vyv_cox
Well-known member
I have now had an opportunity to carry out some more tests and work on my installation and now report back. New readers: It's a Yanmar 3GM30F installed winter 2000, shaft knocking on the stern tube from day 1 at certain conditions led to my fitting an Aquadrive last winter. First sea trals a couple of weeks ago very disappointing. There's a sketch that is something like my installation at http://www.leviathanmarine.com/halyard_aquadrive.htm Now read on!
Following the previous long thread, ParaHandy kindly put a question to the Yanmar engine forum. Reply received was:
"Reply from Gordon Torreson, Yanmar USA: http://forums.sailboatspecialists.com/eThreads/view/Yanmar/view_post?id=3199
There is virtually no way that the damper disc, or the reversing gear, would contribute to a vibration only in rough weather. The disc is used to absorb some shock loading that usually only occurs near idle RPM.
When you think about the dynamics of a boat in rough seas you see a lot of things constantly changing. As the bow drives into a wave a person at the back of the boat sences a great reduction in weight, often to the point of leaving the deck. As that wave passes under the boat, the opposite sensation is experienced, enough to bend your knees. The engine is experiencing these same accelerations. The engine is mounted on flexible mounts. As the first acceleration is experienced, the engine weight on the mounts approaches zero or even a negative number. In the second acceleration the weight felt by the mounts could go to more than double the static engine weight. The engine moves about with respect to the engine beds.
While this is going on the propeller is moving up and down through different densities of water, sometimes pushing the boat up a steep hill and then feeling the boat slide down a wave.
Depending on the boat, the exhaust outlet could also be in deeper water at different times, which would affect a lot of things, including strange engine noise.
The knocking that your friend hears is probably the prop shaft hitting the inside of the stern tube when the engine is changing its position in response to the various accelerations. The positional changes coupled with not starting the alignment proceedure with the shaft in the center of the tube could make the rap, and possibly the thrum.
I hope that I have given you something that leads to the solution to your problem. The noise may be something that you will never be rid of. If you can correlate it with some certain set of events, understanding what is happening may let you live with it, or find a way to fix it."
This was very much in accordance with my own thoughts. I had a damper disc on the original installation but took it off when the Aquadrive was fitted, so this was definitely eliminated. I saw the noise/vibration that I was experiencing as having two quite separate components: the knocking and the transmitted noise. My first check was on the flange fitted to the shaft end. This was always suspect as it appeared to be second-hand when the Yanmar was fitted. Despite my best efforts I could not get this to run true. I now think that it had previously been used on a 25 mm shaft (mine is 1 inch) and been distorted. I purchased a new Aquadrive one from Halyard, not straightforward as they cannot deliver to Holland, so it went via my daughter in UK. Cost an arm and a leg and part of another foot.
Next I disassembled the drive train. I found some marks on the Aquadrive bearing housing that suggested that it was contacting the thrust plate. There is clearance when static, so I assume the rubber bushes deform enough to let the housing move forward a few millimetres under load. I ground a bit more clearance into the thrust plate, not very easy and quite dirty in situ, then reassembled. The new drive shaft flange went on with no problems and appeared to rum completely true.
Sea trials show that both problems have been largely overcome. There is no knocking at any states of power, turning, crash stops, so the new flange seems to have eliminated that problem. At engine revs below 2000, 6 knots on flat water, the installation is very quiet. At higher revs there is progressively more transmitted noise and more again during hard turns. However, this is probably acceptable although noisier than it was without the Aquadrive (excluding the knock). We now wonder whether if we had realised the problem with the flange we could have got away without the Aquadrive. We shall never know!
My thrust plate is made of mild steel. It may be that a wooden or GRP one would transmit less noise to the hull but I didn't want to compromise on strength.
Thanks to everyone for their previous input. We intend to follow up suggestions that the Aquadrive works better with some deliberate misalignment and will report back.
Anyone interested, I have many photographs of the installation process. Post me your e-mail as a private message and I will send some.
One little tip as regards stern tube seals and checking. I have removed mine by pulling the hose off the stern tube countless times, with the boat in the water, to check alignment of the shaft to the tube. Water flows in at a fair rate, but not enough to worry about. An assistant with a bilge pump will control it, but my engine bilge holds up to 5 gallons and I have left the seal off for times up to a couple of minutes without pumping on several occasions. The flow can almost be stopped by wrapping some rag around the shaft/tube, if needed.
