Calculating Forces on Anchor Rode

Discussion in 'Boat Design' started by bristol27, Oct 20, 2012.

  1. bntii
    Joined: Jun 2006
    Posts: 731
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    Location: MD

    bntii Senior Member

    Over thinking the problem and defining your terms with the ocean and conditions you will face.
    This is a recipe for disaster.

    Its a cruising boat- bring the heaviest gear you can get back on board and use it- you will sleep better at night.


    Tall tales-

    I have sat at anchor in a near surf for a night when a hard blow caught us too close to shore in Castro bay. The heavy 65' motor sailor was fetching up on the gear so hard that I thought we would throw the spars. The only thing that kept us off the beach was managing to get a long snubber rigged to reduce the loads.

    Many benign summer cruises with line squalls that lay the boat right over at anchor and sending the cruisers who "do not want to oversize the components, as oversizing adds unnecessary weight and cost to an anchor rode." dragging towards the beach and docks.

    I watched a blast come through in the sheltered creek where my boat is moored just a couple of weeks back. Every single anchored boat drug off at high speed towards the docks and luck had it that all crew on board to sort out and keep off before they did damage.

    I have a tactic when I see a blow coming in- I get geared up in the foulies get the engine running and warmed up. Standing at the helm I can clearly see if the boat drags off on the first strong blast and motor into it if needed. I haven't dragged anchor (knock on wood), but have watched many many times and nearby yachts have and roll out in panic as they careen towards peril- trying to get the engines started and light rodes clear of the props.

    You never know when you might need to take a exposed anchorage. Even inshore if you need to drop the hook dragging might put you on rocks or a shoal where pounding will cost you the boat.

    A 35' C&C came into the yard just last summer.
    Family cruiser (and a family who put everything they had into the boat to 'live the dream'..).
    First time out and they fetched up on a sandy bit of shallows. The gear they had on board couldn't keep them from dragging down on a small jetty and the boat pounded on the rocks taking off the rudder and sinking the uninsured boat at a total loss.
    A 30' live-aboard this summer and exactly the same end- tore up a a main in a hard blast of wind while sailing and the gear on board couldn't hold the boat off some rocks- total loss.

    Learn to sleep with a weather eye on the boats movements during blows on the hook- the boat should tack with a routine through the night. If she misses the tack- wake the hell up and get out on deck as she is dragging..

    Get heavy gear and use it.
    If near docks it will save you a insurance claim & if in the wilds it may save you the boat.
     
  2. Frosty

    Frosty Previous Member


    Yes,--- rule of thumb is get the biggest anchor you can handle and the biggest chain to go in the winch.
     
  3. Submarine Tom

    Submarine Tom Previous Member

    Bristol,

    Calculate your windage and you'll have your average loading on your rode in varying wind conditions. You're peaks wont likely exceed double the average depending on sea swell and boat design/size (i.e. heaving).
     
  4. jonr
    Joined: Sep 2008
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    Location: Great Lakes

    jonr Senior Member

  5. bristol27
    Joined: Oct 2011
    Posts: 15
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    Location: Portland, OR

    bristol27 Junior Member

    This is just what I was looking for. This answers my questions about forces.

    Good to know and something that also didn't seem right to me. This is actually a quote from a website I used for research, so I will remove this data point.


    Thank you all!
     
  6. bpw
    Joined: May 2012
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    Location: Cruising

    bpw Senior Member

    For a 9000 lbs boat you you are really only going to have two choices in chain, 1/4 high test or 5/16 BBB or high test, so you can't really size your gear with any precision. Both would probably be capable of ripping off your foredeck so I would not worry too much about the chain strength so much as what you attach it to. For line you will be likely be limited by how small of line you can comfortably handle, not strength issues.

    And I disagree with Frosty on going to largest chain possible, it is just extra weight that in a place that is very inefficient compared to putting the weight on the anchor. And before everyone jumps on the "heavy chain for catenary" band wagon, think about last time you where anchored on a small boat in a blow, The catenary goes away very quickly and stops doing any good long before a modern anchor will break out. Why else would would snubbers be so important?

    Personally, my preference is for big anchors, small but strong chain, lots of nylon and enough of it to deploy several anchors at once or replace lost gear. This set-up keeps weight down while giving you lots of options for different bottoms or if you need several anchors out to prevent swinging, which will often allow you to anchor somewhere smaller and more sheltered but still have lots of scope out.

    Our boat is similar to yours in size, though heavier so you might be interested in what we carry:

    15kg Rocna, our new primary, only used a couple times but looks very promising.

