Ok folks ... been around boats for most of my 62 years and never really thought about it much before the current 7m ex small prawn trawler rebuild ....
Beside precision alignment requirements, and yes there is a fixed drive (shaft) train, but can any of you gurus from this font of knowledge explain why anti-vibration engine mounts aren't used with in-board marine engines, particularly slow revving diesels ... lets say below 2000 / 3000 or so RPM....
My theory being they are used in large land applications, both mobile and fixed so ????
Quite simple really, all the thrust from the prop comes up the shaft, to the gearbox/motor, the thrust is transferred to the engine mounts to "push" the boat along through the engine beds to the hull, soft mounts would just tear apart, the only soft mounts are on V drives, because all the thrust is to the rigid mounted gear box, and then via a universal jointed drive to the soft mounted motor.
that's a great explanation Noelm !
bonnevile
I sell diesels for a living. 99% have flexible rubber mounts.
In commercial use we recommend that a torsional is done on the whole drive train, ensuring that even the coupling between the tranny and engine is correct match.
thrust is more prevalent in ratios above 2.5:1
these usually use an island style gearbox mount which then has. 50/50 fixing on solid and flexible
maybe you need to look at ages more installs
Cheers
Steven
"ages more installs"?
Don't know how much vibration is too much, but my diesel it doesn't vibrate any more then the Optimax I had b4.
I could say it is more quiet, around 10-15% I'd say.
I should add, there is ways around it, but in general commercial boats tend to go for simple and robust, and nothing is much more simple that bolts to hold the engine direct to the hull.... different types of drive, like pods and stuff like new tugs have, make mounting the engine a whole new ball game.
I would think that one wouldn't want any movement causing the shaft to move and chew out the bearing where it goes through the hull.
More than happy with your explanation in particular Noelm, but I still can't differentiate that much between heavy marine applications as SteveM suggests. If I were to use a heavy truck for an example, it still reflects a massive amount of torque to the ground to move its weight via its driveshaft, which reflects "strain" (Newton's Law) back to the driveshaft, transmission and engine via flexible mounts to the trucks chassis. Not suggesting you are wrong at all Noelm, or maybe I just can't seem to differentiate between the two applications. I most definitely can see the importance of the through alignment for both the Stuffing Box and the Cutless bearing of the Skeg which wouldn't be happy with any torsional movement that could possibly be there to some extent with flex mounts ..
In a truck, the drive is via the rear diff, that is not "pushing" the tail shaft, the force is in a different direction, which is transferred to across the vehicle, which wants to curl up the springs, and twist the engine sideways, next time you are stopped beside a truck, take note of the springs when he takes off, a boat does not have a diff, all the thrust is straight up the shaft, with rotational torque too of course. Also in a car or truck, alignment is not an issue, you have two universal joints to allow for suspension travel when loading and unloading, which does not happen in a boat.
Last edited by Noelm; 28-08-2014 at 07:51 AM. Reason: spelling
I just rebuilt a merc with a bravo leg. The front engine mount has rubber bushes. I guess this is for a little front movement and vibration. So some inboard engines do have anti vibration mounts.
yes, indeed they do, BUT... a stern dive does NOT drive via the main engine crankshaft, that has rotational torque only, it has a couple of right angle changes in thrust, and the main prop thrust is actually on the lower leg (like an outboard) and not to the output shaft like a shaft drive.
Right you are Noelm. :-)
Do you mean the shaft is fixed to the bearers independently of the motor/gearbox?yes there is a fixed drive (shaft) train
Like the last picture?
http://www.gknservice.com/global/mar...ve_system.html
The number one reason that flexible mounts are installed is to minimise vibration throughout the hull / vessel.
Anyway, from an engineering perspective, here is some simple do's and donts.
Flexible Engine Mounting
Flexible engine mounts use either rubber or spring isolators to absorb engine vibration before it is transmitted to the hull. This will reduce noise and vibration in the vessel.
There is a wide variety of flexible mounts available on the market. Any mount selected must hold the engine in alignment and provide for acceptable mount life. The isolator manufacturer should be consulted for further recommendations.
The engine should be installed with sufficient clearance on all sides so that the
allowable engine movement will not cause structural or component damage.
Snubbers may be required on softer mounts to prevent excessive engine
movement due to propeller thrust.If a front power take-off clutch is required on a flexible mounted engine, the clutch and engine should be mounted on common base rails with isolators between the base rails and engine bed.
On flexible mounted systems, the vibration isolators must be installed
parallel to the engine centerline in both the vertical and horizontal
directions. The mount must be free to deflect and must not be fully
compressed under a static load.
If the engine bed is not parallel to the engine crankshaft, it may be necessary to
use wedges beneath the isolators to assure the isolator springs or rubber bushings are properly and evenly compressed.
Solid Engine Mounting
Solid mounting an engine is usually done by using brass or steel shims, pourable
chocking compound or Fabreeka type washers and pads. The use of pourable chocking compounds is the simplest and preferred way to solid mount the engine. When using a chocking compound, the alignment of the marine transmission and propeller shaft is accomplished using jacking screws between the support brackets
and the engine bed. The mounting bolts can be loosely put into place at this point or a hole can be drilled through the chocking compound later. The jacking screws, mounting bolts and bottom of the engine bracket should be coated with a grease or
anti-bonding substance to allow them to be removed later. Temporary dams are put on the engine bed and should extend approximately 13mm (0.5") above the bottom of the engine beds. The chocking compound is poured in to fill the space between the bracket and engine bed. Once the compound has solidified, the jacking screws can be removed or left in place and the final mounting bolts are torqued down.
When using jacking screws on wood or fiberglass engine beds, steel plates should be used under the jacking screws to prevent damage to the engine bed.
The chocking compound manufacturer should be consulted for further
recommendations.
Fabreeka type washers and pads consist of layers of rubber-impregnated canvas and will provide a small amount of flexibility for minor misalignment and a degree of
protection from shock loading. A steel plate is required between the nut and Fabreeka pad to protect the pad from wear and should be the same size as
the pad. It is still necessary to use brass or steel shims to align the engine and gear with the shafting.
Transverse cross bracing on the engine bed and stringers should be used to prevent lateral engine movement on solid mounted systems
Cheers
Steven
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