Anchor rode/bridal/snubber?Proper placement?

[psneeld]

Well....I think it changes the scope ratio based on where the snubber/rode connection hangs.

Granted as the wind or current increases...it steadily becomes a smaller ratio untill the snubber/rode is a straight line from where it touches the boat to the anchor

It doesnt change the atrachment point for ultimate conditions, but it changes the math for the scope at any moment or average conditions.

 

[Steve Bedford]

“Well....I think it changes the scope ratio based on where the snubber/rode connection hangs.

Granted as the wind or current increases...it steadily becomes a smaller ratio untill the snubber/rode is a straight line from where it touches the boat to the anchor

It doesnt change the atrachment point for ultimate conditions, but it changes the math for the scope at any moment or average conditions.”

Sorry, I must be missing something here. If i anchor in 10’ of water and the distance from the bow roller to surface of the water is 5’, I use 15’ as my depth to calculate the amount of rode to lay out to achieve the desired ratio. In this hypothetical, if I want a 7:1 ratio, I need to let out 105’ of rode.

Now in this hypothetical, if i want to add a 10’ snubber, and I attach that snubber at the 95’ mark on the chain rode, I still have a 7:1 ratio. If I attach the 10’ snubber at the 105’ mark on the rode then yes, the rode is now 115 in length and the ratio is 7.67:1.

If I allow more chain to hang freely (not resting on the bottom) as dead weight after the attachment point of the snubber onto the chain then:
I have created basically a kellet effect in helping to keep the chain “down”
This helps to better maintain the cantenary.
I “could” re-consider if I really need a 7:1 ratio with this added weight of chain hanging down but I probably would not do that. I would rather have the scope I want and with the added weight to better maintain the cantenary.

 

[psneeld]

The trigonometry is there...so I wont debate my point further....too simple.

If I thought I was going to anchor some place where my chain was going to become a straight line....I think I would find a better anchorage or marina.

If broken down and facing a big bliw coming up....I might be letting most of my chain out and who cares what the theoretical or real scope is....its all I got to give.

People who try and make anchoring too scientific or difficult I believe either need more experience or wind up in anchoring situations I prefer not to be in.

 

[cafesport]

Snubbers are like bilge pumps,you don’t need them until you do, and all snubbers are not equal. Here is a photo of a snubber that doesn’t even snub.

 

[dhays]

I understand psneeds point, a Steve pointed out, the use of a bridle or snubber can often act like a lightweight kellet. This changed the angle that the rode takes, but not the scope.


OTOH, I attach my bridle at my forward hawse holes. This is about 18" below the bow roller. So this does change the scope ratio a tiny bit. Certainly not enough to matter. My depth sounder is set at -5' and my bow roller is about 6' above the water. To calculate scope I just add 10' to what my depth sounder says. Not exact, but close enough.

 

[ben2go]

Snubbers are like bilge pumps,you don’t need them until you do, and all snubbers are not equal. Here is a photo of a snubber that doesn’t even snub.View attachment 82008

 

[Steve Bedford]

“The trigonometry is there...so I wont debate my point further....too simple.

If I thought I was going to anchor some place where my chain was going to become a straight line....I think I would find a better anchorage or marina.

If broken down and facing a big bliw coming up....I might be letting most of my chain out and who cares what the theoretical or real scope is....its all I got to give.

People who try and make anchoring too scientific or difficult I believe either need more experience or wind up in anchoring situations I prefer not to be in.”

That is a little strong psneeld! From someone who believes “people who try and make anchoring too scientific or difficult (you) believe either they need more experience or wind up in anchoring situations (you) prefer not to be in.” You brought up the trigonometry which does not simplify things.

The length of rode (chain, chain/rope, or rope) that goes from the attachment point on the vessel to the anchor makes up the scope. This is calculated from the vertical distance from the attachment point on the boat to seabed forming a 90 degree angle. For example: distance from attachment point on boat to sea level (5’) and sea level to seabed (10’) for a total of 15’. Using a 10:1 ratio, that length of rode needs to be 150’. Regardless of the catenary. Sagging rode in the water on the seabed or pulled straight bar tight, the scope is the same. Sure, with all chain and totally calm waters, the boat may indeed be positioned right on top of the anchor because there is no tension on the rode. The scope you laid out is still 10:1. As the wind picks up the vessel moves away from the anchor. The centenary begins to form an arc. The heavier the rode is or with a kellet, the more resistance to pulling straight. This continues to keep the angle of the anchor rode attachment point on the anchor shank to the seabed as low as possible for as long as possible. The stronger the winds, eventually the rode will be bar tight. The scope has not changed. Still 10:1.

Pretty simple.

 

[psneeld]

Pretty simple but could be wrong....depending what scope really is...point where the angle is formed or attachment on the vessel.

I think you are missing the basics....

No use after two attempts....

 

[Steve Bedford]

Correct, psneed, I must be missing the basics. This is on me for not understanding what you are sharing.
Steve

 

[psneeld]

Here's my take on it...

 

[MurrayM]

Here's my take on it...

What if the wind comes up?

