Weight of boat with moorings
- Thread starter Franklin
- Start date
But the boat doesn't accelerate down the wave in a way that opposes the anchor. The boat is pulled away from the anchor by the lifting face of the wave. Once the boat is lifted, it does have more potential energy, but assuming the waves are coming from the direction of the anchor, the PE will not be converted into KE in a direction away from the anchor.
Moorings are analogous to anchors, right?
Try the tables, here:
Anchor System Sizing Tables (Reply #6) & Why Swivels are a bad idea http://c34.org/bbs/index.php/topic,4990.msg30400.html#msg30400
Try the tables, here:
Anchor System Sizing Tables (Reply #6) & Why Swivels are a bad idea http://c34.org/bbs/index.php/topic,4990.msg30400.html#msg30400
You are absolutely right! What though is exactly your point? I could certainly imagine a nice streamlined narco-sub with barely any superstructure wouldn't need the same mooring as a normal yacht. So now how much weight to you need for your boat? Rules of thumb have been built up over the years the hard way at other peoples expense. You seriously might want to pay some heed.
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Ok, I'm not an expert on the physics of all of this but one other thing to keep in mind. All of the equations I've seen here talk about the load as a single event. You must remember to factor in the waves have a period, i.e.: number per minute. Each of these forces as calculated must be measured and calculated for each wave and then added (I'm sure with some factoring) to get a total load on the mooring anchor and mooring pendant. Only then will you get an idea of what happens to a boat in an anchorage. Anyone can put an anchor down even at a 3/1 scope and hold a boat for 1 or 2 minutes. However, when measured over the life of a storm, 3/1 wouldn't hold most dingies. Now go to a large mooring anchor, a 10k lb weight wil hold most boats just fine but put it into a storm and you would be amazed at how far a 40' boat can move the same mooring anchor.
Time is the factor that must be added to every one of the calculations.
Interesting conversation. It's a good one to follow
Victor
Time is the factor that must be added to every one of the calculations.
Interesting conversation. It's a good one to follow
Victor
I have yet to see any theoretical evidence that equals reality, especially when it comes to anchoring a boat, and I'll wager that includes a mooring, though I can only use anchoring as my basis, as I rarely use moorings.
Given the proper tackle in the water I really couldn't care how much wind there is when I'm anchored in a place where there are no waves. I've been anchored in well over 100 knots of wind more times than I can count and never been overly concerned that my anchor tackle will break from the wind alone.
However, introducing waves brings a whole new stress into that equation. As no vessel ever has chain that is actually strong enough to hold the vessel once the chain is straight between the anchor and the bow, the surge as one reaches the top of the wave will easily break the chain where straight wind pressure will not. I've seen it happen.
There are many instances where a sudden jerk will break something that constant pressure will not. Every shade tree mechanic knows that.
I really do not understand the point of your post as I can't see even you building a mooring that is any lighter than the heaviest mooring you could possibly put down.
Given the proper tackle in the water I really couldn't care how much wind there is when I'm anchored in a place where there are no waves. I've been anchored in well over 100 knots of wind more times than I can count and never been overly concerned that my anchor tackle will break from the wind alone.
However, introducing waves brings a whole new stress into that equation. As no vessel ever has chain that is actually strong enough to hold the vessel once the chain is straight between the anchor and the bow, the surge as one reaches the top of the wave will easily break the chain where straight wind pressure will not. I've seen it happen.
There are many instances where a sudden jerk will break something that constant pressure will not. Every shade tree mechanic knows that.
I really do not understand the point of your post as I can't see even you building a mooring that is any lighter than the heaviest mooring you could possibly put down.
My point is this...when trying to figure out if a mooring can hold a boat, it is not worth asking how much the boat weights, but what is the windage of the boat.
To further my explanation of this: I am sure most of you understand the rule that Energy can't be created, it can only be transferred. The energy comes from the wave. The size of the boat doesn't magnify the energy transferred to the mooring rope, it just transfers what the wave pushed on the boat. If the boat is heavier, the boat moves slower so it is the same as a lighter boat that moves faster. Same energy. The boat can't produce energy. The boat just moves as far and fast as the result of the wave and weight of the boat colliding. If it is a light boat it moves farther and faster than the heavier boat, but the pull on the rode is the same.
You know what...as I type this I just figured out the flaw in my own theory.
A heavier boat sits lower in the water and therefore catches more of the wave (assuming the power goes deep enough...some waves like dinghy wakes have very little depth) and therefore a heavier boat will absorb and transfer more energy to the mooring rode.
Ok...with that last statement I wrote there I am convinced I was wrong...thanks for helping me fix my own theory by forcing me to dig deep into it.
