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The Benefits of a Straight Driveline

When Stars (Help You) Align

The experts weigh in on the benefits of a straight driveline and how to get there.

boat bottom

Arawak, a 1996 Grand Banks 42 Motoryacht, hopscotched her way up the east coast of the United States with Capt. Tommy McCoy at the helm and his youngest son Jacob riding shotgun. The long days of summer were beginning to wane, and McCoy had a schedule to keep. 

The voyage began in St. Thomas, where Arawak was rescued from a derelict condition and given new life thanks to a pair of new 220-horsepower Yanmar diesels and the the help of Mastry Engine Center. 

After making the voyage to Florida, Arawak stopped at Rybovich Marine Center in Palm Beach, Florida, where, among other things, American Bow Thruster installed a Vetus bow thruster and Glendinning added an integrated joystick control system.

Arawak then made her way five hours up the Florida coast to American Custom Yachts in Stuart, Florida, where some additional issues stemming from the repower came to light. “I had to run the boat 1,500 miles to get up there,” McCoy says. “We knew something was wrong all along, but this was going to take a higher degree of intellect to help us resolve these problems.”

Unnecessary engine noise and vibration can take a lot out of a crew, particularly on a long-legged bluewater cruiser like Arawak. And extended stretches in open water can mean prolonged exposure to these nerve-wracking issues. The job of reducing their impact began with addressing the alignment between engines and propellers.



The key to making sure your powertrain and shafts stay in alignment is getting familiar with the way your boat runs (and then staying familiar) so you can notice subtle changes in sound and feel. If you encounter an unfamiliar vibration while cruising at the rpm you have long used for comfortable cruising, you may need to have a closer look at the situation.

“I’m used to seeing where the motion of the so-called shaking would be higher or lower,” McCoy says. “We found our sweet spot and it served us well as we did 1,200 miles [from St. Thomas to Florida] and the boat did beautifully, but it’s not only a feel—you always want to eyeball the engine room for the kind of motion the engines are making.” McCoy would make a point of getting into the engine room and watching the new engines as Jacob would slip each one into gear. The sounds and the motion of each engine as it is put under load can tell you when something has changed—but of course it’s most helpful when you have a baseline from which to draw your conclusions.

“You’ve got to look and watch to see if anything has changed—negative or positive—over distances,” McCoy explains. “We took it up to 2000 to 2100 rpm, and we could tell that the boat was shaking a little more up on the flybridge—we were more apt to feel the shake up there.” Onboard Arawak, the distance of the flybridge from the engines seemingly created a more pronounced effect.

Not every vibration and shake means the prop, shaft, transmission, and engine have all somehow fallen out of alignment, however. “There were times when I thought we were having some sort of failure or damaged a prop,” McCoy says. “Came a time when I went to the aft cabin to lay down and I noticed that the joinery work was shaking a little more—there was a little bit more noise and action going on—and I said, What the heck is failing? Honestly… But I had noticed as we went across the Mona Passage that the Sargassum was thick—and then we were cruising the Gulf Stream and it was still thick out there. At some point we realized how much grass we’d gone through and decided to put it in neutral and back down and see what would happen. More gulfweed came off those props then you would ever believe.” 

Backing down occasionally became McCoy’s routine to prevent the weed from building up and affecting performance. “Get in the zone and you can tell something’s not right,” he said. Start observing and you will start noticing subtle changes—it can be very beneficial.”


After hauling out at American Custom Yacht (, McCoy set to looking over the running gear since he wanted to prep the boat as much as he could before ACY’s crew dug into the alignment project and the shafts were pulled the following day. “So I look and I’ve got a hairline crack on the hub of one of my props,” McCoy says. “After communicating with General Prop, they had a driver come by and take the props over to Bradenton for evaluation.”

The team from ACY simultaneously pulled the shafts and sent them to Lauderdale Prop in Ft. Lauderdale ( to be assessed. In the meantime the ACY crew wanted to take stock of things as they were. “What they ended up doing was cutting aluminum plugs in the machine shop, so you could plug a centerline in the rear strut,” McCoy explains. “Then, with the shaft out of the way, they could pull a monofilament line all the way through the next strut, the shaft log, the stuffing gland, and then all the way to the coupling that’s at the rear of the gearbox installed on the engine.” A line pulled taut is straight, and so anywhere it comes closer to the side of the shaft run indicates a potential trouble spot with the shaft alignment.

Over the course of the next two to three days, ACY’s team worked on squaring away the driveline. “You know when you start out with a bent shaft or a problem with the driveline, you can’t align anything properly,” says Dave LaCombe, yard manager at American Custom Yacht. “So we sent out the shafts—one of them was pretty bad when we got the report back from Lauderdale Prop, it was close to 30 thousandths out—and we refit and refaced the couplings so everything was running true.” It was the port shaft that was out of alignment, by the way—the same one that spun the prop with a cracked hub. 

