- 1). Decide how many frame ribs are required to form the tugboat's hull shape. There is a minimum number of ribs that act as cross sections required to form any particular hull shape. Whether you use cold molding construction or carvel planking, the hull material lays across the frame ribs, interpolating the curves of the hull by naturally bending over the ribs. For example, a basic boat may only require one profile in the center to determine it's shape. With boats that are asymmetric and more complex, more ribs are required to determine those shapes.
- 2). Determine how many ribs are needed in addition to the number required to form the hull, if any. Larger boats -- like tugs -- will require more than the minimum shaping ribs. Look at other tug boats of the same or similar length, built with similar materials and construction method. For example, if you're building a 48-foot carvel planked tug using spruce over oak, look at tugs in the 50-foot vicinity using softwood planks over white oak. After considering several similar tugs, you'll have a good idea for the number of frames required, or a ratio of ribs per lineal feet.
- 3). Evaluate the the dimension -- both the width and the depths of the ribs. Again, look at other vessels as a guide. Naval construction is more dynamic that home, construction, for example, so a good-performing durable example to use as a model is a great engineering guide. Once you get a good idea for the width and depth of the timber used in successful designs, make some adjustments based on differences in your materials, design or building method.
- 4). Scale your frame rims down, if you like, if your frame ribs will use multiple laminations with ultra-strong modern adhesives. You can scale your ribs down slightly if your hull will be coated in fiberglass and polyester resin.
- 5). Scale your frame ribs up, if you're comparing timbers that have tight ring patterns -- usually from slow-growing northern trees -- relative to your materials. Scale up if your pilot house, decking or superstructure will be heavier.
- 6). Scale up if your engine produces more torque than those in the tugs you're comparing to. Pulling load is a consideration unique to tugboats. Each tug has to have enough strength to withstand the maximum load its engine or engines are capable of exerting. This process of starting with design model, then making appropriate adjustments in number, size, construction method and any other attribute will produce usable frame dimensions.
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