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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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Engine Mounts Perkins 4.108 with BW Velvet Drive Dear All, I have been corresponding with the Professor about engine mounts for my engine and I am missing an important piece of information; does anyone with the same set-up, Perkins 4.108 with BW Velvet Drive transmission, know the weight (static load) at the forward and aft mounts? Fair Winds, Chris Mabel's Casse Tete 003/43 |
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Join Date: 30 January 2007
Posts: 461 |
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I do have a Perkins 4.108 with Velvet Drive BW transmission in my 411 and a couple of years ago I did measure those characteristics. I ignore if those measurements can be applied to your boat but Lars will know it and he should also have my measurements; of course he has got my total agreement to pass them to you if this can be of help. Daniel, 411/004 Luna Menguante |
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Join Date: 15 April 2011
Posts: 393 |
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Dear Daniel, Fantastic news! I have been trying to find a dynamometer to rent but possibly I won't have to do that! I'm sure the Professor will see this and reply. Do you have the measurements youself or do they reside only with the Professor? Thanks again for replying. Fair winds, Chris Mabel's Casse Tete 43/003 |
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Join Date: 02 January 2008
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Dear Chris and Daniele I have the numbers, but they are not very accurate. At the time Daniele estimated an error in the order of 20%, and the sum does not agree with the specification weight of engine and gear.
If you choose to use them the numbers need to be adjusted upwards.
Kind regards
Lars
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Join Date: 03 February 2011
Posts: 39 |
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When replacing mounts on Ravn, 411/36, with engine and gear as descibed above, I was recommended to increase rubber stiffness on the two aftmost mounts to stabilize shaft. On the original mounts there was a number "45" referring to stifness and on new ones same number was 55. Result for shaft through hull was good but the stiffer mounts gave new sounds/vibrations in hull unless running at rpm 1400 or more. So if was to do it again would find those with original stiffness since they had been okay for long period of time, abot 6000 hrs of running. This not a very sophisticated approach or observation, but it told me one is better off sticking to original set-up. Ravn 411/36 Thorbjorn |
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Join Date: 15 April 2011
Posts: 393 |
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Dear Professor, Thank you. I'll try to locate a dynamometer this weekend and report back with results. If I am successful, I plan to place a support across the companion way, attach a chain lift to it at the top and to the engine at the motor mount, raise the engine just off the bed log and measure the weight. 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 Thorbjorn, Thank you for your note. That must be very frustrating. Possibly you can replace the aft mounts? 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 Thorbjörn Thank you for this information. 55 means stiffer mounts than 45, but if vibration now is a problem below 1400 rpm the new mounts are clearly too stiff. You may consider replacing them in order to have a more pleasant time on board.
Would you know the manufacturer and type of the mounts? Photos should also help.
Chris - if the specifications can be determined for Ravn's mounts this would be helpful for your case.
Kind regards
Lars
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Join Date: 15 April 2011
Posts: 393 |
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Dear Lars, Good idea...I'm really not the optimistic about my chances of finding a small dynamometer for rent! Chris Mabel's Casse Tete 43/003 |
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Join Date: 30 January 2007
Posts: 461 |
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Chris, I had a very long and helpful exchange with Lars but I think I did not send to him my last measurements which, hopefully, were pretty good; I will be very happy to compare them to yours if you happen to do them. Total mass: 300 kg; mass on fwd mounts: 80 kg; mass on aft mounts: 220 kg. I used a spring scale, a dynamometer, with full scale at 1 ton and, probably, 5-10% error. Please notice that, as the overhang toward the aft is important, the measurement is pretty critical. In particular you should lift the measured mounts by displacing the engine from its original slope as little as possible. Just as a note: if you use the dynamometer on the fwd mounts keeping the aft mounts down (which is right and rather easy) and then use the dynamometer on the aftmost part of the transmission keeping the fwd mounts down, you will find that the sum is not the total mass of the engine. Odd but this is right and can be explained. In fact this is the way I eventually used because it is easier and less prone to error; the mass on the aft mounts can thereafter be computed from it. Another note is that the static load on the fwd versus the aft mounts is so different that a choice of dampers with different characteristics seems desirable. I had problems in finding flexible mounts with the required height so I eventually did not change them and I still have the original ones: DF2200 Series by Bushings Inc. Daniel, 411/004 Luna Menguante |
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Join Date: 02 January 2008
Posts: 1547 |
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Dear Daniele Thank you for this important information.
Engine mounts are usually chosen based on their share of the total static engine weight + a part of the shaft weight. But this does not disclose anything about how they perform in their main duty of isolating vibration.
It is also necessary to know the mount's static deflection, as this determines to what extent they isolate vibrations in the idling condition. As you may have noticed, idling sets the highest demands on the softness of the mounts.
For the 4-108 engine idling at 500 rpm the required static mount deflection for 50% of the vibrations transmitted to the engine seatings is about 3 mm. This would be a noisy installation, for an excellent one with only 20% transmitted the deflection requirement is 7 mm. Here every millimeter really counts.
Lets see if such information can be extracted from Thorbjörns findings.
