Ok guys, anchors question

[Adelie]

Originally Posted by Izikalvo

Most boats usually have a weight/length ratio that is common,



what is your way of choosing anchor size when your boat is beyond "normal" say, you have a 40Ft boat that weighs 20 tons, or the opposite, you have an 80 Ft long boat that weighs only 20 tons.

Obviously, when considering only length your are doing something wrong.



usually, anchor charts are related to boat length, and some have also weight, but where does the weight factor come from?



Any thoughts?

Boats at anchor are subject to 2 steady loads:
Wind
Current

Wind loads are based on the windage of the hull (beam & height) and rigging, wind speed and how much of the side of the boat is presented to the wind as the boat slews back and forth at anchor.

Current loads depend on the current speed, beam, canoe draft, area of the fins and how much of the side of the hull is presented to the current as the boat slews back and forth.

Although the wind and current are dynamic, they are steady state loads and the boat will be in equilibrium between wind and current loads approximately downwind and anchor rode loads approximately upwind. So the boat won’t move much fore and aft.

As such the mass of the boat need not be considered, it’s an approximately static equilibrium.

Boats at anchor are also subject to 2 non-steady loads:
Wind gusts
Waves.

In both cases there is a spike in the loads imparted on the boat which relatively quickly abates.

As the wind/wave load spikes up it is out of balance with the resisting load from the anchor there is a net load aft. As the boat moves aft the nylon stretch’s or the chain catenary straightens out increasing the road load until it equals the new wind/wave load which is probably decreasing by this point and now the net load has reversed and the boat comes to a stop and starts accelerating forward.

All of the loads are determined by the dimensions of the boat.

Here is where mass enters the equation, acceleration is dependent on mass. For 2 boats with the same dimensions the heavier one will be accelerated to a lower speed aft than the lighter one but being heavier will take longer to come to a stop than the lighter one. Longer in time but not distance.

The total energy and momentum absorbed from the gust/wave will be about the same for each boat regardless of mass. So the total movement aft will be about the same, which means that the peak load on the anchor rode will be about the same for both boats.

It won’t be exactly the same because of the increased speed of the lighter boat means it will ride the wave slightly longer absorbing a bit more energy that needs to be discharged into stretching the rode.

While mass has an effect on peak anchor loads, it is a small fraction of the affect that boat dimensions have. This is why the ABYC load table is only dependent on dimensions.

[kmacdonald]

Can we safely ignore the displacement value in the Rocna suggested anchor size table if it's dependent mainly on dimensions?

[Adelie]

I would.

I forgot to indicate that I would upsize the anchor 1 step or better 2 steps from the size recommended by the manufacture.
To keep the weight from getting out of hand I’d upgrade the chain grade.

[MathiasW]

Originally Posted by Adelie

Boats at anchor are subject to 2 steady loads:
Here is where mass enters the equation, acceleration is dependent on mass. For 2 boats with the same dimensions the heavier one will be accelerated to a lower speed aft than the lighter one but being heavier will take longer to come to a stop than the lighter one. Longer in time but not distance.

The total energy and momentum absorbed from the gust/wave will be about the same for each boat regardless of mass. So the total movement aft will be about the same, which means that the peak load on the anchor rode will be about the same for both boats.

It won’t be exactly the same because of the increased speed of the lighter boat means it will ride the wave slightly longer absorbing a bit more energy that needs to be discharged into stretching the rode.

While mass has an effect on peak anchor loads, it is a small fraction of the affect that boat dimensions have. This is why the ABYC load table is only dependent on dimensions.

Well, I do not think that the energy and momentum of the two vessels will be similar. For once, it is the apparent wind that accelerates the vessel, not the true wind, and so the vessel that will get accelerated faster will see less wind force.

More formally, the additional gust force leads to F(t) = m * a(t)

Assume the gust acts over a time T and in this time will displace the vessel by a total distance S

Then the energy absorbed is force integrated along the path s from 0 to total length S

E = Integral{0,S} F(s) ds = Integral{0,T} F(t) ds/dt dt = Integral{0,T} F(t) v(t) dt

or

E= m * Integral{0,T} a(t) v(t) dt = m * Integral{0,T} dv/dt v(t) dt

and thus

E= m * Integral{0,v_max} v dv = 1/2 * m * v_max^2

which is simply the kinetic energy obtained at the end of the gust.

