When I wiggle my stanchions, the surrounding deck flexes with it. There doesn’t seem to be much room to add a sufficient backing plate. But at least it doesn’t look like water will get into the core in this area. Has anyone else encountered this/have a solution?
37C Deck flexing from stanchions
- Thread starter o0o0o0o0o
- Start date
Deck flexing is usually a sign of rotten core - core that has been wet long enough for bacteria to reduce it to mush.
There is NO coring under the stanchion bases. Two layers of solid glass due to the hull/deck joint. Bigger backing plates would be nice but as you said there is little access without major interior surgery.
The trick to all backing plates is to understand what they’re being asked to do.
Things in shear (sideways load) - mooring cleats, halyard stoppers, winches - need THICK backing plates well-agreed to the underside that provide a strong bearing surface for the sides on the bolts. Thin or no backing plates in this application - I’ve seen the bolts rip sideways through the ‘glass (elongated holes). And of course there’ll be a water leak.
Things in tensile (vertical load) - U-bolts, eye plates - need large-area backing plates that provide more area against the tendency to distort and even rip up through the ‘glass. I’ve seen that happen too.
Things in flex (load at some arm away from mounting flange) - helm pedestals, table pedestals, stanchions and railings - need stiff backing plates of large area securely bonded to the underside of the deck that reinforce the whole neighboring structure. In my not-inconsequential experience the number-one cause of rotten deck core is seat bases in motorboats and helm pedestals in sailboats. Imagine the load they take when some 220-lb guy lands a hip-shot against them 30” off the floor in a seaway. The backing plate must be broad enough and well-adhered to transmit as much of that load as possible over as wide an area as possible.
Stanchions on sailboats usually land two bolts through the aluminum toerail, or at least all of them through the flange. Quality boats (like mine) are made with actual ‘ears’ on the flange to accommodate them (I moved all of mine so I’m not using these any more). Failing that, an overly-long, wide-as-possible backing plate should be provided. Sheet fiberglass (like from McMaster.com) is best for all sorts of reasons. Plywood is just awful (will rot and crush). Coosa is not puncture-resistant enough (will crush; bolts will pull through).
My trick is to locate the hardware, drill a mounting hole or two as a tell-tale, and then affix the backing plate using a dam of 5200 around the whole perimeter and one sheetrock screw up into (and through) the deck. When the 5200 is cured, back out the screw, tape all relevant holes and gaps from the underside, and fill the void within the 5200 ‘dam’ from above with epoxy. This results in a very strong, rigid backing block that you can then drill through and (again, using 5200, the only thing for strength, water resistance, and pliability) crank those mounting bolts home.
Underestimating the relevant structural loads is all too common amongst even experienced sailors. Avoid anecdotal evidence and investigate the way well-seasoned boatbuilding pros (like me) do it. Then go to sea confident you’ve solved a problem that in well longer than your lifetime won’t recur.
Things in shear (sideways load) - mooring cleats, halyard stoppers, winches - need THICK backing plates well-agreed to the underside that provide a strong bearing surface for the sides on the bolts. Thin or no backing plates in this application - I’ve seen the bolts rip sideways through the ‘glass (elongated holes). And of course there’ll be a water leak.
Things in tensile (vertical load) - U-bolts, eye plates - need large-area backing plates that provide more area against the tendency to distort and even rip up through the ‘glass. I’ve seen that happen too.
Things in flex (load at some arm away from mounting flange) - helm pedestals, table pedestals, stanchions and railings - need stiff backing plates of large area securely bonded to the underside of the deck that reinforce the whole neighboring structure. In my not-inconsequential experience the number-one cause of rotten deck core is seat bases in motorboats and helm pedestals in sailboats. Imagine the load they take when some 220-lb guy lands a hip-shot against them 30” off the floor in a seaway. The backing plate must be broad enough and well-adhered to transmit as much of that load as possible over as wide an area as possible.
Stanchions on sailboats usually land two bolts through the aluminum toerail, or at least all of them through the flange. Quality boats (like mine) are made with actual ‘ears’ on the flange to accommodate them (I moved all of mine so I’m not using these any more). Failing that, an overly-long, wide-as-possible backing plate should be provided. Sheet fiberglass (like from McMaster.com) is best for all sorts of reasons. Plywood is just awful (will rot and crush). Coosa is not puncture-resistant enough (will crush; bolts will pull through).
My trick is to locate the hardware, drill a mounting hole or two as a tell-tale, and then affix the backing plate using a dam of 5200 around the whole perimeter and one sheetrock screw up into (and through) the deck. When the 5200 is cured, back out the screw, tape all relevant holes and gaps from the underside, and fill the void within the 5200 ‘dam’ from above with epoxy. This results in a very strong, rigid backing block that you can then drill through and (again, using 5200, the only thing for strength, water resistance, and pliability) crank those mounting bolts home.
