Anchor snubber sizing... check my math please?

[ohthetrees]

Recently bought a 2015 50’ Hanse 505. Going through now and taking care of all the issues I can find.

High on the list is an anchor snubber. The previous owner’s “snubber” was about 1m long, and quite thin, maybe 12mm. Obviously, I need an upgrade. But searching through old CF threads, I see many discussions on snubbers, many of which I’ve read. Here is how I think I’m thinking about it. I’m looking for input, and for thoughts about the size I’ve chosen.

From what I understand just choosing a super large rope diameter is not only costly and heavy, but you might not have enough stretch to help relieve load on both anchor and cleats/bow roller.


My plan then is to have two snubber bridles. A light duty and a heavy duty.

The heavy duty will be a bridle snubber made of a single length of rope with an eye splice joining the two legs. I want this one to survive 90kn.
The light duty will also be a bridle snubber made of a single length of rope with an eye splice joining the two legs. I want this one to easily survive 40kn.

This approach will allow me the ease of handling a light weight bridle day-to-day, and provide extra stretch that the heavy duty snubber would not provide in < 40kn wind.

Additionally, I will have a short strop/penant/thingy to take load off the winch, and/or back up the snubber.

Some baseline numbers…

My boat is 50’ (15.25m) long and about 16 tons (14,500 kg).
PO upgraded (mixed feelings on this one) to a 10mm stainless chain offered by Hanse at the time. It’s beautiful, no rust, but I fear crevice corrosion. Will probably have to retire long before “necessary” because I’ll probably lose confidence in another year or two. Anyway, looking a specs of a similar chain working load is 11,000lb (5,100kg), and a breaking load of 22,000lb (10,200kg).
ABYC shows my boat in 60kn wind might see 5000lb load on anchor gear. However, I found a few convincing sources (e.g Sail Delmarva: Actual Anchor Loads) that say that in real world conditions, with a snubber, I can expect to see 1/4 of that (1250lb).
Doing some rough math, if I want my gear to survive 90kn, I can expect double the load of 60kn wind. Pesky math! So that is 10,000/4 = 2500lb real world load.
If I want my heavy snubber to work at 20% of breaking, I need a snubber with a breaking strength of 12,500lb.
For the light snubber at 40kn, I can expect to see 3200/4 = 800lb real world load. A snubber with 800lb working load will have a 800/0.2 (20%) = 4000lb breaking strength.


Those are my base line numbers.


For the heavy duty, on defender, I find Yale 8plait that is 12,200lb breaking load n the 5/8” (~16mm) size. Each leg would be 50’.
For the light duty, I find Yale 8plait that is 4000lb breaking load in the 3/8 size. 3/8"(~9.5mm). Each leg would be 25’.
For the strop, I find 3/8” (9.5mm) has a breaking load of 19600lb. I’d probably use a piece about 3’ long at most.

I intend to construct two legged bridles. It will be one continuous piece of rope, with either an eye splice or a thimble in the middle. I’ll use dyneema soft shackles to attach snubber to chain.

I’m interested in general opinions. But I’d really appreciate feedback on a few specifics.

1) If I want a two legged bridle, should I size the rope so that each leg could alone take the entire load? Or should I assume the legs are working together, and size each to take 1/2 the calculated load? If I make each leg full sized, won’t stretch be too low under most conditions?

2) I prefer not having any metal on the snubber. I prefer a soft shackle. With my bridle, will the eye splice lose too much strength compared to putting a thimble in when using a soft shackle?

3) I I size my heavy duty snubber for 90 knots of wind, will I have too little elasticity for when it is blowing 40kn?

4) My “gut” says that I want larger line than my calculations yielded. What do you think?

Thanks in advance for your feedback!

[Simi 60]

As a reference we also have a light duty snubber and a storm snubber on our 70 tonne high windage full time cruiser.

Light duty snubber is 20mm 3 strand nylon about 3.5m long with a victory hook and it does not touch the water so does not get green.
This , or its previous equivalent, has seen 80 knots.

Heavy duty one is also 20mm 3 strand nylon at 2x the length, heavy duty galv thimble and soft shackle connection.
It has seen no more than 45 knots.

[rslifkin]

Personally, I like dock lines as a snubber. Use the pre-existing eye to prusik hitch it to whatever your rode is made of, then cleat off the other end.

I'd also size your snubber to be more in the 10 - 15% breaking load range under expected conditions for each light and heavy. If you need more stretch than you get at that load, make the snubber longer. Running it at a bit lower load will reduce the risk of it failing in heavy weather and should also make chafe a little easier to manage.

