Pacific MotorYachts Ltd

When T.C. Watson first began designing steel boats back in the 1950s they were shaped the same as a wooden boat would be; that is with a round bilge, flared bow sections and as in the case of the vessel Esperance (1962) shown below, a round or “cruiser” stern.

This was perfectly natural as the type has well proven sea-keeping qualities. However this style of hull form proved difficult to execute in steel and required an amount of man hours not greatly different to that of wooden boat construction.

A popular alternative and one that certainly reduced man hours was single chine construction, commonly referred to as “square bilge”.

The major problem with square bilge construction is the difficulty it imposes on the designer to achieve a seaworthy shaped hull. The single chine forces straight line frame shapes; the hull cross section shape and more often than not a straight line shear, neither of which have been shown to be beneficial to sea keeping ability.

We have found a far better alternative to be a double chine configuration which with a careful design approach can be used to effect all the advantages of round bilge construction yet also reduce the man hours required in construction. 

We now prefer to design for steel construction solely as a double chine hull form; particularly as our major cost is labour. The combination of the double chine hull form and building the vessel in sections rather than one piece results in a major saving in the total cost of the vessel.

The new W60, along with the rest of our fleet of passagemakers incorporate all these advantages without sacrificing any seaworthiness or propulsive efficiency; propulsive costs (diesel) being the major cost for the vessel owner.

There is still definitely shape in the vessel as can be seen in these photos of the W60.

Note how the shape of the bilge closely resembles a round bilge vessel. Also the transom corners which are rounded to a “yacht like” shape.

We are however still dealing with compound curves as in round bilge construction, but not quite to the same degree. Where the steel shell plate is curved in two directions it requires “stretching” mechanically.

For this job we use a wheeling machine which is able to “point load” the material causing it to distort to the shape we require.

The trick is in knowing were to stretch the material. This is the work of experienced tradesmen who have developed expertise from practice. A competent tradesman can determine from the loft floor where the plate will need shaping and perform this task without needing to offer the plate up to the job.

In the photo above an angle iron is having an edge stretched to form the camber required for the boat deck of the superstructure as shown below.

And here we see a bilge plate wheeled neatly to shape. This piece features lengthwise curvature along with crosswise curvature and is shaped to fit exactly!

The photo below shows the aft corner of the swim platform. This is built integral with the hull, a feature we look at in detail later.

The plate to cover this corner is very difficult to shape as it has curvature in three different directions.

And here is the closing plate fitted which makes a tidy arrangement at the confluence of the two chine lines, bottom and transom plate.

Here we have the aft starboard corner showing both the transom and swim platform closed in.

The view on the inside of the aft starboard tank shows the plating fits perfectly against the frames. The lines that are visible on the shell plating are the “wheel lines”.

Something we are proud of is that our vessels are 100% New Zealand designed and built. And they are built entirely by “time served” tradesmen.

The steel work that is shown here combines 60 years involvement in the design of steel vessels, a commitment to the use of modern technology in the preparation of materials, modern shipbuilding practices, along with the most up to date welding equipment and procedures in the hands of a skillful team of fabricators who between then have many tens of thousands of hours experience.