Reinforcing a portion of deck to take seat pedestals.

My speedboat seats were on aluminium framed ply boxes (previous owner)
They started to rot so I removed them.I have aquired tubular pedestals with a 8" square base plates,with only four fixing holes. I'm not sure if the fix will hold.The seats swivel and I turn them facing the stern when I fish.
I have some clear wired 1/4" thick fibreglass sheet.The deck is about 3/8" thick and is a sealed unit between the deck an the hull,with a small sealed cavity inbetween.
I am thinking of sanding the deck to take 12" squares of it,and fibreglassing over it with a few layers of csm.Hopefully this will enable me to use wide dia screws to screw the pedestals down with.
There are only two seats,I removed the rear seats to gain more space for fishing.
The other option is to use the chines where they step out on the inside of the boat,and add fibreglassed over steps to take a 9" x 15" plank to screw the seats to.I would need to add a bit of height on the plank to gain the correct seat height.
Any opinions?

 

Good morning VB,
Re your pedestals, are they the same height as the previous plywood boxes?
Re how they have 'only' 8" square base plates, I would be rather dubious about just using screws to secure them - there is going to be a LOT of leverage and force applied to the baseplate when you are underway.
I appreciate that you cannot through-bolt the base plates, but maybe you could bond a base plate to the deck with epoxy (perhaps even recess it flush in the deck), and then use bolts tapped into the base plate to secure the pedestal base plates?

I am not too sure what you mean here - would this plank span the full width of the boat in the cockpit? And do you really mean for it to have a cross section area of 9" x 15"?

 

Hello BJ,
Yes,the baseplates are the problem,I have some galvanised steel sheet that I can enlarge the baseplates with,then drill more fixing holes.
The wire reinforced fibreglass sheet I mentioned is what I was going to use fibreglassed to the deck,which would thicken and reinforce the deck where the seats go.If I make the sheet a 12" square,fibreglassed to the deck with CSM,that would give me a thicker portion of deck,plus the extra area.
The pedestals have a swivel incorporated at the top,which the seats are attached to with bolts.The pedestals actually raise the seat hight slightly,which I prefer.
Slight typo with the plank,the measurement was 1-1/2" thick.I used a decimal point-- 1 point 5".
Yes,the plank would go the full width of the boat from chine step to chine step.
That will make the seats lower,so I would add packings to the top of the plank to gain the neeeded height for the seats (just under the seat area)
Many thanks for your interest BJ.

 

I have acquired some large dia, deep fast thread screws that are used for screwing plastic together to fix the pedestals with.