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Old 06-04-2014, 05:54 PM   #21
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Super-in a traditional setup- engine connects to tranny, tranny to prop haft. the drive energy from the prop is transmitted to the tranny and thus to the engine. In essence, the propulsive force of the prop is transmitted tot he hull through the engine mounts. The alignment has to be almost perfect and any movement by any piece of the drive train will cause vibration or damage to the drive train. With AQ, a thrust bearing is mounted on a plate attached to structural hull members (stringers). The thrust bearing absorbs the propulsive force of prop. The prop shaft is aligned with the thrust bearing plate. Since the bearing plate is hard mounted, that alignment only needs to be done once and there is no movement to upset the alignment. The engine/tranny is attached to the thrust bearing via a CV joint. This allows some freedom in engine mounting as it need not be in a direct line to the prop shaft. It also allows the use of very soft engine mounts as engine movement is no longer an issue with the CV joint. This eliminates or greatly reduces any engine vibration. The engine/tranny are isolated from the prop shaft. All propulsive force is absorbed by and transmitted to the hull be the thrust bearing and plate.
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Old 06-04-2014, 10:47 PM   #22
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Well said Salty Dog and THD.

Every boat should have one but the effects of not having one cannot be felt on many installations.

One can make a strong case for saying they are or should be required for any engine installation that has flexible mounts. Less so when the mounts are stiff or very stiff. It's insane or at least stupid to have quite to very flexible engine mounts and no Aqua Drive Or equivalent. When an engine moves in the yaw or pitch axis extreme forces must be present at the thrust bearing in the transmission, coupling and fwd end of the prop shaft. If the TX bearing, coupling and shaft are strong enough then the shaft would prevent the engine from moving in the yaw and pitch axis. Doubt if that could ever happen.

If the propeller shaft is in line w the center of rotation of the engine when subject to torsional movement the movement would be only in accelerating and decelerating the rotational movement of the shaft. Propeller blade tips would accelerate and decelerate slightly but the shaft would not move up or down. But almost always the shaft is well below the rotational center so torsional vibration will move the shaft back and forth from side to side. Nothing but an Aqua Drive can absorb those movements gracefully.

Any way you stack this up the standard straight shaft drive w flexible engine mounts is an undesirable way to transmit power to the prop in a small boat. My boat falls into this category but it's plenty smooth enough to keep using the straight drive.
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Old 06-04-2014, 11:53 PM   #23
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If the shaft gland has some room to accomodate some motion of the shaft, then the normal vibes of the engine do not cause alignment problems. Even soft mounts designed for propusion engines do not displace very much under load. Most shaft lines can handle that displacement without issue.

Also, some vibration energy from the prop comes up the shaft, but is absorbed by the mass of the engine on the soft mounts. Solid mount a thrust bearing to the hull with aquadrive, and vibes coming up the shaft get right into the hull.

If the shaft has a cutless bearing where it comes through the hull, or a solid mounted packing gland, then the motion of the engine on soft mounts can really cause trouble. Aquadrive et al are very handy there. But those installs are pretty rare in trawler type boats.

These types of thrust bearings and drives have their uses, and do well there. But they are not the be-all, end-all type of fix for vibration and noise issues.
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Old 06-05-2014, 07:07 AM   #24
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Folks that install a set with more flexible mounts MUST look at all the other items , that attach to the engine , like the water inlet and exhaust and perhaps re doo them to handle the new engine motion.

Its the complete power system that is being revamped.
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Old 06-05-2014, 03:00 PM   #25
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FF is correct.
When motor mounts are flexible enough, everything else attached to the motor must follow that flexibility or, pay the price!
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Old 06-05-2014, 04:01 PM   #26
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FF is correct as usual,
I agree. And I personally have a stone to uncover on Willy in that regard. Willy has a tall exhaust riser connected to a FG lift muffler via a very stiff exhaust hose. I haven't brought it to the fore yet as I have little vibration and so many other priorities.

Anyway the engine vibration will be greatest on the attached parts furthest away from the center of rotational/torsional vibration. The attach point of the hose is such a point but can hardly move. I think I need to attach a flexible short pipe (accordion) of SS to allow the engine to move.

And of course there is that possibility that that rigid hose is keeping the engine from moving and reducing vibration. But one would think that would just transfer the vibration to the lift muffler or what it is mounted to.

My engine mounts are fairly rigid. Polly Flex plastic. So this thread is a reminder to get w the yard guys and "get-er-done".
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Old 06-05-2014, 07:21 PM   #27
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I've decided to go with the Aquadrive (s). I met with the mechanic yesterday and we're going to finalize the modifications in about 3 weeks . Aquadrive says the system can handle 15 degrees of misalignment but prefer 10 or less. I've calculated that I can lower the front of the engine about 3.5" and end up at 9 degrees. This will give me the ability to take off the rocker cover to adjust valves, service injectors, etc; which is impossible now.

