posted 04-21-2009 04:39 PM ET (US)        

Drawing the sheer line of a boat's hull has always been a crucial element of the design. Classic yacht designers were known for their artful eye and skillful drawing of the sheer line. However, Boston Whaler boats, particularly the earliest versions, tended to have a straight sheer line, something drawn with a straight edge, without a hint of curve.

To make a Boston Whaler boat with its Unibond hull construction technique, two molds have to be constructed. One mold is for the outer hull, and a second for the inner hull. When the two are combined, the uncured laminate of each hull section must mate to the corresponding face on the other section with an air-tight and pressure-resistant seal. The tight seal is required so that the expanding foam introduced between the hulls can fill the space between them in a controlled manner, only escaping via designated vents, sprue holes, or other secret inventions employed in the construction.

What better surface to design for two structures to meet and bond together than a straight, flat, regular surface? This straight edge may have also helped to simplify the arrangement of clamps needed to fasten the two mold pieces together. As the foam expands, a great deal of pressure is exerted against the molds.

With the development of advanced manufacturing techniques such as computer-aided design, computer controlled machines, and cutting of designs in foam by multi-axis tools, the designers of Boston Whaler boats may have been freed from the constraint of the straight sheer line. Molds and tooling could be made with a mating surface that followed a complex curved shape with as much ease as a straight shape. And the two molds could be made with a certainty they would fit together.

There is some correlation in the design element of the Boston Whaler hull sheer line and the arrival of modern computer-aided design and manufacturing. I believe there may be a linkage between them.

Here is a c.1986 design in which there are three large molded elements that are joined: the outer hull, the inner hull, and the superstructure cap. Notice that the surface at which they meet is perfectly straight.

http://continuouswave.com/whaler/cetacea/images/86Revenge512x305.jpeg

By 2002 the sheer line of new Boston Whaler designs had acquired a seldom seen curvature, as shown here:

http://continuouswave.com/whaler/cetacea/images/2002-255Conquest668x350. jpeg
http://continuouswave.com/whaler/cetacea/images/2002ConquestRear590x397. jpeg

posted 04-21-2009 05:23 PM ET (US)            

http://continuouswave.com/whaler/cetacea/images/loProfile19a-505x214. jpeg

Now that is a sweeping sheer line!

Here it is with the upper cap in place.

http://continuouswave.com/whaler/cetacea/images/21Rescue550x342.jpeg

The upper cap does not have to meet with such perfection as the inner and outer hulls, as there is generally an adhesive bonding material used to glue them together. In some cases mechanical fasteners are also added. The seal can be caulked to be water-tight, but it does not have to withstand the pressure of the expanding foam.

posted 04-21-2009 10:48 PM ET (US)            

However, if you look at car design in the 1950's and 1960's it is clear that complex shapes could be fashioned and turned into giant stamping press molds. But for a small boat builder, I have to think they might not have had the time and money to invest in extremely complex mold shapes for the joining line.

posted 05-05-2009 11:33 AM ET (US)            

Both of these boats represent the ealiest Whalers and no feature on the sheer line interface between the outer hull and inner hull is a straight line without a curve. This interface curves in both the water-line direction (the sheer line) and the butt-line direction as you move fore and aft along this interface. As you cross the stern on each boat this interface rises near the outboard ends and dips in the motor notch. All areas have a near perfect fit with any imperfections covered by the narrow white plastic "L" shaped rub strip.

I do agree that most of these lines are relatively straight when compared to traditional boats of the '50s and '60s but since the ealiest Whalers still used complex curved surfaces at the inner to outer hull interface I think it shoots a hole in the theory that perfectly straight lines where used or needed to facilitate the fit between the inner and outer hull tooling.

I have never witnessed the early Whaler tooling but I expect the perfect fit between the inner and outer hull tooling was accomplished by skilled craftsman either making a splash for each tool off of a plug that represented assembled hull or they made one tool first and then splashed the interface to start making the other tool.

posted 05-05-2009 03:10 PM ET (US)         

Although it’s obvious shape, form function and design are locked together and limited only by constriction methods and materials.

As BW is made of GRP formed in moulds simplistically we could say we are limed only by the complexity of the mould.

Although there are many considerations in mould construction within reason only the cost of manufacture would be the biggest restriction, especially for repeat processes.

The complexity of the joint at the inner and outer hull is probably no harder on straights or curves once the basic method and configuration is established.

But I would agree with you general observations from first principles straighter lines would seem a natural place to start.

It’s true that more modern tools available today CAD/CAM and moulds cut from foam on huge CNC type beds and special formulation of materials have changed what is possible. This is probably truer for steels and alloys than GRP.

GRP moulds may have become cheaper to make especially one off complex ones but not necessarily quantum leaps forward of what a very skilled worker could have accomplished given enough time, over todays methods.

I think the biggest factor in the change from straight lines to more curves is simple education.

I don’t mean we were thick and now smart; it’s more we have been taught over the last 40 years about curves.

We now all have new words in our vocabulary of design like ergonomics, natural forms, human factors, bio-metrics etc

We all have been progressively changed into what is beautiful and what is not along with this has come more curves.

Of course development in manufacturing and materials has played its part but the appreciation of curves has been leaned slowly by the main stream.

Strangely we learn very quickly sometimes as when a new design develops often considered ugly at first, once proven is soon added to our repertoire of beauty/appreciation , in hind sight.

Although I can’t agree with the maybe the new style of war ships that have minimum radar silhouettes that are probably the epitome of joining straight lines together. They have their place.

I simply feel BW had no though/demand for the complex curves most of us expect or appreciate today more so than they could not do it, we just did not expect/want them then and no doubt some still do not :)