How to use a snubber with a bowsprit?

[Kettlewell]

I just don't understand not using a snubber with all-chain rode. My 38-foot boat is around 22,000 lbs. and I use 5/16" chain. If I do not use a snubber for some reason I can feel it through the entire boat, even in lightish conditions. The ride on the chain is just horrible! In heavier air it feels like something will break. But, we also rarely anchor in more than 15 feet of water, and often in a lot less, so we might only have 75 feet of rode out, or less. I need at least 15 feet of lighter nylon, or 25 feet of heavier stuff in order to make the boat not jerk back on the chain. This summer I moved back to using 50 feet of chain and then 7/8" nylon, and I usually have around 25 feet of that out. The ride is noticeably smoother than using all chain with a short snubber, especially if the wind gets above 20 knots.

[Dockhead]

Originally Posted by Kettlewell

I just don't understand not using a snubber with all-chain rode. My 38-foot boat is around 22,000 lbs. and I use 5/16" chain. If I do not use a snubber for some reason I can feel it through the entire boat, even in lightish conditions. The ride on the chain is just horrible! In heavier air it feels like something will break. But, we also rarely anchor in more than 15 feet of water, and often in a lot less, so we might only have 75 feet of rode out, or less. I need at least 15 feet of lighter nylon, or 25 feet of heavier stuff in order to make the boat not jerk back on the chain. This summer I moved back to using 50 feet of chain and then 7/8" nylon, and I usually have around 25 feet of that out. The ride is noticeably smoother than using all chain with a short snubber, especially if the wind gets above 20 knots.

Yeah, I didn't understand it either and religiously used a snubber for years and years in situations where I now know it was completely unnecessary. Our previous boat was the same size and weight as yours and you couldn't imagine not using a snubber. I've had this boat since 2009, and it has 1/2" rather than 5/16" chain, weighing 330kg (a third of a metric ton) for a 100 meter length, compared to 200 feet of 5/16" chain weighing about 90kg. It weighs about 50,000 pounds with cruising gear and full tanks. Poiu's boat is even that much bigger than mine.



All I can say is there's is just some tipping point in the physics where it takes a whole lot more wave action to jerk the chain short. Like a WHOLE lot more.

[Kettlewell]

All I can say is there's is just some tipping point in the physics where it takes a whole lot more wave action to jerk the chain short. Like a WHOLE lot more.

I'll take your word for it, but I would be worried that everything would be fine until suddenly it isn't. In my experience that point is usually reached at about 0200 on a black night when you are suddenly roused from a nice sleep and have to clamber out on deck in the driving rain to rig the snubber.

[Dockhead]

Originally Posted by Kettlewell

I'll take your word for it, but I would be worried that everything would be fine until suddenly it isn't. In my experience that point is usually reached at about 0200 on a black night when you are suddenly roused from a nice sleep and have to clamber out on deck in the driving rain to rig the snubber.

I agree with you.


Therefore I'm very careful about taking this risk. I have gradually experimented and satisfied myself that under 30kn of wind, there is no snatching unless there is very heavy wave action. If I think there is the slightest risk of that, I put on the snubber. During the day I do take more risk for the sake of experimentation.

[dlj]

Originally Posted by Dockhead

Yeah, I didn't understand it either and religiously used a snubber for years and years in situations where I now know it was completely unnecessary. Our previous boat was the same size and weight as yours and you couldn't imagine not using a snubber. I've had this boat since 2009, and it has 1/2" rather than 5/16" chain, weighing 330kg (a third of a metric ton) for a 100 meter length, compared to 200 feet of 5/16" chain weighing about 90kg. It weighs about 50,000 pounds with cruising gear and full tanks. Poiu's boat is even that much bigger than mine.



All I can say is there's is just some tipping point in the physics where it takes a whole lot more wave action to jerk the chain short. Like a WHOLE lot more.

I imagine this may well be right that at some point where the weight of the chain is more that X% of the weight of the boat, that the catenary effect is such that the boat is moving in a way that there is not the jerk experienced because of the weight of the chain.

For my boat, the downside of that is I would have to have less chain length available because of the physical increase in size/weight for the larger chain. If I carried the same length of chain, then the weight in my bow anchor locker would be that much more. Then there would be other issues with performance. If I carried the approximate same weight of chain, then I'd have significantly less chain rode.

It's all a compromise.

I've chosen to always use a nylon line attached to my chain as a snubber and be able to carry a significant length of chain in my chain locker.

