Boom: Sam L Morse Co hulls 100 to 116 (and perhaps more): Documentation

Forespar Products Corp, which manufactured spars for many of the more recent Sam L Morse Co BCCs, was kind enough to scan their file copy of the build specs for the boom of BCC 116 Zygote.

  1. Revisions
    Forespar kept a master build document on spars, making a photocopy of the master document for each build. The build specification document therefore has a list of revisions. The build spec for Zygote’s build references a major revision at hull #100 (? 8 Mar 1994 ?).

Revisions noted include:

19 Jul 1995 change to 1.15" track (for the clew car) and the appropriate car;

18 Mar 1999 lengthen boom by 3" to 15’ 3.5" and change length of outhaul accordingly;

15 Sep 2000 add a mid-boom bail at 7’ along the extrusion.

  1. Decoding some of the specs
  • 327016 is the serial number of Zygote’s boom - that number is stamped on one of the lugs of the boom inboard end fitting.

  • The extrusion is 152 tube (depth 6 inches, width 4 inches) 184 inches long (15’4”) with a linear mass of 2.80 lbs per foot (wall thickness is 0.13 inches) and Area Moment of Inertia of 11.5 inch^4 (vertical deflection) and 5.6 inch^4 (horizontal deflection). (where inches^4 means inches raised to the power of 4)

  1. Outhaul
    Note the diagram of the mainsail clew outhaul just below the schematic of the boom.

My own notes on maintaining the boom include the note: “Do not unscrew simultaneously both of the two stainless steel bolts forward of the outhaul control line exit slot; they hold the internal eye strap (and the standing end of the outhaul control line) in place.”

See attached BCC 116 Forespar boom.pdf, about 50 KB.

BCC 116 Forespar boom.pdf (49.5 KB)

Here are the notes I wrote in Zygote’s operating manual about the outhaul:

Outhaul: The mainsail outhaul gear has a 2:1 mechanical advantage and is largely inside the boom:
• 5/16” white with blue fleck polyester double braid has a spliced eye captured by a stainless steel eye strap (installed on the inside lower port wall with two bolts) as its standing end. From the eye strap the double braid runs aft and is reeved through a Ronstan RF41101 single block that turns it forward to exit the boom through a slot on the port side, with a 5” Forespar Marelon MF763/5 cleat forward of the slot to belay the tail of the outhaul;
• an eye-splice in 3/32” wire is made by a shackle to the aft end of that internal Ronstan RF41101 block, from where the wire runs aft through the boom, exits via a Ronstan RF457 exit box, and turns forward to attach by shackle to the clew of the mainsail. (The wire was originally attached by eye-splice to the Ronstan 689 car to which the mainsail clew is made. That arrangement was better for moving the car but the wire cut into the end of the track on which the car runs.)

Full adjustment of the outhaul tail only amounts to a fist width. Slight easing is about half of that.

Significance: The outhaul is a marginal primary sail control for depth in the lower one-third of the mainsail and the shape of the lower portion of the leech. The foot of the cruising mainsail is attached by eight slides, secured to the sail by webbing through grommets evenly spaced along the foot, in a slot along the top face of the boom.

Easing the outhaul closes the lower leech and adds draft to the bunt of the sail increasing lift and draft (and increasing windward helm slightly). Trimming the outhaul opens the lower leech and flattens draft in the central bunt by pulling sailcloth into a tension curl parallel to the foot of the sail.

When running or close hauled, the effect of the outhaul is marginal. The effect of the outhaul on draft control is slightly more than marginal when reaching.

Evolution: With the low 2:1 mechanical advantage, easing the outhaul is easier than trimming the outhaul. Orthodox advice is:
• first to hoist the mainsail with a slack outhaul and to trim the halyard (as discussed above); and
• second, while the sail is not powered, to tension the outhaul to create the first sign of a tension curl parallel to the foot of the mainsail;
• trim the outhaul:
o in light air, ease the outhaul but not so much as to close fully the lower luff;
o in wind up to 16 knots (until first reef) trim the outhaul to open the lower luff;
o when reaching in lumpy sea (waves dominating wind) ease the outhaul slightly, so the lower luff starts to close, for extra power.
• Ease the outhaul after dropping the mainsail to stow.

Maintenance: Flush salt and dust from the mainsail foot slot with freshwater after each sail. Remove the mainsail foot slides from the slot periodically, then:
• run a tuft of cotton along the slot to clean it;
• spray each slide with a silicone lubricant (McLube Sailkote); and
• let the lubricant dry before reinserting the slides in the slot.

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Mea culpa!

19 Jul 1995 change to 1.15" track (for the clew car) and the appropriate car;

Should of course read “change to 1.25” track".