Patrick Bray The hull structure of this shapely motor yacht is based on widely spaced heavy transverse frames intersected by numerous light longitudinal stringers defining the design’s waterline curves.
In Part 1 (Professional BoatBuilder No. 210) I explored design of the hull form, the shell plating, and choosing efficient, cost-effective build methods. While it may seem counter-intuitive to obsess about cost effectiveness in the construction of expensive toys for clients who don’t view cost as a limiting factor, price is always a vital consideration for the purchaser and more importantly for the builder. Most yacht builders barely clear a 10% profit margin on a new boat construction project. Lamentably, we’ve all known of builds that go significantly over budget and have even seen clients take over a yard’s finances just to get their boat finished.
Making a boat more efficient to build by embracing effective systems from the start of planning saves on material and labor and goes a long way towards successfully managing the construction and trimming the bottom line. For some builders such systems are common practice, to others they are a mystery. I recently lost a large design contract because neither project manager nor builder could fathom the process of designing for a cost-effective build using the framing principles discussed below. Conversely, on another project a shipyard embraced these techniques and saved months of construction time and created a fairer, stronger hull.
Framing Strategically
Having looked at developable hull forms in Part 1, it’s time to take a detailed look at the framing system. Large commercial bulk carriers are typically framed with the closely spaced transverse frames serving as the major structural members. This works well for them as over 50% of the hull is the same shape through the parallel middle body where each frame is identical.
It’s not the same for the average yacht or smaller commercial vessel characterized by a sleek hull form and compound curves. For those boats, a longitudinally framed system is stronger per pound, quicker to build, and yields a naturally fairer hull form. This framing system features widely spaced transverse frames, often at 36″ (.91m) apart, combined with closely spaced longitudinals, often at 18″ (457mm) apart. Each frame is different in shape, so they are more labor intensive to produce than stringers and comprise numerous parts to accommodate the changing complex overall shape required for each one.
The wider frame spacing of the longitudinal system requires higher strength frames, which can be satisfied by increasing frame depth slightly for a significant improvement in the section modulus. Adding a flange or flat face to the frame creates an I-beam effect that yields far greater strength for the same frame depth.
By limiting the number of frames, you reduce the labor time to build them all and shorten the set-up time to align them at their proper spacing. The greater spacing is also much more forgiving of any minor discrepancies in the frame size and shape that would otherwise compromise hull fairness.
Once the frames are stood up in the correct location, it takes very little time to lay all the closely spaced longitudinal stringers into place in the notches cut for them, using long lengths of stock material. Immediately, the installation of the stringers helps to align the frames. When properly fitted, these longitudinals will adopt a natural bowed shape, which assists in creating a more even curve and a fairer hull. The longitudinals should be made of T-, or at the very least, L-shaped stock. Flat-bar stringers do not bend well and even if precut to shape tend to trip and buckle under load before reaching their calculated ultimate strength.
Patrick Bray Flanged frames can be efficiently cut and welded to define hard-chined hull sections with developable surfaces.
Patrick Bray With judicious cutting, flanged frames are effective in conical and even compound curved hull sections.
Flanged Frame vs Plate Frames
When building a chined hull form, you can easily assemble frames from cut angle bar, or they may be computer cut in flat stock and mechanically flanged on one edge before installation. Both methods for flanged frames eliminate a huge amount of welding required to add a face plate to every plate frame, thereby reducing labor cost and weld distortion of the many parts.
Flanged Frames
Patrick Bray With transverse flanged frames, only the joints need to be welded. Consequently, each half section of this hull framing requires about 1m (3.28’) of total weld length.
Plate Frames
Patrick Bray Transverse plate frames with welded flat face (or rider bar) require approximately eight times the weld length as comparable construction with flanged frames.
Patrick Bray Welding face plate onto cut plate frames is a labor-intensive task.
Choosing a cut plate frame with welded face plate presents several disadvantages, the foremost being that it requires more than eight times the total weld length per frame. That translates to eight times the amount of welding labor per frame, and in aluminum, leads to significant distortion of the metal and significant strength reduction compared to the flanged-frame system. That all adds up to a massive difference in labor cost and build time.
Plate frames installed without a welded face plate, when first stood up, are very floppy and hard to keep aligned without some temporary support. Plus, they require considerably greater depth to achieve the same modulus as a flanged frame and are very likely to trip and buckle under stress thereby never providing their calculated ultimate strength.
The increased frame depth of flat frames impinges on the enclosed hull space making the interior fitting more challenging. In contrast, widely spaced flanged frames can be hidden or disguised in lockers and cabinets, allowing the usable interior volume to extend right out to the longitudinals in many areas.
In addition to the structural and space benefits of flanged frames, they tend to support a naturally fair hull with far less distortion and need for grinding of welds and application of fairing compound. The tedious job of fairing and sanding a hull can take several additional months and is ongoing for most of the build time. One of our builders claimed a one-month reduction in build time for the hull alone thanks to the specification of flanged frames.
Conclusions
Common sense should prevail in the engineering and layout of any yacht to optimize turnaround time for the client, less time tying up the shop, and a minimum of labor hours, while producing a quality product. Every design and engineering decision must be weighed against the advantages and disadvantages to meet the stated goals of the vessel. It is all about making the right compromises during the design phase to save money and guard against disappointment in the finished build. Hours spent on the shop floor or in a refit yard working through problems and issues that were not addressed in the design stage are an avoidable inefficiency and a waste of time and material.
A vessel’s framing is what all the other elements of any boat project hang upon. As such, it demands that designer, builder, and client make careful calculations and deliberate decisions during the earliest stages of planning. In my experience, for most custom-built metal yachts, flanged hull frames offer tangible benefits in vessel construction and operation.
About the Author: Patrick Bray is a licensed engineer and naval architect with more than 50 years of experience in design and new construction, including three years as on-site naval architect for Crescent Yachts (Surrey, British Columbia) doing shop drawings and ensuring compliance with the classification societies. He filled a similar role at Trites Marine Services, another B.C.-based aluminum commercial vessel builder. Doing business as Bray Yacht Design and Research Ltd, he has worked directly with many other yards as designer and project manager.
