What is your Bigness Factor?

[Kettlewell]

I don't think there would be any particular increase in tension on the anchor rode if boats were going up and down in large swells, and I have observed this myself while at anchor in 10-foot swells down in the Virgin Islands. The swells were so long and big that we were just going up and down almost as on an elevator. If you didn't look out the ports you really didn't mind the motion, but if you watched what was happening it was very seasick inducing. However, there wasn't any additional pressure on the anchor rode due to this. We just went up and down.

When waves create additional forces is when they are very steep and breaking, which can happen with shorter waves in shallow water. I too have felt that surge when anchored during a hurricane with some short breaking waves even though land was only maybe 1/4 mile to windward of us. However, I am uncertain what displacement effect there would be in short breaking waves like that. The heavier boat would resist moving back, but once moving would do so with more momentum.

[Snowpetrel]

Originally Posted by Andrew Troup

Snowpetrel:
I'm not considering orbital currents. That's trickier, and in any case I think it falls into the majority of forces, which are largely independent of displacement (because of the "supply matching demand" balance referred to earlier)

I'm considering the boat travelling, under the influence of gravity, down the flank of a long swell.

It would do this regardless of the orbital motion of the water particles: they will add to it, but in inverse proportion to mass (which I think is why a surboard initially accelerates quicker without anyone on it).

The situation I'm singling out is more like the situation of a skier. Heavy skiers do not travel faster than light skiers on a given slope, but they punch a bigger hole in the wall when they 'hit the daylodge'.

I think 'travelling downhill due to gravity' is the probably the one mechanism where a 2 tonne boat applies roughly twice as much force as a 1 tonne boat when it gets pulled up short by the chain. And I think it applies only in roadsteads, ie virtually unlimited fetch.

Agreed. I guess I am thinking that a longer boat is less likely to be affected by waves. Hence putting lwl into the formula. It also has the handy effect of fixing the scaling issue. So it can apply to vessels from supertankers to small yachts. It is hard to get hard data on this due to events being pretty rare. The thailand tsunami is all I can really think of. Cabo san lucas had severe wind and wave action acting together. Pretty hard to work out the ratio's of each factor but in this case much of the stretch in the system is taken out by the wind load so the wave loads would have a big part to play. I am betting a long light boat would have less anchor load in this case than a long heavy boat, but I am unsure how much by maybe Alain's 20%. A long enough boat would just pitch and not really be effected by the waves at all. The formulas show me that for most vessels if an anchor is sized correctly for windage it should be ok for displacement as well in most decent anchorages. If the wave length (not swell length) gets to the same as your waterline length get the **** (insert expletive of choice) out.

[Pelagic]

I try to keep things simple when dealing with dangerous stuff, so a few related questions:

• What Does Bigness Factor mean on an anchor?
• Does it relate only to physical size, or does it also apply to Safe Working Load (SWL)?
• If SWL is also a factor, when that anchor break-out point is reached and exceeded for the exact same anchoring scenario, does the anchored boat’s displacement acting on all ground tackle components.. not come in to play?
• If no, please explain to this simple mariner...

[Bash]

Originally Posted by Pelagic

[*]If no, please explain to this simple mariner... [/LIST]

I think the basic theory behind the OP's calculous is: big = gooder; small = badder.

Gotta love "bigness factor," regardless. (Even if mine only comes out to 1.19. Have I mentioned I've never dragged this anchor?)

[Delfin]

Originally Posted by Andrew Troup

Delfin

It's evident you didn't read or reflect carefully on what I wrote in my last post, but I guess it would be churlish not to try one last time.

eg your "wave hitting...". I quite agree, but this has nothing to do with lifting on a long swell, which is the ONLY thing my post considered

and "looking at different boats at anchor..."

If you were to do this in an open roadstead exposed to a long swell train, boats of different mass would go up and down harmoniously, all floating on their marks. So would a ping-pong ball, for that matter.

Where would you draw a demarcation between rising to a tide and not rising to a chop?
Because you're talking about situations within that demarcation, and I'm talking outside it.

I wonder if you see any difference in the movement of these two vessels in this gale, one smaller and lighter than the other? It appears to me that consistent with the laws of physics, the heavier vessel is pitching less and on a more subdued period than the lighter vessel. Or maybe it's just me.

Gale force winds slam yachts and boats in Baltimore, West Cork, Ireland. - YouTube

But I'm sure you are quite right that if you anchor in a roadstead with large breaking waves, you'll need a larger anchor than if you merely anchor in gales as in the video.

[Delfin]

Originally Posted by Pelagic

I try to keep things simple when dealing with dangerous stuff, so a few related questions:

• What Does Bigness Factor mean on an anchor?
• Does it relate only to physical size, or does it also apply to Safe Working Load (SWL)?
• If SWL is also a factor, when that anchor break-out point is reached and exceeded for the exact same anchoring scenario, does the anchored boat’s displacement acting on all ground tackle components.. not come in to play?
• If no, please explain to this simple mariner...