I totally agree with you about a chain brake and I'm looking into how to do that - geometry complications on my boat don't make that an easy one. For the time being, once my anchor is set with the snubber line, I attach the loose end of the chain to a cleat.

dj

[Dockhead]

Originally Posted by dlj

I imagine this may well be right that at some point where the weight of the chain is more that X% of the weight of the boat, that the catenary effect is such that the boat is moving in a way that there is not the jerk experienced because of the weight of the chain. . . . j

I think it's not the percentage of weight of the boat represented by the chain -- it's a scaling effect. We have a genius German mathematician on this Forum who did a fantastic analysis of anchoring and scope and the forces involved -- I bet he could figure it out, but the question is beyond my own meager abilities in math and physics.


Ships don't use snubbers in any conditions at all, and I'm sure that the chains are not a greater mass as a % of their weight; in fact probably much less.


There is some point at which the wind and/or wave action required to create a snatch load -- required to take all the elasticity out of the catenary -- starts to exceed loads felt in even stronger conditions.



Also depth is important. I would always use a snubber if anchored in less than 6 meters or so of water. I'm normally in at least 10 or 15 meters of water, where the catenary effects are much stronger.

[Cheechako]

I cant tell .. no bobstay I guess? There really can't be much chafe with the snubber near parallel to the side of the bow?
I'd just use a bridle snubber anchored to a cleat on both sides, will rub on the boat bow at times but nothing abrasive there really.
My 47 foot boat had 3/4 snubber and my 44 had 5/8 snubber. It stretches plenty in 20 knots or more of wind if you use nylon 3 strand.

[dlj]

Originally Posted by Dockhead

I think it's not the percentage of weight of the boat represented by the chain -- it's a scaling effect. We have a genius German mathematician on this Forum who did a fantastic analysis of anchoring and scope and the forces involved -- I bet he could figure it out, but the question is beyond my own meager abilities in math and physics.


Ships don't use snubbers in any conditions at all, and I'm sure that the chains are not a greater mass as a % of their weight; in fact probably much less.


There is some point at which the wind and/or wave action required to create a snatch load -- required to take all the elasticity out of the catenary -- starts to exceed loads felt in even stronger conditions.



Also depth is important. I would always use a snubber if anchored in less than 6 meters or so of water. I'm normally in at least 10 or 15 meters of water, where the catenary effects are much stronger.

Good point - but I also don't think ships and small sail boats are a good equivalent.

While comfort is what has been discussed here, from my perspective safety is the more pressing need. The fact is most of us are sailing fiberglass boats with the connection points being hard points that are attached to a fiberglass structure. The impact loading that comes from lack of elasticity could have a long term detrimental effect on that fiberglass structure. The endurance limit of fiberglass construction seems to be about 25% of it's static load strength (Reference: Fatigue Prediction Verification of Fiberglass Hulls, Pat Miller, Marine Technology, Vol. 38, No. 4, October 2001, pp. 278-292).

Now in my boat, I'm not to worried about that as my boat is build like the proverbial brick sh!t house... But on more "production type" boats, I think that would be a serious concern.... I still treat my boat with great care - hence always use a snubber.

Now in steel ships, those connection points I'm sure are engineered to have that loading below the endurance limit such that ripping out a connection point would not be a concern, neither would long term degradation effects be a concern. Also, given the general size difference, I doubt those ships are particularly able to feel those shock loads, so from a "comfort" perspective, it probably doesn't matter.

So I don't really know if the "comfort" aspect is due to relative size of chain to ship or not. But I don't think the real concern is "comfort", but rather safety and long term structural integrity of your boat.

ABYC Standard H40 requires in line 40.6.2.2 "Strong Points shall have a safe working load greater than, and be fastened to withstand, twice the design loads in TABLE 1." For a 40 foot boat that would be 3600 pounds. Twice that is 7200 pounds and frankly, I think the shock loading that can occur without a snubber can easily approach 7200 pounds on a boat this size. Meaning you are exceeding the endurance limit of your fiberglass structure. That in my mind is a serious problem.

It would be great to get your German math genius to chime in on this one...

dj

[wrwakefield]

We have to use a bridle to avoid the chain rode contacting the bobstay on our bowsprit in all but the calmest weather. It also stops any bottom noise [e.g., chain dragging over rock, etc.] being conducted up the chain and transmitting to the hull via the bow roller.

