You’re taking barges up river when you hear the first of several quick “thumps” along the bottom. That’s followed by a problem with the steering. The guy on the back deck sees a couple of logs pop to the surface, having just slammed the running gear.
At the wheel, you work to keep the boat under control, knowing full well it will have to be drydocked and the prop, rudder and probably the nozzle repaired or replaced. It will be time consuming and expensive. There’s also no guarantee that once back in the water, the same thing won’t happen again within the hour.
Prop damage from debris has been a problem for riverboats for a long time, but a new bow design from CT Marine in Edgecomb, Maine, shows promise for mitigating the problem.
It’s a bow design intended to keep logs to the side of a tug or towboat instead of going under it. Basically it changes the water flow “from buttocks flow to waterline flow,” said CT Marine’s Corning Townsend. “They [the logs] don’t go under the towboat, they go to either the port or the starboard and when they are outside, they are no longer a risk to the propellers.”
The design should also result in less damage to a boat’s bow area.
The first boat built with the CT Marine bow was for the Army Corps of Engineers some 10 years ago. The first commercial towboat application is a 4,400-hp, 103'×38' vessel being built for Tidewater Barge Lines, Vancouver, Wash., at Vigor Fab’s yard in Portland, Ore. Two sister vessels are also on order and all three will operate on the Columbia and Snake rivers.
CT Marine has a patent pending on the bow design, which underwent model testing. During testing, a model of a conventional triple-screw towboat and a towboat with the CT Marine bow had numerous logs coming at them. The logs had three different diameters, two different lengths and two different densities.
“We visually counted the number of strikes of various types of logs that would go into the propellers,” Townsend said. “It’s about a 40 percent reduction if we use our bow.”
On the river, you won’t notice much difference between a CT Marine towboat with the new bow design and a conventional towboat. “It’s 40 feet wide on deck and looks like a towboat,” Townsend said.
Near the waterline there starts to be a difference. “It goes from 40 feet wide on deck down to a couple of inches wide in the water. Under water it’s like a ship,” said Townsend.
The stern is what Townsend calls a “cow catcher.” It’s a 2"-thick plate that takes a log’s direct hit on the hull’s centerline, causing it to go off to the side.
He thinks most any riverboat could be designed with this bow. On existing boats that have had heavy bow damage, it might be worth retrofitting, though Townsend isn’t sure how cost effective it would be.
CT Marine is not the first to try this bow design. River packets in the early 1900s had a similar bow to keep logs out of the paddlewheel. “Yet they had a big wide deck so they could push coal barges or load cotton,” Townsend said. “There’s nothing new in naval architecture.”
ENCLOSED FOREDECK
If the critical lines of CT Marine’s bow are mostly near and below the waterline, it’s what’s above the waterline that counts with a new bow designed by Seattle’s Guido Perla & Associates for four platform supply vessels being built for Jackson Offshore Operators.
Instead of building the PSVs with an open foredeck and a lot of unprotected space between the stem and the superstructure, the bow rises up from the bulbous bow, flares out before reversing direction towards the wheelhouse and then wrapping around behind it.
That presents an enclosed foredeck protected from the weather and waves, while offering a lot more room for accommodations.
“Generally the new real estate and the enclosed foredeck was the primary motivation” for the GPA 675J PSV design, said GPA’s Stefan Wolczko. It’s the largest GPA-designed offshore service vessel for the U.S. Gulf market.
The Breeze, the first of the four 252'×60' DP-2 PSVs for New Orleans-based Jackson Offshore, was delivered in September. BAE Systems, Jacksonville, Fla., is building all four vessels.
Despite the seeming bulkiness of the bow’s design, Wolczko said there’s “zero effect” in terms of resistance to the boat’s forward motion. And the design doesn’t affect the lines of the rest of the hull. From the foc’sle deck down, a vessel with the GPA-designed bow or one with a conventional bow and superstructure will have the same hull.
“It makes no difference, particularly below the waterline,” he said, adding that any change in the vessel’s seakeeping or heavy-seaway resistance “is probably not significant.”
Wolczko said structure optimization or economic analysis haven’t been performed to determine the design’s “low end of efficiency.”
“But it would be an effective technique for designing a bow down to a pretty small size.”
One benefit of maximizing the superstructure’s volume is being able to put the diesel-electric engine room up higher. On the vessels for Jackson Offshore, the engine room is between the main deck and the fo’c’sle deck.
“Putting it up forward and above the tank farm you are really maximizing the amount of space you have below deck for having a full tank farm,” said Wolczko.
While the four PSVs have a highly unusual bow, they also have a relatively new propulsion system moving them through the water. Matched up with a diesel-electric system including two 2,815-hp Caterpillar 3516C generators and two 1,220-hp Caterpillar C32 generators are a pair of 2,613-hp Rolls-Royce Azipull thrusters.
The forward facing thrusters provide benefits people are looking for in terms of “directional thrust and maneuverability for a DP vessel that’s in close proximity to other floating structures or fixed platforms,” said Wolczko. Plus, there’s the “straight-line transit efficiency you get with a straight shaft.”
Change is hard for many vessel owners, witness the reluctance of operators in the Gulf of Mexico to switch from straight-line shaft propulsion to Z-drives, but if the GPA-designed bow and CT Marine’s towboat bow prove successful, making these changes may be a lot easier.
