I call engine bearers the structural part of the engine room which supports the engine. Some write « engine stringers » and both should not be confused with the engine feet. In the three pictures below, the engine bearers are clearly shown in the first picture while the third picture shows one engine foot. Engine bearers are the hull extension that attaches the engine to the boat. For most boat, this is where the thrust from the propeller is transfered to the hull, moving the boat forward. As the engine can go back and forth in close quarters, the engine bearers must be structurally solid, enough to withstand several newtons (pounds) of force due to changes in the thrust direction.
I found four designs of engine bearers. The first one is more of a category and referes to custom fiberglass mold attached to the hull. I did not investigate those. The three other designs start with wood mounts fiberglassed into the hull. The differences between them stem from how the engine is fastened to the bearers. With the first design, the engine is through bolted in the wood (first picture below). In the second design, the engine is through-bolted on a 90° steel angle bar, itself through-bolted on the side of the wood bearers (second picture below). With the third design, the engine is bolted directly in the bearers with wood fasteners (third picture).
In my views, through bolting is superior to the use of wood fasteners (or lagbolts) because its the whole bolt and nut that holds the engine in place rather than the threads on the fasteners. There is less chance that the engine will detach from the bearers because of vibration. Compared to the 90° steel bar design, the first design is the most structurally sound: there is a minimum number of pieces and the engine sits on top of the bearers, reducing any lateral stress. However, the first design makes it harder to change a through bolt if it breaks.
On the second design, because through bolts are accross the engine bearers, they can be replaced with ease. Likewise, the 90° steel bars can be replaced if they are fatigued. I would thus argue that the second design is easier for maintenance. However, because the engine sits in between the bearers, the bearers must be made smaller to fit in the engine bay. Furthermore, the engine generates a lateral force on the bearers, which increases the stress on the fiberglass holding the bearers. If it breaks the mechanical bond « sideways », then the bearers must be rebuilt from scratch.
The first design is the hardest to implement as the position of the engine feet must be known exactly prior to the install. This may be a good replacement if the same engine is used and if it is known where it stands in relation to the shaft. Starting from scratch with a new power train would however rely on theoritical measures to make it work, which is a risk.
The second and third designs are structurally similar and allow for drilling the holes once the engine is in place. They are thus easier to implement. However, the second design requires thinner bearers and so, more precision as to where they should be fiberglassed in the first place. In contrast, the third design allows for wider bearers and thus, more sideways manoeuver for engine alignment. It further supports the engine load straight from the top. Thus, the only weak point of the third design, compared to the first one, is the use of wood fasteners. It also has the advantage of being easier to implement. Because there was no engine prior to the install, I used the third design.
I knew from previous analyses that the engine would sit at an angle of roughly 11° down bubble. I designed the engine bearers using a laser protracter so as to fit this measure, pointing down at the sterntube hole (picture at the top).
From there, I measured the height of the bearers at the center of the two engine feet. I took several steps to ensure the design measurement was within the adjustment range of the engine feet. By doing so, I guaranteed myself that any built-in error could be corrected by a few turns of screw on each engine feet. The design for the engine bearers is shown in the pictures below. The first picture shows the overal sideview of the bearers. The other pictures splits the design into pieces of 1 5/8″ x 3 1/2″ (laminated 2″ x 4″).
I chose white oakwood for the engine bearers. The logic is to choose a strong wood. It took me two good days to find a supplier, and it took them a week to provide the pieces, so its best to plan ahead. The pieces were screwed together and bonded with wood glue, paying extra attention to not place screws where the engine feets were planned to be. The resulting design is shown in the two pictures below.
Fiberglassing the bearers to the hull was done in two parts. The first part is about filling the gap under the bearers and the hull. This was done in three steps. First layering thickened epoxy (peanut buttery) on the corners of the bearers, both under and on the sides, so as to have a first good layer of epoxy resin. Then, I used polyester resin putty (« thickened polyester resin ») to fill most of what is under the bearer. I then finished with thickened epoxy for the final innermost layer of resin under the bearers. Thus, the underside of the bearers is an « ice cream sandwich » of two sides of thickened epoxy and one center of polyester putty. The whole thing was grinded to offer a smooth surface over which the fiberglass would be laid down.
The second part was layering the fiberglass down. In order to have enough surface of contact to the hull, especially on the exterior side of the bearers, I decided to remove the sides of the engine room. The design for the fiberglass pieces is shown in the picture below. It follows the idea exposed in BoatWorks Today’s video on « How to Fiberglass Over Plywood » (see the fiberglass section), that is three pieces of fiberglass that overlap on the plane sides of the bearers: the top piece, the outer piece, and the inner piece. The inner and outer piece of course extends much wider than the bearers themselves, increasing the bond to the hull by a wider surface of contact.
Once the surfaces grinded, cleaned with acetone and the fiberglass mat pieces cutted to the proper dimensions, the bearers where fiberglassed in less than 30 minutes. I used three layers of 10 oz fiberglass mat.
Hindsight is 20/20
If I were to do it again with the same information I had on the engine position, I would probably change three things. First, instead of assembling the oak alltogether prior to its installation, I would through bolt the bottom piece so that the bolts would gel in thickened epoxy. Second, I would use a mold to pour all the thickened epoxy below the bearers in one application. Then, I would assemble the remaining pieces of the engine bearers using glue and screws.
That approach would increase the strength of the bond between the hull and the bearers. Third, I would increase the height of the bearers at the fore engine feets by roughly 6 milimeters (a quarter inch) so as to have a better alignment from the bearers themselves. This increase is well within the tolerance factor built in the early measurements, but it would have been better than quarter inch steel plates added during the alignment.
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References
Casey, D. (2005). Complete Illustrated Sailboat Maintenance Manual, McGraw Hill, 892 pages, ISBN 0-07-146284-8.
CruisersForum.com (2021). Fiberglass reinforcements to restore an odd engine bed structure, retrieved online in October 2023 at this url.