Amidships Chain Storage

Discussion in 'Sailboats' started by Ted Royer, Jul 20, 2018.

  1. MikeJohns
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    MikeJohns Senior Member

    You can check anything I post with another professional to see if it's factual, instead of looking for even more disinformation that you imagine supports a laymans opinion. How can any technical thread with intellectual content run sensibly when you do that ?

    It's within the thread that errors need to be addressed. You keep posting misinformation and when you are told it's wrong instead of learning you come up with something else just as incorrect. So lets address this here and now and stop it occurring in the future.

    Not only can you not know what you don't know, but if hubris stops you even trying to understand why you are wrong you'll never learn. Dunning Kruger is often prevalent when laymen disagree with specialists within the specialists field. Usually people learn and move on.

    You want to keep re-defining a complex dynamic system as a vibration. Dynamic systems such as moorings are not vibrations they are complex systems, they have a complex transfer function that has to be analysed as a whole.
     
  2. Ted Royer
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    Ted Royer Junior Member

    Okay next (somewhat related) question. Please let me know if I should report this new question on a separate post.

    On the same boat in this original post, using an all chain rode and utilizing a 5/8" nylon snubber (say of 30') I would like to secure the snubber to the location identified in yellow, just above the water line to a new well supported towing eye, instead of to a cleat on the deck. I would just pick it up with a boat hook and secure it on deck when we got ready to weigh anchor and leave it tied to the towing eye permanently. I realize this has the disadvantages of UV degradation, inaccessibility, could fall off the deck while underway and get caught in the propeller, etc. but it has two advantages that I can see:
    1) It does not chafe across the deck or anywhere else that I can see. If this is true, then in bad weather or when the boat is on the hook while we are away, this feature is significant to me.
    2) It lowers the required scope by being several feet lower than a similar system pulling from the deck. This is less significant and I realize that if the snubber parts, then my scope calc is in error.

    Anyway, please weigh in on it (pun intended) if you have thoughts, Thanks again.

    upload_2018-8-6_11-7-34.png
     
  3. gonzo
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    gonzo Senior Member

    One of the advantages of making the rode fast lower on the bow, is that it will decrease the pitching.
     
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  4. Ted Royer
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    Ted Royer Junior Member

    To understand the extent of my paranoia & insanity, I bought a 55 Lb Mantus for a 8,000 Lb, 30 ft boat. In other words, my anchor has a small boat attached to it.

    From the SV Panope videos the Mantus seemed to have the one of the top setting/resetting features (as apposed to ultimate holding power per pound of anchor) in my price range. Then I up-sized the anchor for a boat four times my displacement to increase the ultimate holding power and further increase the setting ability (since I assumed that across a wide variety of seabeds, the larger the anchor (for identical anchor types) the higher the chance that it can punch through the seabed surface and dig in. It will be on 275 ft of 5/16" G4 chain.
     
  5. Ted Royer
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    Ted Royer Junior Member

    Thanks. I will add that to my list of reasons to install the mother-of-all-towing-eyes and try it.
     
  6. philSweet
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    philSweet Senior Member

    Sounds about right, I bought a 50# CQR for my Cal 28, and kept a spool of 1 1/2 nylon on board for misc. tasks in addition to 3 250' hard nylon 5/8 rodes. This was in the Florida Keys. We were averaging about 20 days per year with 100+ mph wind gusts during several years of el nino. Only about half of those were forecast, the rest were just surprise squalls with NOAA still chirping out it's Camber-of-Commerce forecast.

    Regarding the low snubber mount - Don't do that - you'll regret it. The problem is that not only is the snubber lower, it is also further aft, and that is never a good thing on a fin-keeler like yours. What you want is a great big stainless anchor roller for the bow that pokes out about 18" and holds a pair of 35# anchors. It also has a big bail for the snubber line. Run the snubber line inside about 4' of 1" heavy duty rubber water hose. Seize the hose to the snubber line. The hose protects the snubber from chafe from the bail, anchors, chain, and everything else. Now she'll hang like a duck. I expect her to tack all over the anchorage with the low and aft mount. But it's easy enough to jury-rig it for a few days each way and see the difference.
     
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  7. gonzo
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    gonzo Senior Member

    I added a bow pulpit with roller to a O'Day 27, and the boat would pitch and bury the bow a lot more than when the line came off further aft. I expected a change but it was a lot more. As you increase the distance from the CG, the moment increases, which in turn increases pitch.
     
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  8. TANSL
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    TANSL Senior Member

    Amazing.
    You mean the moment about the CoG, right?
     
  9. MikeJohns
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    MikeJohns Senior Member

    I suspect what Tansl means by amazing is this:

    The pitch RAO is directly related to radius of longitudinal gyration . If you add weight at the bow you shift the natural frequency particularly since it's the mass times the square of the distance from the centroid. If that new frequency is close to a fundamental or harmonic of the wave spectra you greater pitch amplitude. You might just as easily find you move the RAO peak away from resonance on another boat and reduce pitching.
     
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  10. Ad Hoc
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    Ad Hoc Naval Architect

    Mike

    There are plenty of papers and studies showing the effects of changes in the radius of gyration with seakeeping. In large slow displacement vessels, small changes can have a noticeable effect.
    I can try and dig out a lot of my old references on the subject (papers in paper format) if you're interested?
     
  11. TANSL
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    TANSL Senior Member

    MikeJohns, I find amazing that someone believes that the ship rotates around its center of gravity and therefore explains how he managed to vary the pitch of a ship by varying the moment of a force with respect to the CoG of the ship, without taking into account at all the water plane, the rotating axes of the boat, etc.
    That's what I interpreted from Gonzo's explanation. Maybe I misunderstood he.
     
  12. MikeJohns
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    MikeJohns Senior Member

    Thanks, I'll remember that, unless someone else wants them posted here. I might get them off you privately at some stage.

    On a small light boat too the effects can be quite considerable.
     
  13. Ad Hoc
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    Ad Hoc Naval Architect

    Then I am equally amazed you do not understand the basics of seakeeping.

    When a rigid body has rotational motion, the motion is perpendicular to the axis of rotation. A rigid body is made up of a single element or many elements. If it is a simple steel rod, then it is simple single element (mass), but when it is a vessel, it is the summation of all the elements to obtain the CoG - the centre of its rotation. The rotation of the body or ship is centred around its CoG. That's basic mechanics. It is why the radius of gyration is important to understand in seakeeping.

    So yes, you very clearly misunderstood. Again.
     
    Last edited: Aug 6, 2018
  14. BlueBell
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    BlueBell Ahhhhh...

    "To understand the extent of my paranoia & insanity, I bought a 55 Lb Mantus for a 8,000 Lb, 30 ft boat. In other words, my anchor has a small boat attached to it."

    Mantus anchors are great at setting quickly, but, as mentioned, don't have the highest holding power.
    However, the anchor that doesn't set, has no holding power...
     
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  15. Ad Hoc
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    Ad Hoc Naval Architect

    Indeed.
    Anyone who has been in a small dinghy (or a small RIB) notices the change in seakeeping behaviour when you are sitting in the centre of the boat compared to at either extreme ends.
     
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