Our bridle needs to extend at least 7m minimum from bow fairleads [port and stbd] to chain connection point when not loaded to minimize contact between bridle and bowsprit if conditions will cause the boat to yaw a bit at anchor…. The bridle is lengthened for sportier conditions.

Of course, the geometry is different on every boat, but this works well for us.

A view of our fair weather bridle from the starboard anchor roller [hence the bowsprit is off center in photo]:



In case any of this is useful…

Cheers, Bill

PS: If anchoring in water shallower than the bridle/chain attachment point depth [very rare for us] then we secure the bridle further back on the boat- still fairleading at the bow- maintaining its length for the elasticity. This keeps the bridle off the bottom, but we may have to put up with the bridle chafing on the bobstay in yawing conditions in such circumstances.

[Don C L]

The Downeast I sailed long ago had a bowsprit and the owner had a snubber attached to the bobstay chainplate. It is a good location but I was never convinced the chainplate was stout enough for that. Though it may be unsightly I'd vote for engineering a beefy one suitable for anchoring the next time you haul out. It may have been mentioned but you can put a Prusik loop on the chain and run a snubber to it. If you are lucky enough to have two rollers there side by side, one can be for snubber, the other for chain.

[wrwakefield]

Originally Posted by Dockhead

I think it's not the percentage of weight of the boat represented by the chain -- it's a scaling effect. We have a genius German mathematician on this Forum who did a fantastic analysis of anchoring and scope and the forces involved…

He bundled his treatise into a wonderful anchor rode catenary app that contains all the math and physics.

Don’t leave home port without it!

Cheers, Bill

PS: It requires reading and understanding the in-depth ‘Help’ files to get full benefit of the app.

[dlj]

Originally Posted by wrwakefield

He bundled his treatise into a wonderful anchor rode catenary app that contains all the math and physics.

Don’t leave home port without it!

Cheers, Bill

PS: It requires reading and understanding the in-depth ‘Help’ files to get full benefit of the app.

Great reference! Thanks for the link. Reading through his work, looks like I may be overly conservative regarding peak loading values (impressions). But I'm still using a snubber every time I anchor (also his recommendation).

dj

[Dockhead]

Originally Posted by Don C L

The Downeast I sailed long ago had a bowsprit and the owner had a snubber attached to the bobstay chainplate. It is a good location but I was never convinced the chainplate was stout enough for that. Though it may be unsightly I'd vote for engineering a beefy one suitable for anchoring the next time you haul out. It may have been mentioned but you can put a Prusik loop on the chain and run a snubber to it. If you are lucky enough to have two rollers there side by side, one can be for snubber, the other for chain.

I think this is a very good approach.


If you've belayed the chain above then the snubber chainplate doesn't need to be as strong as the chain -- only as strong as the snubber.


You can do this at the same time you install chainplates for your Jordan Series Drogue, if you're going to do that.

[dlj]

Originally Posted by Dockhead

I think this is a very good approach.


If you've belayed the chain above then the snubber chainplate doesn't need to be as strong as the chain -- only as strong as the snubber.


You can do this at the same time you install chainplates for your Jordan Series Drogue, if you're going to do that.

I would definitely make that at least as strong as required for a hard point per ABYC. Plus, your snubber should be pretty darned close to as strong as your chain.

dj

[poiu]

Originally Posted by Dockhead

I think this is a very good approach.


If you've belayed the chain above then the snubber chainplate doesn't need to be as strong as the chain -- only as strong as the snubber.


You can do this at the same time you install chainplates for your Jordan Series Drogue, if you're going to do that.

You are right as mentioned above you need a chain stopper of some sort, but here, imagine the snubber stretches to max, which is rather a lot. 20% roughly. Then it snaps and you need the chain stopper to save you, but the boat is accelerating backwards under what will be extremely high loads and u less you have sized the length of the chain stopper line just right so momentum does not build the shock load is going to extremely high and likely to break something. Chain stoppers on big ships are never slack, so this doesn’t happen to them. You need either or I think.

I suppose I can answer my own doubts - if the chain stopper line was in nylon, triple strength of the snubber for example that may save the day. Not dyneema.

Unfortunately many cruising boats aren’t set up perfectly for anchoring as I am discovering and the vertical bow, bowsprit, only one anchor roller, no chain stopper, as per most new boats is not as good as it looks.

I still need to use some kind of snubber though as last night made clear if for no other reason to stop than the chain graunching.