Tuesday, July 29, 2014

Just in from Tom Fish, Voodoo Child 41LN23 -- Making an Engine Bed

Before work
Here's why Tom Fish, Voodoo Child 41LN23, replaced his engine beds (excerpted from a 23 July '14 email).

The old engine beds needed to be reworked for several reasons. A previous owner had modified the height apparently to allow for a new offset transmission to match the height of the stern tube. And, for some reason, the centerline between the engine beds did not line up with the stern tube either.

Also, there was a small amount of rotten wood coring beneath the fiberglass layer of the starboard engine bed which probably occurred due to the previous modification.

The Beta Marine 60 engine is narrower than the old BMW. The Beta also needed to sit lower for the shaft to line up with the stern tube and it needed the engine beds closer together (or thickened toward midline of the vessel) for the mounts to reach.

The new engine bed.
 We considered fabricating custom brackets to reach the old beds but due to the shape of the engine block there was not enough clearance to turn brackets down and then midline again and we knew we had to rebuild the beds anyway due to the rotten core.

What we ended up doing was cutting about 2 inches off the height of the beds which got us down to solid wood and put us just below the height needed for the Beta. Then we thickened the beds inward toward midline by lag bolting and epoxying thick beams of mahogany. This was all then glassed in with epoxy including over the tops. Finally, 1/2 inch thick stainless steel plates were lagged down using 3M 4200 in and around the pre-drilled screw holes.

An advantage of using the stainless steel plates on top of the glass was the ease of sliding the engine around during installation and alignment. They also protect the new beds from future rot.

All done and lookin 
The Beta engine mounts will only allow for a small amount of adjustment during engine alignment so the most difficult part of the project was predicting the final trajectory of the transmission shaft during these major modifications. We used an engine mockup with fore and aft centering holes and a laser pointer that was centered and shot thru a new cutlass bearing from outside the hull. End result was accuracy within a millimeter with the motor mounts at mid-adjustment. I think we got really lucky. Surprisingly, Beta engine mounts only allow about 7mm of height adjustment which, at mid-adjustment, translates to only 3.5mm up or down. 

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