As a relative newcomer to boating (without your years of experience) I tend to make scope calculations with a worst case scenario mindset, so I'm with Steve on this one.

 

[psneeld]

If you read my posts, I agree that when the rode becomes straight ..it's true...but that's not where the discussion started.

I just try and keep the facts straight, not just what someone thinks based on part of the discussion.

And also like I posted before, If I think my rode of all chain is going to be pulled straight, I feel like I have already screwed up anchoring for the night.

 

[Steve Bedford]

Thanks psneed. I agree with your diagrams and agreed earlier.

However, the “ratio” is inaccurate. You are labeling the angle of the shank as the ratio of the rode to the seabed. The ratio in anchoring is the length of the rode to the depth (height of the attachment to the vessel to the seabed). That is the scope and that is what I have been saying has not changed. Somewhere earlier, someone said the scope changed. It has not! Yes, that shank/rode to seabed angle changes with added weight pulling the rode downward making a stronger more resisting centenary.

Thanks for clarifying why we were disagreeing. I am not proclaiming to be a “know everything” but I do believe there are folks reading the posts and not posting. They are trying to learn. I try to explain in simple terms what will help them better understand what is happening underwater during the anchoring process.

Steve

 

[Steve Bedford]

“As a relative newcomer to boating (without your years of experience) I tend to make scope calculations with a worst case scenario mindset...”

With that approach, I will gladly anchor in the same area as you. That is the best way to proceed.

I have been in sitiuations where my entire deployed chain and snubber are tight. With a scope of 10:1 or more, that angle of the shank/rode to seabed is still small and with a good anchor, holding well.

Steve

 

[psneeld]

And I disagreed with the way you were explaining reality to the newbies....sorta still do.

I'll just let the simple diagrams tell the story.

 

[Steve Bedford]

Okay psneed, you get the last word. I am done.

I refuse to call anyone “newbies.” I believe that is disrespectful and not helpful. If anyone has further questions for me for me about this, I encourage them to send me a private message.
Steve

 

[MurrayM]

Words are slippery things, and while clearly describing something in the originators head they can have varied success depending on who's reading it.

As far as explaining scope goes, here's the best description I've found: Anchor Scope Illustrated Photo Gallery by Compass Marine How To at pbase.com

 

[dhays]

The diagrams help. We tend to think of scope in terms of a ration such as 3:1, 5:1 or 7:1. As psneed was trying to point out, is that those scopes really describe the angle between the rode and the sea floor. A 5:1 scope is a bit over 11 degrees. What matters as far as holding is that angle.

Of course with the catenary, the actual angle is a lot less. As psneed’s diagram shows, the bridle decreases the angle. If scope is really the angle, then in that regard the bridle has the effect of increasing the scope.

Of course, if the conditions worsen and the catenary decreases, then it won’t matter. Most of us choose a scope with the idea that conditions won’t eliminate the catenary. If so, as psneed has said, then we either have chosen the wrong spot or chose inadequate scope.

 

[sunchaser]

To calculate scope I just add 10' to what my depth sounder says. Not exact, but close enough.

Don't forget to add in the added height from current depth to high tide. I've added as much as 15 a 18 vertical feet when anchoring at low tide in many BC and AK locales. That can amount to a lot of rode.

On a separate note, I've "lost" cantenary in gales and large waves. Surprising how common it is for commercial fisherman, or so they tell me.

 

[dhays]

Don't forget to add in the added height from current depth to high tide. I've added as much as 15 a 18 vertical feet when anchoring at low tide in many BC and AK locales. That can amount to a lot of rode.

On a separate note, I've "lost" cantenary in gales and large waves. Surprising how common it is for commercial fisherman, or so they tell me.

Very true on both counts.

 

[Sabre602]

...Of course, if the conditions worsen and the catenary decreases, then it won’t matter....

It's an interesting discussion, though it seems to me somewhat academic to split hairs. In the absence of a stiff wind or current, the boat will likely lie nicely to her anchor without it even digging in.

In such conditions, scope matters very little. It's simply the weight of the anchor and rode lying on the bottom that holds the boat "at anchor."

What we're really interested in is what happens when the wind and/or current kick up. These are the situations where the bridle will be straightening in line with the rode. The catenary won't necessarily be out of a chain rode, of course, but the point is that the effect of the bridle under these conditions will have little to do with changing scope.

It will, however, if long and springy enough, dampen surging loads on the anchor and on the point of attachment on the foredeck.

 

[BruceK]

It's an interesting discussion, though it seems to me somewhat academic to split hairs. In the absence of a stiff wind or current, the boat will likely lie nicely to her anchor without it even digging in.

In such conditions, scope matters very little. It's simply the weight of the anchor and rode lying on the bottom that holds the boat "at anchor."

What we're really interested in is what happens when the wind and/or current kick up....

Indeed. If the anchoring procedure leaves the boat lying to "the weight of the anchor and rode" "without it even digging in", expect unpleasant unsafe consequences.
Surely no one drops the anchor and leaves it at that, even in innocuous conditions. No laying out the rode, no helping the anchor dig in (if yours needs help), no testing the set?

