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| S&S Swan Maintenance - Engine Mounts Perkins 4.108 with BW Velvet Drive |
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Join Date: 15 April 2011
Posts: 393 |
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Dear Daniel, Thank you for your note. Yes, that first digit looks very strange in the photo but is actually the exact same as the second digit. I first thought it looked like a backwards 3. Optical illusion. And you are correct that the Bushings website does suggest a the larger -07 motor mount. What to do! I will say that the owner of TAD is very knowledgeable and that these are the ones he recommended...at least that's what I think. I guess I'll need to call them too! I'm certainly learning a lot about motor mounts! Fair Winds, Chris Mabel's Casse Tete 43/003 |
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Join Date: 15 April 2011
Posts: 393 |
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Dear Lars, Your post on shaft drop and down angle is very interesting. Thankfully, I don't have to worry about this but I wonder how the measurement would be made at cruising speed. Is this a physical test or a theoretical calculation? Did you notice Daniel's note? Is the weight rating provided for the motor mount applicable to total weight, for the forward or aft mounts, or individuall? With warm regards, Chris |
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Join Date: 02 January 2008
Posts: 1547 |
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Dear Chris Measurement of the engine position at cruising speed can be made for example by attaching laser pointers forward and aft, and taking readings on bulkheads - the further away the better.
The supplier should specify the maximum load each mount can take, vertically and horizontally. In your case the forward heavily loaded mount vertical dynamic load is 1.7 times the static, assuming the mounts are 0.457 m apart transversely.
For some light high-power engines the dynamic factor may be over 2, and for them the loads on the lightly loaded side are negative, i.e. the mounts are subject to pulling.
Using engine weight as the only criterion is a simple way of choosing mounts, but as you can see there is more to it.
Kind regards
Lars
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Join Date: 15 April 2011
Posts: 393 |
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Dear Lars, Yes, there certainly is a lot more to motor mounts than I had thought. I really need to re-read this entire thread and try to determine if the mounts I currently have, and the ones that Daniel is using, are the correct ones. Unfortunately, I may not be able to get the necessary information. In which case, I guess we will have to use experience, meaning, they seem to wark well for Daniel and they seemed to work well when I used the boat briefly, so I would keep what I have. Fair winds, Chris Mabel's Casse Tete 43/003 |
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Join Date: 02 January 2008
Posts: 1547 |
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Dear Chris Suggest you take a look at the diagram at the end of this document, and consider if this enables you to choose mounts from this manufacturer.
Item 2) in the instruction is not clear enough though - for a 4-cylinder 4-stroke engine the interfering frequency is rpm x 2 / 60. For idling at 500 rpm the frequency is 16.67 Hz.
This manufacturer also has newer slightly different mounts with added features, but it appears there are no diagrams for them.
Kind regards
Lars
http://www.missionsupplyonline.com/pdf/Cushyfloat_eng.pdf
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Join Date: 03 February 2011
Posts: 39 |
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Lars, thank you for good advise. Can`t see any specifications on mounts other than this "55" reference. These on Ravn not a Vetus make but as probably well known, Vetus also have a wide range of mounts. Kind regards, Thorbjorn |
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Join Date: 02 January 2008
Posts: 1547 |
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Dear all
Chris suggested that I post this information related to his engine installation.
The topic:
How to choose Cushyfloat mounts using the supplier Diagrams 1 thru 3,
For checking maximum loads additional calculations of torque and thrust are required.
Idling loads at 500 rpm - as per earlier information there is 110 kg load on each aft mount.
Diagram 1: Choose the sloping line for mount 17-1609-45 and note that the 110 kg load level crosses at 7.8 Hz. In Diagram 2 read static deflection 5.0 mm for this mount and load line .
Diagram 3: interfering frequency is 16.67 Hz, at 7.8 Hz degree of isolation is 72%.
In the earlier discussions vibration transfer rate was mentioned, this is the complementary to isolation - here transfer is 100 -72% = 28%
40 kg load on forward mounts when idling. Note the big difference in load compared to the aft end.
Diagram 1: 17-1600-45 at 8.3 Hz Diagram 2: static deflection is 4.4 mm.
Diagram 3: interfering frequency 16.67 Hz, degree of isolation 70%
These mounts would be fine for idling, but also powering loads must be checked. The mount supplier gives limits for these loads in the table above the diagrams.
Powering condition loads - cruising @25 hp/2600 rpm assumed. This is a continuous load, full power loads are higher, but used for short spells only, and considered insignificant here.
Aft mount heavy side sees 130 kg vertical load, and 1095 N thrust. (Based on calculated torque and thrust)
The Cushyfloat vertical load limit with thrust is 95 kg and 1000 N is the thrust limit, and both calculated loads exceed the given mount limits. Longitudinal deflection 2.4 mm, this is near the limit.
Forward mount heavy side sees 60 kg vertical, and 365 N thrust
Vertical load limit is 35 kg,and thrust limit 370 N, vertical load exceeds mount limit.
Most suppliers suggest the same size mounts forward and aft, but in this case it would result in a very low deflection forward, and ending up in the Resonance area on the diagram - must be avoided.
These mounts chosen for the idling condition are not acceptable for powering, try next size up with the notation 55 instead of 45. The notation gives the rubber hardness.
Harder mounts Idling at 500 rpm
Aft, Diagram 1: 17-1609-55 at 10 Hz. Diagram 2 static deflection 3.3 mm
Diagram 3: degree of isolation 48%.
