Eye splice vs splice w/thimble vs direct splice vs soft shackle

[thinwater]

Are there any known cases of the U-bolt breaking on that cat? Bending? There are certainly a lot of them out there, enough for experience-based evaluation.



(I agree the design is poor, but that does not mean it will fail. With a snubber, perhaps the load is not that high, probably not much over a ton even in a storm.)

[crayiii]

I don't know of any failures beyond the initial design. ��

I've only had the boat for a month and this is almost the only area that concerns me. I've sailed a few other cats (non Lagoon) and picking up a mooring ball was easy since I could use the front cleats. Not so on this boat because of the cleat location.

I'll take more photos when I get back in a couple hours.

I really really appreciate all the help with this!

[Breaking Waves]

Originally Posted by thinwater

Are there any known cases of the U-bolt breaking on that cat? Bending? There are certainly a lot of them out there, enough for experience-based evaluation.



(I agree the design is poor, but that does not mean it will fail. With a snubber, perhaps the load is not that high, probably not much over a ton even in a storm.)

I agree with your implication - for typical usage, it is most probably 'good enough' in factory configuration (smallish line cow hitched) . . . but the lashing approach is not really much harder to do and is 'more proper'.

The way it is set up from the factory, the snubber is almost certain to fail at the small bend radius before the U-bolt breaks, although I could well imagine some u-bolt deformation at the bend radius breaking load.

I did not comment on bridle material. You probably have a better idea on loads on this size cat - what would you figure for proper size - ? And 8-Plait 12 or 14mm prefered?

[crayiii]

Here are some photos. Most owners I have heard from run the line from the outboard factory cleat under the pulpit out to the ball.

There are aftermarket inboard cleats that have been added but I don't know if I should trust the backing.

[thinwater]

Originally Posted by Breaking Waves

...I did not comment on bridle material. You probably have a better idea on loads on this size cat - what would you figure for proper size - ? And 8-Plait 12 or 14mm prefered?

I didn't mean to imply that the lack of failure means it is OK. I just wondered if it was a known failure point. It seems like quite a few have been hurricane in the islands, so we should know what worked and what did not. I know from studies I have been involved in with Florida mooring fields that the cause of failure is pendant chafe in about 98% of the losses. That is one big advantage of the eyes, even if they do seem small. No chafe.

ABYC table 1 is based on monohulls, but the 50' x 60 knot WLL for chain and fastenings is 4800 pounds. However, that number will ONLY be reached in shallow water (<10 feet) with all chain and some exposure to waves (several mile fetch). The actual load, with a nylong snubber and catenary is probably 3-4 times less, or about 1800 pounds, to be safe. The WLL of nylon is about 10% of BS for long life, so about 18,000 pounds, which is 3/4" nylon. I might use 5/8" for anchoring, to reduce strain on the anchor (dragging is FAR more likely than the bridle failing), but I'd go 3/4" on a mooring... if it were me.

I had a 34' cat and I would use 9mm to anchor and 1/2" to moor. Similar logic.

What about stronger winds? Loads go up with V^2, so just do the math. If you are thinking a huricane my side swipe the island, I would probably add a second set of lines to the cleats, sharing the load, since the bolt is going to become suspect at over 2 tons. More likely, I would reengineer the whole thing and give the mooring itself a REAL close look.
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One thing I did find encouraging is that the washers appear to be what are called "extra thick" fender washers, not the thin, flimsy standard fender washers that West Marine sells. Something to watch for an ALL boats is bent washers. They will slowly bend into a cone, giving warning that they are undersized and core damage is in process. Replace with backing plates or extra thick washers immediately. This is a pet rant of mine, since I have seen a lot of core damamge that resulted from standard fender washers bending. Good boat don't use them.

[crayiii]

Another thought is to replace that shackle with an eye bolt that bolts through the bracket and has a better pulling angle.

[thinwater]

Originally Posted by crayiii

Another thought is to replace that shackle with an eye bolt that bolts through the bracket and has a better pulling angle.

What is the U-bolt size? Typically, the rating is reduced 50% for side loading, so in this case about 40%. A standard 5/8" shackle (I don't know the size--this is the size of the bolt, not the bow) has a WLL of about 4000 pounds, or about 2400 pounds at this angle, which would be fine up to about 80 knots, but would fall short of the 4800-pounds ABYC recommendation. That said, there are two, but I like to engineer bridles for single leg loading, because it does happen. But if you figure two legs... you are close as is. They are probably figuring 2 legs and a 45 degree agle (short bridle), in which case the combined working load is 6000 pounds/1.41 = 4300 pounds. And that, results in a 10:1 safety factor at 60 knots.


