How to anchor fly bridge seat

[NWpilot]

I have a 1984 Bayliner 3870 and I'm replacing the fly bridge seat. When removing the old one, held down by nuts on the base, three of the six bolts embedded in the deck started turning. I don't want to tear into the deck so I thought why not just cut the bolts off, build a wooden platform out of mahogany and anchor the seat base to it. I plan to anchor the wooden platform to the fiberglas deck with fiberglas epoxy rather than trying to screw anything into the deck.
Does anyone here think this approach is worthwhile or am I crazy for trying this.
Thanks in advance for your opinions.

 

[tiltrider1]

it will work.

Or,

You could epoxy in a nutsert.

 

[SIBERNUT]

Depending on how much twisting & force it is subject to, just use what is called "pine trees" . Plastic round things with hairs sticking off. Resist pulling out. Pic of you need one.

 

[SIBERNUT]

.......and a fecal pox on "spell check".......

 

[CaptTom]

just use what is called "pine trees" . Plastic round things with hairs sticking off.

Never heard of them. Google images doesn't seem to help.

 

[Bmarler]

Not sure what the access is like, but I might use a dremel to slot the bolts so a screwdriver can hold them while removing the nuts. Then, squirt some epoxy down the bolts to re-secure them.

 

[SIBERNUT]

Pine tree

 

[Max1]

I have a 1984 Bayliner 3870 and I'm replacing the fly bridge seat. When removing the old one, held down by nuts on the base, three of the six bolts embedded in the deck started turning. I don't want to tear into the deck so I thought why not just cut the bolts off, build a wooden platform out of mahogany and anchor the seat base to it. I plan to anchor the wooden platform to the fiberglas deck with fiberglas epoxy rather than trying to screw anything into the deck.
Does anyone here think this approach is worthwhile or am I crazy for trying this.
Thanks in advance for your opinions.

When I rebuilt my flybridge deck last year, I had a new rule: *nothing* would ever be screwed to the deck. This was because the original deck had rotted through in numerous places due to water intrusion through screw holes, access holes, etc. I wanted the new flybridge deck to be 100% water tight with the exception of the upper wheel chain tunnel which has a watertight surround (and is inside the flybridge brow)
I used bonding for everything else. The deck-rails are screwed to a wood base that is bonded, not screwed to the deck. Ditto for the brow hold-down nut and flange assemblies. For something like the handrail that is adjacent to the ladder, I did *exactly* as you are thinking. I got a piece of teak wood 12" x18" and varnished it nicely, screwed the stainless handrail to it and then bonded it to the deck. I used PL premium adhesive and it works like a charm. and best of all, the new upper deck is 100% water tight with 0 chance of rot.
If you need to debond anything, just use an oscillating saw with a smooth, flat blade. It will cut the adhesive without tearing up the deck.

 

[NWpilot]

The responses got me to thinking a little more about using epoxy. It's supposed to be 101 degrees tomorrow, not good epoxy weather. I'm building a 15" x 21" base from Sapele Mahogany so I'll have over 300 sq.in. of surface to bond to. I think I'm going to go with a mastic instead.
As to the existing bolts, I'm going to cut them flush with the deck and seal them with Sikaflex 291. (I did get the existing seat off)
New chair

 

[lwarden]

I may be late to this project, but thought I would add my 2c. When I remodeled my flybridge we added two captain chairs. The deck of the flybridge is 1" honeycomb with about 1/8" of glass on both sides. I ended up making 3/8" G-10 plates a bit larger than the base of the seat and using SS inserts on the backside of the G-10. These inserts were imbedded in epoxy pockets I cut into the honeycomb to stitch the top and bottom layers together locally. The G-10 plates were bonded to deck with thickened epoxy. Then I simply screwed the bases through the G-10 to the SS inserts. The G-10 spreads the load across the floor and the force to pull a single insert through the 3/8" G-10 would be in the thousands of pounds. Ended up being very solid and after coating the disks with gelcoat it looks original.

 

[Brooksie]

Sounds sketchy to attach seats this way IMO. Epoxy, especially G-10 is very strong for sure but no stronger that what it is bonding to (gelcoat, paint, wood, polyester FG) and it's all secondary bonding.) Something like a seat will come adrift when you most need it and least expect it.

 

[JosieWhales]

I have a 1984 Bayliner 3870 and I'm replacing the fly bridge seat. When removing the old one, held down by nuts on the base, three of the six bolts embedded in the deck started turning. I don't want to tear into the deck so I thought why not just cut the bolts off, build a wooden platform out of mahogany and anchor the seat base to it. I plan to anchor the wooden platform to the fiberglas deck with fiberglas epoxy rather than trying to screw anything into the deck.
Does anyone here think this approach is worthwhile or am I crazy for trying this.
Thanks in advance for your opinions.

We thru bolted the base and used backing plates on the ceiling.

 

[DennisB1]

I agree with Brooksie. The seat appears to be about 18 inches high. Add a 200 lb person on the seat the reaction loads on the base will be quit high in a rolling confused sea. Every thing is depending on the thread engagement in the wood or epoxy surrounding the screw. A sealed through bolt with washers and backing block would be a safer solution.

 

[lwarden]

Bond strength of epoxy is ~2Kpsi to raw polyester (all gelcoat was removed). Seat base is 10" so 78 square inches. If my math is right that's 157K lbs. in tensile strength, or 4X the loaded weight of my boat.

I didn't bother to run the numbers, but since it's a Friday and I could use a distraction here is a FEA of the seat pedestal with a 18" height loaded with a 3G inertial load with a 200lb passenger. The aluminum tube holding the seat up is by far the weakest link in the design, but it's still a factor of 10 below the yield strength of 6061-T6 Al. The stress on the G10 plate to deck doesn't even register.

I think I'll be safe.