Anchor Snubber

[MM5AHO]

Here's a simpler method that Alain F's

WIND FORCES
The main force which is experienced by a yacht at anchor comes from wind resistance. It is proportional to the wind speed squared and to the frontal area presented by the yacht. Broadly speaking the frontal area will be proportional to the square of the length. Provided snatching does not occur, the maximum force on the anchor cable is given by the approximate formula:

Maximum force in kg = (yacht length in metres)2 x (wind speed in knots)2/500


F(wind) = L2xV2/500 (units kilograms,metres and knots) Formula 2


Force is measured in kilograms; wind speed in knots, yacht length in metres.
So with a 10 metre yacht anchored in a 30 knot wind, the maximum force on the anchor cable is expected to be about 180 kg if the chain is not tensioned beyond the borderline state. (borderline state is when its about 2 links for fully tensioned with no catenerary) The force will be much larger if snatching occurs.

 

[Saguday]

Thanks Geoff, I've seen that before but what I haven't figured out is why when I put that into my spreadsheet to compare with Alain's numbers they come out at approximately half the value:

For a 10m boat (wind in Knots, F in kgF):

Wind.... AF........G

30........ 366......180
40........651.......320
50........1017.....500
60........1465.....720

For my 12m boat (wind in Knots, F in kgF):

Wind.... AF........G

30........ 496......259
40........881.......461
50........1376.....720
60........1982.....1037

I've checked and both are doing the sums right, so I'm scratching my head a bit.

 

[MM5AHO]

I agree, that these two don't agree.
One of these is wrong, but I don't know which one! (neither of these is my calculations!)

 

[Polly's Kettle]

I think the only advantage is that it keeps the snubber dry. Not particularly compelling, I grant you, but mine is rigged that way. One day I may persuade the wife out of a marina and actually get to use it!

Right, let's say you have a 10m length of suitable sized nylon as a snubber. What, if any, are the perceived advantages of having it rigged along the deck, as opposed to having it all overboard? Personally, I can see several disadvantages of having it rigged on deck: chafe, clutter, not having a clear line for it etc. What good points are there?

 

[Saguday]

Just digging around into this, Vyv_Cox has faith in John Knox's numbers which seem to correlate with real-world measurements. Alain Freysse's and the ABYC seem to be very pessimistic and since posting this I've come across a lot of disagreement about all the numbers - so it's not just us

I'll use Prof Knox's I think

 

[MM5AHO]

Just digging around into this, Vyv_Cox has faith in John Knox's numbers which seem to correlate with real-world measurements. Alain Freysse's and the ABYC seem to be very pessimistic and since posting this I've come across a lot of disagreement about all the numbers - so it's not just us

I'll use Prof Knox's I think

One justification for those numbers is a loadcell system to measure the load on the anchor rode that John Knox developed. Many measurements in a wide variety of conditions give reasonable agreement with his formula.

 

[Saguday]

Yes, experimental back-up is a Good Thing.

Is the anchor watch unit still available to buy or has it gone? A modern day twist on it would be to make the data collectable via an app and then crowd source a whole load of real-world anchoring data for all sorts of different anchors, boats and conditions.

 

[vyv_cox]

Right, let's say you have a 10m length of suitable sized nylon as a snubber. What, if any, are the perceived advantages of having it rigged along the deck, as opposed to having it all overboard? Personally, I can see several disadvantages of having it rigged on deck: chafe, clutter, not having a clear line for it etc. What good points are there?

The disadvantage of having all the rope over the bow is that if/when the wind slackens sufficiently for the hook to touch the bottom there is an almost certainty that it will be wiped off the chain. Some will say that a Dyneema shackle, or even a rolling hitch, instead of a hook would overcome that particular problem but I am very happy with the advantages a hook gives me.

 

[RichardS]

I'm back on my pontoon now and walked round to the marina services to pay for my lift and saw this snubber on a large mobo (60 foot-ish), registered in Dartmouth so a long way from home. I've never seen one like this.

It must be seriously resilient to be suitable for such a big boat and must be very strongly made as there seems to be no failsafe if the rubber bellows should break. :ambivalence:

I don't know how much they cost but the boat had 4 of them.

Richard

 

[RupertW]

I'm back on my pontoon now and walked round to the marina services to pay for my lift and saw this snubber on a large mobo (60 foot-ish), registered in Dartmouth so a long way from home. I've never seen one like this.

It must be seriously resilient to be suitable for such a big boat and must be very strongly made as there seems to be no failsafe if the rubber bellows should break. :ambivalence:

I don't know how much they cost but the boat had 4 of them.

Richard

There must be something strong inside surely - a spring, slightly coiled wire?

 

[RichardS]

There must be something strong inside surely - a spring, slightly coiled wire?

I couldn't see anything but why would it be that complex shape if it had a spring inside it? Could be a coiled wire.

There's presumably something on the internet about them but I've never come across it?

It's difficult to tell from the photo but it's a big dude .... that's at least 10mm chain.

Richard

 

[jordanbasset]

I'm back on my pontoon now and walked round to the marina services to pay for my lift and saw this snubber on a large mobo (60 foot-ish), registered in Dartmouth so a long way from home. I've never seen one like this.

