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Project update - transom rebuild

December 17, 2017

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Project update - transom rebuild

December 17, 2017

Almost ready for the water - putting the finishing touches on our recent transom rebuild project


Recently a customer here at Springwood Marine purchased a boat online, without getting it checked out professionally. When he took delivery of the boat and launched it, he had a major water leak. On checking it out we found that his y pipe on the exhaust had rotted out. We removed the outboard to replace the pie and check what else might be trouble. In doing so we discovered that his transom was soft, and full of rot.


This is a story on how the fix was done.


One of the biggest problems facing older fiberglass boats is the transom rotting out from the inside. Transoms are rarely solid fiberglass layups, and use an inner core between two thin layers of glass to make up the desired thickness. The cores on older boats were usually plywood, marine or otherwise. Ply is great if it doesn’t get wet.


 The damage we discovered after we removed the outboard and outer skin.


When the transom is drilled to for fitting of trim tabs, bung holes, or penetrations made for running of sounder cables, or in this case the opening for the stern drive, if the end gran of the ply, once exposed by drilling/cutting is not sealed, the water will migrate through the grain and rot will start from the inside. Over time the entire transom can become nothing but a wet, soggy mess. Some newer boats may use a composite core, and while this levitates the problem to an extent, unless any penetrations are sealed correctly, you can still have trouble.


After removing all the hardware, the first job was to remove the outer skin of the transom in one piece that we could reuse on the new build. The inner skin would be left in place. To do this we used a Finn saw to make a continuous cut about 40mm in from the edge of the transom. We ran masking tape all the around the edge to use as a guide. The width of the cut is not to much of a concern, but needs to be kept down to about 3mm. We then dug out the old ply core.


After digging out the old core, we were left with the inner fiberglass skin. We then thoroughly sanded and cleaned the inner surface, and vacuumed out all the debris.


We used the outer skin as a template to cut the new core. For the core, we used a composite product called Thermo-lite board. It is an extremely light and strong composite board, with excellent stiffness and limited compressibility. It is also impervious to rot. We used to laminates of 19mm to make up the total required thickness of 38mm. To easy inserting the core into the “pocket” formed by the cutting of the outer skin, we made the inner and outer laminate in three sections.


The inner laminate was glued in place using WEST system epoxy, thickened with micro ballons. We covered the entire surface of the laminate with epoxy, using a tile trowel to spread the epoxy in a thin, even layer. The outer laminate was glued to the inner laminate in the same manor, then the outer skin we had removed earlier was replaced in the same manor. Time was an issue here as we had about a 45 minute window before the epoxy started to cure. We dry fitted everything, and worked out gluing sequence well in advance before actual gluing started.


Once the outer skin was fitted, we used existing screw holes to insert screws which in effect clamped the four layers together.


The cut was then glassed over and fared, and the whole transom sanded. The cut-out for the stern drive was done, along with the transom drain hole. The raw edge left by cutting the drive hole was sealed using epoxy.


The transom was resprayed using two-pack epoxy and the drive and hardware refitted.


This is just one example of the many services we offer at Springwood Marine that goes above and beyond just the service and sales of outboard motors. Contact us to see how we can help keep you out on the water this summer.



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