Updated March 11
The largest Steamship Authority ferry, the 255-foot double-ender MV Island Home, appears to be a hole digger, according to Woods Hole project manager Bill Cloutier. Cloutier told the SSA board during its February meeting that a project to armor one or more slips with riprap and scour pads is under consideration, to prevent repeated undermining by the Island Home. Cloutier said some of the slips are being deeply scoured out.
“We believe it’s the Island Home and its props. Some engineers are actually studying that boat now … wherever it goes, it seems to make big holes with its props,” he said. The subject arose after Barnstable board member Robert Jones said he wanted to make sure any potential problems with the waterside portion of the Woods Hole project are addressed while construction is afoot, and not discovered years later.
Cloutier said if need be, part of a given slip will be studded with riprap for “about 12 feet” and another portion, “another 65 feet,” would be covered with a mat made up of concrete pads.
Engineer Chris Colletti offered a more detailed description. “The mat itself is an articulated concrete mat. Basically like concrete blocks end to end, with cable or rod going through it so it can be flexible, and that is just laid along the bottom,” he said. “The size, the thickness, are dependent on the velocities you get from the prop. And, as Bill said, we’re looking at the boat now to see what kind of velocity we have to design for.”
Driscoll later told The Times no decision has been made on slip armoring. “We have applied for permission to install riprap and/or scour pads at the Woods Hole terminal with the appropriate permitting agencies, in the event they need to be installed,” he wrote, “but that decision has not been made.”
Driscoll pointed out one of the main reasons the Island Home is taking blame for burrowing, as opposed to other ferries, isn’t because of its draft — how deep its hull goes below the waterline — but the fact it has props at either end. Ostensibly this means twice the capacity to churn up the seabed. He also said concern is primarily focused on the middle slip, formerly Slip 1, soon to be Slip 2.
“The Island Home typically uses the middle slip,” he wrote about the Woods Hole terminal, “while the Governor (which has a deeper draft) typically uses the southern slip. The Island Home also has propellers on both sides, whereas the MV Martha’s Vineyard, MV Nantucket come straight into the slips, so their propellers would not be on the bulkhead side of the slip.”
The Governor is also a double-ender. Driscoll told The Times Tuesday morning the vessel doesn’t have props at either end, but he walked that back Tuesday night, and wrote in a text message that it indeed did have props at either end. Nevertheless, it hasn’t been mentioned as a culprit, despite its deeper draft. Driscoll noted the design draft of the MV Governor is 11 feet, three and thirteen-sixteenths inches, whereas the design draft of the MV Island Home is 10 feet, 6 inches.
Concerning Vineyard Haven Harbor and area around the SSA terminal there, Tisbury harbormaster John Crocker said, “There are deep holes out there from prop wash, in the neighborhood of 35 feet.”
Crocker said the SSA dredged the north side of the terminal a few years back, and shoaling-wise, he doesn’t believe the bottom topography is overly problematic. Nevertheless, the holes are there, he acknowledged, and said scour pads in Vineyard Haven “couldn’t hurt.”
Asked if pads will be installed at the Vineyard Haven terminal, Driscoll emailed “no.”
The reason for this, he later said in a phone conversation, is because there is concern sheet piles that form the bulkhead around the new Slip 2 in Woods Hole may shift because of scouring action from Island Home. Driscoll said sheet piles have already moved because of vibrations from pile-driving work.
“There’s no similar phenomenon happening in Vineyard Haven,” he said. “If there’s a hole in the harbor, that’s different to a hole in the harbor next to a bulkhead.”
Updated to include a correction issued by the Steamship Authority. –Ed.