Under platform attachment methods

[b_arrington]

I'm planning on installing a hydraulic step under my swim platform. The "standard" way to attach it is to through-bolt it from the top with nuts below. However, I'm not too keen on a large number of bolts on the platform so I'm looking for other options.

So far I've come up with attaching 3-4" wide x 1/4" thick aluminum plate to the underside of the platform, then tapping it for bolts that would hold the platform. My current challenge is how do I secure the aluminum to the underside of the platform. The platform is cored with 1.5" Airex core, and vacuum infused.


A couple ideas so far. All of the below would be in addition to using some 5200:

• Pop rivets. Use some large-ish aluminum or stainless closed-end pop rivets. Would need a line of them down each side of the aluminum plate, spaced 4" apart or so. Drill into the plate, through the fiberglass, and rivet. Not sure if the fiberglass is strong enough for a rivet?
• Rivnuts. Similar to a pop rivet, but allows to thread a bolt in to hold the plate in place. Similar question to the pop rivet - is the fiberglass strong enough to not be crushed by the rivet action?
• Jacknut. Kind of like a hollow wall anchor with fingers that spread out on the blind side. Might be easier on the fiberglass?
• Epoxy in a bolt anchor. Haven't gotten this one all worked out yet.

Any thoughts if any of these are good / bad ideas, or some other idea, quite welcome.

This is a pic of the underside of my swim platform, and a mockup of the what I'm thinking.

Pic of the step in a boat show display. Shows the rails and many attachment points.

CAD drawing of step.

 

[Stee6043]

I'd be hesitant to do anything other than thru-bolt. I imagine they have the 16 bolts based on what they assume for the strength of the fiberglass and the rated load of the step.

You could always seadeck the platform after installation, cover up the carriage bolt heads? The plate idea sounds moderately terrifying. I think you'd be better off (structurally) using small(ish) beams, perhaps a sandwich of 1x1 or 2x2 structural AL beams.

As for attachment, perhaps build a "shelf" on your transom to support the forward edge of the step. Then fabricate an aesthetically pleasing clip/bracket to support the aft edge over the top of the platform, full-length of the step. Ask the manufacturer if that would be sufficient support for the step (nothing in the middle). This would eliminate the thru-bolts with the exception being where the clip/bracket would attach to the aft edge of your platform.

Or an alternate for the aft clip/bracket idea would be a tube/box beam supported directly from the transom. Add a couple struts below the waterline. Less desirable than bolts on the platform in my opinion, however. And you'd still need a connection or four thru the platform I'd think.

 

[Skybolt]

I'd be hesitant to do anything other than thru-bolt. I imagine they have the 16 bolts based on what they assume for the strength of the fiberglass and the rated load of the step.

You could always seadeck the platform after installation, cover up the carriage bolt heads? The plate idea sounds moderately terrifying. ...

+1

My exact thoughts.

 

[Morgan Jane]

I would also take into account the additional cantilever effect you will create on the platform when the steps reach out beyond the platform. A 250 lb man stepping on a step 2 ft. out is like a 500 lb man stepping on a step 1 ft. out.

 

[Henry Boyd]

Brad,
I installed a trim fence on the transom of Serenity this winter. It’s a permanent trim tab that Sabre now puts on all of its boats, basically a ¾” thick slab of resin & glass that is attached to the transom end that sticks down about an inch. Sabre had me use an industrial adhesive called Plexus. They told me it’s what they use to glue the helm cabin structures to the top of the topsides. I only used screws to hold the piece in place and provide clamping force while the adhesive cured.

I’d avoid using an aluminum mount for corrosion avoidance reasons. If it were me I’d make a mount out of fiberglass & epoxy with stainless nuts and stainless fender washers embedded. Then this could be adhered to the platform using the appropriate industrial adhesive.