Re: Storm Anchors
What I gotten from my reading and listening to discussions is that
drogues and sea anchors serve 2 different purposes. The drogue streamed
off the stern is used for stability/comfort when conditions are such
that you can still safely make-way. The sea anchor is used streamed off
the bow as a last defence when conditions have deteriorated to the
point where the vessel is in danger of capsizing and cannot continue to
safely make-way.
Mike, what you and Maurice have said is traditional logical thinking (my
thinking too) but the CG report suggests otherwise i.e. when things get REAL
bad, chances for survival are enhanced by running the sea anchor off the
transom and take the beating on the stern rather than the bow. Because that
approach is so contrary to my understanding, is the reason that I posed the
question. Thus far all the responses have inferred that I don't understand
the difference between anchors & drogues and their normal uses. I do
understand their normal uses that you and Maurice have discussed. What I
don't understand is what I first stated. That is, how can it be that
holding the stern to the wind is better in storm conditions than keeping the
bow to the wind? It does not make sense to me and I am asking if anyone can
make sense of that approach. Normally, I would not consider the idea but CG
tests seem to indicate that capsize can better be avoided by using this
seemingly crazy approach.
I think it would be helpful if some of you folks that are hydro-dynamic
savvy or understand the physics of hulls in heavy seas would read the entire
article and comment on the CG findings.
Thanks,
Glen
I read the article and I thought it was pretty clear.
First of all, it referred only to sailboats and so its conclusions may be
different for powerboats, although I think not. The whole objective of the
drogue or sea anchor was to keep the boat oriented perpendicular to the
crests of breaking waves. If the boat gets sideways, it is liable to broach.
Secondly, it was very clear that the issue was the inability of even the
largest sea anchor (a 28' parachute was mentioned) to maintain continuous
tension on the rode sufficient to keep the bow from yawing away from the
wind. As soon as the bow yaws away from the wind, the boat is vulnerable.
My inference is that streaming from the stern works better because the boat
is much more directionally stable when pointed downwind. This would be for
two reasons: the bow has more windage because it has more freeboard (and
probably the mast is forward of the center of yaw) so tends to damp any
yaw, secondly the boat has better directional stability moving forward
because the rudder is at the stern and will tend to damp yaw (also because
the hull is designed for directional stability moving forward).
I have no idea what is changed for a powerboat with a large square transom.
Sailboats usually have slender hulls and expose less transom area. On the
other hand, it is clear that the area of the transom is still much smaller
than the area of the side of the boat.
Mike T.
In article <01be56be$6a384480$In article 01be56be$6a384480$ac75accf@len, Glen & Karen wrote:
Normally, I would not consider the idea but CG
tests seem to indicate that capsize can better be avoided by using this
seemingly crazy approach.
Glen,
I went back and reviewed the website again. What I noted this time was
that 1) the study was from 1986, 2) the website is sponsored by a company
selling drogues and 3) the conclusions about stern-to were based on model
testing only. Laboratory testing is fine as a starting point, but the
real world examples in DDDB don't seem to support those conclusions. I'm
suspicious that the length of the tether off the bow in the tests was too
short, but since that stat isn't in the study can't prove it.
Hoping not to have to ever find out the reality
CHeers EH
Mike Eedy
M/V Autumwynde
North Bay Ontario
Mikey@efni.com