For my 26k lbs, 38 foot boat I decided on 30 foot long 1/2" 3 strand (can always be tied shorter if needed) as a primary snubber for good to moderate weather when anchored anywhere shallow enough to be on all chain. Haven't decided for sure what I want for heavier weather, but I do have some 50' 5/8" dock lines aboard I could use in a pinch (they're double braid, so less stretchy, which isn't ideal). The 1/2" 3 strand should stretch a little over 5% (1.5 - 1.6 feet over a 30 foot length) at 10% load (630 lbs) and about 10% (3 feet over 30 feet) at 20% load (1260 lbs).

[DefinitelyMe]

1) Yes. The load will cyclic, on one leg and then the other as the boat 'tacks' through the wind.

2) Yes. Without doing any maths I think the sharp bend will significantly reduce strength compared to a thimble.

3) I think not at that size, because:

4) I think yes. I once did similar calculations to the ones you have done and decided, based on those, to use a 1/2" (12mm) snubber on our 36' monohull. It snapped a couple of times in sudden gusts. Upgraded to 5/8" and have been happy ever since.

[Breaking Waves]

Commenting in reverse order . . . . .

Originally Posted by ohthetrees

4) My “gut” says that I want larger line than my calculations yielded. What do you think?

Yes, you do. For two reasons. (a) most snubber failures are due to chafe - raw size gives you time to identify and deal with chafe. And further, in severe weather, the elasticity of nylon causes it to heat internally, which significantly weakens it, so that rope is weaker exactly when you need it to be strong. and (b) you need to plan for 'bad case' situation especially with your heavy snubber. You need to plan for waves and snatching and such. That is what the abyc numbers do, and what the '25% of abyc' numbers do not do. There is a good reason the abyc numbers are what they are.

3) I I size my heavy duty snubber for 90 knots of wind, will I have too little elasticity for when it is blowing 40kn?

Not if you have sufficient length. You in fact do not need huge amounts of elasticity to provide good mitigation of surges, and 'too much' elasticity is also not great (increased chafe and internal heating). You just don't want 'no stretch' like short all chain with no snubber.

2) I prefer not having any metal on the snubber. I prefer a soft shackle. With my bridle, will the eye splice lose too much strength compared to putting a thimble in when using a soft shackle?

Inside an eye splice you have double line strength. so you can have almost a 1:1 bend ratio (which reduces strength of the eye by 50%) inside a splice and in fact lose almost no system strength - because you have a 50% reduction of 200% = 100%.

1) If I want a two legged bridle, should I size the rope so that each leg could alone take the entire load? Or should I assume the legs are working together, and size each to take 1/2 the calculated load? If I make each leg full sized, won’t stretch be too low under most conditions?

Each side needs to be able to carry the full load. and no that will not be 'too little' elasticity.

............

[AiniA]

I didn't do the calculations but started with a half inch, three braid bridle for use in all conditions. Each leg was about 30 feet long, and connected to a chain hook, but we generally only would have about 10 to 15 feet out from the chocks on either side of the bow. This was on a Bristol 45.5 that cruised at about 40,000 pounds. My thought was that I would probably want heavier line but would try the lighter one since I had it. The snubber lasted for a four year circumnavigation including some windier bits like Easter Island and South Africa (only anchored once there in about 50 knots). I found it no problem to use a snubber all the time and got redundancy since each side of the bridle was independently. I used chafe gear if we were staying somewhere for a longish time or if the conditions were sporty. Even in 10 to 15 knots the snubber snubbed efficiently.

[svspirited]

Length sounds about right, 3 strand nylon will be a good fit, less expensive and easier to splice than 8 plait and has pretty good chaff resistance and chaff is the main enemy here. Ours is 5/8" and 25' for a 44' boat weighing around 17 tons loaded up with 3/8" chain for the anchor. I would not recommend using soft shackles to attach to the anchor line, again chaff is the enemy, but instead do a splice at the anchor chain end around a stainless thimble with a heavy duty load rated shackle and a chain hook. You can use something fancy like the Mantus or just a simple stainless chain hook. We also have an eye splice on the end that connects to the cleat. I plan on building a longer one for when we are in deeper water, most of the places I currently anchor are in less than 20' but in deeper water I would want a longer, say 40' snubber.



You should also do something about chafe protection, we use recycled fire hose where the snubber comes to the cleat as well as where the snubber will rub against the chain while swinging. The second bit of anti-chaff may not be needed in a double bridle setup but it can't hurt.