Kind regards
Lars
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Join Date: 15 April 2011
Posts: 393 |
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Dear Daniel, Thank you for the thoughtful and informative note. I think, given the information you provided, I may not need to weigh the engine and tranmission. Thank you again. The motor mounts that were on the boat when I bought it were the same ones you currently have and also the ones that I bought 2 years ago at the recommendation of TransAtlantic Diesel, the company that sold me the new transmission. In the brief time that I ran the boat before the interminable rebuild project, the mounts seemed good. I may keep these! I was not able to track down deflection specifications for these mounts but I'll keep trying. If I do find it, I will post again. In the meantime, we may hear additional interesting comments. 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, I remember that the engine vibrated considerably at very low RPM. You mention 500 RPM which may be the speed that my engine was set. Once the idle moved up to say 800, the engine smoothed out and quited down. Based upon your note, it seems that although we now have the weight distribution data, you would still like the deflection specifications. I called the Bushings Company and they promised to send it to me. I will say, however, that the representative with whom I spoke did not seem to really understand what I was asking (possibly my fault). If I do not hear back from him Tuesday, I'll call again. Thank you for all of your help, Professor! It's always a learning experience. With warm regards, Chris
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Join Date: 15 April 2011
Posts: 393 |
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Dear Lars, I remember that the engine vibrated considerably at very low RPM. You mention 500 RPM which may be the speed that my engine was set. Once the idle moved up to say 800, the engine smoothed out and quited down. Based upon your note, it seems that although we now have the weight distribution data, you would still like the deflection specifications. I called the Bushings Company and they promised to send it to me. I will say, however, that the representative with whom I spoke did not seem to really understand what I was asking (possibly my fault). If I do not hear back from him Tuesday, I'll call again. Thank you for all of your help, Professor! It's always a learning experience. With warm regards, Chris
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Join Date: 02 January 2008
Posts: 1547 |
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Dear Chris Your information that 800 rpm smoothed out the vibrations is helpful here, it indicates that the mount static deflection was about 3 mm.
Perkins literature gives the idling rpm 500, but because of vibration it is likely that many 4.108's are set to higher idling - this is an easy remedy. However, the clutch is then loaded more by the stronger impact when engaging the gear.
Enclosed a diagram showing the numbers graphically for an excellent installation. The diagram refers for all 4-cylinder 4-stroke engines
Kind regards
Lars
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Join Date: 30 January 2007
Posts: 461 |
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Dear Lars, relating the static compressional displacement of the mounts to the expected transmitted noise is very interesting, simple and helpful. I do not remember discussing this matter with you previously but I know that my brain cells are getting old and I assume I forgot about it... :-) After some thought I found that theoretically it makes very much sense and it is a very straightforward experimental way to solve the problem without complicate measurements and calculations. Great to mention the fact and thank you as always! Daniel, 411/004 Luna Menguante |
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Join Date: 15 April 2011
Posts: 393 |
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Dear Lars, Thank you. I am a little embarrassed to admit that I had not thought about the extra pressure a higher idling RPM would transmit to the pressure plate.
The chart you provided is very helpful. If we decide to keep the idle at a relatively low rate, this implies a stiffer motor mount, I think. So, back to the Bushings Company to see if I can get the static deflection information. Today at the boat, I remembered to take a picture of the mounts that I currently have (they are new but 2 years old!). I think these are likely the same ones that Daniel is using. Thanks for all your help! 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
Pls note that low idling rpms require soft mounts with large deflection.
Kind regards
Lars
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Join Date: 02 January 2008
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Dear Daniele, Chris, and Thorbjörn An additional comment: I believe you all have an in-line shaft and engine according to the original set up. In this case the above diagram applies, and you can use the normal procedures for shaft alignment.
On many boats, however, there is a reduction gear with shaft drop or down angle, and this makes a difference with softer mounts - the propeller thrust causes the engine to go out of alignment and the shaft to bend. Therefore the suppliers tend to offer stiff mounts.
I would suggest, however, that soft mounts may be used if the shaft alignment is adjusted so it becomes straight in the working condition at the preferred cruising speed. This differs very much from the established standard approach of only aligning the shaft with the engine stopped.
In principle the engine out-of-alignment needs to be measured at cruising speed, and the reverse applied with the engine stopped. It is also possible to calculate this accurately enough if complete information for the installation is available.
Kind regards
Lars
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Join Date: 30 January 2007
Posts: 461 |
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Dear Chris, discarding a printed digit that looks specular, your flexible mount appears to be a DF 2205-2 which, according to the manufacturer specs, corresponds to a four-point mounting for a 150 to 450 lbs engine with 5/8" stud: exactly as mine but also apparently slightly undersized. I tried to discuss the matter with the manufacturer in order to understand what they meant precisely by "150 to 450 lbs" but also in my case they did not understand what I was talking about. Does it mean that the single mount is supposed to work at 37 to 112 lbs? A positive answer could suggest, for example, to use two DF 2207-2 (i.e. 112 to 180 lbs each) for the aft mounts. Daniel, 411/004 Luna Menguante |
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Join Date: 02 January 2008
Posts: 1547 |
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Daniele, Chris, Thorbjörn In addition to the static loads on the mounts from the engine weight there are also additional dynamic loads created by the torque and thrust when motoring. It is necessary to check with the supplier that these larger loads are acceptable.
The torque increases the mount loads on the "heavy" side, and reduces them on the "light" one, and some engine suppliers because of this specify different capacity mounts each side. Should be asked if this would be appropriate here.
Daniele and Thorbjörn have a reduction ratio of 2.91:1, and this increases their torque loads slightly over Chris having 2.57:1. On the heavy side the aft mount maximum load is then 305 lbs, while the forward mount maximum load is 150 lbs. In addition to this there is a maximum horizontal thrust load of 232 lbs on each mount.
It is also necessary to check how much the engine moves forward due to the thrust loads. Particularly for boats with drip-free propeller shaft seals the seal may begin to leak because of this movement.
Kind regards
Lars
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