[Rhodes22-list] Sacrificial Toothpicks

[Michael Hellyar]

It's been a while since Strength of Materials, force vectors and calculus. Thanks for bringing back the memories of the torture ; - ). I get it. Masts can come down and good design seeks to minimize the damage.

Mike Hellyar
sent from my iPad

On Jun 21, 2013, at 9:07 AM, "Lowe, Rob" <rlowe at vt.edu> wrote:

> Mike,
> Dennis's discussion is a good one and Mary Lou's follow up does relate the practical side of the issue.  I've lost my mast step twice (on previous boat).  First time was when the mast got dropped during stepping the mast (inexperienced owner) and the second was when I was raising the mast and didn't get it off the stern rail high enough before pulling it up with the halyard.  - rob
> 
> 
> -----Original Message-----
> From: rhodes22-list-bounces at rhodes22.org [mailto:rhodes22-list-bounces at rhodes22.org] On Behalf Of Dennis
> Sent: Friday, June 21, 2013 12:29 AM
> To: 'The Rhodes 22 Email List'
> Subject: Re: [Rhodes22-list] Sacrificial Toothpicks
> 
> 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
> 
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