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Bertram31.com General Bulletin Board
Clarifications.........
Posted By: Capt Partick McCrary In Response To: Glassing bulk heads / Hard spots (Jim Kilbane)
Date: Monday, 12 December 2005, at 8:32 p.m.
All,
1. My statements regarded bulkheads, (ie: vertical partitions that seperate one compartment from another), connected at a perpendicular to the side of the hull.
2. ONLY the area of the hull sides above the chine & below the shear line, from the transom corner to the bow is done with spacing and bridge tabbing.
3. Battery boxs, rudder shelves, etc. can be hard glassed, but any hard spot that falls within the area described in #2 would be better if the item was not hard bonded.
Rudder shelves normally don't extend all the way to the hull sides. At least not on the B31. The rudder shelf begins & terminates inboard of & level to the top of the outboard stringers.
4.As JP points out, "Pillasters and window trims, non structrual type add ons, If they aren't wedged in its safe to say you can build to fit"
5. Deck beams, being a bridge from port to starboard are just like very short bulkheads. If they are hard bonded to the hull, as described in #2, they will damage that spot. Even more so because the same forces are being applied, but to a smaller surface area. Like a broad with spiked heels walking on your teak deck.
Deck beams should be designed to carry the load of a deck without the need of any connection at the extremities if they are properly and firmly affixed and bonded to the underside of the deck.
Should that deck beam also run through a hot engine room it will have considerable longitudinal expansion and contraction. Good way to poke holes in the hull sides.
A perpendicular deck beam, be it for a house floor, patio deck, or a real deck on a real boat, should have positive crown, either naturally or by by design. The height to width ration should be approximately 3:1 or greater. If the beam needs to be shorter in height than this ratio, it should be done with an I-beam design. An I-beam can carry the approximate same load at 2:1 as a plain beam at 3:1.
The same physics applies to fiberglass composite panels & decks. The top surface is the loaded surface and under compression while the bottom surface is under tension. As the core material seperating the to exterior surfaces is increased in thickness, the strength of the panel is increased geometrically. This is assuming that all elements are structurally bonded or integral.
When a load is applied to either an I-beam or a composite panel, the top surface is applying a diagonal force, (strech), to both of the two underlaying elements.
Br,
Patrick
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