Fwd, Diagram 1: 17-1600-55 at 10.9 Hz, Diagram 2 static deflection 2.8 mm
Diagram 3: degree of isolation 40%, this is a low value, means noisy idling
Harder mounts powering condition
Aft, vertical load limit with thrust 140 kg, thrust limit 1500 N, both loads OK. Longitudinal deflection 1.6 mm, OK
Fwd, vertical load limit with thrust 55 kg, thrust limit 560 N, vert load not quite OK, engine cruising rpm may be lowered slightly.
Mount load capacity is almost met at the expense of a lower degree of isolation
Checking two alternative Cushyfloat types, information for them can be found on the internet
Cushyfloat HT
17-2182 35 aft mount static deflection 4.3 mm, better isolation than 1609 above, Degree of isolation about 64%, but height is 91 mm compared to 50 mm for the above. Longitudinal deflection is 0.55 mm, OK
The vertical load limit with thrust is 140 kg, acceptable, a thrust limit not stated, but the letters HT mean high thrust.
Fwd mount static 1.8 mm deflection, 1600-55 is preferable
Cushyfloat HD
17-4726 40 aft mount static defl. 3.9 mm, not as good isolation as for HT above, and the height is 101 mm
Forward mount static defl. 1.3 mm, not as good as HT above
Effect of higher idling rpm, Diagram 3 with 1609-55 and 1600-55 mounts
600 rpm 20 Hz aft mounts degree of isolation 69%, fwd mounts 56%
700 rpm 23.3 Hz aft mounts 76%, fwd mounts 72%
800 rpm 26.7 Hz aft mounts 84%, fwd mounts 80%
Higher idling rpms reduce vibration, a well known fact
Conclusions
Because of the big difference in mount loads fwd and aft end the recommendation is to not use the same size mounts both ends.
Cushyfloat has probably been on the market for the longest time. There are also others presenting similar diagrams, it appears sometimes with the same mount characteristics. Some display simple calculators.
There may possibly be other mounts better suited to this particular installation, but a comparison against the cases above is necessary.
Kind regards
Lars
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Join Date: 15 April 2011
Posts: 393 |
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Dear Lars, Thank you for providing a comprehensive, informative and interesting narrative on the selection process of motor mounts. This is a topic that is clearly important but one that I had previously largely ignored. In addition to the engineering aspects of the motor mount selection, there are practical implementation issues; for me, I will need to move the engine forward about ¾ inches since the 17-1609-55 mount is longer front to back. I have examined my photos and am confident that I can do this. Further, since I am in the process of replacing my drivetrain from transmission to propeller, it’s easy for me to make accommodations for the larger mount. Others may not have the same flexibility. I think it’s interesting to note that this selection process is extremely complicated even when using the 3-graph-method provided by Cushyfloat which is referenced in your post. For example, had you not provided the interfering frequency of 16.67 Hz, how would one make a good selection? So, as usual, I have learned a lot about a new topic; just don’t ask me to repeat it. Finally, if I understand your conclusions, I will purchase 2 17-1609-55 mounts for the rear requirement and 2 17-1600-55 mounts forward requirement. Further, to improve the isolation resonance, I will run my engine at a 700-800 rpm idle. Running the engine at load, I may run at a slightly lower speed because the vertical load limit of the forward mounts is only 55kg in comparison with the 60kg heavy side load when cruising at 25hp/2600 rpm. Thank you for all of your help!
Chris Mabel’s Casse Tete 43/003 |
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Join Date: 30 January 2007
Posts: 461 |
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Very interesting discussion indeed but I am puzzled by a small particular in the long explanation by Lars and I would like to understand better. It regards the thrust which I believe is the horizontal component of the force applied to the mounts and, if so, should be rougly equal for all the mounts and 1/4 of the total thrust along the shaft. This is not exactly true because the transversal/horizontal resisting torque of each mount may vary depending on the height of the mount and its elastic characteristics; I wonder how one takes into account this complication. Daniel, 411/004 Luna Menguante |
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Join Date: 02 January 2008
Posts: 1547 |
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Dear Chris and Daniele Thank you for the comments.
Would like to suggest an alternative approach enabling you to use the four mounts you already have.
Add weight to the forward end of the engine, so there is the same mount load forward and aft. Preferably add this to the engine support brackets, or to a cradle attached to them.
Would be good to ask your mount supplier if the maximum vertical load given is with thrust considered, is there a maximum thrust, and compare with Cushyfloat.
For Cushyfloat the maximum vertical load is reduced by 1/3rd if there is simultaneous thrust.
It is possible that thrust is more or less ignored if your mounts are intended for pleasure yachts, as their engine hours usually are very low in comparison to commercial yachts.
The rubber can be expected to come loose from the metal, or crack as a result of shear loads caused by thrust, but this may take many years in pleasure applications.
Daniele has read my post carefully, and noted that the thrust was not equally shared between the mounts. This is correct, the mounts are of different size, and my assumption was that they take thrust in the same proportion as their vertical load capacity.
If you use four identical mounts they share the thrust equally.
Kind regards
Lars
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Join Date: 15 April 2011
Posts: 393 |
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Dear Lars, Thank you. That's a very interesting idea. I am not clear about how the weight would be added. I'll give that some thought. I wonder if the batteries just ahead of the engine could be used. In the meantime, I have written to Bushings, Inc. again and have requested detailed information about their motor mounts. We will see. Chris |
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