hydraulic anchor winch (pt 2)

sarabande

sarabande

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OK, so the scenario is a handpowered winch for heaving up the anchor.

I have a technical firm who are interested in developing a test rig.

We need to specify some operating parameters, so, let me start with the following:

Anchor weights 10 kg to 50 kg

Chain weight for 60 metres
12mm 230kg
8mm 90 kg

So deadweight lift for big anchor with 60 m chain out is 280 kg (less displacement, thank you Mr Archimedes)


What I need to know is:


what power does the Forum think will be needed to recover an anchor well and truly stuck in the mud ? (above the deadweight of anchor and chain)

A typical electric winch has a motor rated at 0.9 kW.
 
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what power does the Forum think will be needed to recover an anchor well and truly stuck in the mud ? (above the deadweight of anchor and chain)


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I think an anchor 'well and truly stuck in the mud' is best broken out by shortening the scope and letting a rising tide do the heavy work. But I know I'm old fashioned.
 
This may sound pedantic, but it's intended to be helpful ultimately. Power is force times velocity so winch power is significant only if you are in a hurry to lift a stuck anchor. It's my expectation that as you increase the speed of rasing the anchor, the force needed to break it out will increase too (don't know by how much, but experience suggests a lot) so the power needed goes up hugely. The answer as Ken says is to drag it out slowly - just a bit more force than the straight lift requiires, going slowly, will get it free from the mud, but not very quickly.
My own experience is that it takes tens of seconds to get it moving. Using an electric winch, I pull the chain tight then stop and wait a while; the load eases a bit, pull tight etc. 3 or 4 cycles are generally sufficient. Power used - negligible since velocity is almost 0. Electric power into the winch could be quite high as the motor stalls.
I reckon winch power is set by the speed you want to pull the chain in. My 1000 watt Lofrans is bigger than needed (6 ton boat, 30m 8mm chain, 35lb anchor) - it can pull the boat up to the anchor in moderate wind, say F3.

Derek
 
thanks, Derek.

The plot thickens in one way, and becomes more transparent in an other.

So I need to find out what force is exerted by a boat "lifting" an anchor with a taut chain ? I have broken out anchors on a 16 tonne displacement motor sailer by winding the chain in the way that Ken McCulloch suggests, but have no idea of the actual force created by this method. Any suggestions ?

The engineers also have initial concerns that the use of hand'/hydraulic winch in cold weather (say less that 10C) will adversely affect the performance as most hydraulic systems are expected to work at around 30C to 60C. Not the normal marine environment !

A human being can work at about 250 W to 500W, so what I am trying to design is a system where that "power" (?) can be used to provide a reliable source for an anchor winch.

Once broken out, I guess that most people will be content to wind in at a steady rate, probably using less force than when breaking an anchor out.
 
That Maxwell Marine site has loads of the key data, and some very practical assumptions.

Many thanks.

Back to the engineering workshop !
 
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So I need to find out what force is exerted by a boat "lifting" an anchor with a taut chain ? I have broken out anchors on a 16 tonne displacement motor sailer by winding the chain in the way that Ken McCulloch suggests, but have no idea of the actual force created by this method. Any suggestions ?

[/ QUOTE ]I'm not sure what you're asking.

If you're wondering at the forces when the boat is pitching in swell, e.g. you pull the rode tight when the boat is low then it tries to pitch upward, then the force is related to the net buoyancy of the boat together with its pitching moment. I.e. the force is essentially given by whatever bit of the boat's bow is underwater that shouldn't be. A dumb thing to do unless you're desperate and are prepared to risk breaking something. But you can guess a ballpark figure by examining the boat's displacement, its hull shape, and the height of the swells.

If you're wondering at the forces generated by the boat motoring forward trying to drag the anchor up, then it's related to the boat's momentum (if moving) or the boat's bollard pull (if static), whatever horizontal force is in play, divided by the cosine of the acute angle between the horizontal and the rode. In other words a lot if the rode is pulled in as far as possible. Short scope is a force multiplier. But again, as above, you can make some ballpark guess at the figure if you want.