But I'm guessing on the size.

[crayiii]

Originally Posted by thinwater

What is the U-bolt size? Typically, the rating is reduced 50% for side loading, so in this case about 40%. A standard 5/8" shackle (I don't know the size--this is the size of the bolt, not the bow) has a WLL of about 4000 pounds, or about 2400 pounds at this angle, which would be fine up to about 80 knots, but would fall short of the 4800-pounds ABYC recommendation. That said, there are two, but I like to engineer bridles for single leg loading, because it does happen. But if you figure two legs... you are close as is. They are probably figuring 2 legs and a 45 degree agle (short bridle), in which case the combined working load is 6000 pounds/1.41 = 4300 pounds. And that, results in a 10:1 safety factor at 60 knots.


But I'm guessing on the size.

The bolt takes a 17mm socket.

So I should keep each bridle leg to about the beam of the boat it sounds like.

[Breaking Waves]

Originally Posted by thinwater

which is 3/4" nylon.

If 3/4” is the baseline bridle, then I think you clearly need to do something better than the cow hitch to that u bolt, because you are going to have a big bend radius strength reduction with a direct connection (it’s going to be quite significantly below 1:1 - u section looks like 10mm rod in the photo, so like 1:2).

I agree about chafe being large fraction of morning attachment failures, and this is a good chafe free solution. But I also think it is a rather weaker rope to boat connection that vast majority - very few would have this sort of huge bend radius strength loss.

I’m less up on the hardware/shackle strength issues. This shackle type looks to be called “wide d shackle”, I believe this size would be rated somewhere in the 8 - 20k lbs range (more if it is ht, but I doubt it is) in square pull, so potentially 50% less in 90 degree pull. So perhaps 4 - 5k lbs on this application. I wonder if an off axis cyclic load would egg out those aluminum holes the shackle is pinned thru. Idk - but it does look relatively easy to move part or all the load to the beam, which would seem better to me (but I do have less test experience with odd hardware loading issues).

The internal backing ”plates” and glass reinforcement do actually look pretty decent - better than I expected.

[crayiii]

Would 5/8" nylon 3-strand offer enough strength?

[Breaking Waves]

I guess if it were me, I would initially do a Dyneema lashing to low friction ring, from the existing shackle. That would easily solve the obvious and significant bend radius issue.

And I would keep noodling the attachment/angle issue around. It may well become more important as the lashing “fixes” the bend radius weakness, and then moves the “weak link” to the off axis shackle.

[crayiii]

Okay, so just so I'm sure, you mean something like this (only to scale)?

[Breaking Waves]

Originally Posted by crayiii

Would 5/8" nylon 3-strand offer enough strength?

I believe thinwater is suggesting it has enough strength new. And it will be more elastic than 3/4”. But will have a shorter life and safety margin than 3/4”. Seems like you are sort of on the boundary between the two.

I don’t have enough experience with multihull loads to have an opinion.

[Breaking Waves]

Originally Posted by crayiii

Okay, so just so I'm sure, you mean something like this (only to scale)?

Yes, exactly, only you want to try to equalize the length/load between all the turns as much/closely as you can.

Use Dyneema smaller in diameter than the shackle rod diameter - I’m guessing 6 mm Dyneema would be good.

[thinwater]

That is smaller than a 5/8" shackle, perhaps 10 mm (DIN), maybe 12 mm (JIS or proprietary) which is way smaller than I had hoped. The bow is probably 10 mm. BS ranges from 12,000-13,500 pounds, give or take, not knowing what it is, or a WLL of about 2000-2500 pounds at that angle. ABYC calls for about 3400 pounds per leg at 60 knots, before single leg loading is considered, so I would try to find out. Yes, stress on the aluminum is a factor. I've inspected a few Lagoons for cracks, as well as some Leopards (very similar) and never found any. I'd be asking around, since the engineering is... not conservative.


Lots of good suggestions. I would hesitate to add metal hardware, since that concentrates loads and adds wear. The Dyneema lashing is unusual but smart... but it will move wereas the luggage tag will not.


Interesting problem. I'd be asking around. Have there been shackle failures or bridle failures?