Coamings Question

[commanderpete]

Michael,  for the board part of my Commander coamings I got somewhere around 10 feet 2 inches at the longest part and 13 1/8" max height

[Commander 274]

Thanks commanderpete!  You saved me a very long drive.
 
I aspire to make my Old Glory as beautiful as your Grace.

[ebb]

//www.wlfuller.com (type in search window: wood screw chart)
Three perfect charts on drilling for screws from #2 thru #18
using their taper point drill bits and countersink OR straight bits.
Have to get the chamfer/countersink just so if you want the screw head to be flush with the mahogany.  Or just below the surface so you don't sand the patina off when prepping for a fresh coat of varnish.
With an expensive probably one time use taper bit you would be buying the matching countersink so that you'd have the option to drill for plugs or just deep enough for the head.  The countersink bit is oversized for plugs - so you have to eyeball the chamfer  for exposed heads - difficult with a machine.

With a set of regular drill bits you can get the holes correct (with the coaming braced in place).
Jamestown 2009 holiday catalog (#54) on page 124 has an Austrian HAND COUNTERSINK -RLC-5706100- for $20.61.  With this you could finesse the countersinking of the #14 or #16 screw head to perfection.  And you'd have this tool for any size flat head screw you need to get flush with out a special countersink.

Somebody above here mentioned having problems getting the screws in.  Could be that unforgiving fiberglass is right under the coaming.  Might relieve the hole in the glass with a slightly larger bit than the shoulder of the screw. But continue into the wood backing with smaller strate bits or the appropriate taper.  
Appropriate taper means you have matched the screw length to the taper bit, which come in short, regular and long, see chart.

Like the idea of the larger deeper threads of #16.  More holding power.

No doubt you have varnished ALL sides of the coamings before putting them on.  Imco dipping the screws in varnish when driving them home is tradition.
Recommend butyl for the bedding, because you will  be taking the coamings off again...... sometime!
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Traditional tapered screws are becoming a thing of the past. This goes for slotted heads too.   McMasterCarr, who carry a range of every imaginable type of fastening, have 304 and 316 wood screws.
 I use non-tapered, sometimes sold as 'sheetmetal' screws (which also come flathead.)  They are usually skinny with prominent threads, so that in the small sizes (#6, 8, 9, & 10) I feel they do a better job grabbing than tapered.  And you predrill with strate bits.
Tapered screws have to be driven all the way home to work.  Straight shank hold throughout their length.