Re: [PUP] Sea Anchors deployment
I am curious about the 400 feet of 7/16" H T chain--this weighs about
2.5 lbs a foot or around 1000 lbs. Is it really necessary to have the
400 feet of chain?
The 400 ft of chain is my main anchor chain. 400 ft is standard from
Nordhavn. I sometimes use all 400 ft, the sad truth is it is very hard
to find room in an anchorage, much less one with good holding in the
Med during season and I "anchor" or (depend on the chain drag to
hold me) in some pretty poor spots.
What I am aware of deployment of sea anchors, short lengths of
chain are desirable--
both near the anchor to weight it down and from the boat for
chafe protection and keeping the line down--and preventing
it being pushed back toward the boat--but 400 feet seems a lot.
ParaTech (our Sea Anchor company) said to use 400 ft of 7/8" line
and anywhere from 100-300 of chain as our sea anchor rode.
You let out sufficient rode to have better ride. The chain holds the
line in the water and the resulting catenary curve functions as a
shock absorber. This is something they added to their
recommendations in the last couple of years. In addition as you
note it helps control the line chaffing issue, now I neeed to make
sure the chain does come off the bow roller and saw thru the hull!
Also how do you let this 400 feet of 7/16 chain run out, with
out risking damage to your boat or yourself?
Off the bow roller.
I obviously did a bad job describing the system, sorry,
here is a second attempt-I hope it is better!
I have 400 ft of chain for the main anchor.
In prep for bad weather, I disconnect the 7/16" anchor chain
and attach it to the sea anchor 7/8" line. The sea anchor line
runs thru the chain roller (it has a "keeper" bar across the top)
then runs over the top of the railing that runs around the bow.
The line then goes over foredeck and over the Portuguese
bridge, into a locker that is on the inner wall of the bridge.
The locker door opens aft. The sea anchor line is wire
tied to the railing on the bow and again at the railing around
the Portuguese bridge.
Scott,
I am trying to following your words of how your sea anchor is rigged and deployed. I think I understand most of it.
Am I correct that when the Sea Anchor is completely set that a typical configuration starting from the furthest end would be:
- The "fetch line" with a small float
- A large float to support the crown of the chute
- The chute itself
- The chute bridle
- A swivel shackle
- Up to 400' of nylon rode
- A shackle
- Up to 400' of BBB chain
- Contact with the bow roller
- Tie off at the bow near the anchor windless with a secure chain hook.
Items 2 and 6 are adjusted to produce a two wave length separation between boat and chute
If this is near correct I am surprised at two things:
A) That the up to several under pounds of chain weight doesn't pull the float and chute under.
B) That the boat rides comfortable, and in the "shadow" of the chute, when attached at the bow roller rather than off to port. (or strbd for the southern hemisphere)
Corrections and comments appreciated.
Regards, John Harris
At 11:35 AM 1/3/05 -0700, you wrote:
Items 2 and 6 are adjusted to produce a two wave length separation between
boat and chute
I think it is generally understood that a single wavelength of rode is not
enough. But there may be some who have not understood even that. However,
my reading of the relevant literature does not conclude that 2 wavelengths
are necessarily "magical". Part of the problem is that it is hard to
simulate by computer or models what would constitute the best length.
Thence comes the really hard part. Go to sea and test this.
First there is the little item of having enough rode to fully test various
lengths of say 1, 2, 3 or more wavelengths. It may not be practical to do
more than one test in any one storm. This severally limits the number of
tests that one can do in ONE lifetime. After all, one mistake could result
in the proverbial "truncated life". It is difficult to test 1, 2, 3, etc.
wavelength rodes even in one storm as it may be impossible to arbitrarily
put out 1 then 2 then 3 wavelengths at various intervals.
As if this was not enough, wavelengths are not static and may very well
change during the duration of the storm. Shorter wavelengths are typical
during the initial stages and longer wavelengths may predominate later, but
the wave that will get you, may very well be completely out of character
and from another direction.
I have no quarrel with the notion that planning and preparing for the
typical storm is useful. What I disagree with is allowing the assumption
that conditions will be typical and therefor that it is not necessary to
take into account the "what if" scenario.
Getting down to some hard facts. Nylon can stretch up to about about 50%
before breaking. 400-900 feet of rode will stretch 200 to 450 feet before
breaking. Let's not argue that it is 40% ,the difference is not worth the
debate. A boat accelerated by a breaking wave will come near to the speed
of the wave, say 40 feet per second. Meanwhile the rode is stretching. If
the breaker is large enough in comparison to the boat a rollover can occur
before the rode can halt the acceleration and force the boat to ride up and
over the wave. Jordan suggests that the boat should be decelerated such
that there is about 15 feet per second difference in the speed of the wave
and that of the boat.
A single point drogue/parachute using a nylon rode will stretch such that
the boat will not be restrained before the rollover/pitchpole will occur. A
rode with drogues spaced all along the length will start the restraining
process almost immediately and has the best chance of preventing the
rollover/pitchpole. This is not to imply that other dangers do not exist,
even as this is happening.
More later.
Capt. Mike Maurice
Tualatin(Portland), Oregon