If the anchor's holding, your winch can't be expected to pull against it... you lock it off, stop the chain, work the anchor free, then get the windlass back in action.
 
Hi Craig

I was thinking of the amount of deflection from the boat's normal "at rest" position, that is necessary to lift a stubbornly or well set anchor.

Just need to get some idea of the difference between the "dead weight" of the ground tackle (in the putative worst case of 50kg anchor and 230 kg of chain), and the force needed to lift an embedded anchor. From reading lots of anchor tests, I know that modern anchors are known to be very difficult to break out sometimes becasue they set so well, and I want a figure for that break out to give to the engineering shop.

In my case it takes a 0.9kW winch to haul the chain tight (yes, I do know enough NOT to use the winch to bring the boat vertically above the anchor !) plus some of the pitching moment of the boat. It's an estimate of the force provided by the boat: is it 500 kg, 150kg, or 1000kg.

I know there are lots of variables, but some techy type must be able to estimate, or even with a load cell or deflection meter on the chain, give a realistic figure ....


I don't understand the bit about a short scope being a multiplier, sorry !
 
I don't understand the short scope being a multiplier unless we are talking about velocity ratios. This is all about gearing where as we all know the longer distance moved on the input side as opposed to the distance moved on the output side the less force required but the slower the lift.
 
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I was thinking of the amount of deflection from the boat's normal "at rest" position, that is necessary to lift a stubbornly or well set anchor.

Just need to get some idea of the difference between the "dead weight" of the ground tackle (in the putative worst case of 50kg anchor and 230 kg of chain), and the force needed to lift an embedded anchor. From reading lots of anchor tests, I know that modern anchors are known to be very difficult to break out sometimes becasue they set so well, and I want a figure for that break out to give to the engineering shop.

In my case it takes a 0.9kW winch to haul the chain tight (yes, I do know enough NOT to use the winch to bring the boat vertically above the anchor !) plus some of the pitching moment of the boat. It's an estimate of the force provided by the boat: is it 500 kg, 150kg, or 1000kg.

[/ QUOTE ]The forces involved can be a lot. We've bent solid 20mm steel plate shanks in bad surge with the anchor fouled. 25 tonnes of boat pitching against it, and an eye on 12mm G40 as to whether it's going to hold up... Now that's a fouled anchor, not a deeply buried one, but the point is you can estimate the potential loads by looking at what your boat is actually capable of generating. A bit of experience will tell you what the boat needs to do to recover the pick.

You should make sure all your gear, bow-rollers etc, is rated above the breaking load of your chain, not much point in going too much higher obviously because the chain will fail first. Forget about the windlass, try powering against a stuck anchor or even leaving it locked off, and you'll just do expensive things.

The windlass should essentially be powerful enough to winch in at whatever speed you find convenient the weight of the entire rode deployed, plus anchor, in deep water. That's how you spec it.

[ QUOTE ]
I know there are lots of variables, but some techy type must be able to estimate, or even with a load cell or deflection meter on the chain, give a realistic figure ....

[/ QUOTE ]It's easy to give worst case ball-park figures, but your experience as above needs to speak to the more every-day stuff. If you know the bollard pull of your boat and the revs needed to pull out the anchor, then you can estimate... but as above, you spec the windlass according to the rode weight. When recovering the anchor, you try pulling with the windlass, but if it's struggling, stop the chain and use the boat's power.

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I don't understand the bit about a short scope being a multiplier, sorry!

[/ QUOTE ]Your boat can only apply a horizontal force on the rode, the vertical force comes from the boat's buoyancy (resistance). It's like leaning on a tent pole, much easier to pull out the stay peg than pulling on it directly.

The tension in the rode is greater than the horizontal load courtesy of the boat. If you're at 2:1 scope, which is a 30 degree angle, and say your boat's being loaded up at maybe 100Kg (backward or forward), then the tension in the rode is actually 115Kg. Reduce the scope further and it escalates rapidly. Meaning: even the bollard pull of your boat can gear up to quite some force in a rode at minimized scope.
 
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