Yes, once a vessel breaks free and is no longer anchored, then displacement matters as greater force is required to stop it. As long as it is anchored, however, displacement matters only slightly more than the color of your dodger when comparing vessels of equivalent windage in the same anchorage.

But then, that is a topic for a thread on stopping moving boats, not on getting boats that aren't moving to remain that way.

[Snowpetrel]

Originally Posted by Pelagic

I try to keep things simple when dealing with dangerous stuff, so a few related questions:

• What Does Bigness Factor mean on an anchor?
• Does it relate only to physical size, or does it also apply to Safe Working Load (SWL)?
• If SWL is also a factor, when that anchor break-out point is reached and exceeded for the exact same anchoring scenario, does the anchored boat’s displacement acting on all ground tackle components.. not come in to play?
• If no, please explain to this simple mariner...

The original Kettlewell bigness factor has a scale issue, it unfairly penalises small boats and favours bigger boats with a much higher number. But is applicable for comparing vessels of a similar size. Kettlewell's formula was sized for vessels around 35-38 feet from memory. The further you go from this the less accurate the results become.

A way to get around the scaling effect is to use windage which scales in a perfect world by four times if you double the size of a vessel keeping everything else the same (displacement would be eight times).

length is probably the best indicator of windage, though not the only one (or a perfect one), but for simplicity lets use it. So take length x length in ft then divide this answer by the anchor weight in pounds.

Length(ft) x Length (ft)
Anchor weight (lbs)

For a BIB anchor you should be getting an answer of less than 25 or so. It is strictly for comparing different vessels anchors. And is of use when Jedi says he has a 176 lb bruce on his sundeer 64 and it held him in a hurricane, I can punch the numbers and go WOW 23.3 definitely in the BIB but about the same as a 50lb anchor for my 34 footer, OK.

this simple formula neglects the differences in vessel type, shape, freeboard, displacement SWL etc. It doesn't tell you anything about loads. In reality a sundeer 64 scaled exactly to 34foot would be a much smaller boat than snowpetrel so I would probably need to go to a 55 or 60lb anchor for the same holding.

Your vessel comes out as 65x65=4225 4225/97.5 = 43.3 which is pretty small about the same as my 34 footer having a 25lb anchor. However it obvously works fine for you, as a 25lb anchor might work for Snowpetrel, but it is not BIB for your vessel. For your 65 footer to get into BIB territory you would need 65 x 65=4225 4225/25= 169lb anchor

Of course bigger boats aren't double in all ways, masts are shorter beam is less and freeboard is reduced so real scaling is about ^1.7ish, but it doesn't take into count displacement which can add about 20% to the loads in some cases, and anchors also don't double in Holding power with twice the weight(^1). they scale by about ^0.93. So there are lots of further complexities that you as the boat owner need to take into account.

I wouldn't ignore the manufacturers guidance. And wouldn't want to go under there recommended values. Different anchors have different holding powers and this also takes no account off.

[downunder]

Originally Posted by Bash

I think the basic theory behind the OP's calculous is: big = gooder; small = badder.

Gotta love "bigness factor," regardless. (Even if mine only comes out to 1.19. Have I mentioned I've never dragged this anchor?)

Then we have to conclude that with your anchor, a different anchor type is a more important factor than bigness.

Better anchor type otherwise bigness reguired.

[Delfin]

Originally Posted by Snowpetrel

The original Kettlewell bigness factor has a scale issue, it unfairly penalises small boats and favours bigger boats with a much higher number. But is applicable for comparing vessels of a similar size. Kettlewell's formula was sized for vessels around 35-38 feet from memory. The further you go from this the less accurate the results become.

A way to get around the scaling effect is to use windage which scales in a perfect world by four times if you double the size of a vessel keeping everything else the same (displacement would be eight times).

length is probably the best indicator of windage, though not the only one (or a perfect one), but for simplicity lets use it. So take length x length in ft then divide this answer by the anchor weight in pounds.

Length(ft) x Length (ft)
Anchor weight (lbs)

For a BIB anchor you should be getting an answer of less than 25 or so. It is strictly for comparing different vessels anchors. And is of use when Jedi says he has a 176 lb bruce on his sundeer 64 and it held him in a hurricane, I can punch the numbers and go WOW 23.3 definitely in the BIB but about the same as a 50lb anchor for my 34 footer, OK.

this simple formula neglects the differences in vessel type, shape, freeboard, displacement SWL etc. It doesn't tell you anything about loads. In reality a sundeer 64 scaled exactly to 34foot would be a much smaller boat than snowpetrel so I would probably need to go to a 55 or 60lb anchor for the same holding.

Your vessel comes out as 65x65=4225 4225/97.5 = 43.3 which is pretty small about the same as my 34 footer having a 25lb anchor. However it obvously works fine for you, as a 25lb anchor might work for Snowpetrel, but it is not BIB for your vessel. For your 65 footer to get into BIB territory you would need 65 x 65=4225 4225/25= 169lb anchor

Of course bigger boats aren't double in all ways, masts are shorter beam is less and freeboard is reduced so real scaling is about ^1.7ish, but it doesn't take into count displacement which can add about 20% to the loads in some cases, and anchors also don't double in Holding power with twice the weight(^1). they scale by about ^0.93. So there are lots of further complexities that you as the boat owner need to take into account.

