Through Hull Backing Plates - Bedding For Load

[Canuckjetboater]

Checked this forum's AWESOME FAQ and Control F7 before posting this thread to avoid duplication. Before I post I can't say enough about the versatility and ease with which this site can be navigated. I have used a lot of software in my time and this forum is top shelf! Now to business. From reading member posts a lot of you are doing neat mods. I am contemplating adding the same metal receptacle for the cockpit table that I have in the main cockpit of my SX195 to an area of the swim platform (like on a lot of Yamaha boats) - if I can find a suitable mounting location. I'm sure all of you would use a metal backing plate to secure this part thereby avoiding load damage to the fiberglass that just screwing it in could do. A trick I learned a long time ago - when doing ANY through hull accessory mounting - was to make a bed of Sikaflex or 3M about 1/8" thick slightly larger than the area of the backing plate. This ensures total contact of the plate to the inner fiberglass wall instead of having it rest on a couple of higher spots, which ensures much greater spread of the load. Of course, I would use ss aircraft type nuts and bolts with washers.

 

[Darkside]

Using Sikaflex or anything that remains pliable will just cause a point load across the top of the backing plate when the table is cantilevered from the bracket. A mixture of thickened epoxy with either cotton flocking or chopped FG fibers mixed in and applied to a roughened (60 grit) plate will work much better. If at all possible the backing plate should extend between bends (curves) in the hull. I have quit using metal backing plates and use G10 with some fender washers. G10s coefficient of expansion is the same as the boat and will never break loose.

 

[Canuckjetboater]

Using Sikaflex or anything that remains pliable will just cause a point load across the top of the backing plate when the table is cantilevered from the bracket. A mixture of thickened epoxy with either cotton flocking or chopped FG fibers mixed in and applied to a roughened (60 grit) plate will work much better. If at all possible the backing plate should extend between bends (curves) in the hull. I have quit using metal backing plates and use G10 with some fender washers. G10s coefficient of expansion is the same as the boat and will never break loose.

@Darkside .......I appreciate your perspective. I have never seen evidence from my installs that co-efficients of volumetric or linear expansion have created any gaps, stress-release phenomenon or diminished their integrity. I used 3/16" aluminum plate in most applications 1/4" in some extreme situations - that I sanded on the caulking side for added cohesion (and a reason I stopped using SS for plates.) I have found that Sikaflex and 3M harden sufficiently to load spread and being a semi-cured liquid only, IMO, enhances their strength as anti-compressive and load-spreading abilities of liquids are, IMO, superior to a rigid installation behind fiberglass, something that I have seen result in stress cracking. The fact that Sikaflex and 3M do not become rigid is a compelling reason for their universal use with through hull fittings as they expand and contract with whatever product they are employed with, particularly materials of disparate composition. I have never had stress cracks using the method I described with installations going back decades in situations where either constant load or random, variable loads were applied. I have avoided any sort of chopped FG or woven roving, of any nature - even when mixed with vinylester resins, as that composition attracts water and can lead to hull damage.