Sizing an anchor windlass

So I want to add an anchoring system to the tug.  It never had one before as it was used as a harbor tug that was always tied up to a pier when it was not pushing or pulling.  She did not have berths for sleeping either – she was what was known as a “bucket” boat, because you brought your lunch or dinner bucket onboard every day.

So there is a very nice example of a low deck profile vertical anchor windlass on the fireboat, John J. Harvey.  The anchor chain comes in through the hawsepipe, through a chain stopper and then 270 degrees around the vertical windlass and then falls into the anchor locker.

DSCF6395.JPG

And here is a pix of the bow of New York Central No. 13.  I got this hawsepipe from Jimmy Gallagher when he sold the Yankee.  I have just sandblasted the iron to a dull grey color and the primer coat is on its way!

IMG00433

So how much weight must this windlass pull up to raise all the anchor chain and the anchor?

Lets start with the anchor chain.  I think I want to use 3/4″ stud link chain.  This is the size used on the tugboat Bertha and she is about the same length and displacement as NYC 13.

Using the dimension charts here:

http://www.anchormarinehouston.com/~kerry/amarine/catalog/chains/stud-link-anchor-chain

3/4″ stud link is 5.33 pounds per lineal foot.

Anchor chain is measured in a unit called shots.

1 fathom = 6 feet

1 shot = 15 fathoms = 90 feet

weight of one shot = 480 lbs

The best scope to use when anchoring is about 7 to 1.  Scope, which is the ratio of length of chain to the depth measured from the highest point (in our case, the hawsepipe) to the seabed. For example, if the water is 50 ft deep, and the hawsepipe is 10 ft above the water, the scope is the ratio between the amount of chain let out and 60 ft.

Height from hawsepipe to waterline = 10 feet

Max depth to anchor in = 50 feet

=60 feet  x scope of 7 = 420 feet of chain needed to anchor safely in 50 feet of water

So how many shots is that?

420 feet / 90 feet per shot = 4.6 shots,  so lets round up to 5 shots.

Lets add in an extra shot, in case there were some very deep anchorage.

So 6 shots of chain  = 6 x 480 pounds = 2880 pounds

So what about the weight of the anchor?  We have to lift that up too.

I have onboard already, two different standard stockless anchors:

One is a small “lunch hook” anchor with the number “5″ on one of the flukes.   Judging from its dimensions and comparing to the specs for new anchors, it weights 500lbs.

IMG00013

The other is a bigger one and given its measurements, it weights 1000 lbs.

So the total weight of the ground tackle (bigger anchor plus chain) is 2880 lbs + 1000 lbs = 3880 lbs

Lets add in a 3x safety factor so:

Needed windlass pulling power = 3 times the ground tackle = 3380*3= 10,140 lbs


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3 Comments

  1. Amber Johnson
    Posted November 2, 2009 at 11:51 pm | Permalink

    Eric, Hi there. I am delving into the world of sailing (yes, I know tugboats don’t ’sail’ exactly) around the turn of the twentieth century to do research for a novel I am writing. Is there any chance you are one of those friendly internet fellows willing to have a crazy writer’s random questions bounced off your seaworthy noggin’? If so, would you mind sharing with me any ideas you might have on how one (fictional character) might subtly and silently sabotage or jam a manual anchor windlass so that it will not hoist anchor? A thanks in the acknowledgement section (if I ever get published) is what’s in it for you :)
    Oh, and great blog. If I were a tugboat, enthusiast, I’d be an Eric’s tugboat enthusiast :)

    • Posted November 3, 2009 at 12:16 am | Permalink

      Hi Amber – Thanks for the nice words about the blog! I don’t really know much about manual anchor windlasses. The ones I know are all driven by an electric or a hydraulic motor. But wherever there is a machine, there is a way to foul it up so it wont work. I know that truth to be universal. There is a pawl inside most windlasses that all allow it to be clicked in one direction only, allowing the motive power to take a break without having to hold the load in mid hoist. You could somehow have that break, causing it to be incredibly difficult for the anchor to be raised. Also, I once heard about an anchor chain being lifted to find that a violent storm had somehow (??!!??) created an overhand knot in the chain. I cant even imagine how that would happen. That would definitely add a level of difficulty to raising the ground tackle. I hope this helped! Cheers – Eric

  2. Eldho
    Posted April 6, 2012 at 11:35 am | Permalink

    Hi,

    I would like to do the sizing calculation for an electric driven double anchor windlass on a GL class survey vessel. The anchor is HHP type with a mass of 1440 kg each. Anchor chain is of stud link type with a dia of 34 mm and K3 grade. The length of the chain is 220 m each. (The specifications for the anchor windlass installed on a similar vessel having an anchor mass of 1300 kg and chain dia of 32 mm were 61 kN pulling power and 10 kW electric motor). Kindly suggest me the pull on the windlass and the power required for the electric motor. It would be much helpful, if you could post the calculation steps.

    Thanks & Regards

    Eldho

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  1. By Sizing a hydraulic motor for the windlass on September 3, 2009 at 7:36 pm

    [...] is a follow to the post Sizing an anchor windlass where we calculated the mass that our anchor windlass must lift given the scope, chain and anchor [...]

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