Engine mounts and flexible couplings I have just removed my engine and am in the process of renewing my engine mounts. I also propose to fit a flexible coupling between the shaft coupling and the engine coupling. This should reduce the vibration and noise.
The manual for my Perkins engine only states that the bearers should be able to support a static load of 5 to 8 times the weight of the engine.
1. Should I go for very stiff bearers given that I will fit a flexible coupling?
2. Should I have the aft bearers stiffer that the front?
Dear John
Engine rubber mounts are usually chosen for smooth performance at idling speed. The mounts need to have a suitable flexibility in order to avoid excessive vibration transferred to the hull, but not so soft that the engine moves around too much under load. At higher rpms the vibrations are reduced, as you may have noticed.
As you have a larger somewhat rough low rpm engine it is particularly important to consider smooth idling.
Referring to your questions:
- A flexible coupling does not affect the choice of engine rubber mounts
- The center of gravity position for the engine with reduction gear determines whether there is more weight aft requiring different carrying capacity.
If you could inform which reduction gear there is and the idling rpm I can provide information about the vibration isolation efficiency for Cushyfloat type mounts. The isolation efficiency determines the amount of vibration transferred to the engine bed, and out into the hull.
Kind regards
Lars
Lars. Happy New Year.
The gearbox is a Borg Warner Velvet Drive 72CR 1.88:1 ratio
The idling revs are approximately 700
The total weight of the engine unit is 263 KG or 579 Lbs (from the manual I am not sure if this includes the gearbox)
The weight of the 72CR gearbox is listed as 153 Lbs
My local suppliers of mounts and coupling is R&D Marine www.randdmarine.com
Thanks
John
Greetings to you too!
Thank you for the information
153 lbs is the dry gearbox weight, oil approx 10 lbs.
This gearbox is very long, nearly 18", and this loads the aft mounts more. Assuming the aft engine mounts are used (no mounts on gearbox) the mount load proportion is about 68/32, i.e. much higher load on the aft mounts.
The recommendation is to aim at about 70% isolation efficiency when idling - the better the efficiency the softer the mounts need to be.
R&D Marine should be able to choose suitable mounts.
The engine brackets should be close to the mount stud lower end, this can be adjusted by adding spacers under the mounts.
Kind regards
Lars
Hello John and Professor.
May I add my best wishes for 2016 to yours?
Are you speaking of a Perkins 4108 here? If so I would definitely be interested by John's findings.
Many Thanks.
Philippe 41/022
Dear Philippe
Thank you for the greetings
John's engine is a Perkins M65, so the findings are not directly applicable to the 4.108, but may give guidelines.
Best regards
Lars
on my S&S 40 I have a Perkins 4-108 with the big and heavy Borg Warner gearbox. The mounts are close together and placed such that most of the weight is on the aft mounts. I went by deflection of the rubber when the weight was placed on the mount. There is a direct correlation between static deflection and best absorption of vibration. I did aim for about 1/8 inch deflection. Because the mounts must also absorb the propeller thrust I found engine mounts from a Buick suitable for the rear mounts and Vetus mounts for the front mounts. I also found that, over the years, the mounts sag a little and I have to re-align the engine.
Dear Peter
The rubber mount static deflection is a simple way of evaluating vibration characteristics, but additionally there are two very important numbers which must be considered - the number of cylinders, and the idling rpm.
The engine vibration excitation frequency is directly related to these two numbers, and a low number of cylinders as well as low idling rpm both require more deflection, i.e. softer mounts.
The rubber mount on the other hand has a natural vibration frequency related to the static deflection under load, and finally it is the relation engine/mount frequency which decides the isolation efficiency of the mount.
The natural frequency of a flexible mount with 1/8" deflection is close to 10 Hz, and then the following table can be compiled for a 4-cylinder 4-stroke engine assuming various idling rpms. Perkins old literature suggests 500 rpm idling for the 4.108, but I believe that higher numbers are generally used.
An engine/mount frequency relation of at least 2 is considered suitable. A 2-stroke engine doubles the frequency, while a higher number of cylinders increases it in proportion, both change the frequency relation radically, and increase the isolation efficiency.
It is important to keep the longitudinal deflection caused by propeller thrust within limits, maximum 3 mm is recommended, and the mounts need to be stiffer in that direction. This is usually achieved by increasing the longitudinal dimension of the rubber element.
Kind regards
Lars
Of course, we all love him!
Two years ago as I performed a major refit of the engine and of the engine room, I was planning to replace the old original mounts of the Perkins 4.108.
Lars and I had a very long exchange of letters dealing on technicalities regarding the mounts and I can assure that he is the most prepared and helpful person I ever met.
At the end he worked out the optimal choice for the characteristics of the mounts but I had some geometrical constraints that, at that time, I did not want to change; namely the distance of the bolts that fix the base of the mount on the stringer (4"), the distance from the stringer to the lower surface of the engine brackets (from 59 mm up), and the vertical stud diameter (5/8").
Unfortunately I was not able to find proper mounts that would fit the constraints with the exception of the original ones which are still in production (DF-100 by Bushing Inc.) but which are not the optimal choice regarding other aspects. At the end, I decided to postpone their substitution but I am still very interested in knowing if someone finds proper mounts that do not need major changes in the engine room.
Daniel, 411/004
I have just removed my engine and am in the process of renewing my engine mounts. I also propose to fit a flexible coupling between the shaft coupling and the engine coupling. This should reduce the vibration and noise.
The manual for my Perkins engine only states that the bearers should be able to support a static load of 5 to 8 times the weight of the engine.
1. Should I go for very stiff bearers given that I will fit a flexible coupling?
2. Should I have the aft bearers stiffer that the front?
Anybody got any experience's they can share.
John B
Swan 411 0101
Hi John,
I installed the Perkins-Sabre M65 in my Swan 47 12 years ago and it continues to run like new. Three years ago I installed the Swedish-made Aqua-Drive constant velocity unit, bought from Mack Boring & Parts, a major Yanmar dealer in the US. I also bought and installed matching softer motor mounts recommended by the dealer.
Without doubt this upgrade has been the most satisfying of all I have done to our 'wonderful machine,' as the previous owner often referred to her. At startup the difference in reduced noise and vibration at idle was startling. It only got better at higher rpms. We usually cruise at around 1,800 revs at 7 or so knots and often the only way we know the motor is on is hearing the exhaust splash.
All loads formerly transmitted directly to the motor are absorbed at the new bulkhead that is installed to contain the unit's massive main thrust bearing, aft the CV joints. For the bulkhead, I used two pieces of 3/4 inch marine ply, glassed together, then encased in glass/cloth, and custom cut and glassed to the hull.
Good luck with your unit, let know if u have questions.
All, as per normal, our great research and plans never work out as we expect. The new R&D mounts did not fit the space at the gearbox end without substantial alterations, (time / money) so we have had to revert to plan B, new Perkins mounts. The flexible coupling will be fitted this week, we might have to shorten the prop shaft slightly.
I have also filled the cavity around where the bulkhead was cut away to allow the gearbox / shaft entry with closed cell foam. This looks better and will hopefully reduce the noise also.
when I installed a new engine and a new gearbox on board CHINOOK about 15 years ago, a Yanmar 4JH3 with mechanical gearbox, I installed a constant velocity drive unit called Python Drive. This system includes a bearing which takes all the engine propulsion loads and keeps the engine mounts free of propulsion loads.
The system works absolutely fine - engine, gearbox and Python Drive - as new, after 15 years!
