How do you calculate "pull" for a boat? - SailNet Community
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post #1 of 12 Old 07-21-2008 Thread Starter
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How do you calculate "pull" for a boat?

As it pertains to anchors. I see numbers batted around about a boat pulling X amount in 20 kts or y in 30, etc. How do you get those numbers? I assume it is for bare poles, nose into the wind. My almost 18' boat weighs about 1000# before anyone is aboard, probably 1500# with people and gear, has about 3' of freeboard at the bow and the cabin only bumps up another foot or so at an angle to reduce windage. I have never had trouble holding the bow line, but I haven't been in really heavy winds.

-Andy
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post #2 of 12 Old 07-21-2008
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Most calculations go out the window when you are anchored in a gentle breeze and a nasty storm with gusting winds crops up - if that hasn't happened to you yet, it will. So, just get a couple of bigger anchors than you think you will ever need (most info sheets on anchors tell you what size or weight boats they are for, go up a size or two), put 6-12' of chain on them and heavy nylon rode and don't worry.

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Slipped in Bahia Marina, easy access to Corpus Christi Bay and the Gulf of Mexico.
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I found a chart on this page:

Anchoring

My concern is whether or not I need to just get rid of my current anchor and get a new one or if it does have some usefulness. According to that chart, it is a "working acnhor" (barely) but not a "storm anchor".

-Andy
Newport 17 - "Kohanna"
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post #4 of 12 Old 07-21-2008
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Andy, I had 9 boats sitting on my 25 lb danforth this weekend in Bodkin Creek (near Baltimore) for a little better than 100,000 pounds of boat, with a beam of at least 100 feet.

Had steady wind at 12, gusts to 16. Didn't move an inch - the calculated holding power of my anchor is 1600 pounds and the chart in your link says I need 1800 for storms (33.5 ft by 14 foot beam). I set the anchor with 2500 rmp for 10 seconds of 0.00 kph on my GPS.

Charts are nice, experience is better.

To the purists out there, I also had a 40 pound claw sitting on 100 ft of chain (slack chain at the bow, my anchor held the 'pack') as the back up in the primary had tripped - our sterns were only 100 feet away from the piers.
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post #5 of 12 Old 07-21-2008
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How much your boat weighs or displaces isn't generally as important as how much windage it presents. The more surface area the boat has to push on, the greater the wind's force against the anchor will be. My boat isn't very heavy given its LOA... about 5000 lbs. all up. However, it does have an 18' beam and more windage than a 28' monohull would... and as such, I got the same anchor that is being used by another member on this site for his Canadian Sailcraft 36T, which is substantially larger and heavier than my boat.

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post #6 of 12 Old 07-21-2008
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Anchors/Windage

There are simple formulas to calculate the windage a boat creates in steady wind-Bow-on with the perfect holding.

That is not what you buy an anchor for.
Your anchoring system has to hold during gusts, when the boat is sailing back and forth at anchor and increasing its windage considerably, and then you add the waves that shock load the anchoring system as they pass under your boat. All these things tremendously increase the load that your system has to sustain. Since waves also lift the bow as they pass they reduce your scope making the anchor work that much harder as well.

Different anchors work for different conditions and bottom types. Using the right anchor with its favorite bottom type makes it seem like the anchor can do anything. Then that same anchor in another situation seems like the worst anchor in the world. That is why most cruisers buy anchors considerably larger than any manufacturer suggests and have several different types aboard.
Remember that scope should be measured 1) from your bow, 2) at the top of the highest wave you may encounter 3) to the sea bottom 4) during the highest tide you may encounter.

5 to 1 is a lunch hook
7 to 1 is an overnight in a very protected anchorage
10 to one is the minimum for rough water high wind high current, etc.

Make sure you know the bottom your anchor is trying to hold onto
Make sure your swinging circle does not intercept anyone elses swinging circle
Take bearings

Don


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post #7 of 12 Old 07-21-2008
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There is no advantage in a rode over 10:1 and a 7:1 rode is very nearly as good as a 10:1 ...I don't see any need for more than 7:1 in anything less than a full gale in most anchorages...not just the very protected ones. Here's the actual holding power of various scopes compared to 10:1
7:1=91%
6:1=85%
5:1=77
4:1=67
3:1=53
2:1=35
From Don Casey.
Note that these %'s are somewhat dependent on anchor type in the LOWER ratios as Danforth & Bruce types do better than plows on short scope...but this is largely irrelevant to the discussion of how much scope is needed.

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post #8 of 12 Old 07-21-2008 Thread Starter
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One conclusiong I am coming to is that one of my favorite ways to anchor is impossible under anything but ideal conditions. For beach combing and what not, I like to pull into knee deep water (wth the keel up, I draft under a foot) and wade in to shore. This is usually within a few feet of shore, so it wouldn't take much scope at all to allow it to swing to shore. Sometimes I anchor at the stern and carry a bow line in if there is something to attach to. You don't even want to know how little scope I generally use for this. But this tends to be in very light or no wind.

A big part of this is probably my "evolution" from lakes to sounds. The wind is generally better and while sailing I love that. But while rigging or taking a break on a beach, it is a PITA! You can't just pull in and throw it in park...

-Andy
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Nothing wrong with beaching on the sand as long as the tide doesn't go out!!

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post #10 of 12 Old 07-21-2008
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Quote:
Originally Posted by sailingdog View Post
How much your boat weighs or displaces isn't generally as important as how much windage it presents. The more surface area the boat has to push on, the greater the wind's force against the anchor will be. My boat isn't very heavy given its LOA... about 5000 lbs. all up. However, it does have an 18' beam and more windage than a 28' monohull would... and as such, I got the same anchor that is being used by another member on this site for his Canadian Sailcraft 36T, which is substantially larger and heavier than my boat.
SD, that's why I said 100 feet of beam. I had 3 power boats (28 ft each) 3 sailboats over 30 (including mine) and 2 at 30 feet. Combined windage should have been much more than these charts indicate my anchor will hold.

Something about that Ches. Bay muck, I mean mud.
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