DESIGNING NEW ENGINE BEDS (by Clyde Phillips)

[bob mitchell]

by Clyde Phillips

                    The engine beds serve one purpose: a place to put the engine to sleep.  No, no, no.  They hold the engine in its place.  The engine’s place is between the shaft and the hull, the beds are between the engine and the hull.  Your main problem is the shape of the beds and where to put them.  The beds are hull pieces, but to know the shape you need to know where the engine needs to be so that the beds can be made to fill the place between engine and hull.  So you need to mentally (with the help of drawings) picture where the engine will be and then design the beds to fit the engine mounts (the top of the beds) and the hull (the bottom of the beds).  The spread of the beds (distance apart) is determined by the spread of the mounts.  Here’s an off the cuff discussion of the drawing to help you mentally see where the engine and bed will be and how the beds are shaped to do their job.

 

The place to start is at the stern bearing for the shaft.  That is pretty well set; what you have is what it is.  The shaft passes through the stern bearing to the stuffing box—straight line because it shouldn’t bend.  On the forward end of the shaft (you know all this) is the shaft’s half of the coupling.  Every boat blueprint is developed on a set of lines, and all these lines relate to a “base line” from which everything is measured.  Now to your drawing.  Full size would be best.  Draw both a side view and a top view of the engine.  Extreme detail is not needed.  I would use the extended centerline of the shaft as the base line for the engine drawing and the beds drawing.  What you need to draw as accurately as possible is the position of the engine mounts above or below the side view of the shaft centerline, and the distances left and right of the top view of shaft centerline (forward and aft mounts may have different up/down and left/right distances).  Using this method you can find the position of the four mounts of the engine which will tell you the position of the beds at these points.

 

Now you must locate the extended centerline of the shaft on the boat.  String a small diameter cable from behind the stern tube to forward of the engine room (or engine).  This line should be of a non-stretch construction and it should be able to stand a lot pull tension; it has to be tight to be straight.  Plan some sort of adjusting mechanism that will be used at each extreme end of this line; you must center the line at the outboard end of the stern tube and the inboard end to the shaft tube or ally.  The adjusting mechanism on this inboard end must be forward of the engine location.  So far we haven’t talked of the angle that this line makes in relation to the hull or any part of the hull.  But I don’t think we have to worry about that.  (See next)

 

Consider now if what I say here will work for you.  You have two different two dimensional drawings that will give you the three dimensional positions of the bottoms of the engine mounts.  It should now be possible to built a mock up of these “pads” so that their positions in relation to each other will be fixed, immovable.  You now must locate on this mock up the position of the extended centerline of the shaft and permanently affix some means to indicate the centerline on this mock up jig.  (If needed I’ll draw a picture of how I see this done.)  This jig now has four places in a relationship that matches the engine mounts and has an indicator of the centerline of the shaft.  This needs to be mounted in the engine room in such a way that allows you to work under it to get measurements to fashion the beds.

 

For the mock up jig of the mounts (these are guesses, use your best judgments) use 2 X 4’s held vertically athwartship, one for the forward and one for the after sets of mounts.  These should be longer than the outside spread of the beds, maybe by a couple of feet, to allow room for working under this jig.  These are to be 90 degrees to the shaft centerline and held so their spread equals the fore/aft spread of the mounts.  Maintain the up/down difference between the mounts, if there is any.  Mark the center of each mount on each of these cross pieces at the appropriate distances left and right of center (of shaft line) on the forward side of the cross pieces.  Erect 1 X 6’s vertically on the center line of the cross pieces (to go under the shaft center line) to the exact height of the shaft centerline minus the radius of the wire (that will represent the bottom of the wire.  Mark on the after face of this upright the centerline of the shaft.  Note that the forward face of the cross piece and the after face of these verticals are on the same plane of the center of the engine mounts that are marked on the forward face of the cross pieces.  Though not mentioned above, it is on this plane on each cross member the height of the centerline of the extended shaft is measured to get its heights above the mount levels.  Mount this mock up jig to the hull so it will not move with the after faces of the verticals touching the wire and the cross pieces level (or corrected to “level” considering the amount the boat may be off level in its bracing).

 

  Once you have such a jig (that indicates the center of the engine mounts) in position all you have to do is draw up the lengthwise shape of the top of the beds to match the bottom of the mounts and clears the engine in all other places.  Then draw lengthwise shape of the bottom of the beds to match the hull, keeping the frames, floors, etc., in mind and remembering the bottom of the beds will have an angle that matches the rise of the frames.  Look ‘engine beds’ up in your favorite boatbuilding books for their recommendations for bed length and believe what they said. but cover as many frames as you can with whatever length of bed you can get in the boat.  (Looking at a picture Bob sent me it looks like the bed sit on four frames and overlaps at each end by a few inches.) 

 

I wrote the above off the top of my head, with an idea of what I wanted, so it is clear to me.  You are reading it (have read it) trying to figure out what I was thinking.  So if there are any questions, ask them.  I will try to get a drawing of my idea to you, but you may learn enough from the above to get your idea that will do what I’m suggesting.  Or you will come up with your own plan of action, which is fine with me, because if I went to the shop today to build one for me I would likely change things myself.  I see what I have said as a simple, easy project.

 

[Clyde A. Phillips]

The drawing associated with this article does not include three things I want you to know:

1.  There is no scale to the drawing.

2. The drawing shows the engine mounts on the same level.  Yours may not be, so adjust your jig to match your engine.

3. The red lines indicate the center line of the shaft.  The top red line goes with the side view (upper left).  Mentally raise the side view up to the red line and you'll see how that moves the mounts to a position parallel to the shaft line.  Since the uprights are to be only high enough to match the distance between the shaft center line and the bottom of the mounts and a lot of shafts are close (vertically) to the bottoms of the mounts (some times even under the mounts), the uprights will be short or nonexistant.  You may even not need the upright and need to notch out the 2 x 4's (brown in the drawing) either from the top or from the bottom.

AND EVEN NUMBER 4 ! -- All this about building the jig may not be needed.  If you have the inclination, just do a drawing on paper to figure the relationships between the shaft and the engine mounts.  I think that if I by-passed the jig I'd still locate the mount locations with something so as to figure the full shape and the angles of the bottoms of the new engine beds. 

 

And remember, you are "on the scene" with your boat and I'm working with nothing physical before me, so your ideas will likely be better than mine.  Enjoy!