Re: [Amel Yacht Owners] some ideas (2)


Jean-Pierre's MacBook Air <jgermain@...>
 

Hi Jeff,

Totally agree with your math.  The best option is avoidance of the risk.

Hence why I’m in Curacao for the Hurricane season.

Fair winds,

Jean-Pierre Germain
SY Eleuthera, SM 007



On 30 Sep 2017, at 08:28, jmkraus@... [amelyachtowners] <amelyachtowners@...> wrote:


Stay In Town:

There are 2 forces acting on a boat tied or anchored.

1:

Force the boat exerts due to it's own mass 

F=ma. m=mass of boat a=(v2-v1)/(t2-t1). What that means is as your boat moves, and tugs on your lines, the harder the wind blows, and the faster it swings and bobs, more force is exerted on your lines. 

2:

Force of the wind on your boat 

F=APCd. a=area, P=(a coefficient)v2  where v=velocity of the wind  Cd are coeficients.

SO...force is equal to the square of the wind velocity.

A 30mph wind exerts a 900unit force.

A 70mph wind exerts a 4900unit force 

A 100mph wind exerts a 10000unit force

Bobbing and tugging more drastically, along with double the wind force, it's easy to see why a hurricane with winds of 100mph will exert TWICE the combined forces on your boat as tropical storm force winds of 70mph would.


You need to counteract these forces obviously.
At anchor:
Winds will swing counterclockwise (backing) as the storm passes (assuming the storm passes from east to west). As a result, your boat will swing with it. You need to deploy anchors at angles to account for this.
Most harbors and bays bottom sediment is either sand, mud, clay, or a combo of this, but generally on the soft side. I prefer an anchor that will present a perpendicular surface on it's flukes, driving them into the bottom, and presenting maximum resistance to the pull of the anchor line.
I used a combination of a dansforth  (which held) and a plow (which dragged) while Spirit was tied up and also anchored between piers at the marina to weather Irma's tropical force winds.

I plan to buy a fortress anchor. Look them up.

In the marina: Between piers, NEVER in a slip.
Anticipate the wind shift and tie up so you will have enough lines holding you as this occurs.
Get some tires. Slip them over the pilings you plan to tie to. Wrap a piece of chain around the tire, and tie up to the chain. The tires will flex, and dampen the motion, lessening the force on the lines (see F=ma)

I plan to buy some spectra line to supplement my line inventory. Expensive, but light and easy to store. 
I was lucky that Jose Mendes (owner of Marina Pescaderia) had enough heavy nylon line for me to purchase to tie Spirit up to weather Irma. Damn lucky. Get it now, so you have it when you need it, you might not be so lucky. 400 feet of Spectra would take up less space then 100 feet of good nylon line. 

Next up...Get out of Dodge. But not today.
I hope this stuff is helpful to someone. 

Jeff Spirit Amel 54 #14




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