Replacing engine mounts

David Kennett and Megan Blakie share some of the lessons they learned replacing the engine mounts on their Yanmar diesel.

It was a race against time and tide when we replaced the engine mounts on our 15-year-old Yanmar 18hp motor. We had begun to notice a vibration at the wheel when helming, and to hear an unfamiliar noise at certain revs. Initially we thought it was the rattle of some clutter stowed in the back lazarette, but with a bit of Sherlock Holmes detectivework, we discovered the engine wasn’t aligned properly. One of the four mounts holding the engine was out of line and twisted. The movement of the engine was affecting our propeller shaft; with it out of line, there was potential to damage the propeller shaft seal and excessively wear the cutlass bearing, which is why we rose very early on a very chilly wintry day to careen our yacht — a 10.3m Van de Stadt. We wanted a spring tide with a low tide in the middle of the day so that the hull was out of the water during daylight hours.

Preparation

We made the mistake of not adequately soaking the bolts on the engine mounts with penetrating oil (eg CRC). If we had done this prior to the day of the job — maybe even a week ahead — it would have been easier and quicker to undo them.

Unfortunately, after 15 years of doing hard labour, our bolts had seized up. They took a long time to undo, which put us under time pressure. Tide waits for no man — or woman!

We selected some strong off-cuts of wood to use as chocks under the engine. (We placed them under the sump, as a safety back-up in the unlikely event the hoist failed.). We also borrowed sturdy planks to hold the chain hoist for lifting the engine and hired a suitable chain hoist and two Acrow props.

Steps

• We secured the boat on the tidal grid using ropes, two springs and two Acrow props on the stern (see careening tips below).

• Electrical connections and other attachments such as the exhaust hose and solenoid and alternator wires were disconnected from the engine.

• Once the tide went out sufficiently (about knee high), we knocked the prop shaft out of the gearbox.

• The chain hoist was secured to strong planks that rested across the open hatchway.

• We set the height of the new engine mounts to the height of the old ones, by using the adjustment nut. This helps to roughly align the engine.

• After undoing the old engine mounts, we hoisted up the engine and chocked it up (with wood off-cuts) as a safety back-up. Each mount was replaced in turn.

• After lowering the engine, we reconnected fittings and the prop. Aligning the engine is recommended when the boat is refloated (because boats tend to bend when on the hard or sitting on their keel).

Alignment of engine

Although the alignment of the newly mounted engine is best done once the boat is floating, we did it while the boat was still out of the water. We have a deep-sea propeller shaft seal, which makes it difficult to adjust on the water.

• Centre the propeller shaft in the stern gland. It’s helpful to use a piece of wood with a V cut in it to hold the shaft in place.

• Put the coupling flange on the end of the propeller shaft and check if it aligns with the flange on the gearbox. Adjust the engine mounts (using the adjustment nuts) so that the two flanges are aligned.

• Bolt together the two coupling flanges (with the prop shaft in) and use a feeler gauge to check the gap. Ensure the gap is even all the way round the two flanges. If it isn’t, adjust the engine mount heights until the gap is even. This can be time-consuming and fiddly.

• It may be prudent to get an engineer on board to do final adjustments; this shouldn’t be too expensive because you will have done most of the hard work already.

• Once aligned, unbolt the flanges and put the flexible coupling back in. We took the advice of an engineer and installed a new one.

How often and with what?

This is the first time Bee’s Knees has enjoyed new engine mounts since her launch 15 years ago; she has clocked up about 6,000 engine hours. Manufacturers often recommend replacing mounts every few years but we think this is overkill and unrealistic on the wallet.

We ended up buying an Australian-manufactured mount (Poly Flex) and a new flexible coupling. Yanmar mounts were comparable in price but arguably may not meet cat one and three requirements because they are not through-bolted.

Lessons learned

• Engines mounts get a hard time, not least because yacht engines spend most of their life on a lean, being tossed up and down as the boat bashes through waves. Although replacing mounts is fairly expensive, it’s worth doing to avoid consequential damage and wear.

• Careening the boat is a much cheaper option than hauling out but it does make the whole exercise a race against time. Also, on tidal grids you may be prohibited from cleaning the hull even though it’s exposed (environmental regulations).

• Before D-day, spray the bolts with penetrating oil.

Tips for careening

• Don’t take the boat too far up the tidal grid or ramp in case the boat bounces further up than intended and gets stuck (ie there’s insufficient water at the next high tide to take it off).

• If the boat is leaning slightly, make sure the lean is towards where the boat is tied up.

• Tie up the boat without much slack in the ropes, but adjust the tension as the boat settles (as the tide goes out). We put on two spring lines to stop movement forwards and backwards with the wave roll.

• If — like us — you have a fin keel, use Acrow props to stabilise the boat. We hired them fairly cheaply. Place them under the stern (one on each side).

• Place old carpet between the boat and the Acrow props, to protect the hull and paintwork.

• Tie the props to the boat with string/lashing to make them easier to retrieve at high tide.