To further my explanation of this: I am sure most of you understand the rule that Energy can't be created, it can only be transferred. The energy comes from the wave. The size of the boat doesn't magnify the energy transferred to the mooring rope, it just transfers what the wave pushed on the boat. If the boat is heavier, the boat moves slower so it is the same as a lighter boat that moves faster. Same energy. The boat can't produce energy. The boat just moves as far and fast as the result of the wave and weight of the boat colliding. If it is a light boat it moves farther and faster than the heavier boat, but the pull on the rode is the same.
You know what...as I type this I just figured out the flaw in my own theory.
A heavier boat sits lower in the water and therefore catches more of the wave (assuming the power goes deep enough...some waves like dinghy wakes have very little depth) and therefore a heavier boat will absorb and transfer more energy to the mooring rode.Ok...with that last statement I wrote there I am convinced I was wrong...thanks for helping me fix my own theory by forcing me to dig deep into it.
Franklin gets it. the anchor/mooring resist the motion of the boat AND the wind force pushing the boat. Worse case loading is just before the top of the wave where the angle of the boat is greatest WRT the horizon and the wind pushing it in the same direction (steady state wind making waves for a while all going in the same direction). Since the boat is now at an angle to the horizon but still normal to the water surface the normal (to the wave face) force is just the weight supporting the boat (at an angle and trying to accelerate it in the now not aligned to the gravity field). the force horizontal (assuming sufficient scope) is W*sin(wave angle measured from the horizontal). At 0 (calm seas) the force is 0, at 90 the force is equal to the weight of the boat.......hey the weight does in fact have an impact. Ask yourself the following; if I where holding the painter of a dinghy and had the choice of holding an empty dingy or the same dingy loaded with junk and people which would be easier to pull up the boat wake?
To franklin's point of big ships you do get a benefit from having a LWL that is much greater than the wave wave length as the angle of the boat due to the wave is much lower.
No analysis for wind or the acceleration that the weight force (not aligned with the vertical) was attempted or implied as that was not the question of the OP.
To franklin's point of big ships you do get a benefit from having a LWL that is much greater than the wave wave length as the angle of the boat due to the wave is much lower.
No analysis for wind or the acceleration that the weight force (not aligned with the vertical) was attempted or implied as that was not the question of the OP.
Good argument but the sizing of moorings has pretty well been addressed by years of trial and error. I think there are just too many variables to be able to improve on the local knowledge passed from generation to generation on how to set up an adequately safe mooring.
If you read my last post you would have realized I found the flaw in my theory. Also, I am not the type of person to just take somebody's word for anything; it has to make sense to me. It makes sense to me now. Sometimes I never fully understand something until I try to argue my point and then it becomes clear I am either right or wrong. I have no problem admitting I am wrong because it is the truth I am after, not me being right.
This one should work... Up to 22,000 LBS. Measure it and let us know.
http://www.ebay.com/itm/Digital-Pus...External-Sensor-HF-100K-1-100KN-/271359113070
http://www.ebay.com/itm/Digital-Pus...External-Sensor-HF-100K-1-100KN-/271359113070
Sometimes I can get an understanding of something by greatly exaggerating the factors involved. Instead of thinking about a heavy 40' boat vs. a light 40' boat, let's take it to an extreme.
And, never mind how a boat gets moved by wind and waves. We know for certain that the boat gets moved.
Let's go from water to land, and look at two large objects. Object one is a "Big Boy" steam locomotive. Object two is an air filled thin skin balloon identical in size and shape to the locomotive. Object one is on wet ice, has a rope attached to it, and you are on the other end of the rope on land, wearing cleated shoes. The locomotive is moving at six mph, and you have to stop it by pulling on the rope. If you don't stop the locomotive, it's going to crash into your home.
Or, you have the option of being on the end of a rope attached to the balloon on the same wet ice, bearing down on your home at 6 mph.
There is no wind. Never mind how I got items one and two to move.
Which of the two objects would you prefer to attempt to stop in order to save your home?
Seems like it's all about how much weight is attached to the line. And when that weight moves, takes all the slack out of the line, and comes to a sudden stop....OUCH.
And, never mind how a boat gets moved by wind and waves. We know for certain that the boat gets moved.
Let's go from water to land, and look at two large objects. Object one is a "Big Boy" steam locomotive. Object two is an air filled thin skin balloon identical in size and shape to the locomotive. Object one is on wet ice, has a rope attached to it, and you are on the other end of the rope on land, wearing cleated shoes. The locomotive is moving at six mph, and you have to stop it by pulling on the rope. If you don't stop the locomotive, it's going to crash into your home.
Or, you have the option of being on the end of a rope attached to the balloon on the same wet ice, bearing down on your home at 6 mph.
There is no wind. Never mind how I got items one and two to move.
Which of the two objects would you prefer to attempt to stop in order to save your home?
Seems like it's all about how much weight is attached to the line. And when that weight moves, takes all the slack out of the line, and comes to a sudden stop....OUCH.
OK, I'll give it a try.