We will move the engines 2 " forward as well to give the space needed for the Aquadrive and access to the stuffing box for repacking.

Again, all this is subject to many double checks. If every thing works out the work will be done in September. Still time to do sea trials before haul out for winter up here in Ontariario.

At least, those are the plans for the moment.
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Old 06-05-2014, 07:37 PM   #28
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Sounds like it will fix a lot of issues for you. Which model of Aquadrive are they recommending for the twin Lehman 120's.
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Old 06-05-2014, 08:57 PM   #29
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I just finished my installation on a 42ft steel hull and I can say vibration have been reduced by 80%. The engine does not shake at all unlike before the AD. The motor is barely felt when running. Please remember I have a steel boat and I now have 76 inch of free float shaft that is not shaking the hull.
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Old 06-06-2014, 12:00 AM   #30
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As I recall the ideal (and necessary) misalignment is 2 degrees.

I wouldn't consider much more than 3 degrees.

Perfect alignment is to be avoided. A slight misalignment is necessary to cycle or rotate the CV joint parts so they won't stay in one position and wear prematurely.
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Old 06-06-2014, 06:51 AM   #31
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IF the boat is not used for a while , it may take 5 min of shaking for the CV joints to wake up and do their job.

No big deal , just be aware it does happen., and will go away with no effort .
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Old 06-06-2014, 07:04 AM   #32
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Sounds like it will fix a lot of issues for you. Which model of Aquadrive are they recommending for the twin Lehman 120's.
A new model the 6070025. I'd attach a pic they sent with dimensions but can't figure out how! Lol
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Old 06-06-2014, 03:22 PM   #33
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I'd attach a pic they sent with dimensions but can't figure out how! Lol
Set up your account | Help, Tips, etc

Post 6 at the above thread sheds light on the photo posting mystery. Rest of that thread has some great info too.
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Old 06-06-2014, 09:07 PM   #34
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If you think about it transmitting all the trust generated at the propellor, through the shaft, gear box, engine, and then into the hull really does not seem logical and being able to transfer the thrust into the hull as soon as possible appears to be a better engineering solution and should also reduce load on the engine/transmission.

Wonder it it would be practical to use spring mounts similar to generator use in a building for vibration isolation which would isolate 99% of all engine vibration.
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Old 06-06-2014, 10:31 PM   #35
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Yes all the thrust goes to the thrust bearings in the gearbox and 1/4 of that goes to each mount.

Then there's the heaving, pitching and yawing. But the biggest engine movement that produces vibration is probably torsional. Think of a mouse running around the clock taking several steps fwd and then one backwards. And doing it several thousand times a minute. That's why we call it revolutions per minute ... not minutes. RPM is only about what happens in one minute. The expression rpm's is nonsense as a result.

There's something else that may have an effect on vibration and that's the preloading of the mounts from engine torque. Torque results in a force opposite from the torque itself. I think I stated that correctly. So if one had a CCW rotating engine (top of the flywheel moving left as seen from the rear) the force of torque would present a force that pushes down on the stbd engine mounts in fwd gear and down on the port side in reverse gear. That force can be calculated. So on one side the engine mounts are seeing more downward pressure than on the other side. The gear ratio and diameter of the prop has a lot of effect on the amount of downward force from torque on one side and an equal upward force on the other side. Those forces in combination w the weight of the engine result in the effective downward force on each engine mount.

I know engine mounts are classified or rated on how much weight they would support when attached to an engine and a boat. If a pair (L & R) engine mounts are mounted to a bell housing and a point fwd on the engine much more weight will be on the aft mounts especially if a long and heavy transmission like a BW is employed. During my repower I moved my aft mounts back alongside the gearbox to more evenly distribute the engine weight and provide more pitch and yaw stability. So all four of my mounts are rated the same. Probably one of the reasons Willy is rather smooth. If the aft mounts were attached to the bell housing they would be required to carry more weight. Like two 75lb mounts fwd and two 125lb mounts aft.

Now here's my question. Would the torque w a 3-1 gear be enough to warrant different weight load ratings port and stbd. And would fwd gear only be considered ... I would think that would be a given. And I assume this would be largely dependent on the variety of weight ratings of mounts. Has anything like this ever been done or considered?

This is even more of a think tank question/post for me than usual. There's probably a good chance I'll get no response as nobody will make it all the way through the post.
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Old 06-06-2014, 11:03 PM   #36
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You are correct that the torque is multiplied by the gear ratio, and that can squash the mounts on one side, and unload the ones on the other side. But this only occurs when the engine is at a high power setting. Given the typical straight six, that engine makes some serious vibes at 3 hits per rev down close to idle. That is where it is important for the engine to "float". At idle, there is also little torque going down the shaft. So when needed the most, the engine floats the best. Add power and the rpms go up, but the vibes go down, so even if the mounts hit the limits of their travel, it causes little vibe to enter boat structure.