LaCombe discovered another problem: The port rear strut was out of whack as well. “So we dropped the port rear strut,” he says, “and realigned everything.”

Another issue that arose in ACY’s assessment: Although the repower of the boat had resulted in a change to smaller-diameter props, Arawak still had her original 2-inch-diameter shafts. The smaller props had shorter barrels, or hubs, and didn’t fit the shafts’ tapers quite right. The result was that the forward end of each prop hub, when seated on a shaft’s taper, still sat a few

 inches behind the aft end of the strut and cutless bearing. Generally that distance should measure less than the shaft’s diameter, in this case 2 inches.

props positioned


Back in St. Thomas, McCoy had fashioned some rugged engine bearers out of quarter-inch aluminum stock to mount on the the boat’s stringers. To fix the alignment starting at the engines, ACY’s machine shop fashioned new bearers out of half-inch aluminum (“They used some material they had in the yard, which was a little overkill,” McCoy says. “But you know what? It’s a bulletproof installation now.”), and modified them to optimize the angle of the engines on their isolation mounts.

As previously mentioned, however, Dave LaCombe, the yard manager was also dealing with the problematic rear strut on the port side. The rear struts on a Grand Banks 42 have two legs in a V-configuration that rise from the shaft tube and terminate in large flanges that are through-bolted to the hull.

The team at ACY modified these flanges to make them easier to adjust. “They drilled into the flanges at the base of the strut, and then they tapped holes and they put little setscrews in there,” McCoy says. “The setscrews let them adjust up and down and here and there to make sure they had that strut in exactly the right spot. So when the mono line was pulled through to the gearbox they could make sure it was all straight and perfect.”

“We don’t like to go anything over five thousandths [of an inch],” LaCombe explains. “There are lots of variables in the way people set up drivelines, but I’ve got a rule of thumb where we don’t let anything go out of here that’s more than five thousandths. It has a lot to do with the gear ratio and the size of the propeller. To me having the driveline as perfect as you can possibly get it just alleviates all the problems and adds to the longevity of all components.”

For sure, every boat is different. But it makes sense to try to get as close as possible. “There are reasons for a flexible driveline, because some boats are made to flex and twist,” LaCombe says. “Those old Grand Banks boats—they’re pretty well-built boats so they don’t flex much.”

Once the strut was precisely aligned, the properly adjusted setscrews left a gap between the flanges and the hull. Rather than shim the flanges to keep them in their new, proper position, ACY injected epoxy, effectively molding the perfect shim in place.

“The epoxy creates a foundation that sets things just right,” LaCombe says. “We let it cure overnight, and then we use 5200 for the seal, and 5200 all the bolts back up.”

In the meantime, General Prop in Bradenton offered to replace the prop with the cracked hub, but said they would have to fabricate a replacement, a job requiring two months. McCoy’s contact at General Prop listened as McCoy outlined a tight delivery schedule that entailed delivering Arawak to the fall boat shows and came up with a different solution: They would machine the hub of a stock prop made for a 1½-inch shaft to fit on the 2-inch shaft, then adjust the pitch to make it work.

But, of course, there was still the matter of the distance from the prop hub to the strut. After the shafts had come back from Lauderdale Prop (treated with a dye wash that fortunately showed no cracks), LaCombe asked the team at the prop shop if they would recommend cutting the shafts. The response was a resounding no. The reasoning made sense: If the shafts were cut down, and another set of props with full length hubs were to be eventually fitted on Arawak, the hubs would rub up against the shaft tubes of each strut.

New Yanmars

To deal with the issue, ACY spooned out the keyways of each shaft—that’s a technical term for grinding the forward end of the keyway very carefully—to reduce stress on the Woodruff key/shaft interface and allow the hub and key to seat better. “The prop barrel was about 3 inches off of the rear strut which nobody wants to see,” LaCombe says. “That sort of thing puts too much stress on the shaft. But even though the shaft has a longer taper, the chance of cracking it was minimized by spooning the keyway. When different props are fitted this should alleviate problems. So the stress on the shaft between the leading edge of the barrel of the propeller and the rear end of the barrel of the strut is now a lot less—nothing out of the ordinary.”

With six new cutless bearings (one for each of two struts per shaft, plus one for each stuffing gland in the engine room), newly aligned drivelines, and a new portside prop, Arawak departed ACY to make her way north, “hooked up.” as they say. “I never took the boat over 1900 rpm running from St. Thomas up to Florida and we averaged 7½ knots,” McCoy says. “But now we’ve found 2200 to 2300 is a nice, smooth cruise and we get 9 knots all day long unless we get out in the Gulf Stream, where we were doing 13 knots.”

This article originally appeared in the November 2015 issue of Power & Motoryacht magazine.