Need Help with backing plates for tower. PLEASE

I had a guy install a Aerial Freeride tower fro me (which I love). The only thing is that he used small pieces of wood for backing plates, and they move around a bit because they are not on a flat surface. I have removed 2 of them and want to replace them with starboard and marine epoxy to fill the gaps underneath. I included pics below. Would appreciate input. You can see the hull is not level it has a couple ridges in it in the inside and outside. Its hard to see the gaps under the starboard, but there are some small valleys (maybe 1/4") Is this OK to fill with marine epoxy? Thanks. Sorry pics are sideways but you get the idea.

I dont see an issue with the backing plates being wood. Given the lack of a flat surface, I would remove the bolts and nuts, fill the contours to as level as possible with a heavy glass body filler. Then put the plate back in its place and glass mat resin over the plate making it part of the deck.

What woul dbe a heavy body filler I could use? Never used it before. Would glassing it in be mandatory? Never done that either.

Bondo brand fiberglass body repair filler. Not the regular stuff for metal body panels, but like for repairing an old vette.

Backing plate for sure would not move around if its glassed to the deck.

It is not moving like sliding, it is moving like flexing a bit with weight of tower and with hull.
Was thinking maybe body filler to flatten out the area, use supermend epoxy and attach aluminum 1/4" plate (4"x10") over that then bolt on.
Reasonable or flaws with this?
Is starboard a bad idea?

I don't see much wrong with the way it was installed. glassing it in is one way to do it. a bit overkill IMO. If you were to redo it, your method sounds just fine. the forces put on the feet are basically up and down, not twisting or using any leverage against the glass. use a good bit of epoxy to fill those gaps and it will be solid. will be stronger than the way it is, but not by much. those wood backers are a bit small though...

agreed.
DO you think the starboard is ok?

I would use some aluminum plate and get rid of any plywood there...it will never be tight for long with plywood.

OK, so the plan is to use an aluminum plate (maybe 4"x6")
I will use Supermend epoxy to level it out underneath and fill in the gaps

If I use aluminum, is it OK to put a flat piece of aluminum over a non flay surface (it has a few valleys underneath. I was going to slather it with epoxy to fill in the gaps underneath

Originally Posted by pharmacist25

If I use aluminum, is it OK to put a flat piece of aluminum over a non flay surface (it has a few valleys underneath. I was going to slather it with epoxy to fill in the gaps underneath

No, you need to fill the gaps between the fibreglass and backing plate to apply even pressure to the entire area behind the backing plate. If you don't you will develop stress cracks on the fibreglass.

Cast a base by filling the area with thickened epoxy or fibreglass reinforced bondo.

About two thirds down this page ("Casting a base") has a bit of info: http://www.sterndrives.com/supplies/epoxyresin03.html

Great info. Thanks. This is kind of what I was thinking. Going to mix some fiber with epoxy to thicken then use it to fill in gaps, put on aluminum plate while wet til it oozes out sides. Let cure then bolt down tight once cured. SOUnd good?

If you do go with aluminum plates, round off the corners so you don't create any stress concentrations/points.

Originally Posted by pharmacist25

Great info. Thanks. This is kind of what I was thinking. Going to mix some fiber with epoxy to thicken then use it to fill in gaps, put on aluminum plate while wet til it oozes out sides. Let cure then bolt down tight once cured. SOUnd good?

Sounds like a good plan.

Originally Posted by TomH

If you do go with aluminum plates, round off the corners so you don't create any stress concentrations/points.

This is good advice too.

I don't think you need any leveling goop of any kind...just use some reasonably thick alum plate...it wont need to make up all flush as long as you can get it to tighten well

One thing to keep in mind is that the front mounts are the ones that really need backing plates. When a load it put on the tower from a rider, the front mounts are being pulled up. this is when a backing plate is doing its job. The rear mounts are being pushed down, so backing plates on the inside aren't doing much. You could put them on the outside to distribute the load though.

wheelerwake,

you are correct, IF the rear tower foot is mounted to the top of the deck. In this case, is seems as though the rear feet are mounted to the side of the deck. This puts the downward load on the bolts and bolt holes. Eventually, this is going to oblong the holes and more. This application absolutely needs a backing plate, and one that contacts as much of the inner deck cap surface as possible. Flat surface and glassed in backing plate.

Wouldn't you guys be a little concerned that aluminium plate is too rigid?

I went with regular 1/2" plywood when I did mine. Marine adhesive it to the hull, it contoured to the curve of the hull easily.

My opinion would be to have the backer plate chemically as well as mechanically fixed to the hull.

I'm also concerned about a plate that is too rigid. That's how you can introduce spider cracks.
This is the method I use for boats that were not originally prepped for a tower.
A 3/4" thick birch ply plate with radiused corners. I like softened corners & edges and a plate with just a degree of pliability. Drenched in polyester resin.
An 1/8" thick aluminum back-up plate or some very thick & large washers.
Roughen up the raw interior fiberglass underside of the deck cap. Wipe down with Acetone (easy does it).
Apply a fiberglass reinforced body filler between the deck cap and plate (by Evercoat, 3M/Bondo, etc.). Thoroughly tape up interior areas with sections of a drop cloth. Before it cures, let it ooze out as you tighten the bolts prior to installing the tower. Let it cure for a couple of hours. Remove the bolts and install the tower.

Honestly, I have had so many different replies, its hard to see which way to go. I do worry about the aluminum being rigid and digging in, but I was going to take off the corners and make sure to out a fair amount of fiberglass infused epoxy under it. The fact that it is not a flat surface makes it hard. Now I see a resinated piece of birch with massive washers may do the trick with that same epoxy. Hmmmm. Still looking for guidance. Or maybe there is not right or wrong answer (except for the obvious)

Originally Posted by DavidAnalog

I'm also concerned about a plate that is too rigid. That's how you can introduce spider cracks.
This is the method I use for boats that were not originally prepped for a tower.
A 3/4" thick birch ply plate with radiused corners. I like softened corners & edges and a plate with just a degree of pliability. Drenched in polyester resin.
An 1/8" thick aluminum back-up plate or some very thick & large washers.
Roughen up the raw interior fiberglass underside of the deck cap. Wipe down with Acetone (easy does it).
Apply a fiberglass reinforced body filler between the deck cap and plate (by Evercoat, 3M/Bondo, etc.). Thoroughly tape up interior areas with sections of a drop cloth. Before it cures, let it ooze out as you tighten the bolts prior to installing the tower. Let it cure for a couple of hours. Remove the bolts and install the tower.

I like this. I was worried about the rigidity of the aluminum.
Should I use a hardener in the resin when coating the birch?

Originally Posted by pharmacist25

I like this. I was worried about the rigidity of the aluminum.
Should I use a hardener in the resin when coating the birch?

Most all compounds will come with a catalyst/hardener that must be mixed in per the instructions.
The right mix ratio is always very important. Never get impatient and mix things too hot.

OK. Great. What size/grade of washers do you recommend?
The holes for the 2 bolts are already in the fiberglass hull. Would you recommend putting the plate in place and letting it cure and then drill the holes through for the bolts?
Want to be able to remove bolts if I need to.

I will have a polyester resin coated piece of birch. I am going to use Bondo-Hair to level out the playing field. Should I then use a fiberglass impregnated epoxy to attach the wood to the hull, or should I just put the piece of wood down on top of the Bondo-Hair filler and bolt it on?