[Rhodes22-list] Rebedding Mast Raising Gin Pole Fitting On Cabin Roof

[Jay Friedland]

Nuff said. Through bolting now makes more sense. Roger, do you remember 
bolt length as the barrel nut has the limited depth? Is compression 
between the liner and cabin top a problem- air space, etc.? Launching 
Sat. so this should get the job done.
Thanks,
Jay


On May 12, 2004, at 8:35 AM, Roger Pihlaja wrote:

> Jay,
>
> A couple of years ago, I retrofitted the GBI single handed mast raising
> setup.  The photo that was enclosed in the earlier post on this subject
> looks like my gin pole setup.  So, I presume the mounting fitting on 
> the
> cabin roof is the same.  I thru-bolted the two bolts on the cabin 
> roof.  Use
> 1/4 - 20UNC SS flat head screws, 5/16 inch ID X 1-1/2 inch OD SS fender
> washers, and 1/4 - 20UNC SS barrel nuts.  Look on pp. 591 - 592 of the 
> West
> Marine Master Catalog, 2004 for these pieces/parts.  The stainless 
> steel
> fender washers & barrel nuts will give you a nice polished fastener 
> that
> will be very strong, look good, will be accessible for maintenence, 
> and will
> not be a head banger on the roof of the cabin in the head area.
>
> When stepping the mast, the mounting fitting under the gin pole is 
> under a
> combination of compression loading and shear loading.  The relative
> proportions of shear load & compression load change as the angle of 
> the mast
> changes.  Compression loading is the dominant force at the beginning 
> of the
> lift when the mast angle is low & the gin pole is nearly vertical, with
> shear loading steadily increasing as the mast gets more vertical & the 
> gin
> pole lays down onto the foredeck.   The structure of the cabin roof 
> resists
> the compression load.  However, the only structural elements resisting 
> the
> shear load are the two 1/4 - 20UNC bolts.  MJM's hypothesis re the 
> screw
> threads being stripped on the bolt that simply turns is probably 
> correct.
> As such, the ability of that bolt to resist shear loading must be 
> considered
> suspect & putting all the shear load on the single remaining bolt is a
> dangerous proposition.  The consequences of the bolts failing under 
> shear
> load would be that the base of the gin pole would come free under 
> load.  The
> gin pole and the mast would go shooting every which way & the forces
> involved are enormous!  Since there would very likely be someone 
> standing
> beside the gin pole cranking the winch handle, that person would almost
> certainly be injured.
>
> Please do not attempt to use your gin pole until you get this problem 
> fixed!
>
> Roger Pihlaja
> S/V Dynamic Equilibrium
>