    22 lbs Bruce, our old primary, a bit small but was fine for Mexico and we would put out the delta or fortress with it in any sort of blow. Now lives buried in the lazerette as a spare

    22 lbs Delta, our kedge and secondary anchor. I actually like it more than the Bruce but the Bruce would not fit in the cockpit locker and had to live on the stem so it became the secondary. It is always ready to go with 300 ft of rode.

    75 lbs fisherman, a storm anchor, never used but makes me feel good, it lives disassembled under the floorboards. A good rocky or hard bottom back up if we lose the Rocna.

    22 lbs Fortress, Big storm anchor and used to replace the primary in soft bottoms. Always used as part of a two anchor set up since I don't trust it to re-set itself.

    And enough rode and chain to deploy at least 4 of them at a time. We have a mix of 5/16 BBB and high test right now, but will be going back to 1/4 high test when we can. 1/4 high test was unavailable when we replaced our chain in Panama so we had to go up to 5/16, quite a bit more work to haul up than the 1/4.

    Edit: Was just looking at your blog and saw you are going without an engine, We did the same and have been quite happy with our decision over the last couple years. Though I would never give up my heater or sink!
     
  7. philSweet
    Joined: May 2008
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    Location: Beaufort, SC and H'ville, NC

    philSweet Senior Member

    Having kept an assortment of boats in the Florida Keys, I have no idea what Frosty means by you don't anchor in a hurricane. That's what you do down there. You abandon the slip in the marina and go anchor somewhere if you have any sense. If you can get up a creek that's good, but my current boat drafts 7' and just has to deal with it. I am fine with any of my previous boats up through 135 mph. We got unnamed and unexpected 70 knot blows a few times each year. Down in the Keys, you don't want to leave your boat ever if its not good for a 100mph. My old 28'er carried an inventory comparable with the one listed above starting at 50# and working down in 10 percent increments. I carried four lengths of chain leaders that were sized to just reach the primary winches so I could grind in an anchor if needed. I carried three 250' 5/8 hard nylon, plus a floater line.

    Two concerns when dealing with the bigger anchors are can your boat set the things and can you work them from your dingy. My 28' couldn't set the 50#er by itself, but if I ran the 45# out opposite, ran the lines back to the primaries and then ran full power ahead up the middle, they would set. I would end up hanging on a pair set about 60-65 degrees apart. I built my own dingy to be able to work the anchors on the 38'er I now have (pair of 60#ers on 90' 3/8 chain)
     
  8. Frosty

    Frosty Previous Member

    Yes I think that what Frosty means --you don't anchor in a hurricane you go find some where else.

    The thing about hurricanes and how to survive them is not be in them at all.
     

  9. waeshael
    Joined: Mar 2013
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    Location: charleston

    waeshael Junior Member

    You could look at the question of force on the rode by looking at the holding power of your anchor system. If you exceed that holding power then the boat will drift downwind. It doesn't matter much how big the chain and rode is if the anchor slides out of the bottom. SO, first you need to understand the characteristics of sand (never anchor in mud during a big blow.) And then the rode geometry for the type of anchor you are using, so that it will not pull out as the rode tension increases. It is clear that the holding power of a burying anchor is related to the square of the depth that the flukes are buried, and linearly to the area of the flukes. Larger anchors require larger forces (higher winds) to bury them. A small sailboat under 50 feet will probably not have the engine power to fully bury a 35lb CQR. It will take a storm to bury it. By the time the wind has risen to 70kts, the anchor will have dragged and buried at a slope of 7 degrees until the flukes are two feet under at which time it will hold a load exactly equal to the forces on the boat, assuming a rode of 10:1 minimum. To prevent shock loads on the anchor which could dislodge it from the bottom, us a stretchy nylon rode that will stretch an additional wave height x2 as the waves go under the boat - this means a small diameter rode, say 5/8 for a 10,000 lb boat. The actual tension on the rode for a boat with low windage is quite small up to hurricane force (measured at 10M height, which is the standard.) At deck level the wind forces will be much lower, perhaps only a few hundred pounds. It is important that the boat does not yaw and present its beam to the wind as this will produce side pull on the anchor. So you must reduce yawing by anchoring with a bridle, and by anchoring off the stern if you have a counter, or canoe stern and are in protected waters, as this will set the boat in its favorite attitude for the wind with its stern into the wind (most of the windage in a sloop is forward of the CLR.)
    for more info go to my site waeshael.com and select the ocean cruising link, then click on the anchoring link.
     
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