 

[Steve Bedford]

I believe this discussion took a problematic turn over the definition of terms. Scope is the length of rode from the attachment to the vessel to the attachment on the anchor.
The ratio for anchoring is the length of the rode in relation to the depth from the attachment point on the vessel vertically down to the seabed floor.
The angle in question is the angle created from the rode attachment on the anchor and the subsequent rode to the seabed floor.

The scope is not that angle. The only way to change the scope is to change the length of the anchor rode, +/-. The angle is constantly changing from 0 degrees (complete calm) and no stress or pull on the anchor to the maximum angle with the rode pulling bar tight. That is the angle folks focus on when determining the amount of rode to lay out...their scope. One has no idea the angle of the shank/attachment point to the rode and the seabed except under two conditions: dead calm or bar tight pulling. Anywhere in between is a guess.

Yes, the scope (ratio of depth to the amount of rode layer out) effects the angle but the scope is not the angle. I believe that is where the disagreement occurred. The diagrams shown the “angle” labeled as the “ratio” and that is incorrect.

Added weight pulling the snubber down (kellet or added chain weight after the snubber attachment point on the rode) changes (decreases) that angle slightly under conditions other than bar tight. At bar tight pulls the angle is whatever the math tells us according the scope (the ratio) laid out.

Steve

 

[psneeld]

I apologize.

Reread the whole thread again and the definitions given by Steve are accurate and should be used accordingly.

I just wanted to make sure that there is a lot of "talk" about anchoring" that never really fits what I have leaned on the water.

Ultimately its the angle of attack of the anchors flukes that matrers in anchoring....current conditions OR if the wind pipes up.... not scopes or ratios or attachment poiints...though they all exist and people do need to understand their part of every anchoring evolution.

Often left out of the discussion is seabed angle if it could be an issue....., which further complicates the "strict" scope and ratio usage...though as pointed out, doesnt change the terminology.

 

[Steve Bedford]

Well stated. Excellent advice.

Steve

 

[MurrayM]

Often left out of the discussion is seabed angle if it could be an issue....., which further complicates the "strict" scope and ratio usage...

Good point.

Anchoring on steep slopes is 'normal' in my area. Douglas Channel cuts 60 miles into BC's coast mountains, so the main channel and tributary channels are very deep. One is typically forced to anchor at the heads of inlets on the slopes of river or creek estuary outwash fans, and large tides complicate things further. The first couple times we anchored on steep slopes I drew out some diagrams, just to help visualize how the anchor shank would be laying in relation to the bottom if the wind came up.

This summer, if I remember correctly, we anchored in 100' of water, let out a little over 100' of rode, pulled the anchor to set it 'uphill' and had about 10' of water under the stern at low tide. That put the anchor shank at an angle to the bottom about the same as 1:4 or 1:5 on a flat bottom...guesstimating.

Weather was stable in that wind was from the water during the day, but came lightly from shore during the night. Considered stern tying, but decided to trust the Bruce not to get plucked out of the slope...slept at the helm station with the depth sounder alarm set, just in case!

Should add (because someones going to chastise us for risking it) if we did get blown off the slope and the depth sounder alarm didn't work there was no beach for 20 miles in any direction, only a twisting, narrow waterway with near vertical rock walls...the boat bumping its rub rail against rock with hundreds of feet under the keel would have woken us up).

 

[sunchaser]

Murray
You raise a good point. Secure anchorages in your area are hard to find. Improvisation and understanding tides, currents and weather are much more important than text book anchoring.

The commercial guys in Central and Northern BC know this well and have identified nooks and crannies that work for them but defy anchoring logic as surmised in TF postings.

 

[BruceK]

I believe this discussion took a problematic turn....
Added weight pulling the snubber down (kellet or added chain weight after the snubber attachment point on the rode) changes (decreases) that angle slightly under conditions other than bar tight. At bar tight pulls the angle is whatever the math tells us according the scope (the ratio) laid out.
Steve

First,thank you for your polite persistence pressing your analysis to benefit fellow members.
In the climate of discussing kellets, do you have a view about the Anchor Buddy? Anchor Buddy - anchor weights made in New Zealand A proven anchor sentinel / kellet anchoring system . Not a kellet in the conventional sense of weight dampening the rode, the AB weight attaches to and slides down the rode, to be positioned just above the seafloor. Making for a vertical rode between it and the boat, and a horizontal rode along the seafloor to the anchor, leading to a lateral pull on the anchor.

I see tide as an issue to its working, it would change the position of the AB relative to the sea floor, and perhaps interfere with reset after tide reversals.

 

[Bacchus]

A different explanation might help... my thinking...
The extra weight does not change the length of rode it changes the "effective" attachment point (not the actual attachment). The extra weight changes the angle of the rode to match a shallower depth.
Fir the ex previously mentioned
10 ft depth biw 5 ft above water total depth = 15
W weight and catanery the effective attacment point may be closer to 2ft above water so total "effective" depth = 12 ft

 

[Steve Bedford]

“A different explanation might help... my thinking...
The extra weight does not change the length of rode it changes the "effective" attachment point (not the actual attachment). The extra weight changes the angle of the rode to match a shallower depth.”

Excellent explanation Don!

Steve