Whether this is the same for both vessels or not cannot be inferred from this equation alone.

But then consider conservation of energy and momentum of the system vessel plus gust 'cloud', which may not hold exactly, but for now let's assume that.

If you consider a ball in flight (the gust cloud) hitting a heavier ball at rest (the vessel), then these conversation laws have it that the heavier the vessel is compared to the gust cloud, the less energy is transferred.

With this one would conclude that, all other things being eqal, lightweight vessels get more energy transferred to than heavier vessels. Therefore, the dynamic aspects of gusts and waves are more serious for smaller, lighter vessels.

Waves that simply bounce off a large, heavy commercial ship without any effect can wash away a smaller boat completely, thereby transferring a large kinetic energy.

Need to go now to get some provisioning aboard, but perhaps later I will have a look at a series of smaller balls hitting a heavier ball in succession, which may be a bit more accurate model. To see whether that changes the picture. I do not think so, but just to be sure.

Cheers,

Mathias

[Adelie]

Originally Posted by MathiasW

Well, I do not think that the energy and momentum of the two vessels will be similar. For once, it is the apparent wind that accelerates the vessel, not the true wind, and so the vessel that will get accelerated faster will see less wind force.

More formally, the additional gust force leads to F(t) = m * a(t)

Assume the gust acts over a time T and in this time will displace the vessel by a total distance S

Then the energy absorbed is force integrated along the path s from 0 to total length S

E = Integral{0,S} F(s) ds = Integral{0,T} F(t) ds/dt dt = Integral{0,T} F(t) v(t) dt

or

E= m * Integral{0,T} a(t) v(t) dt = m * Integral{0,T} dv/dt v(t) dt

and thus

E= m * Integral{0,v_max} v dv = 1/2 * m * v_max^2

which is simply the kinetic energy obtained at the end of the gust.

Whether this is the same for both vessels or not cannot be inferred from this equation alone.

But then consider conservation of energy and momentum of the system vessel plus gust 'cloud', which may not hold exactly, but for now let's assume that.

If you consider a ball in flight (the gust cloud) hitting a heavier ball at rest (the vessel), then these conversation laws have it that the heavier the vessel is compared to the gust cloud, the less energy is transferred.

With this one would conclude that, all other things being eqal, lightweight vessels get more energy transferred to than heavier vessels. Therefore, the dynamic aspects of gusts and waves are more serious for smaller, lighter vessels.

But this is at odds with the Rocna sizing chart which allows larger lighter vessels so use the same anchor as smaller heavier ones.

Originally Posted by MathiasW

Waves that simply bounce off a large, heavy commercial ship without any effect can wash away a smaller boat completely, thereby transferring a large kinetic energy.

Now you are co-mingling the effects of heavy with large and small with light when the question is what are the separate effects. The larger vessel is going to have a larger anchor and chain so the comparison doesn't work.

Originally Posted by MathiasW

Need to go now to get some provisioning aboard, but perhaps later I will have a look at a series of smaller balls hitting a heavier ball in succession, which may be a bit more accurate model. To see whether that changes the picture. I do not think so, but just to be sure.

Cheers,

Mathias

You are trying to argue from first principles to a conclusion which is OK for a start fi you do not yet having testing data.

I was using first principles to explain the ABYC tables. I understand that the ABYC tables are based on testing with an extremely healthy factor of safety thrown in. Heinz's anchoring book is similar in that vessel mass plays little if any part in the anchor loads calculated.

So why does Rocna include vessel mass in their anchor sizing table? Maybe they did their own anchor testing. More likely I believe they are giving buyers what they expect. When you are trying to sell somebody something, don't argue with them about physics, confirm what they believe and take their money.

[Lodesman]

Sometimes an anchor is used as an emergency brake - then mass plays a big part in the equation. Any manufacturer's guide is just that - a guide. A responsible consumer will weigh that against their individual needs and capabilities.

[Adelie]

Using the anchor as a brake is an edge case that occurs very rarely compared to how often it is used normally.

The point of this thread is for members to hash out and come to understand what goes into the anchor sizing tables so they can use them “responsibly”.

[Lodesman]

Originally Posted by Adelie

Using the anchor as a brake is an edge case that occurs very rarely compared to how often it is used normally.

A primary function of an anchor is as an emergency device, regardless of whether it ever gets used for that or not. When you decide on number, type and size of fire extinguisher(s) on board, you factor in the potential magnitude of a fire. Same thing with sizing an anchor that could potentially be used in an emergency situation.