Underestimating the relevant structural loads is all too common amongst even experienced sailors. Avoid anecdotal evidence and investigate the way well-seasoned boatbuilding pros (like me) do it. Then go to sea confident you’ve solved a problem that in well longer than your lifetime won’t recur.
DianaOfBurlington, I appreciate your comments concerning sailboat helm pedestals causing flex in the cockpit floor due to rotting core underneath. My new-to-me 1979 Hunter 37C (#23) has this problem.
Are you suggesting a conventional core repair, removing the cockpit's top skin and replacing rotting wood with better wood, followed up by McMaster sheet backing plate underneath? What thickness would you recommend for the McMaster sheet?
Thanks in advance,
Brian Dunn
Are you suggesting a conventional core repair, removing the cockpit's top skin and replacing rotting wood with better wood, followed up by McMaster sheet backing plate underneath? What thickness would you recommend for the McMaster sheet?
Thanks in advance,
Brian Dunn
Brian Dunn, I had the same problem of rotted core in my cockpit sole. We removed the top skin and replaced the rotten wood with a stiff foam-like product, then reskinned. Benefit of the foam-like product was improved sound deadening for the engine.
Kappykaplan, thanks for responding. Was your foam-like product StarBoard or something similar? Same thickness as the core it replaced?
Brian and Kappy, sorry I have not seen this till now.
Kappy, I am curious as to what the ’stiff foam-like product’ is. I see no reason to replace the core under the pedestal with anything other than real ‘glass. The reason it was done in foam-cored ‘glass at the factory is that they weren’t convinced they were automatically going with pedestal steering at the time; or they were unsure of where it would be located; or they were just cheap (most likely).
It is true that a well-done foam-cored deck should support a pedestal (or stanchion or anything else). I’m saying solid ‘glass is better - no threat of future water intrusion and no flex due to dissimilar materials. Also no need for a backing plate - the floor itself is the backing plate.
If you retain the foam core, you have two options:
A. Oversize-drill the holes and bush them with blocks or tubes of fiberglass, like G10 from McMaster. Epoxy them in place.
B. Oversize-drill the holes, tape over the bottom, fill with either epoxy or fiberglass resin with microfibers mixed in, and then redrill them at the proper diameter.
Either works. Then add the oversized backing plate, well applied (5200, 610, epoxy & microfiber moosh, whatever you think - not silicone). Then bed the pedestal in 4200.
Brian, I would NOT use StarBoard, EVER, for any kind of structural job like hardware backing plates or deck core. It simply has no strength for anything like that. And no epoxy or sealant or adhesive sticks to it. In general it belongs in the same category as silicone - not on the boat, not in the truck, not in your neighbor’s truck, not within a 45-drive of your boat, ever. Sounds extreme, yes; but the alternative (‘I have a tube or a sheet right here’) leads to too many lapses in judgment.
Kappy, I am curious as to what the ’stiff foam-like product’ is. I see no reason to replace the core under the pedestal with anything other than real ‘glass. The reason it was done in foam-cored ‘glass at the factory is that they weren’t convinced they were automatically going with pedestal steering at the time; or they were unsure of where it would be located; or they were just cheap (most likely).
It is true that a well-done foam-cored deck should support a pedestal (or stanchion or anything else). I’m saying solid ‘glass is better - no threat of future water intrusion and no flex due to dissimilar materials. Also no need for a backing plate - the floor itself is the backing plate.
If you retain the foam core, you have two options:
A. Oversize-drill the holes and bush them with blocks or tubes of fiberglass, like G10 from McMaster. Epoxy them in place.
B. Oversize-drill the holes, tape over the bottom, fill with either epoxy or fiberglass resin with microfibers mixed in, and then redrill them at the proper diameter.
Either works. Then add the oversized backing plate, well applied (5200, 610, epoxy & microfiber moosh, whatever you think - not silicone). Then bed the pedestal in 4200.
Brian, I would NOT use StarBoard, EVER, for any kind of structural job like hardware backing plates or deck core. It simply has no strength for anything like that. And no epoxy or sealant or adhesive sticks to it. In general it belongs in the same category as silicone - not on the boat, not in the truck, not in your neighbor’s truck, not within a 45-drive of your boat, ever. Sounds extreme, yes; but the alternative (‘I have a tube or a sheet right here’) leads to too many lapses in judgment.
I honestly don't recall. The wooden core in the cockpit sole was completely rotted and the pedestal was pulling up on the starboard side. The repair was completed many years ago and I haven't had a problem since.
DianaOfBurlington, went back through my records and found the receipt from August 2015. "Measure, cut and fit core & pvc board." Sorry it's not much to go on.
Sounds like it could have been Divinycell, which is a PVC foam coring material (quite hard) used in a lot of European boats instead of balsa core...