Following the previous long thread, ParaHandy kindly put a question to the Yanmar engine forum. Reply received was:
"Reply from Gordon Torreson, Yanmar USA: http://forums.sailboatspecialists.com/eThreads/view/Yanmar/view_post?id=3199
There is virtually no way that the damper disc, or the reversing gear, would contribute to a vibration only in rough weather. The disc is used to absorb some shock loading that usually only occurs near idle RPM.
When you think about the dynamics of a boat in rough seas you see a lot of things constantly changing. As the bow drives into a wave a person at the back of the boat sences a great reduction in weight, often to the point of leaving the deck. As that wave passes under the boat, the opposite sensation is experienced, enough to bend your knees. The engine is experiencing these same accelerations. The engine is mounted on flexible mounts. As the first acceleration is experienced, the engine weight on the mounts approaches zero or even a negative number. In the second acceleration the weight felt by the mounts could go to more than double the static engine weight. The engine moves about with respect to the engine beds.
While this is going on the propeller is moving up and down through different densities of water, sometimes pushing the boat up a steep hill and then feeling the boat slide down a wave.
Depending on the boat, the exhaust outlet could also be in deeper water at different times, which would affect a lot of things, including strange engine noise.
The knocking that your friend hears is probably the prop shaft hitting the inside of the stern tube when the engine is changing its position in response to the various accelerations. The positional changes coupled with not starting the alignment proceedure with the shaft in the center of the tube could make the rap, and possibly the thrum.
I hope that I have given you something that leads to the solution to your problem. The noise may be something that you will never be rid of. If you can correlate it with some certain set of events, understanding what is happening may let you live with it, or find a way to fix it."
This was very much in accordance with my own thoughts. I had a damper disc on the original installation but took it off when the Aquadrive was fitted, so this was definitely eliminated. I saw the noise/vibration that I was experiencing as having two quite separate components: the knocking and the transmitted noise. My first check was on the flange fitted to the shaft end. This was always suspect as it appeared to be second-hand when the Yanmar was fitted. Despite my best efforts I could not get this to run true. I now think that it had previously been used on a 25 mm shaft (mine is 1 inch) and been distorted. I purchased a new Aquadrive one from Halyard, not straightforward as they cannot deliver to Holland, so it went via my daughter in UK. Cost an arm and a leg and part of another foot.
Next I disassembled the drive train. I found some marks on the Aquadrive bearing housing that suggested that it was contacting the thrust plate. There is clearance when static, so I assume the rubber bushes deform enough to let the housing move forward a few millimetres under load. I ground a bit more clearance into the thrust plate, not very easy and quite dirty in situ, then reassembled. The new drive shaft flange went on with no problems and appeared to rum completely true.
Sea trials show that both problems have been largely overcome. There is no knocking at any states of power, turning, crash stops, so the new flange seems to have eliminated that problem. At engine revs below 2000, 6 knots on flat water, the installation is very quiet. At higher revs there is progressively more transmitted noise and more again during hard turns. However, this is probably acceptable although noisier than it was without the Aquadrive (excluding the knock). We now wonder whether if we had realised the problem with the flange we could have got away without the Aquadrive. We shall never know!
My thrust plate is made of mild steel. It may be that a wooden or GRP one would transmit less noise to the hull but I didn't want to compromise on strength.
Thanks to everyone for their previous input. We intend to follow up suggestions that the Aquadrive works better with some deliberate misalignment and will report back.
Anyone interested, I have many photographs of the installation process. Post me your e-mail as a private message and I will send some.
One little tip as regards stern tube seals and checking. I have removed mine by pulling the hose off the stern tube countless times, with the boat in the water, to check alignment of the shaft to the tube. Water flows in at a fair rate, but not enough to worry about. An assistant with a bilge pump will control it, but my engine bilge holds up to 5 gallons and I have left the seal off for times up to a couple of minutes without pumping on several occasions. The flow can almost be stopped by wrapping some rag around the shaft/tube, if needed.
