Momentum is a purpose-built stell offshore cruising cutter built to the very highest standards.

On display at the recent Boating World Big Boat Show in Auckland were two of the finest examples of steel boatbuilding. Both were from the drawing board of Bob Salthouse and both were built by Johnson Yachts, a company that over the last decade has developed a reputation as one the finest builders of steel yachts and launches in the country.

The 17m (56ft) Rory Mohr will be well known to many readers, being named SeaSpray Boat of the Year in 1989. However, attracting just as much attention was the more modest but no less outstanding 11m (36ft) cruising cutter Momentum, which has only recently been launched and is Ray Johnson's own boat.
The name is not without a touch of irony: Momentum has taken Johnson no less than fourteen years to complete. A marriage break-up and having to fit between Johnson Yachts' various large assignments slowed progress - but never halted it. Momentum also took a staggering 14,000 man-hours to complete, such is the quality and detail.
"I could build two ordinary 36 footers in that time," muses Johnson, a perfectionist by nature who was prepared to take the extra time to ensure everything was just right.
The Bob Salthouse design first caught his eye in the pages of Seaspray when it was entered in a design competition aimed at finding "a cruiser/racer capable of fast ocean passages." The Salthouse entry was highly commended and Johnson figured the large volume and the reasonably heavy displacement would be well suited for building in steel.
Construction
The hull has 4mm plating down to the waterline, 5mm below that with 6mm used for the keel. 6000lbs of lead was poured inside the keel fabrication - roughly 30% of Momentum's nine-tonne displacement.
A big fan of steel boats of many years, mostly for the high strength and the security they offer, Johnson says with modern epoxies and paints the maintenance problems traditionally associated with steel are no longer an issue.
All plating was sandblasted and primed with Epiglass Pa10 before welding and then reblasted. The inside of the hull was then coated with an anti-corrosive zinc silicate, an epoxy undercoat and finally a marine coat to give a surface that is easy to keep clean and assists spotting any potential problems in the future.
The outside is coated with epoxy red oxide, faired with microballoons and sheathed in epoxy/dynel down to the waterline. A dynellarninate is unusual on a steel boat but was used with considerable success on Rory Mohr. Johnson says it allows a higher quality and more durable finish that can be "whacked with a hammer and still not chip the Epithane Marine Gloss."
In the bilges, the edges of all frames are sealed off with Sikaflex to keep air and moisture out.
The fairness of the hull is such that people viewing the boat for the first time are surprised to learn it is steel. "It's all in the technique", according to Johnson, who says that only five litres of bog was used to fair the topsides.
The deck is 12mm ply with a 10mm teak overlay and, along with the coamings and toerail, is fastened to a steel carlin and beams. The cabin super-structure is laminated kauri beams topped with kauri and plywood.