It's also not a bad idea to add one of those Sea Dog mooring snubbers to give greater shock absorption ability. You can use them on your dock lines when not anchored up.

[slug]

You will dead end your ground tackle to a cleat ?

Cleat size. 6 in, 8in, 10 in ...,

The rope size you choose is s function of cleat size

The calculation is cleat size shall be 6 x rope circumference
The breaking strength of mooring cleats

https://www.rocaindustry.com/wp-cont...ing_Cleats.pdf

Breaking strength double braid polyester

https://www.cortlandcompany.com/site...le-braid_1.pdf

[Cheechako]

My snubber on a 47 foot mono was 5/8 nylon. 20-25 feet long, albeit in normal conditions I didn't use all that. But by the time you tied to a cleat on each side of the bow and get to the chain hook you've probably used up more than 10 feet. The Snubbers were double (ie: Chain Hook in the middle and 20+ feet each side.
In strong winds those stretched a lot more than I thought they would! There were times I thought it should be 3/4"
Maybe something like this, amended for your boat measurements.

[Wallaby]

Sailboat is 50 ft 36000 lb.

Snubbers (all with chaffing):
1: 5/8" 3-strand nylon one leg of about 16 feet (spare/light duty)
2. 3/4" 3-strand nylon two legs of about 22 feet to 1/2" stainless slotted plate for 3/8 G4 Acco chain (medium duty) I have two of these for backup

3. 7/8" 3/strand nylon one leg of 60 feet to aft winch (heavy duty)

[PoppaEddie]

We lived on our sailboat for 8 years and had great adventures; one of which was a hurricane while attached to a dock. A curious thing was noted after, the 1/2" spring lines all held; the 1" lines even long ones 50% broke. Strength is not the only factor! All my 3 strand had stretch that endured the entire storm. Many braided lines did not. My dock broke up and my boat kept the dock together until we were able to add chains to hold it. The longer version notes my boat was left in my wife's care and I was absent and even somewhat famous locally for many years after because of it. My 36' boat is much smaller; but you are on the right track and the slack in the chain is the main item you can expect the line to soften (stretch) the shock from.

[Cheechako]

One thing I forgot to mention is too small a snubber will have a lot of "knuckles" from overstretching in the line after a strong blow. They wont go away.

[MJH]

Originally Posted by ohthetrees

Recently bought a 2015 50’ Hanse 505. Going through now and taking care of all the issues I can find.

High on the list is an anchor snubber. The previous owner’s “snubber” was about 1m long, and quite thin, maybe 12mm. Obviously, I need an upgrade. But searching through old CF threads, I see many discussions on snubbers, many of which I’ve read. Here is how I think I’m thinking about it. I’m looking for input, and for thoughts about the size I’ve chosen.

From what I understand just choosing a super large rope diameter is not only costly and heavy, but you might not have enough stretch to help relieve load on both anchor and cleats/bow roller.


My plan then is to have two snubber bridles. A light duty and a heavy duty.

The heavy duty will be a bridle snubber made of a single length of rope with an eye splice joining the two legs. I want this one to survive 90kn.
The light duty will also be a bridle snubber made of a single length of rope with an eye splice joining the two legs. I want this one to easily survive 40kn.

This approach will allow me the ease of handling a light weight bridle day-to-day, and provide extra stretch that the heavy duty snubber would not provide in < 40kn wind.

Additionally, I will have a short strop/penant/thingy to take load off the winch, and/or back up the snubber.

Some baseline numbers…

My boat is 50’ (15.25m) long and about 16 tons (14,500 kg).
PO upgraded (mixed feelings on this one) to a 10mm stainless chain offered by Hanse at the time. It’s beautiful, no rust, but I fear crevice corrosion. Will probably have to retire long before “necessary” because I’ll probably lose confidence in another year or two. Anyway, looking a specs of a similar chain working load is 11,000lb (5,100kg), and a breaking load of 22,000lb (10,200kg).
ABYC shows my boat in 60kn wind might see 5000lb load on anchor gear. However, I found a few convincing sources (e.g Sail Delmarva: Actual Anchor Loads) that say that in real world conditions, with a snubber, I can expect to see 1/4 of that (1250lb).
Doing some rough math, if I want my gear to survive 90kn, I can expect double the load of 60kn wind. Pesky math! So that is 10,000/4 = 2500lb real world load.
If I want my heavy snubber to work at 20% of breaking, I need a snubber with a breaking strength of 12,500lb.
For the light snubber at 40kn, I can expect to see 3200/4 = 800lb real world load. A snubber with 800lb working load will have a 800/0.2 (20%) = 4000lb breaking strength.