The way a boat moves in waves is governed by its Froude number. Boats with the same Froude number will respond the same way to waves. Froude number is a function of velocity (speed of the wave in this case) and the inverse of the square root of length. Mass doesn't enter in to the equation.
The length in this case is your lwl. The reason a big ship does not move as much in the same waves is its great length, not its greater mass. A boat's mass goes up as the cube of its length. The force and energy of the boat's movement are both linear functions of mass. So as length goes up, response to waves only goes up by the inverse of the square root of the length, while force and energy go up by its cube. So longer boats need heavier ground tackle, whether an anchor, or mooring.
By the way, I would think, it would be the max force through wave cycles that would be the biggest danger to ground tackle, because some force will overwhelm some component of the system, for instance, pulling it out of the bottom, or breaking a component. Energy tends to be a problem only for line, which can accumulate increasing heat or abrasion as a result of repetitive stretching, leading to fiber breakdown.
As a result, the West Marine (and other) recommendations for anchor rode strength, and anchor weight increase with boat length. The recommendation for nylon rode thickness is 1/8" per 9 feet of lwl. (Strength goes up with the square of thickness.)
So a longer, heavier boat will require heavier mooring gear.
While this is a general, dimensional analysis, reading more detailed analyses and experiments will produce the same conclusion. Detailed dynamics are complex. Wind speed can be important at higher velocities. Wind acts on the cross section of the boat and varies with the square of the velocity. In open water, (that is infinite fetch) wave height and energy increase with wind speed, and waves remain the dominant issue.
The way a boat moves in waves is governed by its Froude number. Boats with the same Froude number will respond the same way to waves. Froude number is a function of velocity (speed of the wave in this case) and the inverse of the square root of length. Mass doesn't enter in to the equation.
The length in this case is your lwl. The reason a big ship does not move as much in the same waves is its great length, not its greater mass. A boat's mass goes up as the cube of its length. The force and energy of the boat's movement are both linear functions of mass. So as length goes up, response to waves only goes up by the inverse of the square root of the length, while force and energy go up by its cube. So longer boats need heavier ground tackle, whether an anchor, or mooring.
By the way, I would think, it would be the max force through wave cycles that would be the biggest danger to ground tackle, because some force will overwhelm some component of the system, for instance, pulling it out of the bottom, or breaking a component. Energy tends to be a problem only for line, which can accumulate increasing heat or abrasion as a result of repetitive stretching, leading to fiber breakdown.
As a result, the West Marine (and other) recommendations for anchor rode strength, and anchor weight increase with boat length. The recommendation for nylon rode thickness is 1/8" per 9 feet of lwl. (Strength goes up with the square of thickness.)
So a longer, heavier boat will require heavier mooring gear.
While this is a general, dimensional analysis, reading more detailed analyses and experiments will produce the same conclusion. Detailed dynamics are complex. Wind speed can be important at higher velocities. Wind acts on the cross section of the boat and varies with the square of the velocity. In open water, (that is infinite fetch) wave height and energy increase with wind speed, and waves remain the dominant issue.
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NotCook
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- Dec 29, 2008
- 806
Exactly my thoughts.
However, like Franklin, I too have frequently pondered what are the physics involved and what are the forces in play on the anchor chain and mooring. In my practical experience, it is the bow of the boat raising on the wave that puts the strain on the chain, whether an anchor or a mooring. That point where the chain straightens and comes to an abrupt stop with a profound jerk is what makes me nervous. In our case, we have a 75# Bruce, with 400 feet of 5/8" chain. If we have 200' of chain out, we have 675# of chain and anchor hanging there. Yet waves beating on the bow will yank that chain straight with a snap and a it is frightening how it jars the whole boat with a frightening bang - and that is after lifting all that chain until is straightens out. A snubber with a line with some stretch is essential, yet we lost the big galvanized chain hook from our snubber last spring when the snapped straight and parted the 5/8" snubber line we used on it. I was standing at the bow watching when it happened. Likewise, our mooring has 7 5'8" chains, each 40' in length, arrayed out in an arc predominantly in the direction of the prevailing wind, each with a 6' sand screw at the end.
A commonly understood anchoring principle is that chain holds the rode down at a n angle that keeps the anchor properly embedded in the bottom. Now, interestingly, we frequently find that even in a good breeze, our anchor chain hangs straight down even in only, say, 20 feet of water. That is only about 60# of chain actually hanging there, yet the boat doesn't move. In fact, as the breeze changes, we frequently just swing around the point where the chain touches the bottom.
Here is the interesting and, I think, significantly relevant detail: our vessel is 80,000#. My point, and my feeling, irrespective of whatever is the applicable formula (and which is the applicable formula is a great question...), is that the waves and the mass of the vessel are the relevant considerations.
NotCook
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- Dec 29, 2008
- 806
You and me both, Franklin. Some people, like my Admiral, don't seem to get that! I tend to argue a point until you prove me wrong, and at that point I appreciate the new understanding. This has been an interesting thread.