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25-03-2013, 05:37
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#1
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Registered User
Join Date: Mar 2010
Posts: 236
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Setting the anchor
Once the anchor has taken a bite, I let out some more rode and then put transmission in reverse. When the slack is out, I increase rpm to about 1500 and hold it for several seconds. If that holds, I assume I have a pretty good set. I wonder how much pull I'm putting on the anchor? Would 1500 rpm equal 25 knots of wind? Or 50? Is there a way to figure this out? I guess I could go to a slip, tie off the bow roller with a weight scale in the line and find out how many pounds I'm pulling. Am I over-thinking this?
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25-03-2013, 05:44
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#2
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Moderator Emeritus
Join Date: Oct 2009
Location: Manchester, UK
Boat: Beneteau 473
Posts: 5,589
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Re: Setting the anchor
Thats going to depend on the size of engine (HP), type and size of prop, hull form etc.
But yes, a weight scale tied off between boat and a fixed point is about the only way you will find out what the bollard pull is
__________________
Nigel
Beneteau 473
Manchester, UK
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25-03-2013, 05:52
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#3
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Registered User
Join Date: Dec 2009
Posts: 2,959
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The dead pull of the typical sailboat propeller is surprisingly few "pounds". That said, I am somewhat under powered at 38hp and 10T but know I can just barely motor into a 40 knot headwind. In reverse a Max Prop is as good as forward so one might guess that a dead pull on the anchor represents maybe 50 knots for me.
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25-03-2013, 05:57
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#4
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Registered User
Join Date: Dec 2007
Boat: Mahe 36, Helia 44 Evo, MY 37
Posts: 5,731
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Re: Setting the anchor
Quote:
Originally Posted by F51
Am I over-thinking this?
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Yes you are over thinking this.
My only recommendation would be to back down on it as you said.
Let it slack, then back down on it again.
Let it slack, then back down on it again.
Three times will really bury the anchor deep
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25-03-2013, 06:16
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#5
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Senior Cruiser
Join Date: Aug 2009
Posts: 4,033
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Re: Setting the anchor
Quote:
Originally Posted by F51
I wonder how much pull I'm putting on the anchor?
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There are various formula to estimate approximate bollard pull.
One of the simplest is: Pull (in lbs) = 11.87 * (shaft HP * prop dia in inches) ^ .67
so, for example, with my engine at full revs, it generates 75 hp, about 95% goes to shaft HP, so lets say 70 SHP, and I have a 20" prop dia. So, 11.87 * (70 *20)^.67 = 1500lb bollard pull.
Maxprops and autoprops will pull the same in forward and reverse, while fixed props will be less efficient in reverse (lets just say 80%).
How much wind/waves 1500lbs corresponds to is a whole other and rather more difficult question. There are formula for that also, but it's a much more complex question with many more variables.
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25-03-2013, 06:28
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#6
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Registered User
Join Date: Sep 2011
Location: Good question
Boat: Rafiki 37
Posts: 14,143
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Re: Setting the anchor
Our approach is similar Cotemar's. We lay out the anchor and chain (all-chain rode, 3/8") while moving back slowly, usually by wind only. When we get to about 5:1 we'll start to set the anchor, but will often go to more (7, 8:1) if there is any slippage.
Once the anchor starts holding we go through a three or four-stage dig where we ramp up the RPMs in steps: something like 600, 1200, 1900, 2200 (this from memory). At each stage we stop and hold until we're confident we're not moving; usually 10 to 30 seconds. The first few steps produce a bite, and then a slackening, due to the weight (catenary) of the rode. By the time we get to full (normal) throttle we're generating enough force to remove all catenary (taught rode). We hold this until we are 100% confident of no slippage.
It's an interesting question you pose F51. I think the only time we've had zero catenary is in blows around 50 knts, but it's hard to know for sure due to the large snubbers.
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25-03-2013, 06:33
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#8
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Registered User
Join Date: Apr 2006
Posts: 4,413
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Re: Setting the anchor
I was always under the impression that once the flukes set into the bottom... sand or mud etc. the applied forces cause the anchor to dig down inter the bottom... But obviously this is going to be influenced by the bottom, the scope and the force (load) on the rode. The scope is self limiting in combination with the design of the anchor. The holding force (resistance) is related to the bottom characteristics as well as the geometry.
What is the functional difference in terms of real world performance of an anchor which as *taken a bite* (boat no longer makes weigh down wind (or down current)... and one that is fully set?
It an anchor which has not been fully set but only taken a bite more prone to breaking out? Would this be from a change in direction of pull? Wave action rocking the boat?
My own experience over 27 years is that anchoring in any moderate winds the anchor will set without the use of the engine from the forces of the wind alone. With no wind on has to back down to set the hook.
I've had the anchor drag when it fouled on something on the bottom or set in bottoms with poor holding... like eel grass.... or a very sudden and dramatic wind shifts with strong winds.
I anchor with all chain and scope of not less than 5/1
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25-03-2013, 06:36
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#9
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Long Range Cruiser
Join Date: Dec 2007
Location: Australian living on "Sea Life" currently in England.