I wouldn't ignore the manufacturers guidance. And wouldn't want to go under there recommended values. Different anchors have different holding powers and this also takes no account off.

Good summary.

On the last, a further complication is that the efficiency of anchors changes with size. One anchor that is a poor performer in lighter weights can, with the benefit of gravity, be a stellar performer in heavier weights. Evans Starzinger's tests on a large Bruce type, Manson and Rocna brought this home. Another complicating factor is where the wind resisting surfaces are on the vessel. In the video posted above, the sailboat lies more quietly in the same wind as the lighter power boat. However, its center of the windage is forward of the centerline, so it exerts some significant pressure on its ground tackle when it veers. However, as a rule of thumb your formula seems very useful since it helps explain why the Mirabelle broke free and why your anchor is far proportionally larger for your boat and why you likely won't break free. Or me for that matter.

[Andrew Troup]

Originally Posted by Kettlewell

I don't think there would be any particular increase in tension on the anchor rode if boats were going up and down in large swells, and I have observed this myself while at anchor in 10-foot swells down in the Virgin Islands. The swells were so long and big that we were just going up and down almost as on an elevator. If you didn't look out the ports you really didn't mind the motion, but if you watched what was happening it was very seasick inducing. However, there wasn't any additional pressure on the anchor rode due to this. We just went up and down.

When waves create additional forces is when they are very steep and breaking, which can happen with shorter waves in shallow water. I too have felt that surge when anchored during a hurricane with some short breaking waves even though land was only maybe 1/4 mile to windward of us. However, I am uncertain what displacement effect there would be in short breaking waves like that. The heavier boat would resist moving back, but once moving would do so with more momentum.

I can't find anything to disagree with. Your account reminds me of something I'd forgotten: I recall watching the echosounder in Rada Vinapu roadstead, clocking 3m or more of rhythmical rise and fall, and we didn't even require a snubber. (Admittedly on a mixed rode with a heavy chain)

My argument did not aim to predict what would happen in practice, but an attempt to erect a theoretical and metaphorical fence around an exceptional class of situations which, in theory, COULD mean that a heavy boat imposed a proportionately heavy demand on the anchor.

I'm entirely comfortable with you pointing out that, even in such cases, the steepness of the swells is generally so minimal that the hypothetical 'skiing' problem does not arise.

What you are pointing out is that the fence I drew does not, in most real cases, contain any lions, only lambs.

Which I think provides added consolation for those favouring simple guidelines like SnowP's, which neglect to consider displacement when sizing anchors and rodes.

Those formula and charts and rules of thumb are outside the fence.

[Pelagic]

I agree Delfin... once you are starting to drag onto a lee shore and you hope anchor will reset, your displacement becomes a major factor in choosing your ground tackle.

But isn't that a key part of choosing your anchor system?.... when you are in marginal conditions
and dragging because of extreme weather and/or just poor holding?

Yacht Bewitched anchor dragging - YouTube

[Delfin]

Originally Posted by Pelagic

I agree Delphin... once you are starting to drag onto a lee shore and you hope anchor will reset, your displacement becomes a major factor in choosing your ground tackle.

But isn't that a key part of choosing your anchor system?.... when you are in marginal conditions
and dragging because of extreme weather and/or just poor holding?

The point of an anchor that is as big as you can carry is so you don't find yourself in Bewitched's situation where you need to have the hook "reset", so of course picking an anchor that is capable of resisting extreme forces is prudent and cost efficient insurance. And, unless you think that anchoring in the Alenuihaha channel is prudent, the forces you have to deal with are almost entirely related to your windage and the profile of surfaces in contact with the top foot or so of water. These are calculable, and while SP's formula doesn't directly address these, it is accurate enough for a rule of thumb.

[s/v Jedi]

Here is some anchor size vs displacement:

Anchor Fail - YouTube

[Pelagic]

Originally Posted by Delfin

The point of an anchor that is as big as you can carry is so you don't find yourself in Bewitched's situation where you need to have the hook "reset"....

Yachts can and do compromise on safety, because of lack of mandatory equipment regulations on ground tackle.

However, I find it hard to believe that applying same calculations as Classification Societies, which does include displacement and SWL, is foolish.

Also any active cruising sailor who believes they will never find themselves in a dragging situation, believes too much in the Vendors hype.

[Delfin]

Originally Posted by Pelagic

Yachts can and do compromise on safety, because of lack of mandatory equipment regulations on ground tackle.

However, I find it hard to believe that applying same calculations as Classification Societies, which does include displacement and SWL, is foolish.

Also any active cruising sailor who believes they will never find themselves in a dragging situation, believes too much in the Vendors hype.

Actually, after 40 years of cruising, I have confidence that if I undersize my ground tackle, or anchor places I shouldn't I could easily drag, as I have I think 3 times before. So I carry a big anchor and try not to do stupid things so I won't drag again.