[Rhodes22-list] Sacrificial Toothpicks

[Dennis]

Hi Mike -

First, apologies for the following boring dissertation ->

The mainsail and the Genoa both provide drive for the boat, and both try to
pull the top of the mast overboard. The forestay, backstays, and shrouds all
stabilize the top of the mast, but they have to pull down fairly hard on the
head of the mast to do so - so most of the force at the foot of the mast is
straight down. The compression post (in the cabin) lives to carry that
downward force to the keel.

A lot of the Genoa's drive is picked up at the base of the forestay and by
the Genoa's sheet - but forget those forces for a moment, since they don't
bear directly on the mast. Also ignore the forward and aft lowers for the
moment and the support they offer to the mast.

Most of the remaining force on the mast is caused by the mainsail as it
pushes the boom against the mast, causing some lateral force at the base of
the mast. It's not a huge force, given that the mainsail is relatively small
and some of its drive is absorbed by the sheet and traveler. The lateral
force at the foot of the mast is made still smaller by the fact that the
boom is generally a few feet up from the foot of the mast, so some of the
boom's force is actually carried aloft, and resisted by the stays and
shrouds at the top of the mast. 
 
The remaining lateral force at the foot of the mast is resisted by the
screws in the cabin top. The relatively small lateral forces are trying to
slide the foot of the mast forward or sideways, trying to shear off the
heads of those screws. Not gonna happen. 

On the other hand, if the mast is out of control when it comes down, it's
best to have a 'fuse' in the system - a weak link which is designed to fail
and allow the force of the falling mast to dissipate rather than bending the
mast or the push pit railing. Those screws provide that weak link, allowing
the base of the mast to break free of the cabin top. It's the only time the
screws will ever see an upward force.

End of boring dissertation. It's not nearly so boring if you have a chance
to watch it in action; the adrenalin rush is surpassed only by having your
life pass before you. 

Or so I'm told ;-)


Dennis 

www.great-loop.us
s/v Magic Moments
berthed in Gibraltar, Michigan


-----Original Message-----
From: rhodes22-list-bounces at rhodes22.org
[mailto:rhodes22-list-bounces at rhodes22.org] On Behalf Of Michael Hellyar
Sent: Thursday, June 20, 2013 11:36 PM
To: The Rhodes 22 Email List
Subject: Re: [Rhodes22-list] Sacrificial Toothpicks

I appreciate all the feedback and we will consider all the comments. I
explained our logic for through bolting and I hope someone will explain why
we are wrong.

Mike Hellyar
sent from my iPad