Boat: Beneteau 393 "Sea Life"
Posts: 12,822
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Re: Setting the anchor
No you are not over thinking it.
My normal cruising revs are from 1800 to 2100 on an engine that goes to 3,300 rpm.
I lay out 3:1 or LESS when setting after it bites I put it in astern at 1,500 rpm for those few seconds you are talking about and then bring the revs up to 2,000 for about one minute. So I give it a good BIG tug in! Then I can lay out the extra scope.
No use having the pick pop out when the wind drifts up to 15 knots, but if its gunna be hitting 25 to 30 knots and you have just dropped you are likely to stay aboard and forgo the bar for a while!
Mark
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25-03-2013, 06:46
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#10
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Moderator
Join Date: Jul 2007
Boat: Bestevaer.
Posts: 14,678
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Re: Setting the anchor
We gradually increase the revers in reverse and finish with about 20 second in full reverse (maxi prop)
Don't forget to monitor some transits, many people don't realise they dragging backwards when trying to set the anchor.
This seems equilvant to aout 30-35k of wind. If the wind is stronger than this the anchor will set deeper
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25-03-2013, 07:23
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#11
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Registered User
Join Date: Aug 2011
Location: Lower Chesapeake Bay Area
Boat: Bristol 27
Posts: 10,458
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Re: Setting the anchor
What he said.
I've never used the engine to set an anchor..............in 40 years with different boats (both power and sail) and different anchors ...........
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25-03-2013, 07:49
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#12
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Registered User
Join Date: Oct 2010
Location: The boat - New Bern, NC, USA; Us - Kingsport, TN, USA
Boat: 1988 Pacific Seacraft 34
Posts: 1,454
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Re: Setting the anchor
Quote:
Originally Posted by estarzinger
There are various formula to estimate approximate bollard pull.
One of the simplest is: Pull (in lbs) = 11.87 * (shaft HP * prop dia in inches) ^ .67
so, for example, with my engine at full revs, it generates 75 hp, about 95% goes to shaft HP, so lets say 70 SHP, and I have a 20" prop dia. So, 11.87 * (70 *20)^.67 = 1500lb bollard pull.
Maxprops and autoprops will pull the same in forward and reverse, while fixed props will be less efficient in reverse (lets just say 80%).
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In addition to the loss in efficiency that my 2 blade fixed propeller suffers in reverse, my transmission has a ratio of 2.14 in forward and 2.50 in reverse making a full speed 3200 engine rpm 1498rpm at the propeller in froward and 1280 rpm in reverse. That has to reduce the propeller zero speed thrust in reverse.
Standing on my anchor chain as it crosses the bow in full reverse it seems less tight than when the wind is blowing and the boat is at the end of one of its swings, pivoted near broadside to the wind, and bouncing up on a wave. I've never been really impressed with the amount of pull I can put on the anchor in reverse.
An Excel based propeller bollard thrust calculator that includes the transmission ratio is given as a link in
Propellers and Albergs
You don't get to choose the propeller. It does the choosing.
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25-03-2013, 08:01
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#13
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Senior Cruiser
Join Date: Aug 2009
Posts: 4,033
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Re: Setting the anchor
^^
Nice, thanks.
But regarding Bollard Pull, if you go look at the cell in the spreadsheet that does the bollard calculation you will see it in fact uses exactly the formula that I posted above.
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25-03-2013, 08:53
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#14
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Registered User
Join Date: Apr 2011
Location: Vancouver, BC
Boat: Niagara 35
Posts: 1,878
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Quote:
Originally Posted by wsmurdoch
In addition to the loss in efficiency that my 2 blade fixed propeller suffers in reverse, my transmission has a ratio of 2.14 in forward and 2.50 in reverse making a full speed 3200 engine rpm 1498rpm at the propeller in froward and 1280 rpm in reverse. That has to reduce the propeller zero speed thrust in reverse.
Standing on my anchor chain as it crosses the bow in full reverse it seems less tight than when the wind is blowing and the boat is at the end of one of its swings, pivoted near broadside to the wind, and bouncing up on a wave. I've never been really impressed with the amount of pull I can put on the anchor in reverse.
An Excel based propeller bollard thrust calculator that includes the transmission ratio is given as a link in
Propellers and Albergs
You don't get to choose the propeller. It does the choosing.
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Shouldn't the chain be slack when you're bouncing up on that wave? You're right that that's the dangerous part, so that's why the snubber should be taking the load.
__________________
Chris
SailMentor.com - Become the Confident Skipper of Your Own Sailboat
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25-03-2013, 09:09
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#15
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Senior Cruiser
Join Date: Aug 2009
Posts: 4,033
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Re: Setting the anchor
As an aside, the ABYC says the anchor loads on my boat in 30kts should be about 1600lbs (I believe they are usually much less than that in a protected small fetch harbor), and that's about what the Bollard pull formula says I should be able to pull (And I suspect I pull less than that because I have a feathering prop optimized for motor sailing rather than one maximized for load speed torque).
But all caveats aside, and for a very very crude ballpark, the suggestion is that generally full reverse will load an anchor very roughly like gale force (but not storm force) winds will.
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