Boat Vibration: Chasing Down the Culprits

Engine Mounts
The four rusty mounts were still intact after 30 years, but they were the wrong size so we replaced them anyway.

Fiberglass boats built in the 1980’s are numerous and, with routine maintenance, fun to own. The routine maintenance caveat is a cautionary tale. Let me tell you about one problem that a competent do-it-yourself sailor, my friend Jeff, encountered — and the can of worms it opened.

Jeff bought his 1984 30-foot sailboat about 12 years ago. While he has spent many years focused on structural issues (see Fiberglass Core Repairs, Part One, and Rotten to the Core: Fiberglass Core Repair, Part Two), and optimizing his boat for racing, one thing he did not address was the vibration issues he had with his small Yanmar 2GM engine. Oh, he was going to fix that problem, but sailing was always a priority and he and I both thought it was a simple engine alignment problem. To that end, he borrowed my good box end wrenches many years ago with the idea that once the boat was floating he could perform a final alignment. Note: engine alignments done on land require a second final alignment after the vessel is afloat, due to the flex of the hull.

A couple of years ago, Jeff also started having problems getting his two-blade folding prop to open. He’d gun the engine forward and back, trying to get the centrifugal force to open the blades, but more often than not just one blade would open, the boat would shudder, and whatever alignment problems might have existed paled in comparison to this new vibration. Determined to find the problem he consulted with another friend who had had similar problems. He told Jeff that the problem was that the transmission was slipping and wasn’t engaging with enough force to spin the prop open.

tranny open
The inside of the transmission for a Yanmar 2GM engine has two conical disks (center) that needed cleaning and lapping to work properly.

This past winter, after checking the prop and cutlass bearing, Jeff bit the bullet and removed the transmission. It was not too difficult to do except for the confined space of his engine compartment. He soaked the bolts with some PB Blaster and, with a small pipe to act as a breaker bar, was able to remove the shaft coupling and exhaust hose to access the transmission. When the transmission was out of the boat and its case was open, sure enough, the two conical disks that engage the engine were shiny smooth. Cleaning the conical disks with emery cloth and then using lapping compound to mate the surfaces seemed to do the trick, giving the tranny the friction it needed to operate. (Previously, with the engine off but in gear, he could turn the shaft in either direction by hand). Even on the bench there was a huge difference, and so I was called to help put everything back together on his boat.

Since Jeff accepted that a number of small things beyond the transmission could contribute to the problem, and the hassle of working in the confined space meant he’d rather not revisit any of them,  he determined to also replace the engine mounts and address other opportunities for preventative maintenance while he had the engine compartment apart, such as installing a new dripless shaft seal (the old stuffing box was out of wadding and leaked profusely), swapping out the original 30-year old Racor fuel filter, installing a new exhaust hose, and fixing a broken through-hull ball-valve handle.

pulling tail shaft
Pulling the shaft out to clean it with wet-dry sandpaper so the dripless shaft seal would work properly.

The forsythia was just about to bloom here in the Northeast as I arrived at the boatyard to help install the new engine mounts. The job took us only a couple of hours. Using a scissor jack and a couple of small pieces of plywood to protect the boat’s sole and the engine from damage, we were able to lift the engine just enough to remove the old mounts. The rubber in the old mounts appeared to be sound and although rusty, the metal on the mounts was OK. One potential problem may have been that the engine mounts were all the same. Yanmar specifies that the front mounts on a 2GM be bigger than the rear mounts—so maybe having the wrong, smaller-than-specified mounts contributed to our vibration problems.

engine mounts
An engine mount should last

We got the new mounts in, refit the tranny, and did a rough alignment with the coupling (with the drive shaft centered in the shaft log). The next day, the dripless shaft seal was fitted and the coupling bolted back together. Things are looking good and with only three weeks before launch Jeff is focused on fixing the ball valve handle, installing new exhaust hose, and his new Racor before hopefully buttoning up the engine space for the next 30 years.

We’ll do the final alignment when the boat gets launched and see if we solved the myriad problems. I did get my box end wrench set back as the new mounts turned out to be metric instead of standard thread, and so the wrenches were no longer needed. I don’t know how much one issue might have caused another, but I am satisfied that fixing all the problems while we had the lid up will contribute to a better, more vibration-free and worry-free boating season.