Engine mounts
Obviously a question for Lars...
I need to renew at least two of the mounts of my Perkins 4.108 and I am tempted not to buy the original ones which are very expensive. First I would like to know if this is a wise choice or not and second, if yes, I would like to have some technical indication as I am rather confused by the many different kinds of commercially available engine mounts.

Daniel, 411/004

Hi
You may find what you need here for a sensible price
http://www.asap-supplies.com/search/engine+mounts

Dear Daniel
I can agree with you that the range of engine mounts is outright confusing, with lots of different types and promotional information for them. Some vendors specify the engines for which their mounts are suitable, but things are perhaps not that straightforward any more.

In the old days the mounts were specified based on the total weight of the engine, but now the mounts have become much more sophisticated, and can be different forward and aft, and also sideways. Some high power engines actually lift the mounts on one side, and Saildrive and V-drive installations lift the forward mounts, and double the load on the aft ones, so they must be able to cope with such forces.

Also, propeller shafts used to be aligned with the crankshaft, but now reduction gears have shaft drop or down angle, or both, and this affects the loads on the mounts in working conditions. It is possible to calculate what happens based on propeller thrust and torque, but as the loads and their distribution varies with the rpm it is necessary to consider the entire rpm range, as well as reverse, making evaluation of mounts complicated to say the least.

I would suggest that one of the marine engine manufacturers' primary goals is to have proper engine mounts properly installed, because engine induced vibrations on board are highly dependent on this, and the crew experiences the results first-hand.

This means that the engine manufacturer or distributor should have the best knowledge about which mounts are suitable, and it would be generally advisable to use their specifications. It is likely that this is considered confidential information, and they just tell you to buy the mounts they offer.

Your engine installation, however, represents the traditional approach with shaft and engine in line, and if you know the actual weight on the mounts in question it could be tried with some mounts specifically for this engine, the choice is yours.
Kind regards
Lars

Dear Daniel
Prepared these example diagrams for providing more insight into your engine mount question.

Engine not working: With the yacht upright the loads on the mounts are fairly equal. When the yacht heels the loads change towards the next diagram.

In-line shaft: With the engine working and an in-line shaft the propeller torque lifts one side evenly, and depresses the other. The diagram is for another engine, but your installation is in-line and behaves in the same way.
For your engine at full power the mount loads are approx 950 and 400 N, but I believe that you never use full power. For lower outputs the mount loads go towards the average of the above numbers, i.e. the static load.
It should be noted that the reduction ratio affects the propeller torque, and the loads can be different on another yacht with the same engine due to this. A light and powerful engine can cause upward loads in the mounts on the light side, and the mounts must be able to cope with this.
In a situation with a rope caught in the propeller the mounts will have a hard time if the propeller shaft is strong enough and does not break.

With shaft drop: This causes very big changes, the front end is depressed, and the back end lifted, but the effect can also be the other way around depending on the geometry. Here the yellow bar means negative load, i.e. the port side aft mount is lifted.

Shaft down angle tends to lift the engine fairly evenly, and if applicable reduces the loads for in-line and shaft drop installations slightly, in these examples about 200 N per mount.
For clarity the propeller thrust forces are not shown, vector presentation would be needed.
The diagrams show the vertical loads in Newton on the mounts for a 75 hp engine at full power with reduction 2.74:1, shaft drop 0.1 m, and down angle 8 degrees.
Kind regards
Lars

Dear Lars,
interesting indeed and more complicated than one could guess.
You made me think about it and I guess that if one would want to take into account almost every factor, they are many. For example boat roll and overhang/mass of the transmission. Moreover there are shear stresses due to propeller thrust when the thrust bearing is within the motor block.
What would be interesting to know is how much the high frequency vibration absorbing specification of the mounts is affected by different quasi-stationary stresses due to the above changing factors.

Daniel, 411/004

Dear Daniel
Suggest that you take a look at the calculation section in the catalog of this company:

http://www.paulstra-industry.com/engines-a17-en.html

Kind regards
Lars

After a long exchange of letters with Lars in which he was, as always, very kind and patient with my questioning, he explained to me the theory behind the anti-vibration mounts and computed the optimal specifications of the ones needed for my Perkins 4.108.
Namely the rear mounts should hold a static force of 95 kgf and a dynamic force up to 130 kgf while the forward mounts 40/70 kgf respectively. These numbers are the ones needed to choose the mounts along with the used idling speed, 500 RPM.
Now I have another problem I propose to other 411 owners who still have a Perkins: did you ever buy new mounts for your engine? I have difficulties in finding a brand that will fit the two critical points for assembling the mounts with minimal work: height of the engine feet from the beam (approximately 62mm) and distance between the fixing screws on the beam (101.6mm).
Unfortunately none of the most known marine mounts fit these specifications - in particular the height.
I would appreciate your suggestions.
Daniel, 411/004

Hello Daniel think vetus.com have such products. Didn`t buy there when replacing in my boat since found identical with same dimensions in local store but think those vetus carries also are identical to those that came with the boat. There is a reference to rubber hardness and the original ones was in my boat`45`.
Was recommended to replace with harder rubber, `55` to stabilize shaft through sterntube since two aftmost gets heavy load. When starting up few days back, there was no difference with regards sound or vibration.The 4.108 now at almost 7000 hrs started up after two and a half years in lay-up like stopped the day before. There was a fisherman here that had a 4.108 replaced after 22000 hrs for the reason more power. Of course a lot of continious operation on his boat, but still, they must be very good motors. Ravn 411/036 Thorbjorn

Thank you Thorbjorn,
indeed there is a Vetus mount, Type K, which have the required geometrical specifications (height range, stud diameter and base length) and, in particular, the K75 with the right rubber hardness for the forward mounts with working loads 38/75 kgf. Unfortunately there is no model that will fit the load requirements of the aft mounts.
In the meanwhile I discovered a new thing: the present mounts of my engine are DF-100 by Bushings Inc. and are still produced and available in the US. At that time there was little choice in hardness but the same manufacturer is nowadays producing also an improved model with a range of different hardness values, the 2200 series.
It may be interesting to know if the 411 and other boats with the Perkins 4.108 where provided by the yard at build time with the same DF-100 mounts.
Daniel, 411/004