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post #25 of Old 11-01-2009
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To respond to David Dennis, on a pound for pound basis, nothing (except the high tech composites like kevlar and Carbon) are stronger then wood. If you were building a purpose built, one off, small cruiser, colded molded wood construction with a protective barrier of epoxy/kevlar laminate would be a tough, low maintenance and inexpensive way to go.

On the other hand, with regards to Johno's point of three years ago, while wood itself is buoyant, by the time that you add a ballast keel (even without the weight of an engine) to a wooden sailboat, a wooden boat would not float without additional buoynacy chambers. A good design might include some buoyancy chambers, and they would buy you some time to self rescue.

But even with fiberglass construction, there are few production boats that are optimized for offshore use, and many of those rely on out dated construction modes that result in unecessarily heavy hull weights, compromising the ability to carry ballast, which in turn compromises stability, which in turn compromises the ability to carry sail, which in turn compromises the overall sailing ability of the boat other than for offshore distance cruising.

This topic of an appropropriate Fiberglass structure and construction details for offshore work came up in an earlier discussion with Stede and here is the response that I had written on that topic....

No matter what you do you just about can't put enough conventional fiberglass in the hull of a boat to prevent it from being pierced in a collision with a floating container or other heavy, solid, small contact area object. For that matter, if you are going to end up with a reasonable weight boat, you typically don't end up with enough steel either. For a serious 'go anywhere cruiser', the key to building a safe go anywhere structure consists of a variety of factors. Small panel areas, by that I mean that the boat should have a series of longitudinal frames and athwartship frames. Forward of the main bulkhead these should be quite closely spaced. [I will use my boat as an example (which was after all built for offshore work despite her light weight) the biggest unsupported hull sections below the waterline forward of the main bulkhead are about 4" by about 16" in area.] There should a ‘crush block’ at the stem at the waterline. [On my boat the crush block extends 6" above the waterline and extends back 16" to the first transverse frame.]

The area forward of the main bulkhead should be compartmentalized with watertight bulkheads that extend vertically above the waterline that would result if the boat had at least two of the compartments flooded. [On my boat these bulkheads appear to extend over a foot above the flooded waterline.] Ideally the tops of the longitudinal frames and the athwartship frames are on the same plane so that you can screw plywood into the tops of the frames to slow or stop the flooding. Ideally one of these bulkheads are on the centerline of the boat because should the boat ride up on something the sharp ridge at the centerline of the vee’d sections at the forward end of an offshore boat would really have to stand up to a lot of abuse. That whole bulkhead system should be heavily glassed into place. [That pretty much describes the construction of my boat.)

In the area of the keel there should be massive and closely athwartship ‘floor frames’ (this applies on fin keel or full keel, encapsulated or bolted on). [On my boat the ‘floor frames’ are over 8” deep and 4” wide and taper out to 4” deep above the waterline terminating at the waterline longitudinal except on the areas near the two main bulkheads where they extend to the rail.] On a boat with an encapsulated keel, the membrane across the top of the ballast needs to be as heavy as it would be on a boat with a bolt-on keel.

There should be no liners blocking access to the skin of the boat (at least forward of the main bulkhead and on the leading edges of the keel) up to the height of the flooded waterline mentioned above. All decks and flats in this area should be quickly removable so that access to make repairs can occur. [Here my boat gets a ‘B’ I can get to everything under the berths and forepeak quite quickly but the deck of the forward cabin is not removable. That is something I plan to change if I ever take the old girl offshore.]

Seacocks should not rely on backing blocks. Instead the hull should be built up to a thickness that locally reinforces the area under the seacock and distributes this localized stiffness out into the hull.

Once you have done all of that, coring or non-coring becomes less important. But coring above the waterline tends to produce a better offshore hull. For example, to quote from the Shannon website,” The most important feature of Shannon's fiberglass work is the use of composite core construction techniques. Composite core construction uses a layer of structural foam sandwiched between two thicknesses of laminated fiberglass. A composite hull can be both lighter and stronger than a conventional hull made with only solid fiberglass laminates. Cored hulls can remove unwanted weight above the waterline and have tremendous impact strength to absorb a blow from a piling, another boat or in a grounding. Solid laminate hulls are heavier in the topsides and when hit, tend to fracture and fail along the filament lines of the laminate. Shannon hulls use 1/2" to 1" semi-rigid PVC closed-cell linear foam material. Linear foams do not shear internally under impact, as has been found in the less expensive cross-linked PVC foams. Extensive testing has proven that foam core materials have better memory than balsa wood cores, enabling them to spring back into shape after a concussion. Unlike balsa wood, foam cores not allow water migration and rot if water penetrates into the core material from a skin fracture.”

In a composite boat, I believe that a couple layers of Kevlar, ideally in a vinylester or epoxy resin, and located in the outer plies, is critical to approaching the kind of abrasion resistance exemplified by steel but at a tiny fraction of the weight.

Then there is the main bulkhead. I don't care how a boat is constructed, at the mast and shroud area there needs to be either a massive ring frame or bulkhead to address the kind of loads that come from the rigging and keel loads. Without some kind of athwardships rigidity the boat will flex in a way that will ultimately weaken it through fatigue.

Similarly, there is the rudder area. Again, I don't care what kind of rudder you have, the area around the rudder post should have sturdy knees or bulkheads extending transversely and fore and aft. In my opinion, the rudder tube should extend well above the waterline and should have support at or near the deck level. In my opinion the rudder tube (and perhaps the shaft log), should be in their own watertight compartments or at least a compartment that is tight against the hull but extends above a partially flooded waterline. (This may require two shaft seals for the prop shaft or a flooded engine compartment neither of which is too easy to achieve.)

There should be substantial knees or bulkheads at shrouds attachment points and these should be tied into substantial longitudinal framing.

Hull to deck joints are another area of concern. I am a big believer in a belt and suspenders approach. I personally like a large inward facing hull flange with frequently spaced, comparatively small diameter bolts that is backed up by a slightly resilient adhesive caulk. The bolting should pass through an uncored section of the deck. At that point, glassing the interior of the joint becomes extraneous and makes future repairs much harder to perform.

Respectfully submitted,

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Curmudgeon at Large- and rhinestone in the rough, sailing my Farr 11.6 on the Chesapeake Bay

Last edited by Jeff_H; 11-01-2009 at 09:19 AM.
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