a 25' (8M).
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Old 30-03-2013, 05:12   #158
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Re: What is your Bigness Factor?

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Originally Posted by Don L View Post
This is probably time to start a new thread, but do you really use a 20-30M snobber? I use a 25' (8M).
That sort of length is fine (and what i use) for mild / moderate conditions, but for bad conditions it does need to be longer.
A good solution is to have a long snubber, but normally only use a short length. If the wind picks up you can then easily let out another 20m of chain, just by releasing the clutch on the anchor winch. This gives you more scope and a longer snubber ,a good combination.
It's not always the right answer, but it is often an easy solution to conditions that have deteriorated unexpectedly.

Even if the conditions have not reached the stage where such a snubber is strictly necessary the longer snubber (or two lengths that are joined when needed) allows for easy lengthening of the scope without having to start the main engine.

Some boats rig their snubber from the stern cleats. That way they have a whole boat length of stretch before it reaches the bow.
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Old 30-03-2013, 05:17   #159
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0.4 ?!? Not very big. Not in the poll range. Works fine. 20 pound Fortress on a 50 foot boat. 30 feet of 5/16 chain then just rode. Not dead yet. Scandalous.
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Old 30-03-2013, 05:27   #160
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Re: What is your Bigness Factor?

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Originally Posted by Don L View Post
This is probably time to start a new thread, but do you really use a 20-30M snobber? I use a 25' (8M).
If you use a shorter snubber you can add these Mooring Line Snubbers
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Old 30-03-2013, 05:30   #161
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Quote:
Originally Posted by Pelagic View Post

I agree!
This is an interesting thread but I still believe the poll calculation is incomplete in considering all the key factors:..

From that...Quite clear what is missing in this poll is ‘Displacement and windage

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
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.

I checked in van dorns book. Same deal. If I double the length of a boat I get 8 times the displacement. According to his charts this only gives four times the load on the anchor. This is in agreement with my formula based on length squared.

This suprised me initally and it still doesn't make intutive or fit entirely with my own crude observations, but it seems to me that any attempt to scale anchors directly to displacement is quickly going to end up with far to big anchors on big boats and far to small anchors on small boats eg take a 5 ton 34 footer. If she had a 40lb anchor as a 68 footer she would need a 320lb anchor to compare. Vs a 160lb if we scale to area. If we scale down we end up with a 16 footer lying to a 5lb anchor.

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.

The supertankers hit this problem years ago. A suitible anchor sized to hold them in a strong gale could not remotely stop them if they ever got to a speed of 1.5 or so knots. Making them useless in an emergency situation because the ship would be moving or drifting to fast.

Heavy boats have often have bigger masts. More beam etc so maybe that needs to be accounted for someplace. But we need a simple formula that works passibly.
(Loa x loa)/anchor weight does this supprizingly well for any sized vessel. See the post on anchor design and misnomers.
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Old 30-03-2013, 07:17   #162
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Re: What is your Bigness Factor?

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Originally Posted by Snowpetrel View Post

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.
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.
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Old 30-03-2013, 07:39   #163
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Re: What is your Bigness Factor?

Wind load is the factor until you get significant wave height creating large surges. Displacement "inertia" does not come into play unless you are sailing wildly around the anchorage on the end of your tether. Think about it, how is inertia created if you are connected to your anchor? The boat moves side to side because of wind gusts, but it isn't stopped at the end of its swing by the anchor--it is stopped by a wind shift blowing the boat back the other way.

Here's another thought experiment. You have two identical steel plates, say 10 feet x 10 feet square. You anchor both of them with floats along the bottom that will hold the plate vertical, except one set of floats only lets 2 feet of steel plate show above the water, and the other set of floats lets 8 feet of steel plate show above water. Identical weights. Which plate will create the most pull on the anchor? The one with the greater windage above water.

I can confirm this in the real world in comparing a 30-foot mono I owned vs. a 32-foot cat I owned. I used the same anchoring gear on both, they displaced the same, but the cat had much greater windage. Since I had no windlass on the mono and only a small manual on the cat I pulled in both anchors by hand, when I could. I couldn't do it on the cat above 20 knots of wind, but on the mono I could do it up to 30+.

Re-read Van Dorn's Oceanography and Seamanship and this article by Don Dodds is interesting too.
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Old 30-03-2013, 08:39   #164
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Re: What is your Bigness Factor?

I will never forget the time we took our lady friend on a day sail with a lunch at anchor. After lunch my wife started on clean up and I asked the lady friend to help me get the anchor up. She was to keep the boat into the wind and add a little power to help us get over the anchor.
It seemed harder than I expected. Finally cleated off over the anchor and rested with a strain on the anchor. In a little while the anchor broke loose and I stowed it.
"Steer into the wind", I called. We fell off. I scurried back and took the wheel. Added power and looked down at the transmission lever. SHE HAD NEVER PUT IT IN GEAR! I had pulled the boat against the 25 knot wind all the way!
38 foot boat. 35 lb lunch anchor. BF=.9 A scope of about 4:1. Second hardest manual anchor retreval I ever did.
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Old 30-03-2013, 08:42   #165
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Re: What is your Bigness Factor?

Proof that wind load is the biggest factor, as is the size of the skipper's forehead (thanks boatman for putting this on my FB wall ) :

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