There’s usually no problem when developing specs to match a waterjet with a particular hull. That’s especially true for larger boats. 

“There’s so much engineering and design talent. Multiple people are reviewing it,” said HamiltonJet’s Steve Peake. 

It’s with smaller boats, under about 60', that issues arise between the anticipated benefits of waterjets and reality when the boat goes into the water. And generally the problem is not with the jets. 

“We tailor the jet on what we get back from the builder or the naval architect,” said Marine Jet Power’s Jordon Tilton. The questions have to do with such things as a boat’s dimensions, weight, horsepower and gearbox — if one will be used. The answers determine the waterjet and the impellor that goes in it. 

If changes are made at the boatyard, the waterjet supplier can probably adjust its original selection as long as they are notified of the changes. But when the boat goes in the water and it’s, say, 40,000 lbs. and the jet and impellor were sized for a 30,000-lb. boat, “We can’t guarantee what it’s going to do,” said Tilton.

Peake said he sees one or two boats a year where because “of a bad design or lack of control of the customer, the customer, after the fact, puts everything but the kitchen sink on the boat.”

Most of the time those are smaller boats. However, he remembers one 100' catamaran where the “builder had one concept and the owner had another concept.” By the time they were through, the owner made several changes and the boat had grown 50 percent relative to the original number. “The boat wasn’t light and fast anymore, and the owner complained,” Peake said.

Matching a waterjet with a hull is a function of thrust versus resistance. “If the hull resistance is ‘X’, we’ll give you enough thrust to do whatever it is you want it to do,” Peake said. “You change the criteria and you don’t have enough thrust anymore. The problem is you are overweight. It’s not the jet’s problem.”  — M. Crowley