Engine mounting girders support

I'm finding GL rules a bit vague on this part. A better word would maybe be "unclear".

In mixed framing (transverse with some longitudinal elements) main engines up to 400kW are to be mounted on longitudinal bottom girders. The frames just "up front and beyond" the engine have to be web frames, stronger than regular transverse frames (if memory serves, I think 5 times stronger).

But, do the longitudinal bottom girders have to end on a bulkhead, or can they just end on a strong (mixture) of a floor and a frame?

I can't see the sense on ending on a bulkhead, since that would just be a waste for long enginerooms.

I'm asking this because I have the engine on a high position (much higher than the old one, and lenghtening the longitudinal girders all the way up to the bulkhead would be very impractical.

 

What size vessel and build materials? - better still ask the designer/NA/engineer who consulted on the design?

 

Vessel of some 16m or so, built in steel. Motor yacht.

 

OK thanks - sorry I cannot help

 

based on what you posted I see no requirement to end on a bulkhead. Why do you think they have to end at a bulkhead? There are other requirements to isolate the engine compartment from the rest of the below deck area, but that is not a structural requirement.

Without looking at the requirement myself, in my professional opinion, the intent of this requirement is to provide adequate anchorage for the engine. I suspect they are attempting to compensate for fatigue, vibrations, and transient loads by calling out a much higher ultimate design load. this is a really obsolete way to design for transient and vibration loads, different materials act very differently in these conditions. But it was not an uncommon way to design for fatigue and indeterminate loads caused by running engines in heavy seas.

 

masalai, Petros - thanks for answering. I know that recently I've flooded your board with stupid questions, if not that, then overly simple ones. But I'm currently working on two projects, my first two, and until one deals with the first two I guess, and finally gets an answer from the class society, a lot of things that should, don't always make sense.

I'm really grateful to all in here (included) for sharing their time to explain some of the simple things. Naval architecture is not a big discipline, and sometimes there isn't anyone one can ask

Regarding the engine mounting - yes, I'm aware of the isolating the engineroom requirements (bulkheads, fireproof/vapours/bulkheads ... and so on). However, what in particular confuses me is this (I-3-3-1-F book):

(8.6.2.) Engines shall be mounted on the floors via longitudinal engine girders. Low power engines ... <snip>
(8.6.3.) Higher longitudinal girders of high-propulsion-power engines shall at least be long enough to carry the engine, the gearbox, ... transfer the forces ... Longitudinal girders of seatings shall be connected to the machinery space end bulkheads.
(8.7.3.) The seating long. girders shall be adequately supported athwartships by web frames in accordance with (9.3.)
(9.3.1.) ... <snip> ... Under no circumstance the section modulus of the web frme may be less than according to (Table 1.44.)

(9.4.1.) In the case of propulsion units up to 400kW the web frames shall be arranged at the forward and after end of the engine.
(9.4.2.) In hulls build on transverse principle with prop. power up to 400kW, the section modulus of the web frame shall be 5 times that of the transverse frame in accordance with ...



So, what baffles me is:
- do long. girders need to end at bulkheads or can they end at the reinforced web frames?
- how much stronger do those web frames need to be? (for transverse framing, 5 times? What about for mixed transverse/longitudinal?)

 

I see the confusion, you have to watch the terminology: I take the longitudinal girders as running fore and aft (parallel to the keel line), and transverse frame as side to side in beam direction. what is not clear is the relationship with the "web frame" and the "transverse frame". Is a web frame a frame in the form of a truss to allow for the hollow space for the engine and transmission? If so it appears these web frames have to be 5 times stronger than the non web frames and there has to be one of these extra strong frames at the front and back of the engine installation. frames always run transversely, so these appear to be different type of structures that are transverse in the hull. Perhaps you should look in the "definition of terms" section to see if the difference between a web frame and transverse frame is clear. It looks like to me that you have to have "web frames" that are 5 times stronger than the normal frames adjacent to the engine installation.

It appears the longitudinal girders have to be laterally braced per 9.3 (to prevent them from buckling). The longitudinal girders have to be connected to the bulkheads, therefore they have to be at least as long as the machinery compartment (they could be longer, extending past the bulkhead, but they have to be attached to it). It could end at a frame, but it still must be attached to the bulkhead. There is appears no requirement to make the longitudinal engine support (or "bed") girders to be any stronger, only the transverse "web frames" that go in front and behind the engine installation.

Does this make sence?

 

Yes, very much. My underdstanding was similar.

I would still like someone who had this dilemma to confirm, whether the class societies understand it that way also.

 

Generally Engine girders can be whatever you want them to be. BUT, they must be supported and you need to demonstrate that to Class.

However, as a 'norm' most Class socities, unless shown otherwise, shall require engine girders taken to the WTB at each end. DNV for example, also requires a taper of said girder over 2 frames past the WTB.