All commercially available resilient mounts are designed to float the engine within a certain force per mount. Once that force exceeds a certain level, it starts to "ground", but gradually. Same if force is reduced. But as described, usually when package is making serious torque and mounts beginning to ground, engine is also at point that it is making very little vibe.

If running a 4cyl without balance shafts, it has a nasty second order vibration that increases with rpm, unlike the smooth six. In that case the grounded mounts can cause the rather obnoxious vibe to enter the hull. As a 2x vibe, it ends up being detected more as a noise vs a vibe.

Designing a mount system for a 3cyl or 4cyl is much more of a challenge. A six is super easy as at cruise rpm it makes almost no vibes. Inherently smooooooth. Except at idle!!
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Old 06-06-2014, 11:25 PM   #37
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Now here's my question. Would the torque w a 3-1 gear be enough to warrant different weight load ratings port and stbd. And would fwd gear only be considered ... I would think that would be a given. And I assume this would be largely dependent on the variety of weight ratings of mounts. Has anything like this ever been done or considered?

This is even more of a think tank question/post for me than usual. There's probably a good chance I'll get no response as nobody will make it all the way through the post.
Eric,

As you know, Yanmar marine diesels are known for their rather soft engine mounts. Some have different mounts left and right. For example, the 4JH3E has mounts rated at '150' on the port side and '200' on the starboard side. This is to counteract the torque when operating normally in forward gear. I don't know what those ratings represent numerically, but they do relate to stiffness. On the 4JH3E, the engine mounts are the same regardless of transmission type or gear ratio. The turbo version of the this engine uses '200' port and starboard. They are supposedly tuned to each particular engine.

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Old 06-07-2014, 06:36 AM   #38
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Wonder it it would be practical to use spring mounts similar to generator use in a building for vibration isolation which would isolate 99% of all engine vibration.

Fisheries boats that attempt to be very quiet for fish counting use a simple setup.

The mount the engine on a heavy frame with floating mounts , then mount the frame , on floating mounts to the vessel.

National Fisherman has drawings .

Takes lots of engineering to handle the motion of everything hooked on the engine, and only works well at some speeds and engine RPM.
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Old 06-07-2014, 10:23 AM   #39
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FF,
That's interesting about the National Fisherman ... they have some good stuff from time to time.
Re the spring mounted engine side and fore and aft forces are not dealt w w springs well. And I think they are only practical when the torque is contained within a unit like a gen set. If one had the main engine hooked to a hydraulic pump springs may work. I did an experiment quite a few years ago building a wood cage frame around the engine and rigging springs in tension from above to carry the weight of the engine so my very flexable Yanmar mounts only had to handle the vibration and not the weight of the engine. Didn't seem to make much difference. I had high hopes the overhead springs could make my 3 cyl as smooth as an eight. I wanted to get to the "what vibration" point. Was a big effort for something that flat didin't work.

Larry,
Yes indeed those Yanmar mounts are very soft. I ran a Yanmar in my previous boat. My fwd and aft mounts were load rated different. Before the Aqua Drive the prop shaft would really get whipping around .. to the point where it would rattle against the stern tube. Not good. Another reason I went to the AD. The thing that amazes me most about the soft Yanmar mounts is that they recommended replacing them every two years.

Ski n NC,
Thanks Ski that's what I was really looking for. Wondering if anybody really thought about this stuff and attached any mental energy to it. I think this explains it best "But this only occurs when the engine is at a high power setting". However most FD boat engines should be at high power (50 to 80%) most all the time. Excluding SD trawlers that operate at 25 to 40%.
Another point you make that's a good one is "So when needed the most, the engine floats the best". In engineering sometimes one just gets lucky.
About the six cyl engine smoothness they are certainly much smoother than a three or four but I don't think you get serious smoothness w less than 8 cylinders unless balancers are used. There are 6 cyl engines that are smooth but none are light. I just bought an 8 cyl PU truck (350 GMC) w stick shift and am amazed at how smooth it is lugging around in top gear. Someone just said the smoothness of the Lehman goes south past 1800rpm. I know from an engineering and theoretical viewpoint the six is smooth but in a seat of the pants real world there's nothing like an 8 ... straight or bent.

Thanks all for reading my post and responding.
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Old 06-08-2014, 07:00 AM   #40
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Most of the engine mount MFG folks will give the weight loading per mount .

Some also have torque specs .

Look at the No Noise site for an idea.
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