Mechanical update 10-10-2018

This post provides information on the transmission replacement along with some “fun facts” on marine transmissions.  (if you are not interested in marine mechanical components, I recommend you skip this post)

Marine Transmissions – a little background

In a boat like ours, a transmission serves a relatively simple function. It transfers energy from the flywheel on the back of the diesel engine to the propeller shaft. A marine transmission comes set with a specific gear ratio (ours is 2.03:1) and only has three modes of operation: forward, reverse and neutral. It does not change gear ratios like a car transmission. Some refer to the transmission as a marine gearbox which is a more accurate description. Marine transmissions typically last a very long time unless they have encountered some sort of trauma (propeller hits a log, ship runs aground, etc.).

The gear ratio is determined when the boat is designed to account for the hull shape, engine configuration, propeller size, etc.   Our 2.03 gear ratio turns the propeller one revolution for every two revolutions of the engine flywheel. Our engines normally operate between 800 to 2200 rpm, so the propellers rotate between 400 and 1100 rpm.  Turning our propellers any faster than 1100 rpm would be inefficient and possibly harmful due to cavitation created by the propellers. 

Marine transmissions come in different configurations to meet the space constraints of the boat and relevant locations of the propeller shaft and engine. The transmission is connected to a bell housing (adaptor plate) that is then connected to the back of the engine. The bell housing supports the transmission gearbox and transfers the propeller shaft thrust load to the engine and the connected engine mounts. The engine mounts are connected to the engine support beams that allows everything to get pushed along by the propellers. The bell housing and transmission therefore need to be compatible. (see photos associated with this post)

The information above explains why we decided to have new transmissions shipped from England. The original transmissions and engines were manufactured in England in the 1980’s. Fortunately, like-for-like replacements are still being made by a company in England. This allows us to change-out each transmission without having to undertake even more time-consuming modifications that would have been needed to install a transmission with different design characteristic. Another alternative was to remove the old transmissions, ship them to a marine gear shop and wait for someone to try to rebuild the damaged units (which we now know would have been highly unlikely given the condition of the port transmission). 

Replacement Timeline

Our new transmissions, damper plates (**) and oil coolers arrived at Green Turtle Bay September 28. The only local mechanic qualified to do the replacement was booked up until October 3rd. Our port transmission (the one that completely failed coming out of the Olmsted Lock) was removed on October 4th. It was obvious it had endured some serious trauma during its 30-year life. The new damper plate and transmission were installed October 5. The starboard transmission was removed October 8th.  While this unit was still working, we already knew it had experienced some sort of impact damage in the in the past when we found 2 of the 3 coupling bolts broken during the shaft seal repair in Port Charles MO. That knowledge caused us to go ahead and replace both transmissions.  The new starboard transmission installation was completed October 8. Final testing of both units was completed the morning of October 9. This allowed us to be underway by 1:45pm that afternoon. 

** Damper plates - I had never heard about or seen a damper plate before. It is a very interesting piece of equipment. (see photo associated with this post) It has a large flat circular plate that bolts to the engine flywheel. It contains a spline hub for the transmission shaft to fit into. There are bolts around the hub connecting it to the damper plate. The interesting part is the shock absorbing material interwoven around the bolts to absorb some of the shock when the transmission is engaged. Even at idle speed, the flywheel is spinning around 800 rpm. When the transmission is engaged, the propeller shaft and propeller start spinning at 400 rpm. 

Replacement Process

Fortunately, our boat was designed with enough clearance to disconnect the propeller shaft from the transmission and push it back out of the way while the boat remained in the water. Otherwise, we would have needed to move off the boat, have it lifted out of the water, and the repairs would have been undertaken while on land. 

To perform the transmission replacement, the mechanic places temporary shims under the engine and existing transmission to hold them in place. He uses a hydraulic ram to push the propeller shaft back as far as possible. He disconnects the bell housing, transmission and rear engine mounts by removing numerous connecting bolts, ancillary hoses, oil coolers, exhaust hoses and other items that are in the way. 

The final removal of the bell housing/transmission unit is accomplished with a section of chain connected to the lifting lug on the transmission, a stout metal bar and three strong men from the marina lift to the 230 pounds of bell housing/transmission through the engine room hatch, move it across the salon and out the side salon door onto the dock. It is placed on a hand track, rolled to the mechanic’s work boat, and lifted onto his boat.   (I was the forth member of the lifting team during this process, but they seemed determined to carry most of the load.) Once the old transmission is out, the mechanic removes the existing damper plate from the engine flywheel and installs the new one. Back at his shop, the mechanic mounts the engine bell housing on the new transmission, then performs the previously described moving process in reverse to get the new transmission in place. He then makes sure the engine, transmission and propeller shaft are properly aligned, adds oil to the transmission and tests the system. 

Everything appears to be working. We traveled 45 miles yesterday from Green Turtle Bay to Paris Landing Marina TN without any problems.