Small Solutions... Big Results

VALUE-ENGINEERING AS A BOATBUILDING TOOL

Apr 29, 2023

Value-engineering is an approach to boatbuilding that seeks to deliver maximum utility value per unit cost. Not the cheapest boat per se, but rather the most advantageous ratio of benefit to cost. Value-engineering isn’t, however, simply about the design of a product but, just as often, about the planning and execution of its manufacture.

For example, several years ago, I contracted to plan and supervise the ship-fitting of a 45-metre high-performance littoral patrol vessel from the keel to the main weather deck. The work included the first-ever installation of two Hamilton HT1000 water-jet drives in a fiber-reinforced epoxy plastic composite hull.

The drive units, which were capable of absorbing more than 5,000 horsepower each, were massive ― about 13 tons each. Even the cast aluminum water intake grates weighed more than half a ton each. And both the drive units and their water intake grates had to be bedded and bolted to the vessel's composite hull with literally hundreds of 19mm and 25mm diameter stainless steel cap screws (hex-head bolts).

Since the drive train components would be highly stressed under load, installation needed to be flawless. In metal hulls, the fit-up of these units had previously been fairly straightforward, and final attachment had been completed by welding casing components to the adjacent metal hull structure.

However, mating metal drive-train components to a molded composite hull promised to be much trickier ― partly because elastomeric bedding/bonding compounds would have to be employed to fully seal the joints between the drive components and the hull … and such compounds have limited working lives once applied.

Another complicating factor in this case was that a molded vessel hull has an outer surface that is smooth and locally uniform where it has been controlled by the machined inner surface of a female mold (the “!tool”). In contrast, the inner surface 9f a composite hull is generally undulating and lovally prety rough and uneven due to the nature of building up a fiber-reinforced composite structure, layer by layer. This relatively uneven inner surface works against being able to properly sear the backup washers that distribute the loading in a through-bolt configuration. Moreover, the sheer number of bolts involved called for very accurate pre-boring of the bolt holes, if there was to be any chance of getting them all to line up fully at final assembly, whilst maintaining minimum clearances in the bolt holes themselves. There would be zero wiggle room for lining up at final assembly. Sounds like a job for a numerically-controlled robot borer, costing perhaps a couple million dollars or more ... right?

Well, maybe. However, being a long-time advocate of value-engineering, I naturally thought there had to be a better, more cost-effective way. So, instead of looking for a high tech solution, I opted for designing, building, and employing some purpose-specific low-tech tools.

I directed my crew to use the castings that needed to be bolted to the hull as self-templates for boring the hundreds of holes required.

To make that possible, I designed and had my crew build some fully-articulating electro-hydraulic fixtures that would enable us to lift, maneuver, and hold the heavy water-jet intake grates in place against the hull while we bored the holes.

And we built a temporary steel-tracked rolling gantry crane inside the aft machinery space of the hull to accept the roward ends of the drive units whilst an external crane inserted them through their respective transom openings.

Then came the most elegant and inexpensive solution I think I've ever developed for assuring relative boring precision in such circumstances.

I had some Delrin bushings machined to fit inside the holes that were factory-bored into the cast flanges being bolted to the hull. These bushings acted as guides to center and normalize the axis of a 6mm pilot drill bit.

My crew began on each flange by drilling, from the outside inward, four evenly-spaced pilot holes, using the flange as a self-template. We then moved to the inside of the hull and, using a spot-surfacing tool, specifically designed and built for the job, we ground a perfectly centered and very flat round landing area for the wide heavy duty "fender" style washers that would act as backing for the bolted-on assembly.

We purpose-built this tool using a standard hole saw and mandrel whose integral pilot bit had been replaced by a smooth 6mm rod. The hole saw itself was stuffed with marine grade plywood filler blocks, cut using the hole saw and built up until the stack stood slightly proud of the saw teeth. This packing stack was then covered with a 16-grit flexible resin-backed sanding disc material. (Total materials cost was less than $200 to build a half dozen of these surfacing tools.)

By inserting the pilot rod of the tool in the pilot hole from the inside outward, we were able to grind a perfectly centered and normalized landing flat into the inside of the composite hull skin — whose lam schdule had included an extra layer of material to allow us to proceed without compromising hull strength in the least — and enabled us, thereby, to achieve full contact between the oversize load-bearing backing washers and the interior surface of the composit hull..

Once the initial locating holes had been bored, we then proceeded to bore and prepare the rest of the bolting holes and insert, on a trial basis, all of the bolts. We then removed all of the bolts except for the four over-length "locator" bolts, which we back off enough to allow the mating surfaces of the hull skin and the casting's flange to be "buttered" with a high-strength elastomeric bedding/bonding compound. After which a half dozen pairs of hands tightened the locator bolts and reinserted and tightened the hundreds of additional bolts (threads bathed in the same bedding/bonding compound) into their respective pre-bored holes ― all completed well within the working time for the bedding/bonding compound.

Every bolt hole lined up perfectly with its mate in the matching casting and every backing washer sat fully flush with maximized contact area on the inside of the hull skin. Later, extensive QC inspection and testing failed to disclose a single leak, demonstrating that, with proper planning and preparation, big results can be achieved well within scheduling and budgetary constraints, provided only that you choose the right small solution.

― Phil Friedman

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