It must be seriously resilient to be suitable for such a big boat and must be very strongly made as there seems to be no failsafe if the rubber bellows should break. :ambivalence:

I don't know how much they cost but the boat had 4 of them.

Richard

It's one of these
http://seascrew.com/browse.cfm?DOCK-LINE-SHOCKABSORBER-RUBBER---STAINLESS&p=0000006047
''Mooring Shock Absorber.
Very strong, Noiseless and long life.
Made of stainless steel 316 and rubber
UV & salt resistant, Stroke 110mm
For Boat 18' to 30' Breaking load 6 Tonne
OVER ALL LENGTH 256mm
END OF LINE 1 LEFT''

I assume they come in different sizes

 

[RichardS]

It's one of these
http://seascrew.com/browse.cfm?DOCK-LINE-SHOCKABSORBER-RUBBER---STAINLESS&p=0000006047
''Mooring Shock Absorber.
Very strong, Noiseless and long life.
Made of stainless steel 316 and rubber
UV & salt resistant, Stroke 110mm
For Boat 18' to 30' Breaking load 6 Tonne
OVER ALL LENGTH 256mm
END OF LINE 1 LEFT''

I assume they come in different sizes

Crikey ..... £112 for one snubber! And unless there are larger sizes, I can understand why mobo-man has 4 of them as 4 x 30 ft = 120ft ....... presumably?

Richard

 

[lw395]

There must be something strong inside surely - a spring, slightly coiled wire?

I think that has two (pairs of?) stainless rods running through it and the rubber bits are under compression?
I have seen something like this before, but I think it was made out of BL mini suspension 'doughnuts'!

Them shackles do want mousing.

 

[jordanbasset]

Crikey ..... £112 for one snubber! And unless there are larger sizes, I can understand why mobo-man has 4 of them as 4 x 30 ft = 120ft ....... presumably?

Richard

My thoughts entirely, seems over engineered to me, which I guess is the reason you see so few about.

 

[jimbaerselman]

Hmm. In line snubbers. A triumph of marketing over rational mechanics.

There are rubbers which thread on your mooring rope to induce 2 or three twists in the rope over a distance of 30 - 40 cm. The twist adds 30 degree diversion from the straight. Cos 30 is near enough 15% - 4.5cm for 30cm (or 6cm for 40cm) of twists.

Metal snubbers with springs collapse from about 25cm to 10cm - giving you 15cm of elasticity. But they're noisy, and a standby loop of chain is needed to prevent the spring from "bottoming out" - a frequent cause of failure.

Nylon warps give you 5% to 10% stretch before deforming. Few are shorter than 5m. That's 25cm to 50cm of spring. Rig them longer in places with more surge.

End on, midships to shore, uses 10m. Good for a metre of surging. Yes, squeaks can be cured with a little olive oil.

Same logic for anchor springs. 15m nylon was my minimum. That stretched along the deck to an aft winch to add another 10m if needed.

So I don't see any benefits added by short rubber or metal snubbers, compared with long springy nylon.

Marketers! Some are good at persuading you they can provide your solution. Without mentioning the competition, of course. Suckers, aren't we.

 

[Robin]

I used to make my own snubbers in the 1970s from strong shock cord lengths whipped together and hooked across a slack loop of anchor chain. When rubber dog bone snubbers came out commercially I moved to using those, in various sizes and elasticity, with multiple turns of line around to suit the occasion. I personally think the amount of measurable stretch is not the point at all, it is merely the last second extreme jerk that they prevent that does the job, a stretchy nylon line alone does not stop those final jerks, it just delays them..

 

[NormanS]

Where do you folk get all this jerking and snatching? Are you anchoring out in the open sea? :disgust: Or are you allowing the boat to yaw excessively? If necessary, I rig an anchor sail to avoid that. :encouragement:
Still, what would I know, the most wind that I've ever been anchored in was 74 knots.

 

[Robin]

Where do you folk get all this jerking and snatching? Are you anchoring out in the open sea? :disgust: Or are you allowing the boat to yaw excessively? If necessary, I rig an anchor sail to avoid that. :encouragement:
Still, what would I know, the most wind that I've ever been anchored in was 74 knots.

How do you rig an anchor sail? I tried leaving the mizzen set on one boat we owned and it didn't help, other boats had no backstay free of stuff to hoist a sail on. my point about jerking is that is that which breaks anchors out. When at one YC we re launched bilge keelers and others ( max draught to lift out there was 4ft) after being wintered ashore, we often did it into marginal water depths for them to get away. The club launch was then used to pull/drag them into slightly deeper water and in order to start them moving it took a run at it and jerked them off the pudding. The warps used were stretchy nylon which did not break, nor stretch enough tostop the jerk from working, although an occasional bow cleat got removed unintentionally:ambivalence:. Jerks rule OK?

 

[NormanS]

Anchor sail. I have a ketch. My anchor sail is like two small mizzens with a common luff. It has slides which go into the track gate, above the stowed mizzen, and each clew is sheeted to the outer corners of the taffrail. There is a spreader (telescopic boathook with a spike) between the two clews. The double sail seems much more efficient than a reefed mizzen, as it is effective even when head to wind. Takes seconds to rig.