Those are my base line numbers.


For the heavy duty, on defender, I find Yale 8plait that is 12,200lb breaking load n the 5/8” (~16mm) size. Each leg would be 50’.
For the light duty, I find Yale 8plait that is 4000lb breaking load in the 3/8 size. 3/8"(~9.5mm). Each leg would be 25’.
For the strop, I find 3/8” (9.5mm) has a breaking load of 19600lb. I’d probably use a piece about 3’ long at most.

I intend to construct two legged bridles. It will be one continuous piece of rope, with either an eye splice or a thimble in the middle. I’ll use dyneema soft shackles to attach snubber to chain.

I’m interested in general opinions. But I’d really appreciate feedback on a few specifics.

1) If I want a two legged bridle, should I size the rope so that each leg could alone take the entire load? Or should I assume the legs are working together, and size each to take 1/2 the calculated load? If I make each leg full sized, won’t stretch be too low under most conditions?

2) I prefer not having any metal on the snubber. I prefer a soft shackle. With my bridle, will the eye splice lose too much strength compared to putting a thimble in when using a soft shackle?

3) I I size my heavy duty snubber for 90 knots of wind, will I have too little elasticity for when it is blowing 40kn?

4) My “gut” says that I want larger line than my calculations yielded. What do you think?

Thanks in advance for your feedback!

Practical Sailor has a number of articles related to snubbers. Check their website for "What is the ideal snubber size?", updated 4/16/2020.

~ ~ _/) ~ ~ MJH

[SV__Grace]

Here's a wealth of bridle info from my friend Bill who has a sistership to our 20ton Nauticat ketch. He's in Alaska and has weathered more storms at anchor than anyone I know- https://svdenalirosenc43.blogspot.co...has-their.html

A storm bridle needs a larger diameter line that must be pretty long to get enough stretch. Your 50' 5/8 line should be fine unless it's cleated at the bow and much shorter.

One of Bill's strategies for a storm snubber that I like and have implemented is to add a length of dyneema line on both sides and lead it aft through a block to the sheet winches (our boat has a OE turning block that works well, a snatch block or Antal low friction hook would work as well).

The purpose of the dyneema is to lengthen the snubber without adding more stretch.

So here's my setup using Bill's system with my own adaptations-

- 100' of 5/8 nylon 3-strand cow hitched to a Mantus chain hook, each bitter end is eye spliced
- Soft shackle attaching 60 feet of eye spliced dyneema line on each side
- Antal low friction hook attached to toe rail on each side to create a fair lead from bow to aft turning block
- 15' of bridle is initially deployed extending forward of the bow (with the full 50' stretched out on each side)

If there's too much of a bungee effect I can shorten the 3-strand.

If the wind kicks up to the point I want more scope, I can let out up to 50 more feet during a storm, from the cockpit.

When the storm passes I can shorten the bridle from the cockpit as well.

It might be overkill, but I also-

- use my lighter, secondary snubber as a lazy backup so it would take up the chain if the primary snubber breaks (if using this snubber in settled conditions, my storm snubber is the lazy backup)

- use a robust line with a chain hook to secure the chain to a cleat so if both snubbers break the force would not be on the windlass (I've heard of boaters losing all their chain overboard when this strategy isn't used)

[contrail]

The key is for the line to be long enough, and light enough. After a really strong blow, it will be permanently deformed, but will have done its job and should be replaced. Steve Dashew and Earl Hinz go into all of this in detail. Dashew notes that they did their original circumnavigation in a Columbia 50 something on a single snubber of 1/2 inch. The heavier lines will snap whereas the lighter ones will stretch, as one poster has already noted. Irma, dockside, snapped quite a few of my lines, but none of the long three strand ones. On a previous 33 footer, I endured the eye passage of Hurricane Marty (cat 1/2 in the Sea of Cortez) on one snubber, 3/8 of an inch, if I remember correctly, certainly no more than 1/2. At the end of the storm, the line was twice the length and looked like string, but it never broke. My neighbor, who had long bragged about the strength of his snubber, had to replace his snapped snubber six times, during the storm! Made a believer of me, although I was, already. I also used a kellet, for those conditions, and I think that damped a lot of the jerkiest motions. By the way, I can't imagine going through a hurricane on a 3.5 meter snubber.....way too short, in my opinion. I will get contradicted on this, but have some experience....