[Auklet]

Originally Posted by Lodesman

A primary function of an anchor is as an emergency device, regardless of whether it ever gets used for that or not. When you decide on number, type and size of fire extinguisher(s) on board, you factor in the potential magnitude of a fire. Same thing with sizing an anchor that could potentially be used in an emergency situation.

Yes. But emergency use implies emergency-capable deployment as well. Such thoughts have arisen while cruising along, and near, the Ventura country coastline between Channel Island and Ventura harbors approaching the head marker leading into Ventura Harbor. The marker is in 35 ft, but if approaching from the south along the shore you will see 30 ft or less @ 1/2 n.mi. from the beach. West swells (not steep) arriving on your beam may be 3 to 4 ft high. This might occur during very light wind. So if motoring and the diesel quits near there you probably have an EMERGENCY! If you cannot get the boat moving properly under sail and your bower is not up to the task to grab and hold under the arriving swells, I doubt you’d have much time to assemble any secondary ground tackle for emergency use and get it over the side. Near that area but inshore by another 500 yds (450 m) the chart indicates Breakers and yes, they do form periodically. So indeed, choose the best anchor and tackle you can as your bower.

And BTW this is when you’d want to get your bower down FAST; Gravity Drop! RE: Posts #103, #135 and related. Potentially not a good deployment situation for a Fortress.

[Auklet]

Originally Posted by Adelie

Using the anchor as a brake is an edge case that occurs very rarely compared to how often it is used normally. .

I think one has to look at it via the frequency vs magnitude dynamic. Events of low magnitude (e.g., most earthquakes, tropical storms, meteor impacts, most wave heights) are relatively commonplace. Conversely, events of high magnitude (Richter 9+ earthquakes, Cat 5 hurricanes, large asteroid impacts or gigantic, 100-ft waves) are rare but they do come along. I guess the question is what do you want to be riding on if one big (high magnitude) event comes your way?

[Icarus]

Originally Posted by Izikalvo

Most boats usually have a weight/length ratio that is common,



what is your way of choosing anchor size when your boat is beyond "normal" say, you have a 40Ft boat that weighs 20 tons, or the opposite, you have an 80 Ft long boat that weighs only 20 tons.

Obviously, when considering only length your are doing something wrong.



usually, anchor charts are related to boat length, and some have also weight, but where does the weight factor come from?



Any thoughts?

Obviously common sense ought to kick in when choosing anchor size?

[Dockhead]

Originally Posted by wingssail

We always "power down" and have done so since we installed the windlass 29 years ago. We've anchored in depths of 12' to 95', on a variety of bottoms. . .

We do too.

Originally Posted by wingssail

We generally keep the boat in position until the anchor is on the bottom. In windy conditions that requires a little planning and skill but it is doable. After the anchor is on the bottom we let the boat carry astern (either naturally or by engine) as we let out chain and then when the desired scope is reached stop deploying chain and set the anchor.. . . .

I let the boat drift slowly back once the anchor hits the bottom, as the chain goes out.


A big issue is keeping the bow from being blown off by the wind during this process. Why God made bow thrusters (one of the reason).

[s/v Jedi]

Thrusters used to deal with problems for electric lowering of anchor while you can do it like it’s supposed to be done without all these troubles? Really!?

Maybe it’s better to use the virtual anchor option that the megayachts have? Just go where you want to be and press the button and you’re done only needs a stern thruster added to the arsenal of gadgets

I actually saw 45’ sailboats with stern thrusters… and they needed them to prevent hitting things in the marina !

[goboatingnow]

Originally Posted by Nicholson58

My autopilot was useless. It requires GPS and forward motion. Hand steering was all that could be done. If you must resort to this, it will take you a while to feed-forward program your brain to anticipate the boat’s next move. It helped that there were bridge lights to aim for.

Most APs don’t require GPS my brand new EVO1 9 axis system doesn’t.

[goboatingnow]

Originally Posted by Lodesman

A primary function of an anchor is as an emergency device, regardless of whether it ever gets used for that or not. When you decide on number, type and size of fire extinguisher(s) on board, you factor in the potential magnitude of a fire. Same thing with sizing an anchor that could potentially be used in an emergency situation.

Very rarely used that way these days , some vertical windless have no gravity drop option at all.