Engineering a High-speed Wooden Hull
October 26, 2007 Comments
Engineering a high-speed hull is not rocket science, nor is engineering a high-speed wood hull that difficult, but there are some basic principals that apply.
By far the largest part of the loads imposed on the structure will derive from the speed of the vessel through (and over) the water. Momentary impact (point) loads on the hull bottom are also dependent on sea state, deadrise angle, and longitudinal position along the hull. A 25 LWL boat traveling at 30 knots might see a bottom pressure load of 25 psi, the same boat at 60 knots might see 54 psi. That's 8000 lbs on a square foot of bottom.
To design the structure you must be able to predict the highest speed the vessel will achieve. To predict the speed you must know the weight and power of the boat.
There are two main elements in any hull structure, the skin which keeps the water out, and the supporting structure (stiffeners). The stiffener layout takes various forms, all transverse, all longitudinal, or a mix of transverse and longitudinal. The spacing of the stiffeners divides the skin into small sections called panels. The location, geometry, and area of these panels determines the load (which may be) imposed on them, and thus the makeup of the skin.
I've found that a stiffener system composed of transverse plywood bulkheads (which become interior structure) supporting deep longitudinal stringers works well to support an all diagonal cold-molded skin in a modern high-speed structure. The longitudinals are usually a mix of solid laminated timbers and plywood webs with top and bottom flanges of solid timber. Often we cut lightening holes in the bulkheads and in the longitudinals, both to reduce weight and to aid air flow through the hull.
Bottom planking is set at about 30 degrees off centerline and in four opposed and alternating layers. Topside planking is also in four (thinner as topside loads are smaller) layers at about 45 degrees to vertical. There are two or three times more stringers in the bottom than in the topsides.
On structural wood joints
Half-laps are not very good because you only have half the thickness of the member running continuous across the joint. Thus for equal strength you need a member twice as thick as the load requires. In a high-speed vessel weight is everything and stiffeners twice the size required should not be done. This is why plywood gussets are a good (abet ugly) solution, they double the material running across the joint just in that required area and add no extra weight where not needed.
Typical finger joints are not structurally adequate, the bond area is far too small. For an epoxy bond that equals the strength of the timber (plus a safety factor) a minimum scarf joint of 8:1 slope is required. So a scarf in a 1" by 2" will be a minimum of 8" long and have a bond area of at least 16 square inches. Finger joints can work, but they must be far deeper than usually seen in household joinery. The Gougeon's have done considerable research into this as they used finger joints in wind-turbine blades. There is a paper available from them on the subject, and it may also be covered in the newest edition of their book, The Gougeon Brothers on Boat Construction.
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